Mineral sands processing involves exposure to external and internal radiation
sources. The level of exposure is associated with the production of monazite, which contains
approximately 6% thorium by weight. External radiation levels may range from less than
1 µGy h-1 to 10 µGy h-1 in the general plant environment, to greater than 150 µGy h-1 in
monazite storage areas. Internal radiation may also be significant since airborne gross alpha
activity levels from less than 0.05 Bq m-3 to about 5 Bq m-3 are found. Recent estimates of
radiation exposure indicate that approximately 15% of workers in the industry exceed
15 mSv y-1. A small percentage of workers are estimated to be receiving radiation doses
approaching or exceeding the statutory 50 mSv y-1 limit. Whilst the majority of the radiation
dose is contributed by inhaled radioactivity, recent reviews of ICRP models and data reveal
that the assessment protocols and default values used in internal dose estimation may be
overly conservative, possibly by more than an order of magnitude. The basis for present
assessment procedures is briefly reviewed, as are the major areas of uncertainty in internal
dose estimation protocols. Priorities for radiation research are suggested and the practicability of obtaining better estimates of internal radiation dose is discussed.
The measurement of the alpha particle activity of the short half-life descendants of radon (218Po, 214Pb and 214Bi) in a sample of the mine atmosphere, is an integral part of determining the exposure of uranium miners to airborne radioactivity. Methods employed to date commonly use the total number of counts in one or more time intervals to estimate the activity. Such methods may attempt to determine the activity of each of the short-lived descendants present (Thomas 1972, Tsivouglou et al. 1953), requiring three counting periods, or may attempt to minimise the inherent errors in estimating the total potential alpha particle energy from a single count (Rolle 1972, Kuznetz 1956). Such counting techniques inevitably discard some of the information present in the counts from the alpha particle detector. An optimal method for the determination of the activities of the important descendants of radon which utilises the times of arrival of the alpha particles from the decay of the descendants of radon, has been devised and tested. The method consists of fitting the theoretical total count function to the experimental total alpha count, by a method of least squares, using the numbers of atoms of each of the descendants of radon in the sample at the start of counting as the parameters of the best fit. The method is shown to be accurate and the statistical uncertainties of the results have been investigated for a variety of total counting times, counter efficiencies, total potential alpha energy present and equilibrium ratios between the descendants of radon. The principal advantage of the method is that it would allow, in principle, real time evaluation of the concentrations of the three important descendants of radon and the total potential alpha energy, as the counting proceeds. It is suggested that the method may be suitable for instrumentation as the calculations required are reasonably simple and achievable with modern microcomputers. This optimal method is compared with two commonly used methods for the determination of the total potential alpha energy of the descendants of radon.
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