Low dose human calcium assay in vivo via the $sup 40$Ca(n,$alpha$) reaction

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of the Ar/sup 38/ ground state. Even in the absence of isotopic spin selection rules, the alpha transition to the O/sup +/ state is expected to be inhibited appreciably by selection rules on angular momentum and parity. A calculation using the statistical model of the nucleus indicates that these decrease the cross section for the 0/sup +/ yields O/sup +/ transition by a factor of about 5 relative to that for the ground state transition. It thus appears that the isotopic spin selection rule inhibits the transition only by a factor of about two. This breakdown of the isotopic spin selection rule can probably be attributed to coulomb effects in the component state. The reduction of the cross section as a result of angular momentum selection rule is probably equally important in other (d, alpha ) reactions that have so far been used to study dsotopic spin selection rules. It appears that further experimental results are needed for an understanding of the ' isotopic spin forbidden'' (d, al

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