Edinburgh Nuclear Physics Group Consolidated Grant Proposal - Equipment

希腊人过去常说物质是不可分割的。当卢瑟福和他的同事们证明元素可以通过核反应转化时，这个想法受到了打击。有一段时间，人们认为所有的元素都是在大爆炸中产生的。贝特和霍伊尔等科学家证明，几乎所有的元素都是在恒星的核反应中产生的，例如，这也使我们的太阳发光。我们仍在寻求理解这些元素产生的方式以及恒星在其一生中如何演变。这个问题正在通过用望远镜对宇宙中恒星化学丰度进行新的详细观测来解决，而在地球上，通过试图重现恒星中发生的反应来解决。元素可以通过在超新星爆炸中发现的高爆炸性，热密度环境中的核反应产生，随后物质被抛入宇宙，最终在我们的太阳或星际介质等位置获得。在爆炸环境中，不稳定核的反应和性质对于理解这些过程中的元素生产和能量产生至关重要。人们可以用一条洪水泛滥的河流来做类比，河水冲破堤岸，然后流向完全不同的方向：正常情况下，生命是静止和稳定的，但往往是在这些暴力事件中，留下了永久的印记。新一代加速器设施能够生产越来越多的这些爆炸过程所涉及的关键放射性核素。所以我们现在可以研究恒星中发生的反应，以及随后原子核的衰变路径，最终形成我们周围看到的稳定同位素。这些稳定同位素的元素丰度提供了关于其经常暴力历史的编码信息。需要这一新的信息来发现这些元素最初形成的爆炸性环境的性质。在恒星较长的静止阶段，它们的演化受核反应控制，这些核反应发生在较低的温度和密度下，并且涉及稳定的同位素。你可能会认为这些反应更容易研究，但由于反应发生在低得多的温度和密度下，核聚变被带正电的核之间的排斥强烈抑制，并且只能通过量子隧穿以非常低的概率发生。这导致了低的实验产额，并且聚变反应的特征被宇宙射线产生的反应淹没。所以我们现在在世界上唯一的地下核天体物理加速器实验室工作，上面的岩石为我们的实验形成了一个保护罩。恒星的结构与核物质的结构密切相关。中子星，超新星爆炸的遗迹，可以被有效地看作是巨大的原子核由引力结合在一起。我们正在用高能点状基本粒子束进行的精密实验揭示了核周围几乎纯中子物质的皮肤，其精确厚度告诉我们中子星的可能结构。这些光束还允许我们窥视质子内部，探索内部夸克重新排列的不同方式。这些排列以不同的激发态的形式出现，称为核子共振。我们认为我们有一个很好的理论，QCD，来理解质子，但事实上它预测的共振比我们观察到的要多得多，所以我们要去寻找新的共振！甚至更奇特的配置，是所谓的杂交，其中粘合夸克在一起的胶水与夸克结合，产生一种新的物质形式。这将是一个重大发现。

The Greeks used to say matter was indivisible. This notion took a beating when Rutherford and co-workers showed that elements could be transformed by nuclear reactions. For a while it was thought that all the elements were produced in the big bang. Scientists such as Bethe and Hoyle showed in fact that nearly all the elements are produced in nuclear reactions in stars, which also for example make our sun shine. We are still seeking to understand the means by which these elements are produced and how stars evolve during their lifetime. This problem is being addressed through new detailed observations of stellar chemical abundances in the cosmos with telescopes, and here on earth, by trying to re-create the reactions occurring in stars. Elements can be produced by nuclear reactions in highly explosive, hot dense environments such as found in supernovae explosions, with the material subsequently thrown out into the cosmos, and eventually fetching up in locations such as our sun or the interstellar medium. In explosive environments it is the reactions and properties of unstable nuclei that are critical for understanding element production and energy generation in these processes. One can make an analogy with a river in full flood bursting its banks and then flowing in completely different directions: normally life is quiescent and stable, but it is often in these violent episodes that permanent imprints remain. New generation accelerator facilities are able to produce an increasingly large number of the key radioactive nuclear species involved in these explosive processes. So we can now study the reactions occurring in the stars and the subsequent decay paths of nuclei that end up in the stable isotopes we see around us. The elemental abundances of these stable isotopes provide coded information on their often violent history. This new information is required to discover the nature of the explosive environments in which such elements were first formed. In the longer quiescent phase of stars, their evolution is controlled by nuclear reactions occurring at much lower temperatures and densities, and which involve stable isotopes. You might think these would be easier to study, but because the reactions occur at much lower temperatures and densities nuclear fusion is strongly inhibited by the repulsions between the positively charged nuclei, and can only take place with very low probability by quantum tunneling. This leads to low experimental yields, and the signature for the fusion reaction is swamped by reactions produced by cosmic rays. So we are now working at the only underground nuclear astrophysics accelerator laboratory in the world where the rock above forms a protective canopy for our experiments.The structure of stars is intimately tied to the structure of nuclear matter. Neutron stars, a relic of supernovae explosions can usefully be viewed as gigantic nuclei held together by the gravitational force. Precision experiments we are performing with high energy point-like fundamental particle beams are revealing a skin of almost pure neutron matter around the nucleus whose precise thickness tells us about the likely structure of neutron stars. These beams also allow us to peer inside a proton and explore the different ways the quarks inside can re-arrange themselves. These arrangements take the form of different excited states known as nucleon resonances. We think we have a good theory, QCD, to understand the proton but in fact it predicts many more resonances than we observe, so we are going to search for the new ones! Even more exotic configurations, are the so-called hybrids, in which the glue binding quarks together combines with quarks to produce a new form of matter. This would be a major discovery.