The Neutron Spectrum Inside the Shielding of Balloon-Borne GE Spectrometers

Understanding the background of balloon borne germanium detectors used in high energy astrophysics is crucial for the design of future space missions. In this paper we present a method which allows to estimate the poorly understood (β- and β+ background components inside the shield of an balloon borne instrument. The method consists of an experimental estimate of the neutron flux by determining the activation of isomers with measured gamma-ray line strengths and known cross sections. The gamma-ray line intensities of typical isomeric transitions in Germanium detectors are calculated for a neutron spectrum model. After comparing the calculated line fluxes with the measurements of balloon experiments the model neutron spectrum is changed appropriately. The procedure is repeated iteratively until an acceptable fit is achieved.