Patent Description:
Radon is a radioactive noble gas, which is colorless and odorless. In nature, its most common isotopes are <NUM>Rn (which belongs to the uranium chain (<NUM>U)), and <NUM>Rn (which belongs to the thorium series (<NUM>Th)). Radon found naturally in the air comes i. from soil, bedrock and from groundwater ( due to presence of watersoluble radium salts), and it can enter into lungs with the inhaled air. Measurements of the specific activity of radon in the air, also referred to as measurements of radon concentration - are important e.g., from the point of view of determination of the level of radiological exposure. This is because the decay products of e.g., <NUM>Rn, which are heavy metals and are also radioactive, may remain in the organism. The potential harmfulness of radon to animals, including humans, is mainly related to the short-lived products of its decay being the emitters of highly ionizing α particles: <NUM>Po and <NUM>Po. Polonium isotopes can be formed in the air and can be inhaled into the lungs (along with aerosols), wherein due to the emission of α radiation, they are considered harmful if they reach the body calls. <NUM>Po and <NUM>Po can also be formed directly in the lungs as a result of the decay of inhaled radon. It is estimated that radon and its daughters are responsible for more than half of the dose that a person takes annually from natural radiation sources. To assess the degree of risk in a given location or room, it is necessary to measure the radon concentration at a level of a few Bq/m<NUM> or lower (wherein for <NUM>Rn 1nJ = <NUM>,<NUM> Bq) - concentrations up to <NUM> Bq/m<NUM> are considered safe.

Radon present in the air can also interfere negatively with some specialized devices, such as those used in laboratories to study extremely rare physical processes (nuclear decays beyond the Standard Model), or in the production of nanoelectronic circuits. For this reason, the so-called radon-free clean rooms are built, i.e., clean rooms with an atmosphere with the radon specific activity close to zero [in practice at the level of ~mBq/m<NUM>], to minimize the effect of deposition of its decay products (radioactive heavy metals) on surfaces. Ultrasensitive detectors are used to measure radon concentration in clean rooms of this type, having sensitivities at the level of <NUM> mBq/m<NUM>.

There are various radon detectors based on different principles: ionization pulse chambers, scintillation detectors with zinc sulphide activated with silver (ZnS(Ag)), α spectrometers with semiconductor silicon detectors, trace detectors of α particles in plastics CR-<NUM> and LR-<NUM> (solid state detectors), detectors with activated carbon, electret detectors etc..

There are also known radon calibration chambers for a controlled exposure of detectors to radon and its daughters. They enable calibration of the radon detector apparatus. However, chambers of this type are not capable of measuring concentration of radon in the environment.

There are known <NUM>Rn and <NUM>Rn detectors that operate based on the principle of an electrostatic collection of ionized atoms of radon daughters (<NUM>Po/<NUM>Po and <NUM>Po/<NUM>Po) on the surface of a semiconductor sensor placed in the measurement chamber and connected to a high voltage, of the order of <NUM> V. The semiconductor sensor converts the energies of α particles to electrical impulses with amplitudes proportional to their primary energies. The number of impulses is proportional to the number of α particles and, consequently, it is proportional to the concentration of <NUM>Po and <NUM>Po created in the detector. Since these radon daughter isotopes have shorter half-lives (<NUM>. /<NUM>) as compared to the parent radon (<NUM> days and <NUM>), they are in a radioactive equilibrium with them (the activity of the derivatives is equal to the activity of radon). The detector system includes a pump for maintaining a preset air flow through the measurement chamber of constant volume. Depending on the air flow and chamber volume, different detection sensitivities can be obtained.

A publication by <NPL>, presents a low background detector for continuous and real-time monitoring of <NUM>Rn concentration in the air. The sensitivity of the presented device is a record <NUM> mBq/m<NUM>. The detector comprises a <NUM> I chamber with an oval shape similar to a flattened sphere, with an air inlet located at the bottom of the chamber and an air outlet located in the vault of the chamber, wherein a sensor for detecting α particles is installed in the air outlet. During the measurement, the air flows through the detector chamber at a predefined flow velocity, wherein the air movement is made of stainless steel to generate an electric field to push the <NUM>Rn daughters (<NUM>Po and <NUM>Po) onto the alpha particle detector.

A Polish patent <CIT> describes a detector for measuring radon decay products in the air, comprising a scintillation chamber having a height of <NUM> - <NUM>, for retaining and collecting radon decay products on internal walls covered with ZnS(Ag) and on a window made of plastic (so-called plexiglass). The chamber is coupled to a photomultiplier which has a photocathode powered by a voltage of <NUM>-<NUM> V, of a negative polarity, and has a grounded cathode. During detection, air is passed through the chamber at a constant volumetric flow rate of <NUM> dm<NUM>/min. , wherein the air stream is sucked into the chamber by a negative pressure pump, and after passing through the chamber, it is removed from its interior via an outlet stub pipe. This detector is installed in a light-tight housing.

A publication "Radon Mitigation Applications at the Laboratorio Subterraneo de Canfranc (LSC)" (by Perez-Perez J et al, XP091173210) discloses a Radon Abatement System that significantly reduces radon levels and describes a similar Radon detector to that in: <NPL>.

A publication "<NPL>) discloses studies of adsorption characteristics of radon in nitrogen, argon, and xenon gases using different charcoals at various temperatures.

A <CIT> discloses essentially a device which is an improvement of pulsed ionization chambers by eliminating the problem of micro-discharges and it is used for measuring radon through pulse detection of alpha particles and, more particularly, for measuring radon wherein ion charges generated due to alpha particles generated during alpha decay of radon are pulsed and detected through a single probe rod, and the number of particles is counted without omission, even when alpha particles are generated in microscopic succession, thereby improving measurement accuracy of radon.

One of disadvantages of the known radon detectors equipped with a measurement chamber is its constant volume, which determines the constant volume of the gas being examined. Radon detectors having measurement chambers of large volumes (~<NUM>) require a lot of space for their installation, as well as corresponding operating equipment, which results in a high material and equipment costs of the detector itself, as well as high maintenance costs. In the case of large chambers, also the exchange of the examined gas volume takes some time, and this may introduce limitations in the use of the detector (limited frequency of measurements). On the other hand, chambers having a smaller volume generally are less sensitive and need significantly longer measurement time.

The aim of the presented invention is to develop a detector for measuring the radon concentration in gases, including air, which will solve most of the problems described above.

The detector according to the invention is according to the appended claims. In particular, it is based on a pressurized measurement chamber with at least one opening for supplying and discharging gas from the environment to the interior of the measurement chamber. Two sensors (alpha particle detectors) are applied for detection of alpha particles resulting from radioactive decays of the radon daughters produced in the gas present inside the measurement chamber and collected on the sensor surfaces. The detector is also equipped with a compressor for pressurizing the gas inside the measurement chamber. The gas inlet/outlet of the chamber are tightly closed, preferably by electromagnetic valves. For each of the sensors, the detector has a separate power supply system comprising high DC voltage supply for creation of an electric field in the chamber and a low DC voltage supply from a battery pack to bias the sensors. Moreover, each of the sensors is equipped with a charge sensitive pre-amplifier connected in series with a spectroscopic amplifier. The spectroscopic chain is mounted together with the battery power supply in a metal housing preventing the penetration of electromagnetic disturbances from the outside. In order to be processed the signal from each alpha detector is separated form high DC voltage by dedicated capacitors.

Such a design allows the gas to be compressed to a high pressure inside the chamber, preferably to a pressure in the range of <NUM> to <NUM> barg, for the duration of the measurement. As a result, high measurement sensitivities of about <NUM> mBq/m<NUM>, and more preferably <NUM> mBq/m<NUM>, can be obtained with a relatively small interior volume of the measurement chamber.

Applying high DC voltage to the alpha particle detectors, with the housing of the detector chamber being grounded (as opposed to solutions from the state of the art, where the housing was connected to high voltage, and the sensors were only biased with low voltage supplied from the battery), provides a significant simplification of the design of the entire detector by eliminating the need for electrical isolation of the housing from other components (e.g. from the gas supply system) by the use of special ceramic inserts, and by eliminating the need of additional protection of personnel against electric shock. The battery pack used to bias the sensors, which is located with other components in an electromagnetic shield also significantly reduces the noise level of the entire system.

Preferably, the signals from the measurement systems (spectroscopy amplifiers) are fed to an adder, wherein the output of the adder is connected to the multi-channel analyzer. The use of two α particle sensors allows for the simplification of the design of the detector chamber and the use of cylindrical geometry, while maintaining a high efficiency of radon detection.

Preferably, the high voltage applied to the alpha sensors to create the electric field in the detector is between <NUM> and <NUM> kV.

Preferably, the low voltage for biasing the alpha sensor is supplied from the battery and has a value from <NUM> to <NUM> V.

Preferably, the measurement chamber is in the form of a cylinder with a volume of <NUM> to <NUM>, with a longitudinal axis, with two bases, wherein the alpha sensors are mounted at each base along its longitudinal axis.

Such a design with cylindrical geometry is relatively simple to manufacture, while the implementation of two sensors improves the efficiency of detecting radon daughters. Moreover, such an arrangement of the sensors allows for an optimal distribution of the electric field inside the cylindrical chamber, and thus a correspondingly high efficiency of radon daughters collection.

Preferably, the detector chamber has two openings, one opening made in each base, on the longitudinal axis of the chamber. Such an arrangement of openings enables faster emptying and filling of the chamber with portions of gas between successive measurements.

The openings are provided with gas-tight ports, preferably equipped with electromagnetic valves. They provide adequate gas tightness of the measurement chamber.

Preferably, each of the alpha sensor is installed axially on one of the flanges. Such design provides convenient and quick access to each of the sensors: by removing the flange attached to the chamber, the sensor is also removed. Thus, maintenance and possible repair or replacement of each of the sensors can be performed more efficiently.

Preferably, the detector is further equipped with a manifold or an appropriate panel for selecting and introducing gases into the measurement chamber. As a result, a single radon detector can be used to sample gases form different locations.

Preferably, the detector is further equipped with the so-called slow control system for controlling and automation of all the operations of the radon detector.

The invention is further related to a method for measuring the activity concentration of radon and its daughters in gases by means of the radon detector described above. The procedure includes the following steps: filling the detector chamber with gas through at least one opening and compressing the gas to a pressure in the range of <NUM> to <NUM>,<NUM> kPa inside the chamber by means of a compressor, closing tightly the chamber, analysis of the signals from the alpha particle sensors generated as a result of radioactivity of radon and its daughters formed in the compressed gas present inside the measurement chamber. Owing to this, a larger mass/volume of a gas can be accumulated in the chamber for the measurement, thus increasing the amount of radon in the chamber, which results in a higher sensitivity of the detector. The possibility of applying different pressures in the chamber, depending on the measurement needs, allows for a virtual change of the chamber capacity, because, depending on the pressure applied, a different mass/volume of a gas may be examined in the chamber of a fixed volume. Owing to this, the detection sensitivity adjusted to the concentration of radon in the examined gas can be obtained, which can reach even <NUM> mBq/m<NUM>.

The invention is shown by means of example embodiments in a drawing, wherein:.

The detector, according to the invention, for measuring radon concentration in gases, including the air, is shown in <FIG> which illustrates the design features of the detector, and in <FIG>, which illustrates the functional elements of the detector.

The detector is based on a measurement chamber <NUM>, preferably in the shape of a cylinder or close to the shape of a cylinder. The volume of the measurement chamber <NUM> is limited by the side wall 10c extending between the two bases 10a, 10b. The bases 10a, 10b are preferably circular or have a shape close to a shape of a circle, such as oval. The measurement chamber <NUM> may have different volumes adjusted for given measurement needs, preferably a volume in the range <NUM> to <NUM>, more preferably a volume of about <NUM>, wherein the chamber is designed as a pressure vessel for holding gas therein for a measurement time, under the pressure of <NUM> - <NUM> barg, i.e., <NUM> - <NUM>,<NUM> kPa.

The measurement chamber <NUM> hosts two alpha particles sensors <NUM>, <NUM>. The first sensor <NUM> is installed at the first base 10a, and the second sensor <NUM> is installed at the second base 10b of the measurement chamber <NUM>. Preferably, both sensors <NUM>, <NUM> are installed along the longitudinal axis h of the measurement chamber <NUM>. Each of the sensors <NUM>, <NUM> is configured to detect alpha particles (α) generated during the decay of radioactive radon <NUM>Rn and its daughters: <NUM>Po and <NUM>Po. The use of the two sensors <NUM>, <NUM> located on the h axis of the measurement chamber <NUM> provides an improvement in the collection efficiency of the radon daughters. Each of the sensors <NUM>, <NUM> has a large active area 11a, 12a, preferably equal to <NUM><NUM>. For example, in the construction of the radon detector, sensors of the Ultra-AS type from Ortec® having a diameter of <NUM> can be used.

Preferably, each sensor <NUM>, <NUM> is a low background sensor - with low noise and low background - i.e., a negligible signal generated by the sensors <NUM>, <NUM> themselves, since each of them is made of materials with a very low (negligible) content of radioactive isotopes.

The measurement chamber <NUM> has at least one, and more preferably two gas-tightly closeable openings <NUM>, <NUM> for transporting the gas between the interior of the measurement chamber <NUM> and the environment, for maintaining the gas under the pressure of <NUM> to <NUM> barg during the measurement and for installing sensors <NUM>, <NUM>. For example, one opening is a gas inlet <NUM> and the other one is a gas outlet <NUM>. The openings <NUM>, <NUM> can be made centrally, each opening <NUM>, <NUM> in one base 10a, 10b of the measurement chamber <NUM>, preferably on the longitudinal axis of the measurement chamber <NUM>. Preferably, each opening <NUM>, <NUM> is gas-tightly closed by a flange <NUM>, <NUM>, for example of the CF-<NUM> type (Con Flate connector <NUM> in diameter and sealed with a copper gasket). For the discussed prototype of the radon detector provided with the flanges <NUM>, <NUM>, each of the sensors <NUM>, <NUM> can be installed centrally in one flange <NUM>, <NUM> with the active surface of the sensor <NUM>, <NUM> located in the measurement chamber <NUM>, preferably in the longitudinal axis of the measurement chamber <NUM> - as presented schematically in <FIG>. This solution provides convenient and quick access to each of the sensors <NUM>, <NUM>: by decoupling the flange <NUM>, <NUM> from the corresponding flange of the measurement chamber <NUM>, the sensor <NUM>, <NUM> is removed together with the flange. Thus, maintenance and possible repair or replacement each of the sensors <NUM>, <NUM> can be performed very efficiently. Moreover, for the measurement chamber <NUM> in the form of a cylinder, the location of the active surfaces of the sensors <NUM>, <NUM>, to which the voltage is applied, on the longitudinal axis h of the chamber and aligned with the plane of the CF flange provides an optimal electric field distribution, and thus a correspondingly high collection efficiency of the radon daughters on the sensors <NUM>, <NUM>. The results of a Monte Carlo simulation presented in <FIG>, confirmed that <NUM>% of the electric field lines (along which charged polonium ions drift) close on both active surfaces 11a, 12a of the sensors <NUM>, <NUM>. The active surfaces 11a, 12a are in the same plane as the openings <NUM>, <NUM>.

In addition, the location of two openings <NUM>, <NUM> of the measurement chamber <NUM> on its opposite sides - each opening <NUM>, <NUM> made centrally in the base 10a, 10b, ensures efficient transport of the gas portion between the measurement chamber <NUM> and the flange and finally the environment, quick introduction and removal of the portion of the examined gas respectively to and from the inside of the measurement chamber <NUM>, and, if necessary, flushing the inside of the measurement chamber <NUM> with radon-free gas between consecutive measurements. The gas is introduced into the flange through the valves (solenoid/electromagnetic valves) Z1, Z2 mounted on the rear covers of the flange <NUM>, <NUM>. These covers also constitute the base on which the sensors <NUM>, <NUM> are mounted through the suitable Teflon supports. The covers also have electrical feedthroughs (SHV type) used to supply high voltage and bias voltage of the sensors <NUM>, <NUM>.

At the inlet and at the outlet of the chamber, on the flanges installed on the openings <NUM> and <NUM>, solenoid/electromagnetic valves Z1 and Z2 are mounted.

The radon detector is equipped with a compressor <NUM> for pressurizing the gas in the chamber during the measurement. Furthermore, the detector comprises a dedicated unit <NUM> for controlling/automation of its operation, including periodical compression of the examined gas in the chamber. An oil-free compressor may be used, preferably with a metal diaphragm to achieve the required pressures in the chamber.

The radon detector may furthermore be equipped with a gas supply system (not shown in <FIG> for the sake of clarity), for example in the form of a manifold or a panel, allowing for selection of the source of the gas to be examined. The gas (air) may be sampled from different locations (e.g., rooms) to one or both of the openings <NUM><NUM> of the measurement chamber <NUM>. The supply system may be connected to the detector control unit <NUM> to automatically, e.g., as programmed, select the location form where the examined gas is received and to couple the gas selection process with the measurement itself. As a result, measurements with a single radon detector can be carried out cyclically from many different locations (supply points). Described gas supply system may be equipped with electromagnetic valves and pumps/compressors, each installed at the inlet at a specified supply location.

Each sensor <NUM>, <NUM> is connected to the power supply and a measuring/spectroscopy system shown in detail in <FIG>, as an example for the sensor <NUM>.

In contrast to most of the prior solutions, in the present invention high voltage generating an electric field in the detector chamber is applied to the alpha sensors wherein the chamber housing is grounded. The sensor <NUM> is biased with a low DC voltage from a battery pack <NUM> at the level of <NUM> to <NUM> V, preferably <NUM> V, via the resistor <NUM>. The sensor <NUM> is further supplied with a high voltage HVIN ranging from <NUM> to <NUM> kV, preferably <NUM> kV, through the filter <NUM>. Thus, on one of the electrodes of the sensor <NUM> (housing) a voltage HVIN (for example, <NUM> kV) is present, and on the other electrode (active surface) a voltage of HVIN + BAT (for example, <NUM> V) is present. There is therefore a potential difference on the electrodes of the sensor <NUM>, which enables its correct polarization, and thus registration of alpha particles.

The signals from the sensor <NUM> are read by an amplifying system <NUM> which comprises a charge-sensitive pre-amplifier (CSA) 115A connected in series with the spectroscopy amplifier 115B which amplifies the weak impulse from the charge-sensitive pre-amplifier 115A up to several hundred times. The capacitors <NUM>, <NUM> filter the constant component of the high voltage, so that only the pulses from the sensor <NUM>, observed after the decay of the polonium ion on the surface of the sensor <NUM>, enter the amplifying system <NUM>.

The amplifying system <NUM> is supplied with the Vcc voltage filtered by the filtering system <NUM>. In addition, the system comprises diodes <NUM>, <NUM> protecting the system against breakdown due to charge that could arise as a result of uncontrolled discharge of the capacitors <NUM>, <NUM>.

Thus, a charge impulse (in the form of a voltage spike with a duration of less than <NUM> microsecond and an amplitude of several tens up to several hundreds of mV) enters the input IN of the amplifying system <NUM>. It is important to filter out any external disturbances that could come from the outside - for this reason, the systems <NUM>-<NUM> are mounted in a common metal cover <NUM>, which serves as an electromagnetic shield (Faraday box) against external disturbances.

An output OUT from the amplifying systems <NUM> supplying the amplified signal from the sensors <NUM>, <NUM> is sent to the adder <NUM> (OR type), wherein the signal from one or both sensors <NUM>, <NUM> is present on the adder output. The signal from the adder <NUM> is sent next to the multi-channel analyser <NUM>, which generates an energy spectrum that can be displayed and analysed by software installed on the controller <NUM> (e.g. on a PC type computer).

The measurement of the radon concentration in the gas by the radon detector according to the presented invention is carried out with a sealed measurement chamber <NUM> containing the examined gas compressed to a pressure of <NUM> - <NUM> barg. More specifically: in order to carry out the measurement, a predetermined volume of gas (e.g. air) is pumped into the measurement chamber <NUM> by the compressor <NUM> until a predefined pressure value (in the range of <NUM>-<NUM> barg) in the measurement chamber <NUM> is reached and the solenoid valves, mounted on the flanges installed in the openings <NUM>, <NUM>, are tightly closed. Then, the measurement begins. Namely: after reaching the required gas pressure in the measurement chamber <NUM>, in a specific time interval, which allows to register an assumed number of radioactive decays, depending on the needs in terms of measurement accuracy. For example from <NUM> to <NUM> decays may be recorded (basically, the more decays are registered, the smaller the statistical error is) simultaneously by means of both sensors <NUM>, <NUM> - to which a suitable voltage is applied in the range from <NUM> to <NUM> kV, preferably <NUM> kV, the alpha particles generated by the decays of polonium isotopes produced in the gas being under pressure in the chamber and deposited on the sensor <NUM> or <NUM>, as a result of drift in the electric field, are counted. After the measurement is completed, the gas is removed from the chamber. According to the abovementioned method, with the aid of a radon detector, measurements can be carried out sequentially. If the measurements are carried out in a sequence, a real time monitoring may be realized.

If the concentration of radon in the examined gas is relatively low, the pressure of the gas in the measurement chamber <NUM> can be increased, thus one increases the effective mass/volume of the gas portion which is examined at a time. This is obtained by the use of the compressor <NUM>, the gas-tight closing solenoid valves mounted on the flanges installed on the openings <NUM>, <NUM>, and the design of the chamber which is suitable to withstand the high pressure. For example, for the air as the gas to be examined, the cylindrical measurement chamber <NUM> with a capacity of <NUM> litters, for the gas pressure equal to <NUM> barg - <NUM> litters of air is introduced, and at a pressure equal to <NUM> barg, for the measurement chamber <NUM> of the same volume (<NUM>), <NUM> litters of air is introduced. Moreover, by adjusting the gas pressure in the range of <NUM>-<NUM> barg in the chamber, one enables optimization of the measurement time -for a given concentration and required accuracy it is possible to measure shorter by applying higher pressure. Moreover, by means of the compressor it is possible to force a rapid gas exchange (flushing) in the measurement chamber <NUM> in order to start a new measurement in case the measured concentrations of the activity of radon and its daughters are high.

Thus, the developed design of the radon detector allows for a high measurement sensitivity equal up to <NUM> mBq/m<NUM>, while maintaining a relatively small chamber capacity. The detection sensitivity of the device can be adopted to the measurement needs. For higher radon concentrations in the examined gas, lower gas pressures in the chamber can be applied - thus subjecting a single detection to a smaller mass/volume of the gas, and for lower radon concentrations, pressures up to <NUM> barg may be applied, thus conducting a one-time detection of radon in a much larger mass/volume of the gas. Moreover, thanks to the possibility of increasing the volume of the examined gas, the measurement can be performed faster, which significantly improves the efficiency of the measurement process.

An additional improvement of the sensitivity of the detector according to the invention can be achieved by electro-polishing of the inner surface of the measurement chamber <NUM>, the use of connectors and valves sealed with metal gaskets, and the use of materials with possibly low radon emanation for the construction of the detector.

Claim 1:
A detector for measuring radon concentration in gases, comprising:
- a pressurised measurement chamber with at least one opening for supplying and discharging gas from the environment to the interior of the measurement chamber; and
- sensors of alpha particles generated as a result of radioactive decays of radon daughters in the gas present inside the measurement chamber and drifted to the sensor surfaces;
characterized in that it further comprises:
- a compressor (<NUM>) for pressurizing the gas inside the measurement chamber (<NUM>), wherein each opening (<NUM>, <NUM>) of the measurement chamber (<NUM>) is equipped with gas-tight ports;
- wherein each of the sensors (<NUM>, <NUM>) is a semiconductor sensor that has a separate power supply system comprising a high DC voltage (HVIN) supply (<NUM>) connected to a housing and to the active electrode of the sensor for creating an electric field in the chamber, and a low DC voltage supply (<NUM>) from a battery pack (<NUM>) connected only to an active surface electrode of the sensor for use as a bias voltage;
- wherein the measurement chamber (<NUM>) is grounded;
- for each of the sensors (<NUM>, <NUM>), an amplifying system (<NUM>) comprising a charge sensitive pre-amplifier (115A) connected in series with the spectroscopy amplifier (115B), wherein the amplifying system (<NUM>) is mounted together with the battery pack (<NUM>) in a metal housing preventing penetration of electromagnetic noise from the outside;
- wherein the signal from each sensor (<NUM>, <NUM>) is fed to a corresponding amplifying system (<NUM>) via capacitors (<NUM>, <NUM>) adapted to filter the output signal of the sensors from the high DC voltage level (HVIN).