Attenuation factors for certain tissues when the body is irradiated omnidirectionally

Experimental values of mean attenuation factors are reported for certain tissues in an anthropomorphic phantom irradiated omnidirectionally with gamma rays from sodium-24, radium-226, iodine-131 and xenon-133. The data are used to relate the mean absorbed dose to the whole body, bone marrow, gonads and skin to (a) the absorbed dose in air and (b) the exposure in air. Corrections are made for the degradation of the radiation by scatter and the photoelectron enhancement effect. It is proposed that for natural background gamma rays a mean attenuation factor of 0.7 may be used for bone marrow. The corresponding rad/R factor is 0.67. It is also estimated that the effective mean depth of bone marrow for omnidirectional irradiation is 5.0-5.5 cm. (author)