Radiation Dosimetry. Part II

Abstract

The practical problem of Radium Dosimetry is simpler than the corresponding X-ray problem for two reasons, namely:— (1) Whereas no two X-ray tubes can be depended upon to give the same output under the same working conditions, the γ-ray intensity from a given amount of radium, with specified filtration, is a constant that may be depended upon to a few per cent.* (2) In a large number of cases, tissue absorption may either be neglected or is easily allowed for, and the calculation of dosage becomes a purely geometrical problem once the dosage rate at a given distance from a given quantity of radium is known. These considerations have induced a large number of physicists to attempt to measure the dosage rate at 1 cm. from 1 mgm. of radium in terms of the X-ray unit, i.e., in röntgens per hour. The large range of numerical values for this quantity to be found in the literature has very naturally been responsible for a general lack of confidence in the results of small ionisation chamber measurements. The present definition of the rontgen, indeed, makes the task practically beyond the resources of the ordinary physics laboratory, since the large “open-air” chamber is virtually specified as the measuring instrument. An open-air chamber of adequate dimensions has, however, recently been constructed at the National Physical Laboratory, and has yielded results agreeing reasonably closely with small chamber measurements made in the same laboratory.