Extremely low-level scintillation spectrometer

A 4 in. x 4 in. NaI(Tl) gamma spectrometer with an extremely low background of 120 cpm above 30 keV is described. The γ detection limit for material in the 2-litre sample container surrounding the crystal ranges between 0.3 and 0.8 dpm over the spectral range from 0.1 to 3 MeV. The spectrometer has been used for the measurement of low gamma activities, such as those due to small traces of impurities in various materials, and in searches for the decay of extremely long-lived naturally occurring isotopes, such as vanadium-50 (fourfold forbidden β transition, lower limits of half-life found to be 9 x 1016 and 6.9 x 1016 yr for the two modes of decay), calcium-48, nickel-58 and tellurium-130 (double β decay, half-life limits found for these three nuclides being 2.0 x 1016, 2.4 x 1019 and 1.6 x 1018 yr, respectively). Recently the spectrometer was used for meteorite measurements. For the better detection of positron emitters, the spectrometer was extended by a γγ-coincidence system by the addition of an ordinary 3 in. x 1 1/2 in. NaI crystal. The detection efficiencies for annihilation radiation, 22Na and 26Al are 0.3, 0.7 and 0.8 dpm respectively. In the St. Severin meteoritic material 16.8 dpm 40K (933 ppm K), 10.7 dpm 54Mn, 9.1 dpm 22Na and 5.8 dpm 26Al per 100 g were found. The material of Esebi contains 11.5 dpm 40K (638 ppm K) and 6.1 dpm 26Al per 100 g. (author)