Tissue-air ratios for narrow 60Co gamma-ray beams.

This study introduces a table of tissue-air ratios (TAR) for narrow 60Co gamma-ray beams. The table is consistent with recently published TAR data for broad 60Co gamma-ray beams [Table 4.1, Br. J. Radiol. Suppl. 25 (1996)]. Narrow-beam TARs are derived analytically from broad-beam data of Table 4.1 and are tabulated for circular fields ranging from 0.2 to 2.2 cm in radius--an approximate equivalent of a 0.4 cm x 0.4 cm to 4 cm x 4 cm square-field range. The extent of depth is from 0.5 to 30 cm in water.