P.J.B.M. Rachinhas

FULL-ENERGY ABSORPTION OF X-RAY ENERGIES NEAR THE XE L- AND K-PHOTOIONIZATION THRESHOLDS IN XENON GAS DETECTORS : SIMULATION AND EXPERIMENTAL RESULTS

Distributions of the number of primary electrons produced per incident mono-energetic x-rays in the 1- to 41-keV energy range, which includes the xenon L- and K-absorption edges, were simulated in xenon gas detectors with the Monte Carlo technique. These simulated full-energy absorption distributions are calculated as frequency plots of the number of primary electrons produced per incident x-ray photon. The simulation includes the absorption of x-rays and the de-excitation of the residual xenon ions, followed by the development of the primary electron cloud. The discontinuities observed in the Fano factor, w-value, energy linearity and energy resolution reflect the discontinuities of the Xe photoionization cross-section at the photoabsorption edges. The simulation results are compared with experimental values measured with a gas proportional scintillation counter, and with recent data from other authors. The discontinuities in energy linearity produce an ambiguity in determining the x-ray energy in certai...

OPERATION OF GAS PROPORTIONAL SCINTILLATION COUNTERS IN A LOW CHARGE MULTIPLICATION REGIME

The operation of a xenon filled gas proportional scintillation counter under low charge multiplication gains is discussed. It is shown that the best energy resolution for X-rays is obtained for a reduced electric field, E/p, of about 8Vc m~1 Torr~1, leading to a charge gain (for inter-grid distance of 5 mm) of about 1.05, as calculated by a Monte Carlo simulation method. A discussion is presented concerning the dependence of the E/p values for better detector performance, on the inter-grid distance. ( 1999 Elsevier Science B.V. All rights reserved.

Monte Carlo simulation study of the Fano factor, w value, and energy resolution for the absorption of soft x rays in xenon–neon gas mixtures

Xenon gas proportional-scintillation counters (GPSC) have many applications in the detection of soft x rays where their energy resolution, R, is comparable to solid-state detectors when large window areas are required. However, R is known to deteriorate for energies Exr below 2–3 keV due to electron loss to the entrance window. Since the addition of a lighter noble gas increases the absorption depth, we have investigated the use of Xe–Ne gas mixtures at atmospheric pressure as detector fillings. The results of a Monte Carlo simulation study of the Fano factor, F, the w value, and the intrinsic energy resolution, R=2.36(Fw/Exr)1/2, are presented for Xe–Ne mixtures and pure Xe and Ne. The results show that the addition of Ne to Xe reduces the intrinsic energy resolution R but this never compensates for the reduction in scintillation yield in GPSC applications, implying that the instrumental energy resolution R will only improve with the addition of Ne when electron loss to the window in pure Xe is significa...

The transmission of photoelectrons emitted from CsI photocathodes into Xe, Ar, Ne and their mixtures: a Monte Carlo study of the dependence on E/N and incident VUV photon energy

Although ultraviolet photosensor devices offer many advantages when used in radiation detectors, there is often a significant reduction in pulse amplitude when the photosensor operates in a detector filled with a noble gas. This is due to the backscattering of electrons by the noble gas atoms. In this study, we investigate the problem of the backscattering of the photoelectrons emitted from a CsI photocathode into Xe, Ar, and Ne and the binary mixtures Xe–Ar, Ar–Ne and Xe–Ne using a detailed Monte Carlo simulation. Results for the photoelectron transmission efficiencies are presented and discussed for the case of a CsI photocathode irradiated with photons with energies in the range Eph = 6.8–9.8 eV (183–127 nm) and for applied reduced electric fields in the range E/N = 1–40 Td. The dependence on incident photon energy, nature of the gas and applied electric field are examined, and the results are explained in terms of electron scattering in the different noble gases.

Xenon–neon gas proportional scintillation counters: Experimental and simulation results

When gas proportional scintillation counters (GPSC) are used to detect very low energy x rays, the addition of the light noble gas neon to the usual xenon filling improves the collection of primary electrons that originate near the detector window. However, xenon–neon mixtures have lower electroluminescence yields than pure xenon. Increasing the scintillation electric field jeopardizes the energy resolution because of the additional fluctuations introduced by electron multiplication. In this work we investigate the effect of a limited amount of charge multiplication on the electroluminescence yield and the energy resolution R of a xenon–neon GPSC using both Monte Carlo simulation and experimental measurements. We consider xenon–neon mixtures with 5%, 10%, 20%, 30%, 40%, 50%, 70%, 90%, and 100% Xe at a total pressure of 800 Torr. Comparing the experimental and Monte Carlo data for 5.9 keV x rays, we conclude that optimum value of R is reached in a region of weak ionization with a charge gain of less than 2...

Energy resolution of xenon proportional counters: Monte Carlo simulation and experimental results

The gas multiplication factor M and energy resolution R of xenon gas cylindrical proportional counters are investigated experimentally and calculated theoretically using a Monte Carlo technique to simulate the growth of single-electron-initiated avalanches. A good agreement is found between calculated and experimental data. The experimental and the calculated results are presented as a function of the reduced voltage K and the reduced anode radius Na, where N is the number density and a the anode radius. The Monte Carlo results for the intrinsic energy resolution R/sub int/ are discussed in terms of the parameter f which characterizes the statistical fluctuations of the avalanche gains. The present calculations have revealed that there is an intrinsic dependence of f on the critical value S/sub c/ of the reduced electric field at the onset of multiplication. This has enabled the parameterization of f and of the intrinsic energy resolution R/sub int/ in terms of the reduced voltage K only and has explained why, for a given range of operating values for M, energy resolution in a cylindrical proportional counter is improved for thinner anode wires and lower pressures.

A Monte Carlo study of backscattering effects in the photoelectron emission from CsI into CH4 and Ar-CH4 mixtures

Monte Carlo simulation is used to investigate photoelectron backscattering effects in the emission from a CsI photocathode into CH4 and Ar-CH4 mixtures for incident monochromatic photons with energies Eph in the range 6.8 eV to 9.8 eV (182 nm to 127 nm), and photons from a continuous VUV Hg(Ar) lamp with a spectral distribution peaked at Eph = 6.7 eV (185 nm), considering reduced applied electric fields E/N in the 0.1 Td to 40 Td range. The addition of CH4 to a noble gas efficiently increases electron transmission and drift velocity, due to vibrational excitation of the molecules at low electron energies. Results are presented for the photoelectron transmission efficiencies f, where f is the fraction of the number of photoelectrons emitted from CsI which are transmitted through the gas as compared to vacuum. The dependence of f on Eph, E/N, and mixture composition is analyzed and explained in terms of electron scattering in the different gas media, and results are compared with available measurements. Electron drift parameters are also calculated and compared with experimental data, confirming the choice of electron scattering cross-sections used in the simulations.

Photoelectron Collection Efficiency in Rare Gases: A Monte Carlo Study

A detailed Monte Carlo simulation is used to investigate the effective collection efficiency of photocathodes in xenon, neon, and mixtures of xenon with neon, in order to obtain a better understanding of the physics involved in the emission of electrons by photoelectric effect from photocathodes in gaseous media in the presence of an applied electric field. The case of the 7.2 eV photon-induced electron emission from a Csl photocathode is analyzed, and results are presented for applied reduced electric fields E/N in the range 0.1 to 40 Td. The calculations take into account the variation of the electron energy along the free paths and the anisotropy of the scattering. The contribution of the cathode electron-image potential to the total potential is assessed.

Monte Carlo simulation of xenon filled cylindrical proportional counters

Single electron avalanche processes in a xenon filled cylindrical proportional counter have been simulated using a detailed Monte Carlo technique. The avalanche gain A and its average value M as well as its frequency distribution and spread parameter b have been calculated for xenon at atmospheric pressure, for a 2.55 cm cathode radius and 12.5 and 50 /spl mu/m anode radii. A discussion is made of the results obtained for M as a function of the anode voltage in terms of prevailing theories. It is found that Monte Carlo calculated M values are markedly lower than those obtained analytically using the first Townsend ionization coefficient calculated under a uniform field. This puts into evidence the relevance of the non-equilibrium nature of avalanche processes. >

Xenon-neon gas proportional-scintillation counters for X-rays below 2 keV: experimental results

The energy spectra of X-rays with energies below 2 keV measured with standard Xe filled gas proportional scintillation counters at atmospheric pressure exhibit a distorted tail towards the low energy region, due to the loss of electrons to the detector entrance window. In this framework, a Monte Carlo simulation study taking into account these losses has previously investigated the use of Xe-Ne mixtures as GPSC fillings. it was shown that the addition of Ne to Xe reduces spectra distortion and improves the overall detector performance. In the present work we present an experimental study of the performance of a GPSC for different Xe-Ne mixtures and X-ray energies in the 200 to 2000 eV energy range and compare the results with the previously obtained Monte Carlo data.