Isomeric Cross Section Ratios in Alpha-Particle Reactions

Isomeric cross section ratios for reactions Mn 55 (α, n )Co, 58 m , g Fe 56 (α, p n )Co 58 m , g and Rb 85 (α, 2 n )Y 87 m , g were measured by the activation method using a stacked-foil technique. The experimental results of the energy dependence of σ m /σ g for Mn 55 (α, n )Co 58 m , g reaction were compared with the calculation using the square-well potential with the interaction radius parameter of r 0 =1.7 fermi and the energy level density parameter of a =2 MeV -1 in Fermi gas model. From the comparison, it seems that as the spin level density parameter either a constant value between σ=3 to σ=4 or the value dependent on nuclear temperature corresponding to 2σ 2 =14.4 T fits comparatively well in experimental results. However, the experimental result on this reaction shows that the σ m /σ g reaches a maximum value at about 25 MeV and then decrease. This behavior may be interpreted by a mechanism of direct interaction. The energy dependence of σ m /σ g for Fe 56 (α, p n )Co 59 m , g reaction qualitatively indicates that the reaction advances through the compound nucleus formation process, contrary to the results for Co 59 ( p , p n )Co 58 m , g reaction measured by Meadows et al. in which the same compound nucleus is formed.