Growth of large size ( 38 mm diameter) KCaI3:Eu scintillator crystals

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Abstract

KCaI3:Eu is a scintillator composition that is promising for national security applications, with a high light yield and good energy resolution (70,000 ph/MeV and 3% E.R. at 662 keV, respectively). In addition to its high performance, we have been successful at growing KCaI3:Eu at larger sizes (38 mm diameter) without cracking. In this work we utilized a multi ampoule growth station to grow four 38 mm diameter crystals simultaneously. Three of these crystals achieved an energy resolution of 4% at 662 keV. The three crystals were hermetically packaged and a collimation study was carried out. This study revealed that light yield at a given irradiation location is dominated by photon path lengths and self-absorption, while the energy resolution dependence is affected heavily by local crystal quality. A 50 mm diameter KCaI3:Eu crystal was grown that achieved an energy resolution of 4.6% at 662 keV. GEANT4 was used to model a gap-style package that results in reduced photon path lengths, and therefore reduced self-absorption probability.

Introduction

Scintillation materials for gamma spectroscopy require a certain set of properties in order to be considered for national security applications including good efficiency, high light yield, and most importantly, excellent energy resolution. Additionally, these compositions must be producible in industrial sizes (>1) and at a reasonable cost in order to be commercially viable. Current compositions that meet some of these criteria include NaI:Tl , SrI2:Eu [1] , LaBr3:Ce [2], and CeBr3 [3]. However, no composition is perfect and research to develop better materials persists. The ABX3 (A = Cs, K; B = Ca, Sr; X = Br, I) ternary halide family has been extensively explored and promising compositions have been identified, including CsCaI3:Eu [4], CsCa(Br,I)3:Eu [5], and CsSrI3:Eu [6]. One material, KCaI3:Eu, has a light yield (>70,000 ph/MeV) and energy resolution (3% at 662 keV) [7] comparable to SrI2:Eu and LaBr3:Ce.

Due to their hygroscopicity, halide scintillator materials are traditionally grown via the vertical Bridgman method, utilizing a vacuum sealed quartz ampoule. This method is not free of challenges, as adhesion to quartz walls and a rigid growth geometry often lead to stress and cracking in the crystal. Additionally, maintaining high-quality, uncracked crystals becomes difficult as the diameter of the crystal increases. A capillary is often utilized to facilitate self-seeding in these materials. KCaI3:Eu tends to grow in a needle-like morphology, often allowing multiple grains to survive past the capillary. This has been previously mitigated by introducing a 15° bend in the capillary to arrest these grains on the ampoule wall [8]. The ionic radii mismatch of Ca2+ and Eu2+ leads to segregation of europium, producing a variation in dopant concentration in large size crystals [8]. Attempts have been made to measure the segregation coefficient of europium in KCaI3:Eu, and a value of 0.45 has been reported [9]. Regardless, KCaI3:Eu has successfully been grown at 38 mm reproducibly while achieving an energy resolution of 4% at 662 keV [9]. Furthermore, 50 mm growth of the cation-alloyed composition KCa0.8Sr0.2I3:Eu with an energy resolution of 4.1% at 662 keV has been demonstrated [10].

The intricate growth process for metal halides increases the production cost, which is ultimately folded into the cost of finished crystals. Traditional Bridgman production consists of a single-bore furnace capable of growing one crystal at a time. Multiple ampoule growth in a single furnace could drastically reduce production costs and time, further improving the commercial viability of KCaI3:Eu. The ability to increase crystal output by simultaneous growth of multiple ampoules has been explored using the Multi Ampoule Growth Station (MAGS) developed at the University of Tennessee. This furnace has been previously used to simultaneously grow four 1 x 2 KCaI3:Eu boules [11]. This growth run yielded six 1 x 1 detector crystals. MAGS has also been used to successfully grow different compositions, such as CsCaI3:Eu and CsCa(Br,I)2:Eu [5].

Similarly to other europium-doped scintillators, large size KCaI3:Eu has deterioration of light yield and energy resolution due to self-absorption of scintillation photons [12], [13]. In addition, self-absorption can lengthen decay times in europium-doped crystals, an effect that is exaggerated in large sizes [10]. This can be mitigated in larger crystals by reducing the nominal activator concentration [14]. Previous experiments have shown that utilizing a “gap” reflector that allows scintillation light to leave the crystal and reflect towards the photomultiplier tube (PMT) can improve both light collection and decay time [14]. By incorporating this into a hermetic package, energy resolution and light yield can be improved for europium-doped crystals. Additionally, digital pulse processing techniques have been developed with SrI2:Eu crystals to improve their energy resolution by correcting the pulse integration using the length of the decay [15].

Similar to LaBr3:Ce, KCaI3:Eu has an intrinsic activity from the naturally occurring 40K. The background count rate grows linearly with volume, and is more of an issue in large crystals. Although the radioactivity can increase the noise and pulse pileup, there is active work toward mitigation of this background contamination in the related composition KSr2I5:Eu [16].

In this work we demonstrate growth of 2 KCaI3:Eu crystals using a conventional single-bore Bridgman furnace and simultaneous growth of four 1.5 KCaI3:Eu crystals using the MAGS furnace. Additionally, we propose a novel package design for europium-doped scintillator crystals, and use light transport software GEANT4 to simulate photon path length inside of a KCaI3:Eu crystal utilizing differently sized reflector gaps.

Section snippets

Crystal growth of 38 mm in MAGS

As the diameter of the crystal increases, so does the importance of thermal uniformity, and therefore an 8-point thermocouple probe was created to measure the radial thermal gradient in the furnace. The MAGS growth station consists of four circular bays aligned linearly inside of a rectangular furnace. A model of the furnace can be seen in the previous publication [11]. For the radial thermal profile, the 8-point probe was translated in one of the exterior bays. Thermocouples were mounted at

Radial thermal profile

The temperature profile collected using the 8-point probe is plotted in Fig. 1. The hot zone of the furnace (>585 °C) contains a fairly large thermal gradient; there exists an almost 10 °C difference between different angles on the probe. However, the radial gradient near the melting temperature of KCaI3:Eu (525 °C) is nearly uniform, which is required for a flat growth interface. The large radial gradient in the furnace in the hot zone should not negatively impact growth; in fact it may

Conclusion

In this work we have demonstrated the feasibility of simultaneous growth of KCaI3:Eu in 38 mm diameter sizes. Three out of four crystals showed excellent energy resolution at 662 keV (4%) and little variation in light yield (<2%) before packaging, even with minor cracking. A collimation study was completed on the packaged crystals to assess variations in performance based on irradiation position. Additionally, we have grown a 50 mm KCaI3:Eu crystal that had a measured energy resolution of

Acknowledgments

This work has been supported by the US Department of Homeland Security, Domestic Nuclear Detection Office , under competitively awarded grant #2012-DN-077-ARI067-05. This support does not constitute an express or implied endorsement on the part of the Government.

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