Standardization of ⁶⁰Co and ¹³⁷Cs Gamma-Ray Beams in Terms of Exposure

At the National Bureau of Standards (NBS), the exposure-rate standards for ⁶⁰Co and ¹³⁷Cs gamma rays were based for a number of years on a weighted average of measurements using a cylindrical ionization chamber and a group of small spherical chambers. Complex setup conditions for the cylindrical chamber, differences between the cylindrical and spherical chamber data, and recognition that the institution of this weighted average exposure-rate standard increased the difference between free-air-chamber and cavily-chamber measurements, led to the development of new spherical chambers. All correction factors for exposure-rate measurements were investigated and updated. Excellent agreement was achieved between independent exposure-rate measurements for six spherical chambers and, as of May 1, 1972, the exposure standards were reduced 0.7 percent for ⁶⁰Co and 0.6 percent for ¹³⁷Cs gamma rays. Recalculation of correction factors since that time indicates that the standard ¹³⁷Cs should be further reduced by 0.2 percent, and this adjustment was made on July 1, 1974. The uncertainties associated with each of the quantities entering into the determination of exposure rate were tabulated and the overall uncertainty of the exposure rates used for instrument calibrations at NBS was found to be about 0.7 percent for addition in quadrature.