Isovector giant resonances in He-6, B-12, Y-90, In-120, and Tl-208 observed in the (Li-7,Be-7) charge-exchange reaction

The (<SUP>7</SUP>Li,<SUP>7</SUP>Be) and (<SUP>7</SUP>Li,<SUP>7</SUP>Be gamm a) reactions have been studied at bombarding energies of 50 A MeV on target s of <SUP>6</SUP>Li,<SUP>12</SUP>C, <SUP>90</SUP>Zr, <SUP>120</SUP>Sn, <SUP >208</SUP>Pb, and polystyrene. The detection of Doppler-shifted gamma-rays from excited <SUP>7</SUP>Be permits the identification of the spinflip and non-spinflip characteristics. Transitions to over 20 discrete states and re sonances have been observed. Angular distributions for <SUP>6</SUP>Li and < SUP>12</SUP>C are well described by microscopic one-step distorted-waves ca lculations with shell-model transition amplitudes. The transitions to the g round states of <SUP>6</SUP>He and <SUP>12</SUP>C exhibit significant Gamow -Teller strength, but similar to 7% and similar to 17%, respectively, of th e calculated cross sections at theta approximate to 0 degrees arise from th e tensor interaction. A broad resonance at similar to 5.6 MeV in the halo n ucleus<SUP></SUP> 6He with J<SUP>pi</SUP> = (2<SUP>+</SUP>, 1<SUP>-</SUP>, 0<SUP>+</SUP>) seems to correspond to a structure predicted at this excitat ion energy containing soft modes of excitation with J<SUP>pi</SUP> = 0<SUP> +</SUP> and/or 2<SUP>+</SUP>. A resonance in <SUP>12</SUP>B at similar to 4 .5 MeV is described by a superposition of known 2<SUP>-</SUP> and 4<SUP>-</ SUP> states, and another one at similar to 7.5 MeV is dominated by a transi tion to a 2- state with weaker contributions from two 1- States (spin dipol e and giant dipole). Four to six resonances each, mostly spin-dipole resona nces, were identified in the final nuclei <SUP>90</SUP>Y, <SUP>120</SUP>In, and <SUP>208</SUP>Tl. The non-resonant quasi-free charge-exchange reaction p<INF>bound</INF> (<SUP>7</SUP>Li,<SUP>7</SUP>Be)n<INF>free</INF> was obse rved for all targets up to effective excitation energies of 50 MeV. The mec hanism is related to that on free protons. The effective number of particip ating protons was determined. (C) 1999 Published by Elsevier Science B.V.