Download LA SCOPERTA DELLE PARTICELLE SUBATOMICHE

Laboratorio di Fisica Nucleare
SIS – A. A. 2004/2005
Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
LA SCOPERTA DELLE PARTICELLE SUBATOMICHE
Contesto
L’argomento proposto verrà presentato in una classe 5 a di liceo scientifico con
programma PNI, nell’ultima parte dell’anno scolastico.
Tempi
Sono previste 6 ore di lezione: 4 per poter spiegare adeguatamente i concetti
prefissati e 2 ore da dedicare alle letture proposte, magari in copresenza col docente
di lingua.
Metodologia
Sono previste per lo più lezioni a carattere frontale, per poter trattare adeguatamente
gli argomenti proposti. Per quanto riguarda le letture di brani originali, data anche la
partecipazione del docente di lingua, è invece preferibile una lezione a carattere
dialogato, con possibilità di lavori di gruppo.
Prerequisiti
Per la trattazione del tema preso in esame sono stati individuati i seguenti
prerequisiti, suddivisi in tipo matematico, fisico e chimico.
Prerequisiti matematici
 Il calcolo algebrico
 I logaritmi e le funzioni esponenziali
 Il calcolo delle derivate
Prerequisiti fisici
 Le leggi del moto di Newton
 La legge di conservazione della quantità di moto
 La legge di conservazione dell’energia
 La forza elettrica e il campo elettrico
 La forza magnetica e il campo magnetico
 Il moto delle particelle cariche
 Lo spettro elettromagnetico e le sue radiazioni
Prerequisiti chimici
 Gli isotopi
 Le reazioni chimiche e il loro bilanciamento
 La massa atomica
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Laboratorio di Fisica Nucleare
SIS – A. A. 2004/2005
Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
Obiettivi
Gli obiettivi didattici che ci si prefigge di raggiungere mediante questa unità didattica
si possono suddividere in obiettivi di carattere generale e più specifici, nel modo
seguente.
Obiettivi generali
 Conoscere aspetti di storia della fisica atomica
 Capire che la fisica non è una scienza immutabile ma una scienza in continua
evoluzione e aggiornamento
 Conoscere le basi sperimentali dei vari concetti fisici presentati
 Leggere, capire ed interpretare testi di fisica in lingua originale (lingua inglese)
Obiettivi specifici
 Conoscere le basi sperimentali e teoriche che portano alle scoperte degli elettroni,
del nucleo atomico, dei protoni e dei neutroni.
 Conoscere i dettagli delle varie scoperte presentate, riuscendo a coglierne gli
aspetti essenziali.
 Conoscere e capire le conseguenze delle scoperte presentate, dando loro il giusto
peso nella storia della fisica.
 Analizzare articoli scientifici originali, cogliendone i punti essenziali e capendo
come si strutturavano le ricerche scientifiche di inizio 1900.
Collegamenti interdisciplinari
Poiché riteniamo che sia importante anche la lettura di testi di fisica originali, è
opportuna una collaborazione con il docente di lingua inglese per permettere una
lettura attenta e approfondita dei brani scelti e proposti agli studenti. Questo è,
inoltre, un aiuto dato ai ragazzi in vista della terza prova dell’Esame di Stato.
Materiale e sussidi
Il materiale di lavoro fondamentale risulta essere comunque il libro di testo adottato.
Per i dettagli inerenti i vari esperimenti esaminati in modo più approfondito verranno
fornite agli studenti delle schede preparate dal docente, nel caso che questi non siano
presenti nel libro adottato.
Verranno inoltre fornite agli studenti le letture dei testi originali che saranno trattate a
lezione e commentate insieme.
Valutazione
La valutazione dell’unità didattica proposta avverrà al termine della stessa mediante
un questionario, della durata di un’ora, inerente gli argomenti trattati e le letture
storiche presentate, anche con domande in lingua inglese: tale verifica potrà essere
corretta insieme dai docenti di fisica e di lingua, come ulteriore preparazione alla
terza prova dell’Esame di Stato.
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Laboratorio di Fisica Nucleare
SIS – A. A. 2004/2005
Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
Presentazione dell’argomento
L’unità didattica che tratteremo, sviluppata seguendo una linea guida di tipo storico 
cronologico, tocca i seguenti punti:
o La scoperta dell’elettrone
o La scoperta del nucleo
o I numeri atomici
o La scoperta del protone
o La scoperta del neutrone
1. La scoperta dell’elettrone
Per introdurre questo argomento riteniamo utile proporre agli studenti la lettura L.1
(in allegato), dell’articolo scritto da Thomson, in lingua originale.
In questo modo offriamo ai ragazzi un modo alternativo di studiare la fisica,
analizzando gli scritti degli stessi scopritori.
A seguito della lettura ci proponiamo di chiarire i nodi concettuali degli esperimenti
eseguiti da Thomson, utilizzando come traccia la seguente trattazione.
1.1 Gli esperimenti di Thomson
Presupposti fondamentali per la scoperta dell’elettrone furono la scoperta e l’utilizzo
dei raggi catodici: essi furono trovati in una serie di esperimenti eseguiti nel 18581859 da J. Plücker sulla conduzione dell’elettricità nei gas a bassa pressione. In tali
esperimenti Plücker osservò che, formando il vuoto in un tubo di vetro contenente
due piastre metalliche caricate elettricamente tramite un generatore, poste ai due
estremi del tubo considerato, la luce spariva all’interno del tubo mentre si formava
un bagliore verdastro vicino al catodo (piastra caricata negativamente) e inoltre che
tale fenomeno non dipendeva dalla posizione dell’anodo (piastra caricata
positivamente).
A tale fenomeno fu attribuito il nome di raggi catodici dal fisico E. Goldstein nel
1870, che dimostrò anche che tali raggi non dipendono dal materiale utilizzato per
costruire il catodo.
J. J. Thomson decise di studiare la natura dei raggi catodici: nel 1894 misurò la
velocità di tali raggi, trovando il valore di 200 Km/s, ma si accorse di aver sbagliato il
metodo utilizzato e abbandonò tale ricerca. Nel 1897, effettuando altri esperimenti,
individuò una deflessione dei raggi catodici dovuta a forze elettriche tra i raggi e il
catodo: tale deflessione era rivolta verso l’anodo e molto lontano dal catodo,
confermando così che i raggi catodici sono caricati negativamente.
Per poter trovare una relazione generale che permettesse di misurare
quantitativamente la deflessione dei raggi catodici, Thomson effettuò vari
esperimenti introducendo campi elettrici e magnetici all’interno del tubo a raggi
catodici.
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Laboratorio di Fisica Nucleare
SIS – A. A. 2004/2005
Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
Nel tubo a raggi catodici le particelle passano attraverso una regione, detta regione di
deflessione (la cui lunghezza sarà indicata con L1), in cui sono soggette a forze
elettriche o magnetiche, agenti ad angolo retto rispetto alla loro direzione originale, e
che poi passano attraverso una regione più lunga e priva di forze, detta regione di
drift (la cui lunghezza sarà indicata con L2), in cui si muovono liberamente fino ad
urtare il fondo del tubo, quando urtano il fondo appare un alone luminoso. Questa
osservazione permise a Thomson di misurare lo spostamento del raggio di particelle
prodotto dalle forze agenti su esso, misurando la distanza tra le posizioni dell’alone in
presenza delle forze ed in loro assenza. La formula ricavata dal fisico risulta essere:
Deflessione dei raggi
Forza sul raggio × L1 × L2
(1)
=
al fondo del tubo
Massa del raggio × (velocità del raggio)
2
In un altro esperimento Thomson, per poter determinare il campo elettrico lungo il
cammino dei raggi tra anodo e catodo, introdusse delle forze elettriche prodotte da
piastre metalliche tra loro parallele e cariche.
L’esperienza fu semplificata tenendo conto che le dimensioni delle piastre metalliche
utilizzate erano maggiori della distanza che intercorreva tra esse, fatto che permise
così di ignorare gli effetti di bordo.
In questo modo Thomson potè affermare che il campo elettrico tra le piastre si
trovava ad angolo retto con le piastre stesse e che ogni punto era ugualmente soggetto
al campo indipendentemente dalla sua distanza dalle piastre.
Fu possibile, quindi, concludere che in questo tipo di esperimento la forza elettrica si
trovava ad angolo retto con l’asse del tubo ed aveva grandezza pari alla carica
negativa per una costante, cioè il campo elettrico E . La formula (1) diventa in questo
caso specifico:
Deflessione dei raggi
Carica del raggio × E × L1 × L2
(2)
=
dovuto ad E
Massa del raggio × (velocità del raggio)2
Per determinare il modulo E del campo elettrico E Thomson utilizzò la conoscenza
del voltaggio V del generatore usato per caricare le piastre metalliche e la loro
distanza d:
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Laboratorio di Fisica Nucleare
SIS – A. A. 2004/2005
Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
E
V
d
Nella relazione (2) restano ancora incogniti i valori della massa e della velocità del
raggio catodico: per ovviare a tali misconoscenze, Thomson, effettuando un’ulteriore
esperimento con i raggi catodici, misurò la deflessione prodotta da una forza
magnetica.
In tale esperimento Thomson fece passare il raggio catodico attraverso una regione in
cui creò un campo magnetico uniforme B perpendicolare rispetto alla direzione
originale del raggio arrivando ad ottenere il seguente risultato:
Deflessione dei raggi
Carica del raggio × B × L1 × L2
(3)
=
dovuto a B
Massa del raggio ×
velocità del raggio
Per poter determinare il valore del campo magnetico, Thomson utilizzò il risultato di
W. Weber relativo al calcolo del valore della forza magnetica su una singola
particella carica che si trova su di un filo percorso da corrente elettrica:
(4)
Forza = Carica della particella × velocità × B
Thomson conosceva i valori dei campi elettrico e magnetico all’interno del tubo a
raggi catodici e le lunghezze delle regioni di deflessione e di drift, e aveva misurato
le deflessioni prodotte da forze elettriche e magnetiche: gli era anche noto che non
sarebbe riuscito ad ottenere alcun risultato indipendente per carica e massa del raggio,
ma solo informazioni sul loro rapporto. Un altro problema che si presentò a Thomson
fu quello di conoscere il rapporto di carica e massa del raggio, poiché non conosceva
il valore della velocità: a tale problema ovviò misurando le deflessioni elettrica e
magnetica insieme. Facendo il rapporto delle formule (2) e (3), con le dovute
semplificazioni, ottenne la seguente relazione:
Deflessione magnetica
B
 Velocità
(5)
=
Deflessione elettrica
E
In questo modo, essendo note l’intensità dei campi e misurate le deflessioni
corrispondenti, fu semplice per Thomson ricavare il valore della velocità: con questo
gli fu possibile determinare il rapporto carica su massa (o massa su carica) del raggio
catodico da una delle formule (2) o (3).
Thomson misurò le deflessioni dei raggi catodici dovuti ai campi elettrici e magnetici
per un numero di casi diversi con valori diversi dei campi, differenti valori della
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Laboratorio di Fisica Nucleare
SIS – A. A. 2004/2005
Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
pressione del gas nel tubo, diversi materiali nel catodo e velocità diverse dei raggi
catodici. I suoi risultati sono riportati nella seguente tabella T.1:
Gas in
cathoderay
tube
Air
Air
Air
Hydrogen
Carbon
dioxide
Air
Air
Material
of
cathode
Aluminum
Aluminum
Aluminum
Aluminum
Electric
field
(N/C)
1.5 x 104
1.5 X 104
1.5 x 104
1.5 x 104
Electric
deflection
(m)
0.08
0.095
0.13
0.09
Magnetic
field
(Niamp.m)
5.5 x 10-4
5.4 x 10-4
6.6 x 10-4
6.3 x 10-4
Magnetic
deflection
(m)
0.08
0.095
0.13
0.09
Deduced
velocity
of ray
particles
(mlsec)
2.7 x 107
2.8 x 107
2.2 x 107
2.4 x 107
Aluminum
Platinum
Platinum
1.5 x 104
1.8 x 104
1.0 x 104
0.11
0.06
0.07
6.9 x 10-4
5.0 x 10-4
3.6 x 10-4
0.11
0.06
0.07
2.2 x 107
3.6 x 107
2.8 x 107
Deduced
ratio of
particle
mass
to charge
(kgIC)
1.4 x 10-11
1.1 x 10-11
1.2 x 10-11
1.6 x 10-11
1.6 x 10-11
1.3 x 10-11
1.0 x 10-11
In tutti i casi riportati in tabella, Thomson utilizzò raggi catodici tali per cui la
distanza percorsa dal raggio sotto l’influenza delle forze elettriche e magnetiche era
di 0.05 m e la lunghezza della regione di drift era di 1.1m.
E’ interessante notare che Thomson, nel pubblicare i risultati da lui ottenuti nei vari
esperimenti, non ha mai corredato tali valori con le incertezze sperimentali delle sue
misure. Tuttavia, esaminando il valore del rapporto massa su carica, oggi è possibile
concludere che tali valori sono affetti, in ogni direzione, da un errore statistico pari a
0.210-11kg/C.
Per determinare il valore del campo elettrico esistente tra le due piastre di alluminio
cariche del tubo a raggi catodici, Thomson effettuò ripetuti esperimenti collegando le
piastre ad una batteria da 225 Volt (che corrisponde al lavoro fatto per far passare una
carica elettrica da una piastra all’altra) e posizionando le piastre a 0.015m l’una
dall’altra riuscì a ricavare che
225 J / C
 1.5  104 N / C
0.015m
Tale espressione è esattamente il valore del campo elettrico, come si può leggere
nelle prime righe della tabella T.1.
Il metodo utilizzato da Thomson per ottenere il valore del rapporto carica su massa
dell’elettrone è il seguente: il raggio catodico viene diretto in un collettore metallico
che misura la carica elettrica delle particelle e la loro energia cinetica, convertendola
in calore. Il rapporto tra il calore e la carica elettrica depositata nel collettore dà
l’energia cinetica e la carica di ogni raggio cioè
½  massa  (velocità)2
Calore
=
Carica depositata
(6)
Carica elettrica delle particelle
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Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
La combinazione dei parametri del raggio che si trovano a secondo membro della (6)
è la stessa che si trova nella formula (2); dunque essa può essere determinata
misurando il rapporto tra calore e carica depositata piuttosto che la deflessione dovuta
al campo elettrico o il voltaggio tra catodo e anodo.
I risultati ottenuti da Thomson utilizzando tre diversi tubi a raggi catodici sono
riportati nella tabella T.2:
Gas in cathode
ray tube
Tube 1:
Air
Air
Air
Air
Air
Air
Air
Hydrogen
Hydrogen
Carbon dioxide
Carbon dioxide
Carbon dioxide
Measured ratio
of heat energy
to charge deposited
(J/C)
4.6x 103
1.8 x 104
6.1 x 103
2.5 x 104
5.5 x 103
10 4
104
6 x 104
2.1 x 104
8.4 x 103
1.47x 104
3 x 104
Mass x Velocity
Electric charge
(Kg M/sec C)
Deduced
velocity
(m/sec)
Deduced
mass/charge
ratio
(Kg/C)
2.3x 10-4
3.5 x 10-4
2.3 x 10-4
4.0 x 10-4
2.3 x 10-4
2 .85x 10-4
2.85x 10-4
2.05x 10-4
4.6 x 10-4
2.6 x 10-4
3.4 x 10-4
4.8 x 10-4
4 x 107
108
5.4x 107
1.2x 108
4.8x 107
7x 107
7x 107
6x 107
9.2x 107
7.5x 107
8.5x 107
1.3x 108
0.57x 10-11
0.34x 10-11
0.43x 10-11
0.32x 10-11
0.48x 10-11
0.4 x 10-11
0.4 x 10-11
0.35x 10-11
0.5 x 10-11
0.4 x 10-11
0.4 x 10-11
0.39x 10-11
Tube2:
Air
Air
Air
Hydrogen
Air
Carbon dioxide
Air
Hydrogen
Hydrogen
Air
Air
2.8 x 103
4.4x 103
3.5x 103
2.8 x 103
2.5 x 103
2 x 103
1.8 x 103
2.8 x 103
4.4 x 103
2.5 x 103
4.2 x 103
1.75x 10-4
1.95x 10-4
1.81x 10-4
1.75 x 10-4
1.60 x 10-4
1.48 x 10-4
1.51 x 10-4
1.75 x 10-4
2.01 x 10-4
1.76 x 10-4
2 x 10-4
3.3x 107
4.1x 107
3.8x 107
3.3 x 107
3.1 x 107
2.5 x 107
2.3 x 107
3.3 x 107
4.4 x 107
2.8 x 107
4.1 x 107
0.53x 10-11
0.47x 10-11
0.47x 10-11
0.53 x 10-11
0.51 x 10-11
0.54 x 10-11
0.63 x10-11
0.53 x 10-11
0.46 x 10-11
0.61 x10-11
0.48 x10-11
Tu be 3:
Air
Air
Hydrogen
2.5 x 103
3.5 x 103
3 x 103
2.2 x 10-4
2.25 x 10-4
2.5 x 10-4
2.4x 107
3.2 x 107
2.5x107
0.9 x10-11
0.7 x 10-11
1.0 x 10-11
Riteniamo importante affrontare questo argomento anche da un punto di vista più
tecnico, introducendo i risultati ottenuti da Thomson con formule matematiche in
notazione moderna.
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Laboratorio di Fisica Nucleare
SIS – A. A. 2004/2005
Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
1.2 Notazione moderna dei risultati di Thomson
1.2.1 La deflessione elettrica e magnetica dei raggi catodici
Mostriamo, ora, come si possa utilizzare la seconda legge di Newton per calcolare la
deflessione dei raggi catodici e come questa misurazione possa a sua volta essere
usata per calcolare il rapporto massa su carica del raggio di particelle.
Supponendo che una forza F, agente in una diversa direzione rispetto a quella del
moto delle particelle, sia esercitata sul raggio catodico, ed alle particelle sia data
un’accelerazione nella direzione di F, per la seconda legge della dinamica si trova
che a  F m con m massa delle particelle.
Se le particelle sono esposte alla forza F per un generico tempo t, esse acquistano una
componente della velocità perpendicolare alla direzione del loro moto, pari a
v  t  a 
tF
m
(1’)
Supponendo che le particelle abbiano una componente della velocità v nella direzione
originale del raggio, e viaggino con tale velocità attraverso la regione di deflessione
avente lunghezza l ed in cui agisce la forza F allora il tempo risulta essere t 
sostituita nella (1’) dà
v 
l
che
v
lF
mv
(2’)
Lasciata la regione di deflessione, il raggio viaggia attraverso la regione di drift, di
lunghezza L, in una direzione poco diversa da quella originale e con una componente
della velocità pari a v. Il tempo T trascorso nella regione di drift risulta essere T 
L
:
v
in tale tempo le particelle si muovono anche in direzione perpendicolare a quella
originale, dunque il loro spostamento d dal cammino originario quando giungono alla
fine della zona di drift è
d  v  T
Sostituendo la (2’) e la relazione T 
d
(3’)
L
nella (3’) si ottiene:
v
L l  F F l  L


v mv
m  v2
(4’)
Se la generica forza F risulta essere una forza elettrica ( Fe  e  E , con e carica
elettrica ed E il modulo del campo elettrico), la (4’) diventa:
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Laboratorio di Fisica Nucleare
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Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
de 
(5’)
e E l  L
m  v2
mentre nel caso di una forza magnetica ( Fm  e  v  B , con e carica elettrica, v velocità
perpendicolare al campo magnetico e B il modulo del campo magnetico), la (4’)
diventa:
e B l  L
(6’)
d 
m
mv
Facendo il rapporto tra la (6’) e la (5’) si ha
e B l  L
dm
Bv
da cui si ricava la
 mv 
e E l  L
de
E
m  v2
dipendenza della velocità dal campo elettrico e magnetico:
E d 
v      m 
 B   de 
(7’)
Sostituendo la relazione appena ottenuta nella (6’) :
dm 
e  B  l  L e  B 2  l  L  de

E  dm
m  E  dm
m
B  de
che esplicitata rispetto ad m/e dà
m B 2  l  L  de

e
E  d m2
(8’)
In questo modo si è ottenuto il rapporto massa su carica delle particelle dei raggi
catodici a partire dalla misurazione della loro deflessione.
1.2.2 La conservazione energetica negli esperimenti sui raggi catodici
Mostriamo qui di seguito come si possono utilizzare i principi di conservazione
dell’energia per calcolare le proprietà dei raggi catodici.
Thomson mise un collettore al termine del tubo a raggi catodici e misurò sia la carica
elettrica Q sia il calore H depositato su esso. In accordo con la legge di conservazione
dell’energia il calore energetico è uguale all’energia cinetica del raggio di particelle
che colpisce il collettore. Se le particelle sono N e viaggiano alla velocità v si ha
H 
1
 m  v2  N
2
(1’’)
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Laboratorio di Fisica Nucleare
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Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
Poiché la carica si conserva, la carica totale nel collettore deve essere uguale alla
carica di tutte le N particelle del catodo cioè Q = eN . Facendo il rapporto tra queste
due relazioni si ottiene
H m  v2

Q
2e
(2’’)
Dividendo il valore di dm dato dalla relazione (6’), per i valori noti di B, l ed L si
trova
I
mv
e
chiamando I l’inverso del rapporto calcolato.
Dividendo tra loro la (3’’) e la (2’’), si ricava la velocità come v 
(3’’)
2H
che sostituita
QI
nella (3’’) dà il rapporto cercato tra massa e carica:
m
I2

e 2 H
(4’’)
Q
2. La scoperta del nucleo
Fin dall’antichità i fisici sapevano che gli atomi sono elettricamente neutri, ma la
scoperta degli elettroni ad opera di Thomson introdusse una carica negativa
all’interno dell’atomo. Per questo, a partire dalla scoperta dell’elettrone, gli sforzi dei
fisici si sono concentrati nella ricerca di un qualche materiale avente carica positva e
quindi della sua collocazione all’interno dell’atomo.
Nel 1903 Thomson era giunto a ritenere che gli elettroni fossero bloccati entro una
matrice continua di materia caricata positivamente, come l’uvetta all’interno di un
panettone (da qui il nome di “modello a panettone”).
Quindi si susseguirono alcune teorie atomiche, fino a giungere agli esperimenti
eseguiti da Rutherford all’Università di Manchester nel 1909-11 da cui si evince la
presenza del nucleo degli atomi. Grazie a questo si capisce che la carica positiva è
tutta concentrata nel nucleo intorno al quale ruotano gli elettroni, così come il fatto
che tale nucleo contiene quasi tutta la massa dell’atomo.
Dalton J., più tardi, scoprì che gli atomi hanno masse generalmente vicine a multipli
della massa dell’atomo di idrogeno e che i nuclei degli atomi constano di particelle
pesanti cariche positivamente, identificabili con il nucleo dell’atomo di idrogeno.
L’esperimento fondamentale che segnò la scoperta del nucleo atomico fu quello
eseguito da Rutherford, Geiger e Marsden dello scattering  nel 1911.
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Ferrari Trecate Irene
Da tale esperienza si capisce allora che l’atomo è, per lo più, costituito da spazio
vuoto, con un piccolo nucleo massivo e carico positivamente, contornato da elettroni
orbitanti.
Alla base di tale esperienza vi è l’ipotesi, elaborata da Rutherford, che l’atomo sia un
microscopico sistema solare in cui gli elettroni, simili ai pianeti, ruotano intorno ad
una massa positiva, il nucleo.
Tale esperienza evidenziò che una particella positiva , invece di procedere diritta, o
quasi diritta, lungo la direzione originaria subiva delle forti deflessioni. Tale
mutamento di direzione era spiegata solo se questa particella interagiva con una
distribuzione di cariche positive non disposte su tutto il volume atomico ma
concentrate in un nucleo centrale piccolo e pesante.
3. I numeri atomici
Il primo calcolo dei numeri atomici fu eseguito da Rutherford, Geiger e Marsden in
occasione dall scattering .
In tale caso Geiger e Marsden nel 1909 trovarono per il numero che indica la carica
elettrica del nucleo dell’oro il valore approssimativo di Z= 180.
Nel 1911 Rutherford usò dati più precisi di Geiger e Marsden, trovando il valore di
Z= 97 in un caso e Z= 114 in un altro. Rutherford usò poi i dati noti relativi allo
scattering  per determinare lo Z di altri elementi: tali valori sono riportati nella
seguente tabella.
Element
Aluminum
Copper
Silver
Platinum
Atomic weigbt
27
63
108
194
Nuclear charge Z in
units of electron charge
as deduced
as known
by Rutherford today
22
13
42
29
78
47
138
78
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La scarsa precisione dei risultati ottenuti da Rutherford è irrilevante rispetto
all’importanza del risultato principale raggiunto da lui stesso: l’esistenza di un nucleo
piccolo, pesante e carico.
La scoperta del nucleo ha prodotto immediatamente una importante conseguenza: la
teoria atomica, elaborata da Bohr, nel 1913 basata sulle ipotesi quantistiche.
Nello stesso tempo H.G.J.Moseley misurò la lunghezza d’onda dei raggi , con
grande precisione utilizzando dei cristalli per produrre una lunghezza d’onda
dipendente dalla curvatura del raggio.
Dopo la pubblicazione del lavoro di Bohr del 1913, Moseley iniziò a misurare la
carica del nucleo di una serie di elementi, di medio peso atomico, che emettono raggi
 in un conveniente range di lunghezza d’onda.
Element
Nuclear charge
(in units of electron charge)
Calcium
20.00
Scandium
not measured
Atomic weight
40.09
44.1
Titanium
21.99
48.1
Vanadium
22.96
51.06
Chromium
23.98
52.0
Manganese
24.99
54.93
Iron
25.99
55.85
Cobalt
27.00
58.97
Nickel
28.04
58.68
Copper
29.01
63.57
Zinc
30.01
65.37
Il fatto che la carica fosse un multiplo della carica dell’elettrone diede nuova fiducia a
Moseley sulle proprie misure e sulla teoria di Bohr.
Il risultato inatteso fu che la carica del nucleo aumenta di una unità nel passaggio da
un elemento al successivo, avente peso atomico superiore: come riconosciuto da
Moseley tale modello si estende oltre gli elementi finora noti.
Quindi, a parte poche eccezioni, il numero che indica il posto di un elemento nella
tavola periodica degli elementi, se essi sono ordinati in base al loro peso atomico, è lo
stesso della carica elettrica del nucleo, secondo l’unità di carica degli elettroni: tale
quantità è chiamata numero atomico.
E’ dunque possibile, a questo punto, determinare la carica nucleare di ogni elemento
e, per deduzione, il numero degli elettroni di tali elementi (atomi) solo leggendo la
tavola atomica degli elementi.
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4. La scoperta del protone
Per quanto riguarda la scoperta del protone abbiamo pensato di presentare tale
argomento dapprima mediante un approccio storico e in seguito trattare più
approfonditamente la fisica dell’esperienza di Rutherford corredata dalla lettura di un
brano dello stesso.
4.1. Esperienza di Rutherford
4.1.1. Approccio storico
Nel 1815, ancora prima della scoperta dell’elettrone, il chimico inglese W. Prout
aveva osservato che le masse atomiche dei vari elementi erano quasi tutte molto
vicine ad un multiplo intero della massa dell’idrogeno: questo fatto gli suggerì che gli
atomi degli elementi più pesanti fossero composti da più atomi di idrogeno.
Tuttavia data la presenza di alcune eccezioni alla regola del multiplo intero, come
quella rappresentata dal rame, e considerata la scarsa precisione dei dati di cui Prout
poteva disporre, la sua congettura non fu tenuta in considerazione dagli studiosi
dell’epoca, ma un secolo più tardi la scoperta dell’esistenza degli isotopi dimostrò
che la congettura di Prout aveva dei fondamenti validi.
Quando si scoprì che il rapporto fra la massa dell’elettrone e quella del protone è pari
a 1/1836 divenne chiaro che la massa dell’atomo è tutta concentrata nel nucleo,
pertanto la regola scoperta da Prout si poteva applicare ai nuclei.
Fu del tutto naturale, pertanto, concludere che i nuclei dei vari nuclidi dovevano
essere composti da più nuclei di idrogeno; al nucleo d’idrogeno venne dato il nome di
protone, particella elementare con carica uguale ed opposta a quella dell’elettrone e
di massa circa 1836 volte più grande.
La conferma del fatto che i nuclei sono composti di protoni si deve ad uno degli
esperimenti di bombardamento della materia con particelle  compiuti da Rutherford.
Egli osservò che, usando l’aria come bersaglio, si producevano particelle cariche
positivamente caratterizzate da un percorso particolarmente lungo e che risultarono
essere nuclei di idrogeno. Questo fatto fu dapprima interpretato pensando che le
particelle osservate fossero atomi di idrogeno presenti nell’aria e ionizzati dalle
particelle . Tuttavia tale spiegazione non soddisfò Rutherford, il quale ripeté con
sempre maggior cura tale esperimento per tre anni, giungendo nel 1919 alla
conclusione che un atomo di azoto colpito da una particella  si disintegra in due
parti, di cui una è un nucleo di idrogeno.
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idrogeno
azoto
e/io
+
OSSigeno
Nella reazione da lui osservata un nucleo di azoto (con numero atomico Z=7 e
numero di massa atomica A=14) assorbe una particella , ossia un nucleo di elio
(Z=2, A=4): il nuovo nucleo così formato si spezza in due parti: un nucleo di un
isotopo dell’ossigeno (Z=8, A=17) ed un nucleo d’idrogeno (Z=1, A=1), secondo la
reazione chimica
14
4
17
1
7N + 2He  8O + 1H
Come si vede, la somma dei numeri atomici e dei numeri di massa dei due nuclei
iniziali è uguale alla somma di quelli dei nuclei prodotti dalla reazione, il che è in
accordo rispettivamente con le leggi di conservazione della carica e della massa.
Dopo avere illustrato tale esperienza ci proponiamo di leggere in aula con gli studenti
alcuni passi dell’articolo di Rutherford del 1919, in cui egli stesso illustrava i risultati
ottenuti bombardando con una particella  un atomo di azoto. Tale lettura L.2 è posta
come allegato al nostro lavoro.
4.1.2. Approccio fisico
Per i vent’anni successivi alla scoperta del nucleo i fisici credettero che i nuclei di
tutti gli elementi conosciuti consistessero di nuclei d’idrogeno ed elettroni. Infatti, ad
esempio, l’elio (He) ha peso atomico A = 4 e numero atomico Z = 2 dunque il suo
nucleo (che è una particella ) consisteva, secondo tale teoria, di due nuclei di
idrogeno e due elettroni per poter avere una carica pari a due unità elettriche.
Rutherford, nel 1917, eseguì un’esperienza che lo portò a notare come una sorgente
metallica provvista di un emettitore di particelle  da radio C emette particelle che
producono delle scintillazioni su uno schermo di solfuro di zinco ad una distanza
superiore rispetto alla portata delle particelle  in aria.
Rutherford studiò tale fenomeno immergendo l’apparato sperimentale in un campo
magnetico: in questo caso potè concludere che le particelle responsabili delle
scintillazioni osservate sono dei nuclei d’idrogeno, da lui chiamati protoni.
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Quello che però ancora non sapeva era se tali protoni rinculavano dagli atomi
d’idrogeno presenti sulla sorgente metallica e venivano bloccati dalle particelle ,
oppure se essi erano colpiti da elementi diversi dall’idrogeno ma più pesanti.
Rutherford, allora, studiò tale fenomeno considerando una sorgente di radio C
all’interno di una scatola metallica, in cui praticò il vuoto, con un foro ricoperto da
una lamina d’argento.
Tale lamina d’argento permise alle particelle  di uscire dalla scatola e colpire uno
schermo di solfuro di zinco, ma anche di mantenere il vuoto all’interno della scatola.
In questo modo fu possibile osservare la variazione del numero di scintillazioni a
seconda del materiale di un foglio metallico inserito tra la lamina d’argento e lo
schermo, o a seconda del tipo di gas immesso nella scatola: Rutherford trovò così che
nella maggior parte dei casi la percentuale di scintillazioni decresceva, in
proporzione, con il potere frenante dei fogli metallici e dei gas interposti. Nel
momento però in cui nella scatola veniva inserita dell’aria secca, la percentuale di
scintillazioni aumentava.
Ripetendo, allora, tale esperimento immettendo nella scatola di volta in volta i vari
costituenti dell’aria, Rutherford capì che l’effetto osservato era dovuto alle collisioni
delle particelle  della sorgente di radio C con i nuclei di azoto dell’aria: il processo
così scoperto da Rutherford fu dunque la disintegrazione del nucleo di azoto ad opera
di una particella  che ne estrasse i protoni.
La ragione per cui tale fenomeno non era mai stato osservato prima è che la
repulsione elettrica tra una particella , caricata positivamente, ed un nucleo pesante
come quello d’oro, avente carica positiva pari a 79 unità elettroniche, era troppo forte
per permettere alla particella  di passare vicino al nucleo.
5. La scoperta del neutrone
Per introdurre questo argomento si presentano ai ragazzi le tappe che portano alla
scoperta del neutrone. Si tratterà in modo approfondito l’esperimento di Chadwick e
il calcolo della massa del neutrone. A conclusione di questo percorso proponiamo
agli studenti la lettura L3 dell’articolo di Chadwick sulla sua scoperta.
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5.1 L’esperienza di Chadwick
Con la scoperta del protone, tuttavia, i fisici credettero che l’atomo fosse composto di
soli protoni ed elettroni: i protoni confinati nel nucleo e gli elettroni orbitanti intorno;
ma i dati a loro disposizione non confermavano tale ipotesi in quanto se questa fosse
stata verificata si avrebbe sempre, per ogni singolo atomo, A = Z. Invece si osservò
che il numero di massa A è sempre maggiore del numero atomico Z: ad eccezione
dell’idrogeno il rapporto A/Z varia tra 2 e 2.5 a partire dai nuclei più leggeri per
arrivare a quelli più pesanti.
Per spiegare tale discrepanza, Rutherford aveva inizialmente pensato che alcuni degli
elettroni potessero cadere sul nucleo per effetto dell’attrazione reciproca: tale idea fu
da lui scartata in quanto risultava essere in contrasto con il modello atomico di Bohr
(1913) secondo cui le orbite degli elettroni sono stabili.
Secondo un’altra ipotesi il nucleo atomico era composto non solo da protoni ma
anche da “elettroni nucleari” posti su orbite più piccole di quelle degli elettroni: tale
idea non fu presa in considerazione.
Una terza ipotesi, formulata da Rutherford nel 1920, introduceva l’esistenza di nuove
particelle chiamate “protoni neutri” o neutroni, aventi la stessa massa del protone ma
prive di carica elettrica, all’interno del nucleo. La verifica sperimentale di tale ipotesi
si presentava difficoltosa, infatti i metodi usati fino ad allora per rivelare le particelle
erano basati sul fatto che esse erano portatrici di carica elettrica ed inoltre non era
nota nessuna sorgente naturale di neutroni.
Intorno al 1930, il fisico W. Bothe aveva scoperto che, sottoponendo atomi di boro
(B) o berillio (Be) ad un bombardamento con particelle  veloci prodotte da polonio
(Po), questi emettevano una radiazione molto penetrante, molto simile ai raggi 
poiché priva di carica, ma con energia superiore a qualsiasi altra radiazione nota.
I. Curie e F. Joliot inviarono questa radiazione contro un blocco di paraffina, un
materiale ricco di idrogeno, scoprendo che da tale blocco era emesso un numero
elevato di protoni.
Ammesso che la radiazione incidente sulla paraffina fosse composta di raggi ,
l’unico meccanismo noto che potesse giustificare l’emissione di protoni era l’effetto
Compton: gli ipotetici fotoni  incidenti avrebbero estratto i protoni dagli atomi
d’idrogeno allo stesso modo in cui, nell’esperimento di Compton, i fotoni estraevano
elettroni dagli atomi di carbonio. Tuttavia, in base ai calcoli, si dimostrò che per
estrarre un protone un fotone  avrebbe dovuto avere un’energia 5 volte superiore a
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quella posseduta dalla radiazione incidente sulla paraffina ed inoltre il numero di
protoni emessi era di gran lunga superiore rispetto a quello atteso in base all’effetto
Compton; tutto questo sembrava violare le leggi di conservazione dell’energia e della
quantità di moto.
Per cercare di unificare tali scoperte con le leggi di conservazione della fisica
classica, J. Chadwick, nel 1932, studiò le radiazioni emesse dal berillio indirizzandole
contro altri materiali, diversi dalla paraffina.
In questo modo trovò che altri nuclei oltre all’idrogeno rinculavano se colpiti da tale
radiazione, ma con una velocità di molto inferiore rispetto a quella dell’idrogeno. Il
modello di diminuzione della velocità di rinculo, al crescere del peso atomico del
nucleo colpito, era esattamente quello atteso se la radiazione emessa dal berillio
veniva considerata non come radiazione elettromagnetica ma come una particella di
massa simile a quella del protone.
Come nel caso della collisione di una particella  con un nucleo, in una collisione
frontale di radiazioni emesse dal berillio, con massa e velocità note, si avevano
comunque due incognite: la velocità finale del raggio e la velocità di rinculo del
nucleo colpito. Essendo noti i principi di conservazione fu possibile ricavare la
velocità di rinculo secondo la seguente formula:
Velocità
Velocità iniziale
di = 2 
del

rinculo
raggio
Peso atomico raggio
(A)
Peso atomico raggio + Peso atomico nucleo
La velocità iniziale del raggio di particelle non era nota ma, considerando il rapporto
delle velocità di rinculo di due diversi nuclei colpiti, si potè trovare tale valore
basandosi sul peso atomico del raggio.
Utilizzando dati numerici a lui noti, Chadwick osservò che la stessa radiazione
emessa dal berillio causava il rinculo di un nucleo d’idrogeno ( M = 1) con velocità
di 3.3107 m/s ed un rinculo di un nucleo di azoto (M = 14) con velocità di 4.7106
m/s. Allora per una fissata velocità iniziale ed un fissato peso atomico del raggio di
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particelle, la formula (A) mostra che le velocità di rinculo sono inversamente
proporzionali alla somma dei pesi atomici di raggio e nucleo colpito ossia
3.3  107 14  M raggio

4.7  106
1  M raggio
da cui si ricava che il peso atomico del raggio emesso quando il berillio è bombardato
da una particella  vale 1.16.
Sapendo che i dati utilizzati erano soggetti ad un certo errore sperimentale, se pur
abbastanza precisi, Chadwick concluse che la massa del raggio di particelle doveva
essere circa pari a quella del protone. Inoltre il grande potere penetrante del raggio di
particelle di berillio indusse i fisici a supporre che esse fossero elettricamente neutre.
Date le proprietà di massa e carica neutra delle particelle prodotte dai raggi di
berillio, Chadwick, come già nel 1920 Rutherford aveva ipotizzato, ritenne che il
neutrone fosse composto da elettroni e protoni e non fosse una particella elementare.
Un’ ulteriore conferma di quanto appena affermato è data dal calcolo della massa del
neutrone.
5.2 Il calcolo della massa del neutrone
Si consideri un neutrone che si muova con velocità v e colpisca un protone fermo, di
massa mp nota, di un pezzo di paraffina. Supponiamo l’urto elastico e scriviamo le
leggi di conservazione dell’energia e della quantità di moto, pensando che il neutrone
rimbalzi indietro ed il protone venga scagliato in avanti [il che consente di ragionare
semplicemente sui moduli].
paraffina
neutrone
protone
v’
vp
Vp=0
v
Con riferimento alla nomenclatura usata in figura, possiamo scrivere:
mv 2
mv' 2
mpvp


2
2
mv  mv' m p v p
2
2
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Ricavando v’ dalla seconda equazione e sostituendola nella prima, si ottiene:
2 2
mv2 m v  mp v p  2mmp vvp mp v p


2
2m
2
2
2
2
Moltiplicando ora per m e dividendo per mpvp e semplificando, si ottiene:
m p v p  mv p  2mv .
Da quest’ultima si ottiene vp: v p 
2mv
che, tuttavia, anche se si misura vp, non ci
mp  m
consente di trovare m essendo ancora incognita v.
Supponiamo allora di ripetere l’esperimento scagliando il neutrone contro atomi di
azoto allo stato gassoso: con lo stesso procedimento si ottiene: vazoto 
dividendo membro a membro, si ha
m
vazotomazoto  v p m p
v p  vazoto
vp
vazoto

m p  mazoto
mp  m
2mv
e,
mazoto  m
che, risolta rispetto ad m diventa
.
Bibliografia
 BORN M., 1976, Fisica atomica, Boringhieri
 CAFORIO A.  FERILLI A., 1995, Physica vol. 3, Le Monnier
 CALDIROLA P. – CASATI G. – TEALDI F., 1993, Corso di fisica per licei scientifici
vol. 3, Ghisetti e Corvi
 NUVOLI L. – PIANO A., 1997, Fisica Liceo Scientifico vol. 3 , Lattes
 WEINBERG S., 1983, The discovery of subatomic particles, Scientific American
Library
Sitografia
 webscuola.tin.it
 www.aip.org/history/electron/jj1987.htm
 dbhs.wvusd.k12.ca.us/webdocs/chem-history/Chadwick-neutron-letter.htm
 www.to.infn.it/~maina/didattica/SIS/Lab-FN.html
 www.ph.unito.it/~maina/didattica/SIS/CPEP-NP-02.pdf
 www.torinoscienza.it
 www.chemcases.com/nuclear/nc-01.htm
 web.lemoyne.edu/giunta/paperabc.html
In particolare: web.lemoyne.edu/giunta/thomson1897.html
web.lemoyne.edu/giunta/rutherford.html
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Allegati
L.1 Lettura di Thomson
L.2 Lettura di Rutherford
L.3 Lettura di Chadwick
L.1 Lettura dell’articolo di Thomson
J. J. Thomson (1856-1940)
Cathode Rays
Philosophical Magazine, 44, 293 (1897). [facsimile from Stephen Wright, Classical
Scientific Papers, Physics (Mills and Boon, 1964).]
The experiments* discussed in this paper were undertaken in the hope of gaining some
information as to the nature of the Cathode Rays. The most diverse opinions are held as to
these rays; according to the almost unanimous opinion of German physicists they are due
to some process in the aether to which--inasmuch as in a uniform magnetic field their
course is circular and not rectilinear--no phenomenon hitherto observed is analogous:
another view of these rays is that, so far from being wholly aetherial, they are in fact wholly
material, and that they mark the paths of particles of matter charged with negative
electricity. It would seem at first sight that it ought not to be difficult to discriminate between
views so different, yet experience shows that this is not the case, as amongst the
physicists who have most deeply studied the subject can be found supporters of either
theory.
The electrified-particle theory has for purposes of research a great advantage over the
aetherial theory, since it is definite and its consequences can be predicted; with the
aetherial theory it is impossible to predict what will happen under any given circumstances,
as on this theory we are dealing with hitherto unobserved phenomena in the aether, of
whose laws we are ignorant.
The following experiments were made to test some of the consequences of the electrifiedparticle theory.
Charge carried by the Cathode Rays
If these rays are negatively electrified particles, then when
they enter an enclosure they ought to carry into it a charge
of negative electricity. This has been proved to be the case
by Perrin, who placed in front of a plane cathode two coaxial
metallic cylinders which were insulated from each other: the
outer of these cylinders was connected with the earth, the
inner with a gold-leaf electroscope. These cylinders were
closed except for two small holes, one in each cylinder,
placed so that the cathode rays could pass through them
into the inside of the inner cylinder. Perrin found that when
the rays passed into the inner cylinder the electroscope
received a charge of negative electricity, while no charge
went to the electroscope when the rays were deflected by a
magnet so as no longer to pass through the hole.
This experiment proves that something charged with
negative electricity is shot off from the cathode, travelling at right angles to it, and that this
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something is deflected by a magnet; it is open, however, to the objection that it does not
prove that the cause of the electrification in the electroscope has anything to do with the
cathode rays. Now the supporters of the aetherial theory do not deny that electrified
particles are shot off from the cathode; they deny, however, that these charged particles
have any more to do with the cathode rays than a rifle-ball has with the flash when a rifle is
fired. I have therefore repeated Perrin's experiment in a form which is not open to this
objection. The arrangement used was as follows:
--Two coaxial cylinders (fig. 1) with slits in them are placed in a bulb connected with the
discharge-tube; the cathode rays from the cathode A pass into the bulb through a slit in a
metal plug fitted into the neck of the tube; this plug is connected with the anode and is put
to earth. The cathode rays thus do not fall upon the cylinders unless they are deflected by
a magnet. The outer cylinder is connected with the earth, the inner with the electrometer.
When the cathode rays (whose path was traced by the phosphorescence on the glass) did
not fall on the slit, the electrical charge sent to the electrometer when the induction-coil
producing the rays was set in action was small and irregular; when, however, the rays
were bent by a magnet so as to fall on the slit there was a large charge of negative
electricity sent to the electrometer. I was surprised at the magnitude of the charge; on
some occasions enough negative electricity went through the narrow slit into the inner
cylinder in one second to alter the potential of a capacity of 1.5 microfarads by 20 volts. If
the rays were so much bent by the magnet that they overshot the slits in the cylinder, the
charge passing into the cylinder fell again to a very small fraction of its value when the aim
was true. Thus this experiment shows that however we twist and deflect the cathode rays
by magnetic forces, the negative electrification follows the same path as the rays, and that
this negative electrification is indissolubly connected with the cathode rays.
When the rays are turned by the magnet so as to pass through the slit into the inner
cylinder, the deflexion of the electrometer connected with this cylinder increases up to a
certain value, and then remains stationary although the rays continue to pour into the
cylinder. This is due to the fact that the gas in the bulb becomes a conductor of electricity
when the cathode rays pass through it, and thus, though the inner cylinder is perfectly
insulated when the rays are not passing, yet as soon as the rays pass through the bulb the
air between the inner cylinder and the outer one becomes a conductor, and the electricity
escapes from the inner cylinder to the earth. Thus the charge within the inner cylinder
does not go on continually increasing; the cylinder settles down into a state of equilibrium
in which the rate at which it gains negative electricity from the rays is equal to the rate at
which it loses it by conduction through the air. If the inner cylinder has initially a positive
charge it rapidly loses that charge and acquires a negative one; while if the initial charge is
a negative one, the cylinder will leak if the initial negative potential is numerically greater
than the equilibrium value.
Deflexion of the Cathode Rays by and Electrostatic Field.
An objection very generally urged against the view that the cathode rays are negatively
electrified particles, is that hitherto no deflexion of the rays has been observed under a
small electrostatic force, and though the rays are deflected when they pass near
electrodes connected with sources of large differences of potential, such as induction-coils
or electrical machines, the deflexion in this case is regarded by the supporters of the
aetherial theory as due to the discharge passing between the electrodes, and not primarily
to the electrostatic field. Hertz made the rays travel between two parallel plates of metal
placed inside the discharge-tube, but found that they were not deflected when the plates
were connected with a battery of storage-cells; on repeating this experiment I at first got
21
Laboratorio di Fisica Nucleare
SIS – A. A. 2004/2005
Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
the same result, but subsequent experiments showed that the absence of deflexion is due
to the conductivity conferred on the rarefied gas by the cathode rays. On measuring this
conductivity it was found that it diminished very rapidly as the exhaustion increased; it
seemed then that on trying Hertz's experiment at very high exhaustions there might be a
chance of detecting the deflexion of the cathode rays by an electrostatic force.
The apparatus used is represented in fig. 2.
The rays from the cathode C pass through a slit in the anode A, which is a metal plug
fitting tightly into the tube and connected with the earth; after passing through a second slit
in another earth-connected metal plug B, they travel between two parallel aluminium plates
about 5 cm. long by 2 broad and at a distance of 1.5 cm. apart; they then fall on the end of
the tube and produce a narrow well-defined phosphorescent patch. A scale pasted on the
outside of the tube serves to measure the deflexion of this patch. At high exhaustions the
rays were deflected when the two aluminium plates were connected with the terminals of a
battery of small storage cells; the rays were depressed when the upper plate was
connected with the negative pole of the battery, the lower with the positive, and raised
when the upper plate was connected with the positive, the lower with the negative pole.
The deflexion was proportional to the difference of potential between the plates, and I
could detect the deflexion when the potential-difference was as small as two volts. It was
only when the vacuum was a good one that the deflexion took place, but that the absence
of deflexion is due to the conductivity of the medium is shown by what takes place when
the vacuum has just arrived at the stage at which the deflexion begins. At this stage there
is a deflexion of the rays when the plates are first connected with the terminals of the
battery, but if this connexion is maintained the patch of the phosphorescence gradually
creeps back to its undeflected position. This is just what would happen if the space
between the plates were a conductor, though a very bad one, for then the positive and
negative ions between the plates would slowly diffuse, until the positive plate became
coated with negative ions, the negative plate with positive ones; thus the electric intensity
between the plates would vanish and the cathode rays be free from electrostatic force.
Another illustration of this is afforded by what happens when the pressure is low enough to
show the deflexion and a large difference of potential, say 200 volts, is established
between the plates; under these circumstances there is a large deflexion of the cathode
rays, but the medium under the large electromotive force breaks down every now and then
and a bright discharge passes between the plates; when this occurs the phosphorescent
patch produced by the cathode rays jumps back to its undeflected position. When the
cathode rays are deflected by the electrostatic field, the phosphorescent band breaks up
into several bright bands separated by comparatively dark spaces; the phenomena are
exactly analogous to those observed by Birkeland when the cathode rays are deflected by
a magnet, and called by him the magnetic spectrum.
A series of measurements of the deflexion of the rays by the electrostatic force under
various circumstances will be found later on in the part of the paper which deals with the
velocity of the rays and the ratio of the mass of the electrified particles to the charge
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Laboratorio di Fisica Nucleare
SIS – A. A. 2004/2005
Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
carried by them. It may, however, be mentioned here that the deflexion gets smaller as the
pressure diminishes, and when in consequence the potential-difference in the tube in the
neighbourhood of the cathode increases.
Conductivity of a Gas through which Cathode Rays are passing.
The conductivity of the gas was investigated by means of the apparatus shown in fig. 2.
The upper plate D was connected with one terminal of a battery of small storage-cells, the
other terminal of which was connected with the earth; the other plate E was connected
with one of the coatings of a condenser of one microfarad capacity, the other coating of
which was to earth; one pair of quadrants of an electrometer was also connected with E,
the other pair of quadrants being to earth. When the cathode rays are passing between the
plates, the two pairs of quadrants of the electrometer are first connected with each other,
and then the connexion between them was broken. If the space between the plates were a
non-conductor, the potential of the pair of quadrants not connected with the earth would
remain zero and the needle of the electrometer would be deflected. There is always a
deflexion of the electrometer, showing that a current passes between the plates. The
magnitude of the current depends very greatly upon the pressure of the gas; so much so,
indeed, that it is difficult to obtain consistent readings in consequence of the changes
which always occur in the pressure when the discharge passes through the tube.
We shall first take the case when the pressure is only just low enough to allow the
phosphorescent patch to appear at the end of the tube; in this case the relation between
the current between the plates and the initial difference of potential is represented by the
curve shown in fig. 3. In this figure the abscissae represent the initial difference of potential
between the plates, each division representing two volts. The quantity of electricity which
has passed between the plates in one minute is the quantity required to raise 1 microfarad
to the potential- difference shown by the curve. The upper and lower curve relates to the
case when the upper plate is connected with the negative and positive pole respectively of
the battery.
Even when there is no initial difference of potential between the plates the lower plate
acquires a negative charge from the impact on it of some of the cathode rays.
We see from the curve that the current between the plates soon reaches a value where it
is only slightly affected by an increase in the potential-difference between the plates; this is
a feature common to conduction through gases traversed by Röntgen rays, by uranium
rays, by ultra-violet light, and, as we now see, by cathode rays. The rate of leak is not
greatly different whether the upper plate be initially positively or negatively electrified.
The current between the plates only lasts for a short time; it ceases long before the
potential of the lower plate approaches that of the upper. Thus, for example, when the
potential of the upper plate was about 400 volts above that of the earth, the potential of the
lower plate never rose above 6 volts: similarly, if the upper plate were connected with the
negative pole of the battery, the fall in potential of the lower plate was very small in
comparison with the potential-difference between the upper plate and the earth.
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Laboratorio di Fisica Nucleare
SIS – A. A. 2004/2005
Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
These results are what we should expect if the gas between the plates and the plug B (fig.
2) were a very much better conductor than the gas between the plates, for the lower plate
will be in a steady state when the current coming to it from the upper plate is equal to the
current going from it to the plug; now if the conductivity of the gas between the plate and
the plug is much greater than that between the plates, a small difference of potential
between the lower plate and the plug will be consistent with a large potential-difference
between the plates.
So far we have been considering the case when the pressure is as high as is consistent
with the cathode rays reaching the end of the tube; we shall now go to the other extreme
and consider the case when the pressure is as low as is consistent with the passage of a
discharge through the bulb. In this case, when the plates are not connected with the
battery we get a negative charge communicated to the lower plate, but only very slowly in
comparison with the effect in the previous case. When the upper plate is connected with
the negative pole of a battery, this current to the lower plate is only slightly increased even
when the difference of potential is as much as 400 volts: a small potential-difference of
about 20 volts seems slightly to decrease the rate of leak. Potential-differences much
exceeding 400 volts cannot be used, as though the dielectric between the plates is able to
sustain them for some little time, yet after a time an intensely bright arc flashes across
between the plates and liberates so much gas as to spoil the vacuum. The lines in the
spectrum of this glare are chiefly mercury lines; its passage leaves very peculiar markings
on the aluminium plates.
If the upper plate was charged positively, then the negative charge communicated to the
lower plate was diminished, and stopped when the potential-difference between the plates
was about 20 volts; but at the lowest pressure, however great (up to 400 volts) the
potential-difference, there was no leak of positive electricity to the lower plate at all
comparable with the leak of negative electricity to this plate when the two plates were
disconnected from the battery. In fact at this very low pressure all the facts are consistent
with the view that the effects are due to the negatively electrified particles travelling along
the cathode rays, the rest of the gas possessing little conductivity. Some experiments were
made with a tube similar to that shown in fig. 2, with the exception that the second plug B
was absent, so that a much greater number of cathode rays passed between the plates.
When the upper plate was connected with the positive pole of the battery a luminous
discharge with well-marked striations passed between the upper plate and the earthconnected plug through which the cathode rays were streaming; this occurred even though
the potential- difference between the plate and the plug did not exceed 20 volts. Thus it
seems that if we supply cathode rays from an external source to the cathode a small
potential-difference is sufficient to produce the characteristic discharge through a gas.
Magnetic Deflexion of the Cathode Rays in Different Gases.
The deflexion of the cathode rays by the magnetic field was studied with the aid of the
apparatus shown in fig. 4. The cathode was placed in a
side-tube fastened on to a bell-jar; the opening between
this tube and the bell-jar was closed by a metallic plug with
a slit in it; this plug was connected with the earth and was
used as the anode. The cathode rays passed through the
slit in this plug into the bell-jar, passing in front of a vertical
plate of glass ruled into small squares. The bell-jar was
placed between two large parallel coils arranged as a
Helmholtz galvanometer. The course of the rays was
determined by taking photographs of the bell-jar when the
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Laboratorio di Fisica Nucleare
SIS – A. A. 2004/2005
Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
cathode rays were passing though it; the divisions on the plate enabled the path of the
rays to be determined. Under the action of the magnetic field the narrow beam of cathode
rays spreads out into a broad fan-shaped luminosity in the gas. The luminosity in this fan is
not uniformly distributed, but is condensed along certain lines. The phosphorescence on
the glass is also not uniformly distributed; it is much spread out, showing that the beam
consists of rays which are not all deflected to the same extent by the magnet. The
luminosity on the glass is crossed by bands along which the luminosity is very much
greater than in the adjacent parts. These bright and dark bands are called by Birkeland,
who first observed them, the magnetic spectrum. The brightest spots on the glass are by
no means always the terminations of the brightest streaks of luminosity in the gas; in fact,
in some cases a very bright spot on the glass is not connected with the cathode by any
appreciable luminosity, though there may be plenty of luminosity in other parts of the gas.
One very interesting point brought out by the photographs is that in a given magnetic field,
and with a given mean potential- difference between the terminals, the path of the rays is
independent of the nature of the gas. Photographs were taken of the discharge in
hydrogen, air, carbonic acid, methyl iodide, i.e., in gases whose densities range from 1 to
70, and yet, not only were the paths of the most deflected rays the same in all cases, but
even the details, such as the distribution of the bright and dark spaces, were the same; in
fact, the photographs could hardly be distinguished from each other. It is to be noted that
the pressures were not the same; the pressures in the different gases were adjusted so
that the mean potential-differences between the cathode and the anode were the same in
all the gases. When the pressure of a gas is lowered, the potential-difference between the
terminals increases, and the deflexion of the rays produced by a magnet diminishes, or at
any rate the deflexion of the rays when the phosphorescence is a maximum diminishes. If
an air-break is inserted an effect of the same kind is produced.
In the experiments with different gases, the pressures were as high as was consistent with
the appearance of the phosphorescence on the glass, so as to ensure having as much as
possible of the gas under consideration in the tube.
As the cathode rays carry a charge of negative electricity, are deflected by an electrostatic
force as if they were negatively electrified, and are acted on by a magnetic force in just the
way in which this force would act on a negatively electrified body moving along the path of
these rays, I can see no escape from the conclusion that they are charges of negative
electricity carried by particles of matter. The question next arises, What are these
particles? are they atoms, or molecules, or matter in a still finer state of subdivision? To
throw some light on this point, I have made a series of measurements of the ratio of the
mass of these particles to the charge carried by it. To determine this quantity, I have used
two independent methods. The first of these is as follows:--Suppose we consider a bundle
of homogeneous cathode rays. Let m be the mass of each of the particles, e the charge
carried by it. Let N be the number of particles passing across any section of the beam in a
given time; then Q the quantity of electricity carried by these particles is given by the
equation
Ne = Q.
We can measure Q if we receive the cathode rays in the inside of a vessel connected with
an electrometer. When these rays strike against a solid body, the temperature of the body
is raised; the kinetic energy of the moving particles being converted into heat; if we
suppose that all this energy is converted into heat, then if we measure the increase in the
temperature of a body of known thermal capacity caused by the impact of these rays, we
can determine W, the kinetic energy of the particles, and if v is the velocity of the particles,
(1/2)Nmv2 = W.
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Laboratorio di Fisica Nucleare
SIS – A. A. 2004/2005
Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
If r is the radius of curvature of the path of these rays in a uniform magnetic field H, then
mv/e = Hr = I,
where I is written for Hr for the sake of brevity. From these equations we get
(1/2)(m/e)v2=W/Q.
v=2W/QI,
m/e = I2Q/2W.
Thus, if we know the values of Q, W, and I, we can deduce the values of v and m/e.
To measure these quantities, I have used tubes of three different types. The first I tried is
like that represented in fig. 2, except that the plates E and D are absent, and two coaxial
cylinders are fastened to the end of the tube. The rays from the cathode C fall on the metal
plug B, which is connected with the earth, and serves for the anode; a horizontal slit is cut
in this plug. The cathode rays pass through this slit, and then strike against the two coaxial
cylinders at the end of the tube; slits are cut in these cylinders, so that the cathode rays
pass into the inside of the inner cylinder. The outer cylinder is connected with the earth,
the inner cylinder, which is insulated from the outer one, is connected with an
electrometer, the deflexion of which measures Q, the quantity of electricity brought into the
inner cylinder by the rays. A thermo-electric couple is placed behind the slit in the inner
cylinder; this couple is made of very thin strips of iron and copper fastened to very fine iron
and copper wires. These wires passed through the cylinders, being insulated from them,
and through the glass to the outside of the tube, were they were connected with a lowresistance galvanometer, the deflexion of which gave data for calculating the rise of
temperature of the junction produced by the impact against it of the cathode rays. The
strips of iron and copper were large enough to ensure that every cathode ray which
entered the inner cylinder struck against the junction. In some of the tubes the strips of iron
and copper were placed end to end, so that some of the rays struck against the iron, and
others against the copper; in others, the strip of one metal was placed in front of the other;
no difference, however, could be detected between the results got with these two
arrangements. The strips of iron and copper were weighed, and the thermal capacity of the
junction calculated. In one set of junctions this capacity was 5x10 -3, in another 3x10-3. If we
assume that the cathode rays which strike against the junction give their energy up to it,
the deflexion of the galvanometer gives us W or (1/2)Nmv2.
The value of I, i.e., Hr, where r is the curvature of the path of the rays in a magnetic field of
strength H was found as follows:--The tube was fixed between two large circular coils
placed parallel to each other, and separated by a distance equal to the radius of either;
these coils produce a uniform magnetic field, the strength of which is got by measuring
with an ammeter the strength of the current passing through them. The cathode rays are
thus in a uniform field, so that their path is circular. Suppose that the rays, when deflected
by a magnet, strike against the glass of the tube at E (fig. 5), then, if r is the radius of the
circular path of the rays,
2r = CE2/AC + AC ;
thus, if we measure CE and AC we have the means of
determining the radius of curvature of the path of the rays.
The determination of r is rendered to some extent uncertain, in consequence of the pencil
of rays spreading out under the action of the magnetic field, so that the phosphorescent
patch at E is several millimetres long; thus values of r differing appreciably from each other
will be got by taking E at different points of this phosphorescent patch. Part of this patch
was, however, generally considerably brighter than the rest; when this was the case, E
was taken as the brightest point; when such a point of maximum brightness did not exist,
26
Laboratorio di Fisica Nucleare
SIS – A. A. 2004/2005
Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
the middle of the patch was taken for E. The uncertainty in the value of r thus introduced
amounted sometimes to about 20 per cent.; by this I mean that if we took E first at one
extremity of the patch and then at the other, we should get values of r differing by this
amount.
The measurement of Q, the quantity of electricity which enters the inner cylinder, is
complicated by the cathode rays making the gas through which they pass a conductor, so
that though the insulation of the inner cylinder was perfect when the rays were off, it was
not so when they were passing through the space between the cylinders; this caused
some of the charge communicated to the inner cylinder to leak away so that the actual
charge given to the cylinder by the cathode rays was larger than that indicated by the
electrometer. To make the error from this cause as small as possible, the inner cylinder
was connected to the largest capacity available, 1.5 microfarad, and the rays were only
kept on for a short time, about 1 or 2 seconds, so that the alteration in potential of the inner
cylinder was not large, ranging in the various experiments from about .5 to 5 volts. Another
reason why it is necessary to limit the duration of the rays to as short a time as possible, is
to avoid the correction for the loss of heat from the thermo-electric junction by conduction
along the wires; the rise in temperature of the junction was of the order 2°C.; a series of
experiments showed that with the same tube and the same gaseous pressure Q and W
were proportional to each other when the rays were not kept on too long.
Tubes of this kind gave satisfactory results, the chief drawback being that sometimes in
consequence of the charging up of the glass of the tube, a secondary discharge started
from the cylinder to the walls of the tube, and the cylinders were surrounded by glow;
when this glow appeared, the readings were very irregular; the glow could, however, be
got rid of by pumping and letting the tube rest for some time. The results got with this tube
are given in the Table under the heading Tube 1.
The second type of tube was like that used for photographing the path of the rays (fig. 4);
double cylinders with a thermo-electric junction like those used in the previous tube were
placed in the line of fire of the rays, the inside of the bell-jar was lined with copper gauze
connected with the earth. This tube gave very satisfactory results; we were never troubled
with any glow round the cylinders, and the readings were most concordant; the only
drawback was that as some of the connexions had to be made with sealing-wax, it was not
possible to get the highest exhaustions with this tube, so that the range of pressure for this
tube is less than that for tube 1. The results got with this tube are given in the Table under
the heading Tube 2.
The third type of tube was similar to the first, except that the openings in the two cylinders
were made very much smaller; in this tube the slits in the cylinders were replaced by small
holes, about 1.5 millim. in diameter. In consequence of the smallness of the openings, the
magnitude of the effects was very much reduced; in order to get measurable results it was
necessary to reduce the capacity of the condenser in connexion with the inner cylinder to
.15 microfarad, and to make the galvanometer exceedingly sensitive, as the rise in
temperature of the thermo-electric junction was in these experiments only about .5° C. on
the average. The results obtained in this tube are given in the Table under the heading
Tube 3.
The results of a series of measurements with these tubes are given in the following Table:Gas.
Value of W/Q.
I.
m/e
v.
Tube 1.
11
Air
4.6x10
230
.57x10-7
4x109
Air
1.8x1012
350
.34x10-7
1x1010
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Laboratorio di Fisica Nucleare
SIS – A. A. 2004/2005
Air
Air
Air
Air
Air
Hydrogen
Hydrogen
Carbonic acid
Carbonic acid
Carbonic acid
6.1x1011
2.5x1012
5.5x1011
1x1012
1x1012
6x1012
2.1x1012
8.4x1011
1.47x1012
3.0x1012
Air
Air
Air
Hydrogen
Air
Carbonic acid
Air
Hydrogen
Hydrogen
Air
Air
2.8x1011
4.4x1011
3.5x1011
2.8x1011
2.5x1011
2x1011
1.8x1011
2.8x1011
4.4x1011
2.5x1011
4.2x1011
Air
Air
Hydrogen
2.5x1011
3.5x1011
3x1011
Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
230
400
230
285
285
205
460
260
340
480
Tube 2.
175
195
181
175
160
148
151
175
201
176
200
Tube 3.
220
225
250
.43x10-7
.32x10-7
.48x10-7
.4x10-7
.4x10-7
.35x10-7
.5x10-7
.4x10-7
.4x10-7
.39x10-7
5.4x109
1.2x1010
4.8x109
7x109
7x109
6x109
9.2x109
7.5x109
8.5x109
1.3x1010
.53x10-7
.47x10-7
.47x10-7
.53x10-7
.51x10-7
.54x10-7
.63x10-7
.53x10-7
.46x10-7
.61x10-7
.48x10-7
3.3x109
4.1x109
3.8x109
3.3x109
3.1x109
2.5x109
2.3x109
3.3x109
4.4x109
2.8x109
4.1x109
.9x10-7
.7x10-7
1.0[sic-CJG]x10-7
2.4x109
3.2x109
2.5x109
It will be noticed that the value of m/e is considerably greater for Tube 3, where the
opening is a small hole, than for Tubes 1 and 2, where the opening is a slit of much
greater area. I am of the opinion that the values of m/e got from Tubes 1 and 2 are too
small, in consequence of the leakage from the inner cylinder to the outer by the gas being
rendered a conductor by the passage of the cathode rays.
It will be seen from these tables that the value of m/e is independent of the nature of the
gas. Thus, for the first tube the mean for air is .40x10 -7, for hydrogen .42x10-7, and for
carbonic acid gas .4x10-7; for the second tube the mean for air is .52x10 -7, for hydrogen
.50x10-7, and for carbonic acid gas .54x10-7.
Experiments were tried with electrodes made of iron instead of aluminium; this altered the
appearance of the discharge and the value of v at the same pressure, the values of m/e
were, however, the same in the two tubes; the effect produced by different metals on the
discharge will be described later on.
In all the preceding experiments, the cathode rays were first deflected from the cylinder by
a magnet, and it was then found that there was no deflexion either of the electrometer or
the galvanometer, so that the deflexions observed were entirely due to the cathode rays;
when the glow mentioned previously surrounded the cylinders there was a deflexion of the
electrometer even when the cathode rays were deflected from the cylinder.
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Laboratorio di Fisica Nucleare
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Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
Before proceeding to discuss the results of these measurements I shall describe another
method of measuring the quantities m/e and v of an entirely different kind from the
preceding; this method is based upon the deflexion of the cathode rays in an electrostatic
field. If we measure the deflexion experienced by the rays when traversing a given length
under a uniform electric intensity, and the deflexion of the rays when they traverse a given
distance under a uniform magnetic field, we can find the values of m/e and v in the
following way:-Let the space passed over by the rays under a uniform electric intensity F be l, the time
taken for the rays to traverse this space is l/v, the velocity in the direction of F is therefore
(Fe/m)(l/v) ,
so that q, the angle through which the rays are deflected when they leave the electric field
and enter a region free from electric force, is given by the equation
q = (Fe/m)(l/v2) .
If, instead of the electric intensity, the rays are acted on by a magnetic force H at right
angles to the rays, and extending across the distance l, the velocity at right angles to the
original path of the rays is
(Hev/m)(l/v) ,
so that f, the angle through which the rays are deflected when they leave the magnetic
field, is given by the equation
f = (He/m)(l/v) .
From these equations we get
v = (f/q)(F/H)
and
m/e = H2ql/Ff2 .
In the actual experiments H was adjusted so that f = q; in this case the equations become
v=F/H,
m/e = H2l/Fq .
The apparatus used to measure v and m/e by this means is that represented in fig. 2. The
electric field was produced by connecting the two aluminium plates to the terminals of a
battery of storage-cells. The phosphorescent patch at the end of the tube was deflected,
and the deflexion measured by a scale pasted to the end of the tube. As it was necessary
to darken the room to see the phosphorescent patch, a needle coated with luminous paint
was placed so that by a screw it could be moved up and down the scale; this needle could
be seen when the room was darkened, and it was moved until it coincided with the
phosphorescent patch. Thus, when light was admitted, the deflexion of the
phosphorescent patch could be measured.
The magnetic field was produced by placing outside the tube two coils whose diameter
was equal to the length of the plates; the coils were placed so that they covered the space
occupied by the plates, the distance between the coils was equal to the radius of either.
The mean value of the magnetic force over the length l was determined in the following
way: a narrow coil C whose length was l, connected with a ballistic galvanometer, was
placed between the coils; the plane of the windings of C was parallel to the planes of the
coils; the cross section of the coil was a rectangle 5 cm. by 1 cm. A given current was sent
through the outer coils and the kick a of the galvanometer observed when this current was
reversed. The coil C was then placed at the centre of two very large coils, so as to be in a
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Ferrari Trecate Irene
field of uniform magnetic force: the current through the large coils was reversed and the
kick b of the galvanometer again observed; by comparing a and b we can get the mean
value of the magnetic force over a length l; this was found to be
60 x i ,
where i is the current flowing through the coils.
A series of experiments was made to see if the electrostatic deflexion was proportional to
the electric intensity between the plates; this was found to be the case. In the following
experiments the current through the coils was adjusted so that the electrostatic deflexion
was the same as the magnetic:-Gas.
Air
Air
Air
Hydrogen
Carbonic Acid
Air
Air
q.
8/110
9.5/110
13/110
9/110
11/110
6/110
7/110
H.
5.5
5.4
6.6
6.3
6.9
5
3.6
F.
1.5x1010
1.5x1010
1.5x1010
1.5x1010
1.5x1010
1.8x1010
1.x1010
l.
5
5
5
5
5
5
5
m/e.
1.3x10-7
1.1x10-7
1.2x10-7
1.5x10-7
1.5x10-7
1.3x10-7
1.1x10-7
v.
2.8x109
2.8x109
2.3x109
2.5x109
2.2x109
3.6x109
2.8x109
The cathode in the first five experiments was aluminium, in the last two experiments it was
made of platinum; in the last experiment Sir William Crookes's method of getting rid of the
mercury vapour by inserting tubes of pounded sulphur, sulphur iodide, and copper filings
between the bulb and the pump was adopted. In the calculation of m/e and v no allowance
has been made for the magnetic force due to the coil in the region outside the plates; in
this region the magnetic force will be in the opposite direction to that between the plates,
and will tend to bend the cathode rays in the opposite direction: thus the effective value of
H will be smaller than the value used in the equations, so that the values of m/e are larger,
and those of v less than they would be if this correction were applied. This method of
determining the values of m/e and vis much less laborious and probably more accurate
than the former method; it cannot, however, be used over so wide a range of pressures.
From these determinations we see that the value of m/e is independent of the nature of the
gas, and that its value 10-7 is very small compared with the value 10-4, which is the
smallest value of this quantity previously known, and which is the value for the hydrogen
ion in electrolysis.
Thus for the carriers of the electricity in the cathode rays m/e is very small compared with
its value in electrolysis. The smallness of m/e may be due to the smallness of m or the
largeness of e, or to a combination of these two. That the carriers of the charges in the
cathode rays are small compared with ordinary molecules is shown, I think, by Lenard's
results as to the rate at which the brightness of the phosphorescence produced by these
rays diminishes with the length of path travelled by the ray. If we regard this
phosphorescence as due to the impact of the charged particles, the distance through
which the rays must travel before the phosphorescence fades to a given fraction (say 1/e,
where e = 2.71) of its original intensity, will be some moderate multiple of the mean free
path. Now Lenard found that this distance depends solely upon the density of the medium,
and not upon its chemical nature or physical state. In air at atmospheric pressure the
distance was about half a centimetre, and this must be comparable with the mean free
path of the carriers through air at atmospheric pressure. But the mean free path of the
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Ferrari Trecate Irene
molecules of air is a quantity of quite a different order. The carrier, then, must be small
compared with ordinary molecules.
The two fundamental points about these carriers seem to me to be (1) that these carriers
are the same whatever the gas through which the discharge passes, (2) that the mean free
paths depend upon nothing but the density of the medium traversed by these rays.
It might be supposed that the independence of the mass of the carriers of the gas through
which the discharge passes was due to the mass concerned being the quasi mass which a
charged body possesses in virtue of the electric field set up in its neighbourhood; moving
the body involves the production of a varying electric field, and, therefore, of a certain
amount of energy which is proportional to the square of the velocity. This causes the
charged body to behave as if its mass were increased by a quantity, which for a charged
sphere is (1/5)e2/ma ('Recent Researches in Electricity and Magnetism'), where e is the
charge and a the radius of the sphere. If we assume that it is this mass which we are
concerned with in the cathode rays, since m/e would vary as e/a, it affords no clue to the
explanation of either of the properties (1 and 2) of these rays. This is not by any means the
only objection to this hypothesis, which I only mention to show that it has not been
overlooked.
The explanation which seems to me to account in the most simple and straightforward
manner for the facts is founded on a view of the constitution of the chemical elements
which has been favourably entertained by many chemists: this view is that the atoms of
the different chemical elements are different aggregations of atoms of the same kind. In
the form in which this hypothesis was enunciated by Prout, the atoms of the different
elements were hydrogen atoms; in this precise form the hypothesis is not tenable, but if we
substitute for hydrogen some unknown primordial substance X, there is nothing known
which is inconsistent with this hypothesis, which is one that has been recently supported
by Sir Norman Lockyer for reasons derived from the study of the stellar spectra.
If, in the very intense electric field in the neighbourhood of the cathode, the molecules of
the gas are dissociated and are split up, not into the ordinary chemical atoms, but into
these primordial atoms, which we shall for brevity call corpuscles; and if these corpuscles
are charged with electricity and projected from the cathode by the electric field, they would
behave exactly like the cathode rays. They would evidently give a value of m/e which is
independent of the nature of the gas and its pressure, for the carriers are the same
whatever the gas may be; again, the mean free paths of these corpuscles would depend
solely upon the density of the medium through which they pass. For the molecules of the
medium are composed of a number of such corpuscles separated by considerable spaces;
now the collision between a single corpuscle and the molecule will not be between the
corpuscles and the molecule as a whole, but between this corpuscle and the individual
corpuscles which form the molecule; thus the number of collisions the particle makes as it
moves through a crowd of these molecules will be proportional, not to the number of the
molecules in the crowd, but to the number of the individual corpuscles. The mean free path
is inversely proportional to the number of collisions in unit time, and so is inversely
proportional to the number of corpuscles in unit volume; now as these corpuscles are all of
the same mass, the number of corpuscles in unit volume will be proportional to the mass
of unit volume, that is the mean free path will be inversely proportional to the density of the
gas. We see, too, that so long as the distance between neighbouring corpuscles is large
compared with the linear dimensions of a corpuscle the mean free path will be
independent of the way they are arranged, provided the number in unit volume remains
constant, that is the mean free path will depend only on the density of the medium
traversed by the corpuscles, and will be independent of its chemical nature and physical
state: this from Lenard's very remarkable measurements of the absorption of the cathode
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Ferrari Trecate Irene
rays by various media, must be a property possessed by the carriers of the charges in the
cathode rays.
Thus on this view we have in the cathode rays matter in a new state, a state in which the
subdivision of matter is carried very much further than in the ordinary gaseous state: a
state in which all matter--that is, matter derived from different sources such as hydrogen,
oxygen, &c.--is of one and the same kind; this matter being the substance from which all
the chemical elements are built up.
With appliances of ordinary magnitude, the quantity of matter produced by means of the
dissociation at the cathode is so small as to almost to preclude the possibility of any direct
chemical investigation of its properties. Thus the coil I used would, I calculate, if kept going
uninterruptedly night and day for a year, produce only about one three-millionth part of a
gramme of this substance.
The smallness of the value of m/e is, I think, due to the largeness of e as well as the
smallness of m. There seems to me to be some evidence that the charges carried by the
corpuscles in the atom are large compared with those carried by the ions of an electrolyte.
In the molecule of HCl, for example, I picture the components of the hydrogen atoms as
held together by a great number of tubes of electrostatic force; the components of the
chlorine atom are similarly held together, while only one stray tube binds the hydrogen
atom to the chlorine atom. The reason for attributing this high charge to the constituents of
the atom is derived from the values of the specific inductive capacity of gases: we may
imagine that the specific inductive capacity of a gas is due to the setting in the electric field
of the electric doublet formed by the two oppositely electrified atoms which form the
molecule of the gas. The measurements of the specific inductive capacity show, however,
that this is very approximately an additive quantity: that is, that we can assign a certain
value to each element, and find the specific inductive capacity of HCl by adding the value
for hydrogen to the value for chlorine; the value of H2O by adding twice the value for
hydrogen to the value for oxygen, and so on. Now the electrical moment of the doublet
formed by a positive charge on one atom of the molecule and a negative charge on the
other atom would not be an additive property; if, however, each atom had a definite
electrical moment, and this were large compared with the electrical moment of the two
atoms in the molecule, then the electrical moment of any compound, and hence its specific
inductive capacity, would be an additive property. For the electrical moment of the atom,
however, to be large compared with that of the molecule, the charge on the corpuscles
would have to be very large compared with those on the ion.
If we regard the chemical atom as an aggregation of a number of primordial atoms, the
problem of finding the configurations of stable equilibrium for a number of equal particles
acting on each other according to some law of force--whether that of Boscovich, where the
force between them is a repulsion when they are separated by less than a certain critical
distance, and an attraction when they are separated by less than a certain critical distance,
and an attraction when they are separated by a greater distance, or even the simpler case
of a number of mutually repellent particles held together by a central force--is of great
interest in connexion with the relation between the properties of an element and its atomic
weight. Unfortunately the equations which determine the stability of such a collection of
particles increase so rapidly in complexity with the number of particles that a general
mathematical investigation is scarcely possible. We can, however, obtain a good deal of
insight into the general laws which govern such configurations by the use of models, the
simplest of which is the floating magnets of Professor Mayer. In this model the magnets
arrange themselves in equilibrium under the mutual repulsions and a central attraction
caused by the pole of a large magnet placed above the floating magnets.
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A study of the forms taken by these magnets seems to me to be suggestive in relation to
the periodic law. Mayer showed that when the number of floating magnets did not exceed
5 they arranged themselves at the corners of a regular polygon--5 at the corners of a
pentagon, 4 at the corners of a square, and so on. When the number exceeds 5, however,
this law no longer holds: thus 6 magnets do not arrange themselves at the corners of a
hexagon, but divide into two systems, consisting of 1 in the middle surrounded by 5 at the
corners of a pentagon. For 8 we have two in the inside and 6 outside; this arrangement in
two systems, an inner and an outer, lasts up to 18 magnets. After this we have three
systems: an inner, a middle, and an outer; for a still larger number of magnets we have
four systems, and so on.
Mayer found the arrangement of magnets was as
follows:-where, for example, 1.6.10.12 means an arrangement
with one magnet in the middle, then a ring of six, then
a ring of ten, and a ring of twelve outside.
Now suppose that a certain property is associated
with two magnets forming a group by themselves; we
should have this property with 2 magnets, again with
8 and 9, again with 19 and 20, and again with 34, 35,
and so on. If we regard the system of magnets as a
model of an atom, the number of magnets being
proportional to the atomic weight, we should have this
property occurring in elements of atomic weight 2,
(8,9), 19, 20, (34, 35). Again, any property conferred
by three magnets forming a system by themselves
would occur with atomic weights 3, 10, and 11; 20,
21, 22, 23, and 24; 35, 36, 37 and 39; in fact, we
should have something quite analogous to the
periodic law, the first series corresponding to the
arrangement of the magnets in a single group, the
second series to the arrangement in two groups, the third series in three groups, and so
on.
Velocity of the Cathode Rays.
The velocity of the cathode rays is variable, depending upon the potential-difference
between the cathode and anode, which is a function of the pressure of the gas--the
velocity increases as the exhaustion improves; the measurements given above show,
however, that at all the pressures at which experiments were made the velocity exceeded
109 cm./sec. This velocity is much greater than the value of 2x10 7 which I previously
obtained (Phil. Mag. Oct. 1894) by measuring directly the interval which separated the
appearance of luminosity at two places on the walls of the tube situated at different
distances from the cathode.
In my earlier experiments the pressure was higher than in the experiments described in
this paper, so that the velocity of the cathode rays would on this account be less. The
difference between the two results is, however, too great to be wholly explained in this
way, and I attribute the difference to the glass requiring to be bombarded by the rays for a
finite time before becoming phosphorescent, this time depending upon the intensity of the
bombardment. As this time diminishes with the intensity of bombardment, the appearance
of phosphorescence at the piece of glass most removed from the cathode would be
delayed beyond the time taken for the rays to pass from one place to the other by the
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difference in time taken by the glass to become luminous; the apparent velocity measured
in this way would thus be less than the true velocity. In the former experiments endeavours
were made to diminish this effect by making the rays strike the glass at the greater
distance from the cathode less obliquely than they struck the glass nearer to the cathode;
the obliquity was adjusted until the brightness of the phosphorescence was approximately
equal in the two cases. In view, however, of the discrepancy between the results obtained
in this way and those obtained by the later method, I think that it was not successful in
eliminating the lag caused by the finite time required by the gas to light up.
Experiments with Electrodes of Different Materials.
In the experiments described in this paper the electrodes were generally made of
aluminium. Some experiments, however, were made with iron and platinum electrodes.
Though the value of m/e came out the same whatever the material of the electrode, the
appearance of the discharge varied greatly; and as the measurements showed, the
potential-difference between the cathode and anode depended greatly upon the metal
used for the electrode; the pressure being the same in all cases.
To test this point further I used a tube like that
shown in fig. 6, where a, b, c are cathodes made of
different metals, the anodes being in all cases
platinum wires. The cathodes were disks of
aluminium, iron, lead, tin, copper, mercury, sodium
amalgam, and silver chloride; the potentialdifference between the cathode and anode was
measured by Lord Kelvin's vertical voltmeter, and also by measuring the length of spark in
air which, when placed in parallel with the anode and cathode, seemed to allow the
discharge to go as often through the spark-gap as through the tube. With this arrangement
the pressures were the same for all the cathodes. The potential-difference between the
anode and cathode and the equivalent spark-length depended greatly upon the nature of
the cathode. The extent of the variation in potential may be estimated from the following
table:-Cathode.
Aluminium
Lead
Tin
Copper
Iron
Mean Potential-Difference between Cathode and Anode.
1800 volts.
2100 "
2400 "
2600 "
2900 "
The potential-difference when the cathode was made of sodium amalgam or silver chloride
was less even than that of aluminium.
The order of many of the metals changed about very capriciously, experiments made at
intervals of a few minutes frequently giving quite different results. From the abrupt way in
which these changes take place I am inclined to think that gas absorbed by the electrode
has considerable influence on the passage of the discharge.
I have much pleasure in thanking Mr. Everitt for the assistance he has given me in the
preceding investigation.
Cambridge, Aug. 7, 1897.
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*Some of these experiments have already been described in a paper read before the
Cambridge Philosophical Society (Proceedings, vol. ix. 1897), and in a Friday Evening
Discourse at the Royal Institution ('Electrician,' May 21, 1897).
L.2 Lettura dell’articolo di Rutherford
Sir Ernest Rutherford (1871-1937)
Collisions of alpha Particles with Light Atoms. IV. An Anomalous Effect in Nitrogen.
By Professor Sir E. Rutherford, F. R. S. The London, Edinburgh and Dublin Philosophical
Magazine and Journal of Science, 6th series, 37, 581 (1919).
It has been shown in paper I. that a metal source, coated with a deposit of radium C,
always gives rise to a number of scintillations on a zinc sulphide screen far beyond the
range of the alpha particles. The swift atoms causing these scintillations carry a positive
charge and are deflected by a magnetic filed, and have about the same range and energy
as the swift H atoms produced by the passage of alpha particles through hydrogen. These
"natural" scintillations are believed to be due mainly to swift H atoms from the radioactive
source, but it is difficult to decide whether they are expelled from the radioactive source
itself or are due to the action of alpha particles on occluded hydrogen.
The apparatus employed to study these "natural" scintillations is the same as that
described in paper I. The intense source of radium C was placed inside a metal box about
3 cm. from the end, and an opening in the end of the box was covered with a silver plate of
stopping power equal to about 6 cm. of air. The zinc sulphide screen was mounted
outside, about 1 mm. distant from the silver plate, to admit of the introduction of absorbing
foils between them. The whole apparatus was placed in a strong magnetic field to deflect
the beta rays. The variation in the number of these "natural" scintillations with absorption in
terms of cms. of air is shown in fig. 1, curve A. In this case, the air in the box was
exhausted and absorbing foils of aluminium were used. Then dried oxygen or carbon
dioxide was admitted into the vessel, the number of scintillations diminished to about the
amount to be expected from the stopping power of the column of gas.
A surprising effect was noticed, however, when dried air was introduced. Instead of
diminishing, the number of scintillations was increased, and for an absorption
corresponding to about 19 cm. of air the number was about twice that observed when the
air was exhausted. It was clear from this experiment that the alpha particles in their
passage through air gave rise to long-range scintillations which appeared to the eye to be
about equal in brightness to H scintillations. A systematic series of observations was
undertaken to account for the origin of these scintillations. In the first place we have seen
that the passage of alpha particles through nitrogen and oxygen gives rise to numerous
right scintillations which have a range of about 9 cm. in air. These scintillations have about
the range to be expected if they are due to swift N or O atoms, carrying unit charge,
produced by collision with alpha particles. All experiments have consequently been made
with an absorption greater than 9 cm of air, so that these atoms are completely stopped
before reaching the zinc sulphide screen.
It was found that these long-range scintillations could not be due to the presence of water
vapour in the air; for the number was only slightly reduced by thoroughly drying the air.
This is to be expected, since on the average the number of additional scintillations due to
air was equivalent to the number of H atoms produced by the mixture of hydrogen at 6 cm.
pressure with oxygen. Since on the average the vapour pressure of water in air was not
more than 1 cm., the effects of complete drying would not reduce the number by more
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than one sixth. Even when oxygen and carbon dioxide saturated with water vapour at 20°
C. were introduced in place of dry air, the number of scintillations was much less than with
dry air.
It is well known that the amount of hydrogen or gases containing hydrogen is normally very
small in atmospheric air. No difference was observed whether the air was taken directly
from the room or from outside the laboratory or was stored for some days over water.
There was the possibility that the effect in air might be due to liberation of H atoms from
the dust nuclei in the air. No appreciable difference, however, was observed when the
dried air was filtered though long plugs of cotton wool, or by storage over water for some
days to remove dust nuclei.
Since the anomalous effect was observed in air, but not in oxygen, or carbon dioxide, it
must be due either to nitrogen or to one of the other gases present in atmospheric air. The
latter possibility was excluded by comparing the effects produced in air and in chemically
prepared nitrogen. The nitrogen was obtained by the well-known method of adding
ammonium chloride to sodium nitrite, and stored over water. It was carefully dried before
admission to the apparatus. With pure nitrogen, the number of long-range scintillations
under similar conditions was greater than in air. As a result of careful experiments, the
ratio was found to be 1.25, the value to be expected if the scintillations are due to nitrogen.
The results so far obtained show that the long-range scintillations obtained from air must
be ascribed to nitrogen, but it is important, in addition, to show that they are due to
collision of alpha particles with atoms of nitrogen through the volume of the gas. In the first
place, it was found that the number of the scintillations varied with the pressure of the air in
the way to be expected if they resulted from collision of alpha particles along the column of
gas. In addition, when an absorbing screen of gold o aluminium was placed close to the
source, the range of the scintillations was found to be reduced by the amount to be
expected if the range of the expelled atom was proportional to the range of the colliding
alpha particles. These results show that the scintillations arise from the volume of the gas
and are not due to some surface effect in the radioactive source.
In fig. 1
curve A the results of a typical
experiment are given showing
the variation in the number of
natural scintillations with the
amount of absorbing matter in
their path measured in terms of
centimetres of air for alpha
particles. In these experiments
carbon dioxide was introduced at
a pressure calculated to give the
same absorption of the alpha
rays as ordinary air. In curve B
the corresponding curve is given
when air at N.T.P. is introduced
in place of carbon dioxide. The
difference curve C shows the
corresponding variation of the
number of scintillations arising
from the nitrogen in the air. It was
generally observed that the ratio
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of the nitrogen effect to the natural effect was somewhat greater for 19 cm. than for 12 cm.
absorption.
In order to estimate the magnitude of the effect, the space between the source and screen
was filled with carbon dioxide at diminished pressure and a known pressure of hydrogen
was added. The pressure of the carbon dioxide and of hydrogen were adjusted so that the
total absorption of alpha particles in the mixed gas should be equal to that of the air. In this
way it was found that the curve of absorption of H atoms produced under these conditions
was somewhat steeper than curve C of fig. 1. As a consequence, the amount of hydrogen
mixed with carbon dioxide required to produce a number of scintillations equal to that of
air, increased with the increase of absorption. For example, the effect in air was equal to
about 4 cm. of hydrogen at 12 cm. absorption. For a mean value of the absorption, the
effect was equal to about 6 cm. of hydrogen. This increased absorption of H atoms under
similar conditions indicated either that (1) the swift atoms from air had a somewhat greater
range than the H atoms, or (2) that the atoms from air were projected more in the line of
flight of the alpha particles.
While the maximum range of the scintillations from air using radium C as a source of alpha
rays appeared to be about the same, viz. 28 cm., as for H atoms produced from hydrogen,
it was difficult to fix the end of the range with certainty on account of the smallness of the
number and the weakness of the scintillations. Some special experiments were made to
test whether, under favourable conditions, any scintillations due to nitrogen could be
observed beyond 28 cm. of air absorption. For this purpose a strong source (about 60 mg.
Ra activity) was brought within 2.5 cm. of the zinc sulphide screen, the space between
containing dry air. On still further reducing the distance, the screen became too bright to
detect very feeble scintillations. No certain evidence of scintillations was found beyond a
range of 28 cm. It would therefore appear that (2) above is the more probable explanation.
In a previous paper (III.) we have seen that the number of swift atoms of nitrogen or
oxygen produced per unit path by collision with alpha particles is about the same as the
corresponding number of H atoms in hydrogen. Since the number of long-range
scintillations in air is equivalent to that produced under similar conditions in a column of
hydrogen at 6 cm. pressure, we may consequently conclude that only one long-range
atom is produced for every 12 close collisions giving rise to a swift nitrogen atom of
maximum range 9 cm.
It is of interest to give data showing the number of long-range scintillations produced in
nitrogen at atmospheric pressure under definite conditions. For a column of nitrogen 3.3
cm. long, and for a total absorption of 19 cm. of air from the source, the number due to
nitrogen per milligram of activity is .6 per minute on a screen of 3.14 sq. mm. area.
Both as regards range and brightness of scintillations, the long-range atoms from nitrogen
closely resemble H atoms, and in all probability are hydrogen atoms. In order, however, to
settle this important point definitely, it is necessary to determine the deflexion of these
atoms in a magnetic field. Some preliminary experiments have been made by a method
similar to that employed in measuring the velocity of the H atom (see paper II.). The main
difficulty is to obtain a sufficiently large deflexion of the stream of atoms and yet have a
sufficient number of scintillations per minute for counting. The alpha rays from a strong
source passed through dry air between two parallel horizontal plates 3 cm. long and 1.6
mm. apart, and the number of scintillations on the screen placed near the end of the plates
was observed for different strengths of the magnetic field. Under these conditions, when
the scintillations arise from the whole length of the column of air between the plates, the
strongest magnetic field available reduced the number of scintillations by only 30 per cent.
When the air was replaced by a mixture of carbon dioxide and hydrogen of the same
stopping power for alpha rays, about an equal reduction was noted. As far as the
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experiment goes, this is an indication that the scintillations are due to H atoms; but the
actual number of scintillations and the amount of reduction was too small to place much
reliance on the result. In order to settle this question definitely, it will probably prove
necessary to employ a solid nitrogen compound, free from hydrogen, as a source, and to
use much stronger sources of alpha rays. In such experiments, it will be of importance to
discriminate between the deflexions due to H atoms and possible atoms of atomic weight
2. From the calculations given in paper III., it is seen that a collision of an alpha particle
with a free atom of mass 2 should give rise to an atom of range about 32 cm. in air, and of
initial energy about .89 of that of the H atom produced under similar conditions. The
deflexion of the pencil of these rays in a magnetic field should be about .6 of that shown by
a corresponding pencil of H atoms.
Discussion of results.
From the results so far obtained it is difficult to avoid the conclusion that the long-range
atoms arising from collision of alpha particles with nitrogen are not nitrogen atoms but
probably atoms of hydrogen, or atoms of mass 2. If this be the case, we must conclude
that the nitrogen atom is disintegrated under the intense forces developed in a close
collision with a swift alpha particle, and that the hydrogen atom which is liberated formed a
constituent part of the nitrogen nucleus. We have drawn attention in paper III. to the rather
surprising observation that the range of the nitrogen atoms in air is about the same as the
oxygen atoms, although we should expect a difference of about 19 per cent. If in collisions
which give rise to swift nitrogen atoms, the hydrogen is at the same time disrupted, such a
difference might be accounted for, for the energy is then shared between two systems.
It is of interest to note, that while the majority of the light atoms, as is well known, have
atomic weights represented by 4n or 4n+3 where n is a whole number, nitrogen is the only
atom which is expressed by 4n+2. We should anticipate from radioactive data that the
nitrogen nucleus consists of three helium nuclei each of atomic mass 4 and either two
hydrogen nuclei or one of mass 2. If the H nuclei were outriders of the main system of
mass 12, the number of close collisions with the bound H nuclei would be less than if the
latter were free, for the alpha particle in a collision comes under the combined field of the
H nucleus and of the central mas. Under such conditions, it is to be expected that the
alpha particle would only occasionally approach close enough to the H nucleus to give it
the maximum velocity, although in many cases it may give it sufficient energy to break its
bond with the central mass. Such a point of view would explain why the number of swift H
atoms from nitrogen is less than the corresponding number in free hydrogen and less also
than the number of swift nitrogen atoms. The general results indicate that the H nuclei,
which are released, are distant about twice the diameter of the electron (7x10 -13 cm.) from
the centre of the main atom. Without a knowledge of the laws of force at such small
distances, it is difficult to estimate the energy required to free the H nucleus or to calculate
the maximum velocity that can be given to the escaping H atom. It is not to be expected, a
priori, that the velocity or range of the H atom released from the nitrogen atom should be
identical with that due to a collision in free hydrogen.
Taking into account the great energy of motion of the alpha particle expelled from radium
C, the close collision of such an alpha particle with a light atom seems to be the most likely
agency to promote the disruption of the latter; for the forces on the nuclei arising from such
collisions appear to be greater than can be produced by any other agency at present
available. Considering the enormous intensity of the force brought into play, it is not so
much a matter of surprise that the nitrogen atom should suffer disintegration as that the
alpha particle itself escapes disruption into its constituents. The results as a whole suggest
that, if alpha particles--or similar projectiles--of still greater energy were available for
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Laboratorio di Fisica Nucleare
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Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
experiment, we might expect to break down the nucleus structure of many of the lighter
atoms.
I desire to express my thanks to Mr. William Kay for his invaluable assistance in counting
scintillations.
University of Manchester, April 1919.
L.3 Lettura dell’articolo di Chadwick
Possible Existence of a Neutron
James Chadwick
Nature, p. 312 (Feb. 27, 1932)
It has been shown by Bothe and others that beryllium when bombarded by -particles of
polonium emits a radiation of great penetrating power, which has been an absorption
coefficient in lead of about 0.3 (cm)¯1. Recently Mme. Curie-Joliot and M. Joliot found,
when measuring the ionisation produced by this beryllium radiation in a vessel with a thin
window, that the ionisation increased when matter containing hydrogen was placed in front
of the window. The effect appeared to be due to the ejection of protons with velocities up
to a maximum of nearly 3 x 109 cm. per sec. They suggested that the transference of
energy to the proton was by a process similar to the Compton effect, and estimated that
the beryllium radiation had a quantum energy of 50 x 10 6 electron volts.
I have made some experiments using the valve counter to examine the properties of this
radiation excited in beryllium. The valve counter consists of a small ionisation chamber
connected to an amplifier, and the sudden production of ions by the entry of a particle,
such as a proton or -particle, is recorded by the deflexion of an oscillograph. These
experiments have shown that the radiation ejects particles from hydrogen, helium, lithium,
beryllium, carbon, air, and argon. The particles ejected from hydrogen behave, as regards
range and ionising power, like protons with speeds up to about 3.2 x 109 cm. per sec. The
particles from the other elements have a large ionising power, and appear to be in each
case recoil atoms of the elements.
If we ascribe the ejection of the proton to a Compton recoil from a quantum of 52 x 106
electron volts, then the nitrogen recoil atom arising by a similar process should have an
energy not greater than about 400,000 volts, should produce not more than about 10,000
ions, and have a range in air at N.T.P. of about 1.3 mm. Actually, some of the recoil atoms
in nitrogen produce at least 30,000 ions. In collaboration with Dr. Feather, I have observed
the recoil atoms in an expansion chamber, and their range, estimated visually, was
sometimes as much as 3 mm at N.T.P.
These results, and others I have obtained in the course of the work, are very difficult to
explain on the assumption that the radiation from beryllium is a quantum radiation, if
energy and momentum are to be conserved in the collisions. The difficulties disappear,
however, if it be assumed that the radiation consists of particles of mass 1 and charge 0,
or neutrons. The capture of the -particle by the Be9 nucleus may be supposed to result in
the formation of a C12 nucleus and the emission of the neutron. From the energy relations
of this process the velocity of the neutron emitted in the forward direction may well be
about 3 x 109 cm. per sec. The collisions of the neutron with the atoms through which it
passes give rise to the recoil atoms, and the observed energies of the recoil atoms are in
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Laboratorio di Fisica Nucleare
SIS – A. A. 2004/2005
Bertolina Lucia
Di Maggio Luisa
Ferrari Trecate Irene
fair agreement with this view. Moreover, I have observed that the protons ejected from
hydrogen by the radiation emitted in the opposite direction to that of the exciting -particle
appear to have a much smaller range than those ejected by the forward radiation. This
again receives a simple explanation of the neutron hypothesis.
If it be supposed that the radiation consists of quanta, then the capture of the -particle by
the Be9 nucleus will form a C13 nucleus. The mass defect of C13 is known with sufficient
accuracy to show that the energy of the quantum emitted in this process cannot be greater
than about 14 x 106 volts. It is difficult to make such a quantum responsible for the effects
observed.
It is to be expected that many of the effects of a neutron in passing through matter should
resemble those of a quantum of high energy, and it is not easy to reach the final decision
between the two hypotheses. Up to the present, all the evidence is in favour of the
neutron, while the quantum hypothesis can only be upheld if the conservation of energy
and momentum be relinquished at some point.
J. Chadwick.
Cavendish Laboratory,
Cambridge, Feb. 17.
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