Preamble

Design and Prototype Testing of a Portable Liquid Scintillation Measurement System for On-Site Rapid Detection of Radon Concentrations in Water Bodies∗

Xiong-jie Zhang,1, 2 Hai-xian Zhou,2 Hai-tao Wang,2 Hong-ze Liu,2 Lu-jin Chen,2 Qiang Hu,2 Jun-jie He,2 Ye Zhang,2 and Shu-min Zhou2
1National Key Laboratory of Uranium Resources Exploration-Mining and Nuclear Remote Sensing, East China University of Technology, Nanchang, 330013, China
2Engineering Research Center of Nuclear Technology Application, Ministry of Education, East China University of Technology, Nanchang, 330013, China

Traditional liquid scintillation measurement instruments face significant challenges for field measurements of radon in water: their bulky size and reliance on stable power supplies hinder on-site analysis of large numbers of water samples. To address these limitations, this study develops a portable liquid scintillation system for measuring radon concentration in water that enables rapid on-site detection. In the system design, a 3×4 array of silicon photomultipliers (SiPMs) replaces the conventional photomultiplier tubes (PMTs) typically employed in such detection systems, reducing overall size while maintaining high detection efficiency. Following development of the power supply, signal summing, and amplification circuits, signals from the SiPM array are integrated and transmitted to a digital multichannel analyzer (DMA) for counting and statistical analysis. Radon concentration is then calculated using a calibrated conversion factor. The entire system operates from a 5 V portable power supply, enabling measurements in field environments without fixed power sources. Testing via the radon decay method indicates that the instrument exhibits good linearity. Through comprehensive evaluation and verification, the instrument's minimum detection limit at a 95% confidence interval is determined to be 0.2188 Bq/L. This study provides an innovative technical solution for on-site radon detection in diverse water samples (e.g., groundwater, rivers, and lakes), addressing the practical needs of field-based water radioactivity monitoring.

Keywords: Radon concentration detection; Liquid scintillation measurement; On-site rapid detection

Introduction

Radon (222Rn) is a major source of natural radioactive exposure in human life. Uranium and thorium are widely distributed in the Earth's crust [1]; their decay produces radon gas, which dissolves readily in groundwater—a key drinking water source in many regions. In areas with radon-rich aquifers, elevated radon in groundwater-derived drinking water may increase cancer risk [2]. Additionally, variations in groundwater radon concentrations can be utilized for volcanic and earthquake monitoring, as well as uranium resource exploration [3, 4]. Therefore, accurate assessment of radon concentrations in water is crucial for mitigating radiation risks, predicting earthquakes, and exploring uranium resources, ultimately protecting public health and enhancing geological hazard assessments.

When evaluating radon concentrations in environmental water bodies, measurement condition limitations must be considered. Particularly in field environments such as large lakes and mountain streams, collecting and transporting large water samples is challenging, necessitating on-site measurement with portable instruments. Among traditional instruments, the RAD7 can perform on-site measurements of radon concentrations in water due to its compact size and portability. However, it has a long measurement cycle, requires converting radon from water to gaseous form and transferring it to the measurement chamber, making operation complex [5]. Furthermore, accurate measurements require substantial waiting periods between samples, preventing rapid processing of large sample quantities.

In contrast, the liquid scintillation counter (LSC) method, recognized as a standard technique for radon-in-water testing [6], offers advantages such as simplicity and short measurement cycles, enabling rapid analysis of numerous samples. Additionally, LSC's approximately 4π geometry yields high detection efficiency for low-energy, short-range radiation [7]. However, conventional LSCs utilize photomultiplier tubes (PMTs), and this vacuum tube-based design limits miniaturization [8]. The resulting large instrument size complicates transport and poses challenges for meeting stable power supply requirements in outdoor environments, clearly failing to satisfy field measurement demands. Therefore, modifying the existing LSC structure to design a portable detection system for radon concentrations in water is essential.

In recent years, silicon photomultipliers (SiPMs) have gained prominence due to their small size, low operating voltage, high photon detection efficiency, and magnetic field insensitivity [9]. SiPMs can be utilized for fluorescence detection with efficiencies comparable to or exceeding those of PMTs [10]. A single SiPM device is typically smaller than 1 cm², allowing multiple SiPMs to be coupled with scintillators to enhance detection performance. SiPM-based front-end readout designs can significantly reduce scintillation counter size while lower operating voltages decrease power supply requirements. Applications of SiPMs as front-end photodetectors are widespread. For example, Fengzhao Shen, Qibin Fu, and colleagues designed a compact dual gamma-neutron detector using SiPMs coupled with NaI(Tl+Li) [11], while Mark M. Bourne and his team developed a compact time-of-flight detector based on SiPMs for neutron spectroscopy of fission samples [12]. These applications demonstrate that SiPMs' smaller dimensions and lower operating voltages enable significant detector size reduction, making them viable for designing portable scintillation counters.

This article presents the design of a portable liquid scintillation measurement system for detecting radon concentrations in water. Integrated into a portable light-shielding box, the system is compact and easily transportable for on-site measurements. Powered entirely by a mobile power source, it can operate without electrical outlets. When performing multiple water sample measurements in field environments, the system can be readily transported to various measurement points, eliminating the need for collecting and transporting large water samples.

II. Design of the Acquisition System Platform

A. Hardware Platform Structural Design

The liquid scintillation measurement system based on Silicon Photomultipliers (SiPMs) comprises a SiPM array, preamplifier, digital pulse multichannel analyzer (DPMA), and embedded control platform, as illustrated in [FIGURE:1]. Fluorescent photons released from the scintillation vial containing the sample and scintillation liquid are collected by a SiPM array and converted into electrical signals. After amplification, these signals undergo energy analysis by a multichannel pulse amplitude analyzer utilizing a high-speed ADC. Finally, the Microcontroller Unit (MCU) stores the data and transmits it to the host computer [13].

A schematic diagram of the testing device and photograph of the actual testing platform are shown in [FIGURE:2]. The SiPM array consists of four collection boards positioned around the vial, with each board containing three SiPM devices. This arrangement enhances collection efficiency of emitted fluorescence photons. Signals from the three SiPM devices on each board are combined using a summing circuit before transmission to the backend signal processing board. The backend processing circuit primarily includes the power supply circuit, summing circuit from the collection board, two-stage amplification circuit, and subsequent circuitry. The entire circuit operates from a portable 5 V power supply. Due to SiPMs' high sensitivity to photons, the acquisition system must operate within a light-shielding enclosure to eliminate external light interference [9]. The system counts and statistically analyzes acquired signals while interacting with the embedded control platform for real-time data exchange via data cable.

B. SiPM Array Summing Design

Liquid scintillation techniques are commonly employed for detecting low-energy α and β particles. When the test sample decays in the scintillation liquid, the scintillation solvent emits fluorescent photons during energy transfer, which SiPMs collect and convert into electrical signals [9, 14]. In liquid scintillation applications, the standard scintillation vial volume is typically 20 mL [15]. This project utilizes the Onsemi J-60035 series SiPMs. [TABLE:1] presents the main parameters of the J-60035 series SiPMs. [FIGURE:3] illustrates the response wavelength of the J-60035 series SiPMs relative to the fluorescence wavelength range of the liquid scintillation solution. The SiPMs sensitivity wavelength range spans 200–800 nm, with maximum sensitivity at 420 nm. The fluorescence wavelength of liquid scintillation typically falls between 400–450 nm, aligning well with the SiPMs effective detection range.

The single-chip size of the J-60035 series SiPMs is merely 6 × 6 mm², while a standard scintillation vial has an outer diameter of approximately 28 mm and height of about 45 mm. To significantly improve optical contact area between the SiPMs and scintillation vial and thereby enhance detection efficiency, this study utilizes a SiPM collection array as illustrated in [FIGURE:2]. The SiPMs surround the scintillation vial, and signals collected by each SiPM chip are processed through a two-stage summation method. This experiment did not investigate structural designs for optimal detection efficiency; the primary focus was validating methodology feasibility, with the system's final detection limit required to be below 1 Bq/L.

C. Two-stage Amplification Circuit and Power Supply Circuit Design

Due to weak SiPM signals, amplification circuits are necessary before digital multi-channel analysis [9]. Moreover, when detecting numerous photons, signal frequency is significantly high. Therefore, the ADA4891 high-bandwidth operational amplifier was selected to minimize signal loss. Since high-bandwidth chips experience reduced stable operating bandwidth as gain increases, the single-stage amplification factor should remain moderate to ensure distortion-free high-bandwidth operation. This design utilizes a two-stage amplification circuit, with each stage featuring an amplification factor of 2. The amplified signal passes through a voltage follower before connecting to the digital multi-channel analyzer. The voltage follower effectively isolates the front-end analog circuit from the back-end digital acquisition circuit, reducing mutual interference, improving signal-to-noise ratio, and enhancing interference resistance.

[FIGURE:4] illustrates the circuit design schematic for summing the SiPM array. When a SiPM detects a photon, it generates a current pulse that is subsequently amplified by a transistor and integrated into the first-stage summing circuit. The collection array consists of four columns with identical structures. These four collection boards are integrated through the second-stage summing circuit, which combines all signals from the array and provides a unified output.

The overall circuit design must provide power for components such as SiPMs and operational amplifiers. [FIGURE:5] illustrates the schematic of the overall power supply design. Since liquid scintillation signals collected by SiPMs are weak (typically in the millivolt range), stable voltage is essential to prevent signal overwhelm by noise. The external power supply connects to the DC chip via a Type-C interface, providing a reference voltage source for the overall power supply circuit. Using a low dropout regulator (LDO) chip, the 5 V voltage is converted to ±5 V for the inverting operational amplifiers. To ensure SiPM bias voltage stability, the design first employs an LDO voltage regulation circuit to stabilize the reference voltage. Next, a boost converter raises the voltage to approximately 30 V, and finally, a buck converter precisely adjusts it to the operating voltage required by the SiPMs. According to the J-60035 series SiPMs operating voltage range in [TABLE:1], this design employs the TPS7A49 chip to provide a working voltage of 28.2 V.

III. Test Results and Discussion

A. Preparation of Radon Solution

This study uses a water radon solution derived from radium (226Ra) particles as the test sample. [FIGURE:6] illustrates the apparatus for preparing the water radon solution. Radium particles are placed in a glass bottle sealed with a PTFE waterproof and breathable membrane, which allows radon gas to dissolve in water while preventing external water from entering the bottle. This method prevents radium particles from dissolving in the water sample, enabling preparation of pure radon water samples. The entire glass bottle is placed in a container filled with purified water, sealed, and left undisturbed for one week. Prepared water samples are tested using the toluene extraction method, which enables further enrichment of radon in water samples, thereby reducing the measurement system's detection limit. [FIGURE:7] illustrates the toluene extraction procedure: combine 1 L of water sample with 20 mL of toluene scintillation solution in a separatory funnel, shake continuously for 3 minutes, allow to stand for 1 minute for phase separation, drain the lower aqueous waste layer, and transfer the upper toluene scintillation solution containing enriched radon to a scintillation vial.

B. Radon Decay Method for Testing Instrument Linearity

Due to radon's half-life of approximately 3.82 days [17], recording count rates at various time points over a short period should follow the decay formula pattern. [FIGURE:8] illustrates the radon decay curve fitted using count rates over various time intervals, with the horizontal axis representing time and the vertical axis showing the ratio of the current interval's count rate to the initial count rate. Defining the first measurement as the initial moment t₀ = 0 days with total count rate n₀, the basic decay law formula is:

n = n₀ × e⁻ˡᵗ

Exponential fitting of the measurement points yields the equation:

n/n₀ = e⁻⁰·¹⁹⁴⁸²ᵗ

This indicates that data points generally exhibit a downward trend. The fitted formula yields λ = 0.19482 d⁻¹. Using the relationship:

T₁/₂ = ln(2)/λ

The measured half-life T₁/₂ = 3.56 days is shorter than the standard value T′₁/₂ = 3.82 days. The main error sources include statistical and systematic errors. Statistical error arises from inherent radioactive decay randomness. Since continuous uninterrupted measurements are impossible, the overall trend can only be represented by collecting numerous data points, leading to statistical fluctuation errors. Systematic error occurs due to an air layer inside the liquid scintillation bottle, allowing some radon gas to escape from the scintillation fluid. Additionally, if the scintillation bottle is not rigorously sealed, radon gas may escape from the opening, resulting in lower overall counting rates. Despite measurement uncertainties, the system's output curve remains broadly consistent with the theoretical radon decay trend.

C. System Detection Limit

To investigate the proportional relationship between radon concentration detected by the system and counting rate, we employed a bubbling device combined with the FD220 radon-thorium analyzer to measure counting rates and concentrations of water samples [18], as shown in [FIGURE:9]. To accurately measure radon concentrations in water, samples must stand for three hours to ensure radon and its short-lived progeny reach radioactive equilibrium before measurement [19, 20].

[FIGURE:10] illustrates the linear relationship between radon concentration measured by the FD220 radon-thorium analyzer and the net counting rate per unit volume (L) recorded by the system. Based on this figure, the linear relationship formula is derived as:

A = 0.0561n

where 0.0561 represents the conversion factor between the system's radon concentration and net count rate. According to the detection limit formula [21]:

L_D ≈ k² + (k_α + k_β)√B

The values of k_α and k_β are determined by the probabilities of type I error (α) and type II error (β), respectively. In this study, α and β are set at 0.05, corresponding to 95% confidence level and test power, thus k = k_α = k_β = 1.645. Here, B represents the accurate background count value. The study selected the average background count of N_B = 1200 from multiple 30-minute measurements as the definitive background value. Consequently, the detection limit L_D is calculated as:

L_D ≈ 1.645² + 2 × 1.645 × √1200 ≈ 117

The data indicate that under identical measurement conditions, a sample measured for half an hour must satisfy N > 1317 to indicate low-level radioactivity rather than radioactive statistical fluctuations. Taking L_D as the minimum count for sample measurement and utilizing the calibration coefficient, the corresponding concentration is determined to be 0.2188 Bq/L.

D. Testing and Discussion

[FIGURE:11] illustrates the error plot of four validation water sample sets measured by the system against the standard conversion line. All four data point sets fall within the 95% confidence interval. For experimental testing, ambient-level water samples were selected as test specimens. Below 1 Bq/L, a relative error of 5% can be satisfied. Therefore, it can be shown that with this scale factor, the liquid scintillation count rate of unknown radon solutions through the system reflects high accuracy in this measurement model.

IV. Conclusion

This article presents a portable liquid scintillation measurement system for radon concentrations in water based on SiPMs. It primarily discusses the hardware design framework and underlying principles. In the experiment, a radon solution was prepared using 226Ra particles, and the radon decay curve was fitted using the radon attenuation method to assess instrument measurement linearity. The fitting results showed small deviation from actual values, with the system's measured decay constant λ = 0.19482 d⁻¹ being relatively larger than the standard value. The article analyzes error contributions including statistical fluctuations and gas leakage. Overall, the system can accurately reflect radon radioactive events through count rate.

In establishing the system conversion relationship using the FD220 radon-thorium analyzer, a conversion equation was developed relating net count rate per unit volume (L) to radon concentration (Bq/L): A = 0.0561n. Finally, through detection limit calculation, the system was assessed to have a minimum detection limit of 0.2188 Bq/L at a 95% confidence interval. The system utilizes the required transformation equations. Four groups of water samples at different concentrations were measured, with all relative errors within acceptable limits.

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∗ Supported by the National Natural Science Foundation of China (No.42564010,12165001), the Jiangxi Province Major Science and Technology Research and Development (No.20224AAC01012), the Science and Technology Research Project of Jiangxi Provincial Department of Education (No.GJJ2200736) and the Graduate Innovation Fund of East China University of Technology (No.DHYC-202405).