A Large Area Gas-flow Proportional Counter System for Soft X-ray Astronomy

Abstract

A new instrument has been developed for the study of diffuse soft X-ray features and hot spots in the X-ray sky using two large area, gas-flow proportional counters The counter consists of a multi-anode and multilayered proportional chamber with anti-coincidence shield on three sides. The proportional counter is sealed with a very thin (1.5 µm) polypropylene-window supported by a 5° x 5° collimator. The two counters are arranged back-to-back; the window on one is coated with carbon (35 µg/cm²) and on the other with boron (70 µg/cm²). The two windows provide two pass bands, viz. the C-band (0·1 – 0.28 keV) and the B-band (0.1 – 0.28 keV). The counters are filled with pure propane gas at 200 torr. Continuous supply of gas is provided by an on-board gas bottle to replenish the gas diffusing out of the window, and the pressure inside the counter is maintained steady (± 1 torr) by a gas control servo-system. The proportional counters were calibrated in a vacuum system with Al-K X-rays and carbon-K X-rays produced by using an alpha source P₀²¹⁰ to excite AI and C targets. The counters had a basic resolution of 35% at 1'5 keV. The mutual anti-coincidence system employed provided a reduction ill the total background by a factor of 40. Additional noise reduction was achieved by the use of linear gates in each channel. Three mutually perpendicular magnetic flux gate sensors, co-aligned with the detectors provide the aspect at a given instant. The pulse-height, counter identity, magnetic sensor output, pressure inside the counter and other house-keeping data were digitized and transmitted via PCM/FM link at 100 kbit/sec. The complete system was flown successfully on board an RH-560 rocket on 24 June 1979 from Sriharikota. The technical details of the instrument and its flight performance are presented.