MEASUREMENTS OF RESIDUAL RADIOACTIVITY IN NEUTRON TRANSMUTATION DOPEDTHERMISTORS

Germanium wafers exposed to intense neutron beams from a nuclear react or to produce neutron transmutation doped thermistors have been analys ed for residual radioactivity in view of their application in experime nts on rare decays. Measurements have been routinely carried out for m ore than three years with germanium spectrometers of low intrinsic rad ioactivity operating in the Gran Sasso Underground Laboratory in order to suppress the background due to cosmic rays. Four long lived nuclei produced during the irradiation by the fast neutron component of the flux have been identified. It is shown that two of them will contribut e considerably to the background in experiments on rare events carried out with thermal detectors, especially in the direct search of dark m atter. These measurements also enable investigation of the presence of contaminants in the germanium wafer before irradiation. The sensitivi ty can be as low as 10(-14) g/g for some elements.