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COSMOGENIC NUCIJDES IN THE HAMLET METEORITE Anthony Turkevich Enrico Fermi Institute for Nuclear Studies University of Chicago The Hamlet meteorite, which fell at Hamlet, Indiana, October 13, 1959, is a small (~2. 5 kg) chondrite, apparently quite ordinary, of roughly pyramidal shape with an approximately square base. Gross gamma-ray spectra were taken on a portion of the whole meteorite, using a 3" x 3" crystal and 200 channel analyzer. In the spectra taken two weeks after infall, a 500-Kev peak and an 800-Kev peak were clearly discernible, along with a number of less prominent peaks. Gross spectra taken later indicate that the 800-Kev peak decays appreciably with time, while the 500-Kev peak is essentially constant. Under the assumption that the point of the pyramid defines the front of the meteorite during infall, analyses of various cosmogenic nuclides were made and are in progress on material from the base, presumably near the original surface. Preliminary data is available on 16-day V4S, 26-day Cr51, the sum of 77-day Co56 and 71-day Co58, 267-day Co57, ~300-day V49, and 300-day Mn54 activities. The "absolute" activities for the V, Cr, Co and Mn nuclides mentioned range from about 5 to about 100 dpm/kg. The tentative nature of this data does not warrant a detailed listing of the numbers, which will be published later in final form, but some comments on ratios of activities may be made. To facilitate our understanding of the production of the cosmogenic nuclides in Hamlet, a proton irradiation of a 5. 2 g sample of the meteorite was performed at the Brookhaven cyclotron, using the external beam. The sample was mounted directly behind a six-inch thick block of iron; a smaller block of iron was placed behind the sample. According to Na24 activity produced in Al foils, a dose of ~1014 protons, nominally 3 Bev in energy, was received by 'hot' Hamlet. The sample was then fused in the course of extracting noble gases, so that the silicate and iron phases were separated. The gamma-ray spectra of each of these phases were ex- amined, and identifications of the various peaks observed were attempted. It appears that all the Co activities are in the iron phase, although a small fraction of these nuclides must have been formed from oxidized iron in the silicate phase. Peaks assignable to Cr were seen in both phases, indicat- ing a partitioning of this element, while an intense peak, presumably in 51
large part due to annihilation radiation from Na22 and to Be7 gamma radiation, was observed in the spectrum of the silicate phase. Apparently, extraction of trace elements into phases appropriate to their lithophilic or siderophilic character was reasonably complete during fusion of the sample. If one compares the activity ratio of Co57/(Co56 + Co58), corrected to time of fall, in Hamlet with the same ratio in the 'hot' Hamlet, an in- teresting discrepancy comes to light. This activity ratio in the cosmic - ray irradiated specimen is about 1. 3 times that in the sample irradiated in the Brookhaven cyclotron. If the cyclotron bombardment represents a valid duplication of the conditions of cosmic-ray irradiation (see the paper by Schaeffer in this volume for comments on this assumption), one would expect that, for a constant long-term cosmic-ray bombardment, the ratio in Hamlet should be~4 times the value in 'hot' Hamlet. This conclusion is derived from a simple consideration of the effect of saturation of the activity levels. There are a number of ways of interpreting this discrepancy. Per- haps the most likely hypothesis is that the cyclotron irradiation, for one or another of the reasons discussed by Schaeffer, is not a good "mock-up" of the cosmic irradiation. Another possibility, however, is that the cosmic irradiation of Hamlet was more nearly an instantaneous irradiation than a continuous one. Before any serious discussion of these hypotheses can be undertaken, much more data must be gathered. Note especially that the production of Co isotopes from Fe or Ni has a low energy- threshold, so that production by secondaries may be important. Also, the effect of working with material which was presumably quite near the pre- atmospheric surface should be given detailed consideration. Finally, the cosmogenic Co isotopes may have been in large part produced by meson interactions with Ni, which would make comparison with the cyclotron irradiation even more questionable. Nevertheless, it is clear that this approach is potentially capable of yielding important information on cosmic-ray fluxes and their variations with time and in different regions of the solar system. 52
