First observation of $\alpha$-cluster states in the ${}^{14}\mathrm{O}\hspace{0.17em}+\hspace{0.17em}{}^4\mathrm{He}$ interaction

We measured ${}^{14}\mathrm{O}+{}^4\mathrm{He}$ excitation functions for elastic scattering which demonstrate, for the first time, a well developed $\alpha$-cluster structure in the proton rich nucleus, ${}^{18}\mathrm{Ne}$. We present the excitation energies and estimates of the spins for the dominant resonances using an R-matrix approach. A resonance at 9.2 MeV excitation energy in ${}^{18}\mathrm{Ne}$ is particularly interesting. The spin-parity of the state is found to be $3^{-}$ and the $\alpha$ particle reduced width for the state appears to be comparable to the single particle limit. We have found indications for unusually large size of the observed $\alpha$-cluster configuration.

Figure 1

Time of flight-energy particle identification spectrum for the products of the ${}^{14}\mathrm{O}+\hspace{0.17em}{}^4\mathrm{He}$ interaction measured at three degrees in the laboratory frame. The inset shows the same spectrum without discrimination of the ${}^7\mathrm{Be}$ contamination in the beam. See text for details.

Figure 2

Excitation function for ${}^{14}\mathrm{O}+\hspace{0.17em}{}^4\mathrm{He}$ elastic scattering at ${180}^{°}$ c.m. ($0^{°}$ in the laboratory system). The excitation energies in ${}^{18}\mathrm{Ne}$ are shown at the top. The data shown with dots are from the measurements at the 33 MeV initial ${}^{14}\mathrm{O}$ energy. The data represented by crosses are from 42 MeV. The data shown with a boxes represent the measurements at the 57 MeV initial ${}^{14}\mathrm{O}$ energy. The excitation function for $\alpha +{}^{14}\mathrm{C}$ [13] is shown at the top. The bold line gives the Rutherford cross section.

Figure 3

R-matrix fit for the low energy part of the excitation functions for ${}^{14}\mathrm{O}+\hspace{0.17em}{}^4\mathrm{He}$ elastic scattering. The arrows indicate the c.m. angles. The dash-dot line shows the results of calculations without the first $1^{-}$ resonance.