The interpretation of solar system abundances at the N = 50 neutron shell

Abstract

New data on CI chondrite abundances demonstrate a high degree of smoothness for the A= 75-100 mass range for odd A nuclei, except for a single element peak at Y ascribable to the s-process peak for the N = 50 neutron shell. Literature estimates of s-process abundances, N., permit a smooth N. curve to be drawn; however the resultant "non-s" abundance curve (nominally r-process) does not show a peak analogous to peaks associated with the N = 82 or 126 shells. However, both the nominal N. and "non-s" abundance curves show similar rapid increases below mass 70. It is more reasonable to ascribe the "non-s" rise as an artifact from relatively small differences in the N. and total abundances, indicating that a relatively broad r-process peak is indeed present. The solar system even A abundances in this mass region are not smooth but show a saw-tooth structure which is also reflected in neutron capture cross sections, indicating that the saw-tooth is an s-process feature. The r-only even A nuclei define the r-process peak assuming that it is smooth. Assuming the systematics of the r-process even and odd A abundance peaks at the N = 82 and 126 shells apply to N = 50, the odd A r-process peak for N = 50 can be obtained, which in turn permits a new calculation of N. for odd A. The new N, is relatively smooth, but, contrary to expectations, the product of N. and the neutron capture cross section is not a smooth function of A, but contains structure, especially a rise between masses 82-84, which is not compatible with an exponential distribution of neutron exposures.

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