J. Escada

Towards THGEM UV-photon detectors for RICH: on single-photon detection efficiency in Ne/CH4 and Ne/CF4

The article deals with the detection efficiency of UV-photon detectors consisting of Thick Gas Electron Multipliers (THGEM) coated with CsI photocathode, operated in atmospheric Ne/CH4 and Ne/CF4 mixtures. We report on the photoelectron extraction efficiency from the photocathode into these gas mixtures, and on the photoelectron collection efficiency into the THGEM holes. Full collection efficiency was reached in all gases investigated, in some cases at relatively low multiplication. High total detector gains for UV photons, in excess of 105, were reached at relatively low operation voltages with a single THGEM element. We discuss the photon detection efficiency in the context of possible application to RICH.

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.

Measurements of photoelectron extraction efficiency from CsI into mixtures of Ne with CH4, CF4, CO2 and N2

Experimental measurements of the extraction efficiency f of the UV-induced photoelectrons emitted from a CsI photocathode into gas mixtures of Ne with CH4, CF4, CO2 and N2 are presented; they are compared with model-simulation results. Backscattering of low- energy photoelectrons emitted into noble gas is significantly reduced by the admixture of molecular gases, with direct impact on the effective quantum efficiency. Data are provided on the dependence of f on the type and concentration of the molecular gas in the mixtures and on the electric field.

Fragments and radicals in gaseous detectors

Abstract The need for detectors able to provide fast and reliable information, particularly in view of applications for high counting rates, requires a better knowledge of the physics of the processes involved, in order to master the after-effects resulting from the emission of photons. In turn, the identification of radicals and fragments formed in the avalanche is important to understand and eventually overcome the problems related with the ageing of the present gaseous detectors and those arising in the substrata of the new-coming microstrip gas chambers. Emission spectra, in the region from 120 to 450 nm, of the mixtures of argon with quenching gases (methane, isobutane and carbon dioxide) in the proportional and self-quenching streamer modes are reported. Especial attention is given to the carbon lines (7.94, 7.48 and 6.42 eV). For each mixture several fragments and radicals are identified.

A Monte Carlo study of photoelectron extraction efficiency from CsI photocathodes into Xe–CH4 and Ne–CH4 mixtures

The extraction efficiency f for the photoelectrons emitted from a CsI photocathode into gaseous Xe–CH4 and Ne–CH4 mixtures is investigated by Monte Carlo simulation. The results are compared with earlier calculations in Ar–CH4 mixtures and in the pure gases Xe, Ar, Ne and CH4. The calculations examine the dependence of f on the density-reduced electric field E/N in the 0.1–40 Td range, on the incident photon energy Eph in the 6.8–9.8 eV (183–127 nm) VUV range and on the mixture composition. Results calculated for irradiation of the photocathode with a Hg(Ar) lamp are compared with experimental measurements for this lamp. To test the electron scattering cross-sections used in the simulations, electron drift parameters in Xe, Ne and their mixtures with CH4 are also presented and compared with available experimental data.

A Monte Carlo study of the fluctuations in Xe electroluminescence yield: pure Xe vs Xe doped with CH4 or CF4 and planar vs cylindrical geometries

Monte Carlo simulation is used to investigate the fluctuations in the Xe proportional electroluminescence (EL) yield (also known as secondary scintillation) produced by sub-ionization primary electrons drifting in the gas under appropriate electric fields, comparing pure Xe gas with Xe doped with CH4 or CF4. The work is modeled on gas detectors of the gas proportional-scintillation type, where amplification is achieved through the production of EL under a charge-multiplication free regime. The addition of the molecular gases to Xe reduces electron diffusion, a desirable effect in large size detectors where primary electrons drift across a long absorption/drift region. However, the presence of the molecules reduces and increases its fluctuations. In the case of CF4, the effects are very strong due to significant electron attachment in the EL field range, ruling out CF4 as an acceptable additive. The addition of CH4 affects and its fluctuations to a much lower extent, and CH4 concentrations lower than ~1% may be an appropriate choice. In addition, Monte Carlo calculations in pure Xe under cylindrical geometry in a regime below charge multiplication have shown that fluctuations in the EL yield are an order of magnitude higher than for planar geometry. For both geometries, though, the fluctuations have a negligible effect on the energy resolution, and variations of the anode radius in cylindrical geometry or grid parallelism in planar geometry may be a more significant cause of concern.

Application and performance of an ML-EM algorithm in NEXT

The goal of the NEXT experiment is the observation of neutrinoless double beta decay in 136Xe using a gaseous xenon TPC with electroluminescent amplification and specialized photodetector arrays for calorimetry and tracking. The NEXT Collaboration is exploring a number of reconstruction algorithms to exploit the full potential of the detector. This paper describes one of them: the Maximum Likelihood Expectation Maximization (ML-EM) method, a generic iterative algorithm to find maximum-likelihood estimates of parameters that has been applied to solve many different types of complex inverse problems. In particular, we discuss a bi-dimensional version of the method in which the photosensor signals integrated over time are used to reconstruct a transverse projection of the event. First results show that, when applied to detector simulation data, the algorithm achieves nearly optimal energy resolution (better than 0.5% FWHM at the Q value of 136Xe) for events distributed over the full active volume of the TPC.

Experimental measurements of the mobility of methane ions in methane

A recently used experimental technique for measuring the mobility of positive ions in their parent gases is applied to methane (CH4 purity ≥99.995%), where ion reduced mobility values K0 are deduced from the analysis of measured time-of-arrival spectra of the ions produced by a GEM. For CH4 pressures in the range 3 to 10 Torr, two peaks could be observed when reduced applied fields E/N were varied from 10 to 60 Td, where E/N is the ratio of electric-field strength to gas-number density. The corresponding extrapolated zero-field mobility values in CH4 were found to be K0 = 2.58 cm2 V−1 s−1 and K0 = 2.42 cm2 V−1 s−1, belonging likely to CH5+ ions on the 1st peak and to C2H5+ and C3H7+ ions, with similar mobility values and falling both under the 2nd peak.

Photoelectron Collection Efficiency in Mixtures of Noble Gases with CF/sub 4/

The collection efficiency f for the photoelectrons emitted from a CsI photocathode into CF₄ and into Ar-CF₄ and Ne-CF₄ mixtures is investigated by Monte Carlo simulation The dependence of f on the reduced electric field E/N in the 0.1-40 Td range, on incident photon energy E_ph in the 6.8-9.8 eV (183-127 nm) range and on mixture composition is analyzed. Electron drift parameters in CF₄, Ar-CF₄ and Ne-CF₄ are also presented.

Demonstration of Single-Barium-Ion Sensitivity for Neutrinoless Double-Beta Decay Using Single-Molecule Fluorescence Imaging

A new method to tag the barium daughter in the double-beta decay of ^{136}Xe is reported. Using the technique of single molecule fluorescent imaging (SMFI), individual barium dication (Ba^{++}) resolution at a transparent scanning surface is demonstrated. A single-step photobleach confirms the single ion interpretation. Individual ions are localized with superresolution (∼2  nm), and detected with a statistical significance of 12.9σ over backgrounds. This lays the foundation for a new and potentially background-free neutrinoless double-beta decay technology, based on SMFI coupled to high pressure xenon gas time projection chambers.