Mechanism for electric dipole transitions from the broadp-wave neutron resonance inMg24

Abstract

Neutron capture gamma rays from the 84-keV p3/2 wave resonance, 266-keV p&/2-wave resonance, and 431-keV p3/2 wave resonance in ' Mg that have large reduced neutron width have been measured with an anti-Compton NaI(T1) detector, using a time-of-flight technique. Successful extraction of gamma-ray intensities for transitions to low-lying states in 'Mg 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 ( -, '+), 585-keV ( -, '+), 975-keV ( -, ' ), 1965-keV ( -, '+), 2564-keV z and 2801-keV ( -, '+) 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/p wave resonance capture the observed and calculated widths for the transitions to the -, '+ states are in excellent agreement; however, the experimental widths for the transitions to the 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 p, /, -wave resonance. The calculations reproduced the observed values satisfactorily.

Resonance neutron-capture cross sections of stable magnesium isotopes and their astrophysical implications

“…Typically, these experiments covered only a limited energy region, and capture data were taken with large detectors, less suited for capture cross-section studies on isotopes in the mass region of Mg, where the cross sections are by far dominated by the elastic-scattering channel. While the most abundant isotope 24 Mg has been investigated several times [12][13][14][15], there are few neutron data for 25,26 Mg. Because the resonance-dominated capture cross sections of all three Mg isotopes are relatively small, the measurements can be strongly affected by various kinds of background and exhibit, therefore, rather large discrepancies. For instance, spin and parity of the first neutron resonance in 25 in the JENDL evaluation, although the parity assignment of Ref.…”