Experimental study of vanishing of N=20 magicity in neutron rich nuclei

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The excited states in the light neutron-rich nucleus ^<11>Be have been studied through the β-delayed neutron- and γ -decays from spin-polarized ^<11>Li_<g.s>.. The experiment is based on the new method in the β-delayed decay spectroscopy from a spin polarized nucleus, ^<11>Li_<g.e.>(β)→11Be(n)→^<10>Be^* (γ)→^<10>Be_<g.e.>, to unambiguously assign the spins and parities of the states populated in β-decay., by taking advantage of the fact that the allowed B-decay from a polarized nucleus shows an angular distribution of W(θ) 〜1 + AP cosθ, which depends on the spin value of the daughter state. Here, θ, A and P are the emission angle with respect to the polarization axis, the asymmetry parameter of β-transition and the polarization of the parent nucleus, respectively.The level scheme and decay scheme of ^<11>Be^* were established from the β-γ, β-n and β-n-γcoincidences, and the spins and parities for 7 levels in ^<11>Be were firmly assigned from the βdecay asymmetries measured in coincidence with the delayed radiations. The neutron-spectroscopic factors of the levels in ^<11>Be were also determined. Some of the levels show good accord with predictions by the anti-symmetrized molecular dynamics (AMD) theory, which predicts various types of 2α-cluster states in the rotational bands and a single α-cluster state.