L.M.N. Távora

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.

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.

The energy nonlinearity of a xenon gas proportional scintillation counter at the K-absorption edge in xenon

Abstract The energy response of a xenon gas proportional scintillation counter was determined between 16 and 60 keV using the fluorescent X-rays from selected elements. A discontinuity in linearity of 177±9 eV was determined at the K-absorption edge in xenon at 34.6 keV. The measured discontinuity agrees with a recently-reported independent result obtained with a standard xenon proportional counter and a synchrotron radiation source.

Experimental measurement of the mobilities of atomic and dimer Ar, Kr, and Xe ions in their parent gases.

A new experimental technique for measuring the mobilities of positive ions in their parent gases is presented. The technique was applied to the rare gases, Ar, Kr, and Xe, and, for pressures typically below 10 Torr, two different types of positive ions were observed. The reduced mobilities of these ions in their parent gases were measured as a function of E/N, the ratio of the electric field strength to the gas number density, at a temperature of 300±1 K. The results were compared with others available in the literature and the two ions were identified as being the atomic and the dimer rare gas ions. The results are in good agreement with those from other authors. Space charge and impurities effects are discussed.

Measuring the mobilities of the ions formed in P-10 mixtures

We measured the reduced mobilities (k₀ in cm²V^-1s^-1) of the ions that are formed in P-10 mixtures under different pressures and various reduced electric fields. Two types of ions were identified. We then used the extrapolated values of k₀ when E/N¿0 (2.41 and 2.76 cm²V^-1s^-1), the Langevin formula and the Blancs law, to calculate the masses of these ions. Calculations indicate that the two ions have masses of 41 a.u. and 25 a.u., respectively. We propose hypothesis to explain the nature and origin of those ions.

Experimental Measurement of the Mobilities of Argon Ions in Gaseous Argon

We have measured experimentally the mobilities of Ar ions in Ar at different pressures (1, 1/2, 1/4, 1/8 and 1/16 atm) and for low reduced electric fields (E/N < 10 Td). The results are in good agreement with those available in the scientific literature and they indicate that Ar2 + is the dominant ion species. The variation of the mobility with the pressure reveals the presence of the Ar3 + ions. Space charge and impurity effects were accounted.

Experimental measurement of the mobilities of atomic, Ne + , and dimer, Ne 2 + , ions in Ne

We measured the reduced mobilities of the atomic, Ne⁺, and dimer, Ne<inf>2</inf>⁺, ions in Ne, under different pressures and reduced electric fields, at 300 K. The value obtained for the atomic ion, 4.1 cm²V⁻¹ s⁻¹, is in good agreement with other values published in the scientific literature. The value obtained for the mobility of the dimer ion, 7.3 cm² V⁻¹ s⁻¹, is not in accordance with other published data. We discuss the most probable sources of error.

Photopeak shift effects due to the drift electric field in high pressure xenon detectors

The photopeak shift in High Pressure Xe (HPXe) gamma ray detectors due to the energy acquired/lost by the photoelectric and Compton electrons, from the electric field applied in the drift region, is calculated using the PENELOPE code for gamma rays from several radioactive sources. The implications of this effect in HPXe detectors are discussed, namely for TPCs for double beta decay of 136 Xe studies.