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

英国核物理咨询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，核子在中子星星的高密度环境中如何相互作用？虽然对这些问题至关重要的一些同位素可以在实验室中进行研究，但其中大多数都是遥不可及的，科学界必须依赖基于核模型和现有数据的推断。该项目的背景是镍的同位素78 Ni，Z=28质子和N=50中子，这两个数字为原子核提供了特殊的额外稳定性，就像封闭的壳对原子的作用一样，被称为魔法。然而，这些幻数在非常奇异的系统中的持久性受到质疑，并可能影响核模型。在该项目中，正在研究N=50及其附近的铜（Z=29）和镓（Z=31）同位素。它们的基态性质，如自旋，形状和电磁性质（命名为矩）的确定将使人们深入了解它们的量子构型，探索Z=28和N= 50的神奇本质。富中子镓同位素的半衰期也将得到解决。最近已查明这些因素偏离了预测的外推。新的计算是在观测的基础上进行的，这可能对我们太阳系中某些元素的丰度产生高达200%的影响。因此，对现代核物理的主要问题来说，确认这些测量结果并将其扩展到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（来自共线几何结构），同时受益于高效率（来自离子检测）。在CERN ISOLDE设施在线产生的外来放射性离子束被成束输送到实验装置，在那里它们在碱蒸汽中被中和。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的元素的原子结构中的特定跃迁。价电子由此被激发超过电离阈值，并且同位素被电离。然后用光束强度监测器（例如微通道板检测器）或衰变光谱站进行计数。激光频率的高分辨率允许探测百万分之一水平的原子跃迁的核诱导扰动，并确定基态性质，如自旋、电磁矩或质子分布的变化。这种方法也可用于选择特定的同位素，甚至异构体，从一束中提取一个超纯的同位素样本，并将其送到衰变光谱站。该技术的这一特殊方面将用于研究最奇特的镓核的衰变特性，但也可以为在全球范围内生产参考样品或同位素比率监测提供独特的机会。

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.