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

Abstract

Prediction of the thermophysical properties of pure neon, pure argon, and the binary mixtures neon-argon and argon-krypton by Monte Carlo simulation using ab initio potentials Absolute W value measurements for 5.9 keV x rays in Ne-Xe mixtures at atmospheric pressures Xenon gas proportional-scintillation counters ͑GPSC͒ 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 E xr 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, Rϭ2.36(Fw/E xr ) 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 significant. The simulation reproduces the photoionization process of the Xe and Ne atoms, the vacancy cascade decay of the residual ions, and the elastic and inelastic scattering of electrons by the gas atoms. The contribution of energy and charge transfer mechanisms such as Penning, associative, and transfer ionization is discussed in detail. It is shown that Penning and associative ionization are the crucial indirect ionization processes which determine the behavior of F and w at low concentrations of Xe. The importance of the nonmetastable Ne states is also assessed.

Quenched and non-quenched Ar-Xe Penning mixtures as detection media for a Gridded-Microstrip Gas Chamber X-ray detector

“…A sharp decrease in R linked to a sharp increase in the gas gain, G, is observed from very low xenon concentrations. As demonstrated for Ne-Xe mixtures [9], the rapid increase in the total number of electrons produced per incident photon with initial increase in Xe concentration, is related to an additional channel for electron energy degradation, resulting from indirect ionization events from collisions of Ne excited species with neutral Xe atoms, which ultimately consists of a conversion of energy wasted in excitation in extra electrons. Consequently, w decreases abruptly and F goes through a minimum, leading to an improvement in R at certain compositions, similarly to what is Another characteristic which is also a very important requirement in any X-ray is the energy linearity, since its affects its performance.…”

“…Afterwards, above --10%Xe a smooth increase in R with a corresponding decrease of gas gain is observed, because xenon is now competing with argon in the processes of excitation and ionization (as it is a larger atom the small increase in concentrations justifies the competition) so the Penning effect becomes negligible [9]. A compilation of the best energy resolutions is presented in Table I.…”

“…It has also been demonstrated [7]- [9], that by adding a small amount of a heavier noble gas (less than approximately 5% for metastable Penning mixtures and 8-10% for non-metastable ones) with lowest ionization potential than some of the excited states of the lighter component, energy transfer between the two components can take place and extra electrons will be released. From this process, known as Penning effect, parameters wand F decrease [9] and as a result, the intrinsic limit of the energy resolution will improve and the ionization efficiency becomes higher than for pure components at relative low voltages.…”

Xenon-neon gas proportional-scintillation counters for X-rays below 2 keV: a Monte Carlo simulation study

“…Using Monte Carlo simulation, we have analyzed in previous work the scintillation yields, -values, Fano factors, and energy resolutions for the absorption of X-rays with keV in Xe-Ne mixtures [8], [9] and compared the results for 5.9 keV with GPSC measurements [8]. However, these calculations did not then take into account the losses that may occur at the lower energies due to the presence of an entrance window.…”

Introducing electron capture into the unitary-convolution-approximation energy-loss theory at low velocities

“…This choice of a reduced energy accounts for electronic screening effects and maximizes the overlap between computed curves for different initial bound states. Note that these data are directly related to the so-called W value [52][53][54] of ionizing events (the average energy to produce an ion pair in a gas of low atomic density).…”