Description

The general framework of this PhD project is structure of atomic nuclei and more particularly the study of the collective properties of the atomic nucleus. This project is a part of an extensive experimental program on nuclear shapes, which our group has been pursuing for several years. We combine multiple complementary experimental techniques: identification of excited states and analysis of their decay using high-resolution gamma-ray spectroscopy, direct lifetime measurements of the excited states and, finally, Coulomb excitation, which allows a measurement of the charge distribution (i.e. the shape) of the studied nuclei. The information on the structure of the excited states obtained from the combination of the various measurements allows theoretical models of nucleus structure to be tested in detail.

The shape of a nucleus, i.e. the deviation of its mass and charge distribution from sphericity, is one of the fundamental nuclear properties. It is governed by both macroscopic effects (the nucleus behaves like a liquid drop) and microscopic effects, such as the shell structure of the nucleus. Their competition may lead to rapid changes in the shape of nuclei as a function of the number of nucleons.

While nuclei with closed proton and neutron shells are always spherical, those away from the closed shells will deform to minimise their potential energy, and most often assume an elongated ellipsoidal shape. Heavy nuclei, the so-called actinides are strongly deformed in their ground state and their excited states show a particular pattern, so-called rotational bands similar to molecules. These bands can be built upon states with different degrees of deformation (“shape coexistence”) [1,2] or related to other degrees of freedom, e.g. vibrations of the nuclear surface. In the nuclide 232Th many such rotational bands have been observed in the past, but their real nature is a long-standing question [3].

The thesis will focus on the experimental study of the collective properties of 232Th using the powerful Coulomb-excitation technique [4], which is the most direct method to determine the shapes of nuclei in their excited states. In this process of nearly elastic scattering of two nuclei, the electromagnetic field acting between them causes their excitation. The nuclei then immediately de-excite, emitting photons that are measured with gamma-ray spectrometers surrounding the target.

If the distance of closest approach between the projectile and target during the scattering process is sufficiently large, the short-range nuclear interaction can be neglected and the excitation can be described using the electromagnetic interaction, the properties of which are well known. Consequently, the population cross sections of the excited states measured in Coulomb-excitation experiments can be directly related to the static and dynamic moments of the charge distribution (i.e. the shape) of the nuclei under study.

In this experiment we will use AGATA [5,6], a new-generation gamma-ray spectrometer, consisting of a large number of finely segmented germanium crystals, which allows us to identify each point where a gamma ray interacts with the detector material and then, using the so-called “gamma-ray tracking” concept, to reconstruct the energies of all emitted gamma rays and their angles of emission with highest precision. This powerful spectrometer, offering unprecedented detection efficiency and experimental sensitivity, was developed by a large collaboration of researchers from 13 European countries and is currently installed at the National Laboratories of Legnaro (LNL, Italy) where the principal experiment will be realized. A second experiment will take place at the Heavy-Ion Laboratory of the University of Warsaw.

Skills required

The prerequisite for a doctoram thesis is a master M2 in subatomic physics or possible in instrumentation. We are looking for candidates with a strong interest in nuclear structure physics, who enjoy experimental work and are not afraid of complex detection systems. A dedicated nuclear physics course on the master level as well as an internship or master thesis in the domain of experimental nuclear physics are strongly recommended. Some knowledge of computer programming (C, C++, root) is also necessary, as well as at least B2 level in English.

Bibliography

[1] K. Heyde and J. L. Wood, Rev. Mod. Phys. 83, 1467 (2011)
https://doi.org/10.1103/RevModPhys.83.1467
[2] P. Garrett, M. Zielińska and E. Clement, Prog. Part. Nucl. Phys. 124, 103931 (2022)
https://doi.org/10.1016/j.ppnp.2021.103931
[3] W. Korten et al., Phys Lett. B 317, 19 (1994) https://doi.org/10.1016/0370-2693(93)91563-3
[4] M. Zielińska et al., Eur. Phys. J. A 52, 99 (2016) https://doi.org/10.1140/epja/i2016-16099-8
[5] S. Akkoyun et al., Nucl. Instrum. Meth. Phys. Res. A 668, 26 (2012) https://doi.org/10.1016/j.nima.2011.11.081
[6] W. Korten et al., Eur. Phys. J. A 56, 137 (2020) https://doi.org/10.1140/epja/s10050-020-00132-w

Keywords

Nuclear Physics, Nuclear Structure, Gamma-ray spectroscopy, Deformation and Collectivity