A Dose-rate Ratiometer

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Abstract
The need for such an instrument arose in connection with the production of X-ray depth dose charts for the 4 MV linear accelerator. The measuring equipment to hand was the Baldwin Ionex Mk3 with the associated 0·6 c.c. ionization chamber, movement of which in a water phantom was remotely controlled by magslips. Dose-rate readings of the Ionex could only be made when the accelerator output was maintained at a particular value, and although actual output fluctuations were small, continual adjustment of the gun filament current was necessary. Moreover, the information required to produce isodose curves was obtained only after computation and plotting of the dose-rate readings. These difficulties were overcome with the ratiometer working in conjunction with the accelerator's build-in dose-rate meter, which acted as the monitor chamber. An alternative was to construct a comparatortype instrument (Kemp, 1945) but this would not only have been more costly, but would not have made use of the existing equipment. A...

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