Nuclear Physics Rolling Grant

我们周围的所有物质最终都起源于大爆炸，然而大爆炸只产生了最轻的化学元素，氢、氦和锂。这就引出了其他化学元素在哪里以及如何产生的问题。事实上，核物理学支撑着将最轻的元素转化为元素分布的过程，直到我们周围的92号元素（铀）。这种过程大部分是在恒星中进行的，但要产生最重的元素，并重现已知元素的丰度，显然需要更极端的温度、密度和压力条件。其中最引人注目的是一颗垂死的恒星将自己炸开的超新星，但同样重要的是像新星这样的现象，一颗耗尽了所有核燃料的恒星（白矮星）在双星系统中从邻近的恒星那里窃取了物质后突然猛烈地燃烧起来。我们的研究着眼于核物理在这些爆炸物体中工作的细节，以了解产生了哪些元素以及产生的能量。事实证明，尽管这些过程非常复杂，并且涉及大量可能的核反应，但影响最终结果的只有其中的一小部分。因此，有针对性的研究计划是可能的，但我们需要研究的许多同位素在地球上并不存在，必须在实验室中以放射性光束的形式产生。天体物理过程可以产生什么元素和同位素的问题与核存在的极限密切相关——在核系统崩溃之前，它可以添加多少质子和中子？我们的研究集中在所谓的质子滴线上的原子核上——在这个极限上，再增加一个质子是不可能的，原子核就会分裂。这条线非常接近质子和中子数量相等的原子核线（N=Z）；这种核由于这种对称性而具有特殊的性质。此外，核力不区分质子和中子——这是基本的组成部分，由此产生了重要的后果。例如，所谓的镜像核，在质子和中子数量的交换下是相同的，具有几乎相同的性质；然而，正是这些细微的差别告诉了我们核力量平衡的深刻细节。我们的研究详细调查了这些影响，并将这些知识反馈到我们对恒星爆炸的理解中。除了单个质子和中子的作用外，原子核的整体行为通常反映了原子核中质子和中子集体作用的结果。这种集体行为可以表现为原子核的振动或旋转。旋转行为通常与变形的核形状有关，最常见的是延长变形（橄榄球形）和扁圆变形（Smartie形）。某些原子核的一个显著特性是，它们在选择采用哪种形状或构型方面具有模糊性。相对于储存在原子核中的能量（例如，可以通过裂变释放——E=mc^2），向核系统添加非常少量的额外能量，可以促使原子核从球形变为长形或扁形，反之亦然。这种被称为形状共存的现象对所涉及的单个核的性质高度敏感，并且众所周知，理论模型很难预测。因此，研究原子核的形状共存对我们对原子核的理论认识提出了一个强有力的挑战。当然，对核性质的良好理解是上述核物理学各个方面的基础。因此，形状共存形成了我们研究计划的第三个方面。

All of the matter around us originates utimately from the Big Bang, yet the Big Bang produces only the lightest chemical elements, hydrogen, helium and lithium. This begs the question where and how the other chemical elements are produced. In fact, Nuclear Physics underpins the processes that are responsible for transforming the lightest elements into the distribution of elements up to element 92 (uranium) which we see around us. Much of this processing is carried out in stars but to produce the very heaviest elements, and reproduce the known abundances of the elements, it is clear that much more extreme conditions of temperature, density and pressure are required. The most dramatic of these is the supernova where a dying star blows itself apart, but also important are phenomena such as novae where a star which has exhausted all its nuclear fuel (a white dwarf) suddenly flares up violently after stealing material from a neighbouring star in a binary system. Our research looks into the details of the Nuclear Physics at work in these explosive objects to understand which elements are produced as well as the energy generated. It turns out that even though such processes are highly complex and involve a huge number of possible nuclear reactions, it is only a small subset of these which impact on the final results. A focused research programme is therefore possible but many of the isotopes we need to study do not exist on earth and have to be produced as a radioactive beam in the laboratory. The question of what elements and isotopes can be generated by astrophysical processes is intimately related to the limits of nuclear existence - how many protons and neutrons can be added to a nuclear system before it falls apart? Our research focuses on nuclei on the so-called proton dripline - the limit where adding a further proton is not possible and the nucleus breaks up. This dripline lies very close to the line of nuclei with equal numbers of protons and neutrons (N=Z); such nuclei have special properties by virtue of this symmetry. Moreover, the nuclear force does not discriminate between protons and neutrons - the basic building blocks, and there are important consequences stemming from this. For example, so-called mirror nuclei which are the same under interchange of the number of protons and neutrons have near identical properties; it is the subtle differences, however, that tell us profound details of the balance of nuclear forces. Our research investigates these effects in detail and feeds back this knowledge into our understanding of stellar explosions. As well as the action of individual protons and neutrons, the overall behaviour of the atomic nucleus often reflects the result of the the protons and neutrons in the nucleus acting collectively. This collective behaviour can manifest itself as vibrations or rotations the nucleus. Rotational behaviour is ordinarily associated with a deformed nuclear shape, the commonest being prolate-deformed (rugby-ball shaped) and oblate-deformed (Smartie shaped). One of the striking properties of some atomic nuclei is their ambiguity with respect to which shape or configuration they chose to adopt. Adding a very small amount of extra energy to the nuclear system relative to the energy stored in the nucleus (which can be released, for example, through fission - E=mc^2) can prompt the nucleus to change from a spherical to a prolate or oblate shape, or vice versa. This phenomenon known as shape coexistence is highly sensitive to the properties of the individual nucleus concerned and is notoriously difficult for theoretical models to anticipate. Studying shape coexistence in nuclei therefore provides a strong challenge to our theoretical understanding of the nucleus. Naturally, a good understanding of nuclear properties underpins all aspects of Nuclear Physics discussed above. Shape coexistence therefore forms the third strand of our research programme.