Thallium-based scintillators for high-resolution gamma-ray spectroscopy: Ce3+ - doped Tl2LaCl5 and Tl2LaBr5

Journal Pre-proof Thallium-based scintillators for high-resolution gamma-ray spectroscopy: Ce3+ - doped Tl2 LaCl5 and Tl2 LaBr5 Urmila Shirwadkar, Matthew Loyd, Mao-Hua Du, Edgar van Loef, Guido Ciampi, Lakshmi Soundara Pandian, Luis Stand, Merry Koschan, Mariya Zhuravleva, Charles Melcher, Kanai Shah

PII: DOI: Reference:

S0168-9002(20)30247-3 https://doi.org/10.1016/j.nima.2020.163684 NIMA 163684

To appear in:

Nuclear Inst. and Methods in Physics Research, A

Received date : 6 February 2020 Accepted date : 18 February 2020 Please cite this article as: U. Shirwadkar, M. Loyd, M.-H. Du et al., Thallium-based scintillators for high-resolution gamma-ray spectroscopy: Ce3+ - doped Tl2 LaCl5 and Tl2 LaBr5 , Nuclear Inst. and Methods in Physics Research, A (2020), doi: https://doi.org/10.1016/j.nima.2020.163684. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. 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. © 2020 Published by Elsevier B.V.

Journal Pre-proof

lP repro of

Urmila Shirwadkar,1 Matthew Loyd,2 Mao-Hua Du,3 Edgar van Loef,1,* Guido Ciampi,1 Lakshmi Soundara Pandian,1 Luis Stand,2 Merry Koschan,2 Mariya Zhuravleva,2, 4 Charles Melcher,2, 4, 5 Kanai Shah 1 1

Radiation Monitoring Devices Inc., 44 Hunt Street, Watertown, MA 02472, USA Scintillation Materials Research Center, University of Tennessee, Knoxville, Knoxville, TN 37996, USA 3 Oak Ridge National Laboratory, Oak Ridge, TN 37831, USA 4 Department of Materials Science and Engineering, University of Tennessee, Knoxville, Knoxville, TN 37996, USA 5 Department of Nuclear Engineering, University of Tennessee, Knoxville, Knoxville, TN 37996, USA 2

*Corresponding author: E. V. van Loef E-mail: [email protected] Address: Radiation Monitoring Devices, Inc. (RMD), 44 Hunt Street, Watertown, MA 02472, USA Tel.: +1 (617) 668 6984 Fax: +1 (617) 926 9980

rna

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47

Thallium-based scintillators for high-resolution gamma-ray spectroscopy: Ce3+- doped Tl2LaCl5 and Tl2LaBr5

Jou

1

This manuscript has been co-authored by UT-Battelle, LLC under Contract No. DE-AC05-00OR22725 with the U.S. Department of Energy. The United States Government retains and the publisher, by accepting the article for publication, acknowledges that the United States Government retains a non-exclusive, paid-up, irrevocable, world-wide license to publish or reproduce the published form of this manuscript, or allow others to do so, for United States Government purposes. The Department of Energy will provide public access to these results of federally sponsored research in accordance with the DOE Public Access Plan (http://energy.gov/downloads/doe-public-access-plan).

Journal Pre-proof

ABSTRACT

lP repro of

In this paper we report on the crystallographic and scintillation properties of Tl2LaCl5:Ce3+ and Tl2LaBr5:Ce3+, two novel thallium-containing high-resolution scintillators for gamma-ray spectroscopy. Crystals of Tl2LaCl5:Ce3+ and Tl2LaBr5:Ce3+ were grown by the Vertical Bridgman method up to 1-inch diameter and 1-inch long. Single crystals of Tl2LaCl5:Ce3+ and Tl2LaBr5:Ce3+ belong to the orthorhombic system with space group 62 and have a density of 5.16 and 5.98 g/cm3, respectively. The scintillators show high light yields of up to 68,000 photons/MeV, excellent gamma-ray energy resolution of ≤ 3% at 662 keV, a fast scintillation decay, and a proportional response over a wide range of energies from 32 keV up to 1275 keV. Density Functional Theory calculations show that the Ce3+ energy levels are inside the bandgap despite the smaller bandgap of Tl2LaCl5 and Tl2LaBr5 compared to K2LaCl5 and K2LaBr. A systematic Ce3+ concentration study was performed for Tl2LaCl5:Ce3+ and trends observed.

rna

Keywords: Inorganic scintillators; Thallium-based scintillators; Radiation detectors; Gamma-ray spectroscopy; DFT calculations.

Jou

48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64

Journal Pre-proof

65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80

1. INTRODUCTION

81

2. EXPERIMENTAL SECTION

82 83 84 85 86 87 88 89 90 91

2.1 Crystal Growth Single crystals of Tl2LaCl5 and Tl2LaBr5 with different Ce3+ concentrations were grown by the Vertical Bridgman method in sealed quartz ampoules using single zone furnaces. Anhydrous beads of TlCl, LaCl3, CeCl3, TlBr, LaBr3, and CeBr3 obtained from APL Engineered Materials were loaded into quartz ampoules in a nitrogen purged glovebox due to the hygroscopic nature of the starting materials. The purity of the starting materials was 99.99%. After loading, the quartz ampoules were sealed under high vacuum and transferred to the Bridgman furnaces. Typically, the growth lasted for about 4 weeks for a 1” x 1” crystal. For details on the actual crystal growth, see refs. [4, 5]. Note that all Ce3+ concentrations mentioned in this paper are nominal, atomic (mol) percent (relative to La).

92 93 94 95 96 97 98 99 100 101 102 103 104 105

2.2 Thermodynamic Properties The hygroscopicity of each sample was determined using a Surface Measurement Systems Dynamic Vapor Sorption (DVS) instrument. In this instrument, samples are placed on a sample tray suspended from a high-precision balance inside of a temperature and humidity-controlled atmosphere, and the weight is continuously monitored. The samples measured included an ~20 mg piece of each composition plus a NaI:Tl sample, all with similar geometries for better comparison. Samples were placed on an aluminum sample tray and hung on the balance, and the DVS was set at 25°C and 40% relative humidity for 24 hours, or until saturated. The samples were also photographed before and after measurement. The percent mass change was plotted vs time. The thermal properties of each composition were analyzed using a Seteram Labsys Evo differential scanning calorimetry (DSC) instrument. Due to the volatility and toxicity of the materials, the samples were vacuum sealed inside quartz crucibles prior to measurement. The benefits of this method versus standard alumina crucibles have been discussed in depth [6]. The DSC furnace was heated at a rate of 5 K per minute.

106 107 108

2.3 Powder X-Ray Diffraction High-resolution powder X-ray diffraction (XRD) was used to derive the unknown standard structural parameters of the new compounds such as lattice constants, Wyckoff atomic positions,

Jou

rna

lP repro of

Recently, several novel halide scintillators with excellent scintillation properties have been discovered that contain both thallium and rare earth elements. Among these, Elpasolite variants such as Tl2LiYCl6:Ce3+ [1, 2] and Inohalides such as Tl2LaCl5:Ce3+ [3, 4] seem the most promising thus far. These novel scintillators were discovered by simply substituting Cs+ (in e.g. Cs2LiYCl6) or K+ ions (in e.g. K2LaCl5) with Tl+ ions and observing the effects on the effective Z, density, and scintillation properties. As expected, the density and effective Z of Cs2LiYCl6 was increased from 3.3 to 4.5 g/cm3 and 34 to 69, respectively, yielding Tl2LiYCl6. Likewise, the density and effective Z of K2LaCl5 was increased from 2.9 to 5.2 g/cm3, and 44 to 70, respectively, yielding Tl2LaCl5. In both cases, the light yield also increased significantly. In this paper we report on the crystal growth, crystallography, scintillation properties and theoretical modelling of a series of Tl2LaCl5 and Tl2LaBr5 crystals doped with different Ce3+ concentrations grown by the Vertical Bridgman method. X-ray diffraction spectra, Differential Scanning Calorimetry, Radioluminescence, Light yield, and Energy resolution as function of Ce3+ concentration are reported. Theoretical modeling performed using Density Functional Theory calculations corroborate the experimental results.

Journal Pre-proof

and atomic displacements. Due to the hygroscopic nature of the materials measured, sample preparation was carried out inside of a nitrogen-purged glovebox unless otherwise noted. Samples for XRD were taken from the grown crystal and ground using a mortar and pestle. The powdered samples were loaded onto a Si zero-background sample stage and covered with a Kapton film for protection during measurement. The sample stage was then removed from the glovebox and placed into a PANalytical Empyrean 2-theta Diffractometer. The X-rays were generated with a Cu X-ray tube operated at 45 kV and 40 mA and passed through a 0.04 radian Soller slit, a ¼° divergence slit, a ½° anti-scatter slit, and a 10 mm mask. The diffracted beam was once again passed through a 0.04 radian Soller slit and a ¼° anti scatter slit; a 0.02 mm nickel beta filter was used in order to only allow Cu Kα x-rays. A PIXcel3D-Medipix 3 area detector was operated in 1D scanning mode to detect the diffracted X-rays. Scans were taken in the range of 10-70 ° 2-theta with a step size of 0.0131 ° and a step time of 39.5 s. HighScore software was used to compare the measured patterns with published structures [7]. Determination of the lattice parameters was carried out using GSAS-II, an open-source software package for Rietveld refinement [8]. No published structures were found for Tl2LaCl5 and Tl2LaBr5, but their peaks matched well with those of In2LaCl5, which was used as the structure for refinement with Tl substituted for In [9].

126 127 128 129 130 131 132 133 134 135 136 137 138 139 140 141

2.4. Computational Methods Theoretical calculations were performed on Tl2LaCl5 and Tl2LaBr5 using density functional theory (DFT) with a hybrid PBE0 functional [10] as implemented in the VASP code [11]. The interaction between ions and electrons is described by the projector augmented wave method [12]. Experimental lattice parameters were used while the atomic positions are relaxed until the residual forces are less than 0.02 eV/ Å. Spin-orbit coupling is included in all calculations. The optimized ground- and excited-state structures of Tl2LaX5:Ce3+ (X = Cl, Br) as well as the 3+ Ce emission energy were obtained by using the hybrid PBE0 functional [10], which has a 25% non-local Fock exchange. The occupation numbers of the electron- and hole-occupied Ce-5d and 4f eigenlevels are fixed ([∆ self-consistent field (∆SCF) method [13, 14, 15]) for the total energy calculation of the excited-state Ce3+ and throughout the entire excited-state structural relaxation process. The ∆SCF method combined with the hybrid PBE0 functional allows for an accurate calculation of excited-state structural relaxation, and the emission energy can be calculated based on the relaxed excited-state structure following the Franck-Condon principle. The calculated emission energies in many compounds based on the above approach have shown excellent agreement with experimental results [16, 17, 18, 19, 20].

142 143 144 145 146 147 148 149 150 151 152 153 154

2.5 Scintillation Properties Radioluminescence spectra were recorded at room temperature using a Philips X-ray tube with a Cu anode operated at 40 kV and 20 mA. Light was dispersed through a McPherson 234 monochromator equipped with a grating (600 grooves/mm, blazed at 500 nm) and detected with a Burle C31034 photomultiplier tube (PMT). The GaAs photocathode was cooled to -50°C using dry-ice in order to minimize dark counts. Spectra were measured in 1 nm increments and integrated for 1 sec per interval. Spectra were corrected for the spectral response of the system. Scintillation properties such as energy resolution, light yield, and non-proportionality were measured using a super-bialkali (SBA) Hamamatsu photomultiplier tube (PMT) (model #R6233100) and standard nuclear instrumentation modules from Canberra such as a preamplifier (model #2005), a spectroscopy amplifier (model #2022), an ADC (model #8075), and a MCA (model #8000D) from Amptek. Pulse height spectra were collected using 137Cs, 22Na, 57Co, 133Ba, and 241 Am radiation sources.

Jou

rna

lP repro of

109 110 111 112 113 114 115 116 117 118 119 120 121 122 123 124 125

Journal Pre-proof

Crystals were tested in a mineral oil filled quartz container in order to protect the samples from moisture. Optical grease was used to couple the quartz cup to the PMT window. Light yields, expressed in photoelectrons per MeV (phe/MeV), were determined by comparing the peak position of the 662 keV full energy peak in the pulse height spectra with that of BGO. The absolute light yield, expressed as the number of photons per MeV (ph/MeV), was calculated from the number of photoelectrons per MeV by dividing this number by the quantum efficiency of the PMT. Scintillation decay was measured by optically coupling the quartz container to a Hamamatsu PMT (model #H6610) operating at -2000V, and irradiated with 511 keV gamma rays from a 22Na source. A 1GHz digital oscilloscope from Teledyne LeCroy (model #HDO 6104) was connected directly to the PMT anode to collect the data. In some cases the PMT output was connected to a CAEN digitizer (12-bit, 250 MS/s). Timing measurements were done using a fast Hamamatsu PMT (rise time 0.7 ns) and a Teledyne Lecroy 1GHZ 12-bit resolution digital oscilloscope.

167

3. RESULTS AND DISCUSSION

168 169 170 171 172 173

3.1 Gravimetric and Thermal Analysis Small samples of Tl2LaCl5:Ce3+, Tl2LaBr5:Ce3+, and NaI:Tl were measured using a DVS intrinsic instrument to compare their moisture absorption rates. The mass gain during the measurement is plotted in Figure 1. Tl2LaBr5:Ce3+ was fully saturated after 5 hours and was completely liquid. Tl2LaCl5:Ce3+ gained 1% mass during the first hour then only gained an additionally 2% over the next 11 hours.

Jou

rna

lP repro of

155 156 157 158 159 160 161 162 163 164 165 166

Figure 1. Mass gain plot of Tl2LaCl5:Ce3+, Tl2LaBr5:Ce3+, and NaI:Tl. The inset photographs are of each sample after measurement.

lP repro of

Journal Pre-proof

Figure 2. DSC curve of Tl2LaCl5:Ce3+.

Figure 3. DSC curves of Tl2LaBr5:Ce3+.

The sample developed a white, chalky coating after completion of the measurement. The NaI:Tl mass gain was still linear after 12 hours. The comparison between Tl2LaCl5:Ce3+ and Tl2LaBr5:Ce3+ is consistent with previous observations in which bromine samples were more hygroscopic than chlorides of the same formula or structure [21, 22]. DSC measurements of both compositions were carried out using sealed quartz ampoules, which allowed for a clean measurement without volatilization of the sample; however, the ampoules eliminated the possibility of thermal gravimetric analysis measurements. The heat flow curves acquired for each composition are plotted in Figure 2 and Figure 3. The melting points were measured to be 523 and 556°C for Tl2LaCl5:Ce3+ and Tl2LaBr5:Ce3+, respectively. Both samples had single melting and crystallization peaks and no evidence of secondary phases. The degree of supercooling (separation between melting point and crystallization point) was exaggerated on both samples when compared to other metal halide scintillators. This is most likely due to the smooth quartz wall, which does not offer any nucleation sites, requiring the sample to self-nucleate at a lower temperature.

188 189 190 191 192 193 194 195 196 197 198 199 200 201 202 203

3.2 Powder X-ray Diffraction Powder diffraction patterns of 3+ 3+ Tl2LaCl5:Ce and Tl2LaBr5:Ce measured at room temperature are plotted in Figure 4. The two compositions appear to be isostructural due to the similarities in their patterns, albeit with different lattice parameters. Although there was no published structure for either composition, a search in HighScore revealed a match with the peaks of In2LaCl5. The structure of In2LaCl5 was used as initial structure for Rietveld refinement of Tl2LaCl5 with the In atoms replaced with Tl atoms, and for Tl2LaBr5, with the In and Cl atoms replaced with Tl and Br, respectively. The Ce3+ dopant was not considered when refining the structure,

Jou

rna

174 175 176 177 178 179 180 181 182 183 184 185 186 187

Figure 4. Powder XRD patterns for Tl2LaCl5:Ce3+ (bottom) and Tl2LaBr5:Ce3+ (top). Peak indexes are included for the high intensity peaks.

Journal Pre-proof

but may have an effect on the lattice parameters when compared to an undoped crystal. Refinement was carried out with GSASII, and the fits for the refined structures are plotted in Figure 5. The intensities for hygroscopic metal halides are often in disagreement with their simulated patterns due to preferred orientation [23, 24]. Because of this, measurements such as single crystal diffraction must be carried out to further refine the atom positions (which affect the intensity of peaks). However, the lattice parameters are expected to be extremely accurate for these two phases, and the refined values are compared in Table 1. Crystals of Tl2LaCl5 and Tl2LaBr5 have the orthorhombic crystal structure with space group Pnma (62). Based on structure and Figure 5. GSASII Rietveld refinement fits for refined lattice parameters, the calculated 3 Tl2LaCl5:Ce3+ (top) and Tl2LaBr5:Ce3+ (bottom). density of Tl2LaCl and Tl2LaBr5 is 5.16 g/cm and 5.98 g/cm3, respectively. The effective Z of Tl2LaCl and Tl2LaBr5 is 79 and 67, respectively.

lP repro of

204 205 206 207 208 209 210 211 212 213 214 215 216 217 218 219 220 221 222 223 224 225

Table 1. Refined lattice parameters for Tl2LaCl5:Ce3+ and Tl2LaBr5:Ce3+.

a (Ǻ)

b (Ǻ)

c (Ǻ)

Volume (Ǻ3)

Tl2LaCl5:Ce Tl2LaBr5:Ce

Pnma Pnma

12.8355(8) 13.2889(6)

8.9673(5) 9.3504(4)

8.0983(5) 8.4705(4)

932.11(9) 1052.51(8)

rna

Space Group

3.3 Radioluminescence and Ce3+ Concentration Effects The X-ray excited radioluminescence spectra of Tl2LaCl5 doped with different Ce3+ concentrations are shown in Figure 6. The spectra of undoped and 1% Ce3+-doped Tl2LaBr5 are shown in Figure 7. The luminescence is attributed to recombination of e – h pairs/excitons on Ce3+ and subsequent 5d → 4f emission. Note that the emission maximum in the spectra of Tl2LaCl5:Ce is shifting from 355 nm to 385 nm as the Ce3+ doping level is increased from 0 to 100%. The shift towards longer wavelengths is probably due to self-absorption and re-emission by Ce3+. This is a well-known effect that is more typically seen in Eu2+ doped halide scintillators at high (> 1%) doping levels [25]. The long wavelength shoulder at about 450 nm in the radioluminescence spectra of crystals with relatively low Ce3+ concentrations is attributed to emission from self-trapped excitons. A similar emission band was observed in K2LaCl5 doped with low (< 0.1%) Ce3+ concentrations [26]. Compared to Tl2LaCl5:1% Ce3+ (375 nm) the peak emission of Tl2LaBr5:1% Ce3+ is shifted towards slightly longer wavelengths (400 nm). We attribute this to a slightly smaller bandgap and deeper Ce3+ energy levels in the corresponding band gap. As will be shown in section 3.6, Density Functional Theory (DFT) results corroborate this observation.

Jou

226 227 228 229 230 231 232 233 234 235 236 237 238 239 240 241

Composition

lP repro of

Journal Pre-proof

Figure 6. X-ray excited radioluminescence spectra of Tl2LaCl5 doped with different Ce3+ concentrations.

rna

3.4 Gamma-ray Spectroscopy The pulse height spectrum of Tl2LaCl5:10% Ce3+ under 137Cs 662 keV gamma-ray irradiation is shown in Figure 8. To compare, the spectrum of NaI:Tl is shown as well. Note how the X-ray escape peak of thallium (at channel # 930) is well-separated from the main photopeak in the pulse height spectrum. Taking into account the differences in quantum efficiencies and assuming a light yield of about 38,000 ph/MeV for NaI:Tl, we estimate the light yield of Tl2LaCl5:10% Ce3+ to be about 68,000 ph/MeV. For a small sample of Tl2LaCl5:10% Ce3+ (10 mm3), we achieved a “best” energy resolution of 3.2% (FWHM) at 662 keV. Based on photon statistics and the proportional response of Tl2LaCl5:Ce3+ [4], the energy resolution can be expected to be improved down to 2.5% (FWHM) at 662 keV with better quality single crystals. The pulse height spectrum of Tl2LaBr5:1% Ce3+ under 137Cs 662 keV gamma-ray irradiation is shown in Figure 9. Note how the X-ray escape peak of thallium (at 590 keV) is once again well-separated from the main photopeak in the pulse height spectrum. We estimate the light yield of Tl2LaBr5:1% Ce3+ to be about 78,000 ph/MeV. For a small sample of Tl2LaBr5:1% Ce3+ (10 mm3), we achieved a “best” energy resolution of 2.8% (FWHM) at 662 keV. The proportionality of Tl2LaBr5:1% Ce3+ was measured using a variety of gamma-ray sources to cover an energy range from 32 keV up to 1274 keV. After measurement of a pulse height spectrum for each gamma-ray source, the relative light yield to that at 662 keV was Figure 8. Pulse height spectrum of Tl2LaCl5:10% plotted as function of energy and is shown in Ce3+ obtained with a 137Cs source. Note how the Figure 10. As seen in the figure, the response X-ray escape peak of thallium (at channel # 930) of Tl2LaBr5:1% Ce3+ is very linear down to 32

Jou

242 243 244 245 246 247 248 249 250 251 252 253 254 255 256 257 258 259 260 261 262 263 264 265 266 267 268 269 270

Figure 7. X-ray excited radioluminescence spectra of undoped and 1% Ce3+ doped Tl2LaBr5.

is well-separated from the main photopeak.

lP repro of

Journal Pre-proof

Figure 9. Pulse height spectrum of Tl2LaBr5:10% Ce3+ obtained with a 137Cs source. Note how the X-ray escape peak of thallium (at 590 keV) is well-separated from the main photopeak.

Figure 10. Proportionality of Tl2LaBr5:1% Ce3+ in the range from 14.4 keV up to 1274 keV.

keV, with just a 1 % deviation from absolute linearity. Considering the proportional response and higher light yield of Tl2LaBr5:Ce3+ compared to Tl2LaCl5:Ce3+, we expect that the energy resolution of Tl2LaBr5:Ce3+ can be improved to below 2.5% (FWHM) at 662 keV with better quality single crystals.

275 276 277 278 279 280 281 282 283 284 285 286 287 288 289 290 291 292 293 294 295 296 297 298

3.5. Scintillation Decay Scintillation decay time spectra of Tl2LaCl5 doped with different Ce3+ concentrations were measured using a 22Na source. The obtained decay curves were fitted with a two-component exponential decay function. The undoped sample shows a relatively slow scintillation decay comprised of a fast decay component with a lifetime of about 167 ns which contributes about 55% to the total scintillation light, and a longer decay component with a lifetime of about 344 ns which contributes about 45% to the total scintillation light. For samples with Ce3+ doping, a fast decay component with a life time in the range of 30 to 110 ns is observed, depending on the Ce3+ concentration, see Figure 11. Approximately 98+% of the scintillation light is due to this fast decay component. The trends seen in Figure 6 and Figure 11 indicate that the emitted light with a fast decay constant (30 – 110 ns) and an emission wavelength between 375 – 385 nm is probably due to a very fast electron-hole pair diffusion process to the Ce3+ site and not due to direct electron-hole capture by Ce3+, since the observed lifetime of the fast decay component is about twice that what is expected for direct Ce3+ 5d → 4f luminescence (typically 25 – 50 ns). However, this electron-hole pair diffusion process seems to be very efficient even for Figure 11. Scintillation decay time spectra of Tl2LaCl5 doped with different Ce concentrations. relatively low Ce3+ concentrations (≤ 1%) as

Jou

rna

271 272 273 274

Journal Pre-proof

the primary decay component already contributes 98% to the total light yield in the case of Tl2LaCl5:1% Ce3+. Contrast this for example to LaCl3:Ce3+ or K2LaCl5:Ce3+, where for similarly low Ce3+ concentrations the short decay component contributes less than 18% to the total scintillation light in the case of LaCl3:Ce3+ [27] while it contributes less than 40% to the total scintillation light in the case of K2LaCl5:Ce3+ [26]. Note that the lifetime of the short decay component decreases as the Ce3+ doping level is increased from 0 to 100%. This shift towards a shorter lifetime can be attributed to a shorter pathway for electron-hole pair diffusion, simply because there are more and more Ce3+ ions present in the lattice at higher Ce3+ concentrations. Note that concentration quenching may play a role at high Ce3+ concentrations as the light yield of those crystals decreases when the Ce3+ is increased beyond 10%. In this scenario the lifetime is reduced as well, by the same process. To summarize:

lP repro of

299 300 301 302 303 304 305 306 307 308 309 310 311 312 313 314 315 316 317 318 319 320

1) Ce3+ concentration range 0 – 10%: Electron-hole pair diffusion dominant. Light yield increases with Ce3+ concentration, scintillation decay becomes faster with increasing Ce3+concentration. 2) Ce3+ concentration range 10 – 100%: Concentration quenching dominant. Light yield decreases, scintillation decay becomes faster with increasing Ce3+ concentration. Table 2 summarizes the light yield, energy resolution, and scintillation decay times of Tl 2LaCl5 doped with different Ce3+ concentrations. From this table it appears that the highest light yield and best energy resolution can be obtained with a Tl2LaCl5:10% Ce3+ crystal, although slight deviations (± 5%) from this Ce3+ concentration might yield crystals with even better scintillation properties. Table 2. Light yield, energy resolution and scintillation decay of Tl2LaCl5 doped with different Ce3+ concentrations.

Light Yield (ph/MeV)

0%

31,500

0.3% 1% 10% 30%

321 322 323 324 325 326 327 328 329

Scintillation Decay (ns, {contribution})

7.2%

167 (55%), 344 (45%)

57,000

4.5%

107 (98%)

57,000

5.0%

93 (98%)

68,000

3.2%

44 (98%)

56,000

5.2%

32 (99%)

50,000

5.0%

30 (99%)

Jou

100%

Energy resolution (FWHM at 662 keV)

rna

[Ce]

3.6 Theoretical Modeling According to the Density Functional Theory (DFT) calculations using a hybrid PBE0 functional, both Tl2LaCl5 and Tl2LaBr5 have direct band gaps at the Γ point. Their calculated band gaps are 5.01 eV and 4.44 eV, respectively. The density of states (DOS) of Ce3+ doped Tl2LaBr5 is shown in Figure 12. The electronic structure of Tl2LaCl5 is similar to that of Tl2LaBr5 except that the former has a lower valence band energy and consequently a larger band gap. The conduction band of Tl2LaBr5 is made up of Tl-6p, La-5d, and Br-4p orbitals. The conduction band minimum (CBM) consists of mainly Tl-6p orbitals with smaller contributions from La-5d and Br-4p. The empty La-4f states are above 6 eV. The

lP repro of

Journal Pre-proof

Figure 12. Density of states (DOS) of Tl2LaBr5:Ce3+ with Ce3+ at its excited state. 12.5% of La atoms are replaced with Ce atoms in this calculation. The energy of the VBM is set at zero.

rna

valence band is primarily made up of Br-6p orbitals. However, there is a strong Tl-6s contribution near the valence band maximum (VBM) (-1 eV – 0). The Tl-6s orbitals are fully occupied and hybridize with the Br-4p orbitals; the resulting bonding orbitals are located around -5 eV and the antibonding orbitals are near the VBM. Thus, the presence of Tl has a strong impact on both the conduction and valence bands. The Tl-6p peak is slightly lower than the La-5d peak in the DOS of the conduction band while the hybridization of Tl-6s and Br-4p orbitals pushes up the VBM by about 1 eV. Compared to A2LaBr5 (A = alkali metal), Tl2LaBr5 should have a smaller band gap and a higher light yield. Trapping of an exciton by a Ce3+ dopant results in the appearance of empty Ce-4f levels and an occupied Ce-5d level inside the band gap, as shown in Figure 12 for Tl2LaBr5:Ce3+. The deep Ce-4f and 5d levels inside the band gap indicate strong hole and electron localization. The Ce-4f band exhibits two peaks due to the spin-orbit splitting of the Ce-4f levels. The calculated Ce3+ emission energies are 3.33 eV (373 nm) in Tl2LaCl5 and 3.46 eV (358 nm) in Tl2LaBr5, respectively. The experimentally measured Ce3+ emission typically displays two peaks corresponding to the 5d → 2F5/2 and 5d → 2F7/2 transitions. Because we calculate the lowest energy state for both the ground and excited states, the calculated Ce3+ emission energy should be compared to the 5d → 2F5/2 transition in experiment. From the radioluminescence of Tl2LaCl5:Ce3+ and Tl2LaBr5:Ce3+ shown in Figure 6 and Figure 7, respectively, we estimate that the position of the Ce3+ 5d → 2F5/2 emission energies in the experimentally obtained spectra can be found at 365 and 375 nm, respectively, which is in reasonable agreement with the calculated values. These results show that, although incorporating Tl (one of the most electronegative cations) in Tl2LaBr5 results in a relatively small band gap, the electron and hole trapping levels of Ce3+ are still deep, leading to strong exciton localization and emission at Ce3+.

Jou

330 331 332 333 334 335 336 337 338 339 340 341 342 343 344 345 346 347 348 349 350 351 352

Journal Pre-proof

353 354 355 356 357 358 359

4. ACKNOWLEDGEMENT

360 361 362 363 364 365 366 367 368 369 370 371 372 373

5. CONCLUSION

374

6. REFERENCES

lP repro of

Powder X-Ray Diffraction was performed at the Joint Institute for Advanced Materials (JIAM) Diffraction Facility, located at the University of Tennessee, Knoxville. This research was sponsored by the Department of the Defense, Defense Threat Reduction Agency of the United States under grant award no. HDTRA1-19-1-0014. The content of this paper does not necessarily reflect the position or the policy of the federal government, and no official endorsement should be inferred. In this paper we report on the crystallographic and scintillation properties of Tl2LaCl5:Ce3+ and Tl2LaBr5:Ce3+, two novel thallium-containing high-resolution scintillators for gamma-ray spectroscopy. The scintillators show high light yields of up to 68,000 photons/MeV, excellent gamma-ray energy resolution of ≤ 3% at 662 keV, a fast scintillation decay, and a proportional response over a wide range of energies from 32 keV up to 1275 keV. Density Functional Theory calculations show that the Ce3+ energy levels are inside the bandgap despite the smaller bandgap of Tl2LaCl5 and Tl2LaBr5 compared to K2LaCl5 and K2LaBr. The calculated emission wavelengths are in reasonable agreement with the experimentally determined values. Tl2LaCl5:Ce3+ appears to be less hygroscopic then NaI:Tl, whereas Tl2LaBr5:Ce3+ is much more so. With a calculated density of 5.16 g/cm3 and 5.98 g/cm3, respectively, and an effective Z of 79 and 67, respectively, both Tl2LaCl5:Ce3+ and Tl2LaBr5:Ce3+ seem to be very promising materials for high resolution gamma-ray spectroscopy.

Jou

rna

[1] R. Hawrami , E. Ariesanti, H. Wei , J. Finkelstein, J. Glodo, and K. Shah, “Tl2LiYCl6: Large Diameter, High Performing Dual Mode Scintillator”, Crystal Growth & Design 17 (2017) 3960 – 3964. [2] G. Rooh, H. J. Kim, H. Park, Sunghwan Kim, “Crystal growth and scintillation characterizations of Tl2LiYCl6: Ce3+”, J. Crystal Growth 459 (2017) 163 – 165. [3] H. J. Kim, Gul Rooh, and Sunghwan Kim, “Tl2LaCl5 (Ce3+): New Fast and Efficient Scintillator for Xand γ-ray detection,” arXiv (2016) 1607.05486. [4] R. Hawrami, E. Ariesanti, H. Wei, J. Finkelstein, J. Glodo, K. S. Shah, “Tl2LaCl5:Ce, high performance scintillator for gamma-ray detectors”, Nucl. Instr. Meth. Phys. Res. A 869 (2017) 107 – 109. [5] E. V. van Loef, G. Ciampi, U. Shirwadkar, L. Soundara Pandian, K. S. Shah, “Crystal growth and scintillation properties of Thallium-based halide scintillators”, J. Crystal Growth 532 (2020) 125438. [6] R. Král, P. Zemenová, V. Vaněček, A. Bystřický, M. Kohoutková, V. Jarý, S. Kodama, S. Kurosawa, Y. Yokota, A. Yoshikawa, M. Nikl, "Thermal analysis of cesium hafnium chloride using DSC–TG under vacuum, nitrogen atmosphere, and in enclosed system," J. Therm. Anal. Calorim. (2019) 1-9. [7] T. Degen, M. Sadki, E. Bron, U. König, and G. Nénert, "The HighScore suite", Powder Diffraction 29 (2014) S13-S18. [8] B. H. T. a. R. B. V. Dreele, "GSASII: the genesis of a modern open-source all-purpose crystallography software packaged", J. Appl. Crystall. 46 (2013) 544-549. [9] G. Meyer, J. Soose, A. Moritz, V. Vitt, and T. Holljes, "Ternäre Halogenide der Seltenen Erden vom Typ A2MX5 (A = K, In, NH4, Rb, Cs; X = Cl, Br, I)", Z. anorg. allg. Chemie 521 (1985) 161-172. [10] J. P. Perdew, M. Emzerhof, K. Burke, “Rationale for mixing exact exchange with density functional approximations”, J. Chem. Phys. 105 (1996) 9982-9985. [11] G. Kresse, J. Furthmüller, “Efficiency of ab-initio total energy calculations for metals and semiconductors using a plane-wave basis set”, Comp. Mat. Sci. 6 (1996) 15-50. [12] G. Kresse, D. Joubert, “From ultrasoft pseudopotentials to the projector augmented-wave method”, Phys. Rev. B 59 (1999) 1758.

Journal Pre-proof

Jou

rna

lP repro of

[13] R. O. Jones, O. Gunnarsson, “The density functional formalism, its applications and prospects”, Rev. Mod. Phys. 61 (1989) 689-746. [14] A. Görling, “Density-functional theory beyond the Hohenberg-Kohn theorem”, Phys. Rev. A 59 (1999) 3359-3374. [15] A. Hellman, B. Razaznejad, B. I. Lundqvist, “Potential-energy surfaces for excited states in extended systems”, J. Chem. Phys. 120 (2004) 4593-4602. [16] C. Zhou, H. Lin, Q. He, L. Xu, M. Worku, M. Chaaban, S. Lee, X. Shi, M.-H. Du, B. Ma, “Low dimensional metal halide perovskites and hybrids”, Mat. Sci. Eng.: R: Reports 137 (2019) 38-65. [17] M.-H. Du, “Microscopic origin of multiple exciton emission in low-dimensional lead halide perovskites”, J. Chem. Phys. 151 (2019) 181101. [18] M.-H. Du, “Emission Trend of Multiple Self-Trapped Excitons in Luminescent 1D Copper Halides”, ACS Energy Letters 5 (2020) 464-469. [19] D. Han, H. Shi, W. Ming, C. Zhou, B. Ma, B. Saparov,Y.-Z. Ma, S. Chen, M.-H. Du, “Unraveling luminescence mechanisms in zero-dimensional halide perovskites”, J. Mat. Chem. C 6 (2018) 63986405. [20] H. Shi, D. Han, S. Chen, M.-H. Du, “Impact of metal ns2 lone pair on luminescence quantum efficiency in low-dimensional halide perovskites”, Phys. Rev. Mat. 3 (2019) 034604. [21] M. Loyd, L. Stand, D. Rutstrom, Y. Wu, J. Glodo, K. Shah, M. Koschan, C. L. Melcher, M. Zhuravleva, "Investigation of CeBr3−xIx scintillators”, J. Crystal Growth 531 (2020) 125365. [22] H. Wei, V. Martin, A. Lindsey, M. Zhuravleva, and C. L. Melcher, "The scintillation properties of CeBr3−xClx single crystals", J. Lum. 156 (2014) 175-179. [23] M. Loyd, A. Lindsey, M. Patel, M. Koschan, C. L. Melcher, and M. Zhuravleva, "Crystal structure and thermal expansion of CsCaI3:Eu and CsSrBr3:Eu scintillators," J. Crystal Growth 481 (2018) 3539. [24] A. C. Lindsey, M. Loyd, M. K. Patel, R. Rawl, H. Zhou, M. Koschan, C. L. Melcher, M. Zhuravleva, "Determination of thermal expansion of KCaI3 using in-situ high temperature powder X-ray diffraction," Mat. Chem. Phys. 212 (2018) 161-166. [25] Jarek Glodo, Edgar V. van Loef, Nerine J. Cherepy, Stephen A. Payne, and Kanai S. Shah, “Concentration Effects in Eu Doped SrI2,” IEEE Trans. Nucl. Sci. 57 (2010) 1228 – 1232. [26] J. C. van’t Spijker, P. Dorenbos, J. T. M. De Haas, C. W. E. van Eijk, H .U. Güdel and K. Krämer, “Scintillation properties of K2LaCl5 with Ce doping,” Rad. Meas. 24 (1995) 379 – 381. [27] E. V. D. van Loef, P. Dorenbos, and C. W. E. van Eijk, K. Krämer and H. U. Güdel, “Scintillation Properties of LaCl3:Ce3+ Crystals: Fast, Efficient, and High-Energy Resolution Scintillators”, IEEE Trans. Nucl. Sci. 48 (2001) 341 – 345.

Journal Pre-proof

Jou

rna

lP repro of

Urmila Shirwadkar: Writing - Original Draft Matthew Loyd: Formal analysis, Investigation Mao-Hua Du: Software, Validation Edgar van Loef: Investigation, Data Curation, Writing - Review & Editing, Visualization, Supervision, Project administration, Funding acquisition Guido Ciampi: Investigation Lakshmi Soundara Pandian: Formal analysis, Investigation Luis Stand: Formal analysis Investigation Merry Koschan: Resources Mariya Zhuravleva: Supervision Charles Melcher: Supervision Kanai Shah: Supervision, Funding acquisition

Journal Pre-proof

Declaration of interests

lP repro of

☒ The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Jou

rna

☐The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: