Comparison of Exposure Standards for⁶⁰Co Gamma Rays

Comparisons for 60 Co gamma rays between the exposure standards of the Bureau International des Poids et Mesures (BIPM), the National Bureau of Standards (NBS) and the Physikalisch-Technische Bundesanstalt (PTB) are reported. For measurements made at about one meter from the source, and if the same physical constants are used, the differences are 0.26% between the BIPM and NBS standards, and at most 0.4% between the BIPM standard and the PTB standard chambers. The differences are consistent with the estimated systematic uncertainties (≈0.5%). The difference between the NBS standard and the PTB chamber measurements, as determined through the ratios of each to the BIPM chamber, ranges from 0.17 to 0.66%. The difference in exposure standards for NBS and PTB actually ranges from 0.48 to 0.97%, since different stopping power corrections are used by these two laboratories. An indirect comparison of the BIPM and NBS standards indicates that the difference of 0.26% will increase to 0.55% at a distance of two meters.