Neutron Capture Induced Radioactive Background in Spaceborne, Large Volume, NaI Gamma-Ray Spectrometers

Radioactivity induced in alkali halide scintillators by the capture of secondary neutrons can provide an important source of background in spaceborne gamma-ray spectrometers subjected to cosmic rays and trapped protons. Previous calculations which concentrated on spallation interactions are now interfaced with a one-dimensional neutron transport code (ANISN) in a simple spherical model in order to illustrate the relative importance of the two components. For large volume detector systems, the neutrons of interest are those produced within the scintillator itself, and their contribution can dominate the background in the 0.2-2 MeV region. Use of conventional neutron shielding techniques, such as 6LiH, does not appear to be beneficial for current detector sizes.