轻丰中子核的奇特结构研究进展

Progress of exotic structure studies in light neutron-rich nuclei

不稳定核的奇特结构,例如集团结构、闯入态等,是放射性核束物理研究的热点方向之一.直接核反应实验是研究丰中子核奇特结构的重要手段之一.本课题组自主研发了适用于放射性束流在质子和氘靶上发生直接核反应实验研究的带电粒子探测阵列.利用该高精度、大立体角、模块化、多用途的探测阵列,我们在轻丰中子核的奇特结构实验研究方面取得了一系列重要进展:从实验上确认了^(14)C的π型线性链状分子转动带;发现了丰中子核^(16)C的π2σ2构型线性链状分子结构,^(14)C的一个共振态可能有“3α呈线性排列”的线链超形变结构.我们定量研究了^(12)Be、^(13)B等N=8附近丰中子核的s波和d波闯入强度,结果表明:^(12)Be基态中s波占19%,远远小于^(11)Be(~80%),而d波成分却高达57%,是主要的闯入成分;^(13)B基态中的s(d)波闯入成分约为12%(5%),处于从Be到C的过渡.

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(Ex),spin-parity(J^(π)),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 ^(14)C and ^(16)C,and the intruder s-and d-wave strengths in the neutron-rich N=8 nuclei.First ofall from a clustertransfer experiment ^(9)Be(^(9)Be,^(14)C*→α+^(10)Be)^(4)He,we found that a state at 22.5(I)MeVin'C decays predominantly into the states around 6 MeV in ^(10)Be daughter nucleus,in line with the unique property of the predicted band head of the o-bond linear-chain molecular states.Moreover,a radioactive beam C 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 ^(4)He+^(12)Be+d and ^(6)He+^(10)Be+d,which alow the reconstruction of the ^(14)C excitation spectra according to their decay paths.The close-threshold resonance at 16.5 MeV is assigned as the J^(π)=0^(+) band head of the predicted positive-parity linear-chain molecular band with(3/2_(π)^(-))(1/2_(σ)^(-))^(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 ^(14)C was performed.Similar to the ^(16)C 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 0t,2t,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 predictedπ-bond linear-chain molecular rotational band in'4c.The results show that starting from'c 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 ^(12)Be are deduced from a new ^(2)H(^(11)Be,p)^(12)Be reaction,which is dramatically diferent from the g.s.of the one-neutron halo nucleus Be with a dominant s-wave intrusion.Lately,12(2)%d-and 5(2)%s-wave intruder strengths in the g.s.of ^(13)B are determined from a new ^(1)H(^(13)B,d)^(12)B reaction The suddenchange of the intrudersd wave intensty between Band Beneeds fur ther theoretica linterpretation 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 ^(12)Be from the ^(2)H(^(11)Be,p)reaction in inverse kinematics.This state is assigned a spinparity of O 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 ^(2)H(^(13)B,^(3)He)reaction via a I-1 proton transfer for the first time.It most likely corresponds to the En=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.