Underground Study of Stellar Neutron Sources for Trans-Fe Element Production in Stars

原子核科学咨询委员会认为，元素的起源是当今最重要的问题之一。目前的天文观测表明，地球上常见的许多元素首先是在恒星中心合成的，在那里发生聚变和其他核反应。这些核反应控制着相关的能量产生和恒星的演化。然而，恒星的条件很难在实验室中重现。在这里提出的特殊条件下，使用一种放置在地下深处的新型紧凑型加速器，可以测量恒星中发生的一些关键核反应。这项研究的目标是测量核过程，这些过程是导致元素合成的反应链的一部分。 这些元素使地球上的生命成为可能。该项目旨在研究驱动s过程的恒星中子源的强度，s过程是产生重元素的两个主要来源之一。最关键的过程之一是α粒子引起的Ne-22反应，产生Mg-25和中子，但其他α中子反应也可能根据恒星环境而发挥作用。然而，大的宇宙射线引起的中子背景一直禁止推进这些测量到恒星的能量范围和目前的反应速率依赖于理论外推，进行高不确定性。目前还没有设施可以在背景中子自由环境中进行这些测量。在过去的几年里，CASPAR（紧凑型加速器系统进行天体物理研究）仪器已建成在桑福德地下研究设施，以满足这一需求。CASPAR运行着一个100万伏特的高强度、完全翻新的货车德格拉夫加速器，可以提供几百微安的α束强度。拟议的实验依赖于使用固体和再循环气体靶系统和He-3中子计数探测器系统。CASPAR科学计划的成功实施将为美国核天体物理界提供一个保持该领域领导地位的竞争机会。它将提供一个世界范围内的独特的机会，研究恒星氦燃烧反应与合成碳和中子生产的反式铁核。

The origin of the elements has been identified by the Nuclear Science Advisory Committee as one of the most important questions of our day. Current astronomical observations indicate that many elements found commonly on Earth were first synthesized in the center of stars, where fusion and other nuclear reactions take place. These nuclear reactions control the associated energy generation and evolution of stars. However, the stellar conditions are difficult to reproduce in the laboratory. Under the special conditions proposed here, using a novel compact accelerator placed deep underground, some of the key nuclear reactions that take place in stars can be measured. The goal of this research is to measure nuclear processes that are part of a chain of reactions leading to the synthesis of elements. These are the elements that make possible life on Earth. The project seeks to study the strength of stellar neutron sources that drive the s-process, one of the two dominant sources for the production of heavy elements. One of the most critical processes is alpha-particle induced reactions on Ne-22, producing Mg-25 plus a neutron, but other alpha-neutron reactions may also play a role depending on the stellar environment. However, the large cosmic ray induced neutron background has been prohibitive for advancing these measurements into the stellar energy range and the present reaction rates rely on theoretical extrapolations that carry high uncertainties. There has been no facility where these measurements can be pursued in a background neutron free environment. Over the past years the CASPAR (Compact Accelerator System for Performing Astrophysical Re¬search) instrumentation has been constructed at the Sanford Underground Research Facility to address this need. CASPAR operates a 1 million-Volt, high intensity, fully refurbished Van de Graaff accelerator that can provide alpha beam intensities of several hundred micro-Ampere. The proposed experiments rely on the use of a solid and a recirculating gas target system and a He-3 detector system for neutron counting. Successful implementation of a science program at CASPAR will offer a competitive opportunity for the US nuclear astrophysics community to maintain leadership in the field. It will provide a world-wide unique opportunity to study stellar helium burning reactions associated with the synthesis of carbon and the neutron production for trans-iron nuclei.