Spectroscopy of exotic reflection-asymmetric atomic nuclei

原子核可以根据它们所拥有的中子和质子的数量呈现不同的形状。许多原子核是球形的，但有些原子核的形状变形得像橄榄球（拉伸的球体）或南瓜（压扁的球体）。在某些情况下，原子核可以变成反射不对称或梨形。这发生在核图表的某些局部区域，在那里中子和质子占据特定的轨道，这些轨道朝向反射形状。具体地说，感兴趣的轨道在轨道和总角动量上都有3 hbar的差异。占据这些轨道的核子可以通过八极相互作用进行相互作用，从而产生反射不对称或八极变形的形状。在核子图上，这发生在核子数34、56、88和126附近。任何具有这些核子数的原子核都容易受到八极相关的影响，在极端情况下会发生永久的八极形变。具有最强八极相关的核具有接近88的质子数Z和接近126的中子数N，这是轻锕系元素区域。这个区域包括中子缺乏的氡（Z=86）、镭（Z=88）、钍（Z=90）和铀（Z=92）核。在过去的几十年中，这些核中的一些已经在实验中进行了研究，它们已经证明了八极形变的光谱特征，如低的负宇称态，具有相反宇称的交错带，以及强的电偶极跃迁。最近的计算显示，轻锕系元素区域中的八极形变区域可能比以前认为的更广，也就是说，它可能包括缺中子的钚（Z=94）和铥（Z=96）同位素。迄今为止，这些奇异的原子核已经超出了实验研究的范围，但新的实验技术正在使这些原子核变得触手可及。本博士项目将涉及Z值在Z值大于约94的轻锕系元素区域上侧的奇异原子核的研究。博士项目将使用不同的技术来研究这些原子核，例如包括衰变光谱学，其中激发态在重离子聚变蒸发反应中的α衰变后被填充。这些实验将在芬兰于韦斯屈莱大学加速器实验室的RITU反冲质谱仪焦平面上使用一组辐射探测器进行。其他实验也将使用最先进的伽马射线分光计，如芝加哥附近的阿贡国家实验室的数字伽马射线分光计，以及意大利帕多瓦附近的莱尼亚罗国家实验室的新AGATA伽马射线跟踪分光计。在这个博士项目中进行的工作将包括一个或多个这样的实验，包括模拟和建模实验的规划，设置和运行实验，以及数据分析。博士项目的结果将导致更好地了解原子核中奇异反射非对称变形的发展和演变。

Atomic nuclei can assume different shapes depending on the numbers of neutrons and protons that they possess. Many nuclei are spherical but some take on shapes that are deformed like a rugby ball (stretched sphere) or a pumpkin (squashed sphere). In some cases atomic nuclei can become reflection-asymmetric or pear shaped. This occurs in certain localized regions of the nuclear chart, where the neutrons and protons occupy specific orbitals which drive towards a reflection shape. Specifically the orbitals of interest have a difference in both orbital and total angular momentum of 3 hbar. Nucleons occupying these orbitals can interact by the octupole interaction which gives rise to a reflection-asymmetric or octupole-deformed shape. On the nuclear chart, this occurs close to nucleon numbers 34, 56, 88, and 126. Any nucleus with these numbers of nucleons is susceptible to octupole correlations and in extreme cases permanent octupole deformation. The nuclei that possess the strongest octupole correlations have proton number Z close to 88 and neutron number N close to 126, which are the light actinide region. This region includes the neutron deficient radon (Z=86), radium (Z=88), thorium (Z=90), and uranium (z=92) nuclei. Over the past few decades a number of these nuclei have been studied in experiments and they have demonstrated the spectroscopic features of octupole deformation such as low-lying negative-parity states, interleaving bands with opposite parities, and strong electric-dipole transitions. Recent calculations have shown that the region of octupole deformation in the light-actinide region maybe more extended than previously thought - that is, it may include the neutron-deficient plutonium (Z=94) and curium (Z=96) isotopes. To date, these exotic nuclei have been out of the reach of experimental investigation, but new experimental techniques are making these nuclei accessible.This PhD project will involve the study of exotic atomic nuclei in the light actinide region with Z values on the upper side of the region with Z values greater than about 94. The PhD project will use different techniques to study these nuclei, for example including decay spectroscopy where excited states are populated following alpha decay in heavy-ion fusion-evaporation reactions. Such experiments will be performed using an array of radiation detectors at the focal plane of the RITU recoil mass spectrometer at the University of Jyväskylä Accelerator Laboratory in Finland. Other experiments will also be performed using state-of-the-art gamma-ray spectrometers such as Digital-Gammasphere at Argonne National Laboratory near Chicago and using the new AGATA gamma-ray tracking spectrometer at the Legnaro National Laboratory near Padova in Italy. The work undertake in this PhD project will include one or more such experiments, including planning for the experiment with simulations and modelling, setting up and running the experiment, and analysis of data. The results of the PhD project will lead to a better understanding of the development and evolution of exotic reflection-asymmetric deformations in atomic nuclei.