Intensity and Spectrum of the Diffuse X-ray Background

Abstract

THERE have been several experiments at balloon altitudes to determine the intensity and spectrum of the diffuse cosmic X-ray background at energies > 20 keV, and in all of them the method of derivation is based on the growth curves which are obtained by plotting either on a log-linear or a log-log scale the counting rates in different energy channels of an X-ray telescope as a function of atmospheric depth for altitudes higher than ∼100 g cm⁻². A few typical growth curves (refs. 1–3 and A. Danjo et al., to be published) are shown in Fig. 1a and 1b. The counting rate decreases with increasing atmospheric height up to ∼15 g cm⁻², after which there is either a change in the rate of decrease or an increase in the counting rate up to the ceiling altitude. This behaviour is interpreted in terms of two components which vary with altitude in opposite ways. The two components are the secondary X-ray photons and slow electrons produced by cosmic ray particles in their interactions with air nuclei in the atmosphere¹, and the cosmic X-ray photons which are incident isotropically from outside². As, on the average, the cosmic ray particles only have to traverse ∼ 70 g cm⁻² before interacting, the first component is assumed to decrease with increase of atmospheric height and the basic assumption made is that the rate of decrease is given by the slope of the best-fit line fitted to the experimental points between, say, 100 g cm⁻² and 20 g cm⁻² either on the log-linear or log-log scale. The second component is attenuated in the atmosphere as a consequence of the photoelectric effect and Compton scattering, and, because the mean free path for even 100 keV X-ray photons is ∼6.5 g cm⁻², the fraction that can penetrate >20 g cm⁻² is negligible; this is the chief justification for assuming that the rate of decrease of the first component is given by the slope of the curve determined for larger depths.