Progress of exotic structure studies in light neutron-rich nuclei

In order to study the exotic structures in unstable nuclei, including cluster structures and intruder states, we develop a silicon detection array, which is suitable for the direct nuclear reactions induced by radioactive beams on the proton and deuteron targets. We outline the Q-value resolution deduced from the invariant-mass method (IM) and the missing-mass method (MM), the extraction of excitation energy (E-x), spin-parity (J(pi)), and spectroscopic factors, as well as experimental evidences of cluster structures. We present the latest experimental results about the linear-chain molecular bands in the neutron-rich nuclei C-14 and C-16, and the intruder s- and d-wave strengths in the neutron-rich N=8 nuclei. First of all, from a cluster-transfer experiment Be-9(Be-9, C-14*->alpha+Be-10)He-4, we found that a state at 22.5(1) MeV in C-14 decays predominantly into the states around 6 MeV in Be-10 daughter nucleus, in line with the unique property of the predicted band head of the s-bond linear-chain molecular states. Moreover, a radioactive beam C-16 was excited to the highlying excited states (above the cluster separation threshold) by the inelastic scattering of deuterons. For the first time, we deduced good resolution Q-value spectra using the three-fold events of He-4+Be-12+d and He-6+Be-10+d, which allow the reconstruction of the C-16 excitation spectra according to their decay paths. The close-threshold resonance at 16.5 MeV is assigned as the J(pi)=0(+) band head of the predicted positive-parity linear-chain molecular band with (3 / 2(pi)(-))(2) (1 / 2(sigma)(-))(2) configuration, according to the associated angular correlation and decay analysis. Other members of this band were found at 17.3 (2(+)), 19.4 (4(+)), and 21.6 (6(+)) MeV based on their selective decay properties, being consistent with the theoretical predictions. Furthermore, a new proton inelastic excitation and cluster-decay experiment for C-14 was performed. Similar to the C-16 experiment stated above, all three final particles were detected in coincidence, allowing clear Q-value and thus decay-path selection. Based on both the angular correlation analyses and the inelastic scattering differential cross sections analyses, unambiguous spin-parities of 0(+), 2(+), and 4(+) are assigned for the 13.9-,14.9-,17.3-MeV high-lying states, which are consistent with the first three members of the predicted pi-bond linear-chain molecular rotational band in C-14. The results show that starting from C-14 the linear-chain structure can be established and certainly encourage further studies for longer chain configuration in heavier neutron-rich nuclei. 57(7)% d-wave and 19(7)% s-wave intruder components in the g.s. of Be-12 are deduced from a new H-2(Be-11, p)Be-12 reaction, which is dramatically different from the g.s. of the one-neutron halo nucleus Be-11 with a dominant s-wave intrusion. Lately, 12(2)% d- and 5(2)% s-wave intruder strengths in the g.s. of B-13 are determined from a new H-1(B-13, d)B-12 reaction. The sudden change of the intruder sd-wave intensity between B-13 and Be-12 needs further theoretical interpretation. Moreover, a new resonant state at 3.21+0.12-0.04 MeV, located just 40 keV above the one-neutron separation threshold, was observed for the first time in 12Be from the H-2(Be-11, p) reaction in inverse kinematics. This state is assigned a spin-parity of 0(-) according to the systematics of the level scheme of the N=8 isotones and decay width analysis. Recently, another new resonant state at E-x=4.8 +/- 0.1 MeV with an intrinsic width of 0.42 +/- 0.28 MeV is populated by the H-2(B-13, He-3) reaction via a l=1 proton transfer for the first time. It most likely corresponds to the E-n=1.24 MeV state observed in the previous one-proton removal reaction. The spin-parity of 2(+) is tentatively assigned to this resonance according to the analysis of its angular distributions as well as the theoretical calculations. In the future, with the development of accelerator technology, we will study exotic cluster structures and intruder states in heavier neutron-rich nuclei.