Geometry and Barrier Attenuation of Cobalt-60 Gamma Rays by a Vertical Steel Slab

This experiment was designed to test the barrier and geometry attenuation factors for ⁶⁰Co gamma rays developed from moments-method calculations by Spencer as given in National Bureau of Standards Monograph 42. A set of vertical, plane steel barriers was employed. Selection of the detector distance from a given barrier and the degree of collimation permitted exposures to be measured as a function of the solid angle subtended by a constant circular area on the barrier. The effective mass thickness of the barriers ranged from 0 to 73 lb/ft² and the solid angle subtended at the detector varied from 0.2 to 5.5 sr. A uniform plane radiation field was simulated by a traveling ⁶⁰Co source that was pumped through plastic tubing that covered a 100-ft semicircular area. Extrapolation of the experimental data gave estimates of the exposures to be expected from an infinitely extended field. The final results for a collimated detector, located behind a steel barrier, were normalized to the free-field exposure received by a detector located 3 ft above the extended field. The experimental values were compared to a family of curves based on calculated results. For all cases except those for relatively small solid angles, the agreement was within 20%.