Loftus T P, Weaver J T
Institute for Basic Standards, National Bureau of Standards, Washington, D.C. 20234.
J Res Natl Bur Stand A Phys Chem. 1974 Jul-Aug;78A(4):465-476. doi: 10.6028/jres.078A.030.
At the National Bureau of Standards (NBS), the exposure-rate standards for Co and Cs 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 Co and 0.6 percent for Cs gamma rays. Recalculation of correction factors since that time indicates that the standard Cs 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.
在美国国家标准局（NBS），多年来钴和铯伽马射线的照射率标准是基于使用圆柱形电离室和一组小型球形电离室的测量加权平均值。圆柱形电离室的复杂设置条件、圆柱形和球形电离室数据之间的差异，以及认识到这种加权平均照射率标准的制定增加了自由空气电离室和空腔电离室测量之间的差异，促使了新型球形电离室的开发。对照射率测量的所有校正因子进行了研究和更新。六个球形电离室的独立照射率测量之间达成了极佳的一致性，截至1972年5月1日，钴伽马射线的照射标准降低了0.7%，铯伽马射线的照射标准降低了0.6%。自那时以来对校正因子的重新计算表明，标准铯应再降低0.2%，这一调整于1974年7月1日进行。列出了用于确定照射率的每个量的相关不确定度，发现美国国家标准局用于仪器校准的照射率的总不确定度在平方和相加时约为0.7%。
