Instrumental background in balloon-borne gamma-ray spectrometers and techniques for its reduction

Abstract

A study of the instrumental background in balloon-borne gamma-ray spectrometers is presented. The calculations are based on newly available interaction cross sections and new analytic techniques, and are the most detailed and accurate published to date. Results compare well with measurements made in the 20 keV to 10 MeV energy range by the Goddard Low Energy Gamma-ray Spectrometer (LEGS). The principal components of the continuum background in spectrometers with Ge detectors and thick active shields are (1) elastic neutron scattering of atmospheric neutrons on the Ge nuclei, (2) aperture flux of atmospheric and cosmic gamma rays, (3) β − decays of unstable nuclides produced by nuclear interactions of atmospheric protons and neutrons with Ge nuclei, and (4) shield leakage of atmospheric gamma rays. The improved understanding of these components leads to several recommended techniques for reducing the background. These include minimizing the passive material inside the shield and reducing the level of the shield threshold. A new type of coaxial n-type Ge detector with its outer contact segmented into horizontal rings can be used in various modes to reduce background in the 20 keV to 1 MeV energy range. The resulting improvement in instrument sensitivity to spectral lines is a factor of ∼ 2 in this energy range.