Cross-section ratios of isomeric pairs produced in the compound nuclear reactions 41 K (a,n) 44 Sc, 65 Mn (a,n) 58 Co, 93 Nb(a,w) 96 Tc, 93 Nb(a,2w) 95 Tc, 9 *Nb(a,3n) 94 Tc, and 136 Ba(a,3w) 137 Ce are analyzed in terms of the statistical model. In this context Vandenbosch and Huizenga have proposed a formalism for calculating isomer formation from compound nuclei of relatively low excitation energy and spin. It is assumed that the density of levels of spin / in residual nuclei is proportional to (27+1) exp[-J(J-\-l) /2a 2 '} and that in the case of a product formed by neutron evaporation the emission of charged particles is not of importance.Here a is the spin cutoff parameter and is related to the effective moment of inertia #. In this paper the extension of such a model to compound nuclei of much higher excitation energy and spin is considered. Typically by this method our experimental isomeric ratios imply a value of 6 much smaller than that of a rigid sphere, $R, even for nuclei excited to well above nucleon binding energies. The introduction of two additional factors in the formalism leads to more reasonable values of $/$R. First, we invoke a principle of limiting spin. That is, the level density of a residual nucleus is not described by the expression above for all 7; rather, above some critical J value, determined by a Fermi-gas model, the level density is taken to be zero. Secondly, we find charged-particle emission, particularly of a particles, to be of importance in many cases. With the inclusion of these factors experimental isomeric ratios are consistent with an $/$R value of unity when the excitation energy Ef is greater than about 10 MeV. Below 10 MeV, $/$R has essentially the same dependence on Ef for all of the reactions analyzed.
B896FIG. 2. Isomeric cross-section ratios (high-spin product to low-spin product) as a function of bombarding energy in the reactions (a) 93 Nb(a;,2w)-95
As part of an assessment of low-level uranium resources, the U. S. Department of Energy has funded a study of the feasibility of extracting uranium from seawater. This project was jointly conducted by Exxon Nuclear Company and Oregon State University, with Vitro Engineering Corporation serving as a subcontractor. 41 6.3 Flowsheet Criteria 43 6.4 Co-product Flowsheet 45
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