Ionization Detectorsatlas.physics.arizona.edu/~shupe/Physics_Courses/Phys_586_S2015… · 2...
Transcript of Ionization Detectorsatlas.physics.arizona.edu/~shupe/Physics_Courses/Phys_586_S2015… · 2...
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Ionization DetectorsBasic operation
Charged particle passes through a gas (argon, air, …) and ionizes itElectrons and ions are collected by the detector anode and cathodeOften 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) modeSecondary avalanches from strong photo-emission and space charge effects occur (gain = 1010)
Geiger-Muller modeMassive photo-emission results in many avalanches along the wire resulting in a saturated signal
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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==
−=−= −
ενε
.ArneP
primary
nprimary
primarytotal
ii
total
nn
WW
xdxdE
n
⋅−≈
Δ=
42 roughly,
pairion an make energy to ave. effective theis ,
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Ionization( )
cmn
cmn
COAr
p
t
/30342.04.298.0
/9333
30102.026
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/driftThese collisions can lead to recombination thus lessening the signal
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DiffusionRandom thermal motion causes the electrons and ions to move away from their point of creation (diffusion)
From kinetic theory
( ) scmionsvscmelectronsv
mkTv
eVkT
/10~/10~)(
8
giveson distributi Maxwell
re temperaturoomat 04.0~23
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6
π
ε
=
=
10
DiffusionMultiple collisions with gas atoms causes diffusionThe linear distribution of charges is Gaussian
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DriftIn the presence of an electric field E the electrons/ions are accelerated along the field lines towards the anode/cathodeCollisions with other gas atoms limits the maximum average (drift) velocity w
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Drift
A useful concept is mobility μDrift velocity w = μE
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 collisionsThe 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 moleculesThe drift velocity is given by the Townsend expression (F=ma)
Where τ is the time between collisions, ε is the energy, N is the number of molecules/V and ν is the instantaneous velocity
( )vN
meEEw
εστ
τμ
1=
==−
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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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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 fineBut for single particle detectors we need amplification!
Vpfe
dAQ
CQV μ
ε1
10100~
/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
/ln2πε
=
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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 α(E)
α(E) is determined byExcitation and ionization electron cross sections in the gasRepresents the number of ion pairs produced / path length
( )
( )⎥⎥⎦
⎤
⎢⎢⎣
⎡==
−=
==
∫cr
a
drrnnM
xEnn
dxndn
α
αλ
αα
exp
)exp(
1 where
0
0
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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
Nab
lqdr
lrCV
CVqdVV
Naa
lqdr
lrCV
CVqdVV
qEdrCVdVdU
b
Na
b
Na
a
Na
a
Na
ln22
/
ln22
/
0
0
0
0
πεπε
πεπε
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Signal Development
Considering only the ions
( )( )
( ) ( )
( )
( )
( ) ⎟⎠⎞
⎜⎝⎛ +−=
==
== ∫
tal
CVl
qtV
trrl
CVrEdtdr
atr
lqdr
drdVtV
tr
r
20
0
0
1ln4
ngsubstituti and for solving
12
ln2
πεμ
πε
πεμμ
πε
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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 timeUsually a differentiator is used
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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) gasCH4, 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+XNeutralization of the ions at the cathode can occur by dissociation or polymerization
Must flow gasBe aware of possible polymerization on anode or cathode
Malter effectInsulator buildup on cathodePositive ion buildup on insulatorElectron extraction from cathodePermanent discharge