The Diffuse X-Radiation Spectrum Below 4 KE V

Abstract

view Abstract Citations (25) References (4) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Diffuse X-Radiation Spectrum Below 4 KE V Baxter, A. J. ; Wilson, B. G. ; Green, D. W. Abstract More detailed evaluation of the diffuse X-radiation reported earlier by the authors gives a revised spectrum of 15E'68 for 2 <E < 10 keV. New measurements between 0.20 and 4.0 keV on October 8, 1968, show a spectrum above 1 keV consistent with an E~6 form, but below 1 keY the flux increases rapidly to the lowest energies measured. The estimated flux at 0.27 keV was 2160 ± 170 photons cm~ sec' sterad' keV'. No significant change in the low-energy flux was observed between the galactic pole and the galactic plane in the direction of Cassiopeia. In a recent paper (Green, Wilson, and Baxter 1968) we reported preliminary measure- ments of the diffuse X-radiation made with a beryllium-window proportional counter flown at rocket altitudes above Resolute Bay, Northwest Territories, Canada. More detailed evaluation of the detector efficiency has provided a revised spectrum of 15E' .68 for 2 <E < 10 keV. Similar spectra have been determined by groups at American Science and Engineering (Gorenstein, Gursky, and Garmire 1969), Naval Research Labora- tories (Henry et al. 1968), NASA (Boldt, Desai, and bit 1969), and the University of California at Livermore (Seward et al. 1967; Hill, Grader, and Seward 1968), and the spectral characteristics in this octave of the electromagnetic spectrum now appear fairly well established. The data are also consistent with isotropy over the sky, leading to the general conclusion that the radiation is extragalactic in origin. Two measurements have been reported at 0.25 keV (Bowyer, Field, and Mark 1968; Henry et al. 1968), representing integrated fluxes for energies below the carbon absorp- tion edge of the detector window. If we consider only the data uncorrected for galactic absorption, the two results are in fair agreement (with allowance for the experimental difficulties). Serious discrepancies arise only when "source" intensities are evaluated by considering absorption of the radiation in the Galaxy. We report here the results of a spectral determination between 0.20 and 4.0 keV, using measurements from a ~-mi1 Mylar-window, argon-filled proportional counter. The counter was flown to an altitude of 155 km in a Black-Brant III rocket from Resolute Bay (74?6 N., 900 W.) on October 8, 1968, at 0505 U.T. The rocket had a large coning angle (76?5), with coning period 38 sec and spin period 19 sec, so that the counter scanned a path almost through the zenith, with half the scan traversing the Earth. The field of view was 0.117 sterad, and the window size 30 cm2. A major concern in this energy region is the establishment of the value of the local background. A minimum of 1 inch of plastic scintillator surrounded the detector on all but the window side, serving as an anticoincidence detector, whose response was moni- tored separately. The effectiveness of the veto counter was demonstrated by the 15: 1 reduction in the counting rate, for energies above 0.28 keV, when the detector field of view was obstructed by the Earth (Fig. 1). The corresponding reduction at the carbon K transmission band was 2.5:1, owing to some telemetry noise contamination. The flux observed when the counter observed the Earth was taken to be the true counter back- ground and was subtracted from the skyward flux. Preflight calibrations were done by observing the escape peak from Fe55 at 2.9 keV and the sulfur fluorescent line at 2.3 keV. The pulses were height-to-time-encoded in the payload and transmitted as constant-height, variable-length pulses to the ground. The L14 Publication: The Astrophysical Journal Pub Date: March 1969 DOI: 10.1086/180323 Bibcode: 1969ApJ...155L.145B full text sources ADS |