of 108mAg in Samples from Bikini and Eniwetok Atolis Date of collection Concentration of 1°8™Ag (d.p.m./g 0.20 + 0.04 F 3) reas) 9) reas) it June 1969 1.140.3 June 1970 0.75+0.15 July 1969 0.09 + 0.05 a August 1964 * 20) * reas) eer i dry wt) fies the numberof individuals comprising sed. Species: Panuflirus sp. 7° confidence level of the count rate 1 was taken from the site of a large in 1954 (Bravo Crater), the crater of lagoon and sea water and which is ‘Ag for the biota of the area (71°"Ag vetween 1954 and 1969). Thermoin the atoll have also produced silver st at Bikini was in 1958, yet we have ntration of 5.2+0.06 d.p.m./g of soil :toH and thus the amount of ?°8™Ag n and elsewhere as a result of pre-1959 the initial production activity | may be conservative in its As we have said, the initial was assumed to be due only stable silver. There are other yr these two radionuclides, the most yns on stable silver ('°8™Ag) and (n,p) m (1°8™,110mM Ag) The cadmium reactint because the quantity of stable cadould probably be kept to a minimum 1ermal neutron cross-sections. The (n,p) reactions at atomic weights 4tron energies between 10-20 MeV Ulibarns) by contrast with the excitareactions at atomic weights > 100 ‘gies, Which are significantly higher in rather shallow isolated water masses for several years', then we ® about n ref. iction. °8™ Ao tion of We haveirradiated stable silver in a fast neutron beam at the University of Washington cyclotron [*Be(d,n)'°B] and have produced easily measurable quantities of 1°°™Ag ['°’Ag(n,2n) 166mA5] and 1°°Ag ['°’Ag(n,3n)'°>Ag]; '!°8™Ag has not been found so far in the irradiated sample. The energy similarities of the gammarays emitted by '°°™Ag andthe very low specific activity of '°®™Ag will require several months decay time before the presence or absence of '!°8™Ag can be confirmed. The (n,y) cross-sections for '°’Ag and !°°Ag (based on a radiative capture initial activity ratio of 162) seem, however, to be almost equal for 11°™Ag/!°8™Ag and so the nuclear states of the two isotopes seem to be similar. It would be surprising if ?°?"Ag production by (n,2n) reactions on '°°Ag were inconsequential, for (n,2n) reactions on 1°’Ag have been shown experimentally to produce relatively large amounts of 1°°™Ag. Production pathways are important, because the type and design of a nuclear device can generate different ratios of fast and thermal neutron fluxes. lt is difficult to resolve our findings with the close agreement previously shown! between silver radionuclide activity ratios and the time of large scale nuclear testing. If it is possible for the silver radionuclides produced in 1961 and 1962 to remain the same possibility must be extended to silver radionuclides produced before 1959. But the absence of definite knowledge of the rates at which silver is removed from the surface layers of the ocean makes it impossible to decide whether or not 198M Ag from pre-1961-62 testing confuses subsequent dating. The determination of ''°™Ag/'°8™Ag ratios in air filters from the time period of interest would clarify the production activity ratios, and the analysis of undisturbed lichen samples collected annually since 1958 (and earlier) would elucidate the relative amounts of '°8™Ag caused by pre-1959 tests and those in 1961-62. The potential usefulness of the ''°™Ag/'°8"Ag ratio in dating is apparent but we feel that some caution should be exercised in their use for the description of natural processes until the outstanding problems are resolved. We thank C. W. Vick for technical assistance and Dr W. Weitkamp for help with the cyclotron irradiation. The work was supported by the US Atomic Energy Commission, T. M. BEASLEY * FE. E. HELD Laboratory of Radiation Ecology, College of Fisheries, University of Washingten * Present address: Division of Biology and Medicine, US Atomic Energy Commission. Received November 2, 1970.