Nuclear Structure Measurements of Exotic Nuclei using Radioactive Ion Beams

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