Reactions leading to the first excited states ofLi7andBe7and isospin-mixed states in

Abstract

Background Ratios of cross sections for mirror reactions sometimes deviate from the values expected on the basis of charge symmetry. These deviations are attributed to smoothly varying charge dependent effects (proton charge, different Q values) and to the effects of isospin mixed states in the compound nucleus. The effects are large and well known for certain positive parity states in 8 Be, but information is lacking for negative parity states.Purpose To measure the excitation functions of angle integrated cross sections for two pairs of reactions that involve 8 Be as an intermediate state:(1) 6 Li (d, p ) 7 Li (0.478 MeV) and 6 Li (d, n ) 7 Be (0.429 MeV); (2) 7 Li (p, p ) 7 Li (0.478 MeV) and 7 Li (p, n ) 7 Be (0.429 MeV). To measure the ratios of the neutron emitting and proton emitting reactions. To examine the implications for the structure of 8 Be.Method The ratios were determined by observing the isotropically emitted gamma rays from the decay of 7 Li (0.478 MeV) and 7 Be (0.429 MeV). Shell model calculations were performed for both positive and negative parity states. Results were compared to existing information.Results Ratios, usually with an accuracy of ±2%, were obtained for deuteron energies from 0.15 to 7.2 MeV and proton energies from 3.0 to 10.0 MeV. There were relatively strong deviations from expectations based on charge symmetry at the lowest deuteron energies and smaller deviations between E d = 2 and 4 MeV. There were very strong deviations for proton energies near 3 and 5.5 MeV, with strong neutron strength near 3 MeV and strong proton strength near 5 MeV. The shell model calculations were generally in good agreement for the positive parity states, and, with some exceptions, for negative parity states. There is reasonable evidence for the lowest lying 2 − T = 1 state at Ex= 20.3 MeV and for two new isospin mixed pairs, one for 2 + states near 22 MeV and another for 2 − states near 24 MeV.Conclusions The results will constrain future calculations for isospin mixed states in 8 Be.PACS numbers: 26.20.Fj, 26.30.Jk, 26.50.+x