A study of background radiation in the environment in Perth, Western Australia

Efendi Zainuddin

The aim of this project is to develop and apply simple and inexpensive techniques for assessing the total environmental radiation dose for residents of the Perth Metropolitan Area. This dose is assumed to come from a variety of sources including indoor Rn(222) and Rn(220) daughters, radioactivity of building materials, soils and drinking water. Techniques were developed to assess the contribution from each of these sources and a survey was carried out in the Perth Metropolitan Area to determine the annual effective dose equivalent from environmental radiation to members of the public. To measure indoor Rn(222) and Rn(220) daughter working levels, a flexible version of the two count method was developed and adapted for use with a simple " Radon Sniffer " based on a low speed pump and alpha counter. Before applying this method to analyse field data, it was calibrated and tested using standard samples prepared in the laboratory. A computer program has been written to analyse integrated observed alpha counts after sampling, based on the mathematical analysis in this study. The modified version of liquid scintillation technique developed by Prichard (1977) was used to determine the activity of radon, radium and other solid isotopes in groundwater collected from different parts of the Perth Metropolitan Area. Results of indoor Rn(222) and Rn(220) daughter measurements and radioactivity in water indicate that there is a significant correlation between indoor Rn(222) and Rn(220) daughter concentration and radioactivity in groundwater with the geological structure and composition of soils and rocks. The specific activity of building materials was measured by gamma spectrometry using a 2x2 cm Nal(TI) detector. From 8 different kinds of building materials measured, it appears that the radium and thorium contents are within the ranges found in other countries. Most of these materials have a radium equivalent within the standard limit. However red mud (bauxite processing residue) had a radium equivalent above the normal limit. This material is not commonly used as a building material in Perth. Based on the results of measurements of six radiation sources, the average total estimated annual effective dose equivalent for residents from these sources in the Perth Metropolitan Area was 2.3 mSv. Background gamma radiation could add another 0.5 to 3 mSv per annum. This is much more than the public limit for artificial sources (1 mSv) recommended by the ICRP (1990). 76 % of the average total dose is contributed by indoor Rn(222) and Rn(220) daughters with Rn(222) daughters contributing 47.4 % (1.2 mSv) and Rn(220) daughters 28.4 % (0.6 mSv) of the average total dose. Radioactivity of local building materials contributed approximately 19 % (0.4 mSv), while radon, radium and solid isotopes in groundwater contributed only 3 % (0.07 mSv), 0.3 % (0.006 mSv) and 2 % (0.04 mSv) to the average total annual effective dose equivalent respectively. The average total annual effective dose equivalent from all sources in this study could be overestimated since the sampling procedure was not truly random. Many of the samples of indoor Rn(222) and Rn(220) daughters were obtained from an area which contains granite and laterite rocks.

A study of background radiation in the environment in Perth, Western Australia

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