Analog States of ${}^7\mathrm{He}$ Observed via the ${}^6\mathrm{He}(p,n)$ Reaction

Isobaric analog states of ${}^7\mathrm{He}$ have been investigated by a novel technique involving the observation of the resonant yield of neutrons from the ${}^6\mathrm{He}(p,n)$ reaction in coincidence with $\gamma$ rays from the decay of the $(0^{+},T=1)$ state in ${}^6\mathrm{Li}$. The $\gamma$ rays provide a clean signature for the isospin-conserving neutron decay of the low-lying isobaric analog resonances. It is conclusively shown that the analog of the recently observed low-lying spin-orbit partner of the ${}^7\mathrm{He}$ ground state does not exist. Evidence is presented that this state lies at much higher energies, in agreement with microscopic calculations.

Figure 1

The experimental setup.

Figure 2

The spectrum of neutrons in coincidence with ${}^6\mathrm{He}$ and a $3.56\pm 0.30\mathrm{MeV}$ $\gamma$ ray, measured with a $({\mathrm{CH}}_2{)}_n$ target. The inset shows a neutron spectrum obtained with a natural carbon target. The contribution of the carbon background is shown by the dashed curve, and the dotted curve gives the contribution from the random background (a constant in the time-of-flight spectrum).

Figure 3

The excitation function for the ${}^6\mathrm{He}(p,n){}^6\mathrm{Li}(0^{+};T=1)$ reaction measured at $180\pm 8°$. See the text for a discussion of the curves. The inset shows the decay channels for the $T=3/2$ resonances in ${}^7\mathrm{Li}$.