Nuclear Structure Measurements of Exotic Nuclei using Radioactive Ion Beams

现代放射性离子束（RIB）设施允许研究远离稳定性的奇异核的核结构。位于CERN的HIE-ISOLDE和美国的FRIB等RIB设施使用后加速器，其允许通过直接（即，耦合、转移、非弹性和弹性）反应，提供高强度、中等能量的光束[1]、[2]。直接反应是有用的，因为它们是选择性的，并且具有相对简单的实验观测值[3]。RIB可以研究广泛的物理主题。这包括绘制出r，s和rp过程中涉及的不稳定核的单粒子状态，这将有助于理解恒星核合成。在RIB设施中通过Coulex反应对八极形变核进行了测量，为Ra和Rn的同位素具有动态和静态八极形变提供了证据[4]。八极形变搜索在寻找具有永久电偶极矩的原子时很重要，这反过来又表明CP破坏，因此探测标准模型之外的物理[5]。非弹性散射可能是一个有用的探针来测量集体方面的核结构（如八极形变），这是无法通过Coulex反应。对于RIB，重离子束将轰击轻目标（例如，氘代聚酯），其中反应据说以逆运动学发生。在逆运动学中发射的轻离子的能量是高度依赖于角度的，并且在向后的角度处是低的，这使得粒子识别困难。由于传统探测器的有限角分辨率，运动学压缩导致实验室坐标系中激发态之间的分离远小于质心坐标系中的分离[6]。在逆运动学中工作所涉及的挑战导致了螺旋轨道光谱仪（HELIOS）方法的发展，其中大口径磁螺线管被用作粒子光谱仪。在HELIOS的设置中，RIB沿着沿着磁轴进入螺线管，通过一个中空的硅探测器阵列，然后它将轰击放置在磁轴上的轻目标。从反应中发射/散射的轻离子将遵循螺旋轨道，主要是在向后的方向上，回到磁轴。来自反应的重离子在反冲检测器中被检测到，从而允许进行事件重合。这种方法使得在低能量下的粒子识别更加容易，因为回旋加速器周期仅取决于磁场的强度和粒子的质荷比。该技术还消除了在实验室框架中检测到的颗粒的运动压缩[7]。在阿贡国家实验室成功证明HELIOS的原理实验后，位于CERN的Isolde螺线管光谱仪（ISS）被开发出来，并正在领导一系列实验，这些实验将针对广泛的物理主题，已经发表了一些结果[8]，[9]。此外，将在FRIB设施安装名为SOLARIS的类似设计的第三台光谱仪。该博士学位的目的是使用新技术探测外来核的核结构，研究主要基于利用HIE-ISOLDE RIB的ISS。参考文献：[1] P. Reiter和N. Warr，“REX和HIE-ISOLDE再加速束的核结构研究”，Prog Part Nucl Phys，第113卷，第103767页，2020年7月，doi：10.1016/j.ppnp.2020.103767。[2]Y. Kadi等人，“ISOLDE的后加速束”，Journal of Physics G：Nuclear and Particle Physics，第44卷，第8期，第084003页，2017年8月，doi：10.1088/1361-6471/aa 78 ca。[3]C. A. Bertulani和A. Bonaccorso，“直接核反应”，2022年1月。[4]L. P. Gaffney等人，“使用加速放射性束研究梨形核”，《自然》，第497卷，第7448页。199-204，2013年5月，do

Modern day Radioactive Ion Beam (RIB) facilities allow for the study of the nuclear structure of exotic nuclei far from stability. RIB facilities such as HIE-ISOLDE located at CERN and FRIB in the United States use a post-accelerator which allows short lifetime isotopes to be studied via direct (i.e., Coulex, transfer, inelastic and elastic) reactions providing high intensity, moderate energy beams [1], [2]. Direct reactions are useful as they are selective and have relatively simple experimental observables [3]. A broad range of physics topics can be investigated with RIBs. This includes mapping out of single-particle states of unstable nuclei that are involved in r, s and rp processes, which will develop understanding of stellar nucleosynthesis. Searches for octupole deformed nuclei have been performed via Coulex reactions at RIB facilities, providing evidence for isotopes of Ra and Rn to have a dynamic and static octupole deformation [4]. Octupole deformation searches are important when looking for atoms with permanent electric dipole moment, which in turn would indicate CP violation, hence probing physics beyond the standard model [5]. Inelastic scattering may be a useful probe to measure collective aspects of nuclear structure (such as octupole deformation) that is not available via Coulex reactions. For RIBs, a heavy ion beam will bombard a light target (e.g., deuterated polyester), with the reaction is said to occur in inverse kinematics. Energies of emitted light ions in inverse kinematics are highly angle dependent and are low at backward angles which makes particle identification difficult. Kinematic compression causes the separation between excited states in the laboratory frame to be much less than in the centre of mass frame due to finite angular resolution of conventional detectors [6]. The challenges involved with working in inverse kinematics has led to the development of the Helical Orbit Spectrometer (HELIOS) approach, in which a large bore magnetic solenoid is used as a particle spectrometer. In the HELIOS setup, the RIB enters the solenoid along the magnetic axis through a hollow array of silicon detectors after which it will bombard a light target placed on the magnetic axis. Light ions emitted/scattered from a reaction will follow helical orbits, predominantly in the backward direction, back to the magnetic axis. Heavy ions from the reaction are detected downstream in a recoil detector allowing event coincidences to be made. This approach makes particle identification at low energy much easier, as the cyclotron period is dependent only on the strength of the magnetic field and the mass to charge ratio of a particle. This technique also eliminates the kinematic compression for particles detected in the laboratory frame [7]. Following successful proof of principle experiments with HELIOS at Argonne national laboratory, the Isolde Solenoidal Spectrometer (ISS) based in CERN was developed and is leading a campaign of experiments that will target a broad range of physics topics, that has already published some results [8], [9]. Additionally, a third spectrometer of similar design named SOLARIS is going to be installed at FRIB facility. The aim of this PhD will be to probe nuclear structure of exotic nuclei using new techniques, with research primarily based on the ISS that utilises the HIE-ISOLDE RIB. References:[1] P. Reiter and N. Warr, "Nuclear structure studies with re-accelerated beams at REX-and HIE-ISOLDE," Prog Part Nucl Phys, vol. 113, p. 103767, Jul. 2020, doi: 10.1016/j.ppnp.2020.103767.[2] Y. Kadi et al., "Post-accelerated beams at ISOLDE," Journal of Physics G: Nuclear and Particle Physics, vol. 44, no. 8, p. 084003, Aug. 2017, doi: 10.1088/1361-6471/aa78ca.[3] C. A. Bertulani and A. Bonaccorso, "Direct Nuclear Reactions," Jan. 2022.[4] L. P. Gaffney et al., "Studies of pear-shaped nuclei using accelerated radioactive beams," Nature, vol. 497, no. 7448, pp. 199-204, May 2013, do