Precision Tests of the Nuclear Wavefunction using Exotic Beams

使用奇异光束精确测试核波函数

• 批准号: PP/F000847/1
• 负责人: Sean J Freeman
• 金额: $ 1.38万
• 依托单位: University of Manchester
• 依托单位国家: 英国
• 项目类别: Research Grant
• 财政年份: 2007
• 资助国家: 英国
• 起止时间: 2007 至 无数据
• 项目状态: 已结题
• 来源: https://gtr.ukri.org/projects?ref=PP%2FF000847%2F1
• 关键词: Precision; Tests; Nuclear; Wavefunction; using

In 1932, Cockcroft and Walton performed the alchemist's dream of transforming one chemical element into another when they bombarded lithium with low energy protons from their prototype accelerator, disintegrating it into two helium nuclei. Despite the passage of 70 years from this pioneering work, our understanding of nuclear physics is still largely dictated by what can be achieved by inducing nuclear reactions between stable nuclei i.e. those isotopes which are found in Nature. This has mainly restricted precision studies to nuclei which are close to the line of stability. Our knowledge of nuclear forces and how nuclei behave can only be advanced by studying nuclei with very different numbers of protons and neutrons those of stable isotopes (which are comparatively few in number). A better understanding of the underlying mechanism of nuclei is the goal of our research, but it also has consequences beyond nuclear physics. For example it can help our understanding of the processes in supernova explosions where most of the heavy elements found in Nature are thought to be synthesised. At the ISOLDE facility, part of the international CERN Laboratory in Geneva, Switzerland, radioactive nuclei are produced with high intensities in the so-called isotope separation on-line (ISOL) technique where a primary target is bombarded with an intense, high energy proton beam. Using different primary targets, ISOLDE can produce beams of varying intensity of over 700 isotopes of 70 different chemical elements. This facility is unique worldwide in the diversity of available beams which it can produce. A recent advance at ISOLDE has been the so-called REX-ISOLDE facility which accelerates these radioactive nuclei to energies where they start to resist the Coulomb repulsion between the positively charge protons when they come into contact with other nuclei in a fixed target. At such energies, interactions take place which allow us to probe the structure of these exotic radioactive nuclei with high precision. Two of these mechanisms are the focus of this grant application. The first, known as Coulomb excitation, is where some of the energy of the interaction goes into exciting the nucleus into higher energy states. The ease with which this takes place reflects the nuclear collectivity, a property which is generally largest for nuclei which are deformed, typically having a non-spherical shape such as a rugby-ball shape, known as prolate deformation. Coulomb-excitation measurements therefore allow us to study the nuclear shape, in particular, a certain special class of nuclei which exhibit shape coexistence. This phenomenon occurs when different states in a particular isotope have different distinct shapes. The second mechanism we aim to employ is known as light-ion transfer. In this interaction between the accelerated beam nucleus and the target nucleus, particles such as protons and neutrons are exchanged. The transferred particle will then occupy one of a number of allowed energy states in the nucleus to which it has been added / there are only a small set of allowed states due to the importance of quantum mechanics in determining the properties of the nucleus. By measuring the ease with which a particle is transferred in such a reaction, we can infer details about the energy states in the nucleus which it has been transferred onto, with high precision. This is especially interesting for the very neutron-rich nuclei since these states are expected to shift around relative to their location in the less exotic nuclei we have studied in the past. It is suggested that this behaviour could very sensitively affect how much of various heavy elements is produced in supernova explosions. Since we know the relative proportions of heavy elements existing in the Solar System, we have a strong constraint on the changes possible in the nuclear physics and a strong motivation for making such studies.

1932 年，科克罗夫特和沃尔顿实现了炼金术士的梦想，将一种化学元素转化为另一种化学元素，他们从原型加速器中用低能质子轰击锂，将其分解成两个氦核。尽管这项开创性工作已经过去了 70 年，我们对核物理的理解仍然在很大程度上取决于通过稳定核（即自然界中发现的同位素）之间诱导核反应所能实现的效果。这主要限制了对接近稳定线的原子核的精确研究。我们对核力和原子核行为的了解只能通过研究质子和中子数量与稳定同位素（数量相对较少）截然不同的原子核来提高。更好地理解原子核的基本机制是我们研究的目标，但它也产生了核物理之外的影响。例如，它可以帮助我们理解超新星爆炸的过程，自然界中发现的大多数重元素被认为是合成的。 ISOLDE 设施是瑞士日内瓦国际 CERN 实验室的一部分，通过所谓的在线同位素分离 (ISOL) 技术产生高强度放射性核，其中主要目标受到强烈的高能质子束轰击。使用不同的主要目标，ISOLDE 可以产生 70 种不同化学元素的 700 多种同位素的不同强度的光束。该设施因其可生产的可用光束的多样性而在世界范围内独一无二。 ISOLDE 的最新进展是所谓的 REX-ISOLDE 设施，它将这些放射性核加速到能量，当它们与固定目标中的其他核接触时，它们开始抵抗带正电荷的质子之间的库仑排斥力。在这样的能量下，发生相互作用，使我们能够高精度地探测这些奇异放射性核的结构。其中两个机制是本次拨款申请的重点。第一个被称为库仑激发，相互作用的一些能量将原子核激发到更高的能态。这种情况发生的难易程度反映了核集体性，这种性质对于变形的核来说通常是最大的，通常具有非球形形状，例如橄榄球形状，称为长形变形。因此，库仑激发测量使我们能够研究核形状，特别是表现出形状共存的某种特殊类别的核。当特定同位素的不同状态具有不同的独特形状时，就会发生这种现象。我们打算采用的第二种机制称为轻离子转移。在加速束核与目标核之间的相互作用中，质子和中子等粒子发生交换。然后，转移的粒子将占据其所添加到的原子核中的多个允许的能量状态之一/由于量子力学在确定原子核的性质方面的重要性，只有一小部分允许的状态。通过测量粒子在此类反应中转移的难易程度，我们可以高精度地推断出粒子转移到的原子核中的能态细节。这对于非常富含中子的原子核来说尤其有趣，因为这些状态预计会相对于我们过去研究过的不太奇异的原子核中的位置发生变化。有人认为，这种行为可能非常敏感地影响超新星爆炸中产生的各种重元素的数量。由于我们知道太阳系中存在的重元素的相对比例，因此我们对核物理可能发生的变化有很强的限制，也有进行此类研究的强烈动机。