Effects of zirconium codoping on the optical and scintillation properties of SrI2:Eu2+ single crystals

Accepted Manuscript Effects of zirconium codoping on the optical and scintillation properties of SrI2 :Eu2+ single crystals Yuntao Wu, Qi Li, Daniel J. Rutstrom, Ian Greeley, Luis Stand, Matthew Loyd, Merry Koschan, Charles L. Melcher

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S0168-9002(18)31221-X https://doi.org/10.1016/j.nima.2018.09.077 NIMA 61242

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Nuclear Inst. and Methods in Physics Research, A

Received date : 28 July 2018 Accepted date : 17 September 2018 Please cite this article as: Y. Wu, et al., Effects of zirconium codoping on the optical and scintillation properties of SrI2 :Eu2+ single crystals, Nuclear Inst. and Methods in Physics Research, A (2018), https://doi.org/10.1016/j.nima.2018.09.077 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 proof before it is published in its final 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.

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Effects of zirconium codoping on the optical and scintillation properties of SrI2:Eu2+ single crystals Yuntao Wu,*,1,2 Qi Li,3,4 Daniel J. Rutstrom,1,2 Ian Greeley,1,2 Luis Stand,1,2,5 Matthew Loyd,1,2 Merry Koschan,1 and Charles L. Melcher1,2,5,6

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Scintillation Materials Research Center, 2Department of Materials Science and Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA 3 Physical Science Division, IBM Thomas J Watson Research Center, Yorktown Heights, NY 10598, USA 4 Department of Computer Science, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA 5 Bredesen Center for Interdisciplinary Research and Graduate Education, University of Tennessee, Knoxville, TN 37996, USA 6 Department of Nuclear Engineering, University of Tennessee, Knoxville, Tennessee 37996, USA

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*Corresponding author: E-mail: [email protected], [email protected] (Y. Wu)

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Abstract Europium doped strontium iodide (SrI 2:Eu2+) is one of the cutting-edge

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halide scintillators for gamma-ray spectroscopy applications due to its low

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radioactive background, ultrahigh light yield (70,000-100,000 photons/MeV), and

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excellent gamma-ray energy resolution (2.6-3% at 662 keV). In this work, we report

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an improvement in the scintillation characteristics of SrI2:Eu2+ single crystals via

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tetravalent ion codoping, namely Zr 4+. High quality SrI 2:Eu2+ single crystals codoped

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with 0, 0.05, 0.1, and 0.2 at% Zr 4+ were grown by the Multi-ampoule Bridgman

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method. All of the as-grown crystals are transparent, crack-and inclusion-free. They

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have high optical quality with a transmittance of over 70%. The effects of Zr 4+

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codoping on spectral properties and decay kinetics of SrI2:Eu2+ under optical, and x-

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and gamma-ray excitation are studied. An enhancement of gamma-ray spectroscopic

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resolution from 2.8 to 2.5% at 662 keV and 5.4 to 5% at 122 keV is achieved upon

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Zr4+ codoping with a slightly enhanced light yield of 95,000 photons/MeV.

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Keywords: Scintillator, halides, single crystals, codoping.

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1. Introduction

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In order to meet the requirements of advanced gamma-ray spectroscopic

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applications, several notable halide scintillators with superior energy resolutions below

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3% at 662 keV were developed during the past twenty years, such as LaBr3 [1],

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SrI2:Eu [2], KSr2I5:Eu [3], KCaI3:Eu [4], and KCa0.8Sr0.2I3:Eu [5]. The optimization of

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existing scintillators is an important and indispensable strategy for developing

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advanced radiation materials in addition to the discovery of new compounds with

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promising performance. A codoping strategy has been used to engineer existing

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inorganic scintillators for targeted applications [6,7], but the progress made on

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improvement in gamma-ray energy resolution is limited to few compounds, such as

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LaBr3:Ce [8], CeBr3 [9], NaI:Tl [10], and KCaI3:Eu [11]. For example, the energy

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resolutions at 662 keV of LaBr3:Ce and CeBr3 were improved to 2% and 3% at 662

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keV, respectively, by Sr2+ codoping[8,9]. In the case of NaI:Tl, a long-standing

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workhorse scintillator, energy resolution can be improved by Sr2+ or Ca2+ codoping

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due to the suppression of slow scintillation processes [10]. We recently reported a

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beneficial effect of Zr4+ codoping on energy resolution of KCaI3:Eu, achieving 2.7% at

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662 keV due to a strong reduction in scintillation light yield loss [11].

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The potential benefits of shallow charge carrier traps in tailoring non-proportionality and

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energy resolution of inorganic scintillators were recognized in the recent years [11-13]. The

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energetically shallower electron/hole traps formed internally or induced by codopants can

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enable the reduction of the non-radiative Auger quenching through temporally capturing

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carriers to enhance the charge carrier separation [11-13].

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Despite the fact that the cutting-edge SrI2:Eu scintillator has an excellent gamma-ray

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energy resolution of 3% at 662 keV [14], many attempts have been made to further optimize

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its energy resolution by isovalent and aliovalent ion codoping. However, none have been

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successful so far, including codoping with monovalent (Cs+, Na+, and Cu+), divalent (Mg2+,

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Ba2+, Ca2+, Fe2+, and Sn2+), and even trivalent (La3+, Gd3+, and Lu3+) ions [15,16]. Based on

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the density functional theory calculations, SrI2:Eu lacks deep-to-shallow trap behavior upon

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codoping of monovalent and divalent ions [13], unlike the Sr2+ codoped LaBr3:Ce [12].

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Specifically, the acceptor-like codopants and iodine vacancies can form energetically

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favorable acceptor-vacancy complexes in the SrI2 host, but the complex defect is not a

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shallow trap [13]. In this work, the effects of Zr4+ codoping on optical and scintillation

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properties of SrI2:Eu2+ single crystals were comprehensively studied and we report an

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improvement in the scintillation properties.

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2. Experimental

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The multi-ampoule Bridgman growth technique [17] was used to grow 22 mm diameter

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0, 0.05, 0.1, and 0.2 at% Zr codoped (Sr0.97Eu0.03)I2 crystals. The codopant concentrations

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given refer to the initial starting melt, and calculations are based on the assumption

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that the Zr ions substitute for Sr ions. High-purity anhydrous SrI2 and EuI2 beads

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(99.999%) and ZrI4 (99.95%) from APL Engineered Materials Inc. were used. The

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processes of material handling and crystal growth are the same as that of KCaI3 [15]

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because of the similar hygroscopicity and melting point (538C for SrI2 and 540C for

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KCaI3). The only difference is that the translation speed during crystal growth was 1

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mm/h for the Zr codoped SrI2:Eu.

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A detailed description of the measurement setups for optical transmission

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spectrum, absolute light yield, energy resolution, non-proportionality (nPR), X-ray

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excited radioluminescence (RL), and scintillation decay time can be found in Ref. 18.

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Photoluminescence emission (PL) and excitation (PLE) spectra were obtained with a

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HORIBA Jobin Yvon Fluorolog-3 spectrofluorometer. A 450 W continuous xenon lamp was

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used as the excitation source. Photoluminescence decay time was measured on a HORIBA

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Jobin Yvon Fluorolog-3 spectrofluorometer using a time-correlated-single-photon counting

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module. HORIBA Jobin Yvon NanoLEDs (pulsed light-emitting diodes) were used as the

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excitation source. The duration of the light pulse was shorter than 2 ns.

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3. Results and discussion

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3.1 Optical quality of as-grown crystals

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The as-grown boules are transparent, crack- and inclusion-free. The non-codoped

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and 0.05 at% Zr4+ codoped SrI2:Eu single crystals are shown in Fig. 1(a) and (b) as

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representatives. For each composition, the optical quality of a 1 mm thick sample was

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evaluated using optical transmission spectra. As observed in Fig. 1(c), all samples

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have an optical transmittance over 70% between 450 and 700 nm.

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Figure 1. As-grown boules of (a) non-codoped and (b) 0.05 at% Zr4+ codoped SrI2:Eu. (c) Optical transmission spectra of 1 mm thick non-codoped and Zr4+ codoped SrI2:Eu crystal slabs. A typical sample used for the optical transmission spectrum measurement is shown in the inset of (c).

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3.2 Spectral properties and decay kinetics under optical, x- and gamma-ray excitation

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The PLE and PL spectra of non-codopd and Zr4+ codoped samples at room

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temperature are shown in Fig. 2. For all samples, the excitation spectrum monitored at

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an emission wavelength between 431 and 436 nm (Eu2+ 5d1-4f de-excitation) consists

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of several broad bands associated with the transitions from 4f ground state to the 5d

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excited states of Eu2+. The Eu2+ 5d1-4f de-excitation of non-codopd SrI2:Eu2+ peaks at

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around 430 nm [19,20]. With the increase of Zr4+ codoping concentration, the Eu2+

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5d1-4f emission band is slightly redshifted. The emission redshift is also observed in

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RL spectra. The broader full width at half maximum of the emission peak in the RL

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spectra compared to that of PL spectra can be ascribed to a severe reabsorption effect

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caused by use of in-line transmission measurement geometry instead of reflection

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geometry.

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Figure 2. Photoluminescence emission and excitation spectra, and X-ray excited radioluminescence spectra of SrI2:Eu codoped with different Zr4+ codoping concentrations.

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The photoluminescence and scintillation decay profiles of non-codoped and Zr4+

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codoped SrI2:Eu samples were measured, and plotted in Fig. 3(a) and (b). All the decay

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profiles can be fit well by a single exponential function. The PL and scintillation decay

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constants as a function of Zr4+ codoping concentration are plotted in Fig. 3(c). For non-

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codoped SrI2:Eu, PL and scintillation decay constants are 1.00 s and 1.03 s, respectively. 5

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With increasing Zr4+ codoping concentration, both PL and scintillation decay constants tend

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to first decrease and then increase. The shortening of PL decays in the 0.1 at% Zr4+ codoped

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sample is probably related to an enhanced non-radiative recombination occurring at Eu2+

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centers. The prolonged PL and scintillation decay time observed in high-codoped samples can

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be ascribed to the enhanced self-absorption due to a smaller Stokes shift with the increase of

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Zr4+ codoping concentration (see Fig. 2).

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Figure 3. (a) PL decay profiles monitored at ex=370 nm and em=436 nm and (b) scintillation decay profiles under 137Cs irradiation of non-codoped and Zr4+ codoped SrI2:Eu single crystals. (c) The PL and scintillation decay constants as a function of Zr4+ codoping concentration.

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3.3 Scintillation characteristics

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The absolute light yields of 5 mm3 non-codoped and Zr4+ codoped SrI2:Eu samples were

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evaluated by using the single photoelectron peak of the R2059 PMT [21]. The wavelength-

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weighted quantum efficiency of the R2059 PMT is also considered for all samples. The 137Cs

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pulse height spectra are shown in Fig. 4(a). The estimated absolute light yield as a function of

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Zr4+ codoping concentration is presented in Fig. 4(b). The light yield of non-codoped SrI2:Eu

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is 93,000 photons/MeV, consistent with the published results [20,22,23]. Upon 0.05 at% Zr4+

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codoping, the light yield slightly increases to 95,000 photons/MeV. When further increasing

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Zr4+ concentration, it decreases to 85,000-90,000 photons/MeV.

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Figure 4. (a) 137Cs pulse height spectra of non-codoped and Zr4+ codoped SrI2:Eu single crystals acquired by a Hamamatsu R2059 PMT. (b) Absolute light yield as a function of Zr4+ codoping concentration.

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The energy resolutions of 5 mm3 non-codoped and Zr4+ codoped SrI2:Eu2+ crystals at

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122 and 662 keV were evaluated. The reported energy resolution for SrI2 doped with 1 to 5

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at% Eu2+ is between 2.6 and 3.1% at 662 keV for a small size sample, such as a few cubic

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millimeters [20,22,23]. The non-codoped sample studied here has a normal energy resolution

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of 5.36% at 122 keV and 2.85% at 662 keV. These values are improved to 5.0% at 122 keV

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and 2.5% at 662 keV in the 0.05 at% Zr4+ codoped sample. The associated pulse height

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spectra are shown in Fig. 5(a) and (b). When the Zr4+ codoping concentration is further

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increased, the energy resolution at 662 keV degrades to 2.77% for 0.1 at% Zr4+ and 3.15% for

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0.2 at% Zr4+. A similar trend is observed for the energy resolution at 122 keV. The energy

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resolutions at 122 and 662 keV as a function of Zr4+ codoping concentration are plotted in

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Fig. 5(c). For these samples with high light yield (>60,000 photons/MeV) and small size (in

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millimeters), the energy resolution is expected to be mainly determined by light yield non-

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proportionality. The nPR curves of SrI2:Eu single crystals codoped with different Zr4+

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concentrations are shown in Fig. 5(d). The reduced “halide hump” [24] of SrI2:Eu with Zr4+

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codoping can lead to a better nPR. The inverse correlation between the “hump” size and the 7

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Zr4+ codoping level could be associated with the formation of new Zr-related defects and/or

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change in existing defect concentration. However, the variation of nPR as a function of Zr4+

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codoping concentration cannot be well correlated with the variation trend of energy

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resolution. More study is needed to reveal the physical origins, but it is beyond the scope of

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this work and will be investigated in a future publication.

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Figure 5. Pulse height spectra of the 5 mm3 0.05% Zr4+ codoped SrI2:Eu crystal under (a) 137 Cs and (b) 57Co irradiation acquired by a Hamamatsu R6231-100 PMT. (c) Energy resolutions at 122 and 662 keV as a function of Zr4+ codoping concentration. (d) The nPR curves of SrI2:Eu single crystals codoped with different Zr4+ concentrations.

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To study the effect of sample size on energy resolution improvement, the gamma-ray

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spectroscopic performance of a 22 mm  10 mm 0.05 at% Zr codoped SrI2:Eu sample was

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evaluated as well as a non-codoped sample of the same size for comparison. The pulse height

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spectra of 22 mm  10 mm non-codoped and 0.05% Zr4+ codoped SrI2:Eu samples under

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137

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the 22 mm  10 mm SrI2:Eu sample is improved from 3.7% to 3.3% at 662 keV, 6.5% to

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5.7% at 122 keV, and 9.3% to 8.2% at 59.5 keV by 0.05 at% Zr4+ codoping. This verifies the

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energy resolution improvement in larger size SrI2:Eu single crystals by Zr4+ codoping.

Cs, 57Co and 241Am irradiation are plotted in Fig. 6. It is found that the energy resolution of

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Figure 6. Pulse height spectra of 22 mm  10 mm (a) non-codoped and (b) 0.05% Zr4+ codoped SrI2:Eu crystals under 137Cs, 57Co and 241Am irradiation acquired by a Hamamatsu R6231-100 PMT.

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4. Conclusions

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High quality SrI2:Eu single crystals codoped with 0, 0.05, 0.1, and 0.2 at% Zr4+ were

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successfully grown by the Multi-ampoule Bridgman method. All the as-grown crystals are

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transparent, and crack-and inclusion-free. The luminescence mechanism of the Eu2+ centers

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in the SrI2 host lattice is not changed by Zr4+ codoping, except for the enhancement of self-

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absorption due to a reduced Stokes shift. With increasing Zr4+ codoping concentration, both

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PL and scintillation decays show a trend of decreasing first and then increasing. The energy

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resolution of 5 mm3 SrI2:Eu is improved from 2.8 to 2.5% at 662 keV and 5.4 to 5% at 122

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keV with 0.05 at% Zr4+ codoping. The energy resolution improvement in larger size samples

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by Zr4+ codoping has also been verified. The light yield is slightly enhanced to 95,000

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photons/MeV by 0.05 at% Zr4+ codoping, and then decreases with a further increase in Zr4+

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concentration. Future work will aim at understanding the energy resolution improvement

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through theoretical calculations and experimental investigations.

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Acknowledgements This work has been supported by Siemens Medical Imaging.

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References

5

[1] E. V. D. van Loef, P. Dorenbos, C. W. E. van Eijk, K. Krämer, and H. U. Güdel, Appl.

6

Phys. Lett. 79 (2001) 1573.

7

[2] N. J. Cherepy, G. Hull, A. D. Drobshoff, S. A. Payne, E. van Loef, C. M. Wilson, K. S.

8

Shah, U. N. Roy, A. Burger, L. A. Boatner, W.-S. Choong, and W. W. Moses, Appl. Phys.

9

Lett. 92 (2008) 083508.

10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

[3] L. Stand, M. Zhuravleva, A. C. Lindsey, and C. L. Melcher, Nucl. Instrum. Methods Phys. Res. A 780 (2015) 40. [4] A. C. Lindsey, M. Zhuravleva, L. Stand, Y. T. Wu, and C. L. Melcher, Opt. Mater. 48 (2015) 1. [5] Y. Wu, Q. Li, B. C. Chakoumakos, M. Zhuravleva, A. C. Lindsey, J. A. Johnson II, L. Stand, M. Koschan, and C. L. Melcher, Adv. Optical Mater. 4 (2016) 1518. [6] C. L. Melcher, M. Koschan, M. Zhuravleva, Y. Wu, H. Rothfuss, F. Meng, M. Tyagi, S. Donnald, K. Yang, J. P. Hayward, and L. Eriksson, JPS Conf. Proc. 11 (2016) 020001. [7] C. Foster, Y. Wu, M. Koschan, and C. L. Melcher, Phys. Status Solidi RRL DOI: 10.1002/pssr.201800280. [8] M. S. Alekhin, J. T. M. de Haas, I. V. Khodyuk, K. W. Krämer, P. R. Menge, V. Ouspenski, and P. Dorenbos, Appl. Phys. Lett. 102 (2013) 161915. [9] F. G. A. Quarati, M. S. Alekhin, K. W. Krämer, P. Dorenbos, Nucl. Instrum. Methods Phys. Res. A 735 (2014) 655. [10] K. Yang, P. R. Menge, J. Appl. Phys. 118 (2015) 213106.

10

1 2

[11] Y. Wu, Q. Li, D. Rutstrom, M. Zhuravleva, M. Loyd, L. Stand, M. Koschan, and C. L. Melcher, Phys. Status Solidi RRL 12 (2018) 1700403. ber , B. Sadigh, A. Schleife, P. Erhart, Appl. Phys. Lett. 104 (2014) 211908.

3

[12

4

[13] Q. Feng and K. Biswas, J. Appl. Phys. 120 (2016) 213104.

5

[14] R. Hawrami, J. Glodo, K. S. Shah, N. Cherepy, S. Payne, A. Burger, L. Boatner, J. Cryst.

6

Growth 379 (2013) 69.

7

[15] S. Lam, S. E. Swider, A. Datta, and S. Motakef, IEEE Trans. Nucl. Sci. 62 (2015) 3397.

8

[16] K. Nishimoto, Y. Yokota, S. Kurosawa, J. Pejchal, K. Kamada, V. Chani, and A.

9 10 11 12 13 14 15 16 17 18 19

Yoshikawa, J. Cryst. Growth 401 (2014) 484. [17] A. C. Lindsey, Y. Wu, M. Zhuravleva, M. Loyd, M. Koschan, and C. L. Melcher, J. Cryst. Growth 470 (2017) 20. [18] Y. Wu, Q. Li, S. Jones, C. Dun, S. Hu, M. Zhuravleva, A. Lindsey, L. Stand, M. Loyd, M. Koschan, J. Auxier, H. Hall, C. L. Melcher, Phys. Rev. Appl. 8 (2017) 034011. [19] Y. Wu, L. A. Boatner, A. C. Lindsey, M. Zhuravleva, S. Jones, J. D. Auxier II, H. L. Hall, and C. L. Melcher, Cryst. Growth Des. 15 (2015) 3929. [20] M. S. Alekhin, J. T. M. de Haas, K. W. Krämer, and P. Dorenbos, IEEE Trans. Nucl. Sci. 58 (2011) 2519. [21] M. Moszynski, M. Kapusta, M. Mayhugh, D. Wolski, and S. O. Flyckt, Absolute light yield of scintillators, IEEE Trans. Nucl. Sci. 44 (1997) 1052.

20

[22] L. A. Boatner, J. O. Ramey, J. A. Kolopus, R. Hawrami, W. M. Higgins, E. van Loef, J.

21

Glodo, K. S. Shah, E. Rowe, P. Bhattacharya, E. Tupitsyn, M. Groza, A. Burger, N. J.

22

Cherepy, and S. A. Payne, J. Cryst. Growth 379 (2013) 63.

23 24

[23] Y. Yokota, K. Nishimoto, S. Kurosawa, D. Totsuka, and A. Yoshikawa, J. Cryst. Growth 375 (2013) 49.

11

1 2

[24] Q. Li, J. Q. Grim, K. B. Ucer, A. Burger, G. A. Bizarri, W. W. Moses, and R. T. Williams, Phys. Status Solidi RRL 6 (2012) 346.

3

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