High resolution (sup 12)C((gamma),p) experiments at E(sub (gamma)) (approx equal) 25-75 MeV

摘要

Absolute differential cross sections for the (sup 12)C((gamma),p)(sup 11)B reaction have been measured over proton detection angels ranging from 30 to 150 deg, using tagged photons of 25-75 MeV energy, for low-lying regions of residual excitation energy in (sup 11)B. Four experiments were performed at the MAX laboratory in Lund in order to provide data. Previously reported cross sections for the reaction had systematic uncertainties of a magnitude which made them agree, in spite of a large spread in absolute values. The cross sections reported, with a systematic uncertainty of 8%, remove previous ambiguities for E(sub (gamma))=40-75 MeV. A reinterpretation of the states excited in(sup 11)B at E about 7 MeV is also presented. The data are compared with quasi-elastic (e,e'p) results in PWIA in the same recoil momentum range. It is found that the momentum distributions do not scale for the two reaction types. Furthermore, the data are compared with the results for the inverse reaction (p,(gamma)) in the centre-of-momentum system by detailed balance. The comparison with respect to missing momentum indicates an angular dependence in the ((gamma),p) reaction which is not present in the inverse (p,(gamma)) reaction. Recent results from the MAX laboratory for the ((gamma),n) reaction are compared to the ((gamma),p) results. The mirror nuclei (sup 11)C and (sup 11)B have almost identical excitation energy spectra at E(sub (gamma))=60 MeV. It is concluded that HF-RPA calculations with essential contributions of meson exchange currents provide a qualitative description of the angular distributions obtained for the ((gamma),p) reaction. An extension of the spherical symmetric basis for the wave function is suggested for the states at E about 7 MeV in (sup 11)B. 108 refs, 83 figs. (Atomindex citation 27:017460)