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Retrospective Assessment of Radon Exposure from Long-Lived Decay Products


It has recently been shown that a dwelling remembers past radon (Rn-222) levels via long lived decay products which are permanently embedded by alpha particle recoil in glass or other hard surfaces. This radon memory effect implies that there are radon detectors with the potential of being retrospective in all dwellings. There is an urgent need to investigate this new technique in order to clarify its potentials and limitations.

The investigation of long lived radon decay products in the indoor environment comprises a new area in radon research. Suitable detection methods are lacking and our knowledge of the plate out and alpha recoil deposition phenomena in a realistic indoor situation is poor.

The objectives of the project are to study the chain of processes which in the indoor environment leads from airborne radon to embedded long lived daughters and to reveal those exposure conditions in which the surface activity concentration of the long lived radon decay products is a useful estimate of lung cancer risk.
A new type of detector was developed for the alpha spectrometry of polonium-210. It is a lightweight open flow pulse ionization chamber (PIC) where the sample to be measured covers the opening of the chamber. Its alpha counting efficiency and energy resolution are comparable to a closed PIC, it is suitable for the nondestructive measurement of semiinfinite samples (sheets of glass) and it is applicable as a reference detector during field and in situ studies of polonium-210.
4 windows from a private house were investigated for polonium-210 inhomogeneity by placing the open PIC at different positions on the window panes. Relative to the centre value, there was a significant but only moderately enhanced surface activity in the upper part. In an effort to investigate the in house variability of polonium-210 on window surfaces, measurements were taken in different rooms of a house. 3 of the windows were used for ventilation purposes. These 3 windows exhibited a low polonium to radon ratio indicating that the room radon measurement is not relevant close to windows frequently used for airing.

The surface activity of glass samples taken from 22 detached houses was compared to the radon concentrations measured in the room from which the glass sample was taken. The coefficient of correlation was found to be 0.54. The spread in the values was due to several reasons:
inaccurate knowledge of the true radon exposure;
interhouse differences in equilibrium factors and plateout conditions at the same radon concentration;
different cleaning practices of the households.

The plateout experiments were carried out in a laboratory room with a volyme of approximately 120 m{3}. The radon concentration in the room can be monitored to levels up to approximately 5000 Bq m{-3}. The individual daughter concentrations and the unattached fractions can be measured by alpha spectroscopy and the use of multiple screens. The aerosol concentration is measured by a condenstation nucleus counter. During the plateout experiments track etch detectors were placed in 9 selected positions on the walls, floor and ceiling.

The 3 series of measurements had the following characteristics:
low aerosol concentration and no electric fields;
low aerosol concentration and electric fields from a high voltage ionizer mounted in the ceiling;
high aerosol concentration and no electric field.
Results were only obtained from experiments 1 and 3. These experiments have given the general plateout pattern for the room in its electrically undisturbed condition and the results confirm that the plateout rate increases with decreasing aerosol concentration.

The surface activity of polonium-210 on glass exposed in homes can be a measure of the long term exposure to radon decay products. The feasibility of autoradiographic track etch methods for measuring polonium-210 embedded in glass was investigated to find a practical method with adequate sensitivity and signal to background ratio to facilitate the estimation of long term exposure to radon decay products.

Experiments were performed on a variety of glasses. The plastic CR-39 detectors used were of a standard size (13 mm by 37 mm) with a thickness of 1.2 mm. In some cases mylar folio absorbers were placed between the glass and the detector. After exposure the detectors were analysed either by a standard evaluation technique or by a special technique using a specifically developed reading program which for all detected features stores 10 parameters related to size and shape. In the subsequent analysis different acceptance criteria were used to optimize the analysis.

If no absorber is used, there is a mixture of tracks of all shapes. The elongated tracks tend to originate at or near the glass surface. Thus by exclusively studying these tracks, many of the background alpha particles originating within the glass were excluded. If an absorber is used, the alpha particles that reach the detector must have sufficient energy to penetrate the absorber. Thus a proportion of the background alpha particles were excluded.

The results were analysed by computer and the detection limit was estimated at approximately 0.3 Bq m{-2}. The average radon level in Swedish homes is 60 Bq m{-3}. According to preliminary results on plateout rates this would result in a build up of polonium-210 of 0.06 Bq m{-2} per year on glass. Thus the glass would need to be exposed for at least 5 years to facilitate detection.

Theoretical calculations were performed to determine the fraction of absorbed polonium-210 which reappears at the glass surface due to the recoil energy. First, the depth distribution of polonium-210 was calculated from that of lead-210 caused by the alpha decay of deposited polonium-214 and that of lead-210 caused by the alpha decay of absorbed polonium-210. Then the probability of recoiling polonium-210 reappearing at the surface of the glass was calculated from the depth distribution of absorbed lead-214. The resulting probability is 29.8%.

The absorbed decay products are found in a thin layer of less than 100 nm. If the glass is not regularly cleaned, dust covers the glass and traps a fraction of the recoil nuclei. Experimental investigations showed that 15% of the deposited activity remains on the surface of glass when cleaned. This may be due to radon decay products forming chemical bonds to the glass or to dposition of the decay products into microcracks on the surface of the glass.

Theoretical calculations were performed to determine the fraction of the activity absorbed in the glass. The following conditions were assumed to be present:
ventilation rate 1 per hour;
surface to volume ratio 3 per metre;
deposition constant of the attached decay products in 1/100 of the deposition constant of the unattached decay products;
15% of the deposited activity is not cleaned away.
Values were taken for the attachment rate which are typical for rooms with and without aerosol sources and a range of values were taken for the deposition constant. The surface activity of polanium-210 was found to be 3% to 13% of the radon air activity. The surface activity is about a factor of 2 lower if aerosol sources are present. Turbulence can enhance the surface deposition.

A radon decay product plateometer has been developed to measure in real time the specific activity of short lived radon decay products. This new type of device uses the alpha track plastic CR-39 to measure alpha particles arising from the decay of the short lived radon decay products polonium-218, lead-214 and bismuth-214/polonium-214 which plateout from the air onto a glass surface.

A plateout measurement in a radon environment is carried out by rotating the glass surface, on which the radon decay products have reached steady state, beneath a 90 degree sector of CR-39. As the steady state activity on any part of the glass enters the shadow of the CR-39 it is no longer supported by plateout and it decays. The emitted alpha particles are recorded by the CR-39. The speed of rotation is set at one revolution per 6 hours so that the 90 degree CR-39 sector records over a period of 1.5 hours which approximately equals 2 half lives for the plateout activity.

On completion of an exposure the CR-39 is removed, etched and the distribution of alpha tracks on its surface is measured using an image analysis system. Considerable effort has been devoted to the development of software for this system such that ultimately every individual track on the plastic may be given a set of location and time coordinates. Image analysis counting times up to 8 hours may be needed for this and the subsequent processing of the data.

As an indicator of the performance of the plateometer, it has been found that, at an indoor radon concentration of 400 Bq m{-3} in laboratory air, an exposure time of about 2 weeks is needed to make accurate measurements of surface activities. Improvements in image analysis software are expected to reduce this time somewhat.
The study of the embedded long lived decay products of radonA222 is the only presently known method for measuring accumulated radon concentration levels from the past. The results from this investigation will be therefore of importance when assessing past and future indoor exposures to radon and its decay products and will also improve our understanding of the fate and behaviour of radon daughters in the indoor environment. The major potential application area of the long lived radon daughter method is radon epidemiology and as a number of these studies are now underway the development of this technique is very timely and appropriate.

Generally the work will be devoted to :
the development of detection methods for short and long lived radon decay products deposited on large surfaces (part A);
studies of the long term variability in the deposition rate of short lived decay products on macroscopic surfaces and the resulting removable and/or unremovable surface activity (part B).

Part A comprises developments mainly along three different lines.
An instrument based on the track etching technique for real time plate out measurements of short lived radon daughters will be developed by UCD and modified versions suitable to field work will be used by the other participants.
The feasibility of autoradiographic track etch methods for measuring PoA210 embedded in surfaces will be investigated. The image analysis facilities at SSI and UCD for analyzing alpha track densities, track shape and track size distributions over areas up to 15 cm{2} will be used. The alpha energy discrimination ability of these facilities will also be investigated.
Alpha spectrometry using pulse ionization chambers will be a reference method for the PoA210 analysis of large area samples. An open flow transportable ionization chamber will be developed by LU with support from DTH. In this new type of chamber samples can be measured in a nondestructive fashion.

The execution of part B of the project will initially rely on the existing facilities of the participating laboratories. During the programme period the equipment developed under part A will add to this existing battery of instruments. The work during will be focused on plate out exposures under well controlled laboratory conditions, comparisons of the experimental results with model calculations and the development of the nondestructive technique for the measurement of embedded PoA210. Cleaning studies in order to differentiate between adsorbed and absorbed surface activity will be performed by the RUG group with support from the CEN group.

Field studies in dwellings are also in the programme. The room to room variation of the surface PoA210 activity will be recorded in a few houses and the feasibility of the track etch autoradiographic technique will be tested in small scale studies.

The two year goal of the project is to reveal the usefulness of the poloniumAinAglass method, or any other similar technique involving long lived decay products, and to identify areas in which further studies are warranted.


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