利用活性靶技术研究丰中子核引起的熔合反应

Superburst models suggest that the fusion reactions induced by light neutron-rich nuclei in the rust of neutron start, such as 24O and 28Ne, may release significant amount of energy and heat the fuel up to the required ignition temperature. Therefore, these fusion rates are very important for our understanding of the thermal evolution of the neutron star and the X-ray superburst ignition. Due to lack of these fusion cross section data and the complexity of reaction mechanisms, it is very difficult to provide an accurate prediction for these reaction rates at the astrophysical energy range. In this proposal, the fusion reactions induced by the neutron-rich nuclei ^{19,20}O will be studied. Employing the multi-nucleon transfer reactions 9Be(18O,^{19,20}O), the low-energy radioactive beam ^{19,20}O could be produced at the HIRFL-SFC and RIBLL1 facilities. The total fusion cross sections of the systems ^{19,20}O+12C and ^{19,20}O+16O around Coulomb barrier energies will be measured using a novel active target-detector system. These results will provide an important examination and constraint for the fusion models. After recalculating the fusions rates of 24O+24O and similar reactions, their impact for the X-ray superburst will be explored.

X射线超爆模型认为存在于中子星外壳层中的丰中子轻核，如24O、28Ne等，其熔合反应释放的大量能量将会加热中子星的表层物质，使其最终到达X射线超爆的点火温度。因此，这些丰中子轻核间熔合反应率的大小，对我们理解中子星的热演化和X射线超爆现象的点火问题都非常的重要。由于缺乏丰中子核熔合反应的实验数据及其反应机制的复杂性，对其在天体物理能区熔合几率的理论预测变得十分因难。基于此，本项目将针对^{19,20}O+12C和^{19,20}O+16O两个反应系统，利用兰州重离子加速器SFC和RIBLL1次级束流线，通过多核子转移反应9Be(18O,^{19,20}O)产生低能^{19,20}O束流，采用独特的气体活性靶技术，测量这两套系统在库仑位垒附近的总熔合截面，检验和约束相关的理论模型。通过改进后的理论计算，探讨24O+24O及类似的熔合反应对X射线超爆现象的影响。

发生在中子星壳层内的丰中子熔合反应对中子星演化以及X射线超级爆等现象均会产生影响。受限于放射性束流强度和反应机制的复杂性，目前缺乏实验数据对理论模型做出有效约束。基于活性靶技术的时间投影室将工作气体同时作为反应靶，能够实现对反应事件的全记录，与此同时，探测器具备约 4 π立体角接受度和三维径迹重建能力，可以显著提高探测效率，有效进行低流强低反应截面熔合反应的测量。1024 路信号读出的时间投影室已研制完成，并开展了12C+12C反应的验证性实验。近期，利用TPC活性靶，完成了19O+12C熔合截面测量，反应截面将被推到约30mbar，我们的结果将对其是否存在熔合增强现象提供更准确的检验。同时，发展起来的活性靶探测器技术，也为今后核反应测量提供了一个强有力的探测手段。