A cobalt-60 primary dose spread array derived from measurements

The dose contributions arising from primary photon interactions in the region surrounding a point in a body exposed to a cobalt 60 gamma ray beam were measured. A hollow spherical shell serves as the interaction medium which releases electrons which are detected by a thin walled ionisation chamber. The shell is composed of concentric sector pieces, each of which can be mounted separately in its proper geometrical position. The results are expressed in the form of a dose contribution array. The dose contribution array is converted into a dose spread array by a simple application of the reciprocity principle. The results are compared with published results of Monte Carlo calculations. In general, the agreement is good but in some respects there are significant differences which are briefly discussed.