This study reports on the crystal growth, luminescence, and scintillation performance of a novel elpasolite K2LiCeCl6 scintillator. The single crystal of this material was grown by the two-zone vertical Bridgman technique. The luminescence and scintillation performance were studied under X-,
Radiation Measurements 141 (2021) 106524 Contents lists available at ScienceDirect Radiation Measurements journal homepage: www.elsevier.com/locate/radmeas Crystal growth and characterization of K2 LiCeCl6 , a novel elpasolite scintillator J.Y Cho a , H.J Kim a , Arshad Khan a , J.M Park b ,∗ a b Department of Physics, Kyungpook National University, Daegu, 41566, Republic of Korea Advanced Radiation Technology Institute, Korea Atomic Energy Research Institute, Jeongup 56212, Republic of Korea ARTICLE INFO Keywords: K2 LiCeCl6 Elpasolite Luminescence Scintillation Pulse shape discrimination ABSTRACT This study reports on the crystal growth, luminescence, and scintillation performance of a novel elpasolite K2 LiCeCl6 scintillator The single crystal of this material was grown by the two-zone vertical Bridgman technique The luminescence and scintillation performance were studied under X-, 𝛾-rays, and 𝛼-particles excitation at room temperature The fast scintillation response and 𝛼-particles and 𝛾-ray separation capability reveal that this scintillator can be used as a dual-mode 𝛾-rays and thermal neutron detector Introduction Nondestructive inspection of hazardous materials is important in various places such as airport and harbor for national and global security worldwide Scintillators are utilized as radiation detectors for nondestructive inspection because of their radiation spectroscopic capabilities Scintillators are also used widely for applications such as medical diagnostic imaging modalities that use X-rays or 𝛾-rays Moreover, the detection of both neutrons and 𝛾-rays are usually required in homeland security applications In most of these applications, scintillators with high density, high effective atomic number, good energy resolution, high light yield, fast timing response, and emission wavelength matching with the spectral sensitivity response of modern photosensors are required To fulfill these requirements, the search for new inorganic scintillators with superior scintillation performance are actively participated by worldwide researchers The Li-contained inorganic elpasolites are found to be one of the most attractive class of scintillators for dual-mode 𝛾-rays and neutrons detection Moreover, most of the elpasolites possess cubic or pseudo-cubic isotropic structures, and therefore exhibit lower thermal stresses and cracks during crystal growth compared to materials with complex structures The developed Li-contained potential elpasolite scintillators includes halides, such as Cs2 LiYCl6 :Ce (CLYC), Cs2 LiLaBr6 (CLLB), and Cs2 LiLaCl6 :Ce (CLLC), oxides, and fluoride scintillators such as LiAlO2 , LiCaAlF6 , and LiSrAlF6 for dual-mode 𝛾-rays and neutrons detection (Khan and Machrafi, 2014; Machrafi et al., 2014; Yamaji et al., 2011; Pejchal et al., 2011; Yoshikawa et al., 2009; Yokota et al., 2011) Most of the studied elpasolite scintillators usually have high light yield, good energy resolution, proportionality of light response, easy to grow in single crystalline form, and most importantly, good pulse shape discrimination for charged particles and photons (Yang et al., 2009a; Yang et al., 2013) This study focuses on the search for novel Li- and Cl-contained elpasolite scintillators for 𝛾-rays, thermal neutrons with Li, and fast neutrons with 35 Cl detection (Mughabghab, 2003) For obtaining fast fluorescence decay time and a well matching spectral response for traditional photosensors such as photomultiplier tube (PMT), silicon multiplier (SiPM), and avalanche photo diode (APL), Ce3+ is incorporated in the lattice structure The presence of Ce3+ in the host material has the advantage of fast luminescence and uniformity of light yield along the crystal Based on these considerations, we are reporting our preliminary study on crystal growth, luminescence, and scintillation properties of K2 LiCeCl6 (KLCC) crystal The KLCC crystal was grown by the two-zone vertical Bridgman technique Luminescence and scintillation properties, such as emission spectrum, light yield, and fluorescence decay time were measured under X-rays and 𝛾-rays excitation at room temperature The discrimination between the 𝛼particles and 𝛾-rays excitation were evaluated to check the capability of KLCC for neutron detection To the best of our knowledge, this is the first report on the single crystal growth, luminescence, and scintillation properties of KLCC crystal Experimental methods 2.1 Sample preparation The KLCC single crystal scintillator was grown using the two-zone vertical Bridgman technique The high purity ultra-dry KCl (99.999%), ∗ Corresponding author E-mail address: jmp1024@kaeri.re.kr (J.M Park) https://doi.org/10.1016/j.radmeas.2021.106524 Received 26 July 2020; Received in revised form 29 December 2020; Accepted 15 January 2021 Available online 23 January 2021 1350-4487/© 2021 The Authors Published by Elsevier Ltd This is an open (http://creativecommons.org/licenses/by-nc-nd/4.0/) access article under the CC BY-NC-ND license Radiation Measurements 141 (2021) 106524 J.Y Cho et al Fig (left) Photographic view of the grown KLCC crystal and (right) prepared sample immersed in mineral oil to avoid the effect of humidity on the crystal surface LiCl (99.998%), and CeCl3 (99.99%) powders purchased from Alfa Aesar were used as starting materials The natural Li abundance in natural LiCl is 7.6% These powders were put in a 10-mm inner diameter quartz ampule and dried at 250 ◦ C for several hours to remove any residual moisture All handling before and after crystal growth were performed in ultra-low humidity argon-purged glove box to avoid the effect of humidity After drying, the ampule was sealed in a vacuum of 10−6 torr The sealed ampule containing the KLCC stoichiometric charge was transferred to the furnace and sintered at 500 ◦ C To find out the melting point of the KLCC sample, a vertical transparent furnace was used The melting point of the KLCC was found to be about 600 ◦ C After sintering and melting, the ampule was transferred to the two-zone vertical Bridgman furnace for crystal growth The crystal was grown at a rate of 0.5 mm/h at a thermal gradient of 10 ◦ C/cm The KLCC crystal was cut using a diamond-coated stainless steel wire saw and optically polished inside a low-humidity glove box The grown crystal and polished sample of × × mm3 dimension are shown in Fig Because of the hygroscopic nature of the grown KLCC crystal like other halide scintillators (Zhuravleva et al., 2013), scintillation properties were measured inside the low-humidity argon-purged glove box Fig X-ray-induced emission spectrum of the KLCC crystal Result and discussion 2.2 Experimental set up 3.1 X-ray induced emission spectrum An X-ray tube having a W anode with operating voltage of 100 kV and current of 1.5 mA was used to measure X-ray induced emission spectrum of the grown crystal The emission spectrum was recorded using a calibrated QE65000 ocean optics fiber spectrometer The pulse height spectrum, non-proportionality response, decay time, quenching factor, and pulse shape discrimination were measured with a super bi-alkali PMT (R6233-100, Hamamatsu) under 𝛾-rays and 𝛼-particles excitation from various radioactive sources at room temperature The KLCC sample was directly coupled to the photocathode of PMT using optical grease (Ej-550, Eljen technology), covered with several layers of 100-μm thick Teflon tape and irradiated with 𝛾-rays or 𝛼 particles from various radiation sources For the light yield measurement, the analog signal from the PMT was fed to a pre-amplifier (CANBERRA Model, 2005) and then shaped with a spectroscopy amplifier (TC 245, tennelec Co.) The shaped signals were digitized with a 25-MHz flash analog-todigital converter (FADC25, Notice Co.) Output signals from FADC were recorded and analyzed with a customized C++-based analysis code, compiled, and run in ROOT package (So et al., 2008) For analysis of the fluorescence decay time and pulse shape discrimination, the output scintillation signal from the PMT was digitized with 400 MHz FADC (FADC400, NOTICE Co.) (Notice) The FADC400 module samples the pulse every 2.5 ns for a period of up to 64 μs to fully reconstruct the decay time signal The room temperature X-ray-induced emission spectrum of the KLCC sample in Fig shows a broadband emission between 300 nm and 450 nm, which is attributed to the spin- and parity-allowed 5d â 4f transition of the Ce3+ ion In Fig 2, the transition from the lowest 5d excited level to the F5∕2 (356 nm) and F7∕2 (370 nm) levels of the 4f1 configuration of Ce3+ ion can be identified (Yang et al., 2009b) The emission observed for the KLCC crystal best matches with the spectral sensitivity response of modern photosensors and therefore can be used as a radiation detector for various applications 3.2 Energy resolution and relative light yield The pulse height spectrum of the KLCC sample was measured under 𝛾-rays excitation using 137 Cs radioactive source (Fig 3) The photopeak in the pulse height spectrum was fitted by Gaussian function (Tasaka, 1972) An energy resolution of 16% full width at half maximum (FWHM) was obtained at 662 keV 𝛾-rays excitation (see Fig 3) The light yield of the KLCC sample was compared with the reference LYSO crystal whose absolute light yield is 33,000 ph/MeV (Pepin et al., 2004; Rooh et al., 2014) Because the channel number is related with light yield in the pulse height spectrum (Moszynski et al., 1997), the light yield of the KLCC sample was calculated by considering the Radiation Measurements 141 (2021) 106524 J.Y Cho et al Fig Non-proportionality response of the KLCC crystal Fig Pulse height spectra of KLCC and LYSO crystals measured under 𝛾-rays excitation from a 137 Cs source (Kim et al., 2017) Fig The fluorescence decay time of the KLCC sample under 𝛾-rays and 𝛼-particles excitation The inset (a) and (b) in the figure shows the 𝛾-rays and 𝛼 particles decay time fitted with three exponential decay time functions, respectively Fig The X-ray-induced emission spectrum of KLCC and LYSO crystals and the quantum efficiency of R6233-100 PMT The deviation of the non-proportional response of the KLCC sample is less than 7% between 300 keV and 1.1 MeV quantum efficiencies of PMT at the peak emissions of LYSO and KLCC crystals The X-ray-induced emission spectra of KLCC and LYSO and the quantum efficiency response of the PMT are shown in Fig for comparison The light yield of KLCC crystal was estimated using the following equation (Shah, 2010): 𝐿𝑌𝐾𝐿𝐶𝐶 = 𝑃𝐿𝑌 𝑆𝑂 𝑄𝐸𝐾𝐿𝐶𝐶 𝐿𝑌 𝑃𝐾𝐿𝐶𝐶 𝑄𝐸𝐿𝑌 𝑆𝑂 𝐿𝑌 𝑆𝑂 3.4 Fluorescence decay time The fluorescence decay time was measured using 137 Cs 𝛾-rays and 𝛼-particles excitation source at room temperature As shown in Fig (a), the 𝛾-rays decay time curve of the KLCC crystal was best fitted by three exponential decay time functions The obtained values of the fast, intermediate, and slow decay time components are 70 ns (67.9%), 360 ns (19.7%), and 2.0 μs (12.4%), respectively Similarly, the 𝛼 particles decay time curve of the KLCC crystal was best fitted by three exponential decay time functions as shown in Fig 6(b) The obtained value of the decay components are 80 ns (65.9%), 350 ns (21.1%), 1.9 μs (13%), for fast, intermediate, and slow decay time components, respectively The scintillation decay time in this study was measured with a spectrally unresolved method, and therefore can be assigned tentatively to its origin The fast and intermediate decay time components (70 ns and 360 ns) can be attributed to the direct electron hole recombination at Ce+3 and delay transfer of energy to the Ce+3 , while the slow decay time constant can be attributed to the selftrapped exciton luminescence (Budden et al., 2013) To understand the luminescence mechanism in this crystal, detailed spectroscopic investigation at room and low temperature are required, which is considered for future work The average decay time under 𝛾-rays excitation was 241 Am (1) where 𝑃𝐾𝐿𝐶𝐶 and 𝑃𝐿𝑌 𝑆𝑂 represent the photopeak channel number of each crystal, QE refers to quantum efficiency, and LY is the light yield The light yield of the KLCC was estimated to be 21,000 ph/MeV at 662 keV 𝛾-rays excitation 3.3 Non-proportionality response The non-proportionality response in the light yield of the KLCC crystal was studied in a wide energy range from 300 keV to 1.1 MeV at room temperature Similar experimental setup was used for the measurement of pulse height spectrum The relative light yield as function of the 𝛾-rays energy for the KLCC crystal was studied using known 𝛾-rays energies such as 133 Ba (303 keV, 356 keV), 22 Na (511 keV), 137 Cs (662 keV), 54 Mn (835 keV), and 65 Zn (1115 keV) Fig shows the non-proportionality of light yield (normalized to the value at 662 keV) as a function of different 𝛾-rays energies for the KLCC crystal Radiation Measurements 141 (2021) 106524 J.Y Cho et al slightly faster than that under 𝛼-particle excitation, which indicates the possibility that optimized KLCC sample can be used for thermal neutron detection in various applications via Li(n, 𝛼)3 H reactions 3.4.1 Pulse shape discrimination A heavy charged particle causes different excitation than photons These different excitation mechanisms are the basis of pulse shape discrimination A heavy charged particle causes greater specific ionization, which makes the scintillator reach higher excited states that take longer to de-excite A photon causes lower ionization that takes shorter time to de-excite De-excitation causes fluorescence, which via PMT produces electrical pulses As the 𝛾-rays and 𝛼 particles have different interaction processes in the KLCC sample, the decay time components under 𝛾-rays and 𝛼 particles show different values as shown in Fig Using these differences, we can distinguish 𝛾-rays and 𝛼 particles Fig shows the fluorescence decay time under (a) 𝛾-rays and (b) 𝛼 particles The pulse shape discrimination capability of the KLCC was analyzed by the mean time method The mean decay time is given by the following formula (Gerbier et al., 1999; Vuong et al., 2019): 𝑡= 𝛴𝐴𝑖 × 𝑡𝑖 𝛴𝐴𝑖 (2) ,where 𝐴𝑖 is the amplitude of the FADC signal at the time 𝑡𝑖 The figure of merit (FOM), which represents the discrimination ability value, was used to separate 𝛾-rays and 𝛼 particles FOM is defined as (Langeveld et al., 2017) : 𝐹 𝑂𝑀 = |𝑃𝛼 − 𝑃𝛾 | 𝐹 𝑊 𝐻𝑀𝛼 + 𝐹 𝑊 𝐻𝑀𝛾 (3) where P𝛼 , P𝛾 are the peak positions of 𝛼 particles and 𝛾-rays, respectively The discrimination ability value using the FOM for mean time was calculated to be 0.81, as shown in Fig 7(a) The 𝛼/𝛽 ratio shows the quenching factor between the emitted scintillation light under the charged particles and 𝛾-rays excitations (DeVol et al., 2007) It was calculated by the ratio of the peak position of 𝛼 particles and 𝛾-rays considering their corresponding energies The measurement of 𝛼/𝛽 ratio was performed with 5.486 MeV 𝛼 particles from 241 Am and 662 keV 𝛾-rays from 137 Cs radioactive sources The 𝛼/𝛽 ratio was found to be 0.15, which means that 85% of the light from 𝛼 particle was quenched during interaction in material compared with that of 𝛽 particle Fig 7(b) shows 𝛼 particles and 𝛾-rays spectrum, which was used to calculate 𝛼/𝛽 ratio Fig (a) The mean time and (b) the 𝛼/𝛽 ratio spectrum of the KLCC sample with 137 Cs (𝛾-ray) and 241 Am (𝛼 particle) radioactive source Declaration of competing interest 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 Conclusion Acknowledgment In this study, we presented the scintillation properties of a new elpasolite inorganic halide scintillation crystal, K2 LiCeCl6 , which was grown by vertical Bridgman technique The X-ray- induced emission spectrum exhibits a broadband between 300 nm and 450 nm, which is attributed to the spin- and parity-allowed 5d–4f transition of the Ce3+ ion The relative light yield was obtained to be 21000 ph/MeV compared with the LYSO reference crystal The non-proportionality of the K2 LiCeCl6 crystal was found to be within 7% in the energy range of 300 keV to 1.1 MeV The energy resolution of the K2 LiCeCl6 was obtained to be 16% (FWHM) which could be improved significantly after improving the crystal growth quality and the material’s purity The K2 LiCeCl6 crystal scintillator has three decay components: 70 ns (67.9%), 360 ns (19.7%), and 2.0 μs (12.4%) under 𝛾-ray excitation Because 𝛼 particles and 𝛾-rays decay times of the K2 LiCeCl6 crystal were different, the mean time pulse shape discrimination method was used, obtaining an FOM of 0.81 The 𝛼/𝛽 ratio of the K2 LiCeCl6 crystal was measured to be 0.15 These preliminary results show that the K2 LiCeCl6 can be applied for the detection of 𝛾-rays and neutron in the 𝛾-rays and neutrons mixed field using pulse shape discrimination capability These investigations were supported by the National Research Foundation of Korea, funded by the Ministry of Science and Technology, Korea (MEST), (No 2020M2A8A4025470) References Budden, B., Stonehill, L.C., 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