B. G. Taylor

Instrument description and performance of the Imaging Gamma-Ray Telescope COMPTEL aboard the Compton Gamma-Ray Observatory

The imaging Compton telescope COMPTEL is one of the four instruments on board the Compton Gamma-Ray Observatory (GRO), which was launched on 1991 April 5 by the space shuttle Atlantis into an Earth orbit of 450 km altitude. COMPTEL is exploring the 1-30 MeV energy range with an angular resolution (1σ) between 1° and 2° within a large field of view of about 1 steradian. Its energy resolution (8.8% FWHM at 1.27 MeV) makes it a powerful gamma-ray line spectrometer. Its effective area (for on-axis incidence) varies between 10 and 50 cm 2 depending on energy and event selections. Within a 14 day observation period COMPTEL is able to detect sources which are about 20 times weaker than the Crab. The measurement principle of COMPTEL also allows the measurements of solar neutrons

New high energy |[gamma]|-ray sources observed by COS B

LOCALISED γ-ray sources contribute to the overall galactic emission; some of these sources have been identified with known astronomical objects1,2, while several unidentified γ-ray sources have also been reported3,4. We describe here a search for γ-ray sources using data from the ESA γ-ray satellite COS B which revealed 10 new unidentified sources. These sources seem to be galactic with typical γ-ray luminosities above 100 MeV in excess of 1035 erg s−1.

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 Imaging Compton Telescope Comptel on the Gamma Ray Observatory

This instrument is based on a newly established concept of ¿-ray detection in the very difficult 1-30 MeV range. It employs the unique feature of a two-step interaction of the y-ray: a Compton scattering collision in a first detector followed by an interaction in a second detector element. COMPTEL has been designed to perform a very sensitive survey of the y-ray sky. Extreme care has been taken to minimize background so that the detection limits of COMPTEL will be dominated by source counting statistics. It combines a wide field of view (about 1 steradian) with a good angular resolution. The design criteria of COMPTEL and the perforrmance of a Science Model are described.

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 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.