237Np/239Pu atom ratios in integrated global fallout: a reassessment of the production of 237Np

It is estimated that as much as 3 tons of 237Np (T1/2 = 2.14 × 106 years) was released to the environment during the above-ground testing of nuclear weapons. This estimate is based on the analysis of 237Np/239Pu atom ratios (0.7 ± 0.2) in soil samples believed to have captured the entire global fallout record. In the course of several studies dealing with the behavior of transuranic radionuclides in different ecosystems, we measured the 237Np/239Pu atom ratios in a variety of soils throughout the northern hemisphere, including repetitive analyses of the soils used to establish the ratio cited above. Our data argue for a global 237Np/239Pu fallout ratio near 0.45 and a total 237Np production of ˜1.5 tons.