Characterization of mixed halide scintillators: CsSrBrI2:Eu, CsCaBrI2:Eu and CsSrClBr2:Eu

Author’s Accepted Manuscript Characterization of Mixed Halide Scintillators: CsSrBrI2:Eu, CsCaBrI2:Eu and CsSrClBr2:Eu L. Stand, M. Zhuravleva, B. Chakoumakos, H. Wei, J. Johnson, V. Martin, M. Loyd, D. Rutstrom, W. McAlexander, Y. Wu, M. Koschan, C.L. Melcher www.elsevier.com/locate/jlumin

PII: DOI: Reference:

S0022-2313(18)31681-8 https://doi.org/10.1016/j.jlumin.2018.10.108 LUMIN16056

To appear in: Journal of Luminescence Received date: 13 September 2018 Revised date: 23 October 2018 Accepted date: 25 October 2018 Cite this article as: L. Stand, M. Zhuravleva, B. Chakoumakos, H. Wei, J. Johnson, V. Martin, M. Loyd, D. Rutstrom, W. McAlexander, Y. Wu, M. Koschan and C.L. Melcher, Characterization of Mixed Halide Scintillators: CsSrBrI2:Eu, CsCaBrI2:Eu and CsSrClBr2: E u , Journal of Luminescence, https://doi.org/10.1016/j.jlumin.2018.10.108 This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting galley proof before it is published in its final citable form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Characterization of Mixed Halide Scintillators: CsSrBrI2:Eu, CsCaBrI2:Eu and CsSrClBr2:Eu L. Stand1, M. Zhuravleva1, 2, B. Chakoumakos4, H. Wei1, 2, J. Johnson1, 2, V. Martin1, 3, M. Loyd1, 2, D. Rutstrom1,2, W. McAlexander1, 2, Y. Wu1,2, M. Koschan1, and C. L. Melcher1,2,3 1 Scintillation Materials Research Center, University of Tennessee, Knoxville, Tennessee, USA 2 Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee, USA 3 Department of Nuclear Engineering, University of Tennessee, Knoxville, Tennessee, USA 4 Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, TN, USA Abstract 2+ Eu -doped mixed halide scintillators, CsSrBrI2, CsCaBrI2 and CsSrClBr2 were studied for gamma-ray spectroscopy applications. We grew Ø13 mm single crystals via the vertical Bridgman method using a self-seeded technique and investigated physical and scintillation properties. Single crystal X-ray diffraction and thermal analysis reveal that CsSrBrI2 and CsSrClBr2 have orthorhombic crystal structures and melt congruently at 611°C and 752°C, respectively, 2+ while CsCaBrI2 has a cubic structure and melts congruently at 671°C. All three compounds have the typical Eu 5d – 4f single-peak emission centered between 445 and 462 nm with a scintillation decay time of a few microseconds. The 2+ optimal Eu concentration to maximize the scintillation light yield was determined to be ~7-10 %. All three mixed scintillators have an improved energy resolution compared to the non-mixed CsSrI3:Eu, CsCaI3:Eu and CsSrBr3:Eu. The best performing scintillator is CsSrBrI2:Eu 7% with light yield of 65,300 ph/MeV and energy resolution of 3.4% at 662 keV. Excellent proportional response for a wide range of gamma-ray energies was measured for all three scintillators. Keywords Halide scintillators, Single crystal, Bridgman technique, Radiation detection

1. Introduction Scintillators for nuclear nonproliferation applications require excellent energy resolution to distinguish the gamma-ray signatures of potentially dangerous radioactive sources from non-threat radioactive sources. In the last decade, several metal halide scintillators from the perovskite-like ABX3:Eu2+ compositional family, where A is Cs+ or K+, B is Sr2+, Ca2+ or Ba2+, and X is a halogen (Cl, Br, or I), have shown excellent scintillation properties. Reported light yields up to 73,000 ph/MeV and energy resolutions as low as 3.4% at 662 keV make them promising candidates for national security applications [1-11]. Compositional engineering has been successfully used in various oxide and halide scintillators to tune their electronic structure and create a favorable environment for efficient electron-hole recombination at the luminescence centers [12-25]. Thus, improvements of the light yield, energy resolution and nonproportionality of CsI:Tl and BaI2:Eu2+ were achieved via isovalent substitution in mixed CsBr(x)I(1-x):Tl and BaBr(x)I(2-x):Eu compounds [16-19]. Partial bromine replacement in CsCaBr(x)I(1-x)3:Eu stabilized the hightemperature crystal structure and suppressed the low-temperature phase transition, which improved crystal quality and enabled large-scale crystal growth [26]. Solid solutions were found to exist in the CsCa(Cl,Br)3 system and resulted in significant improvement of light yield over the non-mixed scintillators CsCaCl3:Eu and CsCaBr3:Eu [27]. Correlation between scintillation properties and halogen substitution in the majority of inorganic ABX3 compounds has not been studied, and this presents an opportunity for scintillator discovery. In this work we further investigate crystal growth and physical and scintillation properties of the previously reported scintillators CsSrI3:Eu, CsCaI3:Eu and CsSrBr3:Eu, where one of the three halogen atoms is replaced with bromine or chlorine to make mixed halides CsSrBrI2:Eu, CsCaBrI2:Eu and CsSrClBr2:Eu. Our objective was to determine whether mixing of the halogen atoms can form solid solutions and yield efficient scintillation performance. We present results of thermal analysis and report crystallographic information of the mixed compounds obtained via single crystal X-ray diffraction. We also evaluated energy resolution, scintillation light yield and non-proportionality, which are the key parameters for gamma-ray spectroscopy applications.

1

2. Experimental 2.1. Crystal Growth Single crystals of CsSrBrI2:Eu2+, CsCaBrI2:Eu2+ and CsSrClBr2:Eu2+ were grown in evacuated quartz ampoules via the vertical Bridgman technique. Due to the deliquescent nature of these compounds, precursors and grown crystals were stored in a Mbraun glovebox that has moisture and oxygen levels below 0.5 ppm. Anhydrous raw materials in bead form with purity of at least 99.99% were mixed and loaded in stoichiometric quantities into quartz ampoules. The loaded ampoules were dried at 200oC and sealed under vacuum at 10-6 torr. The crystal growth experiments consisted of two stages (1) Eu2+ concentration optimization experiments and (2) scale up of the optimized composition. For the Eu2+ concentration optimization experiments, Ø6 mm single crystals of CsSr(1-x)Eu(x)BrI2, CsCa(1-x)Eu(x)BrI2, and CsSr(1-x)Eu(x)ClBr2 (x = 0.03, 0.05, 0.07 and 0.10) were grown in a three-zone Bridgman furnace, using pulling rate of 4 mm/h and cooling rate of 20°C/h. Once the optimal Eu2+ concentrations were determined, Ø13 mm single crystals were grown in a two-zone transparent furnace using pulling rates of 1 – 2.5 mm/h and cooling rates of 5 – 10°C/h, Table 1. An insulating diaphragm was placed between the hot zone and cold zone of the furnace to produce the desired thermal gradient [28, 29]. The growth process was initiated with a Ø2 mm grain selector connected to the bottom of the ampoule, in which the self-seeding process took place. It is worth noting that all Eu2+ doping levels mentioned in the text are nominal concentrations and are molar percentages. Since the ionic radius of Eu2+ is much bigger than that of Ca2+, Eu segregation is expected in the Ca-containing compounds during directional solidification, leading to a concentration gradient in the grown crystal [3]. Such effects should be negligible in the Sr-containing compounds due to similar ionic radii of Sr2+ and Eu2+. To avoid inconsistencies in the characterization results all specimens were prepared from the same axial position in the boule. Table 1. Growth parameters and precursor materials used in this work. Reaction Hot zone/Cold Pulling rate Precursors Product zone (°C) (mm/h) CsBr + SrI2 CsSrBrI2 680/550 2.5 CsBr + CaI2 CsCl + SrBr2

CsCaBrI2 CsSrClBr2

720/600 830/700

1 1

Cooling rate (°C/h) 10 5 5

2.2. Characterization A variety of techniques were used to measure the physical and the scintillation properties of CsSrBrI2:Eu, CsCaBrI2:Eu and CsSrClBr2:Eu. The melting and crystallization temperatures were determined by differential scanning calorimetry (DSC) using a Setaram Labsys Evo TG-DTA/DSC analyzer. The measurements were carried out on ~50 mg specimens at a heating/cooling rate of 10 K/min, under a flow of ultra-high purity argon gas. Densities were determined using the Archimedes method. Due to the hygroscopic nature of the crystals, mineral oil was used in place of water. To determine the specific density of the mineral oil, a Lu2SiO5 crystal was used as a standard due to its well-known density of 7.4 g/cm3. Measurements were conducted using a Sartorius CP324s analytical balance. Single-crystal X-ray diffraction (XRD) was measured using a Rigaku XtaLAB PRO diffractometer equipped with a Mo K-alpha source, a Dectris Pilatis 200K detector, and Oxford N-Helix cryocooler. Measurements were conducted on crystals approximately 0.001 mm3 in size that were suspended in oil inside a Ø200 µm polyimide loop. Sample temperature was maintained at 250 K while under a continuous flow of nitrogen. Rigaku CrysAlisPro software was used for initial peak indexing and integration. The structures were solved and refined using the SIR-2011 module in WinGX and SHELX software packages. An R1 agreement factor of 0.0380, 0.0505, and 0.0419 were achieved for CsSrBrI2, CsSrClBr2, and CsCaBrI2, respectively. Crystal structure projections were produced with VESTA visualization software.

2

The luminescence and scintillation properties were measured at room temperature, using specimens ranging from 0.016 to 1.3 cm3. The specimens were placed in a quartz container filled with mineral oil to protect them from deliquescing during these measurements. The radioluminescence (RL) spectra were measured under continuous 30 keV X-ray irradiation. The emission spectra were measured in reflection mode to minimized self-absorption effects and the emission light was recorded using with a 150-mm focal length monochromator over a wavelength range of 200 to 800 nm. The steady state photoluminescence (PL) spectra and the PL decay time (lifetime) were measured using with a Horiba Jobin Yvon Fluorolog 3 Spectrofluorometer. The excitation spectrum was recorded over a wavelength range of 250 to ~430 nm, as the crystal emission was monitored. The PL emission spectrum was collected over a wavelength range of 380 to 500nm, using 370 nm as the excitation source. The PL lifetime was measured using the time correlated single photon counting technique. A Horiba Jobin Yvon NanoLed with an emission wavelength of 370 nm and a pulse width of ~1 ns was used as an excitation source. The emission monochromator was set to 449 nm for CsSrBrI2:Eu, 452 nm for CsCaBrI2:Eu and 441 nm for CsSrClBr2:Eu to monitor the Eu2+ emission intensity. The pulse height spectra were collected using a standard bialkali Hamamatsu R2059 photomultiplier tube (PMT) or super bialkali R6231 - 100 PMT connected to a Canberra 2006 pre-amplifier, an Ortec 672 amplifier, and a Tukan 8K multi-channel analyzer. A hemispherical dome of Spectralon was used to improve the scintillation light collection and a 10 μs shaping time was used to ensure the complete integration of the light pulse. The absolute light yield (LY) was measured using the R2059 PMT. Mineral oil was used to couple the crystal to the PMT, and a 137Cs source was used as the excitation mode. The number of photoelectrons was calculated from the centroid position of the 662 keV photopeak, using the single photoelectron technique [30-32]. The energy resolution and the non-proportionality (nPR) of the crystals were measured using the R6231-100 PMT with a standard set of γ-ray sources, 137Cs, 22Na, 133Ba, 57Co and 241Am. The energy resolution was defined as the Full Width Half Maximum over the centroid of the photopeak of energy E (R=ΔE(FWHM)/E [33]). The nPR or relative light yield was defined as the ratio between centroid position of a photopeak of energy E and centroid position at 662 keV. The scintillation decay time was measured under irradiation from a 137Cs sealed source with the timecorrelated single photon counting technique [34]. 3. Result and Discussion 3.1. Physical Properties and Crystal Growth 3.1.1.Melting Point Determination Thermal analysis measurements were carried out to determine melting and crystallization points, and the results were used to design the hot and cold zones in the growth furnace. The DSC curves of all investigated compounds have singular endothermic and exothermic peaks, which indicate congruent melting behavior, as shown in Figures 1a - c. No secondary phases were detected. Melting points of 611°C, 752°C, and 671°C, and a degree of supercooling that ranged from 33 to 52°C were measured for CsSrBrI2, CsSrClBr2 and CsCaBrI2, respectively. As expected, the mixed crystals had a slightly lower melting point than their nonmixed counterparts, as shown in Table 2. Similar behavior has been reported in other scintillators such as and La0.97Ce0.03(Cl0.05Br0.95)3 and BaBrI:Eu [14, 35]

3

200 0.8

600

800 

Tc = 559 C

CsSrBrI2

0.6

Normalize Heatflow (mW)

400

0.4

Cooling

0.2 Tm = 611 C

Heating

0.0 a.

Tc = 717 C

CsSrClBr2

0.8 0.6 0.4 0.2

Tm = 752 C

b. 0.7

0.0

Tc = 632 C

CsCaBrI2

0.5 0.2 0.0 c.

Tm = 671 C

200

400

600

800



Temperature ( C) Figure 1a-c. DSC curves showing the melting points (Tm) and crystallization points (Tc) of the mixed halide scintillators investigated in this work. Table 2. Melting point of mixed halides and pure compounds ABX3 compounds CsSrI3 CsSrBr3 CsCaI3

Tm (ᵒC) 645 [36] 760 [11] 686 [5]

ABX’X’’2 Compounds CsSrBrI2 CsSrClBr2 CsCaBrI2

Tm (ᵒC) 611 752 671

3.1.2. Crystal Growth All crystals were colorless with small purple tint due to the strong Eu2+ luminescence excited by normal day light. No dominant cleavage planes were observed. The thin capillary aided in seeding, however it did not prevent crystals from minor cracking. The last-to-freeze portion of the boule was relatively small in all three compounds, which points to absence of phase decomposition. As-grown Ø13 mm boules in quartz ampoules and small polished samples that were used for scintillation measurements are shown in Figures 2, 3 and 4. CsSrBrI2:Eu 7% was transparent and displayed periodic cracking perpendicular to the growth direction, Figure 2. These cracks occurred during cooling and they are likely due to the increased stress present in the crystal from the fast pulling and cooling rate used, 2.5 mm/h and 10 °C/h. These results led us to decrease the pulling and cooling rates to 1 mm/h and 5 °C/h, respectively, for the growth experiments of CsCaBrI2:Eu 7% and CsSrClBr2:Eu 10%. Most metal halide scintillators are known for their poor mechanical properties, and cracking is a typical issue during crystal growth and machining. Literature indicates that crack-free single crystals of SrI2:Eu and LaBr3:Ce can be grown using even slower pulling rates of < 1 mm/h [37-39]. The single crystal of CsCaBrI2:Eu 7% was crack-free, but had a translucent appearance with white growth defects on the surface. These defects formed when the crystal abruptly detached from the ampoule wall during cool down. This allowed halide vapors to condense on the crystal surface, giving it the translucent appearance. The use of pyrolytic carbon coating on the inner surface of the growth ampoules has been shown to reduce ampoule adhesion resulting in a significant reduction in cracking [10, 40]. The interior of

4

the boule had good optical clarity as evidenced by the polished 8 × 8 × 20 mm3 specimen shown in Fig. 3 – right. Of the three compounds, CsSrClBr2:Eu 10 % had the lowest optical clarity, and the cloudy appearance was uniform throughout the boule, Figure 4. The cloudiness in non-mixed CsSrBr3:Eu crystals has been previously ascribed to Sr-rich inclusions, and it was successfully overcome by adjusting the stoichiometry of melt composition [11, 41]. We hypothesize that the same approach would be beneficial in case of CsSrBrI2:Eu 7% and CsSrClBr2:Eu 10%. The CsSrClBr2:Eu 10 % crystal has the same cracking pattern as CsSrBrI2:Eu2+ despite the fact that it was grown at a much slower pulling rate.

Figure 2. Left – Ø13 mm single crystal of CsSrBrI2:Eu 7% inside a quartz ampoule. Right – a polished 5 × 5 × 7 mm3 specimen under normal fluorescent light. Note that this crystal was grown using a pulling rate of 2.5 mm/h and cooling rate of 10 °C/h

Figure 3. Left – Ø13 mm single crystal of CsCaBrI2:Eu 7% inside a quartz ampoule. Right – a polished 8 × 8 × 20 mm3 specimen under normal fluorescent light.

Figure 4. Left – Ø13 mm single crystal of CsSrClBr2:Eu 10 % inside a quartz ampoule. Right – a polished 6 × 6 × 13 mm3 specimen under normal fluorescent light. 3.1.3. Structural Analysis Single-crystal XRD analysis was used to determine crystal structures of the mixed halide compounds. A summary of crystallographic data is presented in Table 3. Atomic positions for CsSrBrI2, CsCaBrI2, and CsSrClBr2 are listed in Tables 4, 5, and 6, respectively. CsSrBrI2 crystallizes in the orthorhombic structure with Pnam space group, which resembles the roomtemperature orthorhombic Pnma phase of CsSrBr3 and the orthorhombic CmCm structure of CsSrI3 [42]. Two separate anion sites were discovered in CsSrBrI2, each being fractionally occupied by both Br- and I-. The first site has a Br-/I- ratio of 0.446/0.554 and the second has a ratio of 0.266/0.734. As shown in Figure 5, the anion sites form an octahedron around Sr2+. CsSrClBr2 was also found to have an orthorhombic structure with space group Pbmn, which is similar to the room-temperature orthorhombic Pnma phase of CsSrBr3 and the room-temperature phase of CsSrCl3 [42, 43]. Similar to CsSrBrI2, two anion sites were found in CsSrClBr2, which are shared by both Cl- and Br- ions. The Cl-/Br- ratio for site 1 is 0.291/0.709 and is 0.287/0.713 for site 2. The Sr2+ ions are coordinated inside Cl/Br- octahedra, as shown in Figure 6. CsCaBrI2 has a cubic perovskite structure with space group Pm-3m, which is similar to that of CsCaBr3 (Pm3m) [45]. This is quite different from the crystal structure of a non-mixed compound CsCaI3, which has a

5

room-temperature orthorhombic Pnma phase [42] and a high-temperature tetragonal P4/mbm phase [46]. CsCaBrI2 contains only one anion site, with a Br-/I- ratio of 0.36/0.64, which is very close to the nominal ratio. Anion sites form an octahedron around Ca2+, as shown in Figure 7. Due to the structural similarity of CsSrBrI2 and CsSrClBr2 to that of the CsSrBr3 parent compound, cracking and cloudiness of these crystals may be ascribed to secondary-phase inclusions or to strong thermal anisotropy which were recently observed in CsSrBr3 using high-temperature powder XRD studies [11, 45]. Table 3. Crystallographic parameters of CsSrBrI2, CsCaBrI2, and CsSrClBr2 Formula CsSrBrI2 CsCaBrI2 CsSrClBr2 fw (g) 554.24 506.70 415.8 Crystal System Orthorhombic Cubic Orthorhombic T (K) 250 250 250 Space Group Pnam Pm-3m Pbmn a (Å) 8.8341(4) 5.9530(2) 8.1463(6) b (Å) 8.5694(4) 5.9530(2) 8.1463(6) c (Å) 12.1396(6) 5.9530(2) 11.5740(15) α (°) 90 90 90 β (°) 90 90 90 γ (°) 90 90 90 V (Å3) 919.00(8) 210.964(12) 768.08(13) Z 4 1 1 Dcalc(g/cm3) 4.006 3.988 3.596 RF 0.0380 0.0419 0.0505 RwF 0.0947 0.1388 0.1278 Goodness of fit 1.060 1.741 1.013

6

Figure 5. Projections of the CsSrBrI2 unit cell with the distorted perovskite structure (left) and an isolated Sr(Br,I)6 octahedron (right). Table 4. Fractional atomic coordinates and equivalent isotropic displacement parameter for CsSrBrI2. Atom x y z U (Å2) Cs1 0.93919(13) 0.01066(13) ¼ 0.0733(4) Sr1 ½ ½ ½ 0.0447(3) Br1 0.51492(18) 0.07813(17) ¼ 0.0795(6) I1 0.51492(18) 0.07813(17) ¼ 0.0795(6) Br2 0.70191(7) 0.30123(7) 0.45886(6) 0.0568(3) I2 0.70191(7) 0.30123(7) 0.45886(6) 0.0568(3)

Figure 6. Projections of multiple CsCaBrI2 unit cells with the perovskite structure (left) and an isolated Ca(Br,I)6 octahedron (right). Table 5. Fractional atomic coordinates and equivalent isotropic displacement parameter for CsCaBrI2. Atom x y z U (Å2) Cs1 0 0 0 0.0841(17) Ca1 ½ ½ ½ 0.050(2) Br1 ½ ½ 0 0.138(2) I1 ½ ½ 0 0.138(2)

7

Figure 7. Projections of the CsSrClBr2 unit cell with the distorted perovskite structure (left) and an isolated Sr(Cl,Br)6 octahedron (right). Table 6. Fractional atomic coordinates and equivalent isotropic displacement parameter for CsSrClBr2. Atom x y z U (Å2) Cs1 0.49301(11) 0.46519(14) ¼ 0.0583(4) Sr1 0.500 0.000 0.000 0.0258(4) Br1 0.20697(13) 0.20812(13) 0.03118(12) 0.0530(6) Cl1 0.20697(13) 0.20812(13) 0.03118(12) 0.0530(6) Br2 0.5593(3) 0.0086(3) ¼ 0.0657(8) Cl2 0.5593(3) 0.0086(3) ¼ 0.0657(8) 3.2. Luminescence and Scintillation Properties The luminescence properties of the investigated materials are ascribed to the 5d - 4f radiative transition in Eu2+ [46-49]. X-ray excited luminescence and the PL emission/excitation spectra are shown in Figures 8a and 8b. The emission spectra consist of single peaks centered at 455 nm for CsSrBrI2:Eu 7%, 462 nm for CsCaBrI2:Eu 7% and 445 nm for CsSrClBr2:Eu 10%. The PL excitation spectrum of each compound exhibits a broad region of poorly resolved bands from ~250 nm to ~430 nm resulting from the energy levels in Eu2+. Similar to the x-ray excited emission, the PL emission spectrum of each compound consists of a single welldefined peak centered at 449 nm for CsSrBrI2:Eu 7%, 452 nm for CsCaBrI2:Eu 7% and 441 nm for CsSrClBr2:Eu 10%, confirming that the only radiative transition is from the lowest energy 5d excited state of Eu. Since no significant shifts were observed in the emission spectra of the mixed crystals compared to the non-mixed crystals, we assume that the structural changes caused by the halide substitution did not affect the crystal field surrounding the Eu2+ luminescence center [50]. The photoluminescence and scintillation decay curves are shown in Figures 9a and 9b, respectively. The lifetime of the Eu2+ 5d1 excited state was 1.7 µs for CsSrBrI2:Eu 7%, 1.9 µs for CsCaBrI2:Eu 7% and 3.3 µs for

8

CsSrClBr2:Eu 10%, and a single exponential decay function provided a good fit to the PL decay curves. The much longer decay constant observed for CsSrClBr2:Eu 10% is most likely due to the higher Eu doping level and the associated self-absorption effects [51-53]. The scintillation decay times are consistent with the Eu2+ lifetimes previously observed. A single exponential decay function provided a good fit for the time profile, yielding time constants of 1.9 μs for CsSrBrI2:Eu 7%, 2.1 μs for CsCaBrI2:Eu 7%, and 3.5 μs for CsSrClBr2:Eu 10%.

450

1.0

CsSrBrI2:Eu 7% RL Max at 455 nm

Normalized Intensity (a.u)

300

600

0.5 0.0 1.0 CsCaBrI2:Eu 7% RL Max at 462 nm

0.5 0.0 1.0 CsSrClBr2:Eu 10% RL Max at 445 nm

300

450

500

CsSrBrI2:Eu 7% Excitation (em: 449 nm) Emission @ 449 nm (ex: 370 nm)

0.5 0.0

1.0 CsCaBrI2:Eu 7% Excitation (em: 452 nm) Emission @ 452 nm (ex: 370 nm)

0.5 0.0

1.0

0.5

0.5

0.0

0.0

CsSrClBr2:Eu 10% Excitation (em: 441 nm) Emission @ 441 nm (ex: 370)

300

600

Wavelength (nm)

400

1.0

Normalized Intensity (a.u)

300

a.

400

500

Wavelength (nm)

b.

Figure (8a). X-ray excited luminescence spectra of CsSrBrI2:Eu 7%, CsCaBrI2:Eu 7% and CsSrClBr2:Eu 10%. (8b) The PL excitation spectra comprised of multiple unresolved overlapping bands from 250 nm to ~430 nm, the PL emission consisted of a sharp single peak from 449 – 452 nm. Photoluminescence Decay Time

Scintillation Decay Time 1

CsSrBrI2:Eu 7% - 1.77 s

Normalized Counts (a.u)

Normalized Counts (a.u)

1

CsCaBrI2:Eu 7% - 1.92 s CsSrClBr2:Eu 10% - 3.3 s

0.1

0.01

0

5000

10000

Time (ns)

15000

CsSrBrI2:Eu 7% - 1.9 s CsCaBrI2:Eu 7% - 2.1 s CsSrClBr2:Eu 10% - 3.5 s

0.1

0

a.

4000

8000

Time (ns)

12000

b.

Figure 9a – b. The photoluminescence and scintillation decay time curves of CsSrBrI2:Eu 7%, CsCaBrI2:Eu 7 % and CsSrClBr2:Eu 10% fitted with single exponential decay function.

All three mixed scintillators have comparable light yield, decay time and non-proportional response but improved energy resolution when compared to the corresponding un-mixed ABX3:Eu scintillators. Table 7

9

shows the best results obtained for CsSrBrI2:Eu2+, CsCaBrI2:Eu2+ and CsSrClBr2:Eu2+ in comparison with the corresponding un-mixed (parent) ABX3:Eu scintillators and state-of-the-art SrI2:Eu and NaI:Tl scintillators. From the Eu2+ concentration experiments, we observed typical concentration quenching effects for CsSrBrI2:Eu and CsCaBrI2:Eu [37, 53, 54]. This was not case for CsSrClBr2, as its light yield increased monotonically up to the maximum Eu concentration investigated. The europium concentration needed to maximize the light yield was 7% for CsSrBrI2 and CsCaBrI2, and 10% for CsSrClBr2. Note that the light yield of the europium optimized Ø13 mm crystals grown under improved growth conditions (i.e. sharper thermal gradient and slower pulling rate compared to crystals grown for europium concentration experiments) improved by ~13%, as shown in Figure 10. The mixed ABX’X’’2:Eu scintillators investigated in this work had an improved energy resolution and more homogeneous response over the un-mixed ABX3:Eu2+ scintillators. At 662 keV, the energy resolution of <1 cm3 crystals was 3.4% for CsSrBrI2:Eu 7%, 3.8% for CsCaBrI2:Eu 7%, and 3.9% for CsSrClBr2:Eu 10%. Note that the energy resolution of all three scintillators slightly deteriorated as the crystal volume increased to ~1 cm3. The 137Cs pulse-height spectra are shown in Figures 11a – c. The scintillation light yield per unit energy as a function of deposited γ-ray energy is shown in Figure 12. CsSrBrI2:Eu 7% exhibits an excellent proportional response from 32 to 662 keV, with a deviation of ~1% from ideal. CsCaBrI2:Eu 7% and CsSrClBr2:Eu 10% begin to exhibit a non-proportional response at somewhat higher energies compared to CsSrBrI2:Eu. Note that the nPR response of these scintillators is similar to the response of oxide scintillators, in which the light yield is constant for high and intermediate energies and then decreases monotonically at low energies [55, 56]. The trend does not show the familiar “halide hump”, i.e. the often-observed increase of the light yield at intermediate energies [57, 58]. The energy resolution R of a scintillator at energy E is experimentally determined from the FWHM of the full energy photopeak in the pulse-height spectrum. However, several factors [59] contribute to the peak width such that: where RM is the contribution from PMT gain and photoelectron detection Poisson statistics, RnPR is the contribution from the non-proportional response, and Rinh is the contribution from crystal inhomogeneities [60]. Since the light yields and the nonproportional curves of the mixed ABX’X”2:Eu2+ scintillators are comparable to the un-mixed ABX3:Eu2+ scintillators, effects from the statistical contribution (RM) and contribution from the non-proportional response RnPR can be neglected. Therefore, the improved energy resolution of the mixed scintillators studied in this work is solely attributed to the crystal inhomogeneity term (Rinh).

Light Yield (ph/MeV)

70k CsSrBrI2:Eu CsCaBrI2:Eu CsSrClBr2:Eu

60k

50k

40k

30k

20k

3

5 2+

Eu

7

9

11

concentration (%)

Figure 10. Light yield as a function of europium concentration, measured from specimens obtained from the Ø7 mm crystals. The highlighted light yields were measured from specimens obtained from the Ø13 mm crystals grown using improved growth conditions.

10

2.0 4.4% at 662 keV 8 x 8 x 15 mm3

1.6

3

(~1 cm )

1.2

0.8

3.4% at 662 keV 5 x 5 x 2 mm3 (< 1 cm3)

0.4

0.0

450

600

750

a.

Energy (keV)

Normalized Intensity (a.u)

137

Cs - CsCaBrI2:Eu 7%

2.0 5.0% at 662 keV 8 x 8 x 20 mm3 (~1 cm3)

1.6 1.2 0.8

3.8% at 662 keV 5 x 5 x 2 mm3

(<1 cm3)

0.4 0.0

450

600

750

Energy (keV)

b.

Cs - CsSrClBr2:Eu 10%

2.0 5.0% at 662 keV

f 13 x 8 mm3 (~1 cm3)

1.6

1.2

0.8 3.9% at 662 keV 6 x 6 x 13 mm3 (< 1 cm3)

0.4

0.0

450

600

750

c.

Energy (keV)

Figure 11a-c. excitation. Normalized Light Yield (a.u)

137

Cs - CsSrBrI2:Eu 7%

Normalized Intensity (a.u)

Normalized Intensity (a.u)

137

137

Cs Pulse height spectra of CsSrBrI2:Eu 7%, CsCaBrI2:Eu 7% and CsSrClBr2:Eu 10% under

137

Cs

nPR response NaI:Tl (ref.) CsSrBrI2:Eu 7% CsCaBrI2:Eu 7% CsSrClBr2:Eu 10% LaBr3:Ce (ref.)

1.15 1.10 1.05

Ideal Response

1.00 0.95 0.90 0.85

100

1000

Energy (keV)

Figure 12. Non-proportional response of CsSrBrI2:Eu 7%, CsCaBrI2:Eu 7% and CsSrClBr2:Eu 10% scintillators compared with NaI:Tl and LaBr3:Ce.

11

Table 7. Summary of physical and the best scintillation properties for CsSrBrI2:Eu 7%, CsCaBrI2:Eu 7% and CsSrClBr2:Eu 10%. Shown as references are CsSrI3:Eu 7%, CsCaI3:Eu 3%, CsSrBr3:Eu 5%, NaI:Tl and SrI2:Eu of similar size. Density Light Yield RL max ER Scint. Reference Zeff 3 (g/cm ) (ph/MeV) (nm) (662 keV) Decay (μs) This work CsSrBrI2:Eu 7% 50.2 4.0 65,300 455 3.4% 1.8 This work CsCaBrI2 :Eu 7% 50.2 3.9 51,800 462 3.8% 2.1 This work CsSrClBr2 :Eu 10% 39.6 3.6 35,100 445 3.9% 3.5 [1] CsSrI3:Eu 7% 52.3 4.25 73,000 455 3.9% 3.2 [6] CsCaI3:Eu 3% 52.3 4.06 40,000 460 4.5% 2.1 [61] CsSrBr3:Eu 5% 38.5 3.76 31,000 440 4.9% 3.5 [29] NaI:Tl 51 3.67 38,000 415 7.1% 0.23 SrI2:Eu

49

4.59

>90,000

435

2.6%

~1

[42]

4. Conclusions Scintillation properties of new mixed halide scintillators CsSrBrI2:Eu2+, CsCaBrI2:Eu2+ and CsSrClBr2:Eu2+ were investigated in Ø13 mm single crystals grown using the vertical Bridgman technique. Melting and crystallization points determined via thermal analysis were used to establish thermal gradients in the growth furnace. Detailed crystallographic studies using single-crystal X-ray diffraction showed perovskite-like structures with an isolated B(X’X”)6 octahedron motif and fractional occupation of the anion sites by the two corresponding halogen ions, X’ and X”. The results also reveal that the mixed compounds crystallize in the orthorhombic and cubic structures, which are similar to those of their parent ABX3 compounds. These crystals are deliquescent in a similar manner to NaI:Tl and require protective packaging. All three mixed scintillators exhibit favorable scintillation properties and have potential for use in domestic security applications requiring gamma-ray radioisotope identification. The improved Rinh of the mixed scintillators led to an energy resolution improvement over their parent ABX3 scintillators; resulting in energy resolutions below 4% at 662 keV. The best performing scintillator is CsSrBrI2:Eu 7% with light yield of 65,300 ph/MeV, energy resolution of 3.4% at 662 keV, Zeff similar to NaI:Tl, and exceptionally good scintillation proportionality. Future work will investigate the effects of cationic substitution on scintillation properties. Acknowledgements This work was supported by the US Department of Homeland Security, Domestic Nuclear Detection Office, under grant # 2014-DN-077-ARI088-04 and grant # 2012-DN-077-ARI067-05. This support does not constitute an express or implied endorsement on the part of the Government. Research at Oak Ridge National Laboratory was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, US Department of Energy. References [1] K. Yang, M. Zhuravleva, C.L. Melcher, Crystal growth and characterization of CsSr1-xEuxI3 high light yield scintillators, Physica Status Solidi (RRL) – Rapid Research Letters, 5 (2011) 43-45. [2] U. Shirwadkar, E.V.D. van Loef, R. Hawrami, S. Mukhopadhyay, J. Glodo, K.S. Shah, New promising scintillators for gamma-ray spectroscopy: Cs(Ba,Sr)(Br,I)3, in: Nuclear Science Symposium and Medical Imaging Conference (NSS/MIC), 2011 IEEE, 2011, pp. 1583-1585. [3] M. Zhuravleva, K. Yang, Chloride, Bromide, and Iodide Scintillators with Europium Doping, in: U.S.P. Office (Ed.), USA, 2012, pp. 25. [4] V.L. Cherginets, N.V. Rebrova, A.Y. Grippa, Y.N. Datsko, T.V. Ponomarenko, V.Y. Pedash, N.N. Kosinov, V.A. Tarasov, O.V. Zelenskaya, I.M. Zenya, A.V. Lopin, Scintillation properties of CsSrX3:Eu2+ (CsSr1−yEuyX3, X = Cl,

12

Br; 0 ≤ y ≤ 0.05) single crystals grown by the Bridgman method, Materials Chemistry and Physics, 143 (2014) 12961299. [5] A. Lindsey, W. McAlexander, L. Stand, Y. Wu, M. Zhuravleva, C.L. Melcher, Crystal growth and spectroscopic performance of large crystalline boules of CsCaI3:Eu scintillator, Journal of Crystal Growth, 427 (2015) 42-47. [6] M. Zhuravleva, B. Blalock, K. Yang, M. Koschan, C.L. Melcher, New single crystal scintillators: CsCaCl 3:Eu and CsCaI3:Eu, Journal of Crystal Growth, 352 (2012) 115-119. [7] A.Y. Grippa, N.V. Rebrova, T.E. Gorbacheva, V.Y. Pedash, N.N. Kosinov, V.L. Cherginets, V.A. Tarasov, O.A. Tarasenko, A.V. Lopin, Crystal growth and scintillation properties of CsCaBr3:Eu2+(CsCa1−xEuxBr3, 0≤x≤0.08), Journal of Crystal Growth, 371 (2013) 112-116. [8] E.D. Bourret-Courchesne, G.A. Bizarri, R. Borade, G. Gundiah, E.C. Samulon, Z. Yan, S.E. Derenzo, Crystal growth and characterization of alkali-earth halide scintillators, Journal of Crystal Growth, 352 (2012) 78-83. [9] A.C. Lindsey, M. Zhuravleva, L. Stand, Y. Wu, C.L. Melcher, Crystal growth and characterization of europium doped KCaI3, a high light yield scintillator, Optical Materials, 48 (2015) 1-6. [10] A.C. Lindsey, M. Zhuravleva, Y. Wu, L. Stand, M. Loyd, S. Gokhale, M. Koschan, C.L. Melcher, Effects of increasing size and changing europium activator concentration in KCaI3 scintillator crystals, Journal of Crystal Growth, 449 (2016) 96-103. [11] S.S. Gokhale, L. Stand, A. Lindsey, M. Koschan, M. Zhuravleva, C.L. Melcher, Improvement in the optical quality and energy resolution of CsSrBr3: Eu scintillator crystals, Journal of Crystal Growth, 445 (2016) 1-8. [12] K. Kamada, T. Endo, K. Tsutumi, T. Yanagida, Y. Fujimoto, A. Fukabori, A. Yoshikawa, J. Pejchal, M. Nikl, Composition Engineering in Cerium-Doped (Lu,Gd)3(Ga,Al)5O12 Single-Crystal Scintillators, Crystal Growth & Design, 11 (2011) 4484-4490. [13] K. Kamada, T. Yanagida, J. Pejchal, M. Nikl, T. Endo, K. Tsutumi, Y. Fujimoto, A. Fukabori, A. Yoshikawa, Scintillator-oriented combinatorial search in Ce-doped (Y,Gd) 3 (Ga,Al) 5 O 12 multicomponent garnet compounds, Journal of Physics D: Applied Physics, 44 (2011) 505104. [14] B. Han-Bo, Q. Lai-Shun, D. Yan-Guo, L. Zheng-Guo, S. Hong-Sheng, S. Kang-Ying, Growth and Scintillation Properties of La(Cl 0.05 Br 0.95 ) 3 :Ce Crystal, Chinese Physics Letters, 30 (2013) 088101. [15] Y. Wu, Q. Li, B.C. Chakoumakos, M. Zhuravleva, A.C. Lindsey, J.A. Johnson, L. Stand, M. Koschan, C.L. Melcher, Quaternary Iodide K(Ca,Sr)I3:Eu2+ Single‐Crystal Scintillators for Radiation Detection: Crystal Structure, Electronic Structure, and Optical and Scintillation Properties, Advanced Optical Materials, 4 (2016) 1518-1532. [16] E.D. Bourret-Courchesne, G. Bizarri, S.M. Hanrahan, G. Gundiah, Z. Yan, S.E. Derenzo, BaBrI:Eu2+, a new bright scintillator, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 613 (2010) 95-97. [17] G. Bizarri, E.D. Bourret-Courchesne, Z. Yan, S.E. Derenzo, Scintillation and Optical Properties of BaBrI: Eu2+ and CsBa2I5:Eu2+, IEEE Transactions on Nuclear Science, 58 (2011) 3403-3410. [18] L. Swiderski, M. Moszynski, A. Nassalski, A. Syntfeld-Kazuch, W. Czarnacki, W. Klamra, V.A. Kozlov, Scintillation Properties of Undoped CsI and CsI Doped With CsBr, IEEE Transactions on Nuclear Science, 55 (2008) 1241-1245. [19] G. Gundiah, G. Bizarri, S.M. Hanrahan, M.J. Weber, E.D. Bourret-Courchesne, S.E. Derenzo, Structure and scintillation of Eu2+-activated solid solutions in the BaBr2–BaI2 system, Nuclear Instruments and Methods in Physics Research Section A: Accelerators, Spectrometers, Detectors and Associated Equipment, 652 (2011) 234-237. [20] U. Shirwadkar, R. Hawrami, J. Glodo, E.V.D. van Loef, K.S. Shah, Promising Alkaline Earth Halide Scintillators for Gamma-Ray Spectroscopy, IEEE Transactions on Nuclear Science, 60 (2013) 1011-1015. [21] Y. Wu, A.C. Lindsey, M. Zhuravleva, M. Koschan, C.L. Melcher, Large-Size KCa0.8Sr0.2I3:Eu2+ Crystals: Growth and Characterization of Scintillation Properties, Crystal Growth & Design, 16 (2016) 4129-4135. [22] A.N. Vasil'ev, A.V. Gektin, Multiscale Approach to Estimation of Scintillation Characteristics, IEEE Transactions on Nuclear Science, 61 (2014) 235-245. [23] A.V. Gektin, A.N. Belsky, A.N. Vasil'ev, Scintillation Efficiency Improvement by Mixed Crystal Use, IEEE Transactions on Nuclear Science, PP (2013) 1-1. [24] O. Sidletskiy, Trends in Search for Bright Mixed Scintillators, physica status solidi (a), 211 (2014). [25] O. Sidletskiy, A.V. Gektin, A.N. Belsky, Light‐yield improvement trends in mixed scintillation crystals, physica status solidi (a), 211 (2014) 2384-2387. [26] M. Loyd, A. Lindsey, L. Stand, M. Zhuravleva, C.L. Melcher, M. Koschan, Tuning the structure of CsCaI3:Eu via substitution of bromine for iodine, Optical Materials, 68 (2017) 47-52. [27] N.V. Rebrova, A.Y. Grippa, T.E. Gorbacheva, V.Y. Pedash, S.U. Khabuseva, V.L. Cherginets, A.N. Puzan, V.A. Tarasov, Scintillation properties of Eu2+-activated CsCaCl3−xBrx (x=1, 1.5, 2), Journal of Luminescence, 182 (2017) 172-176. [28] C.E. Chang, W.R. Wilcox, Control of Interface Shape in Vertical Bridgman-Stockbarger Technique, Journal of Crystal Growth, 21 (1974) 135-140. [29] T.W. Fu, W.R. Wilcox, Influence of Insulation on Stability of Interface Shape and Position in the Vertical Bridgman-Stockbarger Technique, Journal of Crystal Growth, 48 (1980) 416-424.

13

[30] M. Bertolaccini, S. Cova, C. Bussolati, A technique for absolute measurement of the effective photoelectron per keV yield in scintillation counters, in: Proceedings of the Nuclear Electronics Symposium, Versailles, France, 1968. [31] J.T.M.d. Haas, P. Dorenbos, Advances in Yield Calibration of Scintillators, IEEE Transactions on Nuclear Science, 55 (2008) 1086-1092. [32] M. Moszynski, M. Kapusta, M. Mayhugh, D. Wolski, S.O. Flyckt, Absolute light output of scintillators, IEEE Transactions on Nuclear Science, 44 (1997) 1052-1061. [33] C.W.E. van Eijk, P. Dorenbos, E.V.D. van Loef, K. Krämer, H.U. Güdel, Energy resolution of some new inorganicscintillator gamma-ray detectors, Radiation Measurements, 33 (2001) 521-525. [34] L.M.Bollinger, in: G.E. Thomas (Ed.), Rev. Sci. Instr, 1961, pp. 1044-1050. [35] G. Bizarri, E.D. Bourret-Courchesne, Y. Zewu, S.E. Derenzo, Scintillation and Optical Properties of BaBrI:Eu2+ and CsBa2I5: Eu2+, Nuclear Science, IEEE Transactions on, 58 (2011) 3403-3410. [36] K. Yang, Discovery and Development of Rare Earth Activated Binary Metal Halide Scintillators for Next Generation Radiation Detectors, in: Materials Science and Engineering, University of Tennessee, Knoxville, 2011. [37] E.V. van Loef, C.M. Wilson, N.J. Cherepy, G. Hull, S.A. Payne, C. Woon-Seng, W.W. Moses, K.S. Shah, Crystal Growth and Scintillation Properties of Strontium Iodide Scintillators, IEEE Transactions on Nuclear Science, 56 (2009) 869-872. [38] L.A. Boatner, J.O. Ramey, J.A. Kolopus, R. Hawrami, W.M. Higgins, E. van Loef, J. Glodo, K.S. Shah, E. Rowe, P. Bhattacharya, E. Tupitsyn, M. Groza, A. Burger, N.J. Cherepy, S.A. Payne, Bridgman growth of large SrI2:Eu2+ single crystals: A high-performance scintillator for radiation detection applications, Journal of Crystal Growth, 379 (2013) 6368. [39] W.M. Higgins, J. Glodo, E. Van Loef, M. Klugerman, T. Gupta, L. Cirignano, P. Wong, K.S. Shah, Bridgman growth of LaBr3:Ce and LaCl3:Ce crystals for high-resolution gamma-ray spectrometers, Journal of Crystal Growth, 287 (2006) 239-242. [40] L. Stand, M. Zhuravleva, J. Johnson, M. Koschan, Y. Wu, S. Donnald, K. Vaigneur, E. Lukosi, C.L. Melcher, Exploring growth conditions and Eu2+ concentration effects for KSr2I5:Eu scintillator crystals II: Ø 25 mm crystals, Journal of Crystal Growth, 483 (2018) 301-307. [41] Y. Wu, S.S. Gokhale, A.C. Lindsey, M. Zhuravleva, L. Stand, J.A. Johnson, M. Loyd, M. Koschan, C.L. Melcher, Toward High Energy Resolution in CsSrI3/Eu2+ Scintillating Crystals: Effects of Off-Stoichiometry and Eu2+ Concentration, Crystal Growth & Design, 16 (2016) 7186-7193. [42] G. Schilling, G. Meyer, Ternäre Bromide und Iodide zweiwertiger Lanthanide und ihre Erdalkali‐Analoga vom Typ AMX3 und AM2X5, Zeitschrift für anorganische und allgemeine Chemie, 622 (1996) 759-765. [43] H. J. Seifer, U. Langenbach, Thermoanalytische und röntgenographische Untersuchungen an Systemen Alkalichlorid/Calciumchlorid, Zeitschrift für anorganische und allgemeine Chemie, 368 (1969) 36-43. [44] H. J. Seifert, D. Haberhauer, Über die Systeme Alkalimetallbromid/Calciumbromid, Zeitschrift für anorganische und allgemeine Chemie, 491 (1982) 301-307. [45] M. Loyd, A. Lindsey, M. Patel, M. Koschan, C.L. Melcher, M. Zhuravleva, Crystal structure and thermal expansion of CsCaI3:Eu and CsSrBr3:Eu scintillators, Journal of Crystal Growth, 481 (2018) 35-39. [46] D.H. Gahane, N.S. Kokode, P.L. Muthal, S.M. Dhopte, S.V. Moharil, Luminescence of Eu2+ in some iodides, Optical Materials, 32 (2009) 18-21. [47] P. Dorenbos, Energy of the first 4f7→4f65d transition of Eu2+ in inorganic compounds, Journal of Luminescence, 104 (2003) 239-260. [48] A. Pushak, V. Vistovskyy, A. Voloshinovskii, T. Demkiv, J. Dacyuk, A. Gektin, S. Myagkota, Luminescence of Eu2+-doped Me-containing aggregates (Me= Ca, Sr, Ba) in KI matrix. [49] D.H. Gahane, N.S. Kokode, P.L. Muthal, S.M. Dhopte, S.V. Moharil, Luminescence of some Eu2+ activated bromides, Journal of Alloys and Compounds, 484 (2009) 660-664. [50] D. Kim, S. Park, S. Kim, S.-G. Kang, J.-C. Park, Blue-Emitting Eu2+-Activated LaOX (X = Cl, Br, and I) Materials: Crystal Field Effect, Inorganic Chemistry, 53 (2014) 11966-11973. [51] M.S. Alekhin, J.T.M.d. Haas, K.W. Kramer, P. Dorenbos, Scintillation Properties of and Self Absorption in SrI2:Eu2+, IEEE Transactions on Nuclear Science, 58 (2011) 2519-2527. [52] A.F.M.a.B.P. Oehry, Radiation Trapping in Atomic Vapours, Oxford Science (1999). [53] J. Glodo, E.V. Van Loef, N.J. Cherepy, S.A. Payne, K.S. Shah, Concentration Effects in Eu Doped SrI2, IEEE Transactions on Nuclear Science, 57 (2010) 1228-1232. [54] M.S. Alekhin, J.T.M. de Haas, K.W. Kramer, I.V. Khodyuk, L. de Vries, P. Dorenbos, Scintillation properties and self absorption in SrI2:Eu2+, in: Nuclear Science Symposium Conference Record (NSS/MIC), 2010 IEEE, 2010, pp. 1589-1599. [55] W.W. Moses, S.A. Payne, W.S. Choong, G. Hull, B.W. Reutter, Scintillator Non-Proportionality: Present Understanding and Future Challenges, IEEE Transactions on Nuclear Science, 55 (2008) 1049-1053. [56] M. Balcerzyk, M. Moszynski, M. Kapusta, D. Wolski, J. Pawelke, C.L. Melcher, YSO, LSO, GSO and LGSO. A study of energy resolution and nonproportionality, Nuclear Science, IEEE Transactions on, 47 (2000) 1319-1323.

14

[57] S.A. Payne, N.J. Cherepy, G. Hull, J.D. Valentine, W.W. Moses, C. Woon-Seng, Nonproportionality of Scintillator Detectors: Theory and Experiment, IEEE Transactions on Nuclear Science, 56 (2009) 2506-2512. [58] B.D. Rooney, J.D. Valentine, Scintillator light yield nonproportionality: calculating photon response using measured electron response, Nuclear Science, IEEE Transactions on, 44 (1997) 509-516. [59] P. Dorenbos, Fundamental Limitations in the Performance of Ce3+, Pr3+, and Eu2+ Activated Scintillators, IEEE Transactions on Nuclear Science, 57 (2010) 1162-1167. [60] M.S. Alekhin, D.A. Biner, K.W. Krämer, P. Dorenbos, Optical and scintillation properties of CsBa2I5:Eu2+, Journal of Luminescence, 145 (2014) 723-728. [61] S.S. Gokhale, M. Loyd, L. Stand, A. Lindsey, S. Swider, M. Zhuravleva, C.L. Melcher, Investigation of the unique degradation phenomenon observed in CsSrBr3: Eu 5% scintillator crystals, Journal of Crystal Growth, 452 (2016) 89-94. Highlights  Ø13 mm single crystals of CsSrBrI2:Eu, CsCaBrI2:Eu and CsSrClBr2:Eu were grown via the Bridgman technique.  The crystal structures of CsSrBrI2, CsSrClBr2 and CsCaBrI2 were determined.  The energy resolution of all three mixed scintillators was below 4.0% at 662 keV.  CsSrBrI2:Eu 7% has scintillation light yield of 65,300 ph/MeV and energy resolution of 3.4% at 662 keV.

15