A systematic investigation of reaction cross sections and isomer ratios for neutrons up to 20 MeV on Ni-isotopes and Co-59 by measurements with the activation technique and new model studies of the underlying reaction mechanisms

摘要

Results of new cross section measurements are presented for the following neutron-induced reactions: Ni-58(n,p)Co58m+g, Ni-58(n,p)(58)mCo, Ni-58(n,x)Co-57, Ni-58(n,2n)Ni-57, Ni-60(n, p)Co60m+g, Ni-60(n, p)Co-60m, Ni-61(n, p)Co-61, Ni-61(n, x)Co-60m, Ni-62(n, x)Co-61, and Co-59(n, 2n)Co58m+g with emphasis on incident energies between 14 and 20 MeV. In addition, new results have been obtained for the isomer ratios of the Ni-58(n, p)Co58m+g, the Ni-60(n, p)Co60m+g and Co-59(n, 2n)Co58m+g reactions. Detailed model calculations were undertaken to study the systematics of (n, xp) reactions on Ni isotopes and the use of the measured isomer ratios for the determination of the effective moment of inertia in the level density expression. Good overall agreement was obtained and, in particular, the case of the Ni-58 target isotope now shows a complete physically consistent database that is ideal to test model calculations. The difference in spin between isomer and ground state of DeltaJ = 3 for the Co-58 and Co-60 residual nuclei results in a sensitivity to the spin dependence of the level density in these nuclei. However, at present, measurement uncertainties and uncertainties associated with the decay schemes preclude a definitive conclusion about the appropriate value of the effective moment of inertia. A comparison was made between a locally optimized parameter set based on all available data and a global approach. This approach allowed to identify where problems with the global approach can be expected. The locally optimized parameter set includes for the first time gamma-ray strength-functions based on an improved systematics of the gamma-ray widths. (C) 2003 Elsevier B.V. All rights reserved. [References: 76]