Scintillators with potential to supersede lanthanum bromide - eScholarship (2010)

Scintillators with Potential to Supersede Lanthanum Bromide N.J. Cherepy,* S.A. Payne, S.J. Asztalos, G. Hull, J.D. Kuntz, T. Niedermayr, S. Pimputkar, J.J. Roberts, R.D. Sanner, T.M. Tillotson, E. van Loef, C.M. Wilson , K.S. Shah, U.N. Roy, R. Hawrami, A. Burger, L.A. Boatner, W.-S. Choong and W.W. Moses Abstract—New scintillators for high-resolution gamma ray spectroscopy have been identified, grown and characterized. Our development efforts have focused on two classes of high light yield materials: Europium-doped alkaline earth halides and Cerium-doped garnets. Of the halide single crystals we have grown by the Bridgman method— SrI 2 , CaI 2 , SrBr 2 , BaI 2 and BaBr 2 — SrI 2 is the most promising. SrI 2 (Eu) emits into the Eu 2+ band, centered at 435 nm, with a decay time of 1.2 µs and a light yield of up to 115,000 photons/MeV. It offers energy resolution better than 3% FWHM at 662 keV, and exhibits excellent light yield proportionality. Transparent ceramics fabrication allows production of Gadolinium- and Terbium-based garnets which are not growable by melt techniques due to phase instabilities. While scintillation light yields of Cerium-doped ceramic garnets are high, light yield non-proportionality and slow decay components appear to limit their prospects for high energy resolution. We are developing an understanding of the mechanisms underlying energy dependent scintillation light yield non-proportionality and how it affects energy resolution. We have also identified aspects of optical design that can be optimized to enhance energy resolution. Index Terms— Scintillators, scintillators, gamma ray detectors strontium iodide, ceramic I. I NTRODUCTION Use of gamma ray spectroscopy for radioisotope identification requires pushing the limits of energy resolution, and is also enhanced by a high effective atomic number of the detector material. The commercial inorganic scintillator currently providing the highest energy resolution is LaBr 3 (Ce), of ~2.6% at 662 keV [1-3], but it is highly hygroscopic, Manuscript received June 31, 2008. This work was supported by the Domestic Nuclear Detection Office in the Department of Homeland Security (Alan Janos) and by the National Nuclear Security Administration, Office of Nonproliferation Research and Development (NA-22, Robert Mayo) of the U.S.DOE under Contract No. DE-AC02-05CH11231, and this work was performed under the auspices of the U.S. DOE by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. Oak Ridge National Laboratory is managed for the U.S DOE by UT-Battelle under contract DE-AC05-00OR22725. N.J. Cherepy S.A. Payne, S.J. Asztalos, G. Hull, J.D. Kuntz, T. Niedermayr, S. Pimputkar, J.J. Roberts, R.D. Sanner, T.M. Tillotson are with Lawrence Livermore National Laboratory, Livermore, CA 94550 (phone: +1- 925-424-3492; fax: +1-925-423-6394; e-mail: cherepy1@llnl.gov). E. van Loef, C.M. Wilson and K.S. Shah are with Radiation Monitoring Devices, Watertown, MA (e-mail: kanaishah@yahoo.com). U.N. Roy, R. Hawrami and A. Burger are with Fisk University, Nashville, TN (e-mail: aburger@fisk.edu). L.A. Boatner is with Oak Ridge National Laboratory, Oak Ridge, TN (e- mail: lb4@ornl.gov). W.W. Moses and W.-S. Choong are with Lawrence Berkeley National Laboratory, Berkeley, CA (e-mail: wwmoses@lbl.gov). possesses intrinsic radioactivity due to the presence of primordial 138 La, and its crystal growth is still challenging. Energy resolution of ~2% at 662 keV is desired for assignments of gamma ray lines in a typical spectrum, while intrinsic radioactivity interferes with low count rate measurements, especially for large detector volumes. Finally, the ease of crystal growth is directly correlated with cost of manufacture of large crystals useful for detection of weak sources. To identify new scintillator materials with potential to supersede the performance and cost metrics of LaBr 3 (Ce), we use a “directed search methodology,” which is distinguished from the “combinatorial approach” by aggressively downselecting candidates based on criteria and available information. For first round qualification, criteria are: (1) effective atomic number (Z eff ) equivalent to or higher than LaBr 3 (Ce), for efficient photoelectric effect, (2) no/low intrinsic radioactivity for low background, and (3) expected radioluminescence light yield adequate to achieve energy resolution of 2%. Once high-Z, high light yield materials are identified, we evaluate their prospects for good light yield proportionality, ease of crystal growth, lack of confounding phase transitions, low deliquescence, favorable optical properties, dominant decay time of <3 µs (to avoid signal pile- up with standard shaping electronics) and photodetector spectral match. Figure 1 shows how the periodic table can be analyzed such that only compounds containing at least one of the encircled species are considered, yielding a reasonably short candidate list of materials for growth and evaluation. Fig. 1. Selection of candidate scintillators for gamma ray spectroscopy is straightforward, as only a few high-Z elements (shown encircled) are non- radioactive, not optically absorptive and produce growable compounds.