Mechanism for electric dipole transitions from the broad p-wave neutron resonance in 24Mg

Neutron capture gamma rays from the 84-keV p3/2-wave resonance, 266-keV p1/2-wave resonance, and 431-keV p3/2-wave resonance in 24Mg that have large reduced neutron width have been measured with an anti-Compton NaI(Tl) detector, using a time-of-flight technique. Successful extraction of gamma-ray intensities for transitions to low-lying states in 25Mg was performed by an iterative unfolding method, in order to deduce partial radiative widths. Also, we have made an experimental contrivance separating the kernel of the 266-keV broad resonance from that of the 257-keV overlapping narrow resonance. Radiative widths were obtained for the E1 transitions to the ground (5/2+), 585-keV (1/2+), 975-keV (3/2+), 1965-keV (5/2+), 2564-keV (1/2+), and 2801-keV (3/2+) states, and were compared with theoretical calculations based on the valence capture model that was developed by Lane and Mughabghab. Consequently, we found that in the p3/2-wave resonance capture the observed and calculated widths for the transitions to the 1/2+ states are in excellent agreement; however, the experimental widths for the transitions to the 5/2+ states are 20--50 % of the theoretical ones. These noteworthy features in the retardation of E1 transition are explained in terms of the renormalized effective charge, which depends on the orbital angular momentum for the single-particle component of final bound states, as a result of the coupling of the single-particle transition with the isovector field generated by the giant dipole resonance. Moreover, the nonadiabatic coupled-channel calculation using a particle-rotator coupling model was carried out for partial radiative widths of the 266-keV p1/2-wave resonance. The calculations reproduced the observed values satisfactorily