Dosimetry for Large Animal Experiments Using Multiple 60Co Sources and 1 MVp X-rays

Abstract

A quadrilateral configuration of four movable uncollimated 60Co sources with a total activity of 9000 c was used to obtain exposure measurements in air and depth doses in a masonite phantom for radiation experiments involving sheep. The depth dose distribution in the phantom in the four source exposure is compared to distributions obtained using a single collimated ⁶⁰Co source four times in succession to simulate the four source array and exposure geometry, and to 1 MVp X-rays in a bilateral exposure. Distributions of the quadrilateral gamma and X-ray exposures differed quantitatively by no more than 5 per cent throughout the phantom thickness. Ionization measurements were made in outdoor pens built to house 100 sheep individually during chronic low-level exposures ranging from 500 mR/hr to 4 R/hr. Exposure rates at the two rows of pens were varied by using several source arrangements. Bilateral depth dose distributions in a sheep phantom exposed in the pens showed that the radiation from the uncollimated sources at the distances of the pens was less penetrating than from a collimated ⁶⁰Co ource at a closer distance (corrected for inverse square effect), and more penetrating than from 1 MVp X-rays except at the phantom surfaces.