COULOMB EXCITATION OF LIGHT NEUTRON-RICH NUCLEI

用49 MeV/u的不稳定核束研究了丰中子核~(32)&gt；Mg到2~+态的库仑激发。该束流是由94 MeV/u&lt；40&gt；Ar离子轰击9Be靶引起的弹丸碎裂反应产生的，并由RIPS装置分离，RIPS装置基本上由两个偶极磁体组成的系统，具有能量损失降解器。在靶周围放置了一块-奈(Tl)闪烁体，用于探测来自^&lt；32&gt；mg的2^+态的伽马射线，与由5个/400微米厚的硅探测器组成的硅计数器望远镜探测到的非弹性散射的^&lt；32&gt；镁原子核相符合。硅望远镜提供了粒子识别来选择非弹性散射。将光子能量的多普勒频移、伽马射线探测器和硅计数器望远镜的探测效率与伽马射线产额进行积分，得到了相应的非弹性散射截面，并通过耦合通道分析得到了426 e2fm2的B(E2)值。该值与最新的壳模型预测以及N_Pn_n方案的整体唯象表达式相吻合。这些模型的主要成分是N=20壳层闭合的消失。因此，目前的结果证实了2^+态的低激发能所暗示的^&lt；32&gt；mg的大变形，并指出N=20壳层间隙的消失。本研究的成功为研究不稳定核中大量未知的0^+-2^+跃迁开辟了一个新的深层次的研究领域。

The Coulomb excitation of a very neutron-rich nucleus ^<32>Mg to its 2^+ state was studied using an unstable nuclear beam of ^<32>Mg at 49 MeV/u with a ^<208>Pb target. The beam was generated by projectile fragmentation reaction induced by 94 MeV/u ^<40>Ar ions bombarding a ^9Be target, and separated by the RIPS facility, a system with essentially two dipole magnets with an energy loss degradeer, of RIKEN (the Institute of Physical and Chemical Research). A stuck of 64 NaI(Tl) scintillators were settled around the target to detect gamma-rays from the 2^+ state of ^<32>Mg in coincidence with inelastically scattered ^<32>Mg nuclei detected by silicon counter telescope consisting of five/400 mum thick silicon detectors. The silicon telescope provided particle identification to select the inelastic scattering. The relevant inelastic scattering cross section was obtained by integrating the gamma-ray yield with corrections due to the Doppler shift of the photon energy and the detection efficiencies of the gamma-ray detector and the silicon counter telescope.The B(E2) value of 426(〕SY.+-.〔)34 e^2fm^2 was extracted by a coupled channel analysis. The value is in good agreement with a recent shell model prediction and also with a global phenomenological expression of the N_pN_n scheme. The major ingredient of these models is an obliteration of the N=20 shell closure. Therefore the presenct result confirms a large deformation of ^<32>Mg suggested by the low excitation energy of the 2^+ state, and points to the vanishing of the N=20 shell gap. The success of the present study opens up a new profound field of research to investigate a large number of unknown 0^+-2^+ transitions in unstable nuclei.