  The two-neutron RPA model has been used to describe helium isotopes with N
(the neutron number) = 4, 6, 7, 8 in their ground and excited states. The
properties of all these isotopes are given by a single system of equations so
that their properties are interdependent. Since the ground states of $^{9}$He
and $^{10}$He are still not well established, we have looked how the properties
of $^{9}$He induce the properties of the other isotopes, energies and wave
functions. Our results suggest an inversion of $2s$-$1p_{1/2}$ shells in
$^{9}$He. The corresponding ground states of $^{9}$He and $^{10}$He are
slightly unbound and respectively $1/2^+$ and $0^+$ states while the $1/2^-$
and $0^+$ seen in experiments appear to be excited states. With this assumption
on $^{9}$He, we get not only a nice picture of $^{10}$He but also a very good
two-neutron separation energy in $^{8}$He.

Nuclear Theory;Nuclear Experiment