Final Report Summary - SHAPEXPIUM (Coulomb excitation of ground and isomeric state of neutron-deficient polonium isotopes: a study of shape coexistence in the Z = 82 region)
The atomic nucleus, lying at the core of every atom and comprising over 99.9 % of its mass, is a unique many-body quantum system in which nucleons interact via the strong, electromagnetic and weak interactions. This makes the atomic nucleus a unique laboratory to study these fundamental interactions but also one of the most challenging of quantum systems.
Since its inception, our knowledge of the properties of atomic nuclei was derived mainly from studies of nuclei near the valley of stability; the group of stable isotopes that are available on earth. Theory and paradigms have been developed on the basis of these nuclei and their closest neighbours with similar number of protons (Z) to number of neutrons (N) ratios.
Now, in the last two decades, through technological and theoretical advances, it has become possible to produce and study nuclei far from stability in great detail. These nuclei possess truly exotic, unforeseen properties that challenge these paradigms, the benchmarks of the current understanding for half a century. This is revolutionising the knowledge of the many-body atomic nucleus through new observables and interactions: e.g. new nuclear topologies as halo nuclei, new collective structures, the fragility of the shell model, the proton-neutron interaction at unusual N/Z ratios.
The aim of this project was to determine important experimental spectroscopic quantities in the neutron-deficient isotopes close to Z = 82 closed proton-shell and compare them with state-of-the-art theoretical models. Similar to the electromagnetic interaction in an atom, where the noble gases (helium, neon, argon) are characterised by a magic number of electrons (2, 10, 18), is the strong interaction in the atomic nucleus characterised by magic numbers, 82 being one of them.
The neutron-deficient isotopes at and near Z = 82 exhibit the most extensive manifestation of the shape coexistence phenomenon, whereby different types of shapes, either spherical or deformed (prolate, oblate), coexist at low excitation energy (below a few MeV) within the same nucleus. The phenomenon has been observed by using different techniques, ranging from optical and laser spectroscopy to in-beam spectroscopy and decay spectroscopy. Only recently a wealth of data has become available by using new techniques such as Coulomb excitation of post-accelerated radioactive beams and in-source laser spectroscopy. A large part of the research in this field was, and is presently done, at the ISOLDE facility in European Organisation for Nuclear Research (CERN) by beta- and alpha-decay spectroscopy, laser spectroscopy, and Coulomb excitation reaction. The present project focused on a Coulomb excitation study using state-of-the-art beams of neutron-deficient Po isotopes from CERN's isotopes separator on-line facility ISOLDE and instrumentation like the highly segmented germanium array detector array MINIBALL. This revealed unique information on the structure of these nuclei by measuring the transitional and diagonal matrix elements between low-lying excited states. These quantities are directly related to the static quadrupole moment of the excited states, an indirect measure of the shape of the nucleus. The data are combined with other experimental observables of these nuclei and compared with contemporary theoretical studies.
The project objectives have been fully accomplished over the two years. The MC fellow has participated to many experimental campaigns of Coulomb excitation reactions, laser and beta-decay spectroscopy on nuclei in the region of interest for shape-coexistence, even isomerically purified beams were produced and used for these studies.
The MC fellow has analysed the analysis of Coulomb excitation of isomeric 68,70Cu beams to get acquainted with the Coulomb excitation technique. This was mainly done during the first year of the project and has led to a publication on Phys. Rev. C84, 064323 (2011). Later, mainly during the second year of the project, she has carried out the analysis of 182,184Tl beta-decay to study the occurrence and decay properties of shape-coexisting states in 182,184Hg with Z = 80, two protons less compared to the Z = 82 closed shell configuration. The knowledge on these decay properties has been largely improved by this study especially with the measurement of the internal conversion coefficient of the 2(2,+)-2(1+). The data have been presented at several international workshops and conferences, and are in the process of being submitted to Physics Review C. The information gathered from the beta-decay spectroscopy has been crucial to finalise the analysis of the data coming from the Coulomb excitation experiment on 182,184,186Hg performed at ISOLDE at 2.7 MeV / u. Shape coexistence manifests a subtle interplay between individual aspects of the nucleons building up the atomic nucleus and the collective behaviour of the nuclear core. For the first time it could be experimentally proven that the shape of the ground state in these light mercury isotopes was determined to be weak and oblate, further, the excited 0+ state exhibited a larger deformation. This is in contradiction to the most recent theoretical models that are based on axial symmetry. New developments are underway to investigate the influence of tri-axiality, whereby the atomic nucleus has three perpendicular symmetry axis, on the calculations, which might resolve the discrepancy between experiment and theory. On the basis of the experience gained on the beta- and alpha-decay studies, the fellow has worked on setting up of the experimental apparatus for the laser spectroscopy where neutron-deficient Tl isotopes and recently, At and Au were studied. Finally, new data on the neutron-deficient polonium isotopes have been collected and will shed light on the shape-coexistence phenomena in these isotopic chain that has two protons more compared to the Z = 82 closed proton shell.
Although we have already many different experimental probes at our disposal, a full picture of the phenomenon is still lacking, especially concerning the specific role of the proton and neutron single-particle configurations. Higher beam energies and intensities available at HIE-ISOLDE - CERN, as well as improved purity and beam phase space conditions, will allow to perform multiple-Coulomb excitation experiments up to 5.5 MeV / u and 10 MeV / u probing collectivity and even transfer experiments such as (d,p) and (t,p) directly probing these single-particle structures.
The training received during the Marie Curie fellow in production of radioactive beam through resonant laser ionisation, gamma-ray spectroscopy with highly segmented germanium detectors, digital electronics and read-out, data-analysis procedure for multi-parameters systems and nuclear-structure theory has allowed her to acquire several interdisciplinary competencies both from the physical as well as on technical side. Thanks to this, she has obtained a CERN fellow for the two years. It will allow her to continue the research in the field and to consolidate her activity on the forefront of the international effort in radioactive beam research.
Since its inception, our knowledge of the properties of atomic nuclei was derived mainly from studies of nuclei near the valley of stability; the group of stable isotopes that are available on earth. Theory and paradigms have been developed on the basis of these nuclei and their closest neighbours with similar number of protons (Z) to number of neutrons (N) ratios.
Now, in the last two decades, through technological and theoretical advances, it has become possible to produce and study nuclei far from stability in great detail. These nuclei possess truly exotic, unforeseen properties that challenge these paradigms, the benchmarks of the current understanding for half a century. This is revolutionising the knowledge of the many-body atomic nucleus through new observables and interactions: e.g. new nuclear topologies as halo nuclei, new collective structures, the fragility of the shell model, the proton-neutron interaction at unusual N/Z ratios.
The aim of this project was to determine important experimental spectroscopic quantities in the neutron-deficient isotopes close to Z = 82 closed proton-shell and compare them with state-of-the-art theoretical models. Similar to the electromagnetic interaction in an atom, where the noble gases (helium, neon, argon) are characterised by a magic number of electrons (2, 10, 18), is the strong interaction in the atomic nucleus characterised by magic numbers, 82 being one of them.
The neutron-deficient isotopes at and near Z = 82 exhibit the most extensive manifestation of the shape coexistence phenomenon, whereby different types of shapes, either spherical or deformed (prolate, oblate), coexist at low excitation energy (below a few MeV) within the same nucleus. The phenomenon has been observed by using different techniques, ranging from optical and laser spectroscopy to in-beam spectroscopy and decay spectroscopy. Only recently a wealth of data has become available by using new techniques such as Coulomb excitation of post-accelerated radioactive beams and in-source laser spectroscopy. A large part of the research in this field was, and is presently done, at the ISOLDE facility in European Organisation for Nuclear Research (CERN) by beta- and alpha-decay spectroscopy, laser spectroscopy, and Coulomb excitation reaction. The present project focused on a Coulomb excitation study using state-of-the-art beams of neutron-deficient Po isotopes from CERN's isotopes separator on-line facility ISOLDE and instrumentation like the highly segmented germanium array detector array MINIBALL. This revealed unique information on the structure of these nuclei by measuring the transitional and diagonal matrix elements between low-lying excited states. These quantities are directly related to the static quadrupole moment of the excited states, an indirect measure of the shape of the nucleus. The data are combined with other experimental observables of these nuclei and compared with contemporary theoretical studies.
The project objectives have been fully accomplished over the two years. The MC fellow has participated to many experimental campaigns of Coulomb excitation reactions, laser and beta-decay spectroscopy on nuclei in the region of interest for shape-coexistence, even isomerically purified beams were produced and used for these studies.
The MC fellow has analysed the analysis of Coulomb excitation of isomeric 68,70Cu beams to get acquainted with the Coulomb excitation technique. This was mainly done during the first year of the project and has led to a publication on Phys. Rev. C84, 064323 (2011). Later, mainly during the second year of the project, she has carried out the analysis of 182,184Tl beta-decay to study the occurrence and decay properties of shape-coexisting states in 182,184Hg with Z = 80, two protons less compared to the Z = 82 closed shell configuration. The knowledge on these decay properties has been largely improved by this study especially with the measurement of the internal conversion coefficient of the 2(2,+)-2(1+). The data have been presented at several international workshops and conferences, and are in the process of being submitted to Physics Review C. The information gathered from the beta-decay spectroscopy has been crucial to finalise the analysis of the data coming from the Coulomb excitation experiment on 182,184,186Hg performed at ISOLDE at 2.7 MeV / u. Shape coexistence manifests a subtle interplay between individual aspects of the nucleons building up the atomic nucleus and the collective behaviour of the nuclear core. For the first time it could be experimentally proven that the shape of the ground state in these light mercury isotopes was determined to be weak and oblate, further, the excited 0+ state exhibited a larger deformation. This is in contradiction to the most recent theoretical models that are based on axial symmetry. New developments are underway to investigate the influence of tri-axiality, whereby the atomic nucleus has three perpendicular symmetry axis, on the calculations, which might resolve the discrepancy between experiment and theory. On the basis of the experience gained on the beta- and alpha-decay studies, the fellow has worked on setting up of the experimental apparatus for the laser spectroscopy where neutron-deficient Tl isotopes and recently, At and Au were studied. Finally, new data on the neutron-deficient polonium isotopes have been collected and will shed light on the shape-coexistence phenomena in these isotopic chain that has two protons more compared to the Z = 82 closed proton shell.
Although we have already many different experimental probes at our disposal, a full picture of the phenomenon is still lacking, especially concerning the specific role of the proton and neutron single-particle configurations. Higher beam energies and intensities available at HIE-ISOLDE - CERN, as well as improved purity and beam phase space conditions, will allow to perform multiple-Coulomb excitation experiments up to 5.5 MeV / u and 10 MeV / u probing collectivity and even transfer experiments such as (d,p) and (t,p) directly probing these single-particle structures.
The training received during the Marie Curie fellow in production of radioactive beam through resonant laser ionisation, gamma-ray spectroscopy with highly segmented germanium detectors, digital electronics and read-out, data-analysis procedure for multi-parameters systems and nuclear-structure theory has allowed her to acquire several interdisciplinary competencies both from the physical as well as on technical side. Thanks to this, she has obtained a CERN fellow for the two years. It will allow her to continue the research in the field and to consolidate her activity on the forefront of the international effort in radioactive beam research.