Radon (Rn-222) is a naturally occurring radioactive gas. It is produced by decay of radium (Ra-226) part of the decay chain of uranium which occurs throughout the earth's crust. The short-lived decay products of radon, called radon daughters, in air, attach themselves to aerosols, and if inhaled are partly deposited in human respiratory tract. The radiation dose caused by inhalation of radon daughters in air constitutes in many countries the major part of the natural radiation dose to man. The radiation dose caused by radon itself is minor in comparison with that of the radon daughters.
The levels are normally higher indoors than outdoors, and in uranium mines the levels may be very high. Even in the outdoor air, the man modified levels and releases may under certain circumstances be much higher than normal. The fluctuations in air make the measurement result both time and space dependent. The fluctuation of the radon daughters will be expected to be more pronounced because it is subject both to the variation of source (radon) and the aerosol conditions. For the safety of human beings, the monitoring of radon daughters indoors, in mines and in the environment is becoming increasingly important. Since the estimated risk to human beings is based upon the time averaged value of the radon daughters, a time-averaged dosimeter is ideally required. Because the level of the radon daughters is highly space dependent, whether within dwellings or in mines it is advisable to make a large number of measurements. As a consequence, dosimeters for measuring radon daughters should be simple, accurate, capable of rapid processing, and inexpensive.
Radon or radon daughters instruments are either active (with a sampling pump) or passive (without a pump). With due attention to experimental calibration, radon levels can be measured to a reasonable degree of accuracy using passive devices. Radon daughters measurement, logically favors an active device for the following reasons:
(1) The air sampler will determine how many liters of air are sampled so that a time-averaged value of radon daughters per liter can accurately be obtained. The possibility of plate-out contamination which usually affects the passive device can easily be eliminated in the active system.
(2) Radon daughters in air presents a three-dimensional source. If alpha detection is used for the passive measurement of the radon daughters, a plane detector (either solid state track or TLD detector) is usually employed. Depending upon the geometry of the detector and the alpha source, a wide range of alpha energies and incident angles will be registered. The alpha energies at the detector vary greatly because of the energy losses along the varying path lengths in air due in turn to the three dimensional source condition. In order to achieve the required precision, a fairly complicated processing method is usually required. However, in the case of the active device, the sampler can be considered as a means of converting a three dimensional source distribution in air into a plane source on filter paper. For dosimetric purposes, it is far easier to interpret the detector tracks resulting from this method, since, with a plane source and a plane detector the alphas appear mono-energetic. It is especially important if an image analysis system is used for automatic readout of the tracks. Pattern recognition by the image analysis system is sensitive to both the angle of incidence and the residual energy of alphas.
(3) Another favorable condition for the active device is that the theoretical efficiency can be estimated more accurately. This makes the quality assurance more precisely controlled than in the passive device which depends primarily upon experimental calibrations. For dosimetric purposes, radon daughters are measured in terms of the working level (WL) unit, which can be expressed as: ##EQU1## RA=number of tracks for RaA RC=number of tracks for RaC'
E=alpha counting efficiency (tracks/disintegration) PA1 V liter of air drawn through the filter (1/hr.times.hrs).
Under the condition that every alpha from the source (the filter paper) is detected as a readable track after striking the detector, the geometric efficiency can be adopted without further experimental calibrations.
For indoor radon measurement, there is no special requirement for the differentiation of RaC' from RaA. Instead, most of the current continuous or time integrated instruments adopt the "total alphas" method, in which all alpha tracks are counted without regard to the various energies. This method is acceptable because the total alphas method can only introduce an uncertainty of less than 6%, as compared with the true value from the definition mentioned above. This error is not significant for dosimetric purposes.
For personal mine dosimeters, the simpler total alpha method cannot be used. This is because the ratio of RaA/RaC' under the dusty atmosphere of the mine, is related to the time-averaged ventilation conditions, and can vary quite widely depending upon the region in which an individual miner is working. This is an important consideration in assessing the risks to the miner. If the differentiation of RaA from RaC' is required for dosimetric purposes, the active device is much more practical than the passive one. Although careful analysis of tracks registered by the passive device allows differentiation of the different alpha emitters according to the residual energy, this involves complicated processes which are inconvenient for the dosimetry uses. Due to the three-dimensional nature of the source condition for the passive device, random-oriented tracks are always expected even if a collimator device is adopted. If the range of the individual random-oriented track is used for deducing the residual energy, this usually requires the measurement of 5 parameters under a microscope.
One successful active device (CEA U.S. Pat. No. 3,922,555) makes use of Kodak Pathe LR-115 Type II nitrocellulose film, as the alpha detector LR-115 Type II is what is known as a band-pass detector. Its response to alphas is such that the diameter of the transparent track after processing is a maximum for perpendicular incident/alphas with a residual energy of around 3 MeV, while the diameter of the track drops sharply for alphas of residual energy higher than 3 MeV. For tracks caused by oblique incident alphas, the drop in the diameter is also sharp as compared with the perpendicular tracks. With this prior art device, by carefully selecting the thickness of an absorber fixed at the top of a collimator, the tracks of only one kind of radon daughter can be expected in the region of the detector defined by that particular collimator and absorber For instance, by choosing an absorber with a thickness which will degrade the residual energies from alphas from RaA (6 MeV) to around 3 MeV, only tracks, from RaA will be expected in the region defined by the collimator fitted with that particular absorber, residual energies from alphas of RaC' (7.68 MeV) will be too high to produce a readable track after passing through the same absorber. Similarly by using a different thickness of absorber which will degrade the alpha energy from RaC' down to 3 MeV while degrading that of RaA to zero, the region defined by the collimator fitted with this particular absorber will reveal the track of RaC' only. Therefore, the separate detection of RaA and RaC' described by the CEA patent is an ingenious use of the band-pass characteristics of the LR-115 Type II detector However, the CEA prior art device will not work with any detector other than the Kodak Pathe LR-115 Type II nitrocellulose detector. Another characteristic of the device of the CEA Patent is that there must be at least two collimators in order to selectively detect two alpha particles, as may be gleaned from the above description.
The detecting device described by the CEA patent was originally designed as a personal dosimeter for uranium miners. It was found later that it could be adapted for environmental and indoor radon daughters measurements. Years of field testing experience have revealed the following problems:
(1) Two problems have been encountered regarding the uranium mine personal dosimeter:
(a) Technical difficulty in reducing the volume of the detecting head.
Miners usually carry their headlights and the necessary tools all the way to their workplace. To limit the number and the volume of the articles carried by the miners is a serious problem to be considered. As a consequence, uranium mine companies would prefer to mount the personal dosimeter on the top of the headlight battery. This is not possible with the prior art device of the CEA patent which is too large and must be belt-mounted as a separate unit. As is mentioned above, oblique tracks must be limited in the prior art CEA device because the tracks contributed by oblique and perpendicular incidence alphas are of quite different diameters even if the energies of the alphas are the same. Since a long collimator is needed to eliminate the oblique tracks in the CEA device, the height of the detector head cannot be reduced. Also, since RaA and RaC' are separately detected in different collimators in the CEA device, the overall cross section of the detecting head cannot be decreased either. Both factors (length and cross-section) present problems in mounting the prior art CEA dosimeter on the top of the headlight battery.
(b) Contamination of the collimator system.
The collimator system of the CEA patent is easily contaminated by the alpha-emitting uranium dust, especially in the highly dusty uranium-rich mining environments. Recent investigations reveal that alphas emitted by uranium dust (if allowed to pass to the collimator) will have the same residual energy as alphas from RaA deposited on the filter paper. A false high measurement of RaA will result.
(2) Indoor Surveys.
Normally, the purpose of an indoor survey is to estimate the annual mean WL for a given indoor environment. The so-called time-averaged instrument is designed to allow for diurnal fluctuations as well as variations introduced by the living habits of the occupants. In order to obtain a reliable mean value 10 days' continuous exposure is generally required when using the CEA patent device. Badly radon-contaminated houses present another urgent requirement for indoor radon surveys. The purpose of the survey, usually known as "screening", is to identify houses with high radon levels as quickly as possible, for remedial action or further investigation. The survey requires instruments with a minimal sampling time and therefore detecting heads designed for this purpose generally cannot be used for the longer duration time-averaged exposures previously described, as overlapping of tracks may be found. A more versatile design is needed so that the detecting geometry and hence the efficiency or sensitivity of the instrument can be changed in a simple way so that the optimum exposure time can be varied, enabling measurements to be made in from one day to ten days depending upon the requirements.
(3) Environmental Survey Use.
A high-speed battery-operated pump is needed with the prior art CEA device to compensate for the generally low level of radon in the environment. The power supply required by the high-speed pump is usually not convenient for field use. It is also expensive. To improve the detection efficiency is a logical solution. As mentioned above, the current design of the prior art CEA dosimeter is not flexible enough to allow this improvement.
Two other prior art devices, which are less pertinent than the CEA '555 patent, are briefly described below, however, a detailed analysis of this prior art will be foregone.
U.S. Pat. No. 3,505,523 to Becker shows a personal radon dosimeter worn by uranium miners. Becker discloses an active device, however, it is one which does not provide the capability of selectively detecting alpha particles from RaA and RaC', which is very important for use in a mine environment.
U.S. Pat. No. 4,055,762 to Durkin shows a portable radon daughter dosimeter unit used to measure radon gas alpha daughters in ambient air. The Durkin patent discloses an active device; however, it is a solid state electronic device, which is substantially different from the present invention.