The present invention relates to a passive dosimeter including a diffusion chamber in which is disposed a nuclear trace detector for alpha particles and with which the concentrations of radon and thoron gas and of their decay products as well as the percentages of radon and thoron individually can be detected.
The natural radiation exposure of a human being is determined by two radiation components: extraneous irradiation by cosmic rays and rays of the natural radionuclides of the environment; and internal irradiation by inhalation of natural radionuclides in the inhaled air and by ingestion in foods and drinking water. Civilization influences often lead to increases. The noble gas radon is responsible for a noticeable increase in the inhaled dosage. Radon has three isotopes having the mass numbers 222, 220 and 219. They are produced in the decay series of U.sup.238, Th.sup.232 and U.sup.235. Their half-lives are 3.8 days, 56 seconds and 4 seconds, respectively: since natural uranium is composed of approximately 99% U.sup.238 and only about 0.7% U.sup.235, Rn.sup.219 is of subordinate significance.
The concentration of radon (Rn.sup.222), thoron (Rn.sup.220) and their short-lived decay products in air is subject to considerable space and time fluctuations. In houses it depends on the radium content of the construction materials and on room ventilation. Part of the noble gases formed by the decay of Ra.sup.226 and Ra.sup.224, respectively, enters into the air by way of diffusion. The permeability of the construction materials here plays a significant part. Further sources may be water and noble gases as well as the geological subsoil. In the open air, there is the additional influence of meteorological parameters.
The radon concentration increases considerably with decreasing air exchange rate. Recent energy conservation measures have contributed to a steady reduction of air exchange rates in houses. The decay products of radon are heavy metals and are present in air as free atoms but primarily adsorbed at aerosols. By way of deposition on surfaces, there occurs a de-enrichment of the decay products so that a radioactive equilibrium between radon and its decay products is never reached in the air of living quarters. In the open air, an approximate equilibrium occurs only at some height above the ground.
The aerosol-type, short-lived decay products are generally never in equilibrium with radon or thoron, respectively, in air, due to their being deposited on surfaces. It is therefore appropriate to define an equilibrium factor F as follows: ##EQU1## where c.sub.pFp represents the potential alpha energy concentration of the short-lived decay products of radon and c.sub.pRn represents the potential alpha energy concentration of radon in equilibrium with its short-lived decay products.
If one compares the effective equivalent dose of the decay products with the effective equivalent dose of the noble gas radon, it becomes clear that the percentage of radon in the dose plays a part only in the range of equilibrium factors &lt;0.1. Short-term measurements of the equilibrium factors in houses yielded values of between 0.1 and 0.8 with a median value of 0.3.
For thoron decay products only very few measurements of equilibrium factors, defined in a manner corresponding to F, above, in houses are available. These measurements cover a dispersion range from 0.01 to 0.5 with a frequency maximum at 0.05. Due to the low radioactive half-life of Po.sup.216 (0.15 seconds), equilibrium with thoron (55 seconds) always exists. For that reason, Po.sup.216 is treated together with thoron in the dose determination. In the dose determination for the decay products, the percentage of Pb.sup.212 outweighs the percentage of Bi.sup.212 by a factor of 10.
For the identical potential inhaled alpha energy, the dose for thoron decay products is only about 1/3 of the dose of radon decay products. Short-term measurements of the potential alpha energy concentration in about 100 houses produced a median value for radon decay products of 3.5.multidot.10.sup.-8 Jm.sup.-3, for thoron decay products a median value of 3.1.multidot.10.sup.-8 Jm.sup.-3. The measurements also indicated that in some houses the percentage of thoron decay products exceeds the percentage of radon decay products up to a factor of 2. For that reason, a realistic dose estimate in houses must also consider the percentage of thoron decay products.
Presently employed measuring methods can be divided into two groups: active and passive measuring methods. Active measuring methods require an external energy supply to operate; they operate primarily with pumps and electronic evaluation systems. Passive measuring methods operate without an energy supply, have no moving parts or electronic systems and primarily employ thermoluminescence detectors (CaS0.sub.4 :Dy, LiF) or solid state nuclear track detectors (cellulose nitrate, polycarbonate) as their detectors.
A major drawback of the thermoluminescence detectors (TLD) is their comparatively low sensitivity to alpha particles compared to their sensitivity to beta and gamma radiation. Since the number of recorded alpha particles is significant for the evaluation, a second, shielded TLD must be employed for the difference formation. Solid state nuclear track detectors used for radon dosimeters are primarily cellulose nitrate foils as well as polycarbonate foils (MAKROFOL, LEXAN, CR 39). These detectors are insensitive to beta and gamma radiation.
In principle, there are two types of radon dosimeters: diffusion chambers and the so-called open detectors. Diffusion chambers are composed of a vessel closed by a filter with a detector in its interior. Radon diffuses through the filter into the interior of the vessel; radon decay products contained in the air are retained by the filter. The detector records the alpha particles of the radon decay and of the decays of the decay products produced in the dosimeter.
This method has been used for several years and is distinguished by good reproducibility of the measuring results. Frequently, open detectors are employed to obtain statements about the radon concentration in the air. A detector without housing is exposed and records all alpha particles of a certain volume range and the alpha particles of radon decay products deposited on its surface. Generally, this provides information only about the gross alpha activity without nuclide specific separation, if the equilibrium state between radon and its decay products in the air is not known. Information about the percentages of radon, thoron or their decay products can be obtained only by separate evaluation of certain alpha energy groups. Radon alone can be determined very easily by means of a diffusion chamber. A diffusion resistor permits quantitative separation of thoron due to its short half-life compared to radon. Thoron can be measured in a diffusion chamber only together with radon. A measurement of the thoron concentration is therefore a difference measurement. It would be advantageous to find a suitable filter material which permits thoron to diffuse through in large amounts but simultaneously retains decay product aerosols. A mathematical model for such a diffusion chamber would have to consider the diffusion process of radon and thoron through the filter into the interior of the chamber, the separation of decay products and aerosols by the filter and the diffusion process and surface deposition of the decay products produced in the chamber.