The design and performance of a large-volume spherical CsI(Tl) scintillation counter for gamma-ray spectroscopy

This paper presents details of the design and performance of a prototype large-volume scintillation detector used for gamma-ray spectroscopy. In this detector, a spherical CsI(Tl) scintillation crystal having a diameter of 5.7 cm was 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 provided a very good light collection efficiency, which resulted in an average of 20 electrons being generated per keV of energy deposited in the crystal. One of the key features of the detector design is that it minimises spatial variations in the light collection efficiency throughout the detector. Compared with a standard 3 in. NaI scintillation counter, this feature leads to a much-improved energy resolution, particularly for photon energies above 1 MeV. The results presented in this paper clearly demonstrate that a spherical CsI-photodiode detector could be used as an ideal replacement for the standard 3 in. NaI detectors in many applications, but especially when the incident gamma-ray spectra extend up to ∼10 MeV as in neutron-activated gamma-ray analysis.