Chamber for measuring the exhalation rate of radon and thoron from surfaces

Abstract

The invention relates to a chamber designed to measure the exhalation rates of radon (222Rn) and thoron (220Rn) from surfaces in an air environment. Such surfaces may include soil, rocks, building materials, and floors and walls of buildings, etc. Determining radon and thoron exhalation rates is essential for safeguarding human health against radon and thoron exposure and contributes to climate change research. The invention comprises a chamber (1), optionally equipped with a protective cap (2) against rain. The upper surface of the chamber is covered with a porous material (3) that shields the internal volume from atmospheric turbulence while remaining permeable to radon and thoron. This porous material provides a diffusion gradient for the distribution of radon and thoron activity concentrations along the height of the chamber. Inside the chamber walls, at different heights, no fewer than four highly sensitive sensors (4) are installed. Each sensor includes an alpha particle detector (5) securely attached to an adsorption element (6), which serves as a highly efficient adsorbent for radon and thoron, available as a layer or band. The chamber (1) has an open base (8) that interfaces with the surface being examined (9) below it and is hermetically sealed to prevent radon or thoron entry through the chamber walls.

Description

[0001] CHAMBER FOR MEASURING THE EXHALATION RATE OF RADON AND

[0002] THORON FROM SURFACES

[0003] FIELD OF THE INVENTION

[0004] The invention relates to a chamber designed to measure the exhalation rate of radon (222Rn) and thoron (220Rn) from various surfaces in an air environment. These surfaces may include the earth’s surface, rocks, construction materials, and indoor surfaces such as floors and walls. According to the World Health Organization, radon is the second leading cause of lung cancer after smoking. Recently, regions around the world have been identified with elevated thoron levels, sometimes even surpassing those of radon, which has sparked increased interest in monitoring thoron levels and exhalation rates. Exhalation, or the release of radon and thoron from surfaces into the air, is the primary source of these gases in both outdoor and indoor environments. In addition, recent studies have suggested a link between atmospheric radon levels and climate change-related processes. Determining the radon and thoron exhalation rate (i.e., the activity of radon or thoron released per unit area per unit time) is crucial not only for protecting human health but also for advancing climate change research.

[0005] BACKGROUND OF THE INVENTION

[0006] A known method for determining radon and thoron exhalation rates involves measuring the accumulation of radon / thoron in a confined volume placed over the tested surface. Alternatively, if the material dimensions allow, the sample is sealed within a container, and the accumulation of radon / thoron within this closed environment is tracked over time (N. Jonassen, On the determination of radon exhalation rates, Health Physics 45 (1983) 369-376). However, subsequent studies (C. Samuelsson, A critical assessment of radon-222 exhalation measurements using the closed-can method, in "Radon and Its Decay Products Occurrence, Properties, and Health Effects", ed. P.K. Hopke, American Chemical Society, 1987) have shown that this approach alters the distribution of radon within the material or, in the case of a ground surface, within the topsoil layer. This redistribution changes the exhalation rate and introduces a systematic difference between the measured and actual undisturbed exhalation rates from open surfaces. The same limitation applies to thoron exhalation measurements using this technique.

[0007] Another established method estimates radon exhalation from the earth's surface by constructing a radon concentration profile up to 60-100 cm below the surface and fitting it to a diffusion model (D. Huxtable et al., Measuring radon-222 in soil gas with high spatial and temporal resolution, Journal of Environmental Radioactivity 167 (2017) 36-42; K. Mitev et al., Unperturbed, high spatial resolution measurement of radon-222 in soil-gas depth profile, Journal of Environmental Radioactivity 196 (2019) 253-258). This approach requires air sampling ("soil gas") at various depths or embedding radon-ab / adsorbing samples at different depths, and the subsequent measurement of radon activity in them, or burying radon detectors. Mathematical processing of these results yields two parameters: diffusion length (LD) and radon activity concentration (the activity per unit volume) at an "infinite depth" (a depth much greater than the diffusion length). The diffusion length is calculated using the formula LD=(D / λ)1 / 2, where D is the radon diffusion coefficient and λ is the isotope's decay constant. The diffusion coefficient of thoron is practically the same as that of radon, since these are isotopes of the same element with very close mass numbers. Using these parameters, a radon diffusion profile is constructed, and the average exhalation rate over the exposure period is determined.

[0008] This method assumes a homogeneous distribution of the radon source (226Ra) and a uniform diffusion coefficient over the studied depth. However, in practice, very often the radon source is unevenly distributed in the depths investigated and diffusion coefficient vary widely depending on soil characteristics, humidity, and other environmental factors, leading to deviations from the diffusion profile and introducing systematic errors (K. Mitev et al., ibid). Furthermore, the process of driving sampling probes from different depths or embedding of samples and detectors can disturb the vertical concentration profile. Additionally, the sensitivity limitations of current samples and detectors restrict this method primarily to soils with elevated natural radioactivity levels (K. Mitev et al. ibid). A similar method is impractical for thoron exhalation measurements due to thoron’s shorter half-life, where only the top few centimeters contribute to the exhalation. These limitations result in considerable discrepancies, sometimes by factors of hundreds, among results from different laboratories and methods (MetroRADON Intercomparison Report, 2019. http: / / metroradon.eu / wpcontent / uploads / 2019 / 05 / Report__WP3_3_3_MetroRADON_Intercom parison_final-l .pdf) highlighting the lack of an accurate exhalation rate measurement method.

[0009] The compensatory module for sensors, as described in BG Patent No. 67484, is designed for the measurement of isotopes of radioactive noble gases, particularly222Rn, in various environments such as gas, soil gas, natural gas, and liquid media. This module compensates for temperature influence on the signal received from sensors measuring radioactive noble gases, thereby enhancing measurement accuracy. The module consists of a closed container housing sensor with activated charcoal. This container is tightly sealed in an airtight package that conforms to the shape of the sensor. The packaging material is a polymer foil or plate, allowing radioactive noble gases to diffuse through and be measured.

[0010] Additionally, BG Patent No. 67405 describes a similar compensatory module aimed at improving measurement accuracy by mitigating temperature effects on sensor signals. This module is equipped with a closed container with an ad / absorbing sensor inside. At least one wall of the module is made of a material permeable to radioactive noble gases by diffusion, while the remaining walls are made from materials impermeable to these gases. The material composition, surface area, and thickness of the permeable wall, as well as the module's internal volume, are selected so that the calibration factor remains constant across the sensor's operational temperature range.

[0011] Both compensatory modules described above are intended to solve problems related to temperature effects during measurements, and because those compensatory modules have a closed volume that cannot be in direct contact with the measured surface it is not possible to achieve a diffusion gradient of the activity concentration of radon / thoron in air. Therefore, those compensatory modules are not adapted and do not allow exhalation rate measurements of radon (222Rn) and thoron (220Rn) from the surfaces into air.

[0012] Thus, there is a need for a device capable of accurately measuring radon and thoron exhalation rates from surfaces into air, with sufficient sensitivity and accuracy to deliver reliable, undisturbed exhalation rate values.

[0013] SUMMARY OF THE INVENTION

[0014] The invention relates to a chamber designed for measuring the exhalation rates of radon and thoron from surfaces. This chamber is equipped with radon and thoron sensors and has walls made of material impermeable to both gases. The chamber’s lower base is open to maintain contact with the tested surface, while its upper base is covered with a porous material that allows radon and thoron to diffuse through. This porous material establishes a diffusion gradient of radon and thoron activity concentrations within the chamber’s height. The chamber includes at least four vertically aligned sensors mounted on its wall. Each sensor contains an alpha particle detector with a sensitive surface covered by an adsorbing element. This detector captures alpha particles emitted by radon and thoron, as well as their respective decay products.

[0015] In variations of this invention, the adsorbing element may be configured as a continuous layer or a strip across all alpha particle detectors. Additionally, sensors include an absorbent foil placed between the detector and the adsorbing element. This setup enables sensors with absorbing foil to be selectively sensitive to thoron, or alternatively, the sensors are additionally equipped with a compensatory module, making their signal related only to radon.

[0016] In another embodiment, at least two sensors are radon-sensitive, and at least two are thoron- sensitive. Further options include a seal along the chamber’s contact line with the test surface to prevent gas leakage and a protective rain cap for outdoor use.

[0017] According to another embodiment, the chamber can take the form of a cylinder, cube, parallel- epiped, or straight prism and may be constructed from plastic, metal, or other materials imper- meable to radon and thoron.

[0018] BRIEF DESCRIPTION OF THE FIGURES

[0019] Figure 1 illustrates the chamber of the invention, designed for use on solid, monolithic surfaces.

[0020] Figure 2 shows a variant of the chamber intended for use on loose surfaces, such as soil or granular materials.

[0021] Figure 3 presents different sensor configurations used within the invention.

[0022] Figure 4 depicts an example distribution of radon activity concentration as a function of height above the tested surface for a chamber with a height of 20 cm.

[0023] Figure 5 shows an example distribution of thoron activity concentration as a function of height above the test surface for a 20 cm-high chamber. The solid line represents the distribution at low ambient thoron levels, while the dashed line represents distribution at higher ambient thoron levels.

[0024] Figure 6 provides the experimentally obtained distribution of the sensor signal, along with the calculated radon activity concentration based on height above the surface, for an actual test conducted with the chamber on soil in Sofia, Bulgaria.

[0025] EXAMPLES

[0026] The present invention describes a chamber for measuring radon (222Rn) and thoron (220Rn) exhalation rate from surfaces into air. Unexpectedly, it was found that a non-hermetic chamber, when shielded from atmospheric air turbulence, can establish a diffusion gradient of radon and thoron activity concentrations along its height. This gradient can be accurately measured using modem, highly sensitive radon / thoron sensors with local sensitivity, meaning they provide a signal proportional to the local activity concentration of these gases where the sensor is placed. Within this chamber, the activity concentration of radon and thoron naturally forms a diffusion gradient along its height, allowing the average exhalation rate over the exposure time to be determined based on the activity concentration levels recorded at various heights above the surface. For this purpose, a minimum of four highly sensitive sensors are mounted at different heights within the chamber, each of which measures the local activity concentrations of radon and / or thoron.

[0027] According to Fick’s law, the exhalation rate of radon / thoron from a surface to the air is the product of the activity concentration gradient at the surface-air interface and the diffusion constant of radon in air. Unlike the radon diffusion constant in soil or solid materials, which varies significantly with composition and environmental conditions, the diffusion constant in air is precisely known and minimally affected by meteorological changes. By modeling activity concentrations as a function of height above the surface, the gradient at the interface can be calculated, as the derivative of this function at height 0.

[0028] According to present invention, the chamber may take the form of a cylinder, parallelepiped, or similar volume, with walls made of radon-impermeable material such as plastic or metal. The height of the chamber is between 10 and 25 cm— well below the 223 cm diffusion length of radon in air — to allow for quicker establishment of equilibrium, typically within an hour. The chamber’s lower base is open to the test surface, ensuring a sealed contact to prevent external radon or thoron from entering the chamber, with soil surfaces sealed by driving the chamber 2- 3 cm into the soil and compacting it around the edges, so that there are no small holes or gaps through which radon from the surrounding air can penetrate

[0029] If the surface is a monolithic solid material (e.g. a concrete floor or wall), after placing the chamber, the outer periphery of the contacting edges of the walls is sealed with silicone or other suitable sealant to close any gaps along the line of contact of the walls with the surface, so that radon / thoron enters through the chamber’s lower surface only by the hard surface being tested

[0030] The upper base of the chamber is covered by a porous, low-resistance material (e.g., an aerosol filter or fabric) that permits radon and thoron diffusion while protecting the chamber from atmospheric turbulence. When used outdoors, a protective rain cap covers this porous material to prevent it from getting wet without obstructing air circulation.

[0031] The equilibrium distribution of radon and thoron within the chamber can be described using a steady-state solution of the diffusion equation, accounting for radioactive decay. The equilib- rium distribution has two boundary conditions considered as unknown parameters: the aver- age exhalation rate from the surface and the average activity concentration at the upper end of the chamber. These parameters can be determined through mathematical analysis of sensor data collected at various heights. To solve for a total of four unknown parameters (two for ra- don and two for thoron), including the time-averaged exhalation rates of radon and thoron, measurements should be taken using at least four highly sensitive radon and thoron sensors positioned at different heights. These sensors must be responsive to the local radon and thoron concentrations at their respective locations. For optimal sensitivity, each sensor uses a small alpha particle detector (1-2 cm2) coated with a highly effective adsorbent, such as activated carbon fabric or synthetic zeolites, both known for their high radon and thoron adsorption ca- pacity.

[0032] These detectors are well known to those skilled in the art— for example, surface-barrier semi- conductor detectors and solid-state track detectors. High-performance adsorbents are also available, with the most suitable for use in the proposed chamber being based on carbon fab- ric, which consists of more than 90% activated carbon (D. Pressyanov, New generation of highly sensitive radon detectors based on activated carbon with compensated temperature de- pendence, Scientific Reports 12 (2022) 8479), or on synthetic zeolites (S. Heinitz et al., Ad- sorption of radon on silver exchanged zeolites at ambient temperatures, Scientific Reports 13 (2023) 6811).

[0033] The adsorbent layer should be thicker than the range of the highest-energy alpha particles as- sociated with radon and thoron. This ensures that the sensor will register only the alpha parti- cles emitted by the adsorbed radon / thoron and their progeny. The decay products of222Rn, which contribute to the signal, include218Po,214Pb,214Bi, and214Po. In addition to222Rn, al- pha particles are emitted from218Po and214Po, with214Po always in radioactive equilibrium with214Bi. Similarly, the decay products of220Rn that contribute to the signal are216Po,212Pb,212Bi, and212Po, with alpha emissions from216Po,212Bi, and212Po, the latter always in equilib- rium with212Bi.

[0034] Radon's activity concentration in highly efficient adsorbents is thousands of times higher than in the surrounding air (L. Tommasino et al., Radon-film-badges by solid radiators to comple- ment detector-based radon detectors, Radiation Measurements 44 (2009) 719-723), which accounts for the high sensitivity of these sensors, enabling accurate measurement of low ra- don and thoron levels, even in atmospheric air (D. Pressyanov et al., Influence of humidity on activated carbon fabrics for high sensitivity radon detectors, Applied Radiation and Isotopes 200 (2023) 110941). Since the adsorbed radon and thoron activity is proportional to their local activity concentrations in the air in contact with the adsorbent, the sensor signal directly reflects the local radon and thoron concentrations at the sensor's location. The high adsorption capacity of these adsorbents also enables the sensors to measure atmospheric activity concentrations of radon and thoron with high sensitivity (D. Pressyanov, Ibid.; D. Pressyanov et al., Ibid.).

[0035] EXAMPLE 1

[0036] An embodiment of the invention for measuring radon and thoron exhalation from a monolithic solid surface is illustrated in Fig. 1. The invention provides a chamber 1, shown here in a cylindrical form. When used outdoors, a protective cap 2 may be attached to shield chamber 1 from rain, secured to the chamber 1 by mounting elements 2'. The chamber’s upper base is covered by a porous material 3 that protects its internal volume from atmospheric turbulence. Inside the chamber, highly sensitive sensors 4 are positioned at different heights along the inner wall. These sensors are designed to measure local radon and thoron activity concentrations and consist of an alpha particle detector 5 tightly coupled with an adsorbent element 6, which in this case is shaped as a layer.

[0037] In this embodiment, the adsorbent element 6 is made of a highly effective material for radon and thoron adsorption, such as activated carbon fabric, and the sensors are mounted using assembly elements 7 that do not obstruct the sensor’s exposure to the chamber’s internal volume.

[0038] The chamber 1 according to the invention has an open base 8 of the housing, contacting directly on the tested surface 9, with a contact line 10 sealed hermetically using a suitable sealant 11 such as silicone, to prevent external radon or thoron infiltration.

[0039] Another embodiment of the invention, shown in Fig. 2, adapts chamber 1 for use on loose surfaces, such as soil or loose materials. In this version, the open base 8 is inserted 2-3 cm below the surface level, and the surrounding material is compacted to ensure a tight seal around chamber 1. In this example, the adsorbent element 6 is configured as a strip that covers all alpha particle detectors 5, as depicted in Fig. 2. In one embodiment of the invention, sensors are used to detect both alpha particles emitted by radon and decay products as well as those emitted by thoron and its decay products through detectors 5. In another embodiment, sensors are used that are selectively sensitive to either radon or thoron. The location of the detector 5 and the adsorption element 6 depending on the desired sensitivity are illustrated in Fig. 3. In configuration A, Fig 3, an adsorbent element 6 covers the sensitive surface of detector 1, enabling the sensor to detect alpha particles emitted by both radon and thoron, as well as their decay products. In another embodiment of the present invention, as shown in Fig. 3 configuration B an optional absorbing film 12 is placed between the adsorbent element 6 and detector 5, with a thickness selected to allow only alpha particles from thoron’s decay product,212Po (which has a high alpha particle energy of 8.78 MeV), to pass through, stopping alpha particles from other isotopes. This configuration makes the sensor selectively sensitive to thoron. For this purpose, materials such as 43 pm-thick aluminum foil are suitable. Configuration C of Fig.3 employs a compensatory module 13, as described in patent BG 67484 Bl, where thoron penetration is negligible (D. Pressyanov, ibid.), allowing the sensor to measure only radon and its decay products, thus providing selective radon detection. All sensors must be calibrated individually for radon and thoron. Calibration factors, specific to each gas, represent the ratio of the accumulated signal over exposure time to the product of activity concentration of radon or thoron in air and exposure time.

[0040] The sensors, thus calibrated, are positioned at various heights within the chamber: one detector is centered as close as possible to the test surface, another is near the upper base, and the remaining detectors are arranged at intermediate heights along the chamber wall. According to the stationary solution of the diffusion equation with radioactive decay under the specified boundary conditions, the activity concentration of each isotope (where index i = 1 corresponds to radon and index i = 2 corresponds to thoron) at a height x above the tested surface is given by the expression:

[0041] In this expression, H represents the height of the chamber above the test surface and LDI denotes the diffusion length in air for each isotope (where LDI=223 cm for radon and LD2=2.9 cm for thoron), ∈irepresents the exhalation rate of the corresponding isotope, while denotes the activity concentration of the isotope at height H above the investigated surface. The expressions within the square brackets (labeled here as n and 2 for simplicity) are with known numerical values, calculated based on the parameters involved.

[0042] For chamber heights within the specified range, the dependency in Equation (1) for radon can be approximated linearly with an accuracy of better than 1%. This simplification allows the use of Equation (2), which facilitates more straightforward mathematical processing of the results:

[0043] Here, D denotes the known diffusion coefficient of radon in air (D = 0.105 cm2 / s = 1.05x] 05m2 / s). In this case, the gradient of radon activity concentration along the chamber height remains nearly constant, represented by the coefficient in front of the variable x in the linear dependence (2).

[0044] Figures 4 and 5 illustrate sample distributions obtained from model calculations based on Equation (1) for radon and thoron activity concentrations within a 20 cm-high chamber, constructed according to the invention. Figure 5 shows the distribution of thoron activity concentration, where the solid line represents conditions with low thoron activity concentration in ambient air, and the dashed line corresponds to high ambient thoron levels.

[0045] The signal from a sensor positioned at height x above the investigated surface, generated by the i-th isotope, can be described by where is the calibration factor for the isotope, T represents the exposure duration during which the sensors accumulate signal, and CAI is defined by Equation (1) (or Equation (2) for radon, denoted as CAI).

[0046] In one embodiment, the invention uses four sensors to simultaneously register signals from both radon and thoron, providing four independent inputs from the signals of each sensor. In this version, the adsorbing element 6 may be configured as a strip that fully covers the detectors, as shown in Figure 2. The signal from a detector centered at height x above the test surface and sensitive to both radon and thoron can then be described as follows: where the coefficients are according to Equation (1). This setup enables the calculation of the four unknown quantities: ti, EJ, coi, C02. These values are obtained as solutions to a system of four linear equations with four unknowns, formed by substituting the numerical values of known coefficients and chamber dimensions into Equation (3) for each of the four sensors centered at their respective heights xi. When more than four sensors are used, solutions can be refined using established regression analysis methods.

[0047] In another embodiment, at least two sensors are designed to be sensitive only to radon, while at least two others are sensitive only to thoron. In this configuration, the signal from a detector positioned at a height xi above the investigated surface from a sensor sensitive exclusively to radon can be expressed as follows: and from one sensitive only to thoron:

[0048] When two sensors of each type are used (two sensitive to radon and two sensitive to thoron), the desired quantities can be determined by solving two independent systems of equations, each consisting of two linear equations with two unknowns. These equations are derived using formulas (4) and (5) and can be solved using standard methods. If more than two detectors are used for one or both isotopes, established regression analysis techniques can be applied to refine the results. In cases where only one isotope, either radon or thoron, is of interest, then only two sensors sensitive to that specific isotope are necessary.

[0049] EXAMPLE 2

[0050] The method of operation and the advantages of the invention are further illustrated through the following example, which details a measurement conducted using a cylindrical chamber 1 with a height of 12 cm. On the walls of chamber 1, ten solid-state nuclear track detectors 5 of the Kodak-Pathe LR-115 type II for alpha particles were installed, each measuring 1.5 cm in width and 1 cm in height (see Fig. 2). The center of the first detector is positioned so that, after the exposed base 8 of chamber 1 is driven 2 cm into the soil, it is located 0.5 cm above the ground surface. Accordingly, the centers of the second and third detectors are positioned at heights of 1.5 cm and 2.5 cm, respectively, continuing up to the last detector, which is at a height of 9.5 cm, directly adjacent to the fiber aerosol filter 3 at the upper base. An adsorption element 6 made from a strip of fabric based on activated carbon (type Kynol ACC-5092- 10, Kynol Europa GmbH, Germany) was placed over all the detectors 5, as shown in Fig. 2.

[0051] It was experimentally established that the sensitivity of these prepared sensors 4 is significantly higher for radon than for thoron. The chamber 1 was deployed on soil in a field located in Sofia, Bulgaria, with the exposed lower base 8 driven 2 cm into the ground. The chamber was left in place for an exposure time of 283.6 hours. After the exposure period, detectors 5 were removed and processed, resulting in the determination of the signal from each detector (i.e., the number of alpha-particle tracks per unit area, after background subtraction).

[0052] Within the experimental uncertainties, the relationship between the signal and the distance from the earth's surface was found to be linear (see Fig. 6), showing no statistically significant deviations at the beginning that could be attributed to thoron exhalation. This finding allowed for the analysis to focus solely on radon, for which the linear dependence was confirmed with high accuracy (as seen in Fig. 4). Consequently, the average activity concentrations of radon were calculated for the different installation heights of the detectors (as indicated on the left vertical axis in Fig. 6).

[0053] Standard linear regression analysis methods were employed to determine the slope coefficient of the straight line that approximates the relationship between radon activity concentration and the height at which the detector center is located. This slope corresponds to the gradient of radon activity concentration with height, which was found to be 586 ± 31 Bq / m4. By multiplying the absolute value of this coefficient by the diffusion coefficient of radon in air (as per Fick's law, see also formula (2)), the average exhalation rate of radon from the soil during the exposure time was calculated to be 6.15 ± 0.33 mBq / m2-s, which is equivalent to 22.2 ± 1.2 Bq / m2-h.

[0054] According to data from the European Atlas of Natural Radiation published by the European Commission's JRC (2019), this value reflects a relatively low exhalation rate from soils. This highlights the high sensitivity of the proposed chamber, which can accurately measure exhalation rate across the full range of practical significance. Furthermore, experimental verification confirmed that placing the chamber does not significantly alter the distribution of radon volume activities in the surface soil layer, ensuring that the measured exhalation rate remains accurate and unaffected by the measurement process itself an additional advantage of this invention.

[0055] Note: The research that led to the creation of this invention was funded by the European Union - Next Generation EU, through the National Recovery and Resilience Plan of the Republic of Bulgaria, project No. BG-RRP-2.004-0008-C01.

Claims

PATENT CLAIMS1. A chamber for measuring the exhalation rate of radon and thoron from surfaces, equipped with sensors for radon and thoron detection, characterizing in that all walls of chamber (1) are made of radon- and thoron-impermeable material, where the lower base (8) of the chamber (1) is open and in direct contact with the examined surface, while the upper base is covered with a porous material (3) that is permeable to radon and thoron, establishing a diffusion gradient for the distribution of radon and thoron activity within the height of chamber (1), and there are at least four sensors (4), which are positioned vertically along the wall of chamber (1), where each sensor (4) is equipped with an alpha particle detector (5) covered by an adsorbing element (6), and recording alpha particles emitted by radon, thoron, and their decay products.

2. The chamber for measuring as described in claim 1, characterizing in that the adsorb- ing element (6) is configured as either a continuous layer or strip covering all alpha particle detectors (5).

3. The chamber for measuring as described in claims 1 and 2, characterizing in that the sensors (4) additionally include an absorbent foil (12) located between the detector (5) and the adsorbing element (6), making the sensors (4) sensitive only to thoron, or alter- natively, the sensors (4) equipped with a compensatory module (13), rendering them sensitive exclusively to radon.

4. The chamber for measuring as described in any of the preceding claims, characterizing in that at least two sensors (4) of chamber (1) are sensitive to radon, and at least two are sensitive to thoron.

5. The chamber for measuring as described in any of the preceding claims, characterizing in that optionally includes a seal (11) along the contact line (10) with the examined surface and a protective cap (2) for shielding against rain.

6. The chamber for measuring as described in any of the preceding claims, characterizing in that it has a cylindrical, cubic, parallelepiped, or prismatic shape and made of plastic, metal, or other materials impermeable to radon and thoron.

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