Cyclotron Neutron and γ-Ray Dosimetry for Animal Irradiation Studies

Abstract

During the past year, an extensive animal irradiation program was conducted using the University of California 60-inch cyclotron of the Crocker Laboratory. A wide variety of biological end points were investigated. These included spleen and thymus weight loss, gut weight loss, mortality, hematological effects, and preprotection studies (1, 2). Paralleling these studies was a dosimetry program designed to obtain an accurate description of the radiation fields encountered. The basic problems involved in providing adequate neutron measurements for biological exposures have been reviewed by Moyer (3) and by Rossi (4). Two techniques available for evaluating energy absorption in tissue are (1) direct measurement of energy loss with tissue-equivalent ionization chambers employing the cavity principle and (2) an indirect approach involving measurements of neutron flux density and energy distribution. From considerations of scattering cross section and other physical data, neutron energy absorption in tissue can then be calculated. The method for measuring neutron dose in absolute units with ionization chambers has been developed by Gray (5). Earliest attempts to use ionization techniques for neutron measurements were made by Aebersold and Anslow (6), employing a Victoreen 100-r thimble chamber. Although these measurements did not give a true measure of tissue dose, they provided an arbitrary standard that could be used until better instruments became available. A tissue-equivalent ionization chamber, which allowed a direct measurement of absorbed dose, was developed by Rossi and Failla (7). Using modified chambers, Sheppard and Darden (8) have made physical measurements of the neutron and 7-radiation fields produced by the 84-inch cyclotron at the Oak Ridge National Laboratory. Ionization chamber techniques present the most straightforward approach to the neutron dosimetry problem, since they allow measurements to be made independent of the directional properties and spectrum of the incident flux. However.