Studies on the Dosimetry and Bactericidal Effects of Gamma Radiations from a Cobalt⁶⁰Source

Since 1943 the Department of Food Technology at the Massachusetts Institute of Technology has been conducting studies on the effect of ionizing radiations, produced by particle accelerators, on microorganisms. The object of these studies has been to investigate the fundamental changes involved and, if possible, to establish a basis for ascertaining the feasibility and the practicability of sterilization of foods without the application of heat. Since 1950 research has been underway in these laboratories to evaluate the possibility of utilization of waste fission products as sources of radiation. As a source of ionizing radiations a kilocurie of cobalt60, a “mock” fission source, has been prepared by the Reactor Division at the Brookhaven National Laboratories. The use of cobalt60 for experimental studies is predicated at this time on (a) the ready availability of cobalt60 in multicurie source form and (b) its desirable and well defined physical characteristics of energy, half-life, and the like. As this investigation progresses it is hoped that the actual concentrated fission by-products may be available in the near future for experimental use. The cobalt60 used in this investigation has been well described by Manowitz, in 1951, and will not be described in any great detail here except to state that it is a hollow cylinder of cobalt 13.5 inches long and 2.3 inches in outer diameter, covered by aluminum. The entire source is encased in lead, as indicated in Figure 1. Dosimetry of Source The source has been calibrated by three different technics, which will be described in greater detail in a separate publication. The three dosimeters used for the calibration are (a) an adiabatic calorimeter of the type described by Hochanadel and Ghormley (1952), (b) a methylene blue dosimeter as described by Goldblith et al. (1952), and (c) a ferrous-ferric dosimeter described by Miller (1950), Hardwick (1952), and Weiss (1952). The adiabatic calorimeter was designed and built by Dr. Davison and is illustrated in Figure 1. Essentially it is a glass vial of a physical arrangement that duplicates the actual geometry of the samples to be irradiated. The vial containing purified water is thermally insulated. Five calibration runs have been made with this dosimeter, and the results show close agreement. In addition, dosimetry has been effected by use of the destruction of methylene blue (Goldblith, Proctor, and Hammerle, 1952) and by the use of the ferrous-ferric reaction. The dose rates, as determined, are summarized in Table I. These data show that the dose rates as obtained from the kilocurie cobalt60 source fall within the limits of 2,100 ± 300 rep83 or 1,900 ± 300 rep933 if neither of the two assumed values for ferric ion yield indicated in the table—15.4 µM according to Weiss (1952) and 20.2 µM according to Miller (1950) and Hardwick (1952)—is preferred.