"Scintisphere" - the shape of things to come in gamma-ray spectroscopy

This paper presents details of the design and performance of three prototype Scintisphere detectors for use in gamma-ray spectroscopy. In these detectors, spherical CsI(Tl) scintillation crystals, having diameters ranging from 3.4 cm (20 cm³) to 8.3 cm (300 cm^3´), were polished and packed in dry MgO powder. The scintillation light from the crystal was viewed using a single 1 × 1 cm² silicon PIN diode. A low-noise preamplifier was also integrated within the detector housing. The measured noise level was equivalent to ∼800 electrons (FWHM). Such a configuration results in a relatively high light-collection efficiency. For example, in the 300 cm³ (8.3 cm diameter) detector, an average of ∼10 electron/hole pairs were generated for each keV deposited in the crystal. One of the key features of this detector design is that it minimizes the spatial variations in the light-collection efficiency throughout the detector. Compared with a standard 3-in NaI(Tl) scintillation counter, this leads to a much-improved energy resolution, particularly for photon energies above 1 MeV The results presented in this paper clearly demonstrate that these spherical CsI-photodiode detectors could be used as an ideal replacement for the standard 3-in NaI(Tl) detectors in many applications.