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Ionization Detectors

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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. Ionization Detectors. Modes of operation

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Ionization Detectors

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  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

  2. 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 = 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

  6. Ionization air 33.97

  7. Ionization

  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

  10. Diffusion • Multiple collisions with gas atoms causes diffusion • The 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 m • Drift velocity w = mE • 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. 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) • Where t is the time between collisions, e is the energy, N is the number of molecules/V and n is the instantaneous velocity

  15. Drift

  16. Drift • Large 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. Drift • Ar-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!

  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

  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 a(E) • a(E) is determined by • Excitation and ionization electron cross sections in the gas • Represents the number of ion pairs produced / path length

  25. Proportional Counter • Values of first Townsend coefficient

  26. Proportional Counter • Values of first Townsend coefficient

  27. Proportional Counter • Electron-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

  31. Signal Development • Considering only the ions

  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. 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 de-excitation photons or electrons emitted at cathode walls

  35. Gas • Argon+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 Counters • Many different types of gas detectors have evolved from the proportional counter

  39. Drift • Ar-CO2 is a common gas for proportional and drift chambers

  40. Drift

  41. Proportional Counter

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