Secondary background properties of X-ray astronomical telescopes at balloon altitudes

Abstract

The characteristics of the cosmic ray produced secondary background encountered in X-ray astronomy experiments by balloon-borne scintillation telescopes using NaI(Tl) in the energy range 20-200 kev at small atmospheric depths is derived and discussed in detail. For equatorial latitudes almost all background originates from the electromagnetic interactions, especially through Compton scattering, of photons of energy greater than 200 kev with the scintillation crystal as well as shielding materials. The background originating from nuclear interactions like π° decay gammas, radiative capture, and star production of neutrons is found to be negligible. At high latitudes the atmospheric X rays in the 20- to 200-kev interval also contribute significantly to the background. These features of the background lead to the conclusion that by suitable detector design, particularly by reducing the thickness of the scintillation crystal, it is possible to appreciably improve the sensitivity to cosmic X rays for X-ray telescopes flown on equatorial balloon flights compared with high latitudes.