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
1

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 = 10 3— 10 5 )
Allows measurement of dE/dx


Limited proportional (streamer) mode
 Secondary avalanches from strong photo-emission and space charge effects occur (gain = 10 10 )
Geiger-Muller mode
 Massive photo-emission results in many avalanches along the wire resulting in a saturated signal
2

Ionization Detectors
3

Ionization
 Ionization

Direct – p + X -> p + X + + e -

Penning effect - Ne * + Ar -> Ne + Ar + + e -
 n total
= n primary
+ n secondary
4

Ionization
 The number of primary e/ion pairs is Poisson distributed, being due to a small number of independent interactions
 
1

P

1
 e
 n p rima ry n primary for 1mm Ar

2 .
5 gives
 
0 .
92
 Total number of ions formed is n total
 roughly, dE
 x dx
, W i is the effective
W i n total

2

4
 n primary ave.
energy to make an ion pair
5

Ionization air 33.97
6

For mixtures,
Ionization e.g.
Ar

CO
2

80 : 20
 n t n p


0 .
8
0 .
8

2440
26
29 .
4


3010
0 .
2
0 .
2
33

34


30
93 /
/ cm cm
7

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
8

Diffusion
 Random thermal motion causes the electrons and ions to move away from their point of creation (diffusion)
 From kinetic theory
 
3
2 kT ~ 0 .
04 eV at room temperatu re
Maxwell distributi on gives v

8 kT
 m v ( electrons ) v
 ions

~ 10
4
~ cm
10
6
/ s cm / s
9

Diffusion
 Multiple collisions with gas atoms causes diffusion
 The linear distribution of charges is Gaussian
10

 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
11

Drift
 A useful concept is mobility m

Drift velocity w = m 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 cm 2 /Vs
 The drift velocity of ions is small compared to the (randomly oriented) thermal velocity
12

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
13

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
  m
E
 eE m t
 t 
N

1
  v
Where t is the time between collisions,  is the energy, N is the number of molecules/V and  is the instantaneous velocity
14

Drift
15

Drift
 Large range of drift velocities and diffusion constants
16

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 change with E)
4
(90:10) have a saturated drift velocity (i.e. doesn’t

This is good for drift chambers where the time of the electrons is measured
17

Drift
 Ar-CO
2 is a common gas for proportional and drift chambers
18

Drift
 Electrons can be captured by O by the walls
2 in the gas, neutralized by an ion, or absorbed
19

Proportional Counter
 Consider a parallel plate ionization chamber of
1 cm thickness
V

Q
C

Q

0
A / d
~
100 e

10 pf

1 m
V
 Fine for an x-ray beam of 10 6 photons this is fine
 But for single particle detectors we need amplification!
20

Proportional Counter
C
 ln
2

 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
21

Proportional Counter
22

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
23

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   x )

1

M
 n n
0
 exp r c  a


 a   dr



 a (E) is determined by

Excitation and ionization electron cross sections in the gas

Represents the number of ion pairs produced / path length 24

Proportional Counter
 Values of first Townsend coefficient
25

Proportional Counter
 Values of first Townsend coefficient
26

Proportional Counter
 Electron-molecule collisions are quite complicated
27

Avalanche Formation
28

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
29

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
V
  a

Na dV

 q
CV
0 a

Na
CV
0 l
/
2
 r dr

 q l 2
 ln a
Na
V

V

 b

Na dV

V

 q
CV
0 b

Na
CV
0 l
/
2
 r dr
 q l 2
 ln b
Na
30

Signal Development
 Considering only the ions
V
 r r
 dV dr dr
 q l

r dr dt

m
E
r
 l m
CV
0

 
r
a
V
  q
 l
m
CV
0 l
 a
2 t
31

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
32

Signal Development
 The pulse is thus cut short by the RC differentiating circuit
33

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
34

Gas
 Argon+X



X is a polyatomic (quencher) gas
 CH
4
, CO
2
, CF
4
, 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
35

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
36

Gas
 Polymerization on anodes
37

Proportional Counters
 Many different types of gas detectors have evolved from the proportional counter
38

Drift
 Ar-CO
2 is a common gas for proportional and drift chambers
39

Drift
40

Proportional Counter
41