Measurement of Tissue-Air Ratios and Scatter Functions for Large Field Sizes, for Cobalt 60 Gamma Radiation

Abstract

Currently available data for tissue-air ratios and scatter functions for ⁶⁰Co radiation have been extended to large field sizes and depths of 30 cm following ionisation chamber measurements in a water phantom. The measurements were made using circular fields of radii up to 22·5 cm. A small systematic error in currently published data has been noted and the results are presented first in the form of a table of scatter functions for field radii of 2 to 25 cm and a source-to-surface distance of 80 cm. A more versatile form for scatter data, independent of source-to-surface distance, is suggested and a corresponding table of values is presented. Finally, tissue-air ratios for square fields ranging from 4 × 4 to 35 × 35 cm have been prepared using the Clarkson method. The data for field sizes larger than 20 × 20 cm and depths greater than 20 cm are new, while the remainder represent previously published data slightly amended. More detailed tables are available on request.