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The Nordic Laboratory for Luminescence Dating (Department of Geoscience, Aarhus University) intends to purchase two high resolution intrinsic germanium detectors and associated electronics for use in the energy range 30kev to 1.5MeV. These spectrometry systems will be used primarily to study both natural and fallout radionuclides present at low levels in mineral materials. In order to optimise the sensitivity to naturally occurring gamma rays, especially at low energies, there are two general requirements
The effective sensitive area at low energies (>40keV) of the detectors should be as large as possible. The preferred way of achieving this is by using 'n' type material, as specified in item 1 in appendix 1, so that the sensitive surface of the detector can include the sides and top of the crystal. However the benefits of this material are lost if absorbing material is placed around the sides of the detector. Appendix 1, items 4.3 to 4.6 address this aspect of the specifications.
Because many of the gamma rays of interest are naturally occurring and at low specific activities, the materials used in fabricating the detector housing also act as radioactive sources emitting gamma lines at identical energies to those of interest. We are particularly interested in minimising the gamma ray background at these energies. To this end, we have specified certain peaks of interest in Appendix 1, item 4.7, at which energies the count rates should be indistinguishable from the background continuum.