INFLUENCE OF SHIELD PARAMETERS ON COSMIC-MUON INDUCED BACKGROUNDS OF GE GAMMA-SPECTROMETERS

Computer simulations using GEANT 3.21 with shields of various paramete rs are performed to investigate how shield parameters can influence th e cosmic-muon induced background of a typical low-level HPGe gamma-spe ctrometric system. It was found that the background increases with inc reasing inner shield dimensions to reach a saturated value for very la rge dimensions when the composition of the shield (bulk material, thic kness, lining and proportion between height and width) is not changed. The background for a large shield is almost twice that for a tight on e. The background does not depend significantly on the position of the detector inside a shield and on the shape (cylinder or rectangular) p rovided the inner dimensions are the same. There is a strong dependenc e on bulk material and lining. Backgrounds are lower below approximate to 500 keV but higher above approximate to 500 keV and in the 511 keV annihilation peak for higher-Z materials. Several types of lining are simulated and compared. Best results are obtained for a lead shield l ined with 2 mm of tin.