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Report

Teaser, summary, work performed and final results

Periodic Reporting for period 1 - Shape Evolution (Investigation of shape evolution in neutron-rich nuclei using gamma-ray spectroscopy techniques)

Teaser

Summary

\"The atomic nucleus is a complex, many-body quantal system consisting of protons and neutrons. The study of nucleus is not only of fundamental importance in our quest to understand the world around us and its origin, but it also provides the tools for a variety of applications from energy to medicine. Although the nucleus is being studied for more than 100 years since its discovery by Rutherford, a thorough understanding of its quantum structure is far from being complete. Technological breakthroughs in the last decades have opened up several entirely new and exciting scientific frontiers. One of these frontiers is the production and study of isotopes with extreme neutron to proton ratios (exotic nuclei). These studies have revealed strong modifications in the ordering of single-particle orbitals in exotic nuclei as compared to the ones predicted by the original Shell Model of Mayer, Haxel, Suess and Jensen which in turn have dramatic consequences how the heavy elements, ie. beyond Ni and Fe, were created in the universe in the so-called rapid neutron capture (or r-) process. This research project aims at the study of neutron-rich exotic nuclei in the mass A~100-110 region through gamma-ray spectroscopy using fusion-fission and Coulomb excitation experiments. The experiments are able to determine the shape and deformation of these nuclei. The results of these measurements should help to understand the residual interactions responsible for changing shell structure in neutron rich nuclei and allow for a stringent test of various nuclear structure models, which are used to predict properties of even more neutron-rich isotopes relevant for the r-process.

Neutron-rich nuclei are key players in the creation of elements in several astrophysical scenarios, e.g. supernovae explosions and neutron star mergers. In particular the neutron star merger scenario has attracted strong interest in view of the first experimental observation of this process from the observation of gravitational waves. It should also be noted that the \"\"afterglow\"\" of the merger in the gamma ray spectrum was correctly predicted by nuclear structure calculations as early as 2010. However, the nuclei/isotopes produced in these violent events are far from being accessible in the laboratory for at least decades to come if ever. Therefore, we completely rely on theoretical predictions, which we have to test with (less exotic) isotopes available in the laboratory. The experiments realised in this project and their results are precision tests of nuclear structure models for moderately exotic nuclei. As one example the sudden onset of deformation in the chain of Zr isotopes, although known experimentally for many year, was never described correctly by many different theoretical approaches, e.g. the nuclear shell model or so-called mean-field models with varying effective interactions. Our experiment on 98Zr confirmed that a shape transition occurs suddenly between mass number 98 and 100, and our collaborators from the U. of Tokyo are able to describe this effect as a shape phase transition with their modern version of the shell model, the so-called Monte-Carlo Shell Model.

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Work performed

The project had two experimental parts, first the measurement of lifetimes in neutron-rich nuclei around mass number A~100 produced as fission fragments in a fusion-fission reaction of a Uranium beam on a Beryllium target and secondly Coulomb excitation experiments of the same nuclei used as radioactive ion beams. For the first part two experiments were performed over the last years, both at the GANIL facility in Caen, France, using the VAMOS particle spectrometer for the identification of the fission fragments, but two different gamma-ray spectrometers, EXOGAM and AGATA. EXOGAM is made of traditional High-Purity Germanium (HPGe) detectors, while AGATA is a modern gamma-ray tracking spectrometer, using position sensitive HPGe detectors. Since the realisation of the AGATA experiment was delayed by one year until June 2017, the researcher analysed first the data from the EXOGAM experiment. From these data important lifetime results were obtained for the first time in the isotope 98Zr, showing that the onset of deformation is happening very suddenly between the isotopes 98Zr and 100Zr. In addition, evidence for a triple shape coexistence was found in this nucleus. These results were reported at several conferences, e.g. SSNET 2018 (https://indico.in2p3.fr/event/16782), and were recently published in Physical Review Letters, Vol. 121, 192501 â€“ Published 9 November 2018 (also available at http://arxiv.org/abs/1902.02637)

For the second part of the project several attempts were made at the CARIBU facility at Argonne National Laboratory (USA) to perform Coulomb excitation of different isotopes, namely, 100Zr, 104&106Mo and 110Ru. These experiments are technically extremely demanding for several reasons. First all the elements are refractory and can not be extracted out of a standard ISOL source traditionally used for the production of radioactive isotopes. Therefore, the CARIBU facility uses a very intense 252Cf source for the production of the fission fragments, which are then stopped in a gaseous environment, ionised and reaccelerated in the ATLAS linear accelerator to energies optimal for Coulomb excitation studies. Unfortunately, several incidents did not allow us to perform the experiment on 100Zr at this time. However, after several unsuccessful attempts we were able to perform the experiments on 104&106Mo and 110Ru in September 2018. Due to this long delay (as compared to the original planning) the data from are still under analysis in our laboratory and will be published in due time.

Final results

None of these experimental techniques have been exploited in the past for unstable radioactive isotopes from refractory elements since the experimental facilities became only recently available. Due to technical difficulties important delays had to be overcome which did not allow us to fully exploit the results within the limited time of the project. Nevertheless, an important first publication has been achieved and several more are to come within our collaboration. These results will certainly require that nuclear models to be refined and will therefore become more predictive for isotopes even further from stability. It should also be mentioned that the isotopes of interest are abundantly produced in nuclear reactors and that their structure (e.g. the emitted gamma rays) have an influence on the so-called decay heat, which is still produced long after a reactor has been shut down.