Ionization Detectors Basic operation n Charged particle passes

Ionization Detectors ØBasic operation n 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 Ø Modes of operation n Ionization mode w Full charge collection but no amplification (gain=1) w Generally used for gamma exposure and large fluxes n Proportional mode w Ionization avalanche produces an amplified signal proportional to the original ionization (gain = 10 3— 105) w Allows measurement of d. E/dx n Limited proportional (streamer) mode w Secondary avalanches from strong photo-emission and space charge effects occur (gain = 1010) n Geiger-Muller mode w Massive photo-emission results in many avalanches along the wire resulting in a saturated signal 2

Ionization Detectors 3

Ionization Ø Ionization n n Direct – p + X -> p + X+ + e. Penning effect - Ne* + Ar -> Ne + Ar+ + e- Ø ntotal = nprimary + nsecondary 4

Ionization Ø The number of primary e/ion pairs is Poisson distributed, being due to a small number of independent interactions Ø Total number of ions formed is 5

Ionization air 33. 97 6

Ionization 7

Charge Transfer and Recombination Ø Once ions and electrons are produced they undergo collisions as they diffuse/drift Ø These collisions can lead to recombination thus lessening the signal 8

Diffusion Ø Random thermal motion causes the electrons and ions to move away from their point of creation (diffusion) Ø From kinetic theory 9

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

Drift Ø In the presence of an electric field E the electrons/ions are accelerated along the field lines towards the anode/cathode Ø Collisions with other gas atoms limits the maximum average (drift) velocity w 11

Drift Ø A useful concept is mobility m n Drift velocity w = m. E Ø For ions, w+ is linearly proportional to E/P (reduced E field) up to very high fields n n 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 Ø The drift velocity of ions is small compared to the (randomly oriented) thermal velocity 12

Drift ØFor 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 n Usually occurs in 100 -1000 collisions 13

Drift Ø Electrons in an electric field can substantially increase their energy between collisions with gas molecules Ø The drift velocity is given by the Townsend expression (F=ma) n Where t is the time between collisions, e is the energy, N is the number of molecules/V and n is the instantaneous velocity 14

Drift 15

Drift ØLarge range of drift velocities and diffusion constants 16

Drift ØNote that at high E fields the drift velocity is no longer proportional to E n That’s where the drift velocity becomes comparable to thermal velocity ØSome gases like Ar-CH 4 (90: 10) have a saturated drift velocity (i. e. doesn’t change with E) n This is good for drift chambers where the time of the electrons is measured 17

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

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

Proportional Counter Ø Consider a parallel plate ionization chamber of 1 cm thickness Ø Fine for an x-ray beam of 106 photons this is fine Ø But for single particle detectors we need amplification! 20

Proportional Counter Ø Close to the anode the E field is sufficiently high (some k. V/cm) that the electrons gain sufficient energy to further ionize the gas n Number of electron-ion pairs exponentially increases 21

Proportional Counter 22

Proportional Counter ØThere are other ways to generate high electric fields n These are used in micropattern detectors (MSGC, MICROMEGAS, GEM) which give improved rate capability and position resolution 23

Proportional Counter Ø Multiplication of ionization is described by the first Townsend coefficient a(E) Ø a(E) is determined by n n Excitation and ionization electron cross sections in the gas Represents the number of ion pairs produced / path 24 length

Proportional Counter ØValues of first Townsend coefficient 25

Proportional Counter ØValues of first Townsend coefficient 26

Proportional Counter ØElectron-molecule collisions are quite complicated 27

Avalanche Formation 28

Signal Development ØThe time development of the signal in a proportional chamber is somewhat different than that in an ionization chamber n n 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 d. V 29

Signal Development Ø Surprisingly, in a proportional counter, the signal due to the positive ions dominates because they move all the way to the cathode 30

Signal Development Ø Considering only the ions 31

Signal Development ØThe signal grows quickly so it’s not necessary to collect the entire signal n n ~1/2 the signal is collected in ~1/1000 the time Usually a differentiator is used 32

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

Gas ØOperationally desire low working voltage and high gain n Avalanche multiplication occurs in noble gases at much lower fields than in complex molecules w Argon is plentiful and inexpensive n But the de-excitation of noble gases is via photon emission with energy greater than metal work function w 11. 6 e. V photon from Ar versus 7. 7 e. V for Cu n This leads to permanent discharge from deexcitation photons or electrons emitted at cathode walls 34

Gas ØArgon+X n X is a polyatomic (quencher) gas w CH 4, CO 2, CF 4, isobutane, alcohols, … n n 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 w 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 ØArgon+X n Neutralization of the ions at the cathode can occur by dissociation or polymerization w Must flow gas w Be aware of possible polymerization on anode or cathode n Malter effect w Insulator buildup on cathode w Positive ion buildup on insulator w Electron extraction from cathode w Permanent discharge 36