IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 60, NO. 2, APRIL 2013 955 Instrument Development and Gamma Spectroscopy With Strontium Iodide Nerine J. Cherepy, Member, IEEE, Steve A. Payne, Member, IEEE, Benjamin W. Sturm, Member, IEEE, Owen B. Drury, Sean P. O’Neal, Peter A. Thelin, Kanai S. Shah, Rastgo Hawrami, Michael Momayezi, Brad Hurst, Arnold Burger, Member, IEEE, Brenden Wiggins, Pijush Bhattacharya, Lynn A. Boatner, and Joanne O. Ramey Abstract—Development of the Europium-doped Strontium Iodide scintillator, , involves advances in crystal growth, optics and readout methodology for prototype detectors. We have demonstrated energy resolution of 3% at 662 keV for a crystal, which is equivalent to the performance obtained with Cerium-doped Lanthanum Bromide of equivalent size. Compared to standard analog readout, use of a digital readout method allows improved energy resolution to be obtained with large volume crystals. Comparative gamma spectra acquired with and NaI(Tl) quantitatively depict the value of the high resolution of in discriminating closely spaced gamma lines for radioisotope identification applications. Index Terms—Gamma-ray spectroscopy, scintillators, strontium iodide. I. INTRODUCTION A DVANTAGES of the new single crystal scintillator, Strontium Iodide doped with Europium, , include: (1) no intrinsic radioactivity, (2) high light yield of , (3) density and photopeak efficiency com- parable to , (4) excellent light yield proportionality, and (5) ease of uniform growth, due to its orthorhombic crystal structure and the perfect lattice match between and . Manuscript received June 15, 2012; revised September 10, 2012; accepted October 24, 2012. Date of publication January 09, 2013; date of current ver- sion April 10, 2013. This work has been supported by the US Department of Homeland Security, Domestic Nuclear Detection Office, under competitively awarded IAA HSHQDC-09-x-00208/P00002. This support does not constitute an express or implied endorsement on the part of the Government. This work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Oak Ridge Na- tional Laboratory is managed for the U.S DOE by UT-Battelle under contract DE-AC05-00OR22725. N. J. Cherepy, S. A. Payne, B. W. Sturm, O. B. Drury, S. P. O’Neal, and P. A. Thelin are with the Lawrence Livermore National Laboratory, Livermore, CA 94550 USA (e-mail: cherepy1@llnl.gov; payne3@llnl.gov; sturm1@llnl.gov; drury2@llnl.gov; oneal10@llnl.gov, thelin1@llnl.gov). R. Hawrami and K. S. Shah are with the Radiation Monitoring De- vices (RMD), Watertown, MA 02472 usa (e-mail: rhawrami@rmdinc.com; kshah@rmdinc.com; kanaishah@yahoo.com). M. Momayezi and B. Hurst are with the Bridgeport Instruments, Austin, TX 78759 USA (e-mail: momayezi@bridgeportinstruments.com; Brad.Hurst@BridgeportInstruments.com). A. Burger, B. Wiggen, and P. Bhattacharya are with the Fisk University, Nashville, TN 37208 USA (e-mail: aburger@fisk.edu; bwiggins@fisk.edu; pi- jushb@fisk.edu). L. A. Boatner and J. O. Ramey are with the Oak Ridge National Laboratory, Oak Ridge, TN 37831 USA (e-mail: lb4@ornl.gov; joramey@ornl.gov). Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. Digital Object Identifier 10.1109/TNS.2012.2227796 In our earlier reports [1]–[3], we were able obtain energy resolution of 2.6% at 662 keV with small crystals, but due to optical light trapping by , we found that resolution de- graded for larger sizes. Subsequently, we developed polishing and packaging protocols to optimize light collection uniformity, reliably achieving resolution at 662 keV with 1 cubic inch encapsulated crystals and standard analog readout [4]–[7]. We also compared the properties of Eu-doped and undoped for gamma spectroscopy [8]. is now being grown in single crystal boules of up to . Standard handheld detectors utilizing employ cylindrical scintilla- tors. To obtain gamma spectroscopy with large-size crystals that is comparable to that offered by , we are reducing the Eu concentration, improving the purity of the crystal growth feedstock, and using modern digital pulse processing to acquire gamma spectra. Advantages offered by digital readout, include pileup rejec- tion, direct PMT hit rejection and importantly, this approach allows the analysis of pulse shapes. Not only can “on-the-fly” pulse shape analysis improve the accuracy of pulse height ac- quisition, resulting in better energy resolution for gamma ray spectroscopy, it can be used to obtain spatial information on the gamma ray interaction, and thereby provide directionality as- sessment when searching for a gamma source, without the need for segmented detectors. II. METHODS A. Crystal Growth and Encapsulation Transparent and crack-free single crystals (1–2” diameter) of strontium iodide doped with europium iodide are being grown at RMD, Fisk University and at Oak Ridge Na- tional Laboratory using the vertical Bridgman technique. The crystal growth process starts by loading ultra dry powder with a purity of 99.99% and ultra dry with a purity of 99.9% supplied by Sigma Aldrich into a freshly cleaned and pre-baked quartz ampoule within a glovebox of a pure and dry nitrogen atmosphere. The ampoule is sealed under a dynamic vacuum of . The crystals are grown in a two- zone, high temperature resistance vertical Bridgman furnace. Crystals are cut, polished and packaged using our standard tech- nique [4], [5]. The crystal used here for gamma spectroscopy measurements is shown in Fig. 1. 0018-9499/$31.00 © 2013 IEEE