D.S.A.P. Freitas

The performance of the GPSC/MSGC hybrid detector with argon-xenon gas mixtures

The performance for x-ray spectrometry of neon-xenon gas proportional scintillation counters using a CsI-coated microstrip plate in direct contact with the scintillation gas as a VUV photosensor is investigated for different neon-xenon mixtures. At best operation conditions, the detector gain can reach values about 50% higher than those achieved with pure xenon filling. The highest gains and the best energy resolutions are achieved for xenon contents around 40%. However, the achieved energy resolutions are similar to those achieved with pure xenon. As in pure xenon and argon-xenon mixture gas-fillings, the detector performance is limited by optical positive feedback resulting from additional scintillation produced in the electron avalanche processes around the MSP-anodes. Best energy resolutions are achieved for positive feedback gains in the range of 1.1 to 1.2. The performance achieved with neon-xenon mixtures is inferior to that achieved with argon-xenon mixtures.

A comparative study of microstrip plate geometries as UV photosensors with reflective photocathodes: simulation

A numerical simulation of eight microstrip plate geometries operated in 760 torr of xenon was performed. The cathode widths and anode-to-cathode gaps were varied while the anode widths were held constant at 10 /spl mu/m. Avalanche multiplication factors as a function of position and total gains for all geometries were determined. Since optical positive feedback is a limiting factor in considering microstrip plates as vacuum ultraviolet (VUV) photosensors, the optical positive feedback of a CsI-coated microstrip plate used as a VUV photosensor for a gas proportional scintillator counter was estimated for all geometries. The geometry with the least optical positive feedback in this application was determined to have 160-/spl mu/m cathodes and cathode-to-anode gaps of 55 /spl mu/m.

The gas proportional scintillation counter/microstrip gas chamber hybrid detector

The gas proportional scintillation counter/microstrip gas chamber (GPSC/MSGC) hybrid detector for X-ray spectrometry is described. The detector uses a CsI-coated microstrip plate placed in direct contact with the gas-filling as the photosensor readout for the GPSC scintillation substituting for the photomultiplier tube (PMT). Usable photosensor maximum gain is limited by optical positive feedback due to the additional scintillation produced in the electron avalanche process at the MSP anodes, in the absence of quenching. A low-photoelectron collection efficiency is achieved in the gas atmosphere, resulting in a scintillation conversion efficiency that is about a factor of 5 lower than that achieved with PMT-based GPSCs. However, energy resolutions of 11% for 5.9 keV X-rays are achieved with this detector.

Dependence of the performance of CsI-covered microstrip plate VUV photosensors on geometry: experimental results

The behavior of a xenon-filled microstrip gas chamber instrumented with a microstrip plate with eight different microstrip geometries is reported. The best energy resolution achieved for 5.9-keV and for 22.1-keV X-rays was 13.6% and 7.0%, respectively. The same microstrip plate covered with a CsI film was used in place of the photomultiplier tube to detect the vacuum ultraviolet scintillation light produced in a driftless xenon-filled gas proportional scintillation counter. The relative amplitude, energy resolution,. optical positive feedback, and photoelectron extraction efficiency are discussed. The energy resolution obtained for 5.9-keV X-rays was 11.4%, which is better than that achieved with a standard proportional counter, about 13% for argon based proportional counters.

μ-Strip photosensors for gas proportional scintillation counters

Abstract The results of a comparative study of a xenon gas proportional scintillation counter instrumented with photosensors based on two variants of a CsI photocathode and a microstrip plate are reported. The photosensors were isolated from the gas proportional scintillation counter by a quartz window and operated in P-10 gas at one atmosphere. The CsI photocathode is deposited either in the reflective or semitransparent mode. In the reflective mode, the CsI is deposited directly onto the micro strip plate and we observe gain fluctuations due to a geometric factor that degrades the energy resolution. In the semitransparent mode, the CsI is deposited onto the surface of the quartz window. Both modes of operation were evaluated and their performance characteristics reported.

A comparative study of different microstrip plate geometries for CsI-based UV photosensors working in a reflective mode

A study by numerical simulation of the performance of 8 different microstrip plate (MSP) chambers filled with xenon at the pressure of 10/sup 5/ Pa is carried out. The anode width is fixed at 10 /spl mu/m but the cathode width ranges from 80 to 320 /spl mu/m with variable anode-to-cathode distances (from 10 to 55 /spl mu/m). Simulation results are presented for the avalanche multiplication factors at a certain drift position and gains for the various geometries. The amount of optical positive feedback is also estimated for the various geometries, since this is a determinant factor for the use of a CsI-covered MSP chamber, as a VUV photosensor for detection of the scintillation light of a Gas Proportional Scintillation Counter. For this purpose, the best MSP geometry studied is the one with 160 /spl mu/m cathodes and cathode-to-anode gaps of 55 /spl mu/m.

Tertiary scintillation gas proportional scintillation counter (TS-GPSC): First experimental results

A new concept for a gas detector is presented: the Tertiary Scintillation Gas Proportional Scintillation Counter (TS-GPSC). In this detector the electric field induced secondary gas scintillation is first detected by a CsI-coated GEM-like structure where it releases photoelectrons which are transferred through the GEM holes, with no charge multiplication, to another region where further field induced scintillation (tertiary) is produced and then again detected on a planar CsI-coated photocathode at the backplane of the detector. The electrons therein released are collected at a grid and constitute the detector´s signal. Since there is no avalanche charge multiplication in the detector, the gain will be quite stable and, moreover, as the field induced scintillation yield is very high, and in spite of the low photocathode quantum efficiency, the gain will be high and with low fluctuations, so an improvement in the energy resolution is expected. The first prototype of the tertiary scintillation detector was tested in a xenon atmosphere for hard X-rays and experimental results are presented.

Large area and high-pressure GPSC/MSGC for space solar X-ray spectrometry

The study of solar X-ray emissions can be very useful to predict and prevent the undesirable consequences of charged particles generated by solar flares on Earth, since charged particles take about 20 to 40 hours to reach Earth, while X-rays that are simultaneously emitted by the same solar flares only take about 8 minutes. Herein, we present a large area window (20 cm/sup 2/) and high-pressure (up to 5 atm) hybrid gas proportional scintillation counter/microstrip gas chamber (GPSC/MSGC) for a satellite borne experiment to study solar flare X-ray emissions in the energy range between 20 keV and 80 keV. Xenon filled GPSC/MSGCs are low production cost and low power consumption detectors that provide large detection areas, room temperature operation and optimal energy resolution for solar X-ray observations. Therefore, the performances of a laboratory prototype designed to study solar X-rays was analysed. Namely the energy resolution as a function of the Xe pressure (1 atm to 5 atm), since for energies above 60 keV the gas pressure is a critical parameter to detection efficiency. The results obtained for the energy resolution dependence with the gas pressure (about 6.5% up to 4 atm) will be presented and discussed.

Dual-Cathode CsI Covered Microstrip Plate as VUV High Efficiency Photosensor

A Gas Proportional Scintillation Counter based on a dual-cathode Microstrip Plate covered with a CsI film is described. This new dual-cathode technique has the advantage of increasing the VUV sensitive area of the Microstrip Plate. A detailed description of the technique is presented together with a discussion of the performance. The results obtained for a xenon filled Gas Proportional Scintillation Counter show an improvement of the energy resolution for 5.9 keV X-rays from 12.7%, for a single cathode device, to 11.1% for the dual-cathode device.

Reflective and semi-transparent CsI-photocathodes in microstrip plate gas chamber photosensors for gas proportional scintillation counters: a comparative study

We report the results of a comparative study of a Xe proportional scintillation counter instrumented with photosensors based on a microstrip plate. The photosensor is isolated from the gas proportional scintillation counter by a quartz window and is operated in P-10 gas at atmospheric pressure. The CsI-photocathode is operated either in the reflective or semi-transparent mode. In the reflective mode, the CsI-photocathode is deposited directly on the microstrip plate. Gain fluctuations due to a geometric factor, which degrade the energy resolution, are observed. In the semi-transparent mode the CsI-photocathode is deposited onto the surface of the quartz window resulting in two potential improvements: an increase in the number of photoelectrons produced in the photocathode and a reduction of geometry-related effects. Both modes of operation will be evaluated and their performance characteristics determined.