To improve the metrological traceability of thoron activity concentration, we separated 228Th from industrial thorium nitrate and produced flow-through radon-free thoron sources using two different methods. Four flow-through sources with different activities ranging from 120 Bq to 1200 Bq were prepared, and the activity, thoron emanation coefficient and stability were evaluated. Results show that loading the 228Th solution onto a TEVA resin column gave significantly higher 228Th recovery rates and better stability than immersing TEVA resin directly into the solution. The thoron emanation coefficient increases as absolute humidity increases, which can be used to realize different thoron concentration environments. The average emanation coefficient for the four different thoron sources was 96.37% ± 2.38% at absolute humidity above 20 g/m3.

Building materials have become a predominant source of indoor radon in mid- to high-rise buildings, making in situ measurement of radon exhalation rates from building surfaces essential for identifying radon sources and assessing associated risks. Based on practical survey requirements-addressing sealing leakage at chamber edges and ensuring device portability-this study developed an improved in situ measurement method integrated with leakage compensation through theoretical analysis and experimental validation. The method employs an acrylic accumulation chamber and a portable passive radon detector, adopts a 24 h continuous measurement duration, and processes radon concentration data using an exponential fitting approach. Comparative experiments with the activated carbon method demonstrated good consistency between the two methods. Furthermore, small-scale in situ measurements were conducted in the Beijing area, covering diverse building materials (concrete, brick), surface treatments (cement plaster, coating, wallpaper), and structural components (walls, floors). The results, which varied widely from 0.13 +/- 0.11 to 28.00 +/- 4.87 Bq/m2 & centerdot;h, confirm the reliability and applicability of the method for in situ determination of radon exhalation rates from interior building surfaces.

Ubiquitous natural radionuclides, radon and its progeny, are the largest contributors to public radiation exposure from natural sources and the second leading cause of lung cancer, closely linked to radiation safety. Simultaneously, radon is the most significant source of radiation background in rare-event experiments conducted in deep underground laboratories, which are at the forefront of physics research today. Therefore, studies on environmental or ultra-low-level radon measurements hold substantial academic value and practical application significance. This paper presents the progress and achievements of the Radiation Protection Group, School of Physics at Peking University, in radon and progeny measurement research, detailing high-sensitivity radon measurement results for deep underground laboratories and studies on the particle size distribution of radon progeny for dose estimation purposes. According to the test results of the four radon meters with different volumes developed by our team, the overall trend indicates that the sensitivity and average background value of the electrostatic radon meter increase with the expansion of the measurement chamber volume, while the detection limit and decision threshold decrease accordingly. These findings are consistent with theoretical analysis and the conclusions of international peers, demonstrating that this study has successfully achieved the expected technical indicators. For the second part, due to the presence of various forms of radon progeny (Rn-222/Rn-220) in the environment, such as unattached, nucleation, accumulation, and coarse particle modes, each contributing differently to radiation dose, on-site measurements require distinct separation and sampling methods based on the physical properties of these progeny modes. The progeny aerosols are drawn by a pump into an eight-stage sampling device, with a sampling flow rate of 4 liters per minute (lpm) at each stage. The first stage employs a filter membrane, which collects nearly 100% of particles across all size ranges. The second stage consists of a filter membrane combined with an impactor, designed to capture coarse particle mode progeny with diameters larger than 2.5 mu m. The third stage utilizes an impactor paired with a single-layer wire screen, which filters both coarse particle mode and unattached progeny. Stages four through eight incorporate impactors combined with optimally selected SDB wire screen assemblies, enabling size-resolved sampling of nucleation and accumulation mode progeny aerosols. The activity concentration of unattached and coarse particle mode progeny in the environment is determined by successively subtracting the measurement results from the first three stages. For the latter six stages, the activity size distribution parameters, such as the activity median diameter (AMD) and geometric standard deviation (sigma), are derived through inversion using the expectation-maximization (EM) algorithm and the Twomey algorithm.

Radon and thoron exposure represents the largest component of public radiation dose from natural sources. While radon and thoron gases themselves contribute to this exposure, their short-lived decay products account for the majority of the effective dose. Accurate dose assessment therefore relies on precise measurements of radon and thoron progeny, including their activity concentrations, unattached fractions, and aerosol size distributions. Additionally, due to their radioactive properties and suitable half-lives, radon and its decay products serve as valuable tracers in atmospheric and environmental research, especially in applications requiring long-term continuous monitoring. This paper reviews recent progress and achievements from the Environmental Radioactivity Laboratory of Peking University in the development of measurement methods for radon and radon-thoron progeny. It highlights innovative aspects, fundamental performance characteristics, and key technical specifications of the methods and instruments designed for various measurement purposes and employed in practical applications.

Radon (222Rn) and its isotope thoron (220Rn) are major contributors to public exposure from natural background radiation, making the accurate measurement of their progeny activity concentrations highly importance. This study proposes a method for establishing the β detection efficiency for different progeny based on the calibrated α detection efficiency. This approach establishes a correlation between α detection efficiency and β detection efficiency based on the radioactivity of progeny accumulated on sampling filter membranes. The β efficiency is subsequently fitted via the least-squares method, enabling precise determination of β detection efficiency. The α–β spectrometry method, which features simple instrumentation, integrated sampling and measurement, and the simultaneous utilization of α and β information, offers a concise measurement procedure and high methodological sensitivity. Based on the α–β spectrometry method previously developed by our lab, this study presents an improved version for the precise determination of radon and thoron progeny activity concentrations. The new method adopts a 3-hour measurement cycle and records α–β spectra during sampling, thereby enhancing measurement sensitivity. Additionally, by adjusting and optimizing the sampling and measurement intervals, the uncertainty is effectively reduced. Comparison experiments with the Optimized Wicke method and the PKU-Tn method demonstrate that the proposed α–β spectrometry method achieves comparable accuracy while exhibiting superior performance in terms of sensitivity and uncertainty. In particular, the sensitivity of the new method is approximately 7.3 times higher than that of the PKU-Tn method, and the uncertainties for 218Po, 214Pb, 214Bi, and 212Bi are all lower than those obtained by the two reference methods.

Radon (222Rn, Rn) and thoron (220Rn, Tn) progeny are the primary contributors to natural radiation exposure. Accurate assessment of the radiological hazard from radon/thoron depends heavily on the characteristic parameters of these progeny, with particle size distribution identified as the most critical factor. However, systematic measurements of radon/thoron progeny size distributions remain very limited, mainly due to the insufficient attention given to thoron and the constraints of measurement sensitivity. Extensive field surveys were conducted in diverse environments in China with an 8-stage screen diffusion battery (SDB) system and a newly developed graded screen array (GSA) system. The results show that the activity median diameter (AMD) of attached radon progeny is 209 ± 16 nm (AMD¯±σAMD¯) indoors, 277 ± 26 nm in coal mines, 218 ± 20 nm in the investigated NORM (Naturally occurring radioactive material) environments and 189 ± 16 nm outdoors, with similar values observed for thoron progeny. Analysis of environmental factors indicated that ambient temperature, seasonal variation, and aerosol number concentration had minor effects on the AMD of attached progeny, while carrier aerosol size and sampling location exerted significant influences. The AMD of unattached progeny is 1.10 ± 0.15 nm for radon and 2.20 ± 0.20 nm for thoron. The results are expected to improve the accuracy of inhalation dose assessment and provide important input parameters for radiation protection models.

Naturally-Occurring Radioactive Materials (NORM), such as uranium, radium, and thorium, can lead to elevated radon exposure among workers and the public. In certain mineral processing facilities-such as those handling zirconium and titanium-exceptionally high concentrations of uranium and thorium are present, necessitating field surveys and radon exposure assessments from a radiation protection management perspective. This study investigated three typical factories in Qinzhou, southern China. Measurements were conducted in workplaces for gamma dose rates, radon and thoron progeny concentrations, and key physical characteristic parameters of radon-thoron progeny. Dose conversion factors (DCFs) for radon and thoron were calculated using the TAURUS software. Results indicate an average radon activity concentration of 14.67 ± 0.39 Bq/m3, while the average equilibrium equivalent radon concentration (EERC) and equilibrium equivalent thoron concentration (EETC) were 2.64 ± 0.09 Bq/m3 and 2.45 ± 0.07 Bq/m3, respectively. Elevated gamma dose rates were observed in some workplaces. For characteristic parameters, the average activity median diameter of attached radon/thoron progeny (AMDa) was 218 nm and 206 nm, with geometric standard deviations (GSDa) of 1.7 and 1.8, respectively. The average activity median diameter of unattached progeny (AMDu) was 1.32 and 2.18 nm, with GSDu values of 1.3 and 1.2. The DCF for radon progeny was calculated as 32.5 nSv/(Bq·m-3·h), and for thoron progeny as 149 nSv/(Bq·m-3·h). In workplaces, the estimated annual effective dose for workers primarily stems from gamma radiation and thoron exposure, with contributions of 0.67 and 0.44 mSv, respectively. In contrast, radon exposure contributed only 0.11 mSv. When both workplaces and dwellings are considered, the total estimated annual effective dose was 3.59 mSv, with gamma radiation, radon, and thoron making comparable contributions on average.

Significance Radon(222Rn) is a naturally occurring radioactive noble gas. 222Rn is produced from the alpha decay of radium (226Ra), with a relatively long half-life (i.e., 3.8 days). Thus, 222Rn can diffuse and migrate from the rock and soil where it is generated and can enter the air. According to surveys conducted by the World Health Organization, exposure to 222Rn and its short-lived progeny is the second leading cause of lung cancer. The measurement and evaluation of 222Rn release involves not only occupational exposure but also public exposure. Therefore, the measurement of 222Rn exhalation and the dose assessments of 222Rn exposure have always been important concerns in radiation protection. Progress The entire process of 222Rn moving from the soil to the atmosphere can be divided into three steps: emanation, migration, and exhalation. During the emanation process of 222Rn, 222Rn will obtain a recoil energy of 8.6 u00D7 104 eV from the decay of 226Ra, which will make 222Rn atoms travel through the soil grains at a distance of no more than 50 nm. If produced near the surface of soil grains, 222Rn will leave the grain and stop in the interstitial space (pore), becoming freely movable 222Rn. The fraction of freely movable 222Rn is usually expressed by the emanation coefficient (dimensionless). The emanation coefficient of soil generally ranges from 0.1 to 0.3. In soil with a relatively stable internal environment, the migration of free 222Rn in soil mainly relies on molecular diffusion caused by concentration gradients, eventually entering the atmosphere through the soil-air interface. If only diffusion transport is considered, then Fick's law can be used to describe the migration process of 222Rn and establish a model for 222Rn flux at the soil-air interface. The 222Rn exhalation rate at the soil surface is the 222Rn flux. On-site measurement methods of the soil 222Rn exhalation rate can generally be divided into three categories: accumulation, flow-through, and activated carbon adsorption methods. In actual measurements, different methods can be chosen according to the needs. Conclusions and Prospects The understanding of the 222Rn diffusion exhalation mechanism and influencing factors is becoming comprehensive, and the measurement methods of the 222Rn exhalation rate for different purposes have been developed. Analyzing the physical processes of 222Rn exhalation from soil and measuring and evaluating the exhalation rate of 222Rn are important for assessing environmental radiation and managing uranium tailings and associated radioactive minerals. Moreover, the exhalation rate of 222Rn is closely related to the radiation environmental safety of on-site supervision of naturally occurring radioactive materials. Because of the complexity and diversity of 222Rn measurement sites, even though the amount of 222Rn release can be measured and calculated relatively accurately, nearly no quantitative deterministic correlation is detected between it and the indoor 222Rn concentration. Thus, the 222Rn exposure dose for key populations is difficult to estimate. The 222Rn exposure dose and health risks for key populations can only be estimated and controlled through measurements of the indoor 222Rn levels. Therefore, although the 222Rn exhalation rate is an important parameter that can be measured and calculated on-site, establishing a fixed exhalation rate limit for regulatory purposes is unsuitable.

Coastal surface seawater samples within 30 km around ten Chinese nuclear power plants (NPPs) were systematically investigated. The 239+240Pu activity concentration in the samples varied from 0.226 mBq/m3 to 3.098 mBq/m3, meanwhile the 240Pu/239Pu atom ratios ranged from 0.151 to 0.353. Besides, the Pacific Proving Ground (PPG) close-in fallout and the global fallout were recognized as two primary sources of Pu in these samples. The 239+240Pu activity concentration as well as the PPG contribution showed similar trends as the Kuroshio intrusion path and the coastal currents in the China Seas, illustrating long-range transport and consuming of PPG derived Pu in the coastal China Seas. Moreover, accumulation of PPG sourced Pu in the Beibu Gulf were observed and was attributed to the continuous invasion of the high isotopic Pu that remobilized from the South China Sea (SCS).

Radon is a naturally occurring radioactive gas that exists everywhere in our living or working environments.1 Radon and its decay products are considered to be the second leading causes of lung cancer in tobacco smokers and the leading cause in nonsmokers.2 Therefore,accurately measuring radon concentrations in the environment,understanding its behaviours,and implementing measures to mitigate radon are important for protecting the human health.

For accurate dose evaluation of radon and thoron exposure, it is important to find a more effective method to measure radon and thoron progeny activity concentration in field measurements. For the purpose of improving measurement sensitivity, two new alpha spectrometry methods, the Wicke-Tn and PKU-Tn methods, for radon as well as thoron progeny were proposed and a series of verification experiments carried out in different environments. Results showed that the two new methods both gave accurate radon and thoron progeny activity concentration individually, and the methodological sensitivity and uncertainty were greatly improved. In an experimental mixed radon–thoron environment, the methodological sensitivity of the PKU-Tn and Wicke-Tn methods were nearly 9.0 and 3.6 times higher than that of the Kerr-Tn method, respectively. Among the three methods, the PKU-Tn method had the highest methodological sensitivity and lowest uncertainty, indicating its prospects for use in field measurement.

In modern rare-event search experiments such as neutrino experiments and dark matter search experiments, radon is one of the most important radiation backgrounds since it can emanate from nearly all the materials containing radium and migrate freely in the experiment system. To support the China Dark Matter Experiment (CDEX) at China Jinping Underground Laboratory (CJPL), a series of high-sensitivity radon detection systems with different electrostatic collection chambers were designed, and radon in nitrogen as well as boil-off liquid nitrogen was measured after accurate calibration and enrichment. Results showed that the calibration factors were 2.1 ± 0.2 (counts/h)/(Bq/m3), 21.1 ± 0.7 (counts/h)/(Bq/m3), 186.2 ± 2.2 (counts/h)/(Bq/m3), 387 ± 7 (counts/h)/(Bq/m3) and the 90% confidence level detection limits were 27.22 mBq/m3, 1.89 ∼ 3.06 mBq/m3, 0.41 ∼ 0.68 mBq/m3, 0.44 mBq/m3 for CJPL-HR2, CJPL-HR20, CJPL-HR140 and CJPL-HR300 measurement systems, respectively. Combined with an enrichment system consisting of twenty g CarboACT activated charcoals in a cold trap, the lower level detection limit (LLD) of typical No.1 CJPL-HR140 could reach 1.8 μBq/m3 with three days' enrichment time and three days' measurement time at 20 L/min sampling flowrate. For verification and application, the radon activity concentrations in nitrogen were 0.6 ∼ 1.9 mBq/m3 with an average of 1.1 ± 0.1 mBq/m3.While in boil-off liquid nitrogen, the radon activity concentrations ranged from 0.04 to 0.62 mBq/m3, and they were significantly lower in old-decayed liquid nitrogen compared to newly-filled liquid nitrogen, with a nearly five-fold decrease.

[Objective] The hydrological characteristics of typical forest litters and soil in Southwest Hubei Province were studied, and the water-holding capacity of different forest stands were analyzed and compared in order to provide a theoretical basis and scientific reference for selecting suitable afforestation species and creating reasonable soil and water conservation forests in the region. [Methods] The study was conducted for five typical forest stands in the national forest farm of Jinzi Mountain in Lichuan City. Field survey and sampling, the cutting ring method, and the indoor soaking method were used. The litter storage capacity, water-holding process, and water-holding capacity, as well as the water-holding capacity and infiltration process of the soil layer in each forest stand were compared and analyzed. [Results] ① Litter storage for the five forest stands followed the order of Cunninghamia lanceolata plantation > deciduous broad-leaved mixed forest > Larix kaempferi plantation > Cryptomeria fortunei plantation > artificial plantation of Liriodendron chinense. The maximum water-holding capacity varied from 13.94 to 29.12 t/hm2, which was consistent with the change of litter storage. ② The litter water-holding capacity and immersion time exhibited a logarithmic relationship. The water absorption rate and immersion time followed a power function relationship. ③ The maximum water-holding capacity of the 0—40 cm soil layer ranged from 277.02 to 334.12 t/hm2, and followed the order of artificial plantation of Liriodendron chinense > deciduous broad-leaved mixed forest > Larix kaempferi plantation > Cunninghamia lanceolata plantation > Cryptomeria ortune plantation. The average infiltration rate varied from 6.89 to 22.30 mm/min. The steady infiltration time ranged from 18.40 to 25.73 min. The average infiltration rate of soil varied from 6.89 to 22 mm/min. The steady infiltration time ranged from 18.40 to 25.73 min. The best soil permeability was found in the artificial plantation of Liriodendron chinense, followed by the deciduous broad-leaved mixed forest. ④ From the analysis using the coordinate integrated assessment method, we observed the greatest comprehensive water-holding performance for the deciduous broad-leaved mixed forest. However, in terms of the water-holding performance of litter and soil, the Cunninghamia lanceolata plantation and the artificial plantation of Liriodendron chinense, respectively, were the best. [Conclusion] For the litter layer, Cunninghamia lanceolata plantation had the best water-holding performance. For the soil layer, Liriodendron chinense plantation had the best water-holding performance. Based on the comprehensive evaluation of all indicators, the overall water-holding performance of broad-leaved mixed forest was the best. Therefore, in Southwest Hubei Province, we recommend adoption of a close-to-natural forest culture and management method, planting coniferous and broad-leaved species appropriately, and increasing the proportion of mixed forests so as to increase the hydrological benefits of litter, and to increase soil aeration and permeability, thereby maximizing the water-holding capacity of litter and soil under different forest stands.

以鄂西南鹤峰县鹅掌楸天然林为研究对象,通过对林分空间结构(角尺度、大小比、混交度)和林分非空间结构(径级结构、树高结构、重要值)的研究与分析,直观地反映其林分结构的特征,查清该区鹅掌楸种群的发展现状,为鹅掌楸天然林的合理保护与科学经营提供依据.在鹅掌楸天然林集中生长的代表性地段建立 17 个 20m×20m调查样地,进行每木定位与检尺调查,应用Excel 2019、Winkelmass 2.0 软件对样地调查数据进行处理与分析.结果表明:研究区鹅掌楸天然林中共 73 个树种,鹅掌楸为该群落的优势种,生长处优势地位((U)= 0.17),种群整体呈轻微聚集分布((W) 为 0.56),同时该种群在林分中呈现极强度混交状态((M) = 0.85);种群整体径级、树高分布都呈右偏正态分布,小径级林木株数很少,种群整体呈现稳定型→衰退型.FSSI均值为 0.82,FSSD均值为 0.30,林分空间结构为接近于理想状态(41.18％)或达到理想状态(58.82％).鹅掌楸天然林处于演替后期,林分结构整体上较为理想,可对处于聚集分布的林木进行适当调整,辅以一定人为促进更新的措施,促使鹅掌楸林分结构更加合理.

Variations in soil radon concentrations are a potential precursor of earthquake and volcanic events. However, the unclear migration and variation mechanisms of radon concentrations in soil still limit its effective application. To elucidate the temporal variation and its possible influence factors on radon concentrations at different soil depths, a case study was performed at a suburban site in Beijing. A long-term continuous measurement system consisting of ten radon-in-soil monitors at depths from 0.1 to 5.0 m and other meteorological sensors was employed. The monitoring was carried out from January 8th to July 29th, 2022, covering 3445 h in total. Radon concentrations generally increased with soil depth. Diurnal variation of soil radon concentrations at depths of 1.2 and 1.6 m in winter and spring was observed, and a negative correlation between the soil radon concentration and the residual air pressure was found. This finding indicates a possible air exchange channel between the soil and the atmosphere at the study site. In addition, the soil radon concentration at 4.0 m depth was unexpectedly lower than that of neighboring depths and was steady throughout the measurement period. This is attributed to a possible clay layer in the soil structure at 4.0 m depth. The results of this field study indicate that the complexity of temporal variation of soil radon concentrations should be considered for its application in predicting earthquake and volcanic events.

The concentric impact rings of the Vredefort Crater contain rocks with elevated uranium concentrations resulting from the geological signature of a meteoric impact. The decay of this uranium was estimated to lead to elevated indoor radon concentrations in the Crater, but such a study has never been carried out. This study explores the relationship between the natural radionuclides found in the geology of the Vredefort Crater and indoor radon concentrations. This was achieved through soil sampling and radionuclide surveys conducted on three impact rings, supplemented by indoor radon measurements in dwellings found in the area. In situ measurements revealed that one impact ring had higher-than-average uranium concentrations at 50 Bq/kg. Surprisingly, the measured indoor radon levels were lower than expected (113 Bq/m3). These measurements were taken during the COVID-19 pandemic and colder months, conditions that would typically result in elevated indoor radon levels. Soil samples indicated uranium activity of 30 Bq/kg, comparable to the world average of 35 Bq/kg. However, defunct mine tunnels in the area exhibited elevated radon concentrations, averaging 364 Bq/m3. The disparity between expected and measured indoor radon levels was attributed to the composition of surficial deposits, bedrock, and architectural features of the dwellings preventing radon accumulation.

This paper presents a simple review of the progress and primary achievements of the radiation protection laboratory over its two decades of research. The main research subjects of the laboratory are naturally occurring radionuclides, their environmental behaviors, and the dose evaluation of radon exposure to the public. We developed several precise measurement methods for real environmental surveys and investigations on radon and its progeny. Furthermore, to support related research fields that use radon as a radioactive tracer, in situ continuous measurement techniques for radon in the atmosphere, soil, and water bodies were established. A one-year continuous measurement provided the typical year average (4.9 +/- 2.7 Bq/m(3)) of radon progeny concentration in Beijing, which is consistent with the world average (5 Bq/m(3)) recommended by UNSCEAR. Furthermore, for artificial radionuclides, we conducted wide research on Pu isotopes in Chinese environments. Our study focused on the development of analytical methods for the analysis of Pu in various environmental samples based on ICP-MS. Moreover, we investigated Pu distribution in the downwind area of the Lop Nor nuclear test site and assessed the vertical distributions and migration of Pu in the soil in Southwest China. In the Jiuquan region, we collected surface and core soil samples and determined the Pu239+240 activities and Pu-240/Pu-239 atom ratios in these to assess the level of Pu contamination and the sources of Pu in this region. Using Pu isotopes from the Chinese nuclear tests, we found that in most Jiuquan soil samples, the Pu-240/Pu-239 atom ratios were below the global fallout value, suggesting this area was contaminated by Pu isotopes from the Chinese nuclear test sites. The average Pu-240/Pu-239 atom ratio in the soil samples was similar to 0.16. Besides plutonium, C-14 is another artificial radionuclide we investigated. C-14 is a radioisotope produced as a byproduct in various nuclear facilities and released in the environment under normal operation; it is considered the main dose contributor to the public, making it one of the most important radioisotopes in environmental radiation evaluation. The C-14 analysis of the tree ring samples collected 2 km away from Qinshan NPP revealed that the enhanced values were primarily owing to C-14 discharged from two heavy reactors of Plant III. Moreover, a good correlation existed between the C-14 discharged from Plant III in the growing season and the increased concentration of C-14. We found that almost all C-14 released from Qinshan NPP was deposited within a radius of 6.5 km in 2010, and the maximal C-14 specific activity of most samples is 265.6 Bq/kg C, which is 41.8 Bq/kg C higher than that of the background. For dose contribution to the public, the effective dose resulting from the C-14 discharged from Qinshan NPP was estimated to be 0.5 mu Sv in 2010, which is negligible compared with the annual dose limit for the public (0.25 mSv).

A new-designed measurement device for radon and thoron activity concentration is developed based on gas direct measurement to support their in-situ calibration. It consists of a 2000 mm2 Passivated Implanted Planar Silicon (PIPS) detector, a Multi-Channel Analyzer (MCA), a Micro Controller Unit (MCU), and a small electrostatic chamber with a volume of nearly 23 ml. The device records those alpha particles emitted from radon and thoron gas, and the detection efficiency and the crosstalk factor of 218Po/216Po are determined by Monte Carlo simulation. Measurement results have been compared with AlphaGUARD DF2000 in pure radon and thoron environments, respectively. Results show that the measurement results of the devices and the reference monitor agree well with each other, with an average relative deviation of 0.48% for radon gas from about 3300 Bq/m3 to 38 kBq/m3 and -3.25% for thoron gas from about 25 kBq/m3 to 70 kBq/m3. Uncertainty assessment has also been done, and a relative system uncertainty of radon is about 6.8%, while that of thoron is nearly 7.3%.

The accurate measurement of thoron activity concentration is an important issue in both thoron exposure evaluation and in reducing its influence on radon measurement. For radon monitors based on electrostatic collection technique and an alpha spectrometry analyser, air humidity and sampling flowrate are key factors influencing the sensitivity of thoron activity concentration measurement. For the purpose of improving thoron measurement sensitivity and stability, theoretical derivation and experimental studies were systemically performed in this study. The results show that thoron measurement sensitivity decreases as a negative exponential function with absolute humidity increasing, and the sensitivity of thoron is much lower than that of radon under the same conditions, which is mainly caused by the small value of the concentration ratio of thoron inside to outside of the chamber. When the air exchange rate of the measurement chamber (sampling flowrate/inner volume) increases, the measurement sensitivity of thoron gas first increases rapidly and then decreases slowly after reaching its maximum at the air exchange rate of 0.24 s-1. In practice, in the normal air exchange rate range (for example <0.05 s-1), increasing the sampling flowrate could greatly improve the thoron measurement sensitivity, which consequently suggests an effective way to update thoron measurement under the present conditions of the monitor.