Precision tests of the nuclear wave-function using exotic beams

1932年，科克罗夫特和沃尔顿实现了炼金术士将一种化学元素转化为另一种化学元素的梦想，他们用原型加速器中的低能质子轰击锂，将其分解成两个氦核。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原子核（即自然界中发现的同位素）之间的核反应所能取得的成就。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认识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）非常不同的原子核来提高。更好地理解核结构的潜在机制是我们研究的目标，但它也有核物理以外的后果。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被认为是合成的。ISOLDE设施是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，在所谓的同位素在线分离（ISOL）技术中，放射性原子核以高强度产生，在这种技术中，主要目标被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。该设施在可产生的光束的多样性方面是世界上独一无二的。ISOLDE最近的一项进展是所谓的REX-ISOLDE设备，它将这些放射性原子核加速到能量，当它们与固定目标中的其他原子核接触时，它们开始抵抗正电荷质子之间的库仑排斥。在这样的能量下，相互作用的发生使我们能够高精度地探测这些外来放射性原子核的结构。其中两种机制是本拨款申请的重点。第一种，被称为库仑激发，是相互作用的一些能量将原子核激发到更高的能态。发生这种情况的容易程度反映了核的集体性，这种性质通常对于变形的核来说是最大的，通常具有非球形，如橄榄球形状，称为延伸变形。因此，库仑激发测量使我们能够研究核的形状，特别是研究一类特殊的核的形状共存。当某种同位素的不同状态具有不同的形状时，就会发生这种现象。我们打算采用的第二种机制被称为光离子转移。在加速束核和靶核之间的相互作用中，质子和中子等粒子被交换。然后，转移的粒子将占据它被添加到的原子核中许多允许的能态中的一个/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能态。通过测量粒子在这种反应中转移的难易程度，我们可以高精度地推断出它被转移到的原子核中能量状态的细节。这对于中子非常丰富的原子核来说尤其有趣，因为这些状态预计会相对于它们在我们过去研究过的不那么奇特的原子核中的位置发生移动。有人认为，这种行为可以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，我们对核物理学中可能发生的变化有很强的限制，并且有很强的动机进行这种研究。

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 nuclear structure 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.