The Radiation Field near a Flat Applicator

Abstract

On reading Dr. Robertson's paper it seemed that physical measurements of the radiation received by the organism might be of value in correlating the results with those of other observers, and we therefore undertook to make such measurements. As already stated, the applicator consisted of two silver discs between which were held 400 mgm. of radium in a circular area of 3·5 cm. radius. The two hundred radium cells were unfortunately not fixed in position, but, as will be seen later, little movement was likely under normal conditions as they could not pass over each other, and were fairly tightly packed. Photographic films placed in contact with the applicator when in (a) horizontal and (b) a vertical plane showed that little movement of radium took place on holding the disc vertical, probably not enough to alter appreciably the average value of the dose received by free swimming organisms at the distance at which they were normally irradiated in these experiments. The depth of liquid in which the protozoa were moving was approximately 5 mm., the mean plane of the liquid being at nearly 6·5 mm. from the mean active plane of the radium. Measurements were made with the system already described in this Journal,1 the normal ionisation chamber being replaced by a smaller cylindrical one, 5 mm. deep. The centre of the chamber was 6·5 mm. from the plane of the radium, the chamber therefore corresponding to the position of the medium inhabited by the organisms.