Implementation of techniques for background reduction in low-energy gamma ray telescopes

Abstract

Prometheus I, a low-energy ($\sim$0.06-12.0 MeV), balloon-borne gamma ray telescope has been developed and successfully flown. It consists of a central NaI(Tl) detector that is segmented into an array of 9 x 9 crystals and an active anticoincidence shield of thick plastic scintillator. Implemented on Prometheus are several background reduction techniques which allow it to be 10 times more sensitive to aperture gamma rays than previous generations of low-energy gamma ray telescopes. First, a $\beta$ ray rejection method is used. Next, a low-Z shield minimizes cosmic ray activation. Also layers of Li$\sp6$ absorb slow neutrons, thereby reducing background caused from inelastic neutron scattering and neutron capture. Finally the use of lightweight, low-Z construction materials minimizes locally produced background. Semi-analytical calculations and preliminary in-flight count rate data confirm the usefulness of these techniques.