Exploring The Changing Shell Structure Of Nuclei

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

Abstract

Most of the mass of the Universe that we can see around us is made up of atomic nuclei, the dense cores of atoms which are only about a million millionth of a centimetre in radius. They contain up to several hundred protons and neutrons (collectively called nucleons) held together by strong nuclear forces and influenced by the electrostatic forces between the positively charged protons. The particular ratios of protons-to-neutrons in the stable nuclei we find in nature are determined by a subtle balance between these forces. Most of the characteristic properties of a nucleus are determined by the way nucleons move inside. This is somewhat similar to atomic physics where the electrons in an atom orbit around its centre. Certain atoms, Nobel gases, are more chemically stable than others. This is related to so-called shell gaps in the energy sequence of the electron orbits which makes these atoms more difficult to excite. Similar quantum mechanical effects come into play in nuclei, where nucleons orbit around the centre of the nucleus. The resulting shell structure is very different as nuclei and atoms are bound by different forces. The nucleon numbers related to shell gaps are known as magic numbers. The corresponding magic and doubly magic nuclei, the latter having magic numbers of protons AND neutrons, have properties associated with enhanced stability, they are harder to excite and react, have long lifetimes and spherical shape. It is easy to study magic numbers in stable nuclei as they already exist in Nature and do not need to be manufactured. These numbers, 2, 8, 20, 28, 50, 82.... are well understood and are related to the specific ways in which a nucleon interacts with the others in the nucleus. Practically all nuclear properties, such as shape, the modes of excitation, the spin, magnetic characteristics and so on, depend on the underlying nucleon orbits. Orbitals and magic numbers are therefore fundamental to understanding the way nuclei behave and how they
 
Description This project has produced extensive results in the neutron-rich fp shell nuclei, addressing a variety of issues including
shell models, shapes and new symmetries. Measurements have been made in sd shell nuclei with observations of crossshell sd-fp excitations of importance to the development of shell-descriptions. Heavier nuclei have also been studies with
regards to the evolution of single-particle states around N=82 and the evolution of collective bands close to the rare-earth
valence maximum. A variety of reactions and techniques were used, initially focussing on multi-nucleon transfer reactions
using the CLARA+PRISMA system at Legnaro, but these exciting results were followed up using other techniques at
different facilities. These include fusion of neutron-rich targets and projectiles and isolating the most neutron-rich nuclei
produced at Argonne; the use of radioactive beams from fragmentation to induce Coulex and knockout reactions at MSU;
light ion transfer to study the details of the trends in high-j orbitals at Yale. This has been important for scientific reasons,
to probe additional parameters not accessible in the initial results, as well as pragmatic reasons, to continue scientific
output when the system at LNL was unavailable due to accelerator or equipment downtime. The highlights of are
discussed briefly below; refer to the publication list for more details. / In fp shell nuclei a large number of measurements
have been made beyond N=28 in Fe, Cr, Mn and Ti nuclei (20 publications in total). Multi-nucleon transfer was used to
populate isotopes identified using the PRISMA spectrometer. Although efficiency of the CLARA array is modest,
transitions were identified in 58Cr, 59-63Mn and 61-66Fe. 58Cr was identified as a potential candidate for the E(5)
dynamical symmetry, a critical point of the vibrational to gamma-soft transition. The results on Mn and Fe isotopes have
initiated a theoretical effort to develop the new fpg shell-model interaction by Italian and French theorists. These initial
results have been pursued to high spin with fusion reactions in a more efficient array, Gammasphere, in a series of
experiments involving the fusion of 48Ca with 13/14C and 9Be targets, where weak reaction channels were identified
using a recoil separator, with parallel thick-target deep inelastic studies. This has generated comprehensive data to high
spin on 52Ti, 55-60Cr, 59-63Mn and 59,60Fe. These have been used in collaboration with Japanese theorists to
extensively test the available shell model calculations, indicating that the GXPF1A interaction gives the best current
description due to the correct prediction of the single-particle ordering. It is clear from the experimental measurements
that, in the most neutron-rich species and in the region of high spin, the effects of the g9/2 orbital are present in the form of rotational-like structures up to the expected band terminations. In this larger model space, it is difficult to reproduce the
low-lying structures in isotopes with N>35. This data has had an important impact as an impetus for theoretical
descriptions of the fpg space. Additionally the use of radioactive beams from fragmentation for Coulex and knockout
studies has provided additional experimental probes via transition rates and single-particle content. An important highlight
here has been the study of 64Cr positioned at N=40, which has been determined to have increased collectivity in a
distinct change from the Fe isotopes. / The experimental techniques developed in the fp shell were harnessed in studies
of lighter sd shell nuclei. These have allowed studies of A=30 nuclei out to Mg, at the edge of the island of inversion. The
results have isolated the presence of sd-fp cross shell excitations in all systems, but in 30Mg measurements have
indicated the need to include fp contributions in even the lowest lying states, indicating significant deficiencies in the
current shell model descriptions
Exploitation Route Provides a basis for future scientific work in the field.
Sectors Education,Other

 
Description Contributed to various outreach activities.
First Year Of Impact 2017
Sector Education,Culture, Heritage, Museums and Collections,Other
Impact Types Societal

 
Description STFC Buclear Physics Grants Round 2007
Amount £1,579,024 (GBP)
Organisation Science and Technologies Facilities Council (STFC) 
Sector Public
Country United Kingdom
Start 08/2008 
End 07/2011
 
Description Argonne National Laboratory 
Organisation Argonne National Laboratory
Department Physics Division
Country United States 
Sector Academic/University 
PI Contribution Intellectual input, performing experiments, data analysis, writing publications
Collaborator Contribution Facility provision and intellectual input
Impact Publications.
 
Description Legnaro National Laboratory 
Organisation National Institute for Nuclear Physics
Department Legnaro National Laboratories
Country Italy 
Sector Academic/University 
PI Contribution Intellectual input, performing experiments, data analysis, writing publications.
Collaborator Contribution Facility provision, intellectual input.
Impact Publications
 
Description Michigan State University 
Organisation Michigan State University
Department National Superconducting Cyclotron Laboratory
Country United States 
Sector Academic/University 
PI Contribution Intellectual input, performing experiments, assisting writing publications.
Collaborator Contribution Facility provision, intellectual input
Impact Publications
 
Description STFC Daresbury 
Organisation Daresbury Laboratory
Department Nuclear Physics Support Group
Country United Kingdom 
Sector Academic/University 
PI Contribution Detector development
Collaborator Contribution Joint grant application, electronics and daq, target manufacture.
Impact Publications. Equipment production.
 
Description UWS 
Organisation University of the West of Scotland
Department School of Physics
Country United Kingdom 
Sector Academic/University 
PI Contribution Performing experiments, intellectual input, writing publications.
Collaborator Contribution Intellectual input, publications.
Impact Publications
 
Description Yale University 
Organisation Yale University
Department Wright Nuclear Structure Laboratory
Country United States 
Sector Academic/University 
PI Contribution Intellectual input, performing experiments, data analysis, writing publications.
Collaborator Contribution Facility provision, intellectual input
Impact Publications
 
Description Rutherford Exhibition at Museum of Science and Industry 
Form Of Engagement Activity Participation in an activity, workshop or similar
Part Of Official Scheme? No
Type Of Presentation Workshop Facilitator
Geographic Reach National
Primary Audience Public/other audiences
Results and Impact Three month exhibition Aug to Oct 2011 of nuclear physics past and present to celebrate the Rutherford Centennial. Estimated visitors 200,000.

Manchester played advisory and facilitator role.

Media coverage.
Year(s) Of Engagement Activity 2011
 
Description Variety of activities as Schools Liaison Officer 
Form Of Engagement Activity Participation in an open day or visit at my research institution
Part Of Official Scheme? No
Primary Audience Schools
Results and Impact The PI of the grant was the Schools and Colleges Liaison Officer for Physics and Astronomy. The research outputs of the grant have contributed to talks given as part of the responsibilities this post, to local schools and colleges, at University Open Days held three times a year, and at many one-off events.

Many.
Year(s) Of Engagement Activity 2006,2007,2008,2009,2010