AGB星慢中子俘获s过程中子源反应13C(α,n)16O的直接测量

Current stellar models as well as stellar spectroscopy strongly support the 13C(α,n)16O reaction as the dominant neutron source in the s process in asymptotic giant branch (AGB) stars. The 13C(α,n)16O cross section will be directly measured in this project. There is no experimental cross section data within the Gamow window, and the uncertainty of the low energy data are still quite large and therefore, the reaction rate was calculated by extrapolating high energy data to Gamow energy region. The experiment will be performed at a newly built low-energy nuclear astrophysics terminal quite close to the 320 kV platform in IMP. The He2+ pulsed-beam will be used to bombard the 13C target, and the neutrons will be detected by 4 liquid scintillator detectors. The pulse-shape discrimination is utilized to identify neutrons and gamma ray, together with the massive shields which are consisted of paraffin, Gd and lead. At the same time, the anticoincidence technique of the plastic scintillator and liquid scintillator can also suppress the background. This measurement will reduce present data error, and the cross section in the Gamow window may be measured for the first time. A more reliable reaction rate can be given, which is critical to understand the nucleosynthesis in AGB stars.

本项目将对AGB星慢中子俘获过程（s过程）关键核反应13C(α,n)16O的截面进行直接测量。s过程是宇宙中超铁元素的主要合成方式之一，13C(α,n)16O反应被认为是s过程中中子的主要来源。该反应在Gamow窗口的截面数据尚属空白，且低能区的数据精度很差，其反应率是依靠已有实验数据通过理论外推得到的。本实验将在近物所320 kV平台新建成的低能核天体物理终端上进行：用He2+脉冲束流轰击13C靶，用4台液闪中子探测器测量反应放出的中子，进而得到13C(α,n)16O反应的截面。实验将利用脉冲形状甄别技术鉴别γ射线和中子；用石蜡、镉和铅组成的屏蔽层屏蔽天然中子和γ射线。另外，用塑闪与中子探测器信号进行反符合进一步压低天然本底对实验的影响。本实验将提高已有数据精度，并力争在Gamow窗口内取得截面数据，提高反应率精度，为人们理解AGB星中元素的起源和演化提供可靠的实验数据。

本项目对AGB星慢中子俘获过程（s过程）关键核反应13C(α,n)16O进行了实验研究。s过程是宇宙中超铁元素的主要合成方式之一，13C(α,n)16O反应被认为是s过程中中子的主要来源。该反应在Gamow窗口的截面数据尚属空白，且低能区的数据精度很差，其反应率是依靠已有实验数据通过理论外推得到的。本项目的实验工作主要在近物所320 kV平台新建成的低能核天体物理终端上进行，理论模拟工作在爱丁堡大学进行。各种探测器的详细模拟工作都已经全部完成，基于塑闪和液闪的探测器设计也已经完成并进行了初步测量。