Part-II Wire type detectors 1. Single wire counters

1. Single wire counters a)Prporttional mode of the operation

Geiger counter Proportional counter Output signal Is proportional to deposit energy n 0 =E d /W i

Absorber/ covertor Radiation Drift region Gas amplification External amplification (amplifier) Anode

There are three main processes: 1. Interaction of charged particle or photons with a gas 2. Drift of primary ions 3. Gas multiplication

Small (and simplified) introduction to the interaction of particles and photons with matter 6

Particle tracks is gases In most practical cases charged particles produce well defined tracks inside the gaseous detectors 7

Production of primary and delta electrons 8

How do tracks look in reality? 9

Visualization of particle tracks gives an important information about the nature of the particles and its energy/momentum 10

Interaction of high energy photons and particles with matter-showers 11

Interaction of photons with gases 12

Schematic drawing illustrating the appearance of a fluorescent photoelectron following the transition of an electron from the outer shell to the vacancy in the inner shell

Transitions Argon keV mm Krypton keV mm Xenon keV mm K1K K1K L1L L1L The fluorescent photon mean free path Λ ph at normal conditions originated from various electron transitions in noble gases

Schematic description of the process leading to the ejection of a photoelectron (b) and an Auger electron from an atomic outer shell (d).

Drift of ions The drift properties of electrons and ions are quite different

Drift of electrons

Drift of ions V i =μE. GasIons Mobility (cm 2 V -1 sec -1 ) Ar(OCH 3 ) 2 CH IsoC 4 H 10 (OCH 3 ) 2 CH (OCH 3 ) 2 CH 2 (OCH 3 ) 2 CH ArIsoC 4 H IsoC 4 H 10 IsoC 4 H ArCH CH 4 CH ArCO CO 2 CO

Multiplication M=exp{ ra ∫ rc α(E,p)dr}

Typical dependence of gas gain vs. applied voltage Gas gain A

Pulse shape

Energy resolution (δM/M) 2 ={F+ (δA/A) 2 }W}/Ev={F+ (δA/A) 2 }/n 0 where F is a so-called Fano factor- a constant characteristic to gases (typically it is below 0.2) [11] and (δA/A) 2 is gas gain fluctuation (also typically below 0.75 [12]). Thus typically (δM/M) 2 ≤w/E v =1/n 0. Because ΔM =2.35δ, the ratio FWHM to the mean value of the signal amplitude ΔM/M of the output signal in the case of the 5.9 keV x-rays will be ΔM/M~ 15% and indeed this was well confirmed by many measurements (5.9 keV photons from a 55 Fe radioactive source are usually used to evaluate the energy resolution of the proportional counter)

Because F and (δA/A) 2 depend on the gas type, its pressure and on the gas gain A, one can optimize the single-wire counter geometry and its gas composition. Calculations preformed by Alhazov indicate that: 1. the smaller is the gas amplification, the smaller its fluctuations; 2. the product of the detector’s anode wire on the gas pressure should be as low as possible: 3.one can find gas mixtures having 10—20 % smaller (δA/A) 2 as well as having a small constant F. Indeed, it was demonstrated experimentally that in Penning gas mixtures (F~0.05—0.075) and at gas gains~100, energy resolution of %FWHM for 6 keV photons can be achieved with a low noise preamplifier. At small gas gain however, electronic noise can contribute to the (δM/M) 2, so that (δM/M) 2 ={F+ (δA/A) 2 }W}/Ev+δn 2 where δn 2 is electronic noise fluctuations.

Position-sensitive single wire counters

Secondary processes

What happens when the operating voltage is gradually increased

γ ph N phc ~ ∫ F av (ν)exp {-σ (ν)N mol }dν N ec ~ ∫ F av (ν)exp {-σ (ν)N mol }Q c (ν) dν p.e from the cathode p.e. from the gas N phg ~ ∫ F av (ν)[1-exp {-σ (ν)N mol d g }]dν N eg ~ ∫ F av (v)[1-exp {-σ (v)N mol d g }] Q g (ν) dν -a probability of appearing secondary electron due to photo processes

Most of contribution comes from the avalanche UV photons Surface photoeffect Photoionization Photabsorption

Back diffusion

γ+γ+ a.The electron energy level in a metal, when a positive ion Q + is far away from it. In this drawing ε is the depth of the conduction band b. The energy diagram, when an ion is very close to the cathode surface,. In this case the electron, denoted as “1”, tunnels directly to the ground state of the ion and the energy excess is given to the electron inside the metal, denoted as “2” -probability of secondary electro appearance due to the ion recombination γ + = k gas (E/p) (E i -2φ)

Pulses due to the secondary processes

What limit the gain? Aγ ph =1 or, Aγ + =1, What come first

Corona and unstable corona discharges

Tomorrow MWPC and PPAC detectors

MWPC

Absorver/ covertor Radiation Drift region Gas Amplification External amplification Segmented Anode

Absorver/ covertor Radiation Drift region Усиление внешней электронной цепью Регистратор сигналов Дискретная анодная структура