A. Peacock

Performance characteristics of a gas scintillation spectrometer for X-ray astronomy

Abstract The determination of the optimum electrical parameters of a spherical electric field gas scintillation spectrometer for X-ray astronomy is described. Detailed measurements using a collimated 55Fe 5.9 keV X-ray source are presented leading to trade-offs of drift field, shaping and scintillation field in an endeavour to provide a uniform response in terms of energy and time resolution over the full aperture of 314 cm2. The optimized uncollimated energy resolution at 6 and 60 keV was 11.0 and 4.5% fwhm respectively. The total background rejection efficiency for 60Co gamma ray induced events was 97 and 70% in the energy range 1–10 and 1–60 keV respectively. Finally design improvements which yield and energy resolution as low as 7.5%K at 6 keV are outlined.

The EXOSAT Mission

The origins of EXOSAT (European X-ray Observatory Satellite) can be traced back to the late 1960s when a mission to determine accurately the location of bright X-ray sources, using the lunar occultation technique was studied and described by Collet et al. (1970). The intervening history is briefly highlighted in Table 1, which shows how the main thrust of the mission and in particular the payload complement has evolved. Due to the financial limitations of the ESA scientific programme budget, the mission, approved by Council in 1973, did not start its phase B until 1977. During that year, the political decision to use the Ariane launcher was taken. However, in view of the requirement to maintain compatibility with the Delta 3914 vehicle (as back-up), the mass and envelope constraints, dictated by the Delta, were to be observed, meaning that the technical advantages offered by Ariane could not be fully exploited.

The nature of the light produced inside a gas scintillation proportional counter

Abstract The emission spectrum of the ultra-violet light produced by a xenon gas scintillation counter has been experimentally determined. The continuous spectrum peak intensity occurs around 1670 A. The production mechanism is discussed and is probably molecular in origin. In addition the variation of the total intensity with both the field and gas pressure has been studied so as to optimise the instruments performance.

The in-Orbit Performance of the Exosat Gas Scintillation Proportional Counter

The in-orbit performance of the EXOSAT gas scintillation proportional counter spectrometer is presented. The background rejection capability in the EXOSAT deep orbit is examined as well as the overall detectors response. Typical examples of the spectra from cosmic X-ray sources show that this new type of detector will make a significant advance to the field of medium energy broad band X-ray spectroscopy.

Recent Progress in the Development of a Gas Scintillation Proportional Counter for X-Ray Astronomy

In X-ray astronomy there is a need for a large-area satellite-borne detector, which has a significantly better energy resolution than the classical proportional counter, in order to observe det-ailed X-ray spectral features in a variety of astrophysical objects. A gas scintillation proportional counter, (GSPC), seems to fulfill these requirements. Results are presented demonstrating: (a) an energy resolution of 8.35% (FWHM) at 6 keV, (b) the ability of the GSPC to reject more than 90% of the background events normally falling within a selected energy window. In addition results are presented on the optimisation of the working parameters of the GSPC, and on the development of a large area detector.

The gas scintillation proportional counter on Exosat

The inclusion of a gas scintillation proportional counter (GSPC) within the EXOSAT payload complement significantly improves the spectroscopic capability of the mission. This broad-band medium energy spectrometer used in conjunction with the large area proportioni counter array (ME) should provide additional spectroscopic details on strong X-ray sources at photon energies above ~ 2 keV. The novel type of detector has an energy resolution at least a factor of two better than the ME experiment over a similar bandwidth (2 – 40 keV).

A fluorescent gated gas scintillation proportional counter for high energy X-ray spectroscopy

Abstract Results are presented on a background rejection technique suitable for xenon gas filled detectors measuring X-rays above 35 keV. The technique involves the separate detection of both the xenon K shell fluorescent photon, emitted in most interactions involving the photoabsorption of high energy X-rays above 34.5 keV, and the residual event from the initial photoabsorption. This technique when applied to gas scintillation proportional counters filled with 1 atm xenon yields a background rejection efficiency of ∼97.3% and an energy resolution of ∼2.5% at 60 keV.

High pressure gas scintillation spectrometers for x-ray astronomy

Abstract A comparison of the performance of a gas scintillation counter operating at pressures above one atmosphere is presented. The factors which influence the background rejection efficiency are investigated in detail. In particular the response at higher X-ray energies has been found to improve significantly making this a useful spectrometer for higher energy X-ray astronomy.

A Large Area (300 cm2) Gas Scintillation Proportional Counter for X-Ray Astronomy

A large area gas scintillation proportional counter (GSPC) developed for use as a spectrometer in X-ray astronomy is presented. A crude laboratory version of the xenon cell coupled to a 9 cm diameter ruggedised photomultiplier, has an energy resolution of ~ 11% FWHM at 6 keV over a total geometric area of ~ 300 cm2. The instrument has a high rejection ability for charged particles present in a space environment. A rejection efficiency for Co60 induced events of ~ 90% was achieved, over the energy range 1-20 keV, for an X-ray acceptance of ~ 80%. These results show that the GSPC is a credible improved alternative instrument to a proportional counter with which to study cosmic X-ray sources.

Performance characteristics of a fluorescent gated gas scintillation proportional counter

Abstract Experimental results are presented on the background rejection capabilities of a K gated xenon gas scintillation proportional counter (GSPC) filled to a pressure of 3 bar. A total background rejection of between 99.3 and 99.8% can be achieved over the energy range 35–100 keV. Such results when coupled with an energy resolution of ∼ 2.7% at 60 keV make the K gated GSPC an attractive alternative to sodium iodide and germanium solid state detectors for application in high cosmic X-ray astronomy and other fields.