Precision Tests of the Nuclear Wavefunction using Exotic Beams

1932年，考克饶夫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元素，他们用原型加速器中的低能质子轰击锂，将其分解为两个氦核。尽管这项开创性的工作已经过去了70年，但我们对核物理的理解在很大程度上仍然取决于通过诱导稳定核（即自然界中发现的同位素）之间的核反应所能实现的目标。这主要限制了对接近稳定线的原子核的精确研究。只有通过研究质子数和中子数与稳定同位素（数量相对较少）非常不同的原子核，我们才能进一步了解核力和原子核的行为。更好地理解原子核的基本机制是我们研究的目标，但它也有超出核物理的后果。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都被认为是合成的。在位于瑞士日内瓦的欧洲核子研究中心国际实验室的一部分ISOLDE设施，以所谓的同位素分离在线技术产生高强度的放射性核，该技术用强高能质子束轰击主要靶。ISOLDE使用不同的主要目标，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。该设施在世界范围内是独一无二的，因为它可以产生各种各样的光束。ISOLDE最近的一个进展是所谓的REX-ISOLDE设施，它将这些放射性核加速到当它们与固定目标中的其他核接触时开始抵抗带正电的质子之间的库仑排斥的能量。在这样的能量下，相互作用的发生使我们能够以高精度探测这些奇异放射性核的结构。其中两个机制是本次赠款申请的重点。第一种，称为库仑激发，是相互作用的一些能量进入激发原子核进入更高的能量状态。这发生的容易程度反映了核集体性，这是一种通常对于变形的核最大的性质，通常具有非球形形状，例如橄榄球形状，称为扁长变形。因此，库仑激发测量使我们能够研究核的形状，特别是某些特殊类别的核，表现出形状共存。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被称为轻离子转移。在加速束核和靶核之间的这种相互作用中，交换了诸如质子和中子的粒子。转移的粒子然后将占据它被添加到的原子核中的许多允许的能量状态之一/由于量子力学在确定原子核性质方面的重要性，只有一小部分允许的状态。通过测量粒子在这样的反应中转移的容易程度，我们可以高精度地推断出它被转移到的原子核中的能量状态的细节。这对于非常富中子的核特别有趣，因为这些状态预计会相对于我们过去研究过的较不奇异的核中的位置发生变化。这表明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，我们对核物理可能发生的变化有很强的约束力，也有进行此类研究的强烈动机。

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.