Integrated and Portable Probe Based on Functional Plastic Scintillator for Detection of Radioactive Cesium

applied sciences

Article Integrated and Portable Probe Based on Functional Plastic Scintillator for Detection of Radioactive Cesium

Sujung Min 1,2, Hara Kang 1, Bumkyung Seo 1, Changhyun Roh 1,3,* , Sangbum Hong 1,* and Jaehak Cheong 2,*

1 Decommissioning Technology Research Division, Korea Atomic Energy Research Institute, Daejeon 34057, Korea; [email protected] (S.M.); [email protected] (H.K.); [email protected] (B.S.) 2 Department of Nuclear Engineering, Kyung-Hee University, Yongin-si 17104, Gyeonggi-do, Korea 3 Quantum Energy Chemical Engineering, University of Science and Technology (UST), 217 Gajeong-ro, Daejeon 34113, Korea * Correspondence: [email protected] (C.R.); [email protected] (S.H.); [email protected] (J.C.)

Abstract: The highly reliable and direct detection of radioactive cesium has gained potential interest due to in-situ detection and monitoring in environments. In this study, we elucidated an integrated and portable probe based on functional plastic scintillator for detection of radioactive cesium. A functional plastic scintillator with improved detection efficiency was fabricated including CdTe (cad- mium telluride) material. Monolith-typed functional plastic scintillator having a diameter of 50 mm and a thickness of 30 mm was manufactured by adding 2,5-diphenyloxazole (PPO, 0.4 wt%), 1,4 di[2-(5phenyloxazolyl)]benzene (POPOP, 0.01 wt%), and CdTe (0.2 wt%) materials in a styrene-based matrix. To evaluate the applicability of the plastic scintillator manufactured to in-situ radiological measurement, an integrated plastic detection system was created, and the measurement experiment was performed using the Cs-137 radiation source. Additionally, detection efficiency was compared with a commercial plastic scintillator. As results, the efficiency and light yield of a functional plastic Citation: Min, S.; Kang, H.; Seo, B.; scintillator including CdTe were higher than a commercial plastic scintillator. Furthermore, the Roh, C.; Hong, S.; Cheong, J. Integrated and Portable Probe Based remarkable performance of the functional plastic scintillator was confirmed through comparative on Functional Plastic Scintillator for analysis with Monte Carlo simulation. Detection of Radioactive Cesium. Appl. Sci. 2021, 11, 5210. https:// Keywords: detection; integrated portable probe; Cs-137; plastic scintillator; simulation doi.org/10.3390/app11115210

Academic Editor: Antonio Di Bartolomeo 1. Introduction In general, it is necessary to develop residual radioactivity measurement technology Received: 10 May 2021 that can easily access the site, and the technology in radiological measurements that can Accepted: 2 June 2021 safely and effectively evaluate the environment, contaminated by a nuclear safeguard or Published: 4 June 2021 serious accident at a nuclear facility [1–7]. In addition, there must be verified that the site is not contaminated for reuse of the site after the nuclear facilities are dismantled. Until now, Publisher’s Note: MDPI stays neutral a lot of studies have been conducted to secure technologies that can prepare for serious with regard to jurisdictional claims in accidents such as Chernobyl and Fukushima [7–13]. The total amount of radioactivity published maps and institutional affil- emitted by the Chernobyl accident was about 12 × 1018 Bq [14]. The behavior of radioactive iations. particles largely depends on diffusion factor, particle size, and rainfall, and large particles such as nuclear fuel dust were mostly deposited within 100 km radius of the reactor [13]. In the Chernobyl nuclear accident, radioactive Cs-137 from one of the fission products of U-235 contaminated about 125,000 km2 in Belarus, Ukraine, and Western Russia with a Copyright: © 2021 by the authors. concentration of 37 kBq/m2 [13]. In the Fukushima accident, a large area was contaminated Licensee MDPI, Basel, Switzerland. by the radioactive elements, and one of the major contaminated radionuclides was analyzed This article is an open access article as a Cs-137 [12,13]. Additionally, Cs-137 was easily released from the nuclear power plant distributed under the terms and accident and was rapidly spread to the environments as a mobile element. The decay conditions of the Creative Commons Attribution (CC BY) license (https:// diagram and probability of Cs-137 were shown in Figure1 and Table1. Most of Cs-137 creativecommons.org/licenses/by/ decays to Ba-137 and then gamma decays to a stable state at 0.662 MeV. There are two − − 4.0/). β decay channels. First, 94.6% of β decays proceed from Cs-137 to the excited state

Appl. Sci. 2021, 11, 5210. https://doi.org/10.3390/app11115210 https://www.mdpi.com/journal/applsci Appl.Appl. Sci. Sci. 20212021, 11, 11, ,x 5210 FOR PEER REVIEW 2 of 15 2 of 15

ofmetastable Ba-137m withstate maximumthat decays electron with a 2.552 energy min of 0.514half-life MeV. to the The ground Ba-137m state is a of metastable Ba-137 with emis- state that decays with a 2.552 min half-life to the ground state of Ba-137 with emission of a sion of a 0.662 MeV gamma ray photon. Second, 5.4% of β− decay proceeds directly from Cs- 0.662 MeV gamma ray photon. Second, 5.4% of β− decay proceeds directly from Cs-137 137 to the ground state of Ba-137 with maximum electron energy of 1.176 MeV. Cs-137 has a to the ground state of Ba-137 with maximum electron energy of 1.176 MeV. Cs-137 has ahalf-life half-life of of 30.17 30.17 ±± 0.030.03 years years [15], [15], which cancan causecause long-termlong-term damage damage when when exposed exposed to the toenvironment the environment [16–25]. [16– Cesium25]. Cesium has hassimilar similar chemical chemical properties properties to topotassium potassium (K), (K), so so it is easily itconcentrated is easily concentrated in crops such in crops as rice. such In as addition, rice. In addition,it occupies it occupiesa large amount a large of amount artificial of radioac- artiﬁcialtive gamma-emitting radioactive gamma-emitting isotopes that isotopeshave been that leaked have been through leaked nuclear through tests nuclear and testsnuclear acci- anddents. nuclear In this accidents. regard, cesium In this regard,is an important cesium is radionuclide an important for radionuclide detection and for detectionmonitoring in en- andvironments monitoring and in contaminated environments sites. and contaminated sites.

Figure 1. Decay of diagram of 137Cs. Figure 1. Decay of diagram of 137Cs.

Table 1. β− decay process, electron energy, and probability of Cs-137. Table 1. β− decay process, electron energy, and probability of Cs-137. 137 Cs (Ek)max (Ek)avg (Eγ) Probability 137Cs (Ek)max (Ek)avg (Eγ) Probability − 1 β decay− 0.514 MeV 0.170 MeV - 94.6% ① − β decay 0.514 MeV 0.170 MeV - 94.6% 2 β decay 1.176 MeV 0.420 MeV - 5.4% ② − Internal conversionβ decay 1.176 MeV 0.420 MeV - 5.4% 3 - - - 7.8% ③ Internal(K shell)conversion (K shell) - - - 7.8% Internal conversion Internal conversion (L, M shell)------1.80% 1.80% ④ (L, M shell)γ decay - - 0.662 MeV 85% 4 γ decay - - 0.662 MeV 85% Plastic detectors are widely used as large size detectors. Additionally, over the past few decades,Plastic many detectors studies are widelyhave been used conducted as large size on detectors. plastic scintillators Additionally, composed over the past of polymer few decades, many studies have been conducted on plastic scintillators composed of matrix containing nanomaterials as an alternative to conventional inorganic scintillators polymer matrix containing nanomaterials as an alternative to conventional inorganic scintillators[26]. Plastic [26 scintillators]. Plastic scintillators are generally are generally manufa manufacturedctured by thermal by thermal polyme polymerization,rization, doping, or doping,coating orby coating adding by materials adding materials such as fluorescent such as fluorescent dye. However, dye. However, direct mixing direct mixing of nanomateri- ofals nanomaterials and polymers and is polymersless efficient is less in efficientproduc ining producing transparent transparent nanocomposites. nanocomposites. Because nano- Becausematerials nanomaterials tend to aggregate tend to due aggregate to their due high to their specific high surface specific surfacearea and area surface and surface energy, trans- energy,parency transparency is lost by Rayleigh is lost by Rayleighscattering. scattering. To increase To increase the transparency, the transparency, research research is required is to requiredreduce the to reduce aggregation the aggregation of nanomaterials of nanomaterials [27]. [The27]. Theplastic plastic detector detector is iscomposed composed of a low ofatomic a low number atomic numbermaterial material such as suchC, H, as O, C, so H, its O, accuracy so its accuracy is low due is low to low due stopping to low power stopping power and light yield. Because of these properties, plastic scintillators are used in and light yield. Because of these properties, plastic scintillators are used in applications such applications such as fast neutron detection, charged particle tracking, and non-destructive testing.as fast neutron Plastic scintillators detection, have charged advantages particle over tracking, inorganic and scintillators non-destructive such as fastertesting. decay Plastic scin- timetillators (about have 10 ns),advantages non-hygroscopicity, over inorganic relatively scintilla lowtors manufacturing such as faster cost, decay robustness, time (about and 10 ns), processability.non-hygroscopicity, Using these relatively advantages, low manufacturing it is possible to cost, develop robustness, a plasticscintillator and processability. with Us- improveding these detectionadvantages, efficiency it is possible by adding to develop nanomaterials a plastic of scintillator high atomic with number improved to the detection plasticefficiency matrix by [adding28–36]. nanomaterials of high atomic number to the plastic matrix [28–36]. InIn general, general, Monte Monte Carlo Carlo codes codes are are widely widely used used for radiationfor radiation instrument instrument design, design, per- perfor- formancemance evaluation evaluation of ofprototypes, prototypes, and and detector detector calibration calibration [37–40]. [37–40 The]. The performance performance of the commercial plastic and the CdTe-loaded plastic scintillator was verified through the MCNP simulation. MCNP is a relatively accurate evaluation method for solving a three-dimensional transport equation. In the Monte Carlo method, the spatial structure is simulated exactly as the actual structure and the reaction cross-sectional area is used as a continuous function of energy. Appl. Sci. 2021, 11, 5210 3 of 15

of the commercial plastic and the CdTe-loaded plastic scintillator was verified through the MCNP simulation. MCNP is a relatively accurate evaluation method for solving a three-dimensional transport equation. In the Monte Carlo method, the spatial structure is Appl. Sci. 2021, 11, x FOR PEER REVIEWsimulated exactly as the actual structure and the reaction cross-sectional area is used3 asof 15 a continuous function of energy. In this study, we elucidated an integrated and portable probe based on a functional plastic scintillator for detection of radioactive cesium. In addition, the results were com- paredIn and this analyzed study, we with elucidated the results an ofintegrat the MCNPed and simulation. portable probe based on a functional plastic scintillator for detection of radioactive cesium. In addition, the results were com- 2.pared Materials and analyzed and Methods with the results of the MCNP simulation. 2.1. Chemicals 2. Materials and Methods All chemicals and reagents were commercial products. Styrene (99.9%), 2,5- Diphenyloxazole2.1. Chemicals (PPO, >95%), 1,4-bis(5-phenyloxazol-2-yl) benzene (POPOP, >95%), and CdTeAll used chemicals were purchased and reagents from were Sigma-Aldrich commercial(St. products. Louis, Styrene MO, USA) (99.9%), on a 2,5-Diphe- technical puritynyloxazole grade. (PPO, >95%), 1,4-bis(5-phenyloxazol-2-yl) benzene (POPOP, >95%), and CdTe used were purchased from Sigma-Aldrich (St. Louis, MO, USA) on a technical purity grade. 2.2. Fabrication of a Functional Plastic Scintillator 2.2. FabricationA styrene-based of a Func plastictional scintillator Plastic Scintillator was fabricated by a polymerization method, and its performanceA styrene-based was comparedplastic scintillator and analyzed was fabricated with a PVT-basedby a polymerization commercial method, scintillator and its (EJ-200).performance Plastics was werecompared manufactured and analyzed by the with process a PVT-based shown incommercial Figure2. The scintillator materials (EJ-200). used inPlastics the manufacture were manufactured of plastics by werethe process styrene, show PPOn in (2,5-diphenyloxazole), Figure 2. The materials POPOP used in (1,4the man- di[2- (5phenyloxazolyl)]benzene),ufacture of plastics were styrene, CdTe, PPO and (2,5 plastics-diphenyloxazole), with a diameter POPOP of 50 mm (1,4 anddi[2-(5phenyloxa- a thickness of 30 mm. Styrene was used as the primary solvent, PPO was used as the primary fluorophore, zolyl)]benzene), CdTe, and plastics with a diameter of 50 mm and a thickness of 30 mm. Sty- and POPOP was used as the secondary fluorophore. Although the types of nanomaterials rene was used as the primary solvent, PPO was used as the primary fluorophore, and POPOP are various, CdTe was used. The contents (wt%) of organic dye and CdTe were selected was used as the secondary fluorophore. Although the types of nanomaterials are various, due to previous studies [31]. The amounts of materials added to the styrene were PPO CdTe was used. The contents (wt%) of organic dye and CdTe were selected due to previous (0.4 wt%), POPOP (0.01 wt%), and nanomaterials (0.2 wt%). The plastic scintillator used studies [31]. The amounts of materials added to the styrene were PPO (0.4 wt%), POPOP (0.01 styrene as a basic matrix material, and a nanomaterial having an emission wavelength wt%), and nanomaterials (0.2 wt%). The plastic scintillator used styrene as a basic matrix ma- range of 500–600 nm was selected. CdTe nanomaterial was mixed with the matrix material, terial, and a nanomaterial having an emission wavelength range of 500–600 nm was selected. and stirred at 60 ◦C. After the stirring process for about 2–3 h was completed, the weight CdTe nanomaterial was mixed with the matrix material, and stirred at 60 °C. After the stirring suitable for the thickness of the plastic was measured. The sample was placed in a vial for process for about 2–3 h was completed, the weight suitable for the thickness of the plastic was polymerization and fine bubbles generated during stirring were removed. Fine bubbles measured. The sample was placed in a vial for polymerization and fine bubbles generated can cause cracking due to internal stress during the polymerization process, and since itduring reduces stirring optical were properties. removed. Fine Bubble bubbles removal can cause process cracking of about due 1–2to internal h is required. stress during The de-bubbledthe polymerization sample process, is polymerized and since init reduce a vacuums optical oven properties. at a temperature Bubble removal of up toprocess 120 ◦ C.of Atabout this 1–2 time, h is it required. is important The tode-bubbled increase thesample temperature is polymerized slowly, in and a vacuum if the temperature oven at a tem- is increasedperature of rapidly, up to 120 bubbles °C. At in this the time, sample it is mayimportant be regenerated. to increase The the polymerizationtemperature slowly, process and takesif the abouttemperature 70 h. Plastics is increased that have rapidly, gone throughbubbles in the the polymerization sample may processbe regenerated. are finished The afterpolymerization cutting and process polishing takes the about surface 70 h. to Plastics increase that the have transmittance. gone through the polymerization process are finished after cutting and polishing the surface to increase the transmittance.

FigureFigure 2.2. ManufacturingManufacturing processprocess ofof aa functionalfunctional plasticplastic scintillator.scintillator.

2.3. MCNP Simulation Monte Carlo simulation is a method for solving problems of physical and mathemat- ical systems by generating random numbers and using a probabilistic model. The MCNP code can construct various geometry through the cell card. After modeling the actual geometry through the MCNP computational simulation, the spectrum of the energy accumulated in the scintillator was obtained. And, input data of surface flux in a MCNP 6 simulation was presented in Figure S1. Appl. Sci. 2021, 11, x FOR PEER REVIEW 4 of 15

Figure 3 is the result of calculating the deposited energy for each thickness using MCNP to select the plastic scintillator thickness. The energy of the incident photon is deposited through interaction as it passes through the scintillator. Figure 3a shows the geometry for computational simulation, and the Cs-137 radiation source was placed 20 mm away from the plastic surface. Figure 3b shows the flux by thickness, and Figure 3c shows the energy spectrum by thickness. In this study, a plastic scintillator was fabricated with a thickness of 30 mm showing about 70% energy deposition. Figure 4 shows the geometry for computational simulation of a plastic detector manufactured with a selected 30 mm thickness. The distance of the radiation source was set to 20 mm, 50 mm, and 100 mm under the same conditions as the experiment, and the gamma energy emitted from the radiation source was set to emit only a single energy of interest.

Appl. Sci. 2021Here,, 11, 5210 only one detector and one source are simulated, and the surrounding environment4 of 15 is not considered because it was not expected to have a significant impact on scattering. The size of the radiation source is 2.5 cm in diameter and the location of the source was used as a point in2.3. the MCNP simulation. Simulation It was also set up using a Gaussian energy broadening (GEB) card to obtain the ideal Monte Carlo simulation is a method for solving problems of physical and mathe- Compton spectrummatical of the systems plastic by generatingscintillator. random As the numbers parameters and usingfor GEB a probabilistic in Equation model. (1), The values of a, b, andMCNP c were code applied can construct as 0.00093789, various geometry 0.00498, throughand −0.05999, the cell respectively. card. After modeling And, the all input data of actualenergy geometry spectrum through were the presented MCNP computational in Figure S2. simulation, the spectrum of the energy accumulated in the scintillator was obtained. And, input data of surface ﬂux in a MCNP 6 simulation was presentedFWHM in Figure= a + b S1.√(E+cE2) (1) Figure3 is the result of calculating the deposited energy for each thickness using Here, FWHMMCNP means to select full width the plastic at half scintillator maximum, thickness. E means The gamma-ray energy of the energy. incident photon is The spectrumdeposited derived through through interaction the MCNP as it passessimulation through was the compared scintillator. with Figure the3a actual shows the measurement spectrum,geometry and for computational since the photopeak simulation, did and not the appear Cs-137 radiation due to the source characteristics was placed 20 mm of the plastic scintillator,away from the plasticdetection surface. effi Figureciency3b was shows calculated the ﬂux by using thickness, the andgross Figure counts.3c shows To make the statisticalthe energy uncertainty spectrum byof thickness.the calculation In this study,result a less plastic than scintillator 5%, 109 wasphotons fabricated were with a generated in eachthickness geometry of 30 to mm calculate showing the about measurement 70% energy deposition.efficiency of the plastic detector.

Figure 3. The geometry used in the MCNP computational simulation for the selection of plastic thickness (a), ﬂux by thickness (left), conﬁguration of geometry (right)(b), energy spectrum by thickness (c).

Figure4 shows the geometry for computational simulation of a plastic detector manufactured with a selected 30 mm thickness. The distance of the radiation source was set to 20 mm, 50 mm, and 100 mm under the same conditions as the experiment, and the gamma energy emitted from the radiation source was set to emit only a single energy of interest. Here, only one detector and one source are simulated, and the surrounding environment is Appl. Sci. 2021, 11, 5210 5 of 15

Appl. Sci. 2021, 11, x FOR PEER REVIEW 5 of 15

not considered because it was not expected to have a signiﬁcant impact on scattering. The Figuresize of 3. the The radiation geometry sourceused in isthe 2.5 MCNP cm in computational diameter and simulation the location for the of selectio the sourcen of plastic was thickness used as (aa), point flux by in thickness the simulation. (left), configuration of geometry (right) (b), energy spectrum by thickness (c).

Figure 4. Geometry for computational simulation of 30-mm-thick plastic scintillators. It was also set up using a Gaussian energy broadening (GEB) card to obtain the ideal 2.4. Manufacturing of Integrated Functional Plastic Detection System Compton spectrum of the plastic scintillator. As the parameters for GEB in Equation (1), valuesTo of evaluate a, b, and the c wereperformance applied asof 0.00093789,the CdTe-loaded 0.00498, plastic and −detector,0.05999, a respectively. plastic detection And, systemall input was data constructed of energy spectrum as shown were in Figure presented 5a. Additionally, in Figure S2. the signal generated from the plastic detector is amplified through a preamplifier√ and amplifier, and the amplified analog signal is digitized throughFWHM a multi-channel = a + b (E+cE analyzer2) and then stored. A high volt- (1) age was applied to the plastic detector so that the generated electrons were collected by the electrode.Here, FWHM The system means fullwas width constructed at half maximum,using PMT E(ET-9266KB, means gamma-ray ET-Enterprises energy. Ltd., Uxbridge,The spectrum UK), power derived supply through (DT5423, the MCNP CAEN simulation, Viareggio, was Italy), compared Preamp with (Amcrys the actual 544, Amcrysmeasurement Ltd., spectrum,Kharkiv, andUkraine), since theAmp photopeak (DT5781, did CAEN, not appear Viareggio, due to theItaly), characteristics and MCA (DT5781,of the plastic CAEN, scintillator, Viareggio, the detectionItaly). Additionally, efficiency was the calculatedplastic detector using and the grossdata processing counts. To 9 systemmake the were statistical manufactured uncertainty as an of integrated the calculation system, result so that less the than integrated 5%, 10 photonssystem can were be usedgenerated as a portable in each geometry detection tosystem calculate as shown the measurement in Figure 5b. efficiency Figure 5c of theshows plastic a schematic detector. illustration for plastic detection system. The integrated box can contain a data processing 2.4. Manufacturing of Integrated Functional Plastic Detection System system, which is custom-made using an anti-shock sponge. Additionally, it was stored in a darkTo room evaluate for about the performance 12 h after replacing of the CdTe-loaded the scintillator plastic to remove detector, the a plasticafterglow detection of the PMTsystem that was occurred constructed while asreplacing shown inthe Figure plastic5a. scintillator. Additionally, the signal generated from the plastic detector is amplified through a preamplifier and amplifier, and the amplified analog signal is digitized through a multi-channel analyzer and then stored. A high voltage was applied to the plastic detector so that the generated electrons were collected by the electrode. The system was constructed using PMT (ET-9266KB, ET-Enterprises Ltd., Uxbridge, UK), power supply (DT5423, CAEN, Viareggio, Italy), Preamp (Amcrys 544, Amcrys Ltd., Kharkiv, Ukraine), Amp (DT5781, CAEN, Viareggio, Italy), and MCA (DT5781, CAEN, Viareggio, Italy). Additionally, the plastic detector and data processing system were manufactured as an integrated system, so that the integrated system can be used as a portable detection system as shown in Figure5b. Figure5c shows a schematic illustration for plastic detection system. The integrated box can contain a data processing system, which is custom-made using an anti-shock sponge. Additionally, it was stored in a dark room for about 12 h after replacing the scintillator to remove the afterglow of the PMT that occurred while replacing the plastic scintillator. Appl.Appl. Sci. Sci. 20212021, 11, 11, x, 5210FOR PEER REVIEW 6 of6 of15 15

FigureFigure 5. 5.(a()a Real) Real picture picture of of the the performance performance experime experimentalntal setup setup for for gamma gamma energy energy spectrum spectrum using using a functional plastic scintillator. (b) Real picture of integrated device box and portable probe with a a functional plastic scintillator. (b) Real picture of integrated device box and portable probe with a functional scintillator. (c) Schematic illustration for plastic detection system. functional scintillator. (c) Schematic illustration for plastic detection system.

3.3. Results Results and and Discussion Discussion TheThe transparency transparency of of the the plastic plastic scintillator scintillator decreases decreases due due to to the the coagulation coagulation of of nano- nano- materialsmaterials with with high high surface surface energy energy and and precip precipitationitation during during the the plastic plastic polymerization polymerization process,process, which which may may adversely adversely affect affect the the optica opticall properties. properties. Therefore, Therefore, for for the the transparency transparency ofof the the plastic plastic scintillator, scintillator, it itis is necessary necessary to to be be careful careful in in the the processes processes of of stirring, stirring, bubbles bubbles removal,removal, slow slow heating, heating, slow slow cooling, cooling, and and poli polishing.shing. Figure Figure 6a6a shows shows the the transparency transparency of of thethe CdTe-loaded CdTe-loaded plastic, plastic, and and Figure Figure 6b,c6b,c show show the the UV-Vis UV-Vis absorption absorption and and PL PL emission emission spectraspectra of of the the plastic plastic scintillator. TheThe absorptionabsorption peak peak was was confirmed confirmed at at about about 250–400 250–400 nm, nm,and and 475 475 nm nm emission emission was was observed observed under under 316 316 nm nm excitation. excitation. As mentioned in the introduction, incident photons tend to deposit all the energy on the photoelectrons generated by interacting with CdTe, a nanomaterial of high atomic number, through the photoelectric effect. This energy goes through an energy cascade process, where the plastic matrix and dye are excited to generate excitons. Energy transfer characteristics are maximized through fluorescent dyes. Typically, through the FRET pro-cess, energy is transferred between molecules. As the nanomaterials were loaded, the emission wavelength shifted through the FRET process and the emission intensity increased. This means that compared to conventional fluorescent dyes, quantum efficiency is increased due to nanomaterials, and thus greater fluorescence is generated. Appl.Appl. Sci. Sci. 20212021, 11, 11, x, 5210FOR PEER REVIEW 7 of7 of15 15

FigureFigure 6. 6. (a()a A) A functional functional plastic plastic scin scintillatortillator including including CdTe. CdTe. (b ()b UV-Vis) UV-Vis absorbance. absorbance. (c ()c Photolumi-) Photolumi- nescencenescence emission emission spectrum, spectrum, excitation excitation at at 316 316 nm. nm.

AsTo mentioned compare the in performancethe introduction, of a plastic incident scintillator photons produced tend to deposit by adding all the CdTe energy material on thewith photoelectrons a commercial generated scintillator, by a measurementinteracting with test CdTe, was performed a nanomaterial by placing of high a source atomic at a number,distance through of 20 mm the from photoelectric the scintillator effect. surface. This energy Considering goes thethrough half-life, an energy the radioactivity cascade process,of the Cs-137 where source the plastic is 342.28 matrix kBq. and Unlike dye are inorganic excited scintillators,to generate excitons. plastic scintillators Energy transfer rarely characteristicsgenerate photoelectric are maximized effects, through and Compton fluorescent scattering dyes. Typically, mainly occurs.through Comptonthe FRET pro- edge cess,energies energy of Cs-137is transferred used in between this experiment molecule ares. As 477.3 the keV. nanomaterials Since the plastic were scintillatorloaded, the is emissiondominated wavelength by Compton shifted scattering, through energy the FR calibrationET process was and performed the emission using intensity the Compton increased.edge and This measurement means that testscompared were performedto conventional for 10 fluorescent min. The results dyes, quantum of the measurement efficiency istest increased using a due point to source nanomaterials, for each plastic and th scintillatorus greater arefluorescence shown in Figureis generated.6. Additionally, the resultTo ofcompare calculating the performance the output by of gross a plastic count sc ofintillator the measured produced spectrum by adding is shown CdTe in ma- the terialTable with2. The a relativecommercial efficiency scintillator, of the CdTe-loaded a measurement plastic test scintillator was performed was calculated by placing using a sourceEquation at a (2)distance as the grossof 20 mm counting from methodthe scinti forllator the commercialsurface. Considering plastic scintillator, the half-life, and the asa result, it was confirmed that it increased by 25% compared to the commercial plastic scintil- radioactivity of the Cs-137 source is 342.28 kBq. Unlike inorganic scintillators, plastic scin- lator. However, the measurement result using the point source analyzed that the Compton tillators rarely generate photoelectric effects, and Compton scattering mainly occurs. edge of commercial plastics has better resolution. This is because of the transmission loss Compton edge energies of Cs-137 used in this experiment are 477.3 keV. Since the plastic due to Rayleigh scattering, and aggregation of a solid form CdTe nanomaterials. scintillator is dominated by Compton scattering, energy calibration was performed using the Compton edge and measurement tests were performed for 10 min. The results of the Table 2. Compared relative efficiency for detection of radioactive cesium. measurement test using a point source for each plastic scintillator are shown in Figure 6. Additionally, the result of calculating the output by gross countRelative of the measuredDetection spectrum Net Count is shown in the Table 2. The relative efficiency of the CdTe-loadedEfficiency plastic scintillatorEfficiency was (cps) calculated using Equation (2) as the gross counting method for(%) the commercial(%) plastic scintillator, and as a result,20 mm it was confirmed 3,644,431 ±that495,634 it increased 100.00by 25% compared 2.09 to the Commercial 50 mm 1,332,009 ± 99,208 100.00 0.76 commercialplastic plastic scintillator. However, the measurement result using the point source analyzed that the Compton100 mm edge of commercial 474,203 ± 23,352plastics has better 100.00 resolution. This 0.27 is because of the transmission20 mmloss due to Rayleigh 1,803,557 scattering,± 2657 and aggregation 49.49 of a solid 1.03 form Plastic without 50 mm 781,528 ± 582 58.67 0.45 CdTe nanomaterials.CdTe In addition, the detection100 mm efficiency of 303,323 commercial± 53 plastics and 63.96 the CdTe plastic 0.17 scintillator was measured. The20 mm detection efficiency 4,100,748 ±was558,312 calculated using 112.52 Equation (3). 2.35 In Table Plastic with 2, the gross count rate 50increased mm as the 1,608,902 CdTe ±nanomaterial120,952 was 120.79 added. This means 0.92 that CdTe ± the high atomic number100 of mmCdTe increases 593,654 the reaction331 rate with 125.19photons, thereby 0.34 improving the efficiency. Appl. Sci. 2021, 11, x FOR PEER REVIEW 8 of 15

Relative efficiency % 100 (2) Detection efficiency % 100 % (3) Figure 7 shows the spectrum measured by radiation source distance for commercial plastics, plastics loaded with CdTe, and plastics not loaded with CdTe. The spectrum in Figure 7 shows the net count minus the background radiation, and is a graph normalized to 477.65 keV, the Compton edge energy of Cs-137. Spectral data were obtained by dividing the energy of 3 MeV by a total of 8192 channels. It was confirmed that as the distance of the radiation source increased, the number of counts decreased. Appl. Sci. 2021, 11, 5210Figure 8 represents the measurement data for three plastic scintillators, and it means 8 of 15 the average value of the measured data by distance. As shown in Figure 8, it was observed that in the case of the scintillator to which the nanomaterial was added, the Compton peak was shifted to the leftIn compared addition, the to detection the comme efficiencyrcial scintillator. of commercial Based plastics on andthis, the the CdTe light plastic yield for the CdTescintillator scintillator was measured.was calculated The detection using efficiencythe light wasyield calculated of EJ-200. using Light Equation yield (3). In was calculated basedTable2 on, the the gross data count in Figure rate increased 8 and Equation as the CdTe (4) nanomaterial [41]. was added. This means that the high atomic number of CdTe increases the reaction rate with photons, thereby improving the efficiency.LY LY (4) Total counts (CdTe loaded plastic) Here, LY, CE, and QERelative refer to efficiency the light(% )yield,= the peak channel of Compton’s× 100 edge, (2) Total counts (Commercial plastic) and the quantum efficiency of PMT at the emission wavelength, respectively, and EJ and Net counts(cps) Cd refer to the commercialDetection scintillator efficiency ( and%) = the plastic scintillator loaded with CdTe,× 100re- (3) ( ) × ( ) spectively. As shown in Table 3, the LYEJ valueRadioactivity was usedBq as a Releasereference Probability value of% 10,000 photon/MeV, a valueFigure suggested7 shows by the Eljen spectrum techno measuredlogy. In by addition, radiation sourceQE used distance the values for commercial in the datasheet of PMTplastics, (ET-9266KB) plastics loaded [42]. with When CdTe, CdTe and plasticswas loaded, not loaded it was with confirmed CdTe. The that spectrum the in light yield increasedFigure compared7 shows the to net commercial count minus plastic the background scintillator. radiation, Based and on isthis a graph result, normalized it is to 477.65 keV, the Compton edge energy of Cs-137. Spectral data were obtained by dividing analyzed that thethe CdTe energy nanomaterial of 3 MeV by a not total only of 8192 increases channels. the It wasreaction confirmed rate thatwith as photons, the distance of but also improvesthe the radiation energy source transfer increased, rate. the number of counts decreased.

FigureFigure 7. Normalized 7. Normalized energy spectrum energy by spectrum distance. (bya) Commercialdistance. (a plastic) Commercial scintillator. plastic (b) Plastic scintillator. scintillator (b with) Plastic CdTe. (c) Plasticscintillator scintillator with without CdTe. CdTe. (c) Plastic scintillator without CdTe. Figure8 represents the measurement data for three plastic scintillators, and it means the average value of the measured data by distance. As shown in Figure8, it was observed that in the case of the scintillator to which the nanomaterial was added, the Compton peak was shifted to the left compared to the commercial scintillator. Based on this, the light yield Appl. Sci. 2021, 11, 5210 9 of 15

for the CdTe scintillator was calculated using the light yield of EJ-200. Light yield was calculated based on the data in Figure8 and Equation (4) [41].

Appl. Sci. 2021, 11, x FOR PEER REVIEW CECd QEEJ 9 of 15 LYCd = LYEJ × × (4) CEEJ QECd

FigureFigure 8. 8. GammaGamma measurement measurement spectrum spectrum of of three three plasti plasticc scintillators scintillators using using Cs-137 Cs-137 point point source. source. InsertInsert figure ﬁgure represents represents relative relative counts counts from from channel 1000 to 1800.

TableHere, 2. Compared LY, CE, relative and QE efficiency refer to thefor detection light yield, of radioactive the peak channel cesium. of Compton’s edge, and the quantum efficiency of PMT at the emission wavelength, respectively, and EJ and Cd Net Count Relative Efficiency Detection Efficiency refer to the commercial scintillator and the plastic scintillator loaded with CdTe, respectively. (cps) (%) (%) As shown in Table3, the LY EJ value was used as a reference value of 10,000 photon/MeV, 20 mm 3,644,431 ± 495,634 100.00 2.09 a valueCommercial suggested by Eljen technology. In addition, QE used the values in the datasheet 50 mm 1,332,009 ± 99,208 100.00 0.76 of PMTplastic (ET-9266KB) [42]. When CdTe was loaded, it was confirmed that the light yield 100 mm 474,203 ± 23,352 100.00 0.27 increased compared to commercial plastic scintillator. Based on this result, it is analyzed 20 mm 1,803,557 ± 2657 49.49 1.03 thatPlastic the without CdTe nanomaterial not only increases the reaction rate with photons, but also 50 mm 781,528 ± 582 58.67 0.45 improvesCdTe the energy transfer rate. 100 mm 303,323 ± 53 63.96 0.17 20 mm 4,100,748 ± 558,312 112.52 2.35 Table 3. Compared relative light yield by distance to Cs-137 gamma ray source. Plastic with CdTe 50 mm 1,608,902 ± 120,952 120.79 0.92 Terms100 mm 593,654 20 mm ± 331 50 125.19 mm 100 0.34 mm CE 1295 ch 1298 ch 1291 ch Table 3. ComparedCd relative light yield by distance to Cs-137 gamma ray source. CEEJ 1430 ch 1444 ch 1434 ch TermsQE Cd 20 mm 50 mm0.15% 100 mm QEEJ 0.2% CECd 1295 ch 1298 ch 1291 ch LYEJ 10,000 photon/MeV EJ CERelative LYCd 143012,075 ch photon/MeV 12,103 1444 photon/MeVch 12,037 1434 photon/MeV ch QECd 0.15% QEEJ 0.2% Figure9 shows the result of comparing the spectrum of commercial plastic scintillator (EJ-200)LYEJ and CdTe-loaded plastic scintillator10,000 with photon/MeV MCNP simulation. The average relative Relativeerror in theLYCd Compton 12,075 edge photon/MeV region was analyzed 12,103 toph beoton/MeV within 5%. In12,037 the MCNP photon/MeV simulation results of Figure7b, a signal was detected in a region larger than the Compton edge by CdTeFigure with 9 shows a high the atomic result number.of comparing The reasonthe spectrum that these of commercial signals are plastic not seen scintilla- in the torexperimental (EJ-200) and results CdTe-loaded is likely plastic due to scintillator the aggregation with MCNP due to simulation. the low solubility The average and high rel- ativesurface error energy in the of Compton CdTe. In orderedge region to analyze was the analyzed influence to be of CdTewithin nanomaterials, 5%. In the MCNP the count sim- ulationratio was results calculated of Figure through 7b, thea signal Equation was (5).detected As a result, in a region it showed larger a high than coefficient the Compton ratio edge by CdTe with a high atomic number. The reason that these signals are not seen in the experimental results is likely due to the aggregation due to the low solubility and high surface energy of CdTe. In order to analyze the influence of CdTe nanomaterials, the count ratio was calculated through the Equation (5). As a result, it showed a high coefficient ratio at the Compton edge energy of Cs-137. In addition, a ratio above ‘1’ means that the CdTe nanomaterial improved the light yield in the polymer corresponding to each channel. In this study, according to the Figure 10, it is shown that there is an improvement of Appl. Sci. 2021, 11, 5210 10 of 15

at the Compton edge energy of Cs-137. In addition, a ratio above ‘1’ means that the CdTe Appl. Sci. 2021, 11, x FOR PEER REVIEW 10 of 15 Appl. Sci. 2021, 11, x FOR PEER REVIEWnanomaterial improved the light yield in the polymer corresponding to each channel.10 of 15 In this study, according to the Figure 10, it is shown that there is an improvement of more than 10% by CdTe nanomaterials. The count rate was greatly improved in the 477 keV moreCompton than energy10% by region CdTe bynanomaterials. CdTe nanomaterials. The count In addition,rate was thegreatly results improved analyzed in below the 477 ‘1’ more than 10% by CdTe nanomaterials. The count rate was greatly improved in the 477 keVindicate Compton that the energy effect region of nanomaterials by CdTe nanomateri may be theals. effect In addition, of light the loss results and signal analyzed overlap be- keV Compton energy region by CdTe nanomaterials. In addition, the results analyzed be- lowdue ‘1’ to aggregationindicate that of the nanomaterials. effect of nanomaterials may be the effect of light loss and signal low ‘1’ indicate that the effect of nanomaterials may be the effect of light loss and signal overlap due to aggregation of nanomaterials. overlap due to aggregation ofcount nanomaterials. rate per channel [PS(PPO, POPOP) − CdTe] Count ratio = , (5) Count ratioCount rate per channel [PS,(PPO, POPOP )] (5) Count ratio , , (5)

FigureFigure 9. NormalizedNormalized results results from from measured measured and and si simulatedmulated energy energy spectrum of commercial and Figure 9. Normalized results from measured and simulated energy spectrum of commercial and CdTe-loadedCdTe-loaded plastic plastic scintillator scintillator to to Cs-137 gamma ray source. ( aa)) Point source measurement using CdTe-loaded plastic scintillator to Cs-137 gamma ray source. (a) Point source measurement using commercialcommercial plastic plastic scintillator. scintillator. ( (bb)) Point Point source source measurement measurement using using a a functional functional plastic plastic scintillator scintillator commercial plastic scintillator. (b) Point source measurement using a functional plastic scintillator including CdTe. includingincluding CdTe. CdTe.

Figure 10. Count ratio from gamma spectra from a functional plastic scintillator. FigureFigure 10. 10. CountCount ratio ratio from from gamma gamma spectra spectra from from a afunctional functional plas plastictic scintillator. scintillator. CdTeCdTe nanoparticles have have advantages advantages such such as as the the change of energy gap according to CdTe nanoparticles have advantages such as the change of energy gap according to thethe size size of of nanomaterials, nanomaterials, li lightght emission emission due due to to the the discon discontinuitytinuity in in energy energy state state density, density, the size of nanomaterials, light emission due to the discontinuity in energy state density, increaseincrease in in light light efficiency, efﬁciency, and and light light emission emission (even (even at atroom room temperature) temperature) due due to the to thein- increase in light efficiency, and light emission (even at room temperature) due to the increaseincrease in the in thebinding binding energy energy of excitons. of excitons. Until Untilnow, there now, have there been have a beengreat adeal great of studies deal of crease in the binding energy of excitons. Until now, there have been a great deal of studies forstudies use in for CdTe use nanoparticles, in CdTe nanoparticles, like optical like materials, optical materials,as shown asin Table shown 4, inand Table important4, and for use in CdTe nanoparticles, like optical materials, as shown in Table 4, and important studiesimportant are being studies conducted are being to conducted improve the to energy improve transfer the energy rate by transfer doping rate inorganic by doping par- studies are being conducted to improve the energy transfer rate by doping inorganic par- ticlesinorganic into CdTe particles nanoparticles. into CdTe nanoparticles. ticles into CdTe nanoparticles. Table 4. Progress of CdTe-enabled optical materials. Table 4. Progress of CdTe-enabled optical materials. Nanomaterials Methods Results Year Ref. Nanomaterials Methods Results Year Ref. - Strength increases as energy is - Strength increases as energy is transferred from Eu2+ to CdTe QD. transferred from Eu2+ to CdTe QD. - CdTe QD is weak in X-ray emission, so a - As it can be emitted in the near in- - CdTe QD is weak in X-ray emission, so a - As it can be emitted in the near in- CdTe/BaFBr:Eu2+ nanocomposite was pre- frared range (650–1100 nm), which CdTe/BaFBr:Eu2+ nanocompo- CdTe/BaFBr:Eu2+ nanocomposite was pre- frared range (650–1100 nm), which CdTe/BaFBr:Eu2+ nanocompo- pared to evaluate its property. is a tissue optical window for in 2006 [43] site phosphors pared to evaluate its property. is a tissue optical window for in 2006 [43] site phosphors - Analyze the size and structure of nanopar- vivo imaging, the possibility of us- - Analyze the size and structure of nanopar- vivo imaging, the possibility of us- ticles by TEM. ing it as a semiconductor/phosphor ticles by TEM. ing it as a semiconductor/phosphor nanocomposite material is evalu- nanocomposite material is evaluated. ated. Appl. Sci. 2021, 11, 5210 11 of 15

Table 4. Progress of CdTe-enabled optical materials.

Nanomaterials Methods Results Year Ref. - Strength increases as energy is transferred from Eu2+ to CdTe QD. - CdTe QD is weak in X-ray emission, so - As it can be emitted in the near CdTe/BaFBr:Eu2+ a CdTe/BaFBr:Eu2+ nanocomposite was infrared range (650–1100 nm), which is a nanocomposite prepared to evaluate its property. 2006 [43] tissue optical window for in vivo phosphors - Analyze the size and structure of imaging, the possibility of using it as a nanoparticles by TEM. semiconductor/phosphor nanocomposite material is evaluated. - After synthesis by adding CdTe QD having various emission ranges to PMMA-based materials, the Excellent X-ray emission results applicability to the X-ray imaging field CdTe quantum dots including high resolution, fast decay was evaluated. and polymer time, afterglow prevention, high 2011 [44] - A nanocomposite film containing nanocomposites stopping power, and excellent spectral 0.1–10 wt% of CdTe QD was prepared matching to the CCD detector and the characteristics were evaluated. - Use of bulk polymerization method to secure transparency - The X-ray emission of CdTe QD is very - LaF3:Ce/CdTe nanocomposite was weak, but the emission of prepared to observe the X-ray emission LaF :Ce/CdTe increases (six times) LaF3:Cd/CdTe 3 characteristics of CdTe QD - CdTe QD shows the characteristics of 2012 [45] nanocomposites - Analyze the size and structure of intense photoluminescence and nanoparticles through TEM up-conversion light emission, and short decay time characteristics - CdTe is widely used due to the simplicity of the structure and the - In order to improve the performance of efficiency compared to the doped the CdTe-based solar cell, the developed amount, and the efficiency of the CdCl heat treatment was applied to the CdTe Photovoltaic 2 CdTe-based solar cell is improved CdTe of the upper layer structure in 2012 [46] Devices through the CdCl heat treatment which most of the reactions occur. 2 method. - Improved efficiency by controlling the - By improving the characteristics of the junction of CdTe-based solar cells interface layer (IFL), it shows an efficiency of 10.97%. - A facile one-step synthesis of CdTe quantum dots (QDs) in aqueous solution by atmospheric microwave reactor has been developed using - Green to red-emitting CdTe QDs with CdTe quantum dots 3-mercaptopropionic acid reduction of a maximum photoluminescence synthesized using TeO directly. quantum yield of 56.68% were obtained. 3-mercaptopropionic 2 - The obtained CdTe QDs were - The as-synthesized CdTe QDs were 2013 [47] acid characterized by ultraviolet–visible highly luminescent, which ensures their reduction of tellurium spectroscopy, fluorescent spectroscopy, promising future applications as dioxide X-ray powder diffraction, biological labels multifunctional imaging electron spectrometer (XPS), and high-resolution transmission electron microscopy - A plastic detector is manufactured by - As a result of characterization adding a fluorescent material and a Plastic scintillator evaluation after adding the nano material (Gd O or CdTe) to an containing Gd O or 2 3 concentration of QD by 0.05–1 wt%, 2019 [36] 2 3 epoxy-based material. CdTe 0.1 wt% was analyzed as the most - Evaluating the efficiency using a beta optimized concentration. source (Sr-90) Appl. Sci. 2021, 11, 5210 12 of 15

Table 4. Cont.

Nanomaterials Methods Results Year Ref. - This is a study to use CdTe QD as a - As a result of investigating the release fluorescent probe for Hg2+ detection mechanism of Ag-doped CdTe QD, it - To expand the detection concentration Silver ion-doped CdTe shows multiple release in QD samples range of Hg2+, Ag ions were doped with 2020 [48] quantum dots with high Ag+ doping concentration CdTe QD - The highest PL quantum yield of the - The characteristics of the produced QD QD sample was analyzed as 59.4% were analyzed. - Develop CdTe QD synthesis method to improve quantum yield - Evaluating the characteristics of - Synthesizing CdTe with an emission specific substances (SAL) by wavelength of 540–560 nm and Thioglycolic concentration through fluorescence evaluating the possibility of using it for acid-capped CdTe quenching mechanism SAL detection 2021 [49] quantum dots - Using the ratio of - Evaluating the concentration of ([Cd2+]:[HTe-]:[TGA(Thioglycolic acid)] additives (SAL) using the degree of = 1:0.5:2.4), CdTe QD with high extinction of QD photoluminescence was synthesized at 140 ◦C. - In the PMMA/CdTe thin film without silica coating, non-uniform CdTe - Using CdTe coated with SiO 2 particles act as defects in the matrix. (CdTe@SiO ) and PMMA, a Non-polar PMMA 2 - In the PMMA/CdTe thin film coated nanocomposite material was produced solvent of CdTe with silica, clusters of uniform size were that efficiently exhibits Stokes shift 2021 [50] quantum dots by silica observed. - The characteristics of CdTe coated with coating - The emission spectrum of SiO silica and without silica were compared 2 overlaps with the excitation wavelength and analyzed. range of CdTe QD to increase the response of the optical system - The optical properties of CdTe QD showed the highest emission quantum - Comparison of optical and structural yield when synthesized with a binary CdTe quantum dots characteristics between CdTe QDs solvent. using a binary solvent - Analyzing the optical properties of 2021 [51] - CdTe QD prepared with a Cd:Te molar (water and glycerin) CdTe nanocrystals made with various ratio of 20:1 showed a narrow synthesis parameters photoluminescence band and improved quantum yield - A study on the spectral characteristics - Colloidal graphene quantum dots and of a wide range of CdTe quantum dots CdTe investigated in this work retain and wide-gap graphene QD was their optical and structural properties conducted. when exposed to radiation in the visible CdTe quantum dots - A technology to replace the organic range and graphene quantum shell of CdTe QD of various sizes was - In the case of CdTe QD, the maximum 2021 [52] dots proposed. intensity of the irradiated sample did - Light stability when irradiated with not change within the measurement radiation on CdTe QD and wide-gap error, but in the case of graphene QD, graphene quantum dots was compared the intensity decreased when irradiated and analyzed. with ultraviolet rays. - Monolith-typed functional plastic - Integrated and portable probe based scintillator having a diameter of 50 mm on a functional plastic scintillator for and a thickness of 30 mm was detection of Cs-137. CdTe/PPO/POPOP/ manufactured by adding This - Remarkable performance of the 2021 Polystyrene 2,5-diphenyloxazole (PPO, 0.4 wt%), 1,4 work functional plastic scintillator was di[2-(5phenyloxazolyl)]benzene confirmed through comparative (POPOP, 0.01 wt%), and CdTe (0.2 wt%) analysis with Monte Carlo simulation. materials in a styrene-based matrix. Appl. Sci. 2021, 11, 5210 13 of 15

4. Conclusions In this study, we elucidated an integrated and portable probe based on functional plastic scintillator for detection of radioactive cesium. The performance of the CdTe-loaded plastic scintillator was compared with a commercial plastic scintillator. The performance of the CdTe-loaded plastic scintillator was analyzed using Cs-137 point source. After setting the distance between the radiation source and the detector to 20 mm, the energy spectrum was analyzed, and the reliability of the result was secured by comparing with the MCNP simulation result. When the functional plastic scintillator equipped with CdTe was compared with the commercial plastic scintillator, the relative efficiency increased by up to 25%, and the detection efficiency was also increased. In addition, as a result of calculating the relative light yield based on the light yield of the commercial plastic scintillator, it was confirmed that the light yield increased when CdTe was loaded. It is expected that a high- efficiency and high-sensitivity plastic detection system can be developed by optimizing the amount of nanomaterials in the future and can be applied the measurement data processing methodology. Finally, the highly reliable and direct detection of radioactive cesium based on an integrated portable probe including a functional plastic scintillator will pave the way for potential interest due to in situ detection and monitoring in environmental sites for nuclear facilities and radiation measurements.

Supplementary Materials: The following are available online at https://www.mdpi.com/article/ 10.3390/app11115210/s1. Figure S1: Input data of surface flux in a MCNP 6 simulation; Figure S2: Input data of energy spectrum in a MCNP 6 simulation. Author Contributions: Conceptualization, and original draft preparation: S.M., H.K. and B.S.; supervision and editing C.R., S.H. and J.C. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data available on request from the authors. Acknowledgments: This work was supported by the Nuclear Research and Development Program through the National Research Foundation (NRF) of Korea funded by Ministry of Science and ICT (No. 2017M2A8A5015143). Conflicts of Interest: The authors declare no conflict of interest.

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