Atomic nuclei are the carrier of almost all visible mass in the universe. Nuclei also provide the fuel powering stars and nuclear reactions produce all naturally occurring chemical elements. Despite this fundamental role of nuclei in the universe, their structure and dynamics cannot yet be satisfactorily described on the basis of the fundamental strong interaction. One of the major successes in the description of the properties of atomic nuclei was the introduction of the nuclear shell model. For certain numbers of protons or neutrons, the so-called magic numbers, discontinuities occur, for example in the nucleon separation energies. In analogy to the successful atomic shell model, these magic numbers were interpreted as closed shell configurations, similar to the noble gases. In exotic nuclei however, far away from the stable isotopes, several experimental as well as theoretical investigations found evidence that the shell structure of atomic nuclei can change locally. In order to understand the underlying causes for the migration of nuclear shells, sensitive experimental methods are of paramount importance to investigate rare isotopes far from stability.
Our research focuses the structure of radioactive nuclei. We use direct reactions to populate states in the exotic nuclei and state-of-the-art experimental equipment. Our experiments are performed at world leading radioactive beam facilities such as the RIBF (JAPAN), NSCL (USA) and TRIUMF (Canada). There is plenty of opportunity for graduate and undergraduate students to go and participate in experiments around the world.
Single-particle structure of neutron-rich N=40 nuclei
The region around neutron-rich N=40 nuclei has recently attracted a lot of interest. The highlying 2+ state in 68Ni and its small transition probability to the ground state are a result of the N=40 harmonic oscillator shell gap between the fp shell and the 1g9/2 orbital. This shell gap is reduced for the more neutron rich Fe and Cr isotopes leading to an increase in collectivity for the N=40 isotones 66Fe and 64Fe. We have measured one-neutron knockout reactions in this region to quantify the occupation of the 1g9/2 intruder orbital. This experiment has been performed in October 2012 at NSCL utilizing the S800 spectrometer and the GRETINA gamma detector array.
The experimental data in currently under analysis. A first paper on the experimental method has been published: K. Wimmer et al., "IsoTagger: Identification of isomeric nuclear states produced in fragmentation reactions with radioactive beams" Nuclear Instruments and Methods A 769 (2015) 654
Normal and intruder configurations in the N=20 Island of Inversion
More than 35 years after its discovery, the ''Island of Inversion'' around N=20 in the A=30 mass region continues to be of great interest for the nuclear physics community. In this region of neutron-rich nuclei around 32Mg, deformed ground states have been found and associated with neutron particle-hole excitation across the magic N=20 shell gap. The reduction of the N=20 shell gap results in low-lying collective excitations, deformed ground states and the coexistence of states with different shapes at low excitation energies. The goal of our experimental study at NSCL is to establish firm spin and parity assignments to the excited states in 30,32Mg. Both nuclei were be produced in different nucleon removal reactions, which are expected to either favor the population of normal spherical states or the deformed intruder configurations. Comparison of the partial cross section for each state allows drawing conclusions about the nature of the states.
This experiment was peformed in spring 2013 and is currently under analysis.
Shape coexistence at N=60: single-particle structure in neutron-rich Sr nuclei
Atomic nuclei seem to be the only finite many-body quantum system that exhibits the coexistence of very different shapes near the ground state of the system. The shape transition at N=60 in the Sr, Zr, Mo region is one of the most dramatic transitions of grounds state shape transitions known today.
In this experiment we studied the single-particle structure in 95,96,97Sr via the one-neutron transfer d(94,95,96Sr,p) reaction. The goal is to establish spin assignments and to provide information on the occupation of single-particle states in the Sr isotopes. The experiment will be performed with the TIGRESS + SHARC set-up at TRIUMF to measure energies and angular distributions of the transfer protons and gamma-radiation from the populated excited states.
This experiment was done in August 2013 and July 2014.
Currently we are planning to extend this study to two-nucleon transfer reactions starting using a radioactive tritium target. Our first goal is to test the microscopic structure of shape coexisting state in 96Sr with the t(94Sr,p) reaction.
Single-particle structure at N=28
One of the most interesting forms of shape coexistence is the occurrence of three different shapes within one nucleus. This was first observed in 186Pb, where the lowest three states of the nucleus, all three are 0+ states, exhibit spherical, prolate (rugby-ball shaped) and oblate (disk shaped) deformation. The occurrence of a low-lying excited 0+ state in 44S and the excitation spectrum indicate a coexistence of three different nuclear configurations in 44S. Measurements of single-particle properties of these exotic nuclei give further insights in the wave function composition of states.
This experiment is planned for 2015.