利用电荷交换反应中的Δ共振激发测量中子皮厚度

Accurate knowledge about the neutron skin thickness provides guidance to constrain the density dependence of nuclear symmetry energy, which plays an important role in understanding the properties of exotic nuclei and neutron stars. So far, some experimental approaches have been developed to measure the neutron skin thickness. However, for most of these techniques, the determination of the neutron skin thickness is model dependent and they do not provide conclusive results on the magnitude of the symmetry energy. Charge exchange process in relativistic heavy ion reactions are very peripheral one, in which the Δ resonance excitation channel corresponds to a single nucleon-nucleon collision leading to the excitation of a Δ resonance decaying into pion escaping from the nucleus. According to the intranuclear cascade calculations the probabilities for the Δ resonance excitations in (n,p) and (p,n) reactions will provide a probe sensitive to the neutron and proton radial distributions in nuclei. In this proposal, both the (n,p) and (p,n) charge exchange reactions of 12-15C nuclei, which will be produced with the second Radioactive Ion Beam Line in Lanzhou (RIBLL2), will be studied with the External Target Facility (ETF). The contribution of the Δ resonance excitation can be extracted from the momentum spectra of the ejectiles. Combining the Δ resonance probabilities of (n,p) and (p,n) reactions and the proton radii, the neutron skin thickness of these nuclei can be obtained. Using these studies, we try to propose a novel and model independent method to measure the neutron skin thickness and demonstrate the feasibility and accuracy of this method.

准确测量中子皮厚度可以对核物质对称能的密度依赖性进行约束，这对理解奇特核结构及反应、中子星结构及演化规律等方面均具有重要意义。虽然目前实验上已采用不同的尝试来测量中子皮厚度，但不同方法提取的结果差异还比较大、且具有一定的模型依赖性，难以实现约束核物质对称能的目的。高能区电荷交换反应中的Δ共振激发过程是一种周边的单核子-核子碰撞过程，根据核内级联模型的描述，其(n,p)和(p,n)两种反应的Δ共振激发几率必然能反应原子核核内中子和质子的径向分布情况。本项目计划利用RIBLL2次级束流线产生12-15C同位素，然后利用ETF装置同时测量目标核(n,p)和(p,n)两种电荷交换反应产物的纵向动量分布谱，提取Δ共振激发产额比，并结合已知的质子半径信息，测量其中子皮厚度，寻找出一种新的模型无关的中子皮厚度测量方法，并验证此方法的可行性及测量精度。

在本项目的支持下，利用CSRm提供的能量为280 MeV/u的18O主束，在RIBLL2初级靶点与15 mm厚的Be靶作用，通过弹核碎裂产生了能量~240 MeV/u的目标核12-16C，然后在ETF终端对这些目标核分别与CH2、C、Ag和Pb等次级靶作用产生的核反应产物进行了测量，获得了大量的高质量实验数据。初步实验数据分析显示：在外靶终端可以实现目标核核反应产物的清楚鉴别，在13-15C与C靶的核反应产物的粒子鉴别谱中同时观察到了(n, p)和(p, n)两种电荷交换反应产物，且(n, p)和(p, n)两种电荷交换反应产物的产额比有随入射目标核中子数增加而增加的变化趋势，初步证明了(n, p)和(p, n)两种电荷交换反应产物的产额比可灵敏的反应入射核的中子皮厚度，可能提供一种新的模型无关的中子皮厚度提取方法。