New technical approach to monitoring of low-level environmental gamma radiation using NaI(Tl) scintillation instrument.

概要

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The environmental dose evaluation around a nuclear facility is usually performed by means of calculational methods on the basis of effluent monitoring at a release point. It has been desired to develop practical techniques of direct monitoring in the environment, because the Japan Atomic Energy Commission set up 5 mrem/yr as an objective dose value for the whole-body exposure due to normal operation of LWRs on a single site.<BR>In evaluating gaseous plume doses less than 5 mrem/yr in routine monitoring, there are many difficult problems to be solved. We have developed two techniques for separation of the plume exposures less than 1 mrem/yr from the background radiation, using a NaI(TI) scintillation counter having flat energy response.<BR>One technique is based on the difference in energy spectrum distribution between plume gamma and natural gamma radiation. And, separation of the respective exposures is made by analysis of the two informations obtained by a two-channel monitoring system. Another technique is to reduce the natural gamma radiation components by using a special concave lead shield for the Nal (TI) scintillation probe. A combination of the techniques can much improve the efficiency of separation of 41Ar plume radiation from fluctuating background radiation.
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