A Large number of investigators (Mayneord and Lamerton 1941; Quimby, Marinelli and Farrow, 1938; Oliver and Kemp, 1949; Exner and Packard, 1945; Braestrup, Cameron and McClement, 1949; Mabbs, 1949) have measured the surface back-scatter for radiations in the standard X-ray therapy range. Some of these results are difficult to compare because of difference in the parameters used. However, a few of the results obtained in the previous surveys are shown in Table I together with the values obtained in this investigation. It is seen that variations between extreme values is of the order of 6 per cent. The investigation of back-scatter has been under way for three years in Saskatoon, and preliminary results obtained in 1951 are included in The Physics of Radiation Therapy (Johns, 1953). These data are also shown in Table I. Since 1951 considerable refinement in the experimental technique has been achieved and a number of factors affecting back-scatter have been investigated. In the present paper extensive measu...
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