PROSPECTS OF CSI(T1) PHOTODIODE DETECTORS FOR LOW-LEVEL SPECTROSCOPY

A detailed study of the potential of CsI(Tl)-photodiode detectors for applications in low-level spectroscopy was carried out. The energy resolution of small crystals (few cm3) for gamma-rays in the MeV range can be parametrized as DELTA-E(FWHM)/E = 5% x E[MeV]-0.69. In kg size detectors, where resolution is limited by electronic noise, the use of new large-area low-capacitance pin Si photodiodes led to a significantly improved energy precision of DELTA-E (FWHM) = 150 keV, nearly independent of energy. Unfavourable light collection geometries, like in large-acceptance well detectors, do not substantially degrade the energy measurement. Scintillation light yields vary slowly with temperature and time so excellent long-term stability can be expected. For the first time a systematic investigation of inherent crystal radioactivity was possible due to the improved energy resolution obtained with large crystals. Six alpha-active radionuclides of the Th-decay-chain were identified. The isotopes are in equilibrium with each other but not in secular equilibrium with Th-232. The total alpha-activity varies from crystal to crystal between 2.4 and 5.5 decays/(h cm3) corresponding to Th-228 concentrations of 1.1-2.5 x 10(-18) g/g. Activity from U-238-chain isotopes is not seen at a level two orders of magnitude below the Th-activity. Prospects are discussed for further improved energy resolution and for applications in low-level spectroscopy, specifically for a search for 100 Mo double beta decay.