Elimination of 248Cm as a possible progenitor of carbonaceous chondrite fission xenon

Abstract

It has been suggested by various authors that spontaneous fission of 248Cm may be responsible, at least in part, for the heavy-isotope anomalies, known as the CCF component, in xenon extracted from carbonaceous chondrites. In order to test this hypothesis, we have measured the isotopic spectrum of xenon fission products from a 97% isotopically pure 248Cm sample. The products of spontaneous fission of 248Cm were found to be mixed with the products of heavy-ion-induced fission reactions. The latter component was produced in a prior use of the 248Cm sample as a target for a heavy-element synthesis experiment. Fortunately, the induced fission component accounts for only ∼10% of the observed Xe yields, and its composition can be estimated sufficiently well to allow a fairly precise determination of the isotopic composition of Xe from spontaneous fission of 248Cm. The data also allow two other estimates of the spontaneous fission yields, free of the induced fission component. The first is from a small amount of xenon released at 500°C from the aluminum foil enclosing the sample, and the second is from a reheating performed following a one-month accumulation of new spontaneous fission products. After fairly large corrections for incomplete decay of precursors to 131Xe and 132Xe, these independent estimates of the spontaneous fission yields are in reasonable agreement with the estimates based on the initial outgassing of the sample, corrected as described for the induced fission component. The best estimate of the composition of Xe from spontaneous fission of 248Cm is as follows: 134Xe/136Xe= 1.065 ± 0.015, 132Xe/136Xe= 0.81 ± 0.03 and 131Xe/136Xe= 0.49 ± 0.06. These results are totally inconsistent with any of the various estimates of the CCF composition, implying that if spontaneous fission of 248Cm contributed at all to the CCF component, the contribution was relatively minor.