PARADISE:Purification-assisted radioactive-decay and ionisation spectroscopy of very exotic isotopes of copper and gallium at the CERN-ISOLDE facility

英国核物理顾问小组最近确定了一系列关于原子核的关键问题。例如，反应速率如何决定宇宙中不同元素的最终丰度？或者核子在中子星的高密度环境中是如何相互作用的？虽然对这些问题至关重要的一些同位素可以在实验室进行研究，但其中大多数是遥不可及的，科学界必须依靠基于核模型和现有数据的推断。该项目设置在78Ni附近，78Ni是镍的一种同位素，具有Z=28个质子和N=50个中子，这两个数字为原子核提供了特殊的额外稳定性，就像原子的封闭壳层一样，被称为魔法。然而，在非常奇特的系统中，这些神奇数字的持久性受到质疑，并可能影响核模型。在这个项目中，铜（Z=29）和镓（Z=31）的同位素在N=50和附近进行了研究。确定它们的基态特性，如自旋、形状和电磁特性（称为矩），将使我们深入了解它们的量子构型，探测到Z=28和N=50的神奇本质。富中子镓同位素的半衰期也将讨论。那些最近被确认偏离了预测的外推。在观测的基础上进行了新的计算，这些观测可能会对我们太阳系中某些元素的丰度产生高达200%的影响。因此，对现代核物理学的主要问题来说，确认这些测量结果并使其更接近感兴趣的同位素是至关重要的。为了实现这个项目，将使用共线共振电离光谱（CRIS）技术。它结合了高分辨率（来自共线几何），同时受益于高效率（来自离子检测）。欧洲核子研究中心（CERN） ISOLDE装置在线产生的外来放射性离子束被成群地送到实验装置，在那里它们被碱蒸气中和。原子束与一系列激光束重叠，这些激光束的频率被调谐到感兴趣元素的原子结构中的特定跃迁。价电子因此被激发超过电离阈值，同位素被电离。然后用光束强度监测器（如微通道板探测器）或衰减光谱站进行计数。激光频率的高分辨率允许探测核引起的原子跃迁的微扰，在100万分之一的水平上，并确定基态性质，如自旋，电磁矩或质子分布的变化。该方法还可用于从一堆同位素中选择特定的同位素，甚至是同分异构体，并将鉴定良好的同位素的超纯样品送到衰变光谱站。该技术的这一特殊方面将用于研究最奇异的镓核的衰变特性，但也可以为生产参考样品或在全球范围内监测同位素比率提供独特的机会。

The UK Nuclear Physics Advisory Panel has recently identified a series of key questions concerning the atomic nucleus. For example, how do reaction rates determine the final abundance of the different elements in the universe? Or how do the nucleons interact in the high-density environment of a neutron star? While some of the isotopes that are key to these questions may be studied in the laboratory, most of them are out reach and the scientific community must rely on extrapolations based on nuclear models and existing data.This project is set in the vicinity of 78Ni, an isotope of nickel with Z=28 protons and N=50 neutrons, both numbers providing special additional stability to a nucleus, as closed shells do to an atom, and being referred to as magic. The persistence of those magic numbers in very exotic systems is however questioned and could impact the nuclear models. In this project, the copper (Z=29) and gallium (Z=31) isotopes at and around N=50 are under investigation. The determination of their ground-state properties such as spin, shape and electromagnetic properties (named moments) will bring an insight into their quantum configuration, a probe to the magic nature of Z=28 and N=50.The half-lives of the neutron-rich gallium isotopes will also be addressed. Those have recently been identified to depart from the predicted extrapolations. New calculations were made on the basis of the observation that may have an impact up to 200% on the abundance of some elements in our solar system. It is therefore crucial to the main questions of modern nuclear physics to confirm those measurements and to extend them closer to the isotopes of interest.In order to achieve this project, the technique of Collinear Resonance Ionisation Spectroscopy (CRIS) will be used. It combines high resolution (from the collinear geometry) while benefiting from high efficiency (from the ion detection). The exotic radioactive ion beams produced online at the CERN ISOLDE facility are delivered in bunches to the experimental setup where they are neutralised in an alkali vapour. The atom bunches are overlapped with a series of laser beams, which frequencies are tuned to specific transitions in the atomic structure of the element of interest. A valence electron is hereby excited beyond the ionisation threshold and the isotope is ionised. It is then counted with a beam intensity monitor (e.g. micro-channel plate detector), or a decay spectroscopy station. The high resolution of the laser frequency allows to probe nuclear-induced perturbations of the atomic transition at the level of 1 part in 1,000,000 and determine ground-state properties such as spin, electromagnetic moments or changes in the proton distribution.This method can also be used to select a specific isotope, or even isomer, from a bunch and deliver an ultra-pure sample of a well identified isotope to a decay spectroscopy station. This particular aspect of the technique will be used to study the decay properties of the most exotic nuclei of gallium, but could also offer unique opportunities for the production of reference samples or isotopic ratios monitoring on a global scale.