中能重离子碰撞中的同位旋弛豫由密度依赖的核对称能驱动。同位旋自由度弛豫的快慢和对称能随密度依赖性质紧密相关。最近的研究表明，在中能核反应中，同位旋弛豫可能是一个长时标过程。为了定量测量同位旋自由度在不同密度区域的弛豫时标，我们拟采用HBT方法测量重离子核反应中出射的不同同位旋轻粒子，例如p, d，t，3He等, 其小角关联函数，通过表面发射的假设，利用三体弹道模型得到这些粒子的发射时标，从而给出同位旋自由度从液相至气相的弛豫时标。并结合量子分子动力学模型的计算，进一步约束对称能随密度的依赖行为。

The relaxation of the isospin degree of freedom (IDOF) is driven by the density dependent symmetry energy in heavy ion collisions at intermediate energies. The timescale of isospin relaxation is colsely related to the symmetry energy. It is revealed by recent researches that isospin relaxation is likely a long-time process. In order to measure quantitatively the relaxation timescale of IDOF in regions at different density, it is proposed to measure the small angle correlation functions of the light charged particle pairs, like d-d, t-t and 3He-3He using the HBT technique. With the surface emission assumption, one can deduce the emission time scale as a function of the neutron enrichment of these particles, and therefore shed light on the relaxation timescale of the IDOF from liquid drop to gas phase. Based on the calculations of quantum molecular dynamics, one can further constrain the density dependence of the symmetry energy.