AlphaGUARD PQ2000 Radon Monitor

The AlphaGUARD PQ2000 radon monitor is a portable radon-monitoring device well-known for its high detection efficiency, broad measurement range, rapid response, and sustained long-term stability [21]. Consequently, it is widely used for radon monitoring. The measurement chamber has a volume of 0.62 liters, an effective detection volume of 0.56 liters, and a sensitivity of 50 cpm·kBq^-1·m³ [14]. The AlphaGUARD PQ2000 radon monitor primarily operates in two distinct measurement modes: flow and diffusion [21]. In flow mode, the gas was drawn into the ionization chamber through an external pump. In contrast, the diffusion mode involves the permeation of ²²²Rn gas through a glass fiber filter covering the inlet to the ionization chamber, while simultaneously trapping the ²²²Rn progeny on the filter [7]. The air inlet of the diffusion chamber is characterized by a circular opening with a diameter of 6.5 cm. According to the manufacturer’s specifications, the glass fiber filter had a surface density of 70 g·m^-2, a thickness of 0.35 mm, and an average particle size retention capability of 0.6 μm [22].

In an environment with a mixture of ²²²Rn and ²²⁰Rn, the instrument utilizes the detected α particles to calculate the ²²²Rn concentration, leading to an overestimation of the actual environmental ²²²Rn concentration levels. Consequently, the ²²²Rn concentrations displayed by the AlphaGUARD PQ2000 radon monitor in these environments are inaccurate. Figure 1 shows a schematic diagram of ²²²Rn collection principle by the AlphaGUARD PQ2000 radon monitor.

Fig. 1Full-screen View

AlphaGUARD PQ2000 radon monitor

Establishment of the Experiment for Measuring the Response Coefficient of the AlphaGUARD PQ2000 Radon Monitor

An experiment was conducted in ²²⁰Rn progeny during prolonged measurements using an AlphaGUARD PQ2000 radon monitor in a ²²⁰Rn chamber, as shown in Fig. 2. The rectangular chamber had a volume of 125 liters and used a fan-driven solid-state ²²⁰Rn source with an activity of 6 × 10⁴ Bq. The ²²⁰Rn concentration was measured using a single scintillation cell flow-static method [23]. The concentration from the LM2 ST-203 scintillation cell was decay-corrected to determine the true ²²⁰Rn concentration in the thoron chamber as follows:(1)where C represents the ²²⁰Rn concentration measured using the LM2 ST-203 scintillation cell, λ is the decay constant of ²²⁰Rn gas, and t is the pipeline correction time, defined as the ratio of the pipeline volume V connecting the ²²⁰Rn chamber and scintillation cell to the air flow rate L.

Fig. 2Full-screen View

(Color online) Experimental flowcharts illustrating the response measurement and validation procedures for the AlphaGUARD PQ2000 radon monitor with respect to residual ²²⁰Rn progeny. "AG DIFF" denotes operation of the AlphaGUARD PQ2000 in diffusion mode

Owing to the long half-life of ²¹²Pb, measurements were conducted for at least three days at high ²²⁰Rn concentrations (≥10 kBq·m^-3) to determine the residual ²²⁰Rn progeny response coefficient. The procedure was as follows: valves 1 and 2 were opened, while valve 3 was closed. Subsequently, the AlphaGUARD PQ2000 radon monitor, operating in diffusion mode with a 10 min measurement interval, was placed in the small ²²⁰Rn chamber and exposed until diffusion equilibrium was reached over a period of three days. Afterward, the ²²⁰Rn source was disconnected, allowing the accumulated ²²⁰Rn and its progeny to decay within the monitor in a low-radon background environment. Changes in the counting window data recorded by the AlphaGUARD PQ2000 radon monitor were monitored, and the background values of the environmental ²²²Rn concentrations were subtracted; the resulting values were denoted as C_PQ2000.

The ratio of C_PQ2000 to C_LM-Tn represents the actual value of the response coefficient R₁ of the AlphaGUARD PQ2000 radon monitor relative to its internal residual ²²⁰Rn progeny. The response coefficient R₁ of the instrument’s response to the residual ²²⁰Rn progeny is expressed as follows:(2)To validate the theoretical model of the residual ²²⁰Rn progeny response coefficient in a mixed ²²²Rn and ²²⁰Rn environment, an AlphaGUARD PQ2000 radon monitor was placed (as illustrated in Fig. 2). This experiment required valves 1 and 3 to be opened while valve 2 was closed. The vacuum pump was activated to draw ²²²Rn gas from the ²²²Rn chamber (maintained at a stable concentration of approximately 1600 Bq·m^-3) into the small chamber containing ²²⁰Rn gas, thereby ensuring uniform mixing and establishing a stable ²²²Rn and ²²⁰Rn mixed environment.

The concentrations of ²²²Rn and ²²⁰Rn were evaluated using various ST-203 scintillation cell models. The calibration coefficients for these scintillation cells were determined using standard flow-type solid-state ²²⁰Rn sources with known activities and standard ²²²Rn chambers with known concentrations. The mixed gas containing ²²²Rn and ²²⁰Rn was passed through a high-efficiency filter at a flow rate of approximately 3 L·min^-1 before entering the scintillation cell. After a circulation period of 2 min, the initial counting session began at a counting time of 5 min. The sampling pump was then turned off and the scintillation cell was sealed. Following a 10-min waiting period, the second counting session was conducted, which lasted 5 min. The concentrations of ²²²Rn and ²²⁰Rn entering the scintillation cell can be determined using the following Eqs. [23]:(3)(4)where C_Rn and C_LM-Tn represent the concentrations of ²²²Rn and ²²⁰Rn, respectively. N₁ and N₂ denote the first and second counting rates, respectively. is the calibration coefficient for ²²⁰Rn, valued at 21.69 Bq·m^-3·cpm^-1, with a relative standard deviation of 2.00%. K_Rn,1 is the calibration coefficient for ²²²Rn during the flow measurement period, which is valued at 24.60 Bq·m^-3·cpm^-1 with a relative standard deviation of 2.51%. K_Rn,2 is the calibration coefficient for ²²²Rn during the static measurement period, valued at 20.32 Bq·m^-3·cpm^-1, with a relative standard deviation of 2.70%. The symbol q denotes the counting rate generated per unit concentration of ²²⁰Rn during the second counting period, with a value of 8.61×10^-5 Bq·m^-3·cpm^-1. This methodology achieved a combined uncertainty in the measured ²²⁰Rn concentration, which was controlled within 5% [23].

Consequently, within a mixed environment of ²²²Rn and ²²⁰Rn, the modification of the AlphaGUARD PQ2000 radon monitor’s measurement results using the theoretical model is expressed as the corrected result C_Model-Rn, as:(5)Here, R₂ represents the response of the AlphaGUARD PQ2000 radon monitor to the residual ²²⁰Rn progeny derived from the theoretical model. The calculation of R₂ is described in detail in Sect. 2.3.

Theoretical Model for Evaluating the Response of AlphaGUARD PQ2000 Radon Monitor to Its Internal Residual ²²⁰Rn Progeny in Long-term Measurements

In diffusion mode, the AlphaGUARD PQ2000 radon monitor allows gas from a ²²⁰Rn environment to diffuse through its filter membrane and enter the pulse ionization chamber. According to Fick’s first law, the rate at which molecules cross a unit area per unit time, denoted by J, is directly proportional to the concentration gradient of particles perpendicular to that unit area. In simpler terms [24]:(6)Hence, the gas diffusion process occurring in the pulse ionization chamber of the AlphaGUARD PQ2000 radon monitor involves a time-dependent evolution of the ²²⁰Rn concentration, can be expressed as follows [25-27]:(7)where J denotes the flux of flow, D is the diffusion coefficient of ²²⁰Rn gas in the glass fiber filter, d represents the thickness of the filter membrane, which is assumed to be very thin. C_in and C_out represent the theoretical ²²⁰Rn concentrations inside the pulse ionization chamber of the AlphaGUARD PQ2000 radon monitor and in the ²²⁰Rn chamber, respectively. λ is the decay constant of ²²⁰Rn and γ denotes the air exchange rate.

In accordance with the differential Eq. (7) and under the initial condition C_in(0) = 0, we derive the expression that delineates the temporal evolution of C_in:(8)After a long measurement period, the concentrations on both sides of the filter, specifically, the ²²⁰Rn concentration in the pulse ionization chamber of the AlphaGUARD PQ2000 radon monitor and the ²²⁰Rn concentration in the ²²⁰Rn chamber, reached a stable state and formed a ratio termed the concentration penetration ratio (ξ):(9)In the given equation, as deduced from Eq. (9), it is evident that ξ is related to γ and λ: ξ value of ²²⁰Rn inside and outside the glass fiber filter in equilibrium on the AlphaGUARD PQ2000 radon monitor was 0.14 [28, 29], which was used to calculate the theoretical response coefficient value.

After exposing the AlphaGUARD PQ2000 radon monitor to the ²²⁰Rn chamber for a duration of 3 d, and upon reaching diffusion equilibrium, the internal concentration, denoted as () within the AlphaGUARD PQ2000 radon monitor is described as:(10)Assuming C_out remains constant, after three days, the internal ²²⁰Rn and its progeny within the AlphaGUARD PQ2000 radon monitor can attain long-term equilibrium:(11)In accordance with the series of cascade decays involving ²²⁰Rn and its progeny, the change in the overall radioactive concentration resulting from total α decay within the instrument can be expressed as follows:(12)In the given expression, , , and represent the radioactive concentrations resulting from the α decay of ²²⁰Rn gas, ²¹⁶Po, and ²¹²Bi, respectively. E denotes the average detection efficiency of the internal detector for α particles in the instrument, and T represents the decay time elapsed after the instrument’s exposure. The sum of the radioactive concentrations of ²²⁰Rn, ²¹⁶Po, and ²¹²Bi is detailed in Appendix A.

In fact, the instrument’s detection efficiency for α particles from the decay of ²²⁰Rn gas is nearly equal to α particles from the decay of ²²²Rn gas. Based on the sensitivity of the instrument, the average theoretical E of the α particles measured using the pulse ionization chamber detector inside the AlphaGUARD PQ2000 radon monitor was determined to be 0.496. This value closely matched the simulated detection efficiency obtained by Zhang et al. [30] using Geant4 simulations.

Based on the theoretical model of the residual ²²⁰Rn progeny in the diffusion mode of the AlphaGUARD PQ2000 radon monitor, the response coefficient R₂ of the instrument to its internal residual ²²⁰Rn progeny is intricately linked to ξ. The theoretical expression for the response coefficient R₂ is given as:(13)This formulation embodies a theoretical model describing the instrument’s response to the decay of the residual ²²⁰Rn progeny.

Comparison of Theoretical and Experimental Values for the Response Coefficient of Residual ²²⁰Rn Progeny in Long-term Measurements

AlphaGUARD PQ2000 radon monitor was exposed to a high concentration of ²²⁰Rn in an environment where the ambient temperature was stabilized between 21 °C and 28 °C throughout each exposure period. During exposure, the relative humidity was maintained at 59%±0.71%, the average pressure at (1006 ± 8.49) mbar. The response coefficient was theoretically calculated from a theoretical model describing the instrument’s response to the residual ²²⁰Rn progeny, as established in Sect. 2.3. The response coefficient was obtained via algorithmic fitting. The instrument was placed in a low-radon background environment with a ²²²Rn concentration of (42±15) Bq·cm^-3. As the ²²⁰Rn progeny decayed inside the instrument, three experiments were conducted to measure the response coefficients of residual ²²⁰Rn. The C_PQ2000 values were averaged, and C_R were determined. The theoretical and experimental response coefficients of the residual ²²⁰Rn progeny are presented in Table 1.

The results in Table 1 indicate that, in an environment exposed to high ²²⁰Rn gas concentrations of 4.65×10⁵ Bq·m^-3, as measured by ST-203 scintillation cell, the influence of the residual ²²⁰Rn progeny on radon measurements by the instrument cannot be overlooked. In this experiment, data were recorded every 10 min and subsequently averaged over each 60-min period, as recommended. After the three-day exposure period, the instrument exhibited a response coefficient of 9.78% for the residual ²²⁰Rn progeny within its internal components, consistent with results reported by national agencies such as STUK, SUBG, and IRSN [17]. The relative deviation between the experimental and theoretical values of the response coefficient was at its maximum immediately after the conclusion of the exposure period, exhibiting a deviation of 40.43%. This substantial discrepancy may be attributed to overestimation of the theoretical value. Conversely, the minimum relative deviation is 0.60%. The experimental response coefficient values for the residual ²²⁰Rn progeny ranged from 0.95% to 9.78%, whereas the theoretical values range from 0.82% to 9.10%. All the response coefficient values for the residual ²²⁰Rn progeny within the instrument were below 20%, aligned with the stipulations of the IEC 61577-2 standard.

Figure 3 shows a plot of the experimental and theoretical values of the response coefficients of the residual ²²⁰Rn progeny. The observations suggest that at the 0-min mark, both the fitting results of the theoretical model and the experimental values after 120 min are higher. This discrepancy may be owing to the large uncertainty in the parameter values used in the model. However, after 120 min, the experimental values converged with the theoretical values. The response to residual ²²⁰Rn progeny can be effectively predicted using the theoretical model developed in this study for the AlphaGUARD PQ2000 radon monitor, offering robust technical support for managing residual ²²⁰Rn progeny interference in future applications.

Fig. 3Full-screen View

(Color online) A comparative analysis was conducted between the experimental and theoretical response coefficient values for residual ²²⁰Rn progeny within the instrument. The theoretical model was developed based on curve fitting using experimental data to estimate the relationship between the residual progeny concentration and the instrument’s response

Validation of the Theoretical Model for the Response Coefficient of Residual ²²⁰Rn Progeny during Long-term Measurements

An AlphaGUARD PQ2000 radon monitor was used to measure the ²²²Rn concentration in a mixed environment with low ²²²Rn and high ²²⁰Rn levels, as part of a validation experiment to evaluate its response to the residual ²²⁰Rn progeny. Various models of ST-203 scintillation cells were deployed to determine the concentrations of both ²²²Rn and ²²⁰Rn within a small blue chamber with low ²²⁰Rn levels. Initial measurements indicated a ²²⁰Rn concentrations of 473502 ± 12666 Bq·m^-3. Subsequently, the AlphaGUARD PQ2000 radon monitor was exposed to the ²²⁰Rn chamber for three days, maintaining a stable temperature between 20 °C and 24 °C. After disconnecting the ²²⁰Rn source, the monitor initiated hourly measurements of ²²²Rn concentration within the chamber. Following the methodology outlined in Sect. 2.2 of this paper for the validation experiment, readings from the AlphaGUARD PQ2000 radon monitor were corrected using the theoretical model for residual ²²⁰Rn progeny. The corrected results were then compared with the experimental ²²²Rn concentration values for analysis.

Figure 4 compares the corrected values from the theoretical model for the AlphaGUARD PQ2000 radon monitor in a mixed environment of ²²²Rn and ²²⁰Rn with the actual ²²²Rn concentration values. The theoretical values closely matched the experimental data with minor deviations observed at certain points. These deviations may arise because the AlphaGUARD PQ2000 radon monitor primarily detects α decay energy from ²²²Rn gas, typically around 5.49 MeV, while ²¹²Bi — a progeny of ²²⁰Rn, emits α particles within a similar energy range [17], potentially causing signal overlap at higher concentrations. In this mixed environment, the theoretical model was applied to correct the ²²²Rn concentration values measured by the AlphaGUARD PQ2000 radon monitor. Despite slight fluctuations, the experimental measurements yielded an average ²²²Rn concentration of (1648 ± 41) Bq·m^-3 over 18 cycles. After correction, the theoretical model produced an average concentration of (1631 ± 112) Bq·m^-3 for the same 18 cycles. The close agreement between these values highlights the practical utility of the theoretical model in reducing the influence of residual ²²⁰Rn progeny on the monitor’s performance, thereby enhancing the precision of ²²²Rn concentration measurements.

Fig. 4Full-screen View

(Color online) Comparison between the corrected ²²²Rn concentration results obtained from the validation experiment and the corresponding experimental ²²²Rn concentration values

Influence of Temperature on the Response of the AlphaGUARD PQ2000 Radon Monitor to ²²⁰Rn Gas

Some radon monitors, such as those employing activated charcoal detectors [31], solid-state nuclear track detectors such as CR-39 [32] and instruments with membrane-covered diffusion chambers, where ξ for ²²²Rn gas increases with temperature [19], show decreased sensitivity with increasing temperature. It is essential to account for temperature-induced biases during long-term monitoring using these instruments, particularly in environments with significant temperature fluctuations.

Furthermore, based on previous research findings, γ for the diffusion mode of the AlphaGUARD PQ2000 radon monitor can be mathematically expressed as [33](14)Therefore, for the glass fiber filter, when (where L_D is the diffusion length of ²²⁰Rn gas), combined with Equation (7), ξ for the AlphaGUARD PQ2000 radon monitor can be expressed as(15)In this equation, S represents the effective area of the diffusion window filter in the AlphaGUARD PQ2000 radon monitor; D is the diffusion coefficient of ²²⁰Rn gas in the glass fiber filter, which depends on the air temperature, porosity, and curvature of the glass fiber filter; V is the effective volume of the pulse ionization chamber; d is the thickness of the glass fiber; P is the permeability of the ²²⁰Rn gas through the glass fiber filter by diffusion. In fact, the temperature effect of ξ of the instrument is caused by the temperature effect of P of the glass fiber membrane on the ²²⁰Rn gas [19].

In this study, under a consistent airflow velocity (v=1 m·s^-1) within a disturbed environment, the concentration of the ²²⁰Rn gas diffusing into the instrument was linked to ξ. After exposing the instrument to a stable ²²⁰Rn concentration (255731 ± 9541 Bq·m^-3) at temperatures of 9 °C, 28 °C, and 46 °C for 6 h, the ²²⁰Rn source was then turned off. Measurements were then performed in a low ²²²Rn background to determine the response coefficients of the ²²⁰Rn gas and its internal residual ²²⁰Rn progeny. The relationship between the response coefficient and temperature during short-term measurements was derived using the formula R=signal/C_out. Based on the typical operating temperature range of 0–45 °C for the instrument, the variation in the response coefficient with temperature is shown in Fig. 5(a). Figure 5(b) presents a box plot of the temperature-related response coefficient to the internal residual ²²⁰Rn progeny, where the square indicates the mean and the whiskers show the range from minimum to maximum. Across the three distinct temperature conditions, the median response coefficients of the instrument to its internal residual ²²⁰Rn progeny were 0.685%, 1.038%, and 1.124%, respectively.

Fig. 5Full-screen View

(Color online) a Response coefficient of the AlphaGUARD PQ2000 radon monitor to ²²⁰Rn gas at different temperatures, with the fitted curve described by the equation: y = -11.48 × e^(-T/19.61) + 15.79. b Box plot illustrating the instrument’s response coefficient to residual ²²⁰Rn progeny as a function of temperature

As depicted in Fig. 5(a), as the temperature increases, the instrument response to ²²⁰Rn gas shows an increasing trend because the concentration of ²²⁰Rn gas entering the instrument is affected by different temperatures, leading to different quantities of residual ²²⁰Rn progeny. A box plot illustrating the relationship between the response coefficient of the AlphaGUARD PQ2000 radon monitor and the residual ²²⁰Rn progeny with temperature (see Fig. 5(b)), discernible changes in the instrument response to the residual ²²⁰Rn progeny at varying temperatures were observed. However, the response coefficient values obtained in this study were more than twice those reported by Liu Cuihong et al. [14] under static conditions in 2010. In the present study, a fan-based thorium source was used to generate ²²⁰Rn gas, and the exposure chamber volume was relatively small, which may have contributed to the higher response values. Consequently, the observed response pattern of the AlphaGUARD PQ2000 radon monitor to ²²⁰Rn gas in this experiment was higher than the response coefficient values reported in other studies that utilized membrane-covered diffusion chambers [7].

The Impact of Wind Speed on the Responsiveness of the AlphaGUARD PQ2000 Radon Monitor to ²²⁰Rn Gas

Air exchange through a glass fiber filter (porous medium) depends partly on the pressure difference from the external air. Consequently, the response of diffusion-type detectors to ²²²Rn and ²²⁰Rn gases may vary with changes in the surrounding wind speed intensity [22]. During the extended environmental monitoring of the ²²²Rn concentration using the diffusion mode of the AlphaGUARD PQ2000 radon monitor, the response coefficient to ²²⁰Rn gas within the monitor may be affected by wind speed.

To scrutinize the effect of wind speed on the response of the AlphaGUARD PQ2000 radon monitor to ²²⁰Rn gas, the instrument was placed in a controlled 2700 L ²²⁰Rn chamber containing consistent ²²⁰Rn levels. Environmental temperatures during each exposure were maintained between 15 °C and 19 °C, whereas humidity was carefully controlled between 66% and 80%. Within the temperature and humidity range considered in this study, the potential influence of these environmental factors was deemed negligible [34]. The instrument was exposed for 6 h to a constant ²²⁰Rn concentration of 14501 Bq·m^-3, under varying wind speeds of 0 m·s^-1, 1 m·s^-1, 2 m·s^-1, and 3 m·s^-1. The measurements were conducted over a fixed period of 60 min and the changes in the response coefficient at different wind speeds are listed in Table 2.

Figure 6 shows how the AlphaGUARD PQ2000 radon monitor’s response coefficient to ²²⁰Rn gas related to wind speed. The figure indicates that the wind speed increases, similar to the response coefficient. Through a linear regression of the data within this range of wind speeds, the linear relationship between the response coefficient and wind speed was determined to be . Specifically, as wind speed increased from 0 m·s^-1 to 1 m·s^-1, the response coefficient rose sharply from 4.90% to 10.90%, approximately doubling. This helps explain the higher response coefficient observed in this study compared to previous findings [14], as discussed in Sect. 3.3, which used a fan-driven diffusion-type ²²⁰Rn source in a small-volume ²²⁰Rn chamber. However, as wind speed increased further from 1 m·s^-1 to 3 m·s^-1, the increase in the response coefficient became more gradual, rising from 10.90% to 12.30%. This reduced rate of increase at higher wind speeds can be explained by the principles of Darcy’s law, which governs gas flow through porous media [35-38]. According to Darcy’s law, when the Reynolds number is below a critical threshold, fluid velocity through a porous medium is linearly proportional to its permeability, cross-sectional area, and pressure gradient. However, once the Reynolds number exceeds this critical value, the flow transitions from laminar (Darcy flow) to turbulent (non-Darcy flow), where inertial forces dominate. In this experiment, below a wind speed of 1 m·s^-1, the response coefficient for ²²⁰Rn gas was nearly proportional to wind speed, showing a sharp increase. As the wind speed exceeded 1 m·s^-1, the rate of increase in the response coefficient slowed and eventually stabilized, corresponding to a critical Reynolds number of approximately 1 m·s^-1.

Fig. 6Full-screen View

(Color online) Response coefficient of AlphaGUARD PQ2000 radon monitor to ²²⁰Rn gas at different wind speeds