Structure of neutron-rich nuclei in the sd and fp shells

Lead Research Organisation: University of Manchester
Department Name: Physics and Astronomy

Abstract

This research is concerned with improving our understanding of the nuclear force that holds protons and neutrons together in nuclei and governs their interactions. By studying the gamma rays emitted by particular unstable isotopes, information can be obtained on how these sub-atomic particles behave both individually and collectively. The 'nuclear shell model' views the protons and neutrons in nuclei as occupying discrete single-particle states with different energies and angular momenta. These states appear to be grouped into shells, separated by large energy gaps at so-called 'magic numbers'. Originally, the shell model was based on stable, or near-stable nuclei. As we move away from stability, new effects become apparent that adjust the structure in unexpected ways. One such change is the evolution of magic numbers with increasing neutron excess. Interactions between the constituent particles in neutron-rich nuclei are raising and lowering the energies of states, resulting in changes in their spacing and ordering. These effects can be measured by exciting nuclei, such that their nucleons occupy higher-energy states than usual. Gamma rays are then emitted as the nuclei return to the ground state; this radiation can tell us about the energy spacing and angular momentum of the excited states. In certain circumstances, new nuclei, more neutron-rich than any previously observed, can be created in excited states. This proposal aims to use both methods to determine further information about these exotic nuclear structures. There are three specific objectives of this work. The first is to study magnesium-30 nuclei. With 12 protons and 18 neutrons, this isotope is neutron-rich compared to the stable magnesium-26. Using a particle accelerator, magnesium-30 can be created at high excitation energies by colliding (for example) uranium-238 and magnesium-26 nuclei. If done inside a large array of detectors the resulting gamma rays, emitted as the nuclei relax to the ground state, can be observed. This will allow a detailed picture of the structure of this nucleus to be built up. Similarly, nickel-68 is also neutron rich, with 40 neutrons and 28 protons. This nucleus is also particularly interesting as 40 and 28 are considered magic numbers on the line of stability; however, with increasing neutron excess it is suspected that 40 ceases to be a magic number. Some of the structure of nickel-68 is already known, including long-lived states called 'isomers'. While these states often present problems for spectroscopists, this proposal aims to use them to identify when a nickel-68 nucleus has decayed. This involves using a number of detectors looking for gamma rays at different times, but correlating their data with each other. Finally, this proposal aims to test a new method of producing neutron-rich nuclei. Isotopes have previously been created by fusing two lighter nuclei together, and waiting for 2 protons to evaporate. However, this work plans to push this method further by identifying when 3 protons evaporate from a fused system. This method relies on the observation of known beta decays, which accompany the gamma rays of interest. The benefits of this work for nuclear physics are widespread. There exists a two-way relationship between experiment and theory: whilst providing new testing ground for current theory, the results themselves may be used to constrain shell-model calculations and improve their predictive power for other nuclei. The applications of this work extend to other disciplines. In astrophysics, the understanding of the formation of heavy elements in stars is based on processes that run through very neutron-rich nuclei. Current models rely on the extrapolation of data from moderately neutron-rich nuclei to predict the pathways of these processes. As data become available on ever more neutron-rich species, these calculations improve and, consequently, so does our understanding of the origins of elements.
 
Description Cr-59 collaboration 
Organisation Argonne National Laboratory
Department Physics Division
Country United States 
Sector Public 
PI Contribution Collaboration was led by myself.
Collaborator Contribution Collaborators participated in experiment at Argonne National Laboratory.
Impact Work from other partners in the collaboration may be on-going on related topics, but my personal involvement ended in 2011 with the termination of my STFC Fellowship.
Start Year 2009
 
Description Cr-59 collaboration 
Organisation Michigan State University
Department National Superconducting Cyclotron Laboratory
Country United States 
Sector Academic/University 
PI Contribution Collaboration was led by myself.
Collaborator Contribution Collaborators participated in experiment at Argonne National Laboratory.
Impact Work from other partners in the collaboration may be on-going on related topics, but my personal involvement ended in 2011 with the termination of my STFC Fellowship.
Start Year 2009
 
Description Cr-59 collaboration 
Organisation RIKEN
Department RIKEN-Nishina Center for Accelerator-Based Science
Country Japan 
Sector Public 
PI Contribution Collaboration was led by myself.
Collaborator Contribution Collaborators participated in experiment at Argonne National Laboratory.
Impact Work from other partners in the collaboration may be on-going on related topics, but my personal involvement ended in 2011 with the termination of my STFC Fellowship.
Start Year 2009
 
Description SET for Britain 2009 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? Yes
Primary Audience Policymakers_parliamentarians
Results and Impact A large number of MPs attended, along with scientists from a broad range of disciplines. This has provided me with an established line of contact with my own MP, through whom I have since had correspondence regarding the issue of runding for scientific research.

My MP has written to the Business Secretary (Vince Cable) and the Science Minister (David Willets), representing my views and posing questions to them on my behalf. I received a response in the form of a letter from David WIllets.
Year(s) Of Engagement Activity 2009
 
Description School Visit 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? Yes
Primary Audience Schools_students
Results and Impact I gave a short presentation to an A-level science class (around 20 pupils), which was followed by a discussion session.

N/A
Year(s) Of Engagement Activity 2010