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Tl2LiYCl6: Large Diameter, High Performing Dual Mode Scintillator Rastgo Hawrami, Elsa Ariesanti, Hua Wei, Jeffrey Finkelstein, Jarek Glodo, and Kanai Shah Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b00583 • Publication Date (Web): 12 Jun 2017 Downloaded from http://pubs.acs.org on June 22, 2017

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Crystal Growth & Design

Tl2LiYCl6: Large Diameter, High Performing Dual Mode Scintillator Rastgo Hawrami,* Elsa Ariesanti, Hua Wei, Jeffrey Finkelstein, Jarek Glodo, Kanai Shah ADDRESS: Radiation Monitoring Devices Inc., 44 Hunt Street, Watertown, MA, United States, Zip code: 02472 KEYWORDS: Tl2LiYCl6, crystal growth, dual-mode scintillator, gamma detection, neutron detection, elpasolite

ABSTRACT: Tl2LiYCl6:Ce (TLYC) is a recently discovered dual mode gamma-ray and neutron scintillator. So far small crystals of this composition have been studied, but for practical applications with affordable price, large-scale crystals are required. In this work, we present successful efforts to grow crack-free single crystals with sizes up to ⌀1"×5.5". A variety of experimental techniques were employed to investigate the scintillation properties. A ⌀1"×1.2" TLYC cylinder has a light yield of 25,000 ph/MeV, and its energy resolution is better than 4% at 662 keV. The gamma equivalent energy (GEE) produced by thermal neutron is 1.89 MeVee, along with a neutron induced light yield of 47,000 ph/n. Pulse shape discrimination (PSD) between gamma-rays and neutrons has been successfully shown with a current Figure-of-Merit (FOM) of 2.4. This paper explores the crystal growth, scintillation properties and potential applications of TLYC.

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Crystal Growth & Design

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I. Introduction Scintillators play important role in radiation detection and imaging, more so in recent times with newly developed metal halides such as LaBr3:Ce

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and SrI2:Eu 2. Dual mode elpasolite

crystals have also been intensively investigated in recent studies. 6Li enriched Cs2LiYCl6:Ce (CLYC) and Cs2LiLa(Cl,Br)6:Ce (CLLBC) elpasolite single crystals are proposed as promising neutron/gamma dual mode scintillators

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. The presence of 6Li greatly increases the neutron

cross-section and thus leads to high thermal neutron detection efficiency. CLYC is reported of having a light yield of 20,000 ph/MeV with a 3.6% energy resolution at 662 keV, and its gamma equivalent energy (GEE) is 3.2 MeV 2. While CLYC is an excellent scintillator, its drawbacks are its low density and stopping power, which are comparable to NaI:Tl. In order to improve these parameters, replacement of Cs ions with isovalent Tl ions was investigated. This replacement increases the effective atomic number from Zeff,CLYC = 45 to Zeff,TLYC = 69. The density of TLYC ρTLYC = 4.5 g•cm-3) is also higher than that of its Cs-equivalent (ρCLYC = 3.3 g•cm-3), thus ensuring higher gamma-ray stopping power and photo-fraction. For example, the mean penetration depth of 662 keV gamma-rays in TLYC is 2.4 cm versus 3.9 cm in CLYC. The 662 keV photo-fraction for TLYC is 0.26 versus 0.08 for CLYC. Based on these considerations, TLYC appears to be a promising scintillator for nuclear security instrumentation. RMD first started with growth of 16mm diameter Ce-doped TLYC crystals, which was reported elsewhere

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. Another group also reported the growth of this material 9. In this paper,

growth and evaluation of 1” diameter, high performing Ce-doped Tl2LiYCl6 (TLYC) single crystals is presented. Crystal growth technique, scintillation properties, and gamma-ray and neutron discrimination capability are discussed.

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Crystal Growth & Design

II. Experimental 2.1 Materials Preparation and crystal growth Transparent, crack-free single crystals of TLYC with 5% Ce doping concentration (molar ratio) were successfully grown by the vertical Bridgman technique in ⌀inner 1 inch-fused silica ampoules. For each growth, a stoichiometric mixture of TlCl, LiCl, YCl3, and CeCl3, all from Sigma Aldrich (anhydrous beads, trace metal basis, 4N purity), underwent in-situ material processing and purification prior to being loaded into the silica ampoule inside a glovebox in dry nitrogen gas atmosphere (nominally with