A Comparative Study of Undoped NaI Scintillators with Different Purity

Abstract

The performance of undoped NaI crystal of a high purity of 7 N grade, developed by Saint-Gobain Crystals for a dark matter search, was studied at liquid nitrogen temperature using avalanche photodiode. The measured quantities covered the light output expressed in the electron-hole (e-h) pair number, non-proportionality characteristics, energy resolution and tinally the intrinsic resolution of the studied crystals. In contrast to the previous study, the new crystals showed comparable non-proportionality curves to that known for NaI(TI) at room temperature and a poor energy resolution of about 8% for 662 keV gamma rays from 137CS source. The performed study highlights a role of undoped NaI crystals in a better understanding of limitations of energy resolution in scintillation detectors. A high sensitivity to traces of doping may help to find the way of a modification of non-proportionality of other scintillators by a selective doping or co-doping. A further study of slow components of the NaI light pulses and their influence on energy resolution may clear up its possible deterioration by defects in the crystal structures.

Energy resolution and non-proportionality of scintillation detectors – new observations

“…Moszyń ski et al, 2009). A similar effect observed in the undoped CsI(Moszyń ski et al, 2005c) suggested that accidental doping by impurities improved seriously a non-proportionality and then the energy resolution.Moreover, several observations collected in the last 10 years on the unexpected influence of slow components of the light pulses on energy resolution suggest more complex processes in the scintillators.…”

“…The non-proportionality of the undoped oxide crystals, as BGO (Moszyń ski et al, 2004) and CWO (Moszyń ski et al, 2005a), represents the fundamental properties of the scintillator materials. In case of doped crystals, one observes variation of the non-proportionality characteristic depending on doping agents, particularly for halide crystals (Moszyń ski et al, 2009(Moszyń ski et al, , 2005c. Moreover, the pure, undoped halide crystals, as NaI and CsI, are very sensitive to the impurities, which affect the non-proportionality curves (Moszyń ski et al, 2009).…”

“…In case of doped crystals, one observes variation of the non-proportionality characteristic depending on doping agents, particularly for halide crystals (Moszyń ski et al, 2009(Moszyń ski et al, , 2005c. Moreover, the pure, undoped halide crystals, as NaI and CsI, are very sensitive to the impurities, which affect the non-proportionality curves (Moszyń ski et al, 2009). It suggests that a selective co-doping of scintillators may improve the non-proportionality and their energy resolution.…”

“…A comparison of the non-proportionality curve of the ultra pure NaI-1 crystal to those of scintillation purity(Moszyń ski et al, 2009).…”

The nature of luminescence centers in NaI:Eu single crystals

“…Some traces of OH -, H -, CO 3 2-, O 2 2-are always present even in "pure" [11,12] and Tl-doped NaI crystals [13,[17][18][19]. In some crystals which are "nominally pure" scintillators non-proportionality of the intrinsic emission cannot be properly justified [20]. The total content of the oxygen ions and oxygencontaining impurities in NaI:Eu crystal may be comparable with the concentration of the Eu 2+ ions.…”