核心坍缩超新星中微子驱动风内弱r过程88Sr(α,n)91Zr反应的直接截面测量
Direct cross-section measurement of the weak r-process 88Sr(α,n)91Zr reaction in ν-driven winds of core collapse supernovae
AI总结：
本研究首次在天体物理能区直接测量了弱r过程关键的88Sr(α,n)91Zr反应截面，发现其值比理论预期低3倍，为改进核反应统计模型、提升弱r过程丰度预测可靠性提供了实验依据。
摘要：
已知铁以上的重元素约有一半是通过快中子俘获过程（即r过程）产生的。然而，产生r过程的天体物理位点仍未确定。在多个宇宙位点观测到的化学丰度表明，应当存在不同的作用机制。例如，在一部分贫金属星中测得的银附近元素丰度，表明存在弱r过程。该过程可能发生在核心坍缩超新星的中微子驱动风中，在这类环境的抛射物质中，(α,n)反应主导了原子序数Z~40的元素合成。(α,n)反应的反应率测量数据极少，目前是通过结合α光学模型势的统计Hauser-Feshbach计算得到的，而α光学模型势的约束仍然很差。因此，(α,n)反应率的不确定性是恒星模型计算所得丰度不确定性的重要来源。在本工作中，研究人员首次在天体物理能区对影响弱r过程丰度的88Sr(α,n)91Zr反应进行了探测；直接测量了质心系能量8.37-13.09 MeV（对应温度3.8-7.5 GK）天体物理能区下的总截面。研究人员在ATLAS装置上使用电分段电离室MUSIC，采用逆运动学结合主动靶技术开展了两次测量。结果发现，发生在满壳层N=50处的88Sr上的该α诱导反应的截面，比预期值低3倍，尽管近期的统计计算已通过邻近核的测量得到了验证。这一结果推动学界对N=50处以及向丰中子侧的(α,n)反应开展更多实验研究，以进一步检验此类计算的预测能力与可靠性。
英文摘要：
About half of the heavy elements beyond iron are known to be produced by the rapid neutron capture process, known as r-process. However, the astrophysical site producing the r-process is still uncertain. Chemical abundances observed in several cosmic sites indicate that different mechanisms should be at play. For instance, the abundances around silver measured in a subset of metal-poor stars indicate the presence of a weak r-process. This process may be active in neutrino-driven winds of core collapse supernovae where ($α$,n) reactions dominate the synthesis of Z ~ 40 elements in the expelled materials. Scarcely measured, the rates of ($α$,n) reactions are determined from statistical Hauser-Feshbach calculations with $α$-optical-model potentials, which are still poorly constrained. The uncertainties of the ($α$,n) reaction rates therefore make a significant contribution to the uncertainties of the abundances determined from stellar modeling. In this work, the $^{88}$Sr($α$,n)$^{91}$Zr reaction which impacts the weak r-process abundances has been probed at astrophysics energy for the first time; directly measuring the total cross sections at astrophysical energies of 8.37 - 13.09 MeV in the center of mass (3.8 - 7.5 GK). Two measurements were performed at ATLAS with the electrically-segmented ionization chamber MUSIC, in inverse kinematics, while following the active target technique. The cross sections of this $α$-induced reaction on $^{88}$Sr, located at the shell closure N = 50, have been found to be lower than expected, by a factor of 3, despite recent statistical calculations validated by measurements on neighboring nuclei. This result encourages more experimental investigations of ($α$,n) reactions, at N = 50 and towards the neutron-rich side, to further test the predictive power and reliability of such calculations.
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