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 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- acid used co-doped with 0.2% of one of the following: C ,CO3 ,N , 2− − 3− 2− 2− − − to convert 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 , but these may also be removed as HI(g) [10]Ð[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]Ð[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]Ð[6]. Its limitations include hygroscopicity, gated according to the Hume-Rothery rules of solid , 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 onlyÐ 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. chromatography lined with sodium-filled heat spreaders. The gradient between and inductively-coupled plasma optical-emission spectroscopy the zones was 12◦C/cm. The ampoules were radially symmet- (ICP-OES) measurements were performed by Sigma-Aldrich, ric such that they were equidistant from the inside diameter Urbana. of the heat spreader, and from each other. To accommodate III. RESULTS small variations (<4 mm) in ampoule lengths, they were hung A. Crystallinity well above the previously-calibrated freezing location, and also translated an additional 3 cm at the end of growth. Because the In addition to the co-doped crystals, a standard of diameters of the crystals are small, this multi-ampoule method SrI2(4%Eu) was made. Figure 1 displays the 11 crystals as- typically generates uncracked crystals, or crystals with just 1-2 grown, in their ampoules. cracks, depending on their purity [14]. The standard possessed dark spots on its perimeter, but was otherwise uncracked. The graphite-doped crystal had dark deposits at the tail and 4 parallel cracks that propagated B. Crystal Impurity and Performance Analysis along a cleavage plane, about 60◦ from the growth axis. Oxygen combustion analysis was performed in a custom Yet the carbonate-doped crystal was clear and uncracked. ELTRA ON900 installed at CapeSym. The system’s furnace The white lines evident in its photograph are not cracks but 2832 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 63, NO. 6, DECEMBER 2016

Fig. 2. (a) Red deposits at tail of sulfur-doped boule, which are likely CI4. (b) Deposits believed to be metallic iodine at tail of chlorine-doped boule.

Fig. 3. Light yield (137Cs) from co-doped crystals. Estimated light yield uncertainty is ±2000 ph/MeV.

uncracked. The hydroxide-doped sample exhibited a large radial crack that bisected the boule. In addition it displayed adhesion to the quartz ampoule during later removal. The phosphate-doped boule was clear, but exhibited two cracks, located 5 and 7 centimeters from the nose. The sulfur-doped sample was extremely cracked, and orange at the tail. The cracking pattern was consistent with a concave growth interface, suggesting impurity rejection that served to depress the freezing point in a radial pattern. Dark red deposits were noted at its head and tail, which are likely carbon- tetraiodide, Figure 2(a). In contrast, the sulfate-doped sample was clear and uncracked. However the last 1.5 centimeters of the sulfate-doped boule was cloudy white. The chlorine-doped sample was notably orange at the head and tail, and metallic iodine crystals appeared to be present at the vacant top of its ampoule, Figure 2(b). The bulk of the crystal was clear. The bromine-doped crystal displayed dark deposits at its perimeter and possessed a large crack about 2 centimeters from the crystal’s tail.

Fig. 1. Photographs of standard 1-cm φ SrI2(4%Eu) crystal and co-doped crystals as they appear in their quartz boule, after growth. The first-to-freeze B. Crystal Scintillation Performance section is left. Dopants are noted on each photograph. Using material located 3-4 centimeters from the nose of each boule, 10-milimeter-diameter cylinders were fashioned. periodic (∼ 1 cm), benign deposits at the perimeter. These Our previous work showed that scintillator samples achieved white rings are also present in the O-doped and SO4-doped an optimal 2.8% energy resolution when 2 millimeters thick or crystals. They are likely oxygen rich, but we have not deter- less, because the effects of Eu2+ self-absorption and scattering mined why they form at one-centimeter intervals. centers became minimized. Therefore these samples were The nitride-doped sample possessed some filament-shaped, made 6-8 mm thick, to better observe the effects of co-dopants. dark deposits at its perimeter, and its tail was cloudy and Figures3Ð5displaythemeasuredlightoutput,resolution, chartreuse in color. The oxygen-doped sample was clear and and decay times. Past analysis showed that axial variations SWIDER et al.: THE INFLUENCE OF LIGHT ANION IMPURITIES UPON SrI2(EU) SCINTILLATOR CRYSTALS 2833

Fig. 4. Energy resolution % (137Cs) of co-doped crystals. Estimated energy resolution uncertainty is ±0.2% (absolute percent).

Fig. 6. Emission spectra of five co-doped samples, and the standard, as measured in the Horiba system.

benign fashion with regard to scintillation. One exception was the sulfur-doped sample, which possessed cracks that exacerbated performance. (A thin, uncracked S-doped detector displayed over 80,000 ph/MeV and 3.5% resolution.) The graphite-doped and nitrate-doped crystals were also performed below nominal, although these detectors possessed no cracks. The decay times were all similar.

C. Emission Wavelength Emission wavelengths of the detectors were measured on the in-house spectroscopy system, using an airtight jig with aCaF2 window. Despite surface discoloration in the sulfur, chlorine, and bromine doped boules, emission still peaked at 430 nm, which corresponds to the 5d→ 4f Eu2+ transition. Fig. 5. Decay time (137Cs) of co-doped crystals in microseconds. Estimated uncertainty for decay time is ± 0.2 μ s. Only the nitride-doped material displayed secondary emission. A cylinder fashioned from the cloudy, greenish tail-end exhib- TABLE II ited a small peak centered near 480 nm. Figure 6 shares the emission curves. SCINTILLATION PERFORMANCE SUMMARY D. Oxygen Content After growth, residual oxygen was measured via combustion analysis in the ELTRA apparatus at CapeSym. As noted above, the oxygen content of the anhydrous beads was approximately 600 mol ppm (or 0.06 mol%). After growth, the SrI2(Eu) standard possessed 1500 mol ppm oxygen. Oxygen content of grown crystals was found to be highest at the perimeter and tail. For instance, oxygen values read 2-to-3 times higher if pieces from the crystal’s perimeter were included in the combustion assay. Therefore, boules were dry-sanded with 400-grit silicon carbide paper to remove deposits prior to testing. This step reduced the occurrence of spurious high readings. 2− − 2− Post-growth oxygen content in the O ,OH ,CO3 ,and 2− in Eu content contribute 2.3% (2000 ph/MeV) to light yield SO4 -doped crystals were studied in detail, along with an variance [14]. Table II summarizes the data. undoped standard made with the same SrI2 and EuI2 lots. Most of the crystals were as bright as the un-doped stan- In the co-doped boules, initial O values were about 600 mol dard, if not more so. This result suggests that the anion ppm (background), plus 2000, 2000, 6000, and 8000 ppm, co-dopants either were rejected, or were incorporated in a respectively. In the Eltra glove box, the boules were cleaved 2834 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 63, NO. 6, DECEMBER 2016

TABLE III RESIDUAL ANION IMPURITY LEVELS MEASURED IN CO-DOPED SrI2 (Eu) CRYSTALS.SAMPLES COLLECTED MID-BOULE.

Fig. 7. Residual oxygen content as measured along the length of doped during the aqueous dissolution process for ion chro- SrI2:Eu crystals. Inset: Photograph of CO3-doped crystal showing approxi- matography. Therefore the ion chromatography was deemed mate locations of the axial samples. inconclusive for the nitride-doped boule. As noted in Section C above, oxygen levels were about at 1-cm intervals along the length, in order to examine axial O 2000 mol-ppm (0.2%) regardless of the added amount. concentration. Oxygen values were determined by averaging No phosphate was measured in the PO4-doped sample, which of 3-4 assays at each location. Figure 6 displays the resulting is consistent with that boule’s clear appearance. Sulfur read data. The sharp upward curve at the 8-cm location represents 256 ppm mol in the S-doped crystal. Sulfate values read − very high oxygen readings (ranging from 2% to over 14%) about 90 ppm mol in the SO 2Ðdoped crystal, and also in − 4 collected from white and dark deposits at the very tails of the the N−3,PO 3,Cl−, and bromine-doped crystals, indicating co-doped boules. 4 that sulfates are present in the SrI2 and EuI2 precursors, and Three trends are evident in Figure 7. First, oxygen con- their solid-solubility is about 90 ppm mol. Because the crystals tent measured 1500-2000 mol ppm (0.15-0.20%) on average, were clear and performed well, we conclude SO4 is benign at irrespective of the original amount added. Second, in the 90 ppm levels. co-doped boules, oxygen values remained fairly consistent At 1800 and 2500 mol ppm, respectively, residual chlorine along the length of the boule, but rose sharply in the tail and bromine values remained somewhat high, as compared to deposits. Third, the undoped boule exhibited oxygen values the original doping value of 4000 mol ppm. as high as, and even higher than the oxygen-doped boules; No suitable method for trace carbon analysis was found. but the concentration did not rise as much at the tail. The data Energy dispersive spectroscopy proved useful for determining suggest that excess oxygen exited the melt via volatilization the presence of carbon in dark tail deposits (discussed below), or segregation, until the solubility level was reached. Volatile but this method was not capable of quantitative analysis at ppm oxygen-rich species likely re-deposited at the tail (and perime- levels. Surface measurements such as XPS are not always reli- ter) upon cooldown. able with SrI2, as surfaces readily decompose during handling. Analysis at the midsection of the C, N, Cl, and Br-doped boules yielded 0.15%, 0.20%, 0.18%, and 0.14% oxygen, F. Energy Dispersive Spectroscopy of Black Tail Deposits respectively, confirming the baseline oxygen value for this mini-crystal process was between 0.15% - 0.20%, whether or With the exception of the sulfide, sulfate, nitride, and carbonate-doped samples, the grown crystals possessed a layer not the co-dopant had an oxygen compound. The PO4-doped boule also contained 0.20% oxygen at the midsection. of black ‘smut’ at the last-to-freeze section (tail). When crystal growth is observed in a transparent furnace, this dark material can be seen to agglomerate then float to the top of the E. Additional Post-Growth Impurity Measurements melt within the first few hours after fusing. When removed After crystal growth, residual nitrate, sulfate, phosphate, from the ampoule and exposed to air, the dark smut does chlorine, and bromine levels were measured via ion chro- not display spontaneous reactions or deliquescence in air. matography. Residual sulfur content in the SrS-doped sample Therefore it does not appear to consist solely of strontium was measured by ICP-OES. Samples were collected from the metal or strontium iodide. In ELTRAmeasurements, oxygen midpoint of the boule, about 4-cm from the nose. Table III content in the smut was high, often over the apparatus’ upper below compares the residual quantity to the amount added, detection limit of 14%. In an effort to ascertain the content and to the undoped standard. An example from the oxygen of the dark smut, samples were collected from the O-doped data is added for comparison. Values below the instrument boule and the standard, and analyzed via energy dispersive detection limit have been labeled “ND”. spectroscopy (EDS). Figure 8 shares a representative result. Nitrates and nitrites were not found in any of the grown The EDS recorded carbon, oxygen, strontium, and iodine samples, including the nitride-doped crystal. However, the peaks. Given its high melting point (it is observed floating green-yellow discoloration and spurious emission at the tail of upon melts at temperatures over 750◦C), low density, and the Sr3N2-doped crystal suggests some impurity was present. low deliquescence, we may predict that the composition con- Strontium nitride would not necessarily form nitrate or nitrite tains strontium carbide (SrC2), strontium oxalate (SrC2O2), or SWIDER et al.: THE INFLUENCE OF LIGHT ANION IMPURITIES UPON SrI2(EU) SCINTILLATOR CRYSTALS 2835

Fig. 9. (a) Graphite-doped SrI2(Eu) boule as-removed from its ampoule. (b) SrCO3-doped boule as-removed from its ampoule. The white rings are benign deposits.

substitutions should be considered prevalent [18]. Therefore, when considering mechanisms for anion incorporation, or lack thereof, all of these factors (degree of ionization, liquid and solid solubility, volatility, ionic radii, and net charge balance) should be taken into account. In addition, we must consider interactions with background carbon, and the tendency for the deliquescent SrI2 to lose iodine to hydration, as oxygen has higher electronegativity than iodine: Fig. 8. EDS analysis of black ‘smut’ collected from the tail of an SrI2(4%Eu) standard shows carbon and oxygen peaks, as well as strontium and iodine. SrI + H O → SrI H O(a) Inset: photograph of black smut collected for EDS. 2 2 2 2 heat SrI2H2O → SrO + 2HI↑ (b) graphite, as well as and iodine re-condensed from vapor. The carbon may be a byproduct of strontium Finally we must consider harmful side reactions, such as mineral mining from shale, which can contain 1-3% organic alkali-hydroxide reactions with quartz. Although there are carbon [16]. many factors at play, comparing doped sets can help elucidate the results of the light impurity doping. IV. DISCUSSION Carbon vs. Carbonate: The graphite-doped boule was In this study we have found SrI2(Eu) scintillation perfor- cracked and degraded, yet the carbonate-doped boule was mance to be largely unaffected by the addition of certain light clear and exhibited excellent performance. The difference may impurities. In particular, oxygen-rich (oxygen, carbonate, sul- be attributed to the excess oxygen, which helped volatilize fate, phosphate) and halide (chlorine and bromine) impurities carbon in the latter case. Given the high clarity of the boule, 2− 2− did not degrade performance relative to the undoped standard. we presume the CO3 ionized to CO2↑ and O ,which In some cases these additives appeared to improve scintillation. assisted the removal of background carbon and sulfur via 0 −2 −3 Anaerobic dopants C ,S ,N were more problematic, as oxidation to CO↑ and SO2↑. By contrast, the graphite-doped − was OH . boule possessed dark precipitates, cracks, and poor resolution, In our previous study with cation doping, we observed that suggesting that residual carbon either precipitated within the metallic impurities segregated to the melt when their ionic crystal, or formed strontium carbide, or formed strontium radius was more than 15% larger or smaller than strontium’s, oxalate. The black color of the precipitates suggest the former. per the Hume-Rothery rules for solid-solubility. With anion Figures 9(a) and 9(b) compare these boules as-removed from impurities, the relationship is not as straightforward. Segrega- their quartz containment ampoules. tion is not the only mechanism for impurity removalÐ as many Oxygen vs. Hydroxide: The oxygen-doped boule was clean light impurities also possess low boiling points. with above-average performance. The OH− doped sample Because molten SrI2 is an acidic, ionic liquid, we presume also displayed above-average light output, yet the boule the added dopants presented here dissolved and ionized. Yet it was cracked, and detector resolution was slightly degraded. is not obvious to what degree a dopant such as SO4 becomes We attribute the cracking to adhesions caused by the presence ◦ −2 ionized at 640 C in molten SrI2. It may remain as SO4 ,or of strontium hydroxide. Like most alkali hydroxides, Sr(OH)2 break into components such as: reacts with quartz at elevated temperatures. Adhesions to the − growth ampoule are known to cause strain and cracking. SO + O 2 (i) 3 Sulfur vs. Sulfate:The sulfide-doped boule was highly +2 + SO2 O2 (ii) cracked, suggesting high levels of strain was accumulated by −2 S + O2 + O2 (iii) sulfur substitution for iodine. The 16.3% difference in their − − − − − atomic radii is close to the Hume-Rothery guideline [19]. S 2 + O 2 + O 2 + O 2 + O 2 (iv) However the oxidation state of sulfur is -2 and that of iodine Molten salts do possess local order, such as ion pairs, due is −1. To maintain charge balance in the matrix, interstitial to coulombic screening [17]. Once solidified, ionic crystals strontium, interstitial metallic impurities, or iodine vacancies require net charge of zero, therefore only charge-balanced must be generated. The red deposits at the tail after growth 2836 IEEE TRANSACTIONS ON NUCLEAR SCIENCE, VOL. 63, NO. 6, DECEMBER 2016 suggest the latterÐ the crystal rejected iodine as it incorporated sulfur. The rejected iodine appears to have then combined with residual carbon to make CI4, a bright red compound. In the case of SrSO4 doping, it appears the sulfate did not ionize to the extent of making S−2. The white cloudiness at the tail, the residual oxygen profile, and the lack of dark deposits at the tail indicate the sulfate either segregated or volatilized. Most likely it formed SO2 and O2, and the excess oxygen combined with residual carbon to make volatile CO2. The cloudiness in the last inch of growth may be attributed to dissolved vapors, per Henry’s Law. The dissolved vapors may have also suppressed the freezing point. Nitrate and Phosphate:The nitride doped sample had no dark smut at the tail, suggesting a cyanide (CN) compound was formed with dissolved nitrogen and background carbona- Fig. 10. Boiling points of various light impurities. ceous impurities. The resulting cloudy-green tail material also exhibited parasitic emission. Despite the very high level of oxygen in the phosphate- clear and performed above average. We attribute the advantage doped boule, black smut was still deposited at the tail. The to the high volatility of oxygenated impurities. For instance, smut may have contained black phosphorous. The ionic radius the boiling point of sulfur is 717K, but that of SO2 is 283K. of P+5 is 68% smaller than Sr2+, and the ion is aliova- Carbon sublimes at 4000K, but CO2 is volatile at 194.5K. lent, therefore we predict phosphorus would segregate almost Figure 10 illustrates this effect, by comparing the boiling 3− 5+ entirely, if the PO4 ionized into P and oxygen. This possi- points of various light impurities. Low boiling points indicate bility is reflected in the ICP-MS data, where phosphate content low intermolecular forces, and high volatility. measured below the detection limit. Given the segregation of Anaerobic sulfur impurities proved to be the most harmful, the phosphorous, the added oxygen would form O2↑ as well perhaps due to aliovalent substitution of S2− for I1−.Sulfur as CO↑,CO2↑,SO↑,andSO2↑ with the residual carbon and incorporation at levels of 250 mol-ppm induced cracking and sulfur. The tail smut was not collected for EDS analysis. Future discoloration. Other anaerobic dopants such as C0 and N3− studies will repeat this test and assess its smut composition. were also harmful to crystallinity and performance. Note that Chlorine and Bromine:These dopants displayed substitution sulfur, carbon, and nitride content are not traditionally assayed in accordance with Hume-Rothery rules for solid solubility. in certificates of analysis. Yet, ion chromatography and EDS They are isovalent, and more electronegative than iodine. indicated sulfur and carbon impurities are present in SrI2 and Moreover, chlorine’s ionic radius is within 18% of iodine’s, EuI2 precursors. Therefore it is incumbent upon the crystal and bromine’s ionic radius is within 11% of iodine’s. Bromine grower to apply methods for volatilizing light impurities. Many was incorporated more readily than chlorine, as would be will evaporate on their own once the assay is melted, such predicted. The 4000 ppm doping level for bromine led to as when a melt is fritted. Melts may also be treated with residual values of 2500 ppm at the boule’s midpoint. Near hydrogen gas to assist impurity volatilization, e.g. by forming the tail, the level of substitution appears to have been to CH4 and H2S. be too highÐ a large crack formed, perhaps due to lattice Finally, doping SrI2(Eu) with chlorine and bromine showed strain. That said, the dark deposits along of the perimeter no detrimental effects on performance, despite apparent sub- of the bromine-doped boule also indicate rejection of SrBr2. stitution for iodine in the lattice. In fact the scintillation Orange deposits on the perimeter of the chlorine-doped boule performance was above average. The higher electronegativity suggest microcrystalline SrCl2, as well as the usual oxygen- of these halides may be helpful for scavenging impurities. rich deposits. Alternately, their smaller ionic diameters may provide strain One notable variance in the chlorine-doped boule was the relief. However, levels of 4000 ppm Br− and Cl− appear to be iodine deposits on the ampoule above the crystal (Figure too high, particularly in the case of the bromine-doped boule, 2(b)). The charge-neutral mode of doping relative to the which cracked near the tail. lattice (via SrCl addition) predicts no iodine rejection. One 2 Volatilization of impurities from molten SrI2 may be possible explanation may be the higher electronegativity of inspected via residual gas analysis (RGA), and shall be a chlorine combined with a nominal iodine excess. Secondary subject of future study. halide reactions would favor the chlorine, leaving the iodine unreacted with volatilized and segregated impurities. ACKNOWLEDGMENTS V. C ONCLUSION The authors thank C. Cui for performing ion chromatog- Despite its notoriety, oxygen can have beneficial impact raphy and ICP-OES measurements, as well as other mem- − upon SrI2(Eu) growth, provided it is not in the OH or H2O bers of the Sigma-Aldrich team, L. Coers, E. Donohoe, form. Its solid-solubility in SrI2(4%Eu) appears to be near S. Spencer, and S. Taylor, for helpful discussions regarding 0.2%. Crystals containing this level of residual oxygen were impurities in the starting materials. Also, they wish to thank SWIDER et al.: THE INFLUENCE OF LIGHT ANION IMPURITIES UPON SrI2(EU) SCINTILLATOR CRYSTALS 2837

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