Understanding phase equilibria and segregation in Bridgman growth of Cs 2 LiYCl 6 scintillator
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Stacy Swider and Stephanie Lam CapeSym, Inc., Natick, Massachusetts 01760
Robert S. Feigelson Materials Science & Engineering, Stanford University, Stanford, California 94305 (Received 16 January 2017; accepted 14 April 2017)
Cs2LiYCl6 (CLYC) is a commercial scintillator material having good energy resolution and dual gamma/neutron detection capabilities. CLYC crystals currently used in detectors are grown by the vertical Bridgman method. Boules grown from stoichiometric melts, however, often contain secondary phases, Cs3YCl6 and LiCl, at the beginning and end of the crystal, respectively, suggesting that this composition is incongruently melting. Since no phase diagram containing CLYC existed in the literature prior to this study, the Cs2YCl5–LiCl phase diagram was explored. Several crystals were then grown from various melt compositions. As predicted from the phase diagram, a starting composition of around 60 mol% LiCl did not produce Cs3YCl6 and maintained a low concentration of LiCl.
I. INTRODUCTION
Scintillators are materials that exhibit luminescence when excited by various types of radiation. They are used in an array of applications including medical diagnosis, nuclear detection, and high-energy particle physics. Cs2LiYCl6 (CLYC) is a room-temperature scintillator originally proposed by Combes et al. in 1999.1 It was further developed at the Delft University of Technology.2–4 Early studies reported an energy resolution of ;7% at 662 keV.1 Glodo et al.5 were able to improve the performance significantly, achieving an energy resolution of 3.6% at 662 keV with 10 10 detectors. Furthermore, van Loef et al.2 noted the possibility for pulse shape discrimination with CLYC by detecting core-valence luminescence. Core-valence luminescence is only excited by gamma radiation and has a fast decay time. Excitations from neutron radiation are typically slower, making it possible to distinguish scintillation pulses caused by gamma rays from those caused by neutrons.6 Commercial interest in CLYC developed due to its dual gamma/ neutron detection functionality and good energy resolution. Crystal growth studies directed toward commercializing this compound were started in 2003, and CLYC was a commercial product by 2012.5 CLYC crystals are typically grown by the vertical Bridgman method.5,7 The appearance of bubbles, cracks, Contributing Editor: Scott T. Misture a) Address all correspondence to this author. e-mail: fl[email protected] DOI: 10.1557/jmr.2017.168
and unwanted secondary phases in CLYC Bridgman boules has been an obstacle to the manufacture of highperformance detectors.8 Precipitates were observed forming spontaneously in melts, suggesting that CLYC melts incongruently. As a result, high-quality CLYC crystals with uniform composition are difficult to grow from stoichiometric melts. Since single-crystal growth is considerably more complicated for incongruently melting compounds, knowledge of the relevant phase equilibria is helpful in developing an optimal growth strategy.9 A comprehensive phase diagram can defi
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