(Reprinted from Nature, Vol. 224, No. 5219, p. 573 only, November 8 1969) 407869 Lead-210 Production by Nuclear Devices: 1946-1958 THE possible production of #4*Pb during weapons testing by the reaction **Pb (2n,y) 7°Pb was suggested by Peirson et al.t, who observed unusually high ratios of 219Pb in tropospheric air in 1962 and 1963, and by Jaworowski*?, who reported increased *°Pb concentrations in lichens and deer antlers in 1958-59 and 1962-63. On the other hand, Bhandari et al.? and Crozaz‘ found no corresponding increase in the amount of this isotope in air or glaciers, and Krey® reported no unusually high lJevels of ?4°Pb in the stratosphere in 1966. Resolution of this conflict is important because it has been postulated* that nuclear devices used in excavating a canal.in Central America would produce #!°Pb in amounts comparable with 8Sr—-°Y as a radiological contaminant. If #°Pb is produced in significant amounts during nuclear or thermonuclear explosions, it would be present at former test sites at Bikini and Eniwetok Atolls. The presence there of ?°’Bi (ref. 7) probably formed by reactions such as #°?Pb (p,n), *°’Bi or 7°°Pb (p,y), 2°7Bi rather than by the ?°*Bi (n,3n), *°7Bi reaction, suggests that stable lead was present in shielding or structures adjacent to the nuclear devices. I therefore determined the #4°Pb content of soil and sediment samples from areas of high radioactivity in the Pacific Proving Ground and, for comparison, samples from areas of the Pacific with negligible fall-out. Three of the samples contained 207Bi. Biological samples were not included in this study because the natural levels of 7°Pb and ?1°Po are not well established in tropical biota®; therefore, results would be equivocal, for marine organisms concentrate both ??°Pb and ?}°Po. The samples were taken from locations contaminated with local or intermediate fall-out except for those from Japtan Islet and Palmyra Island. The most radioactive samples were sediments taken from craters produced by test detonations. The soil samples from Kabelle Islet, Rongelap Atoll, were contaminated with fall-out from a thermonuclear device detonated at Bikini Atoll, 80 miles to the west, in March 1954. Gamma-dose rates three feet above the ground at Kabelle Islet were about 20 Rh-! on D+1. The naturally occurring concentrations of #4°Pb in the island soils and sediments can be estimated from published data. Broecker* measured the ?**Ra content of cores drilled at Eniwetok Atoll. Surface samples contained we