Ionization Detectors

Transcription

1 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

2 Ionization Detectors Modes of operation Ionization mode Full charge collection but no amplification (gain1) Generally used for gamma exposure and large fluxes Proportional mode Ionization avalanche produces an amplified signal proportional to the original ionization (gain ) Allows measurement of de/dx Limited proportional (streamer) mode Secondary avalanches from strong photo-emission and space charge effects occur (gain ) Geiger-Muller mode Massive photo-emission results in many avalanches along the wire resulting in a saturated signal 2

3 Ionization Detectors 3

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

5 n Ionization The number of primary e/ion pairs is Poisson distributed, being due to a small number of independent interactions ε 1 P n primary ( 0; ν ) for 1mm 1 e Ar 2. 5 gives ε Total number of ions formed is total roughly, n de Δx dx, W W i total i is 2 4 n primary n primary 0.92 the effective ave.energy to make an ion pair 5

6 Ionization air

7 p Ionization ( 80 : 20) For mixtures, e.g. Ar CO nt / cm n / cm 7

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 8

9 Diffusion Random thermal motion causes the electrons and ions to move away from their point of creation (diffusion) From kinetic theory 3 ε kt ~ 0.04eV at room temperature 2 Maxwell distribution gives v v 8kT πm v( electrons) ~ 10 4 ( ions) ~ 10 cm / s 6 cm / s 9

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

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 11

12 Drift A useful concept is mobility μ Drift 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 collisions The ion mobilities are ~ constant at cm 2 /Vs The drift velocity of ions is small compared to the (randomly oriented) thermal velocity 12

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 collisions 13

14 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 (Fma) w τ N μ σ E 1 ( ε )v ee Where τ is the time between collisions, ε is the energy, N is the number of molecules/v and ν is the instantaneous velocity m τ 14

15 Drift 15

16 Drift Large range of drift velocities and diffusion constants 16

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-CH 4 (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 17

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

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

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

21 Proportional Counter C 2πε 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 21

22 Proportional Counter 22

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 23

24 Proportional Counter Multiplication of ionization is described by the first Townsend coefficient α(e) 1 dn nαdx whereα λ n n exp( α E x) M 0 n n exp ( ) () r α(e) is determined by 0 r c a α dr Excitation and ionization electron cross sections in the gas Represents the number of ion pairs produced / path length 24

25 Proportional Counter Values of first Townsend coefficient 25

26 Proportional Counter Values of first Townsend coefficient 26

27 Proportional Counter Electron-molecule collisions are quite complicated 27

28 Avalanche Formation 28

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 (rna) 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

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 du V V V + + >> V CVdV a Na b Na dv dv qedr q CV 0 q CV 0 a Na CV0 / r l2πε b Na CV0 / r l2πε dr dr q ln l2πε q ln l2πε a Na b Na 30

31 Signal Development Considering only the ions V () t r ( t ) r ( 0) dr μe dt solving for dv dr () r r dr () t q () + μcv0 V t ln 1 t 2 4πεl lπεa q ln l2πε ( t) r a μcv0 1 l2πε r and substituting 31

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 32

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

34 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

35 Argon+X Gas 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

36 Argon+X Gas 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

37 Gas Polymerization on anodes 37