Applied Radiation and Isotopes 203 (2024) 111107Available online 8 November 20230969-8043/© 2023 Published by Elsevier Ltd.Development of a novel low-pressure scintillation cell with ZnS(Ag)-coated clapboard for Rn-220 measurement Zhongkai Fana,b, Tao Hub, Xiangming Caia, Ruomei Xieb, Haoxuan Lia, Fengdi Qina, Shoukang Qiua, Yanliang Tanb, Jian Shana,* aSchool of Nuclear Science and Technology, University of South China, Hengyang, Hunan Province, 421001, China bCollege of Physics and Electronic Engineering, Hengyang Normal University, Hengyang, Hunan Province, 421008, China ARTICLE INFO Keywords: Low pressure ZnS(Ag)-covered clapboard Calibration factor Rn-220 ABSTRACT The high-precision measurement of Rn-220 is essential for assessing and preventing thoron radiological hazards. Prior research has revealed that employing a scintillation cell without a clapboard improves the detection efficiency for both Rn-222 and Rn-220 by reducing air pressure, and the Rn-220 calibration factor is established at an atmospheric pressure of 0.4. However, the decrease in air pressure leads to a corresponding reduction in Rn220 concentrations within the scintillation cell, resulting in lower counts and larger statistical fluctuations. For the purpose of addressing this issue, a ZnS(Ag)-coated clapboard was added to the low-pressure scintillation cell for measuring Rn-222 and Rn-220. Several experiments were conducted in conjunction with Monte Carlo simulations. The results of these simulations, along with experiments using the standard radon chamber, provide valuable references for establishing the Rn-220 calibration factor. The experimental findings demonstrate that saturated detection efficiency of Rn-222 and Rn-220 can be maintained at below 0.7 atmospheric pressure. Therefore, the Rn-220 calibration factor, determined with the detection efficiency of 0.73 ±0.04 at an atmospheric pressure of 0.7, was determined to be 31.98 ±1.83 Bq⋅m3min1 (k =1). 1.Introduction Rn-222 and Rn-220 account for over 50% of the total annual effective dose from natural radiation sources globally (Asano et al., 2001). Multiple surveys have indicated that indoor Rn-220 and its progeny exhibit similar or, in some cases, higher levels compared to Rn-222 and its progeny in numerous regions (Yuan et al., 2000). Radon emissions from the pore spaces in rocks and soils depend on many environmental parameters, such as air pressure, temperature, humidity, and other meteorological parameters (Rogers and Nielson, 1991; Samuelsson and Pettersson, 1984; Schery et al., 1988). Furthermore, the measurement results of radon emissions may potentially be associated with variations in solar neutrino flux on the order of magnitude during solar flares (Jenkins et al., 2010). Accurately measuring Rn-220 is influenced not only by these factors, but its shorter half-life also poses additional challenges for the calibration of Rn-220 measurement devices. Scintillation cells, equipped with an inner coating of ZnS(Ag), are widely employed for measuring concentrations of Rn-222 and Rn-220 due to their insensitivity to humidity (Abdalla et al., 2021). Measurements using scintillation cells are typically conducted under either static or flow-through conditions (Sathyabama et al., 2014; Sensintaffar et al., 1990). The air carrying Rn-222 or Rn-220 gas is sucked into the scintillation cell through a filter. Within the scintillation cell, α particles decayed from radioactive gas strike the inner wall and interacts with the ZnS(Ag) phosphor, generating photons (Eappen et al., 2008). These photons are subsequently detected and counted using a photomultiplier tube (PMT) and its associated electronic module. The resulting net count rate can be converted into respective concentrations using a calibration factor (Sumesh et al., 2014). The calibration factor is obtained through the theoretical calculations based on decay progeny of Rn-222 and Rn-220, derived from their unit pure source. The efficiency of detecting Rn-222 and Rn-220 within the ZnS(Ag)- coated scintillation cell is significantly influenced by the ranges of α particles with different energies in the air. Quindos-Poncela et al. reported a 15% decrease in the detection efficiency of Rn-222 when utilizing an improved scintillation cell, as the pressure increased from 827 to 1013 hPa (Quindos-Poncela et al., 2003). A small scintillation cell (64 *Corresponding author. E-mail address: shanjian0666@163.com (J. Shan). Contents lists available at ScienceDirect Applied Radiation and Isotopes journal homepage: www.elsevier.com/locate/apradiso https://doi.org/10.1016/j.apradiso.2023.111107 Received 3 July 2023; Received in revised form 27 October 2023; Accepted 4 November 2023

Applied Radiation and Isotopes 203 (2024) 1111072mL) was designed to increase the detection efficiency of alpha particles by reducing its internal volume. It was subsequently utilized for calibrating a self-made Rn-220 flow-through source through the flow-through method (Qiu, 2006). A previous study indicated that a scintillation cell system without a clapboard, developed for measuring Rn-222 and Rn-220 by reducing the air pressure, achieved saturation detection efficiency at 0.4 atmospheric pressure (Fan et al., 2023). However, the lower air pressure in the scintillation cell leads to lower Rn-220 concentrations, resulting in lower net count rates and larger statistical fluctuations. In contrast to previous low-pressure scintillation cell systems without cross clapboard, this study aims to attain the equivalent saturated detection efficiency at a relatively higher low pressure by incorporating a ZnS(Ag)-coated clapboard inside the scintillation cell. Monte Carlo simulation was utilized to trace the trajectories of α particles emitted from Rn-222 (α, 5.5 MeV), Po-218 (α, 6.0 MeV) and Po-214 (α, 7.69 MeV) within the scintillation cell at different pressures. The experiments of measuring Rn-222 and Rn-220 were conducted in conjunction with these simulations. The results of simulations and the experiments of Rn222 with the standard radon chamber serve as valuable references for establishing the calibration factor for Rn-220. 2.Monte Carlo simulation for α particles within the scintillation cell The data of Rn-222 and Rn-220 decay chains are shown in Table 1. For the Rn-222 measurement in the static condition, it requires at least 3 h to achieve radioactive equilibrium. The efficiency of detecting α particles within the ZnS(Ag)-coated scintillation cell is influenced by their ranges in the air. Monte Carlo simulation was utilized to track the transportation of α particles in two types of the scintillation cells at different low pressures, one with a ZnS(Ag)-covered clapboard and the other without clapboard (Allison et al., 2016). The count on the internal cell wall or clapboard in the scintillation cell can be obtained with the following assumptions. (1) the scintillation cell is a spherical cell with an inner surface diameter 9.85 cm, and it includes a circular window (transparent glass, diameter 4.66 cm) at the bottom, as shown in Fig. 1; (2) the ZnS(Ag)-coated inner wall of scintillation cell can detect alpha particles except for the bottom circular window without ZnS(Ag); (3) α particles emitted by Rn-222 (α, 5.5 MeV), Po-218 (α, 6.0 MeV) and Po-214 (α, 7.69 MeV) are randomly distributed throughout the airspace within the scintillation cell volume, and the progeny deposited on the inner wall was not considered due to confirm the influence of air pressure on the detection efficiency of α particles; (4) the α particle was considered to be counted if it reached the cell’s sensitive region with an energy exceeding a specified discrimination threshold of 0.13 MeV (Zhao, 2011). The α particles, at an atmospheric pressure, lose some or all of their energy in air before hitting the detector (Sabbarese et al., 2017). The number of α particles impacting the wall or clapboard increases as the air pressure decreases when the same number of α particles were released at different air pressures. The α particle primarily undergo inelastic collisions with particles in the air, leading to ionization reactions. To reproduce this physical process in Geant4, the pre-package physics list QBBC, recommended for simulating processes of electromagnetic interactions of particles, is used (Allison et al., 2006). The simulation process of Geant4 involves customizing a particle source in G4GeneralParticleSource, with an isotropic emission direction and a random distribution of α particles within the air space inside the scintillation cell. As the scintillation cell is less influenced by the absolute humidity (Galli et al., 2019), the other environment parameters of temperature, air pressure and density of air were considered in Geant4 simulation. As shown in Fig. 1, the structure of the scintillation cell was built using Solidworks, and then imported it into G4DetectorConstruction in Geant4. For each energy level of α particles (5.49 MeV, 6.00 MeV, and 7.69 MeV), 105 particles were randomly emitted in any direction at an angle of 4π at different air pressures, respectively. Subsequently, the counts of particles that hit the scintillation cell’s walls and clapboard were recorded. The simulation of α particle tracks within the scintillation cell is shown in Fig. 2 and the results with different pressures are listed in Table 2. Fig. 2 illustrates a decrease in the count of α particles recorded within the scintillation cell due to their emission towards the bottom window. The number of α particles emitted toward the bottom window in Fig. 2d is larger than that in Fig. 2b. The reason is that the cross clapboard in scintillation cell can obstruct the emission of a part of α particles within the cell. The movement of alpha particles in the scintillation cell was simulated five times at the same low pressure. Table 2 reveals that the count on the wall is slightly larger than that on the clapboard at a temperature of 20 ◦C and different air pressures. Furthermore, their detection efficiencies exhibit an upward trend with decreasing air pressure. 3.The verification experiments for measuring Rn-222 and Rn220 A prior research indicated that employing a scintillation cell system without the clapboard can attain saturation detection efficiency of Rn222 and Rn-220 at 0.4 atmospheric pressure (Fan et al., 2023). However, the lower air pressure leads to lower concentrations of Rn-220, resulting in lower counts and larger statistical fluctuations. To address this issue, a ZnS(Ag)-coated clapboard was added to the low-pressure scintillation cell system, as illustrated in Fig. 3. Based on Geant4 simulation results of α particles at various pressures in the two types of scintillation cell, verification experiments were conducted employing the standard Rn-222 chamber at the temperature of 27 ±2 ◦C. This Rn-222 chamber constantly monitored by a PQ2000Pro device (Alpha Guard PQ 2000). The results of detection efficiency of Rn-222 can serve as a reference for establishing the Rn-220 calibration factor. Table 1 Decay series of Rn-222 and Rn-220 (Khan et al., 1993; Sumesh et al., 2014). Radionuclide State Half life Energy (MeV) Decay mode Range (cm) Product of decay Rn-222 Gas 3.82 d 5.49 α 3.95 Po-218 Po-218 Particulate 3.05 min 6.00 α 4.5 Pb-214 Pb-214 Particulate 26.8 min βγ 6.65 Bi-214 Bi-214 Particulate 19.7 min βγ Po-214 Po-214 Particulate 164 μs 7.69 α Pb-210 Pb-210 Particulate 22 a βγ Rn-220 Gas 55.6 s 6.29 α 4.9 Po-216 Po-216 Particulate 0.15 s 6.78 α 5.6 Pb-212 Pb-212 Particulate 10.6 h βγ Bi-212 Bi-212 Particulate 1 h 6.05 α (36%) βγ (64%) Tl-218 Po-212 Po-212 Particulate 3.05 ×107 s 8.78 α 8.5 Pb-208 Z. Fan et al.

Applied Radiation and Isotopes 203 (2024) 1111073Fig. 1.The structure of the scintillation cell (ST-203) with a cross clapboard: (a) the cross clapboard; (b) the whole structure. Fig. 2.Geant4 simulation of α particle tracks in different scintillation cells (α, 7.69 MeV, 104): (a) the scintillation cell with cross clapboard; (b) the simulation results of the scintillation cell with cross clapboard; (c) the scintillation cell without cross clapboard; (d) the simulation results of the scintillation cell without cross clapboard. Z. Fan et al.

Applied Radiation and Isotopes 203 (2024) 1111074(1) the measurement of Rn-222 in the static condition Valves 1 and 6 are in the closed state, whereas valves 2, 3, 4, 5 and control valve are open. Rn-222-rich air, devoid of Rn-220, was transferred from the standard radon chamber into the measurement system and continuously circulated using the vacuum pump. After 5 min, the control valve was fine-tuned to stabilize the reading on the pressure gauge at p1. The pressure indicated on the pressure gauge is equal to the pressure inside the scintillation cell because they are connected via a tube. Valves 2 and 5 are then simultaneously closed, followed by turning off the vacuum pump. To establish a radioactive equilibrium between radon and its short-lived progeny, the low-pressure scintillation cell was left for a duration of 3 h. Subsequently, the counts started to be measured using the BHC336 counter (Beijing Nuclear Instrument Factory) for more than 10 min. The detection efficiency (ηRn) of Rn-222 using the scintillation cell can be represented as follows ηRn=pNRn/603Vp1CRneλt(1)where CRn denotes the Rn-222 concentration (Bq⋅m3), t represents the time duration between the termination of sampling process and the initiation of measurement (s), NRn denotes the net count rate (min1), V denotes the volume (m3), p1 denotes the low pressure, λ represents the decay contant of Rn-222 ((2.1 ±0.0001) ×106 s1) (Bellotti et al., 2015), p denotes the atmospheric pressure (kPa). (2) the measurement of Rn-220 in the flow-through condition Valves 3 and 4 are in the closed state, whereas valves 1, 2, 4, 6 and control valve are open. Rn-220-rich air, devoid of Rn-222, was drawn into the measurement system from a stable Rn-220 source and continuously pumped using the vacuum pump. Subsequently, the control valve was fine-tuned to stabilize the reading on the pressure gauge at p2. After 5 min, the counts were recorded using the BHC336 counter for 10 min. The air pressure in the airflow path before the control valve approximates atmospheric pressure, whereas downstream of control valve, it is maintained at a stable low-pressure p2. The pressure gauge reading corresponds to the pressure within the scintillation cell since they are connected through a tube. Consequently, the actual volume between the outlet of both control valve and the scintillation cell at an atmospheric pressure is approximated as (V2+V)p2p, where V2 is negligible compared to V (around 0.2% of V). The Rn-220 concentration at the scintillation cell’s inlet was calculated using the flow rate, the activity concentration of the Rn-220 source, and the volume V ((5.1 ±0.01) ×104 m3). The pressure inside the scintillation cell varies, and correspondingly, the flow rate also differs, posing a challenge in distinguishing whether the airflow within the cell exhibits smooth or turbulent. The average Rn-220 concentration (CTn) in the scintillation cell can represent the average Rn-220 concentration in both smooth and turbulent airflow, and it can be expressed (Fan et al., 2023) Table 2 The results of Geant4 simulation (The quantities of α particles for each energy level are 105). Parameters Air pressure (kPa) 101.3 81.1 60.8 40.5 30.4 Temperature (◦C) 20 20 20 20 20 Density (kg/m3) 1.205 0.964 0.723 0.482 0.361 The scintillation cell with clapboard 5.49 MeV Count of sphere 42,490 ±96 47,079 ±110 50,262 ±179 50,952 ±201 51,149 ±306 Count of clapboard 40,707 ±118 44,411±132 46,206 ±209 46,597 ±181 46,434 ±312 6.0 MeV Count of sphere 45,624 ±122 49,059 ±148 50,918 ±91 51,023±86 51,105 ±126 Count of clapboard 43,339 ±127 45,620 ±73 46,374 ±111 46,547 ±80 46,457 ±132 7.69 MeV Count of sphere 50,426 ±186 51,045 ±111 51,098 ±134 51,129 ±147 51,201 ±68 Count of clapboard 46,326 ±219 46,481 ±139 46,502 ±133 46,467 ±105 46,363 ±92 The scintillation cell without clapboard 5.49 MeV Count of sphere 51,068 ±158 62,373 ±59 78,137 ±97 94,118 ±44 94,253 ±46 6.0 MeV Count of sphere 58,191 ±51 70,188 ±101 85,618 ±43 94,310 ±60 94,299 ±56 7.69 MeV Count of sphere 80,228 ±108 90,629 ±55 94,313 ±49 94,324 ±68 94,344 ±61 Fig. 3.The devices of the new low-pressure scintillation cell (with ZnS(Ag)-covered clapboard) system for Rn-222 or Rn-220 measurement. Z. Fan et al.

Applied Radiation and Isotopes 203 (2024) 1111075CTn=Ap2Vp2λTn(1eλTn60Vp2L×10−3p)+30pAeλTnV160L×103L×103p+60Vp2λTn(2)where L denotes the flow rate (L⋅min1), V1 denotes the volume between the stable Rn-220 source’s outlet and the control valve, V denotes the volume of the scintillation cell (m3), A denotes the activity of stable Rn220 source (Bq) and it is 20.16 ±0.60 Bq, p represents the atmospheric pressure (kPa), λTn denotes the decay contant of Rn-220 ((1.24 ±0.01) ×10−2 s−1), p2 denotes the low pressure (kPa). At the low pressure of p2 within the scintillation cell, the detection efficiency of Rn-220 can be formulated as follows ηTn=pNTn/602p2VCTn(3)where NTn represents the net count rate (min1). Therefore, the calibration factor K of Rn-220 at the low-pressure p2 is K=p/602p2VηTn(4)where K is the calibration factor of Rn-220 (Bqm3min1). 4.Results and discussion 4.1.The results of Rn-222 measurement In Geant4 simulations, 105 α particles within the scintillation cell were randomly emitted at an angle of 4π for each energy of 5.49 MeV, 6.00 MeV, and 7.69 MeV, under various low-pressure conditions. Fig. 4a illustrates the average count of their three energies of α particles within the scintillation cell under both clapboard-equipped and clapboard-free conditions. For the clapboard-equipped scintillation cell, the average count represents the total collected count including both on the wall and the clapboard. Validation experiments were conducted utilizing the standard radon chamber at the temperature of 27 ±2 ◦C. The findings are illustrated in Fig. 4b, with a comparison to the previous study (Fan et al., 2023). Based on the data from simulations and experiments conducted under the low-pressure conditions, the detection efficiency of the ZnS (Ag)-covered clapboard equipped scintillation cell exceeds that of clapboard-free scintillation cell. At 0.7 atmospheric pressure, it outperforms by approximately 10%. In the same kind of scintillation cell, the detection efficiency increases with pressure decreases. For the clapboard-equipped scintillation cell, the difference of the collection efficiency at the atmospheric pressure between 0.3 and 0.7 is within 4%. Consequently, the detection efficiency (ηRn) of the clapboard-equipped scintillation cell tends to stabilize below 0.7 atmospheric pressure, and the detailed results of Rn-222 experiments are listed in Table 3. 4.2.The Rn-220 calibration factor Fig. 5 illustrates the experimental results of the detection efficiency of Rn-220 within two types of the scintillation cell at different low pressures, incorporating the results of previous experiments (Fan et al., 2023). At 0.7 atmospheric pressure, the detection efficiency obtained by the clapboard-equipped scintillation cell is 14% higher than that of the clapboard-free scintillation cell. Additionally, when comparing the detection efficiency of the clapboard-equipped scintillation cell at 0.3 atmospheric pressure, there is only a 4% difference. Consequently, the detection efficiency (ηTn) of the clapboard-equipped scintillation cell tends to stabilize below 0.7 atmospheric pressure. The experimental results are listed in Table 4. Fig. 6 illustrates the experimental results of the clapboard-equipped scintillation cells for Rn-222 and Rn-220 measurement. The detection efficiency of Rn-222 exceeds that of Rn-220, with an average difference between the two of approximately 5%. The primary reasons are as follows: Rn-222 employs static measurement compared to the flowFig. 4.The results of Rn-222 measurement using two types of scintillation cells at different pressures: (a) the simulation data; (b) the experimental data. Table 3 The data of Rn-222 measurement experiments (k =1). parameters P p1 (kPa) 0.7P 0.6P 0.4P 0.3P 0.8P 101.3 ±1.6 81.1 ±1.3 70.91 ±1.1 60.8 ±1.0 40.5 ±0.6 30.4 ±0.5 CRn (Bqm3) 2880 ±179 3424 ±198 3760 ±218 3104 ±189 4016 ±253 4512 ±251 NRn (min1) 195 ±7 191 ±9 190 ±10 135 ±12 120 ±9 102 ±9 ηRn 0.75 ±0.05 0.78 ±0.06 0.80 ±0.06 0.82 ±0.08 0.83 ±0.07 0.84 ±0.07 Z. Fan et al.

Applied Radiation and Isotopes 203 (2024) 1111076through measurement used for Rn-220, and the wall-attachment probability of Rn-222 and its progeny is greater than that of Rn-220 and its progeny, resulting in higher detection efficiency for Rn-222. Yet their detection efficiencies at lower pressure did not reach 100%. The main reason is the absence of ZnS(Ag) coating on the bottom window, which occupies 6.7% of the scintillation cell’s total internal surface area. Additionally, the detection efficiency of ZnS(Ag) materials for α particles is greater than 80% and there is a loss of α particles in the air layer within the cell that do not interact with the ZnS(Ag) material. The differences between the detection efficiencies of Rn-222 and Rn220 within the clapboard-equipped scintillation cell are less than 4% at both atmospheric pressure of 0.7 and 0.3. They tend to approach saturation at the pressures below 0.7 atm and exhibit a close resemblance, allowing for the calculation of the Rn-220 calibration factor using this detection efficiency (ηTn). Utilizing the measured detection efficiency of 0.73 ±0.04 at 0.7 atmospheric pressure in accordance with equation (4), the Rn-220 calibration factor is established to be 31.98 ±1.83 Bq⋅m−3⋅min−1 (k =1). 5.Conclusions Reducing pressure within the scintillation cell has the capacity to improve its detection efficiency. Nevertheless, the decreased air pressure in the cell also leads to lower concentrations of Rn-220, resulting in reduced counts and increased statistical fluctuations. The new lowpressure scintillation cell system, equipped with a ZnS(Ag)-coated clapboard, was designed for establishing the Rn-220 calibration factor. Monte Carlo simulations and the experiments of Rn-222 with the standard radon chamber provided valuable references for establishing the Rn-220 calibration factor. Experimental results demonstrate that the detection efficiency stabilizes below 0.7 atmospheric pressure, and the Rn-220 calibration factor is established at 31.98 ±1.83 Bqm3min1 (k =1), using measured detection efficiency of 0.73 ±0.04 at this pressure. This technology possesses the capability to offer technical assistance in the development of measurement standards. CRediT authorship contribution statement Zhongkai Fan: Writing – original draft, Software, Formal analysis, Data curation. Tao Hu: Writing – review & editing. Xiangming Cai: Writing – review & editing. Ruomei Xie: Writing – review & editing. Haoxuan Li: Writing – review & editing. Fengdi Qin: Software. Shoukang Qiu: Supervision. Yanliang Tan: Writing – review & editing, Supervision, Resources, Methodology, Funding acquisition, Conceptualization. Jian Shan: Writing – review & editing, Resources, Project administration, Funding acquisition, Formal analysis, Conceptualization. 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. Data availability The data that has been used is confidential. Acknowledgements This work was supported by National Natural Science Foundation of China (Grant No. 12075112), National Natural Science Foundation of China (Grant No. U1865207), Natural Science Foundation of Hunan Province (Grant No. 2023JJ50121), Natural Science Foundation of Hunan Province (Grant No. 2023JJ50091), Science and technology Project (Grant No. 50927030306). Fig. 5.The results of Rn-220 measurement within the two types of scintillation cells at different pressures. Table 4 The data of Rn-220 measurement experiments (k =1). parameters p2 (kPa) 0.8P 0.7P 0.6P 0.4P 0.3P 81.1 ±1.3 70.9 ±1.1 60.8 ±1.0 40.5 ±0.6 30.4 ±0.5 L (Lmin1) 3.82 ±0.15 3.30±0.13 2.75 ±0.11 0.98 ±0.04 0.34 ±0.01 NTn (min1) 10,377 ±198 10,817 ±128 11,223 ±140 20,622 ±359 39,620 ±176 ηTn 0.71 ±0.03 0.73 ±0.04 0.74 ±0.04 0.76 ±0.04 0.77 ±0.04 Fig. 6.The results of Rn-222 and Rn-220 measurement within the clapboardequipped scintillation cells at different pressures. Z. Fan et al.

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