Surface Doses in the Cobalt-60 Facility at Armed Forces Radiobiology Research Institute.
ARMED FORCES RADIOBIOLOGY RESEARCH INST BETHESDA MD
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The surface doses in the Cobalt-60 Facility at the Armed Forces Radiobiology Research Institute were measured using a Capentec Model PS-033 0.5 cm cu parallel plate ionization chamber. With the source in the unilateral mode, the dose in the buildup region 0-5 mm depth was measured and compared to the equilibrium dose for exposure rates ranging from 5 to 3500 roentgenminute. It was determined that the usual skin-sparing effect of cobalt-60 beams does not exist in the AFRRI Cobalt-60 Facility in fact, the dose at a depth of 1 mm was as high as 137 of the equilibrium dose. The surface dose increased relative to the equilibrium dose as the source was moved closer to the chamber, indicating that the increased skin dose is due to the secondary electrons arising within the cobalt-60 structural components. These dose gradients in the buildup region should be considered when using the Cobalt-60 Facility for experimental irradiations.