Fermi能区重离子碰撞中的快裂变过程时标介于统计发射与两体散射之间。所形成的颈部区域中子丰度较高，在至断点的过程中历经亚饱和密度区，将是研究低密区对称能的良好探针。本申请拟在兰州放射性束流线装置(RIBLL)上完成30MeV/u 40Ar+232Th反应实验，关联测量裂变碎片和轻带电粒子，重构碰撞中的快裂变事件。以多角度的多源模型分析得出发射粒子的断前断后同位旋差异，通过垂直裂变轴和平行裂变轴出射的轻粒子得到颈部发射和非颈部发射的同位旋差异，重现颈部区域的同位旋分馏过程。并通过与输运模型计算结果的比较，为约束低密区对称能提供一种新的实验手段。

The time scale of fast fission in heavy ion collisions at Fermi energy regime is between the statistical emission and the two body scattering process. The neck region formed in the fast fission hot nuclei is neutron rich and experiences a stage below saturation on the way to the scission point. Thus it provides a new means to probe the symmetry energy at sub-saturation density because of the mechanism of isospin fractionation. This application is to conduct the collision experiment of 40Ar+232Th at 30 MeV/u at HIRFL-RIBLL. The fission fragments will be detected in coincidence with the emitted light charged particles. The pre- and post- scission multiplicities are deduced by multi-source analysis to different angles for all the light isotopes. The isospin composition of the neck emission will be compared to the non-neck emission through the multiplicities of the particles emitted perpendicular and in parallel to the fission axis. The isospin fractionation process is to be observed. Using these new observables, we expect to constrain the symmetry energy at sub-saturation densities in the framework of a well developed transport model.