1

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

2

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

3

Ionization Detectors

4

Ionization

Ionization Direct – p + X -> p + X+ + e-

Penning effect - Ne* + Ar -> Ne + Ar+ + e-

ntotal = nprimary + nsecondary

5

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 ,

6

Ionization

air 33.97

7

Ionization

cmn

cmn

COAr

p

t

/30342.04.298.0

/9333

30102.0

26

24408.0

20:80 e.g. mixtures,For 2

8

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

9

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

10

DiffusionMultiple collisions with gas atoms

causes diffusionThe linear distribution of charges is

Gaussian

11

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

12

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

13

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

14

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

15

Drift

16

DriftLarge range of drift velocities and

diffusion constants

17

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

18

DriftAr-CO2 is a common gas for

proportional and drift chambers

19

Drift

Electrons can be captured by O2 in the gas, neutralized by an ion, or absorbed by the walls

20

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

21

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

22

Proportional Counter

23

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

24

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

25

Proportional Counter

Values of first Townsend coefficient

26

Proportional Counter

Values of first Townsend coefficient

27

Proportional CounterElectron-molecule collisions are

quite complicated

28

Avalanche Formation

29

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

30

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

31

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

32

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

33

Signal Development

The pulse is thus cut short by the RC differentiating circuit

34

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

35

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

36

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

37

Gas

Polymerization on anodes

38

Proportional CountersMany different types of gas detectors

have evolved from the proportional counter

39

DriftAr-CO2 is a common gas for

proportional and drift chambers

40

Drift

41

Proportional Counter