E. Gerlic, J. Guillot, H. Langevin-Joliot, S. Gales, M. Sakai, et al.. High lying two-neutron hole states in $^{206}$Pb and $^{114}$Sn observed via the ($\alpha, ^6$He) reaction at 218 MeV.
Physics Letters B, Elsevier, 1982, 117, pp.20-24.
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