Ionization Detectors
Basic 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
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Ionization Detectors
Modes 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
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Ionization
Ionization Direct – p + X -> p + X+ + e
- Penning effect - Ne* + Ar -> Ne + Ar+ + e
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 is6
Ionization
Charge Transfer and Recombination
Once ions and electrons are produced they undergo collisions as they diffuse/drift9
Diffusion
Random thermal motion causes the electrons and ions to move away from their point ofcreation (diffusion)
From kinetic theoryDiffusion
Multiple collisions with gas atoms causes diffusion11
Drift
In the presence of an electric field E theelectrons/ions are accelerated along the field lines towards the anode/cathode
Drift
A useful concept is mobility m Drift velocity w = mE
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
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Drift
Electrons 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 t is the time between collisions, is the
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Drift
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-CH
4(90:10) have a
saturated drift velocity (i.e. doesn’t
change with E)
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Drift
Drift
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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 isfine
Proportional Counter
Close to the anode the E field is sufficiently high (some
Proportional Counter
Multiplication of ionization is described by the first Townsend coefficient a(E)25
Proportional Counter
Proportional Counter
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
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Signal Development
Surprisingly, in a proportional counter, the signal due to the positive ions dominatesbecause they move all the way to the cathode
Signal Development
Considering only the ions32
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
Signal Development
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Gas
Operationally 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
Gas
Argon+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
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