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Fysiikan historia
kevät 2011
Luento 13
History of Nuclear Physics
The proton-electron model
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In 1914 it was known that nucleus is the seat of all
radioactive processes. The composition of the
nucleus was, however, still unclear.
Rutherford: ”The hydrogen nucleus in the positive
electron.” This nucleus was called generally the Hparticle, the name `proton´ game later.
To make atomic weights and charges to match, it
was natural to assume that there were electrons in
nucleus (neutrons were unkown). The helium
nucleus was thought to consist of 4 H-particles and 2
electrons.
The nucleus was thought to be bound together by an
intensive electromagnetic force. What else could it
be; electromagnetic force and gravity were the only
forces known at that time.
Binding energy
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In 1905 Einstein suggested that a particle of a mass
m has internal energy E = mc2. This was a new
form of energy, but Einstein was sceptical about the
possibility of measuring it: ”! for the moment there
is no hope whatsoever!”.
Planck pointed out a consequence: a bound system
should weight less than its constituents. (The
binding energy, ie the energy associated with the
interactions of the constituents, is negative.)
The binding energy between electrons and Hparticles due to electromagnetic interactions was
estimated to be much too small to match the data.
In 1932 James Chadwick (1891-1974) found the
neutron. It soon came clear that the nucleus is
made of protons and neutrons. This lead to more
correct values of the binding energies.
Discovery of the neutron
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In 1930 a puzzling reaction was detected:
4
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He+9Be!12C + high energy neutral radiation
Chadwick was able to show that the high energy
radiation consisted of neutral particles with a mass
1.007 times the proton mass. He called the neutral
particle the neutron. He believed the the neutron is
a bound state of a proton and an electron, as
Rutherford had suggested. It took two years to
understand that the neutron is as elementary
particle as the proton.
Induced radioactivity
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French Irene (1897-1956) and Frederic Joliot-Curie
(1900-1958) irradiated aluminium foil with "particles. They saw an amazing thing: the positron
radiation from the target did not end immediately
when the "-source was removed. They had
produced artificially a new radioactive isotope that
decayed via #+ emission:
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Five weeks later Italian Enrico Fermi (1901-1954)
published a paper on radioactivity induced by
neutron bombardment. He became soon the world’s
leading expert in neutron physics.
The energy spectrum of beta decay
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In the first experiments seemed to show that electrons
emitted in beta decay have discrete energies. In some
measurements the spectrum a broad band was seen
instead of sharp lines.
In 1914 James Chadwick found that the energy
spectrum is actually continuos.
The problem was that in the reaction
(A,Z)->(A,Z+1) + e-
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the electron should have a well defined energy
determined by the masses of the mother and daughter
nuclei. People solved the discrepancy by
speculating that what is measured is not the primary
electron but some secondary effect. In 1927 this was
shown wrong.
Bohr suggested non-conservation of energy in beta
decay. He connected this problem with the problem of
the structure of the nucleus (neutron was not
discovered yet).
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In 1930 German-Swiss Wolfgang Pauli (1900-1958)
suggested a totally new solution: In beta decay
another spin-1/2 particle – neutral and very light – is
emitted, a neutrino. The energy is devided between
the electron and the neutrino in all possible ways,
therefore the spectrum is continuous
From his famous letter to a gathering of physicists in
Tübingen:
Dear radioactive ladies and gentlemen,
...
I have hit upon a desperate remedy to save!the law of
conservation of energy. But so far I do not dare to publish
anything about this idea, and trustfully turn first to you, dear
radioactive ones, with the question of how likely it is to find
experimental evidence for such a neutron!
I admit that my remedy may seem almost improbable because
one probably would have seen those neutrons, if they exist,
for a long time. But nothing ventured, nothing gained!
Pauli giving a lecture.
Pauli and Bohr making an experiment
with a Tippe Top in 1951.
Theories of nuclear physics
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In 1934 Enrico Fermi (1901-1954) presented a field
theoretical model for the beta decay, so called four
fermion model:
!
n!
p!
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e
n " p + e# +! e
!e
Never before it was considered theories were new
massive particles can be created in interaction (like
photons). This was a new type of interaction – a
new force - called weak interaction.
The Fermi theory and its successful applications
confirmed the proton-neutron picture of the nucleus.
Nuclear electrons were not needed any more: the
beta decay electrons (and positrons) are created at
the instant of decay.
Fermi was both an experimentalist and a theorist.
Heisenberg’s theory of strong interactions
•  In 1932 Heisenberg presented a quantum
mechanical model for proton-neutron nucleus.
•  Assumed still that the neutron is a kind of bound
state of proton and electron. The electron carries a
strong binding force that keeps nucleons together in
the nucleus.
•  Introduce the concept of isospin: the proton and the
neutron are +1/2 and -1/2 isospin states.
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In 1934 Japanese Hideki Yukawa presented another
theory, where strong force was carried by a specific
heavy particle, the U-quanta (pion). The model could
explain the short range of the strong force.
The birth of accelerator-based nuclear
physics
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In earlier studies of atom and nuclear structure
radioactive isotopes were used as the source of
probe particles (usually "’s). The energy of the
probes was then limited to the typical nuclear
physics scale of some MeV’s.
In 1932 John Cockcroft (1897-1967) and Ernest
Walton (1903-1995) took in use the first particle
accelerator, where protons were accelerated to the
energies of hundres of MeV’s.
They produced the nuclear transformation
Li7 + p -> 2" + 14.3 MeV.
Measuring the kinetic energy of "’s and the masses
known they could test Einstein’s formula E = mc2
within errors. This was the first test of it.
The Cockcroft-Walton Accelerator
was based on a voltage multiplier
that used an intricate stack of
capacitors connected by rectifying
diodes as switches.
Nuclear fission
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Fermi got the Nobel prize in 1937 for demonstrating
the existence of new radioactive elements produced
by neutron irradiation. He had bombarded eg.
uranium (Z=92) with neutrons.
In 1938 Germans Otto Hahn (1879-1968) and Fritz
Strassmann (1902-1980) started a careful
radiochemical analysis of the elements produced in
neutron-U collisions. They staggering result was
that among the products they identified three
isotopes of barium, which have Z=56.
An Austrian Lise Meitner (1878-1968), who had for
a long time worked in the group but who now lived
in exile for Nazis in Stockholm, together with her
nephew Otto Frisch (1904-1979) gave a physical
explanation to the discovery, fission of the uranium.
Large kinetic
energy
Fast neutrons
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This started on intensive study of fission
everywhere. Niels Bohr gave his last scientific
contribution in studying the fission process with
Archibal Wheeler.
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It was immediately understood that a chain reaction
is possible due to the fast neutrons. A new energy
source and also a new weapon, atomic bomb, were
seen available.
During the II World War, a great number of top
physicist worked in the secret Manhattan project in
Los Alamos to produce an atomic bomb, which was
then used in Hiroshima and Nagasaki.
Einstein and Leo Szilard, who
made Einstein to sign a letter to
president Franklin Roosevelt
1939 about the possibility of
atomic bomb.
”Physics lost its innocence.”
History of particle
physics
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The history of particle physics as an independent
research field started with new accelerators after the II
World War.
Also new detection techniques were developed. The
most important ones include:
–  Cloud chamber (Charles Wilson, 1913). In
supersaturated vapor ionizating particles causes creation
of clouds on their track.
–  Bubble chamber (Donald Glaser, 1952). Transparent
liquid heated just below the boiling point. When the
pressure is suddenly decreased, charged particles form
an ionization track around which the liquid vaporizes and
forms microscopic bubbles.
–  Wire chamber (Charles Charpak, 1968). A metal tube
filled with a suitable gas mixture; walls grounded and a
set of wires in high voltage. Ionizing radiation causes
short cuts in the gas and electric current in wires.
–  Scintillators. Materials (some crystals, plastics, liquids,
gases) that absorb energy from ionizing particles and reemit (scintillate) it, usually as visible light. The light is
observed usually with photomultipliers or photodiodes.
–  Cherenkov detectors. A particle passing through a
material at a velocity greater than the velocity of light in
that material emits light (Pavel Cherenkov, 1934). The
light is observed with photomultipliers.
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The particles known before 1950 were the electron,
proton, neutron, positron, muon, kaon. Positron,
muon and pion were discovered from cosmic rays.
With accelerators many new particles were soon
discovered.
Kosmisen säteilyn oli löytänyt alun perin Victor
Hess 1912. Hess halusi osoittaa, että kaikkialla
ilmenevä radioaktiivinen säteily ei ole peräisin
ilmasta vaan kallioperästä. Osoittaakseen tämän,
hän teki mittauksia ilmapallolennoilla (yli 5000
m:n korkeuteen!). Aluksi säteilyn määrä
pienenikin korkeuden funktiona, mutta sitten se
alkoi yllättäen kasvaa. Avaruudesta tuli siis
säteilyä, jota ilmakehä absorboi sitä vähemmän
mitä korkeammalla oltiin.
Hess lähdössä
ilmapallolennolleen vakavailmeisten assistenttiensa
saattelemana.
1920-luvun lopussa ja 1930-luvun alussa useat tutkijat
totesivat kosmisessa säteilyssä olevan läpäisykykyisiä
varattuja hiukkasia.
1934 Carl Anderson ja Seth Neddermeyer totesivat
näiden massan olevan elektronin massaa suurempi
mutta protonin massaa paljon pienempi.
Monta nimeä: mesoni, mesotroni, barytroni, raskas
elektroni, yukoni, µ-mesoni ! myoni (µ)
Carl Anderson
Accelerators
•  In 1931 Ernest Lawrence and David Sloan built the
first cyclotron (1.3 MeV).
A drawing in Lawrence’s patent
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The first syclotron of
Lawrence, the diameter
12 cm.
Cyclotrons were first used in nuclear physics. At
higher energies relativistic effect on kinematics had
to be taken into account. This is done in
synchrocyclotrons.
The first new artifial elementary particle produced in
accelerators was neutral pion $0, which was
discovered in Berkeley’s synchrocyclotron in 1950.
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The next step were synchrotrons, in which the particles are
accelerated in a ring of constant radius. The benefit is that
one does not need one big magnet but separate small
magnets along the ring.
–  Cosmotron, Brookhaven 1953 (3 GeV protons)
–  Bevatron, Berkeley1954 (6 GeV protons)
–  Alternating Gradient Synchrotron (AGS) Brookhaven 1960 (33
GeV protons)
–  Proton Synchrotron (PS) CERN 1959 (28 GeV protons)
–  Super Proton Synchrotron (SPS) CERN 1976 (400 GeV
protons), used later as a collider of protons and antiprotons.
–  Tevatron, Fermilab 1983 (1 TeV proton and antiproton
collider)
–  Large electron positron collider (LEP) CERN 1989 (electrons
and positrons 45 GeV+45 GeV)
–  LHC, CERN 2010 (7 TeV + 7 GeV protons)
–  Etc.
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Linear colliders has the benefit of no energy loss for
synchrotron radiation and no need for bending magnets.
Stanford linear accelerator at SLAC is the largest.
Brookhaven
CERN
Building of CERN started in 1954
SLAC linear accelerator
Tevtron at Fermilab
CMS detector at LHC
Areal view of the
European Synchrotron
Radiation Facility,
ESRF, in Grenoble.
Gargamelle bubble
chamber at CERN
A bubble chamber picture and its
interpretation. The discovery of
the ! particle.
Carl Andersson close to
his cloud chamber.
A wire chamber under
construction.
The heavy water Cherenkov
detector at the Sudbury Neutrino
Observatory.
Qark model
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In the early 1960s the number of hadrons (particles
with strong interactions) was already so large that
particle physicists started to look for fundamental
basic structure of matter.
In 1964 American Murray Gell-Mann (b. 1929 ) and
Russian George Zweig (b. 1937) proposed
independently the quark model of hadrons.
All hadrons are bound states of three quarks
(baryons like the proton, neutron etc.) or
bound states of a quark and an antiquark
(mesons like the pions, kaons etc).
The quark model was based on Gell-Mann’s
earlier discovery of an SU(3) symmetry
among hadrons (Eightfold way –model).
In Gell-Mann’s model hadrons nicely fit in the
representations of the SU(3) symmetry group. The model
predicted eg the existense of the ! particle.
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When the quark model was invented all know
hadrons could be explained in terms of three quarks
(up, down ans strange).
The fourth quark (charm) was discovered in
November 1974 independently by American Samuel
Ting (b. 1936) in Brookhaven ja American Burt
Richter (b. 1931) in SLAC. They found a new
particle J/% (psi), a bound state of charm quark (c)
and antiquark.
This was very important discovery as it confirmed
the prediction of the so called electroweak model
(see later). ”November Revolution”.
Samuel Ting
Burt Richter
3.1 GeV
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In 1977 the group of Leon Lederman (b. 1922)
found another new particle & (ypsilon), which turned
out to be a bound state of fifth quark, bottom quark
(b) and anti-bottom. The rest energy of b-quark is
about 5 GeV.
In 1995 the sixth and so far the last quark type was
found in Fermilab, the top quark (t). It is the heaviest
particle found so far: its rest energy is 175 GeV.
The top quark was discovered
with this detector.
An ypsilon decay event.
Leon Lederman
The Standard Model of particle physics
QED
•  The mother of all the modern particle physics
theories is Quantum Electrodynamics (QED). It is a
quantum field theory that describes the
electromagnetic interactions of particles. Quantum
field theories are generalization of quantum
mechanics to the situations where particles are
created and annihilated.
•  The QED was developed in the 1940s by Richard
Feynman (American, 1918-1988), Freeman Dyson
(Brittish, b. 1923), Julian Schwinger (American,
1918-1994), and Sin-Itiro Tomonaga (Japanese,
1906-1979).
The Shelter Island
Conference (1947) on
the Foundations of
Quantum Mechanics is
considered as one of
the most influential
meetings after the
World War II.
W. Lamb, J.A. Wheeler
(standing), A. Pais, R.
Feynman, H.Feshbach,
J. Schwinger. Willis
Lamb’s discovery of the
Lamb shift in the
hydrogen spectrum
was one of the
mysteries tackled.
Electroweak model
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The QED is technically so called gauge field theory
based on the Lie symmetry U(1).
In order to find a similar theory for other forces,
Americans C. N. Yang (b. 1922) ja Robert Mills
(1927-1999) generalized QED in 1954 to allow for
more complicated interactions, in particularly strong
interactions, and symmetries (the Yang-Mills
theory).
American Sheldon Galshow (b. 1932) suggested
that electromagnetic and weak interactions could be
described with a single Yang-Mills theory based on
the symmetry SU(2)'U(1). The problem was that
the theory predicted the carriers of the weak force,
the W and Z particles, to be massless like photon
which they could not be as the weak force has a
finite range.
C N Yang
Sheldon Glashow
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In 1967 Steven Weinberg (b 1933) invented the way
the mass problem could be solved by using so
called spontaneous symmetry breaking. The
spontaneous symmetry breaking had been earlier
studied by Japanese-American Yoichiro Nambu (b.
1921), British Peter Higgs (b. 1929), and Belgians
Robert Brout (b. 1928) ja Francois Englert (b. 1932).
The spontaneous
symmetry braeking: ”The
dynamics is symmetric but
in the vacuum state
symmetry is broken”.
Yoichiro Nambu holding
his 2008 Nobel medal.
Peter Higgs
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In 1967 Steven Weinberg and Pakistan Abdus
Salam (1926-1996) presented the SU(2)'U(1)
electroweak theory. In it the spontaneous symmetry
breaking was used to get masses for weak bosons
via the so called Higgs mechanism.
The model was generally accepted only after Dutch
Gerald ’t Hooft (b. 1946) showed it renormalizable in
1971.
Abdus Salam
Steven Weinberg
Gerald ’t Hooft
Quantum Cromo Dynamics QCD
•  The quark model did not tell much about the strong
force that keeps quarks together in hadrons. In
particular, why quarks were never observed as free
particles. There were also problems with Pauli’s
exclusion princible: For example, in omega particle
there are three s quarks with their spins parallel, a
state of identical fermions forbidden by the
exclusion princible.
•  American Oscar Greenberg introduced in 1964 a
new quantum number called color charge, which
separates quarks in hadrons.
•  In 1972 Gell-Mann ja German Harald Fritzsch (b.
1943) showed that the color charge can be realized
in a gauge field theory based on the SU(3)
symmetry. This theory, the counterpart of the QED
for strong interactions, is called quantum chromo
dynamics (QCD).
Fritzsch and GellMann quite recently
with an unknown
party company.
The Standard Model
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The electroweak theory and QCD form together the
Standard Model of particle physics.
The carriers of the electroweak force, the W and Z
bosons, were discovered at CERN in 1982 in the
UA1 experiment led by Carlo Rubbia. The carriers
of the strong force, gluons, were discovered at the
German research center DESY in 1979.
An event were a W boson
was produced.
Carlo Rubbia
Particles of the standard model.
The Higgs particlem (H) has
not been discovered yet.