Edinburgh Nuclear Physics Group Consolidated Grant Proposal - Equipment

爱丁堡核物理小组综合赠款提案 - 设备

• 批准号: ST/L005832/1
• 负责人: Philip J Woods
• 金额: $ 1.85万
• 依托单位: University of Edinburgh
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2014
• 资助国家: 英国
• 起止时间: 2014 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=ST%2FL005832%2F1
• 关键词: Edinburgh; Nuclear; Physics; Group; Consolidated

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.

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发现元素可以通过核反应发生转变时，这种观点受到了打击。有一段时间，人们认为所有元素都是在大爆炸中产生的。贝特和霍伊尔等科学家表明，事实上，几乎所有的元素都是在恒星的核反应中产生的，例如，恒星的核反应也使我们的太阳发光。我们仍在试图了解这些元素是如何产生的，以及恒星在其一生中是如何演化的。这个问题正在通过望远镜对宇宙中恒星化学物质丰度的详细观察来解决，在地球上，通过试图重现恒星中发生的反应来解决。元素可以通过核反应在高爆炸性、高温致密的环境中产生，比如在超新星爆炸中发现的环境，这些物质随后被抛入宇宙，最终在太阳或星际介质等地方出现。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产生和能量产生至关重要。我们可以打个比方，一条河在洪水泛滥时冲破堤岸，然后向完全不同的方向流动：正常情况下，生活是平静而稳定的，但往往是在这些狂暴的事件中，留下了永久的印记。新一代加速器设施能够产生越来越多的涉及这些爆炸过程的关键放射性核物质。所以我们现在可以研究恒星中发生的反应，以及原子核随后的衰变路径，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为它们通常剧烈的历史提供了编码信息。要发现这些元素最初形成的爆炸环境的性质，就需要这些新信息。在恒星较长的静止阶段，它们的演化是由核反应控制的，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。你可能认为这些会更容易研究，但由于反应发生在更低的温度和密度下，核聚变受到带正电的原子核之间的排斥的强烈抑制，并且只能通过量子隧道以极低的概率发生。这导致实验产率很低，而且聚变反应的特征被宇宙射线产生的反应淹没了。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提供了保护。恒星的结构与核物质的结构密切相关。中子星是超新星爆炸的遗迹，可以看作是由引力聚集在一起的巨大原子核。我们用高能点状基本粒子束进行的精密实验揭示了原子核周围几乎纯中子物质的外壳，其精确的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自己的不同方式。这些排列以不同激发态的形式出现，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，所以我们要寻找新的！更奇特的构型是所谓的杂化态，其中将夸克结合在一起的胶与夸克结合，产生一种新的物质形式。这将是一个重大发现。