50Cr,52Cr,54Cr(α,p)53Mn,55Mn,57Mn at 18 and 26 MeV

Angular distributions and energies of protons produced by α particle bombardment of the even Cr isotopes were measured at 18 MeV bombarding energy. On the basis of systematic comparison of the 57Mn proton spectrum with the spectra of 53Mn,55Mn, we believe that the Qαp value for the ground state transition to 57Mn is -4.302 +- 0.020 MeV. This disagrees with a previously reported value (-4.170 MeV). This discrepancy can be removed by assuming that the 83% branching mode of the β- decay of 57Mn leads to the 136 keV level in 57Fe, instead of to the 14 keV level suggested. When this is done the Qαp value predicted on the basis of the more accurate measurement of the β- endpoint energy is -4.299 MeV, in agreement with the direct measurement made in our experiments. The angular distributions were analysed using a triton transfer model and DWBA theory. The first excited state of 57Mn at 84 keV has an angular distribution strongly resembling that of the 7/2- g.s. of 53Mn. A level scheme with several spin assignments for 57Mn is proposed. This will be the first information ever presented on the level structure of 57Mn. Spectra of the 52Cr,54Cr(α,p)55Mn,57Mn reaction at 26 MeV is also presented. (author)