Question of low-lying intruder states in8Beand neighboring nuclei

The presence of not yet detected intruder states in ${}^8\mathrm{Be},$ e.g., a ${J=2\mathrm{}}^{+}$ intruder at 9 MeV excitation would affect the shape of the ${\beta }^{\mp }$-delayed $\alpha$ spectra of ${}^8\mathrm{Li}$ and ${}^8\mathrm{B}.\mathrm{}$ In order to test the plausibility of this assumption, shell-model calculations with up to $4ħ\omega$ excitations in ${}^8\mathrm{Be}$ (and up to $2ħ\omega$ excitations in ${}^{10}\mathrm{Be}$) were performed. With the above restrictions on the model spaces, the calculations did not yield any low-lying intruder state in ${}^8\mathrm{Be}.$ Another approach—the simple deformed oscillator model with self-consistent frequencies and volume conservation—gives an intruder state in ${}^8\mathrm{Be}$ which is lower in energy than the above shell-model results, but its energy is still considerably higher than 9 MeV.