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A new caesium 137 beam unit head designed by personnel of the United Kingdom Atomic Energy Authority is described together with modifications to the original control circuit. The activity of the source at installation was 1050 curies, giving a dose-rate in air with a 7 × 7 cm field of 21·8 r/minute at 45 cm from the source (March, 1958). Physical measurements reported include an assessment of the room and head protection, H.V.L.'s, ionization build-up, back-scatter, variation of dose-rate with field size and central axis percentage depth doses. Measured isodose curves are being made available through the Diagrams and Data scheme of the Hospital Physicists' Association.
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Burns et al. (1959) studied this question.
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