Neutron hole states inPb207,206,205isotopes via the (He3,α) reaction at 100 MeV

Abstract

Neutron hole states were investigated in the $^{207,206,205}\mathrm{Pb}$ isotopes up to 25 MeV excitation energy using the ($^{3}\mathrm{He}$,$\ensuremath{\alpha}$) reaction at 100 MeV incident energy with 100 keV energy resolution. Above the well-matched low-lying levels corresponding to high angular momentum transfers, new peaks are identified. In addition, three gross structures, riding on a continuous background, are observed in each of the three isotopes, with some fine structures showing up to 10 MeV excitation energy. For $^{207}\mathrm{Pb}$, angular distributions have been obtained for the low-lying levels as well as for the deeply bound hole states. The data have been analyzed with distorted-wave Born-approximation calculations and spectroscopic factors extracted. It has been found that the low-lying levels do not exhaust the $1i\frac{13}{2}$ and $1h\frac{9}{2}$ neutron hole strengths. Corrections for exact finite-range effects, form factor shapes, and indirect pickup contributions have been calculated, and modify significantly the $2f\frac{7}{2}$ spectroscopic measured strength but not the $1i\frac{13}{2}$ and the $1h\frac{9}{2}$ ones. Most of the $1i\frac{13}{2}$ and $1h\frac{9}{2}$ missing strengths are found in the "bump" located at about 5 MeV excitation energy. The highly fragmented bump observed at about 8 MeV excitation energy is shown to arise from $1h\frac{11}{2}$ neutron pickup exhausting 45% of the sum-rule limit. Finally, the very smooth structure extending to 21 MeV excitation energy is tentatively attributed to $1g\frac{7}{2}+1g\frac{9}{2}$ neutron holes with 80% of the total strength. In $^{207}\mathrm{Pb}$, the four first isobaric analog states are observed as narrow structures around 20 MeV excitation energy.NUCLEAR REACTIONS $^{208,207,206}\mathrm{Pb}$ ($^{3}\mathrm{He}$, $\ensuremath{\alpha}$), $E=101.75$ MeV; measured $\ensuremath{\sigma}({E}_{\ensuremath{\alpha}}, \ensuremath{\theta})$, $^{207,206,205}\mathrm{Pb}$ deduced levels, $\mathrm{Ex}$, $l$, $J$, $\ensuremath{\pi}$, ${C}^{2}S$; $^{207,206,205}\mathrm{Pb}$ inner shell spectroscopic factors: enriched targets, magnetic spectrometer.