2830 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 63, NO. 6, DECEMBER 2016 The Influence of Light Anion Impurities Upon SrI2(Eu) Scintillator Crystals S. E. Swider, S. Lam, and A. Datta, Member, IEEE Abstract— To better identify the influence of light anion impu- as metallic strontium is known to react aggressively with rities on the scintillation performance, small boules of SrI2(Eu) nitrogen Halide impurities such as chlorine and bromine may were grown by the vertical Bridgman-Stockbarger method, each 0 2− 3− be introduced via impurities in the hydrogen-iodide acid used co-doped with 0.2% of one of the following: C ,CO3 ,N , 2− − 3− 2− 2− − − to convert strontium carbonate into strontium iodide. Likewise, O ,OH ,PO4 ,S ,SO4 ,Cl and Br . Residual impurity concentrations were measured, and the scintillation performance residual phosphorous may be present in the hydrogen-iodide of resulting detectors was characterized. Oxygen was tolerated acid, or in the minerals from which strontium is mined. up to 0.2% on a molar basis. Sulfur proved to be highly To maintain and improve purity, crystal growers handle SrI2 detrimental to both crystallinity and scintillation performance. and similar salts in low-moisture, argon-filled glove boxes. Nitrogen produced additional emission near 480 nm. This study They also employ melt-filtration [5] and reactive gasses such suggests that SrI2(Eu) readily incorporates anion impurities, which may substitute for iodine, but these may also be removed as HI(g) [10]–[11]. However, since it is not clear which light before and during growth by volatilization. Purity metrics for impurities are most detrimental to single-crystal growth and starting materials should include sulfur and carbon, as well as scintillation performance, current purification efforts are not oxygen and H2O. thoroughly guided. Some may be unnecessary. Index Terms— Anions, co-doping, crystal growth, impurities, In this study, we examined the impact of light scintillators, strontium iodide. impurities upon crystallinity and scintillation performance of SrI2(4%Eu). Individual crystals of were grown with I. INTRODUCTION 0.2 mol% of the light-impurity containing co-dopant. HE SrI2(Eu) scintillator, first discovered by R. Hofstadter Although co-doping has been previously used to improve the Tin 1968 and rediscovered five decades later [1]–[3], has performance of various scintillators, such as LaBr3 [12] and shown great promise in radiation detection community due CeBr3 [13], this study was concerned with identifying unfa- to its excellent performance. Reported properties include a vorable co-dopants. Resulting data provide improved guidance high light yield ranging from 80,000 to 120,000 ph/MeV at for precursor purification. Previously, we studied the impact 662 keV, resolution as low as 2.6%, and a proportional light of cation co-doping and found many metallic impurities segre- yield response [3]–[6]. Its limitations include hygroscopicity, gated according to the Hume-Rothery rules of solid solubility, a slightly long decay time, and self-absorption [7], [8]. and were rendered benign. Cations with similar ionic radii and Advances in precursor production have led to the availability valences to strontium (e.g. Ca, Ba, Na) became incorporated of high quality, beaded SrI2 starting material. Nevertheless its into the lattice, and degraded scintillation performance [14]. deliquescence makes the material susceptible to inadvertent introduction of light impurities present in air. The 5N classi- II. EXPERIMENTAL METHODS fication of the starting material addresses metallic impurities only– oxygen and moisture are accounted separately. Well- A. Crystal Growth dehydrated strontium iodide typically has oxygen levels of Single crystals of SrI2(4%Eu) were grown from stoichio- 30-50 ppm by weight, which is about 650-1050 ppm on a metric mixtures of anhydrous 99.99% SrI2, and 99.99% molar basis. Residual carbon, sulfur, nitrogen, chlorine, and EuI2, (Sigma Aldrich). During initial compounding, each was 0 2− bromine are not typically measured. Carbon and sulfur impu- co-doped with of one each of the following anions: C ,CO3 , rities may be introduced stochastically from starting materials, −3 2− − −3 2− 2− − − N ,O ,OH ,PO4 ,S ,SO4 ,Cl and Br . In cases which are often mined as sulfides and converted into carbon- where the dopant compound was not listed as anhydrous from ates before synthesis into halide salts [9]. Nitrogen also has the the vendor, it was dehydrated at 175◦Cand8× 10−6 torr potential to be introduced stochastically during salt synthesis, for 2 hours. The specific dopants are listed in Table I. With the exception of SrCl2 and SrBr2, 2000 mol ppm of each Manuscript received March 14, 2016; revised July 20, 2016; accepted October 17, 2016. Date of publication October 26, 2016; date of current anion was added with respect to the SrI2(Eu) matrix. The version December 14, 2016. This work was supported by the U.S. Depart- target value of 2000 mol ppm (0.2 mol%) was chosen because ment of Homeland Security, Defense Nuclear Detection Office, under the it represents the high end of impurity content that might be competitively awarded contract HSHQDC-13-C-00080. This support does not constitute an express or implied endorsement on the part of the Government. present in purchased SrI2. The chlorine, and bromine anions The authors are with CapeSym Inc., Natick, MA 01760 USA (e-mail: were added as 2000 mol ppm with respect to the strontium [email protected]; [email protected]; [email protected]). cation, and therefore those preliminary anion concentrations Color versions of one or more of the figures in this paper are available online at http://ieeexplore.ieee.org. were 4000 ppm. Note that the certain anion molecules contain Digital Object Identifier 10.1109/TNS.2016.2622059 multiple oxygen atoms, e.g. 2000 mol ppm SO4 contains 0018-9499 © 2016 IEEE. Personal use is permitted, but republication/redistribution requires IEEE permission. See http://www.ieee.org/publications_standards/publications/rights/index.html for more information. SWIDER et al.: THE INFLUENCE OF LIGHT ANION IMPURITIES UPON SrI2(EU) SCINTILLATOR CRYSTALS 2831 TABLE I is located within a dedicated argon-filled glove box maintained ANION DOPANTS ADDED IN THIS STUDY at less than 3 ppm moisture. During oxygen analysis, small assays (< 0.5g) are heated to approximately 3000◦Cinthe presence of graphite. As the assay is volatilized, oxygen is released and reacts with the graphite to form carbon dioxide. A high purity helium gas stream sweeps the CO2 to an IR cell. The calibrated CO2 absorption curve is integrated and compared to the original sample weight, for a parts-per-million by weight oxygen reading. After growth, the SrI2(Eu) boules were opened inside the ELTRAglovebox, in order to minimize air exposure before testing. Sectioning was performed in the same glovebox. Small detectors (φ 10 mm ×6 mm height) were made from the midpoint of each boule, and tested for scintil- lation response to 662 keV radiation. Pulse-height spectra were measured using a Hamamatsu R6231-100 photomulti- plier tube (PMT) located in a glovebox at less than 3 ppm 8000 mol ppm oxygen. Finally, we note that the least moisture. PMT output signals were processed by a Canberra pure co-dopant, strontium hydroxide (94% purity), introduces 2005 pre-amplifier, an Ortec 672 amplifier set at 10 μ s 120 mol ppm metallic impurities (6% x 2000 ppm), which shaping time, and an Ortec Easy-MCA-2k multichannel ana- should be within the tolerance of the strontium iodide matrix. lyzer. Photopeak centroids and energy resolutions were deter- We have reported previously that the strontium iodide matrix mined using Ortec Maestro peak-fitting software. Samples rejects most cations during crystallization, according to Hume- were cupped within a specular reflector (Vikuiti ESR, 3M) to Rothery rules for forming a solid solution [14]. improve light collection. Resulting light yield was estimated Starting concentrations of metallic impurities in the SrI2 by comparing the gamma response of the SrI2(Eu) detector to and EuI2 beads were approximately 90 and 60 ppm by mol NaI(Tl) and LYSO(Ce) scintillators made by Hilger Crystals, 3 (30 and 15 ppm by wt), respectively, as determined by each sized 5×5×5mm, with corrections made for the PMT 66-element inductively-coupled plasma mass spectroscopy quantum efficiency. (ICP-MS). The same lot of SrI2 and EuI2 beads was used Decay traces were collected using a Tektronix TDS throughout this study. 784C oscilloscope with a 50 terminator to match the Prior to loading, each ampoule was cleaned with aqua cable impedance. Each recorded trace was an average of regia and acetone, and baked for over 1 hour at 850◦Cat 10,000 decay pulses. Decay times were determined by fitting 2 × 10−5 torr. After vacuum baking, the ampoules were a single exponential decay function from the maximum pulse isolated with a valve, and transferred to the glove box without intensity, I0,to0.1I0. introduction of air. Assays were loaded in an argon-filled Emission wavelength spectra were obtained by exciting the glove box at <2 ppm moisture. After loading, the vacuum- samples at 365 nm with an Hg lamp and collecting the emis- compatible valve was reattached and the loaded ampoules were sion with a Horiba MicroHR spectrometer. The spectroscopy heated to 150◦Cat2×10−5 torr for at least 1 hour, to remove system is described in more detail in Lam et. al. [15]. All residual moisture, before being sealed with a torch. detectors were enclosed in an air-tight optical jig for the For increased throughput, a φ45mm, stainless steel, measurement. multiple-ampoule holder was used to grow four 10-millimeter EDS measurements were made on a JEOL JSM-6300 SEM diameter crystals simultaneously in quartz ampoules, by the system at Geller MicroAnalytical of Topsfield, MA; using vertical Bridgman-Stockbarger method, in a 2-zone furnace carbon tape, KI, SrF2, and CsI references.