Toward High Energy Resolution in CsSrI₃/Eu²⁺ Scintillating Crystals: Effects of Off-Stoichiometry and Eu²⁺ Concentration

CsSrI₃/Eu²⁺ has very promising scintillation properties for gamma-ray spectroscopy applications, but it has proven difficult to grow high quality single crystals in large sizes. This paper reports a composition-engineering strategy, in this case a combination of off-stoichiometric melts and Eu²⁺ concentration optimization, to obtain large-size CsSrI₃/Eu²⁺ crystals with excellent energy resolution. Crystals of a series of off-stoichiometric compositions, Cs_(1+x)(Sr,Eu)_(1-x)I_(3-x) (x = 0, 0.05, 0.06, and 0.1), were grown by the Bridgman method. The Cs_1.06Sr_0.94I_2.94/Eu²⁺ single crystal has the highest optical transmittance between 450 and 800 nm. Cs_1.06Sr_0.94I_2.94 single crystals doped with 0.5, 1, 3, 5, and 7 mol % Eu²⁺ ions were also grown by the Bridgman method. The effects of Eu²⁺ concentration on the phase purity and optical and scintillation properties were studied. X-ray diffraction patterns confirmed the phase purity of all samples with the exception of a hydrate phase formed during measurement. Increasing Eu²⁺ concentration leads to longer decay components due to the effect of self-absorption. An unexpected relationship was found between the Eu²⁺ concentration and the appearance of two photopeaks in a pulse height spectrum acquired under a single gamma-ray energy of 662 keV irradiation. The origins of this phenomenon are proposed from experimental insights. The optimal composition we developed achieved an excellent energy resolution of 3.4% for φ22 mm × 2 mm, 3.9% for φ22 mm × 15 mm, and 4.1% for φ22 mm × 19 mm at 662 keV. The results of this paper lead to a better understanding of the effects of composition-engineering in optimization of nonstoichiometric scintillator compounds.