We report the observation of double isovector giant dipole resonances in (+,-) and, for the first time, also in (-,+) double-charge-exchange reactions at T=292 MeV. The resonances have been observed on nine targets covering a wide range of mass at Q values around -50 MeV in (+,-), but at significantly lower energy (Q=-33.8 MeV) in the inverse reaction on Fe56. The centroid energies in both reactions are very close to the energies at which the "Tz=2 double giant dipole (J=0+,2+,"T=2) states are expected to appear. Angular distributions have been measured for the (+,-) resonances on Ca40,56, Fe, and Nb93 and each is observed to have a quadrupole shape. Based on their energies, angular distributions, cross sections, and the comparison between the (+,-) and (-,+) data on Fe56, we identify the resonances as double giant dipole states (i.e. states arising from a charge-exchange dipole operator acting twice on the ground-state wave function). The data from the present and recent observations indicate that the newly discovered resonance is a general collective feature of all nuclei.
ASJC Scopus subject areas
- Nuclear and High Energy Physics