Transcript phys586-lec18

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
producing amplification
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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 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+ + ePenning 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
  1  P 0 ;   1  e
 n primary
n primary for 1mm Ar  2 .5 gives   0 . 92
 Total number of ions formed is
dE
x
n total  dx
Wi
roughly,
, W i is the effective
ave. energy to
make an ion pair
n total  2  4  n primary
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Ionization
air
33.97
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Ionization
For mixtures,
n t  0 .8
2440
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e.g. Ar  CO 2 80 : 20 
 0 .2
3010
 93 / cm
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n p  0 . 8  29 . 4  0 . 2  34  30 / cm
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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
 
3
kT ~ 0 . 04 eV at room temperatu re
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Maxwell
v
distributi
on gives
8 kT
m
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v ( electrons ) ~ 10 cm / s
v ions
 ~ 10 4 cm
/s
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Diffusion
 Multiple collisions with gas atoms causes
diffusion
 The 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 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
 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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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)

w  mE 
eE
t
m
t 

1
N   v
Where t is the time between collisions,  is the
energy, N is the number of molecules/V and  is the
instantaneous velocity
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Drift
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Drift
Large 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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Drift
Ar-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
V 
Q
C

Q
0A / d
~
100 e

 1m V
10 pf
 Fine for an x-ray beam of 106 photons this is
fine
 But for single particle detectors we need
amplification!
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Proportional Counter
C 
2
ln b / a 
 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
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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 a(E)
dn  n a dx where a 
n  n 0 exp(  a  E  x )
1

rc


n
M 
 exp   a  r dr 
n0
 a

 a(E) is determined by


Excitation and ionization electron cross sections in
the gas
Represents the number of ion pairs produced / path
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length
Proportional Counter
Values of first Townsend coefficient
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Proportional Counter
Values of first Townsend coefficient
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Proportional Counter
Electron-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
dU  CVdV  qEdr
a
V


 dV

CV 0
Na
b
V


 dV
Na
V

 V
q

q
CV 0
a
CV 0 / r

l 2 
Na
b

Na
dr 
CV 0 / r
l 2 
q
l 2 
dr 
q
l 2 
ln
a
Na
ln
b
Na

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Signal Development
 Considering only the ions
V t  
r t 

r 0 
dr
dt
dV
dr
 m E r  
solving
dr 
q
l 2 
ln
r t 
a
m CV 0 1
l 2  r
for r t  and substituti
ng
m CV 0 

V t   
ln  1 
t
2
4  l 
l  a 
q
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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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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 deexcitation photons or electrons emitted at
cathode walls
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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
 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 Counters
 Many different types of gas detectors have
evolved from the proportional counter
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Drift
Ar-CO2 is a common gas for proportional
and drift chambers
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Drift
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Proportional Counter
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