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Abstract

This thesis describes the design and construction of an instrument for comparison of x-ray doses at two different points in an x-ray field. Essentially, it consists of two ionization chambers each fed into a very stable, linear, d.c. amplifier. The outputs of the d.c. amplifiers are coupled in such a way that a fraction of the output of one amplifier is balanced against the total output of the other in a ratio circuit. The scale of the ratio circuit may be set to read directly the dose at any point as a percentage of the dose at an arbitrarily chosen reference point. This percentage is independent of fluctuations of the x-ray intensity with time. The instrument is battery-operated, light in weight and therefore easily portable. The principal use of the instrument is for the comparison of x-ray doses at different points in a tissue-equivalent medium — information required for dose estimates in radiotherapy. The accuracy of the instrument is better than one half percent of the maximum dose in the x-ray field. The instrument can be modified for automatic plotting of "isodose curves".