Abstract
A Compton rejection gamma rays spectrometer is described, formed by a 2″ × 1 3 4 ″ NaI(Tl) crystal placed in a hole drilled in a 29.8 cm dia. × 26.4 cm high cylindrical plastic scintillator. This ensemble is surrounded by a 20 cm thick iron shield. This apparatus gives a peak-to-total area ratio R of 0.610 for Cs 137, 0.542 for Mn 54 and 0.445 for Zn 65. For example, in the quantitative determination with a 9% error of Cs 137 in a mixture of Cs 137 (7.5 10 2 pC) + Zn 65 (4.5 10 4 pC) this device reduces the counting time from 74 to 20 minutes. The crystal background in the range from 0.1 to 3 MeV is also decreased by a factor of 54 by Fe shielding and furtherly of 2.3 by a.c. device. In the specified interval the background counting rate is 37.8 cpm. This equipment is moderately priced and highly versatile: in particular the a.c. mantle can be used as a well type counter or even as a total body monitor for small animals. When it is used as a well counter it is so sensitive as to detect few pC/cm 3 of a gamma emitting nuclide, with gamma energy around 1 MeV, dissolved in a 350 cm 3 solution, like in the case of Mn 54 or Co 60.
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