Search

The Pb-nat(He-3, tp) reaction at E(He-3) = 177 MeV was studied to identify 2hw isovector monopole strength in Bi isotopes. Monopole strength was found in the region -45 < Q < -30 MeV in the coincidence data. The cross section is compared with distorted-wave Born approximation calculations for non-spin-flip and spin-flip isovector monopole resonances. The observed spin-flip and non-spin-flip monopole strengths in the proton-decay channel amount to 20% of the respective non-energy-weighted sum rule.

Gamow-Teller (GT) transitions and MI transitions from or to the ground states are analogous in the pair of T= 1/2 mirror nuclei, if the excited states hold the mirror-symmetry properties. In order to study the mirror-symmetry structure of the A = 27 mirror nuclei Al-27 and Si-27, experimental data on GT transition strengths obtained from Si-27-->Al-27 beta decay and from a good-resolution Al-27(He-3,t)Si-27 reaction at 150 MeV/nucleon and 0 degrees have been compared with M1 transition strengths obtained from gamma decay in Al-27 and Al-27(gamma,gamma') reaction. Good overall correspondence of transitions is observed for states up to nearly 9 MeV in excitation energy, where the particle-decay channel becomes Important. The difference of excitation energies of analog states in the mirror nuclei are about: 250 keV at this excitation energy. From a comparison of B(MI) and B(GT) strengths of the analogous transitions, the contribution of the isoscalar and the orbital terms in the M1 operator is found to become prominent in weak M1 transitions. [S0556-2813(99)03601-8].

Gamow-Teller (GT) states with J(pi) = 1(+) excited by a (p,n)-type charge-exchange reaction on N=Z nuclei with ground-state isospin T = 0 have isospin T = 1, while M1 states excited by inelastic scattering of electrons or protons can have either T = 0 or T = 1. The latter are the isobaric analog states of the GT states. By comparing the GT states observed in the good-resolution (He-3,t) reaction at 150 MeV/nucleon with the M1 states observed in inelastic electron and proton reactions, the isospin structure of the 1(+) states in Si-28 has been determined. From an analysis of the strengths of corresponding 1(+) states observed in the (He-3,t) and (e,e') reactions, the spin and orbital contributions to the M1 excitations have been deduced.

Spin-isospin excitations in Bi-208 have been investigated using the Pb-208(He-3,t)Bi-208 reaction at and near theta approximate to 0 degrees at E(He-3)=450 MeV. The microscopic structure of the Gamow-Teller resonance (GTR), the isobaric analog state (IAS), and the spin-flip dipole (Delta L = 1) resonance (SDR) in Bi-208 has been studied by observing their direct proton decays to the low-lying neutron-hole states in Pb-207. Decay protons were measured at backward angles in coincidence with tritons detected at and near 0 degrees. The total branching ratio for proton decay from the GTR is determined to be only 4.9+/-1.3%. The total branching ratio for proton decay from the SDR amounts to 14.1+/-4.2%. The deduced total widths as well as the total and partial proton escape widths of the GTR and IAS are found to be in reasonable agreement with recent theoretical estimates obtained in the framework of the continuum Tamm-Dancoff approximation.

The Pb-nat(He-3,t) reaction and the subsequent decay of the excited nucleus by proton emission at backward angles were studied at E(He-3) = 177 MeV with the aim to separate the 2 (h) over bar omega isovector monopole strength in Bi isotopes from the continuum background. Since the continuum is largely due to quasifree processes associated with forward-peaked high-energy protons, the observed resonance-to-continuum ratio is drastically improved when coincidences between ejectile tritons and protons emitted at backward angles are required. Furthermore, the cross section for the monopole resonances excited via (He-3,t) is, unlike the cross sections for higher multipolarity excitations, strongly forward peaked. By comparing coincidence spectra at theta (t)approximate to0 degrees and at finite angles. monopole strength was found in the region -45