Absorbed Dose in Biological Specimens Irradiated Externally with Cobalt-60 Gamma Radiation

A persistent problem in radiobiology is the need to express the absorbed dose received by externally irradiated biological specimens of a variety of shapes and sizes when only the exposure (formerly called exposure dose) at some reference point can be conveniently measured. In the case of very small specimens or the desirable but less convenient circumstances in which larger specimens are homogeneously irradiated (e.g., reference 1), the application of conversion factors from exposure to absorbed dose taken from such texts as The Report of the International Commission on Radiological Units and Measurements (ICRU) 1959 (2) may be sufficient to describe the average absorbed dose to the specimen. However, when large specimens are inhomogeneously irradiated, neither the average dose nor the variations in absorbed dose within the specimen can be adequately described in this way. This problem is neglected in many radiobiological experiments, but in some cases it has been solved by detailed study of the particular exposure circumstances employed. The purpose of this experimental study is to provide data which can be directly used with sufficient accuracy for most purposes to determine the magnitude and range of the absorbed dose to biological specimens of varying size and shape irradiated with Co60 y-rays. The data may also be applicable, with only minor modifications, to other y-ray sources such as radium and Csl137. In radiobiological experiments the most convenient and widely used method of exposure is to place the specimen at a point in a large beam or field of y-rays so that the specimen is completely irradiated. The exposure rate at the point is most frequently measured in air with a calibrated ionization chamber. The specimen is usually placed with its center at the point of measurement and its long axis perpendicular to the field or beam direction. The information desired includes: (1) the average absorbed dose to the specimen;