1 ionization detectors basic operation charged particle passes through a gas (argon, air, …) and...
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Ionization DetectorsBasic operation
Charged particle passes through a gas (argon, air, …) and ionizes it
Electrons and ions are collected by the detector anode and cathode
Often there is secondary ionization producing amplification
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Ionization DetectorsModes of operation
Ionization mode Full charge collection but no amplification (gain=1) Generally used for gamma exposure and large fluxes
Proportional mode Ionization avalanche produces an amplified signal
proportional to the original ionization (gain = 103—105) Allows measurement of dE/dx
Limited proportional (streamer) mode Secondary avalanches from strong photo-emission and
space charge effects occur (gain = 1010) Geiger-Muller mode
Massive photo-emission results in many avalanches along the wire resulting in a saturated signal
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Ionization Detectors
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Ionization
Ionization Direct – p + X -> p + X+ + e-
Penning effect - Ne* + Ar -> Ne + Ar+ + e-
ntotal = nprimary + nsecondary
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Ionization
The number of primary e/ion pairs is Poisson distributed, being due to a small number of independent interactions
Total number of ions formed is
92.0 gives 52 1mmfor
1;01
.Arn
eP
primary
nprimary
primarytotal
ii
total
nn
WW
xdxdE
n
42 roughly,
pairion an make energy to ave. effective theis ,
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Ionization
air 33.97
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Ionization
cmn
cmn
COAr
p
t
/30342.04.298.0
/9333
30102.0
26
24408.0
20:80 e.g. mixtures,For 2
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Charge Transfer and Recombination
Once ions and electrons are produced they undergo collisions as they diffuse/drift
These collisions can lead to recombination thus lessening the signal
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Diffusion Random thermal motion causes the
electrons and ions to move away from their point of creation (diffusion)
From kinetic theory
scmionsv
scmelectronsv
m
kTv
eVkT
/10~
/10~)(
8
giveson distributi Maxwell
re temperaturoomat 04.0~2
3
4
6
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DiffusionMultiple collisions with gas atoms
causes diffusionThe linear distribution of charges is
Gaussian
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Drift In the presence of an electric field E the
electrons/ions are accelerated along the field lines towards the anode/cathode
Collisions with other gas atoms limits the maximum average (drift) velocity w
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Drift
A useful concept is mobility Drift velocity w = E
For ions, w+ is linearly proportional to E/P (reduced E field) up to very high fields That’s because the average energy of the ions
doesn’t change very much between collisions The ion mobilities are ~ constant at 1-1.5
cm2/Vs
The drift velocity of ions is small compared to the (randomly oriented) thermal velocity
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Drift
For ions in a gas mixture, a very efficient process of charge transfer takes place where all ions are removed except those with the lower ionization potential Usually occurs in 100-1000 collisions
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DriftElectrons in an electric field can
substantially increase their energy between collisions with gas molecules
The drift velocity is given by the Townsend expression (F=ma)
Where is the time between collisions, is the energy, N is the number of molecules/V and is the instantaneous velocity
vN
m
eEEw
1
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Drift
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DriftLarge range of drift velocities and
diffusion constants
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Drift
Note that at high E fields the drift velocity is no longer proportional to E That’s where the drift velocity becomes
comparable to the thermal velocity
Some gases like Ar-CH4 (90:10) have a saturated drift velocity (i.e. doesn’t change with E) This is good for drift chambers where
the time of the electrons is measured
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DriftAr-CO2 is a common gas for
proportional and drift chambers
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Drift
Electrons can be captured by O2 in the gas, neutralized by an ion, or absorbed by the walls
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Proportional Counter
Consider a parallel plate ionization chamber of 1 cm thickness
Fine for an x-ray beam of 106 photons this is fine
But for single particle detectors we need amplification!
Vpf
e
dA
Q
C
QV
1
10
100~
/0
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Proportional Counter
Close to the anode the E field is sufficiently high (some kV/cm) that the electrons gain sufficient energy to further ionize the gas
Number of electron-ion pairs exponentially increases
abC/ln
2
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Proportional Counter
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Proportional Counter
There are other ways to generate high electric fields These are used in micropattern
detectors (MSGC, MICROMEGAS, GEM) which give improved rate capability and position resolution
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Proportional Counter
Multiplication of ionization is described by the first Townsend coefficient (E)
(E) is determined by Excitation and ionization electron cross
sections in the gas Represents the number of ion pairs
produced / path length
cr
a
drrn
nM
xEnn
dxndn
exp
)exp(
1 where
0
0
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Proportional Counter
Values of first Townsend coefficient
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Proportional Counter
Values of first Townsend coefficient
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Proportional CounterElectron-molecule collisions are
quite complicated
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Avalanche Formation
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Signal Development
The time development of the signal in a proportional chamber is somewhat different than that in an ionization chamber Multiplication usually takes place at a
few wire radii from the anode (r=Na) The motion of the electrons and ions
in the applied field causes a change in the system energy and a capacitively induced signal dV
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Signal Development
Surprisingly, in a proportional counter, the signal due to the positive ions dominates because they move all the way to the cathode
VV
Na
b
l
qdr
l
rCV
CV
qdVV
Na
a
l
qdr
l
rCV
CV
qdVV
qEdrCVdVdU
b
Na
b
Na
a
Na
a
Na
ln22
/
ln22
/
0
0
0
0
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Signal Development
Considering only the ions
tal
CV
l
qtV
trrl
CVrE
dt
dr
a
tr
l
qdr
dr
dVtV
tr
r
20
0
0
1ln4
ngsubstituti and for solving
1
2
ln2
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Signal Development
The signal grows quickly so it’s not necessary to collect the entire signal ~1/2 the signal is collected in
~1/1000 the time Usually a differentiator is used
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Signal Development
The pulse is thus cut short by the RC differentiating circuit
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GasOperationally desire low working voltage
and high gain Avalanche multiplication occurs in noble
gases at much lower fields than in complex molecules Argon is plentiful and inexpensive
But the de-excitation of noble gases is via photon emission with energy greater than metal work function 11.6 eV photon from Ar versus 7.7 eV for Cu
This leads to permanent discharge from de-excitation photons or electrons emitted at cathode walls
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GasArgon+X
X is a polyatomic (quencher) gas CH4, CO2, CF4, isobutane, alcohols, …
Polyatomic gases have large number of non-radiating excited states that provide for the absorption of photons in a wide energy range
Even a small amount of X can completely change the operation of the chamber Recall we stated that there exists a very
efficient ion exchange mechanism that quickly removes all ions except those with the lowest ionization potential I
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Gas
Argon+X Neutralization of the ions at the
cathode can occur by dissociation or polymerization Must flow gas Be aware of possible polymerization on
anode or cathode Malter effect
Insulator buildup on cathode Positive ion buildup on insulator Electron extraction from cathode Permanent discharge
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Gas
Polymerization on anodes
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Proportional CountersMany different types of gas detectors
have evolved from the proportional counter
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DriftAr-CO2 is a common gas for
proportional and drift chambers
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Drift
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Proportional Counter