Correspondence

Abstract

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In view of the increasing interest now shown in the magnitude of the natural background radiation to which man is subjected, determinations have been made of the background counting rate of a Geiger-Muller tube in some hundreds of situations during the past three years. The counting rates have been determined by aural counting with a light-weight portable apparatus incorporating a G10H γ-ray tube with a chrome-iron cathode of effective length 11·8 cm. A standard error of between 5 and 10 per cent is achieved by this method. The counter, shielded from light by black paper and a canvas carrying bag, includes in its response some contribution from energetic β particles. A comparison with and without a 1 mm steel sheath indicated that in typical circumstances about two-thirds of the response above the cosmic-ray counting rate was due to hard γ radiation.