MPhil/Honours Student Projects

Experimental (+Theoretical)

Objective

Neutron-rich nuclei in the A=100 region behave differently to the near-by stable isotopes. Is this exotic behaviour due to the neutron excess?

Description

A frontier in nuclear physics is the study of novel structures in exotic nuclei with many more neutrons than normally exist in nature. In the region near A=100 the magnetic moments (or g factors) of several neutron- rich isotopes have been measured recently [1]. As shown in the figure, the neutron-rich nuclei follow a different trend to that of the stable isotopes. Why? Something is certainly changing. The stable isotopes 100Ru, 102Ru, and 104Ru lie at the fork in the plot. Unfortunately the existing data for these important isotopes does not have the required level of precision or reliability.

In this project precise measurements will be made of the magnetic moments of the Ru isotopes, using the transient-field technique [2]. Magnetic moments are very sensitive probes of nuclear structure. Along with the execution of the experiment and analysis of the data, there will be the opportunity to explore the theoretical interpretation.

References

1. A.G. Smith et al., Phys. Lett. B 591, 55 (2004).
2. P.F. Mantica, A.E. Stuchbery et al., Phys. Rev. C 63, 034312 (2001)

Experimental

Objective

How do ions moving rapidly through a magnetic host pick up polarization from the host? Do the ions interact equally with the orbital and spin contributions to the magnetization?

Description

An ion moving swiftly through a magnetic host has an enormous magnetic field produced at its nucleus (typically several thousand Tesla). In effect, the host’s magnetism is amplified by transferring polarization to the moving ion. However, the polarization-transfer mechanism is not well understood and is controversial. To date all measurements have used hosts like iron and gadolinium, in which the magnetism is carried entirely by the spin of the host electrons. Hosts such as terbium and dysprosium have a similar total magnetization to iron and gadolinium, but it is carried by both the orbital and the spin angular momentum of the host electrons.

The new hyperfine-interactions spectrometer shown in the figure will be used to measure the magnetic fields that act on the nuclei of 20Ne and 24Mg ions as they move rapidly through a terbium host. High velocity Ne and Mg nuclei will be produced using beams from the 14UD Pelletron. The experimental results will be compared with similar recent measurements, which used a gadolinium host [1]. By comparison of the field strengths, it will be determined if the moving ions pick up polarization from the orbital part of the host’s magnetization as efficiently as they do from the spin contribution.

References

1. A.E. Stuchbery et al., Phys. Lett. B 611, 81 (2005)

Experimental/data analysis

Objective

To search for evidence of highly extended shapes in isotopes of iodine and xenon.

Description

The observation of superdeformed states in atomic nuclei, in which the nucleus adopts an extremely deformed, rugby-ball shape with a major:minor axis ratio around 2:1, was one of the most exciting discoveries of modern nuclear structure studies. These states are stabilized by a combination of macroscopic (Coulomb and rotational) and microscopic (shell structure) effects. Because the nucleus is deformed, it can rotate (a spherically symmetric quantum system cannot exhibit roational degrees of freedom), and superdeformed nuclei are identified experimentally by the long sequences of regularly-spaced gamma-ray transitions between excited nuclear levels corresponding to a rotational band. Hyperdeformed states, in which the nucleus is even more deformed (with an axis ratio approaching 3:1) have been predicted for some time now but have not yet been experimentally observed.

The project will involve analysis of gamma ray coincidence data obtained using the two multi-detector arrays, Euroball IV and Gammasphere, in order to search for evidence of such highly deformed states. Depending on the student's preferences, the project can be focussed on either discovering and interpreting the high spin structure of the nuclei under study, or on developing computer algorithms to search for the weak signals of hyperdeformation.

Experimental/data analysis

Objective

To characterise the high-spin structure of neutron-deficient isotopes of polonium, in particular searching for evidence of magnetic rotation.

Description

Magnetic rotation was observed for the first time around 15-20 years ago, and is the subject of much theoretical and experimental investigation. Although it is thought be associated with near-spherical nuclear shapes, magnetic rotation gives rise to cascades of gamma rays with fairly regular energy spacings deexciting connnected states with high angular momentum and excitation energy. Several mysteries persist regarding the precise nature of these states, not least the question of whether the nucleus needs to have some small but finite deformation in order to support them.

Magnetic rotational states have been observed in several isotopes of lead. It is expected that they should also occur in isotopes of polonium, but as yet none have been found. Projects are available for a student to analyse existing data on ²⁰⁰Po or ¹⁹⁶Po with the aim of searching for both superdeformed and magnetic rotational structures. Other possibilities involve an investigation of the Tilted Axis Cranking model used to describe magnetic rotational structures and is application to both lead and polonium isotopes.

The discovery or synthesis of new elements has always fascinated scientists, and currently the search is on to find new “superheavy” elements (elements with atomic number around 120). These elements are synthesised in the laboratory by fusing two heavy nuclei. Fusion leads to superheavy element formation only when the combined many-body quantum system survives the competing processes of fission and quasi-fission, which cause the system to break apart. This depends sensitively on many variables, such as the shape of the interacting nuclei, their mass difference, shell structure, and possibly the number of excess neutrons. The group is currently working on the challenging tasks of isolating the factors that influence the formation of heavy elements, and of theoretically predicting their yields. Using the 15 Million Volt electrostatic accelerator, and a highly efficient fission detector, this research project will involve making measurements of fission cross-sections, and mass distribution of the outgoing fragments. This will allow us to obtain a picture of the dynamics of the fusion reaction. The project can also involve working on theoretical aspects by developing a stochastic model aimed at simulating the process of fusion and quasi-fission. The relative weights of the experimental and theoretical components can be tailored to suit the interests of the student.

Research with exotic radioactive (unstable) nuclei, with 2-3 times more neutrons than protons is a major motivation for the current projects to develop large international radioactive accelerator facilities. For example in exotic 6He, two neutrons are so weakly coupled to the 4He core that their wave-function extends to 10 times the core radius (forming a “neutron-halo”). Not only can they interact with the target nucleus at much larger distances than the core, but they also interact more strongly with, and can bind more strongly to the target nucleus than to the 4He. Their (many-body) interactions with the individual nucleons of the target nucleus constitute environmental interactions and can lead to decoherence. The group’s research involves experimental and theoretical investigations aimed at understanding the different interactions of stable and unstable weakly bound nuclei, and developing a radioactive beam capability at the ANU. A new position sensitive Si detector array is currently being installed to detect the charged break-up fragments at backward angles. The student can work on hands-on developmental aspects and/or experiments and analysis of break-up and fusion reactions, or theoretical modelling and simulations. The project can also have a mix of two of these components, depending on the interest of the candidate.