1

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

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) 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

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==

−=−= −

ενε

.ArneP

primary

nprimary

primarytotal

ii

total

nn

WW

xdxdE

n

⋅−≈

Δ=

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

30102.026

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/driftThese collisions can lead to recombination thus lessening the signal

9

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

4

6

π

ε

=

=

10

DiffusionMultiple collisions with gas atoms causes diffusionThe linear distribution of charges is Gaussian

11

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

12

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

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 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

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DriftLarge 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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DriftAr-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

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

=

22

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 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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Proportional Counter

Values of first Townsend coefficient

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Proportional Counter

Values of first Townsend coefficient

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Proportional CounterElectron-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

−+

+

−

>>

===

−=

−==

==

∫∫

∫∫

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

31

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

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 timeUsually 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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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

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Gas

Polymerization on anodes