Crystal growth and characterization of europium doped KCaI3, a high light yield scintillator
Introduction
Scintillator based detectors of X-ray and gamma rays are utilized within nuclear non-proliferation efforts as well as domestic security applications. The capability to detect and identify radio-nuclides has depended on the energy resolution capabilities of scintillation based detectors such as NaI(Tl) for many decades despite the rather low energy discrimination ability (6–7% FWHM at 662 keV). Advancement of the available technology has relied on either improvements to existing materials or the discovery of new scintillation materials which can attain improved spectroscopic performance over the previous generation of devices. Recently, a new generation of binary metal halide scintillators including LaBr3:Ce [1] and SrI2:Eu [2] have been developed which offer energy resolution of 3% FHWM or better at 662 keV, thus introducing alternative materials for fabrication of radiation detectors with enhanced capability. While technologically superior, these new scintillators remain costly to produce and thus their widespread use has been limited. Nevertheless, comparable yet cheaper alternatives are desired and the search for new scintillators suitable for spectroscopic applications has seen a recent surge of discovery and investigation into new ternary compositions comprised of Group I, Group II elements and the halides. The most promising among them are KSr2I5:Eu [3] and CsBa2I5:Eu [4], two very high performance scintillators reported to obtain <2.6% FWHM at 662 keV. Mixed halides comprised of BaBrI:Eu [4] and Ce(Cl–Br)3 [5] have also shown promising performance with energy resolution of ≈3.5% and <5% respectively.
In this work, we present results from the recent crystal growth and characterization of KCaI3:Eu. The orthorhombic crystal belongs to the ABX3 family of perovskite-type compounds (A = Cs, K; B = Ca, Sr, Ba; X = Cl, Br, I) with a density of ρ = 3.81 g/cm3 and adopts the Cmcm space group (a = 4.561 Å, b = 15.086 Å, c = 11.639 Å) [6]. KCaI3:Eu was first reported as one of the most promising of the ABX3 compounds with a previously reported light yield of ≈70,000 ph/MeV and energy resolution of 3.8% at 662 keV [7]. The earlier report remarked upon our early attempts at crystal growth of KCaI3:Eu which often encountered difficulties in attaining sizable crystals which tend to grow with a needle-like morphology due to the somewhat layered structure. Since revisiting the crystal growth of KCaI3:Eu with an emphasis on controlling the self-seeding process, we have achieved improvements in the quality of Bridgman grown crystals which has resulted in an increase in spectroscopic performance for large and small crystals over previous reports, as we will illustrate.
In order to present a more complete study of the scintillation performance characterization of KCaI3:Eu we include the scintillation lifetime, radioluminescence emission spectra, photoluminescence emission and excitation spectra, and non-proportionality curves.
Section snippets
Crystal growth
Samples were mixed from 4–5 N purity materials, in beaded form, from commercial sources Sigma Aldrich and APL. KI and CaI2 were used as-received while EuI2 was further purified by zone refining to remove excess trace metal impurities in the as-received chemicals. A stoichiometric mixture of 1:0.97:0.03 KI:CaI2:EuI2 was used to attain a nominal incorporation of 3 at.% Eu2+ substituting for Ca2+ sites in the matrix. This initial dopant concentration was chosen due to previous experiences with
Crystal growth
The DSC curves are shown in Fig. 1. The melting point of KCaI3:Eu is 524 °C which is the same as that determined for pure KCaI3 [6]. Under 5 K/min heating and cooling rate, this particular sample exhibited a relatively small degree of supercooling (melting point–freezing point = 20–25 °C). The small exothermic peak at 425° is believed to be a decomposition product formed once the sample is melted. The magnitude of this peak grows with subsequent scans and is accompanied by an endotherm only upon the
Discussion
The compartmentalized aspects of the measured energy resolution, R, can be described by a treatment developed by Dorenbos [13]:where Rstat is comprised of the photon counting statistics, Rin the inhomogeneity of scintillation production and light collection amidst the crystal, reflectors and PMT, and Rnp the contribution from the non-proportionality of the light yield as a function of excitation energy. For KCaI3:Eu, a useful figure of merit, namely the degree of
Conclusion
We have demonstrated that large single crystals of KCaI3:Eu can be obtained through melt growth techniques which possess desirable scintillation properties. For small crystals we have measured an energy resolution of 3% at 662 keV which is comparable to that of the highest performing scintillators SrI2:Eu and LaBr3:Ce (see Table 1). Moreover, the poorer (yet still promising) energy resolution measured in the much larger volume crystal may be attributed to further inhomogeneity due to non-uniform
Acknowledgements
This work has been supported by the US Department of Homeland Security, Domestic Nuclear Detection Office, under competitively awarded Grant #2012-DN-077-ARI067-04. This support does not constitute an express or implied endorsement on the part of the Government.
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