WO1992003748A1 - Process and passive integrating dosimeter for measuring the concentration of radon decay products in the air - Google Patents

Abstract

The invention concerns a process for measuring the radon decay product concentration in the air, using detectors (3) designed to detect the alpha radiation of radon and its short-lived alpha-radiating decay products. The alpha radiation is measured with pairs of identical detectors (3) disposed immediately next to each other, one detector in each pair being isolated from radon decay products in the air. The difference signal from the pairs of identical detectors (3) is subsequently analysed to determine the total radon decay product concentration in the air, or the concentration of any mixture of short-lived alpha-emitting decay products. This is done by using a passive dosimeter with pairs of identical detectors (3) disposed in replaceable geometrically identical chambers (2) located immediately next to each other. The chambers (2) have identical diffusion apertures (5) and one of the chambers (2) is fitted with a filter (6) which isolates the detectors in that chamber from radon decay products.

Description

 Process and passive dosimeter for the integrated measurement of the proportion of radon decay products in the air

The invention relates to a method and a passive dosimeter for integrally measuring the proportion of radon decay product in the air.

Various methods and devices for the detection of radon and its alpha-active decay products are known (reviews and comparisons in "Rad.Prot.Donimetry" 1 (1981) 97; ICRP publication 24; "NEA, Metrology and monitoring of radon, thoron and their daughter products ", Report by a Group of Experts of the OECD / NEA, Paris 1985).

The known methods can be divided into two large groups, active and passive measuring systems.

Active measuring systems are predominantly equipped with electrically operated functional units, such as pumps, control and evaluation electronics, and therefore require an energy supply. They generally consist of a detector-filter combination and an air suction device for filter application or for enriching the radon decay products on the detector surface itself. Semiconductors detectors, scintillation counters, thermoluminescence detectors, electret detectors or solid-state track detectors are used as detectors. With dosimeters of this type, statements can be made about the activity concentrations and / or the potential alpha energy concentration of the donor decay products in the air. Radon gas concentrations are measured by air sampling with special ionization chambers, scintillation chambers, and electret chambers. Activated carbon process or 2-filter methods.

Active measuring systems for radon decay products are mainly used in mining for short-term measurements of a few minutes (SAAS report 267) and as continuously operating devices, due to the high costs, for environmental measurements of up to one month (SAAS report 342). When measuring radon in buildings, active measuring systems are used to determine the weight condition used. Furthermore, active devices for radon decay product measurements for individual dosimetric monitoring of people working underground are known (Feddersen, Ch., Dissertation A of the Faculty of Technical Sciences of the Bergakademie Freiberg, 1986: electronic personal dosimeter with miniature pump and semiconductor detector; "Proc. Of the Specialist Meeting on Personal Dosimetry and Area Monitoring Suitable for Radon and Daughter Products", OECD / NEA, Paris 1979 Personal dosimeter with electrically operated pump, LR 115 as a solid-state track detector and various absorber foils; Burian I. et al., "Pracovni Lekarst-vi ", 1989: Personal dosimeter with an electrically operated diaphragm pump, LR 115 as a solid-state track detector and brake film to optimize the alpha particle registration).

Passive measuring systems mainly use solid-state detectors, in some cases also thermoluminescence detectors, electret detectors and activated carbon. The dosimeters require no power supply, have no moving parts and no electronic assemblies. So-called diffusion chambers are known from the measuring principle and the technical designs. The radon naturally diffuses through an opening, which is closed with a filter for the radon decay products, into a chamber in which one or more passive detectors are located. The radon concentration in the air can be determined from the measurement effect of these detectors and the measurement conditions after suitable calibration of the system ("Hucl.Instr. Meth.", 7_3 (1980) 169; "Health Physics", 40 (1981) 693; "Health Physics Society, 30th Annual Meeting", 1985, TPM-D4; "llth Intern.Conf. On SSNTD's", Bristol 1981, pp. 539/561/569; "Rad. Prot.Dosimetry" 7_ (1984) 341; Riso-M-2483 (1985); "Rad.Prot. Dosimetry" 1_ (1984) 327).

Open, alpha-sensitive detectors without housings or in unclosed housings are also known. With them, only qualitative statements about the existing total alpha activity possible. They are often used, without specially assembled dosimeters, for screening measurements in buildings to determine the order of magnitude of the radon equilibrium concentration. The sensitivity of open detectors shows a strong dependence on the state of equilibrium between radon and its short-lived decay products. Therefore, when estimating the radon equilibrium concentration from the measurement signal of open detectors, deviations of 100 to 200% from the true value can be expected.

Since there is generally no balance between radon and its short-lived decay products in the air due to surface deposition of the radon decay products and air movements, the equilibrium state must be known in order to derive statements about the dose-determining radon decay products from radon measurements alone. Therefore, an average equilibrium factor from comparable stress situations or international recommendations is often assumed or determined by several additional random test measurements with active short-term measuring devices. However, assuming an equilibrium factor can lead to significant errors in dose estimation.

Furthermore, the concentrations of radon and its decay products in the air are subject to strong local, daily and seasonal fluctuations. However, many known active measuring systems only allow short-term statements based on a few minutes of measuring time. Therefore, short-time measurements can easily lead to a complete misjudgment of the actual radiation situation. Reliable statements can only be derived from measurements over longer periods of time, from a few weeks to half a year and longer. Continuously operating active measuring systems for determining the radon decay product content in the air are very expensive and have a complex measuring and o? ^* Ertetechnik. "They are, therefore, ch. Only for selected supervision _^ gsaufgaben used. Long-term integrating in-situ measurements for detecting the existing fluctuations in the radon concentration are preferably carried out with the aid of passive measuring systems using solid-state track detectors, since they are simple in construction, inexpensive to manufacture, energy-independent and easy to handle and evaluate. Small passive, long-term integrating measuring systems were developed especially for personal dosimetric purposes ("Proc. IIth Intern.Conf. On SSNTD's", Bristol 1981, p. 531; Miles, JCH et al., "Radiolog. Prot. Bulletin" 42 _. (1981)), which, however, require assumptions about the equilibrium conditions in the air to interpret the measurement results.

In the known measuring systems, the separation of the radon and its decay products requires consideration of the observation conditions and the registration properties of the detector (Urban, M., "Proc. 13th Intern.Conf. On SSNTD's", Rome 1985) or the use of special nuclide collection or separation methods, such as electric fields ("Health Physics Society, 30th Annual Meetin", 1985, TPM-D3) or absorber films in front of the detectors during the irradiation.

Simple passive long-term dosimeters, which allow a direct determination of the radon decay product concentration in the air and thus enable dose-relevant statements about exposure situations and the actually accumulated dose of people to be accumulated, are not known.

The invention has for its object to provide a method and a passive dosimeter for integrally measuring the radon decay product content in the air, using highly alpha-sensitive, passive detectors for measurement and evaluation and thereby corresponding to different sensitivities or necessary exposure times the application are guaranteed. According to the invention, the object is achieved in that measurements are carried out with pairs of identical detectors located in close proximity to one another, one of the detectors arranged in pairs being isolated from the air from the radon decay products and the detectors subsequently being evaluated, and wherein Statements about the radon decay product portion or the potential alpha energy concentration of an arbitrary mixture of short-lived radon decay products in the air can be derived from the difference signal of the same pair of detectors.

In this case, when evaluating from the measurement signal of the detector isolated from the air in relation to the radon decay products, the radon concentration and the equilibrium factor with regard to the potential alpha energy can be derived from this measurement signal and the difference signal of the detectors which are identical in pairs. In order to guarantee different sensitivities or necessary exposure times, it is expedient if measurements are carried out with identical, interchangeable detectors which are located in close proximity to one another and have different alpha sensitivity in pairs.

It is possible that with alpha-sensitive detectors in the form of solid-state track detectors, preferably polydiethylene glycol bis (allyl carbonate) or / and polycarbonate detector films or thermoluminescence detectors, preferably thin-layer LiF: Mg, Cu, P is measured.

In a further embodiment, the solution comprises a passive dosimeter, in which the same detectors are exchangeably arranged in pairs in geometrically identical chambers of the same volume which are in close proximity to one another, the chambers having the same diffusion openings and one of the chambers with one of the detectors arranged in it Radon decay isolating filter is covered. The diffusion openings are preferably arranged in a cover closing the respective chamber. The chambers can consist of an electrically conductive material, such as carbon mixed or coated with plastic, or a metal or a plastic with a special additive, which ensures a uniform electrostatic charging of the housing.

It is expedient for the detectors in the chambers to be covered by the same thin conductive foil, such as an aluminized plastic foil.

To ensure various sensitivities or necessary exposure times, the filter advantageously consists of a thin, low-density polyethylene film or a hydrophobic glass fiber filter.

It is possible that the surfaces of the diffusion openings can be changed optionally and can also be partially covered on one or more sides.

An expedient form of the formation of the dosimeter, particularly in the form of a personal dosimeter, is that the chambers are arranged directly next to one another.

A further possibility is provided in that the chambers are arranged directly one above the other, the lower chamber being covered by the filter isolating the detectors in it from radon decay products.

In this embodiment, the detector is preferably arranged in the upper chamber in the cover containing the diffusion openings of the lower chamber.

The main advantage of the solution according to the invention consists in the direct determination of the load-relevant radon decay product content of the air without the aid of the highly error-corrected were liable to assume average equilibrium factors in connection with radon measurements alone. Further advantages are the simple structure, the small dimensions and the low weight of the dosimeter, the low manufacturing and detector evaluation costs as well as the negligible influence of the dosimeter measurement signals by environmental influences. The detectors to be used are easy to use and can be evaluated safely. Once the detectors have been replaced, the dosimeter can be reused.

In addition, various sensitivities can be easily realized with the passive dosimeter. Measurements in a completely unknown, elevated and also strongly fluctuating radon / radon decay product atmosphere are possible on a large scale. The simple technical implementation of the method in the form of local or personal dosimeters allows the use of such dosimeters in a wide variety of applications. A real personal dosimeter worn on the chest with forward-facing diffusion openings enables, in particular in a highly moving atmosphere, as is often the case in mines, where load-relevant radon decay product measurements are very error-prone, a much more accurate load determination for people on the basis of the better approximation to the inhalation model. For this purpose, it is expedient to arrange a retaining clip on the back of the dosimeter. Furthermore, it is possible to arrange measuring point & for parallel or further measurements in addition to the chambers receiving the detectors.

A further embodiment of the invention consists in arranging a closure cover which can be displaced in the longitudinal direction on the dosimeter, said cover having openings through which the diffusion openings are opened or closed.

An advantageous solution is that the openings correspond in their position and the number of layers and the number of diffusion openings. It is also possible for a single opening to be arranged, the inside diameter of which is somewhat larger than the outer circumferential line around the diffusion openings.

In order to achieve a safe movement in the longitudinal direction, the closure cover can be connected to the housing via a sliding seat, the closure cover with its side walls encompassing a bead-like covering on the upper edge of the housing.

The invention is explained using exemplary embodiments. In the accompanying drawing:

1: the front view of a passive dosimeter with chambers arranged side by side in section,

2: the front view of a passive dosimeter with chambers arranged one above the other in section,

3: the top view of a lid with diffusion openings,

4: the top view of a lid with diffusion openings and detector,

5: the top view of a dosimeter with a closure lid,

6: section 6-6 according to FIG. 5.

The drawings show the passive dosimeter in a schematic representation. To illustrate the opening, the detectors 3, the filter 6 and the foils 7 in particular are shown thicker. The actual thickness of a poly-diethylene-glycol bis (allyl carbonate) detector 3 is approximately 0.9 mm and that of the filter 6 made of low-density polyethylene is approximately 30 μm and that of the hydrophobized glass fiber filter is 350 μm. As explained below, it is necessary for the chambers 2 to have the same volume. This can be done in a simple manner by increasing the volume of the corresponding chamber 2 by the volume of the filter 6.

1 shows a passive dosimeter, which has two chambers 2 arranged closely next to one another in a housing 1 having. The chambers 2 are geometrically identical and have the same volume. This is the basis of the measuring principle. A detector 3 is arranged at the bottom of both chambers 2. In a manner not shown, a group of detectors 3 of different alpha sensitivity can also be arranged in a chamber 2. It is essential, however, that the same detectors 3 or groups of detectors 3 are arranged in both chambers 2. A film 7, preferably a thin aluminized plastic film, can lie on the detectors 3. This improves the reproducibility of the measurements. A fuse 8, for example a snap ring, is installed above the detectors 3, which fixes the detector 3 to the bottom of the chamber 2 and at the same time enables its easy replacement.

Each of the chambers 2 is sealed at its upper end by a cover 4, as is also shown in FIG. 3. The covers 4 have diffusion openings 5 through which the air enters the chamber 2 and can act on the detector 3.

A useful form of closure is that the cover 4 is placed on the chamber 2 under mechanical pressure until the upper edge of the cover 4 closes with the edge of the housing 1. In order to prevent unauthorized opening of the dosimeter, fuses known per se can be provided.

In one of the two chambers 2, a filter 6 is arranged under the cover 4, so that the diffusion openings 5 are sealed by it. The filter 6 preferably consists of a low-density polyethylene film or a hydrophobic glass fiber filter. While the radon decay products are retained by the filter 6, a radon gas concentration compensation between the ambient air and the interior of the chamber 2 takes place in a few minutes. The number of diffusion openings 5 and thus the gas exchange surface can be different. Different sensitivities or necessary exposure times of the dosimeter can thus be implemented in a simple manner.

Another exemplary embodiment of a passive dosimeter is shown in FIG. 2 in conjunction with FIG. 4. In this dosimeter, the geometrically identical chambers 2 of the same volume are arranged one above the other in the housing 1. The detector 3 is located in the bottom of the chamber 2. In the embodiment shown, the film 7 and the fuse 8 have been omitted. It is of course also possible to use these elements with this structure.

The detector 3 located in the upper chamber 2 is located in the cover 4 of the lower chamber 2, which at the same time forms the bottom of the upper chamber 2. It is also essential for the structure that the structure in both chambers 2 is identical for the same volume. If the detector 3 is arranged in the bottom of the upper or the cover 4 of the lower chamber 2, then the two chambers 2, the diffusion openings 5 can only be arranged in that part of the respective cover 4 which is not covered by the detector 3. The basic structure of such a cover 4 is clear from FIG. 4, diffusion openings 5 being arranged in the edge zone and detector 3 being arranged in the center of the cover 4.

When the dosimeter is constructed in accordance with FIG. 2, the filter 6 is arranged in the lower chamber 2. To replace the detectors 3, the dosimeter can be opened by means known per se, for example the chambers 2 can be screwed together.

In a number of test series, a large number of the passive dosimeters according to the invention, consisting of PVC plastic, were tested about the size of a matchbox, exposed to different atmospheres with increased radon / radon decay product concentrations. The poly-diethylene-glycol (allyl carbonate) detectors of dimensions 13 × 13 mm and 0.9 mm thick were then chemically etched in 4N KOH for 6 hours at 80 ° C. The detector background etch trace density was 20 traces / cm ² with a simple standard deviation of 50%. The etching trace density on the detector in the chamber with filter film in front of the diffusion openings was used as the measuring signal with regard to the radon concentration and the difference in the etching trace densities on the detectors in the chamber without and with fil as the measuring signal regarding the potential alpha energy concentration of the radon decay products ¬ ter film used in front of the diffusion openings. This resulted in a sensitivity of the dosimeter of 2.02 with regard to the radon concentration. 10 "- * tracks cm"" ² / Bq. M" ^. h with a simple standard deviation of 11% and a sensitivity of 1.35 with regard to the potential alpha energy concentration of the radon decay products. 10 "'tracks. Cm" ² / MeV. m ~ ^. h with a simple standard deviation of 18%. The equilibrium factor with regard to the potential alpha energy varied in the calibrations in the range from 0.26 to 0.75. The free portion of the decay products related to the potential alpha energy in the calibrations was in the range from 0.01 to 0.06.

With a visually reliably evaluable etching trace density difference on the derectors of the exposed dosimeter of 300 traces cm ^"2 , a potential alpha energy concentration of the radon decay products in the air of 4.10 MeV m ~ ^ after an exposure period of the dosimeter of 56 h can be demonstrated .

The slight error in the sensitivity of the dosimeter when determining the exposure-relevant potential alpha energy concentration of any mixture of short-lived radon decay products in the air surrounding the dosimeter during the exposure, determined in a wide range of different ways Equilibrium conditions, allows wide use as an individual dosimeter in an elevated and rapidly changing radon / radon decay product atmosphere. The exposure time of the dosimeter is to be selected according to the specific monitoring task.

It is of course possible to design the dosimeter as a personal dosimeter. Here, a holding clip is arranged on the back of a dosimeter according to FIG. 1. As a result, the same can be attached to the clothing. In such a passive dosimeter, in addition to the two chambers 2 for the differential measurement, further measurement points for parallel or other measurements can be provided.

For example, to measure the external gamma radiation dose, it is also possible to arrange exchangeable, highly sensitive thermoluminescent detectors in the housing 1.

To carry out the method according to the invention, it is possible to arrange the detectors 3, which are identical in pairs and are located in close proximity to one another, in fixed measuring points in the rooms to be monitored.

5 and 6, a passive dosimeter, on which a longitudinally displaceable closure cap 9 is arranged, is shown.

The closure cover 9 has openings 10 through which the diffusion openings 5 can be opened or closed. The design and arrangement of the openings 10 can be different. It is thus possible to design these as individual bores (FIG. 5, left side) or as a single large opening 10 (FIG. 5, right side).

The position of the individual bores corresponds to the position of the diffusion openings 5. As a result, only relatively slight displacements are required in their covers. During training as a single large opening 10, its inner diameter is slightly larger than the outer circumferential line around the diffusion openings. This training has the advantage that a secure closure is guaranteed even in dark tunnels. To avoid an unintentional displacement, locking elements known per se can be arranged between the closure cap 9 and the housing 1 in a manner not shown.

FIG. 6 shows a sliding seat 11, the closure cover 9 with its side walls encompassing a bead-like thickening on the upper edge of the housing 1.

With such a cover 9 it is possible to bring a passive dosimeter safely to the measuring point without the detectors having already been influenced by radiation. A corresponding return transport from the measuring point is also ensured.

The examples set out, in particular also the test results explained above, serve to explain the solution according to the invention. However, it is not limited to these examples.

Claims

Claims 1. A method for integrally measuring the radon decay product portion in the air with detectors for detecting the alpha radiation of radon and its short-lived alpha-active decay products, characterized in that measurements are carried out with pairs of identical detectors (3) located in close proximity to one another, whereby in each case one of the detectors (3) arranged in pairs is isolated from the radon decay products from the air and the detectors are then evaluated and statements about the radon decay product fraction or the potential alpha energy concentration of any mixture of short-lived radon decay products in the air can be derived from the difference signal of the same detectors (3) in pairs. 2. The method according to claim 1, characterized in that in the evaluation from the measurement signal of the detector isolated from the air against the Radonzer¬ fall products (3) the radio concentration and from this measurement signal and the Differenz¬ signal of the same pair of detectors (3 ) the equilibrium factor with regard to the potential alpha energy is derived. 3. The method according to claim 1 and 2, characterized in that with pairs of the same, interchangeable, in close proximity to each other detectors (3) is measured, the pairs in different alpha sensitivity besit¬ zen. 4. The method according to claim 3, characterized in that with alpha-sensitive detectors (3) in the form of Festkörper¬ track detectors, preferably poly-diethylene glycol bis (allyl carbonate) or / and polycarbonate detector film or thermoluminescence detectors, preferably thin-film LiF : Mg, Cu, P are measured. 5. Passive dosimeter for integrating measurement of the radon decay product portion in the air with detectors for detecting the alpha radiation of radon and its short-lived alpha active decay products, characterized in that pairs of identical detectors (3) in geometrically identical, in close proximity to one another lying chambers (2) of the same volume are interchangeably arranged, the chambers (2) having the same diffusion openings (5) and one of the chambers (2) with a filter (6) isolating the detectors arranged therein from radon decay products (6) is covered. 6. Passive dosimeter according to claim 5, characterized in that the diffusion openings (5) in a, the respective chamber (2) closing lid (4) are arranged. 7. Passive dosimeter according to claim 5, characterized in that the chambers (2) consist of an electrically conductive material, such as plastic mixed or coated with carbon, or a metal. 8. Passive dosimeter according to claim 5, characterized in that the chambers (2) consist of an electrically conductive material in the form of a plastic with a special additive which ensures a uniform electrostatic charging of the housing. 9. Passive dosimeter according to claim 5, characterized in that the detectors (3) in the chambers (2) are each covered by the same thin, conductive film (7), such as an aluminized plastic film. 10. Passive dosimeter according to claim 5, characterized in that the filter (6) is formed as a hydrophobic glass filter. 11. Passive dosimeter according to claim 5, characterized in that the filter (6) consists of a thin polyethylene film low density. 12. Passive dosimeter according to claim 5, characterized in that the surface of the diffusion openings (5) is optionally changeable and can also be partially covered on one or more sides. 13. Passive dosimeter according to one or more of claims 5 to 10, characterized in that the chambers (2) are arranged immediately adjacent to one another. 14. Passive dosimeter according to one or more of claims 5 to 10, characterized in that the chambers (2) are arranged directly one above the other, the lower chamber (2) through which the detectors (3) located therein are opposite Radon decay insulating filter (6) is covered. 15. Passive dosimeter according to claim 12, characterized gekennzeich¬ net that the detector (3) in the upper chamber (2) in which the diffusion openings (5) of the lower chamber (2) containing lid (4) is arranged. 16. Passive dosimeter according to one or more of claims 5 to 14, characterized in that a retaining clip is arranged on the back of the dosimeter. 17. Passive dosimeter according to one or more of claims 5 to 16, characterized in that in addition to the detectors (3) receiving chambers (2) measuring points for parallel or further measurements are arranged. 18. Passive dosimeter according to one or more of the claims 5 to 17, characterized in that a closure cover (9) which can be displaced in the longitudinal direction is arranged on the dosimeter. net, which has openings (10) through which the diffusion openings (5) are opened or closed. 19. Passive dosimeter according to claim 18, characterized in that the openings (10) correspond in their position and the number of positions and the number of diffusion openings (5). 20. Passive dosimeter according to claim 18, characterized gekennzeich¬ net that a single opening (10) is arranged, the inner diameter is slightly larger than the other circumferential line around the diffusion openings. 21. Passive dosimeter according to one or more of claims 18 to 20, characterized in that the closure cover (9) with the housing (1) is connected via a sliding seat, the closure cover (9) with its side walls a bead-like compression at the upper edge of the housing (1).