Doppler Shift as a Tool for Studies of Isobaric Analog States of Neutron-Rich Nuclei: Application to ⁷He

We have developed a new technique to study exotic neutron-rich nuclei via their isobaric analog states (IAS). We populate high-isospin states in resonant reactions of radioactive ion beams with protons. Characteristic γ rays emitted from excited decay products were used to identify the population of the IAS. We show that information on the differential and total cross section for formation of the IAS can be extracted from the energy spectrum of the Doppler-shifted γ rays. This technique was applied to the study of T=3/2 states in ⁷Li, which are analogs of states in ⁷He. The analog of the ⁷He ground state was clearly observed, whereas the presence of the analog of a narrow 1/2^- state at 0.6 MeV excitation in ⁷He reported by M. Meister et al. [ Phys. Rev. Lett. 88 102501 (2002)] was excluded at the 90% confidence level. Evidence is presented for a broad 1/2^- state at a higher excitation energy in ⁷He.