Fragmentation and splitting of the Gamow-Teller (GT) strength has been observed in a systematic study of the ${(}^3$He,t) charge-exchange reaction at E${(}^3$He)=200 MeV over the entire range of stable Sn isotopes. Triton energy spectra were observed with a high-resolution magnetic spectrometer at angles near θ=0° where ΔL=0 transitions are enhanced. Excitation energies, widths, 0° cross sections, and strengths B(GT) were determined. A theoretically predicted configuration splitting of the main Gamow-Teller component into two components, expected to be dominant near A=118 at the onset of the filling of the 1${\mathit{h}}_{11/2}$ neutron orbital, could not be observed. This may be due to the fact that the total widths of the resonances of 5–6 MeV exceed the predicted splitting. A comparison of the 0° cross sections for the transitions to the Gamow-Teller resonances and the isobaric analog states leads to strengths B(GT) for the main Gamow-Teller components of typically 65% of the sum-rule value of 3(N-Z). Three to four additional Gamow-Teller fragments (‘‘pygmy resonances’’) were observed in all final nuclei at lower excitation energies. The excellent energy resolution of the experiment made it possible to observe a pronounced fine structure in these low-lying resonances which is believed to be due to coupling to two-particle–two-hole doorway states. Also seen with all target nuclei was a systematic sequence of strong ${\mathit{J}}^{\mathrm{\pi }}$=$1^{+}$ states near the ground states in all Sb isotopes (${\mathit{E}}_{\mathit{x}}$=0 to 220 keV). In addition, strong ΔL=1 resonances were observed in all nuclei at excitation energies of typically 20 MeV. Furthermore, nonresonant background from quasifree charge exchange was observed. An average of ∼85% of all excess neutrons seems to contribute to this background in approximate agreement with results from (e,e’p) experiments.

• Received 30 September 1994

DOI:https://doi.org/10.1103/PhysRevC.51.526