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Nesamostatné elektrické
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Nesamostatné elektrické výboje
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• V slabom elektrickom poli môže atmosférou na
zemskom povrchu pretekať len velmi slabý prúd
s hustotami 10-12 až 10-6 A/m2 v dôsledku ionizácie
zpôsobenej radioaktivným žiarením Zeme a kozmickým
žiarením vesmíru.
• ,
• Týmto spôsobom sa v 1 cm3 zemskej atmosféry vytvára
asi 1 000 iónových párov za 1 s. Rovnovážna
koncentrácia náboja je 103 – 104 el. nábojov na cm3. Pre
vedenie elektrického proudu väčšej hustoty je treba
dodatočné ionizačné činidlo a výboje vyvolané týmto
spôsobom nazývame nesamostatné.
5 hlavných zdrojov ionizácie
pozadia:
•
•
•
•
Kozmické (galaktické) žiarenie
Slnečné žiarenie
Rádioaktívne žiarenie zo zemskej kôry
Rádón (v USA je radón po fajčení druhým
najdôležitejším dôvodom k vzniku rakoviny
pľúc)
• Ionizujúce žiarenie v dôsledku ľudskej
činnosti
Svensmark, H., T. Bondo, and J. Svensmark (2009),
Cosmic ray decreases affect atmospheric aerosols and clouds,
Geophys. Res. Lett.,.
• Our results show global-scale evidence of conspicuous influences of
solar variability on cloudiness and aerosols.
• From solar activity to cosmic ray ionization to aerosols and liquidwater clouds, a causal chain appears to operate on a global scale.
Volt-ampérová charakteristika
nesamostatných výbojov
• prúd v dôsledku ionizácie pozadia, Townsendov el. výboj
(tmavý el. výboj)
Aplikácie nesamostatných el. výbojov
Pri prúdoch menších než saturačný prúd, alebo v oblasti
saturčného prúdu – napr. ionizačné detektory dymu
Americium 241
Ionizácia nárazom elektrónu –
1. Townsendov koeficient
• je počet ionizačných zrážok, ktoré elektrón vykoná na jednotkovej
dráhe (1 cm) pri pohybe v smere elektrickéo poľa.
• Elektrónová lavína
Proporcionálne detektory/počitače
• Zachovávajú priamu úmeru medzi počtom elektrónov
vzniklých ionizáciou nárazom častice s vysokou energiu
a počtom elektrónov vzniklých ionizáciou nárazom
elektrónu – t.j. zachovávajú priamu úmeru medzi
energiou častice, ktorá sa uvolní medzi elektródami a
prúdovým signálom.
• Nemeriame prúd vzniklý dopadom nabitých častíc na
elektródy, ale indukovaný prúd !
• Lama-Shockley teorém
Detector theory
•
•
In gas-flow and sealed detectors, X-rays produced from the sample ionize an inert
detector gas, ejecting an outer shell electron to produce an electron-ion pair (Ar –> Ar+
+ e-). The first ionization potentials of the inert gases are small (less than 25 eV);
however, the effective ionization potential that is required to produce an electron-ion
pair is somewhat higher due to competing processes which absorb incident photon
energy without causing ionization.
1st Ionization
Potential
Average
Ionization
Potential
He
24.5 eV
27.8 eV
Ar
15.7
26.4
Xe
12.1
20.8
The average number of electron-ion pairs (n) produced by an X-ray is:
Consider: the energy of Cu-Ka is 8.04
keV, so, using Ar detector gas, n =
8040/26.4 = 304 primary electron-ion
pairs. This number is too small to
detect, but placing a potential across
the gas from wire to tube wall
produces amplification. The electrons
produced by the incoming X-ray are
accelerated towards the anode wire by
the detector voltage and can in turn
ionize other Ar atoms producing
another electron-ion pair and so on
(Figure). This "avalanche" effect
produces an amplification of the initial
signal. The chain of ionizations causes
a momentary voltage across the
detector producing a pulse.
Figure. Principle of detection of an X-ray
photon. Incident X-rays ionize the Ar
detector gas losing an average of 26.4 eV,
then continue to ionize other atoms. The
resulting secondary electrons are
accelerated toward the detector wire,
gaining sufficient energy to ionize other Ar
atoms, producing an electron avalanches.
The Ar ions are neutralized by electrons
donated by methane molecules in the
detector gas mix.
The amount of amplification produced by the gas depends on the amount
of voltage applied to the detector (Figure). At very low voltages in the
region of undersaturation, the detector potential difference is too small
to prevent recombination of electron-ion pairs formed by incident Xrays before they reach the collecting wire. At slightly higher voltages in the
ionization chamber region, the potential is just sufficient to counter
recombination so that the number of electron-ion pairs produced by Xrays equals the number reaching the anode wire, and the gain is 1.
Further increases in the detector voltage produce the avalanche
effect and significant gains. At voltages in the proportional counter
region, the pulse height is proportional to the energy of the incident
X-ray. Too high a voltage drives the detector out of the proportional region
and into the Geiger region.
Figure. The effect of increasing the
applied anode voltage on (a) the gas
amplification factor and (b) the
observed count rate (ignoring pulse
height analysis settings) for a gas
proportional counter. Note the rapid
increase in observed count rate at the
threshold of Geiger breakdown (after
Potts 1987).
•
Gain may be defined as:
•
Detector gains are typically on the order of 104 to 105. With a gain of 104,
the 304 EI pairs formed by a Cu-K X-ray produce 3.04 x 106 electrons that
reach the anode wire. The size of the resulting pulse can be calculated
from:
•
The charge on a single electron is 1.6022 x 10-19 coulomb, and a typical
detector has a capacitance of 10-10 farad. Thus, our Cu-K photon will
generate a voltage of:
•
Recall that we've assumed a gain of 10,000! The resulting pulse is still very
small and needs further electronic amplification.
Nobelova cena za fyziku 1992
Observing the interior of matter
In order to explore the remarkable
processes in the interior of matter,
detectors of very high precision and
performance are needed. Georges
Charpak's invention of a new particle
detector – the multiwire proportional
chamber – has dramatically changed
the exploration of the world of particles.
Georges Charpak
CERN, Geneva, Switzerland
Charpak's invention
• With the multiwire chamber it became possible
to determine the tracks of charged particles –
the smallest constituents of matter – with great
precision. However, the success of Charpak's
multiwire chamber depended mainly on the
enormous increase in data-taking rate.
• Every single wire in the multiwire chamber acts
as a detector. A wire can detect thousands of
particles per second. This makes it possible to
study even very rare processes in the world of
particles.
The electron avalanche in the detector
•
A charged particle passing through a gas ionises the atoms of the gas. The atoms split into a
negatively charged electron and a positively charged ion. In an electrical field the electrons will
move towards the anode and the ions towards the cathode. At the anode an avalanche of
electrons is produced indicating the passage of the original particle.
The particle ionises the gas
In Charpak's invention the
anode consists of a large
number of parallel wires,
normally a hundredth of a
millimetre in diameter and one
or a few millimetres apart. The
cathode consists of an
electrically conductive plane on
each side of the densely
packed anode wires.
The charges move
In the electrical field the
liberated electrons rapidly
move towards the anode wire
and the ions move towards the
cathode planes. The electrons
are accelerated in the strong
field near the anode wire.
The electron avalanche
More electrons are liberated
which in their turn ionise the
gas – an avalanche of charges
is produced, giving rise to an
electric pulse on the anode
wire.
The proportional chamber is
so called because the pulse is
proportional to the original
amount of ions. .
• Each anode wire can handle several hundred thousand
signals per second. This is of great importance when rare
particle collisions are studied. Sometimes only one
particle collision in a million is particularly interesting.
The multiwire chamber
Charpak realised from the beginning that there
were several ways to further develop the multiwire
chamber. The most important development was
the drift chamber. This is used to measure the
time taken for liberated electrons to drift to the
anode. In this way precision was further improved.
The multiwire chamber – both the proportional
chamber and the drift chamber – is now in use in
practically every experiment in particle physics
laboratories. These detectors are also used in
medicine as a complement to X rays.
Six quarks and six leptons
• Charpak's principle has been
used in the DELPHI detector at
CERN. The barrelshaped
detector is 10 m long and 10 m
in diameter. The tracks of the
many charged particles
created when an electron
collides with its antiparticle, the
positron, appear on the
computer reconstructed
picture. By studying such
particle collisions, research
groups at CERN and SLAC,
USA have shown that the
fundamental constituents of
matter are six types of quark
and six types of lepton, one of
which is the electron.
The J/Yparticle
W and Z
At the same time as Burton Richter
discovered the Y (psi) particle, Samuel C.C.
Ting, USA discovered the J particle. They
were shown to be the same particle, now
called the particle. For this discovery Richter
and Ting received the Nobel Prize in Physics
1976.
The observed mass of the particle of 3.1 GeV
corresponds to the mass of slightly more than
three protons. The necessary precision was
The discovery of the W and the Z particles
was rewarded with the Nobel Prize in Physics
in 1984 (Carlo Rubbia and Simon van der
Meer, CERN). The particle collision in which
the Z particle is created and then rapidly
decays into an electron and its antiparticle,
the positron, can be seen in the middle of the
picture. The tracks of all the charged particles
are detected in the central drift chamber. The
Z particle is only created in one particle
obtained thanks to the proportional chamber.
collision in a thousand million.
Y (epsilon) particle
A rat brain
The Y (epsilon) particle was discovered in
1977 by Leon Lederman and his research
group. The 22 proportional chambers played a
very important role in the experiment. In the
two 'arms' muons, into which the very shortlived particle decays, were detected.
A five-thousandth-of-a-millimetre-thick slice of
a rat brain. The colours illustrate the
concentration of molecules sensitive to
radiation. With Charpak's detectors this
picture could be produced in a day
compared to three months with traditional
methods.
Lavínový tranzistor
TOWNSENDOW VZTAH
- uvažujeme nízke tlaky plynu, kde výboj prebieha podľa
Townsendovho ionizačného mechanizmu
APPLIED
VOLTAGE
(VOLTS)
Abnormal
Glow
Townsend dark
500
A
E
B
discharge
F
Transition from
Dark to Glow
discharge
250
Transition from
Glow to Arc
C
Glow
D
G
0
10-15
10-12
10-9
10-6
10-3
CURRENT (AMP)
d.c. voltage current characteristic at an electrical discharge with
electrodes having no sharp points or edges
1
Arc
103