Description of He isotopes using the particle-particle random-phase   approximation model
  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. 
Benoit Laurent and Nicole Vinh Mau
nucl-th nucl-ex