A balloon-borne instrument for high-resolution astrophysical spectroscopy in the 20-8000 keV energy range

The Low Energy Gamma-ray Spectrometer (LEGS), a joint project among NASA/GSFC, CENS, and Rice University, is designed to perform fine energy resolution measurements of astrophysical sources. The instrument can be configured for a particular balloon flight with either of two sets of high-purity germanium detectors. In one configuration, the instrument uses an array of three coaxial detectors (effective volume approx.= 230 cm3) inside an NaI(Tl) shield and collimator (field-of-view approx.= 160 fwhm) and operates in the 80-8000 keV energy range. In the other configurations, three planar detectors (effective area approx.= 53 cm2) surrounded by a combination of passive Fe and active NaI for shielding and collimation (field-of-view approx.= 50 x 100 fwhm) are optimized for the 20-200 keV energy range. In a typical one-day balloon flight, LEGS sensitivity limit (3 sigma) for narrow line features is <= 8 x 10-4 ph/cm2xs (coaxial array: 80-2000 keV) and <= 3 x 10-4 ph/cm2xs (planar array: 50-150 keV). (orig.)