Nuclear Physics & Applications

The Theoretical Nuclear Physics group joined the research effort on shape phase transitions and critical point symmetries at a very early stage in the beginning of 2003. Over the next decade the group played a major role in the development of the field, as it can be seen from the many citations to the work of the group in the relevant review articles. Some of the major early contributions of the group are the introduction of the Z(5), Z(4), and X(3) critical point symmetries. More recent important contributions include the introduction of deformation-dependent masses in nuclear structure, leading to a solution of the long-standing problem of moments of inertia through the use of techniques of Super-Symmetric Quantum Mechanics (SUSYQM).

After 10 years of activity in this topic, a new direction was searched for. A breakthrough idea came through the work on proton-neutron pairs playing a major role in the development of nuclear deformation. This new idea led to the introduction of the proxy-SU(3), which serves as an extension of Elliott SU(3) symmetry. Within proxy-SU(3) the prolate dominance and the prolate-oblate transition have emerged without any parameters. The group currently investigates the abilities of proxy-SU(3) for predictions such as the energy spectrum, the transition probabilities, the lifetimes, the quadrupole moments and the shape phase transitions. Furthermore the group evolves a new mechanism on shape coexistence, which promises to give without any parameters the islands of coexistence on the nuclear map and their observables.

The scientific program in Experimental Nuclear Physics & Ion-Beam Applications is implemented at the in-house Tandem Accelerator Laboratory (TAL) as well as at various European laboratories. The main group activities include systematic studies of nuclear reactions relevant to nuclear astrophysics and the application of ion-beam analysis (IBA) techniques aiming either at the elemental analysis of material surfaces or the study of irradiated materials with technological interest, such as those related to the energy production via fusion. In parallel, the group is active in the field of nuclear structure studies with experiments performed jointly with national and European research teams.

Nuclear Astrophysics

The Nuclear Astrophysics activities of the group focus on the study of nuclear capture reactions relevant to the understanding of a nucleosynthetic mechanism occurring in certain explosive stellar environments, such as supernovae. This mechanism, termed p-process, is responsible for the synthesis of certain 35 neutron-deficient nuclei heavier than iron that lie “north-west” of the stability valley, between ⁷⁴Se and ¹⁹⁶Hg. These nuclides are known as the p-nuclei and have, so far, been observed only in the solar system.

To date, all p-process nucleosynthesis models are able to reproduce most of the solar p-nuclei abundances within a factor of 3 but fail in the case of the light p-nuclei. The observed discrepancies could result due to uncertainties in the purely astrophysics modeling of the p-process. However, on top of any astrophysical model improvements, it is imperative that the nuclear physics uncertainties entering the astrophysical calculations are reduced or set under control at least. Given this challenge, the main goal of Nuclear Astrophysics program is to shed light on nuclear physics aspects of the p-process puzzle that still remain diffuse and contribute, this way, to the understanding of the observed discrepancies of the p-nuclei abundances in the solar-system abundances.

Under these conditions, the major research objectives of the Nuclear Astrophysics program of INPP are two: The first one, required to achieve the overall goal of the program, is the establishment of a proper cross-section database. For this purpose, systematic cross-section measurements of proton and α-particle capture reactions in medium-mass nuclei, from Cu to Cs, are performed. The second objective is implemented in collaboration with Dr. P. Demetriou, Principal Researcher of INPP, who is using the experimental data measured by the group to develop microscopic models of α-particle-nucleus optical-model potentials (αOMP) for compound-nucleus reactions and perform credibility tests of existing phenomenological ones.

Nuclear Structure

The Nuclear Structure studies of the group are carried out jointly with collaborators in Europe. In this context, our group has been seeking for experimental evidence of the critical point symmetries E(5) and X(5) introduced by F. Iachello. E(5) symmetry describes the phase shape transition between a spherical harmonic vibrator to that of a “γ-soft” rotor, while X(5) between a prolate-deformed symmetric rotor and a spherical harmonic vibrator. As in the case of the symmetries introduced by the Interacting Boson Approximation (IBA) model, critical-point symmetries provide parameter-free predictions for excitation energies and their ratios, as well as for reduced transition probabilities. While the X(5) critical point is quite well established with nuclei that exhibit X(5) features in both excitation energy and transition probabilities, this is not the case for the E(5) critical point. Nuclei have been proposed as possible E(5) candidates mainly due to their excitation energies, but in most cases the experimental data are scarce and the verification is not conclusive. These activities were performed within wide European Collaborations at large-scale facilities abroad.

Within national collaborations, primarily with the National Technical University of Athens (NTUA) and the University of Ioannina (UoI), the group has carried out at the local Tandem accelerator activation measurements of cross sections of (n,xn) and (n,f) reactions. A good part of these activities are related to the research program of CERN’s n_TOF collaboration. The data are important for the design of Accelerator Driven Systems (ADS) for the future production of clean and safe nuclear energy as well as for the incineration of nuclear waste. These measurements included the ²⁴¹Am(n,2n) and the ²³⁶U(n,f) reactions, with the latter one being implemented for the first time using a gaseous MicroMegas detector.

Ion-Beam Applications

The group is active in basic and applied research topics related with the interaction of light ions with matter. The research is focused on the measurement of differential cross sections of proton and deuteron induced nuclear reactions at energies and angles suitable for Ion Beam Analysis techniques. These techniques are used in a broad range of applications, including microelectronics, cultural heritage studies, biology and material science.

The INPP research in the field of X-ray Spectrometry is based on the pioneer work of the Institute researchers who developed since mid-70’s innovative and inter-disciplinary applications of XRF techniques in cultural heritage, environmental monitoring and biomedicine. With the technological breakthrough that offered portability and miniaturization of X-ray detectors and sources, the INPP developed custom-made and innovative portable XRF spectrometers motivating the establishment of sustained research collaborative schemes with conservators, curators and archaeologists, but also providing technology transfer and analytical services to end-users. The last twenty (20) years, numerous of field investigations have been carried out by analyzing and studying materials and manufacture techniques from unique archaeological/historical collections in Greece and abroad, including Cyprus, Jordan, Syria and Malta.
The X-ray spectrometry group of INPP has provided strong contribution in the development of an external Ion Beam Analysis station at the INPP Tandem accelerator for integrated characterization of materials and quality control of elemental tagging technologies and of two (2) novel ion-beam based analytical techniques aimed to expand the analytical merits of the well-established Particle Induced X-ray Emission atmospheric pressure (PIXE) technique.
The confocal micro-PIXE (3D Micro-PIXE) technique was first developed, tested and evaluated in vacuum chamber at the Microanalytical Centre of Josef Stefan Institute and in the at the AGLAE accelerator located at the Louvre Museum. Another unique set-up that can produce proton induced monochromatic X-rays for basic and analytical purposes was installed and its merits were exploited at the INPP Tandem accelerator. This novel, linear accelerator-based X-ray source supported the study of Resonant Raman X-ray scattering and atom cascade relaxation processes providing comparable results with those obtained by a synchrotron source. The last few years, the X-ray spectrometry group continues to balance its research activities in basic and applied X-ray spectrometry.

For more information please visit the homepage of the XRF Lab.