A. M. F. Trindade

Polarimetric performance of a Laue lens gamma-ray CdZnTe focal plane prototype

A gamma-ray telescope mission concept [gamma ray imager (GRI)] based on Laue focusing techniques has been proposed in reply to the European Space Agency call for mission ideas within the framework of the next decade planning (Cosmic Vision 2015-2025). In order to optimize the design of a focal plane for this satellite mission, a CdZnTe detector prototype has been tested at the European Synchrotron Radiation Facility under an similar to 100% polarized gamma-ray beam. The spectroscopic, imaging, and timing performances were studied and in particular its potential as a polarimeter was evaluated. Polarization has been recognized as being a very important observational parameter in high energy astrophysics (> 100 keV) and therefore this capability has been specifically included as part of the GRI mission proposal. The prototype detector tested was a 5 mm thick CdZnTe array with an 11 x 11 active pixel matrix (pixel area of 2.5 x 2.5 mm(2)). The detector was irradiated by a monochromatic linearly polarized beam with a spot diameter of about 0.5 mm over the energy range between 150 and 750 keV. Polarimetric Q factors of 0.35 and double event relative detection efficiency of 20% were obtained. Further measurements were performed with a copper Laue monochromator crystal placed between the beam and the detector prototype. In this configuration we have demonstrated that a polarized beam does not change its polarization level and direction after undergoing a small angle (< 1 degrees) Laue diffraction inside a crystal

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.

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.

Ar–Xe mixtures and their use in curved grid gas proportional scintillation counters for X-rays

An experimental study of the scintillation properties of Ar–Xe mixtures has been carried out with a gas proportional scintillation counter instrumented with a photomultiplier tube. Energy resolutions of 7.7, 7.8, 7.8, 8.3, 8.5, 9.1 and 11.5% were obtained for 5.9 keV X-rays respectively for the 100% Xe, 60%Ar–40%Xe, 80%Ar–20%Xe, 90%Ar–10%Xe, 95%Ar–5%Xe, 97%Ar–3%Xe and 99%Ar–1%Xe mixtures at 800 Torr. Thresholds for scintillation and ionization were also determined. The performance of Ar–Xe mixtures as detection media in large detection area applications has been studied for a 52 mm diameter photomultiplier and a 38 mm diameter radiation window. The curved grid technique was used to minimize spectra distortion due to solid angle effects.

A new contribution to the experimental measurement of the N4+ ion mobility in N2 at 298K

We measured the reduced mobility of the nitrogen, N4+, ion in N2, under different pressures and reduced electric fields at 298 K using a new technique. Extrapolation to zero field yields a value of 2.37 cm2V−1s−1, which is in good agreement with other published data.

Polarimetry study with a CdZnTe focal plane detector

Polarimetry has been recognized as a very important observational parameter for high energy astrophysics (>100 keV). Therefore the capability to perform accurate polarimetric measurement of high energy cosmic sources should be included in future space mission proposals. This idea become very appealing in the last three years in the development of a new telescope mission concept based on the Laue focusing technique - the Gamma Ray Imager mission - for the next call for mission ideas in the ESA Cosmic Vision 2015-2025 plan framework. In order to optimize the design of a focal plane for this mission as Compton scattering hard X- and soft ¿-ray polarimeter, a CdZnTe detector prototype has been tested at the ESRF (European Synchrotron Radiation Facility) under an ~100% polarized gamma-ray beam. The prototype detector tested is a 5 mm thick CdZnTe array with the anode segmented into 16×16 pixels of 2.5×2.5 mm2. One of the most important factors that can affect significantly the polarimetric measurements is the direction of the incoming photons from the polarised radiation source with respect to the focal plane of the polarimeter. We have studied and analyzed the polarimetric response of the CZT prototype as a function of the direction of a polarized beam forming different inclination angles with the optical axis of the detector: 0°, 0.5°, 1°, 1.5°, 2°, 3°, 4°, 5° and 10°.

Residual gas analysers in an undergraduate vacuum laboratory: a simple experiment involving direct quantitative measurements

We describe a simple experiment intended for didactic laboratory vacuum classes of undergraduate courses, using a residual gas analyser (RGA): the determination of the natural abundance of the different isotopes of a given element in a gas sample. In addition to the understanding of the RGA operation and handling principles, the experiment allows the students to perform direct quantitative measurements of isotopic abundances and compare with the results tabulated in the literature. As an example, sets of measurements are presented for xenon and neon using a quadrupole mass spectrometer. The experimental results obtained are in good agreement with the values presented in the literature.

Experimental Ion Mobility measurements in Ne-CO

In this paper we present the experimental results for the mobility, K0, of ions in neon-carbon dioxide (Ne-CO2) and carbon dioxide-nitrogen (CO2-N2) gaseous mixtures for total pressures ranging from 8–12 Torr, reduced electric fields in the 10–25 Td range, at room temperature. Regarding the Ne-CO2 mixture only one peak was observed for CO2 concentrations above 25%, which has been identified as an ion originated in CO2, while below 25% of CO2 a second-small peak appears at the left side of the main peak, which has been attributed to impurities. The mobility values for the main peak range between 3.51 ± 0.05 and 1.07 ± 0.01 cm2V−1s−1 in the 10%-99% interval of CO2, and from 4.61 ± 0.19 to 3.00 ± 0.09 cm2V−1s−1 for the second peak observed (10%–25% of CO2). For the CO2-N2, the time-of-arrival spectra displayed only one peak for CO2 concentrations above 10%, which was attributed to ions originated in CO2, namely CO2+(CO2), with a second peak appearing for CO2 concentrations below 10%. This second peak, with higher mobility, was attributed to CO2+ ions. The mobility values of the main peak range between 2.11 ± 0.04 and 1.10 ± 0.03 cm2V−1s−1 in the 1%–99% interval of CO2, while the second peaks from 2.26 ± 0.02 and 1.95 ± 0.04 cm2V−1s−1 (1%–10% of CO2). The inverse of the mobility displays an aproximately linear dependence on the CO2 concentration in the mixture.

_2

In this paper we present the results of the ion mobility measurements made in gaseous mixtures of argon (Ar) and methane (CH4) for pressures ranging from 5 to 8 Torr and for low reduced electric fields (in the 17 Td to 43 Td range), at room temperature. The time of arrival spectra of the several mixture ratios studied revealed that the relative abundance of the ions formed depend on the mixture ratios. For CH4 concentrations in the 2.5–10% range three well defined peaks were observed which were attributed to single-carbon ions (CH5+), 2-carbon ions (C2H4+ and C2H5+), and 3-carbon ions (C3H4+, C3H5+, C3H6+ and C3H7+). The time of arrival spectra for CH4 concentrations of 2.5% (P-2.5), 5% (P-5), 10% (P-10) and the reduced mobilities of the ions obtained from the peaks observed are presented in this paper. The ion mobility study was performed for typical reduced electric fields used in gaseous detectors (E/N > 15 Td).

and CO

We measured the reduced mobilities of the atomic, Ne⁺, and dimer, Ne₂⁺, ions in Ne, under different pressures and reduced electric fields, at 300 K. Extrapolation to zero field yields values of 4.4 cm² V^-1 s^-1 for Ne⁺ and 6.2 cm² V^-1 s^-1 for Ne₂⁺, which are in relative good agreement with other published data. The Ne⁺ ions are converted to Ne₂⁺ by a three-body reaction with a rate constant that was also measured. Its average value for reduced electric fields in the 8 to 12 Td range was (5.6 ± 0.1) × 10^-32 cm⁶ s^-1 at 300 K, also agreeing with the results from other authors using other techniques.