arXiv subjects

P. Vasileiou

Publications and source records attributed to P. Vasileiou.

8 recordsLinked to original sources

arXiv2026

Parameter-free deformation variables of the proxy-SU(3) symmetry in even-even actinide, superheavy and hyperheavy nuclei with Z=82-126, N=82-258

Superheavy and hyperheavy nuclei are one of the frontiers of nuclear structure nowadays, while also for many actinides rather limited experimental information exists. Therefore, theoretical methods providing parameter-independent predictions for these nuclei are of particular interest. Such a method is the proxy-SU(3) approximation to the shell model, which has been adequately tested against experimental data in medium-mass and heavy nuclei up to the rare earth region, and has been found to provide reliable, parameter-independent predictions for the collective deformation variables beta and gamma. Within the proxy-SU(3) approach, the SU(3) symmetry of the 3-dimensional harmonic oscillator, which is destroyed beyond the sd shell by the strong spin-orbit interaction, is restored through a unitary transformation. For each nucleus, the most symmetric irreducible representation (irrep) allowed by the Pauli principle and the short-range nature of the nucleon-nucleon interaction, called the highest-weight (hw ) irrep in mathematical language, is found to suffice, except in cases in which the hw irrep turns out to be completely symmetric, so that the next highest weight (nhw) irrep has also to be included. In this article we provide a full collection of the hw and nhw irreps, as well as of the corresponding parameter-free predictions for the deformation variables beta and gamma, for all atomic nuclei ranging from Z=82, N=82 to Z=126, N=258. Several cases exemplifying the use of the collected results for studying the prolate to oblate shape transition, mirror symmetries, as well as the evolution of the collective variables along the valley of stability are also considered.

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arXiv2026

The Confined beta-Soft rotor model in rare-earth nuclei

Contemporary theoretical descriptions of nuclear structure rely mainly on microscopic, single-particle frameworks often in competition with collective degrees of freedom, especially when deformation plays a dominant role. Such phenomena are prominent in the rare-earth region, where rotational band structures and enhanced electric quadrupole transitions are systematically examined. The Confined beta-Soft (CBS) rotor model, introduced by N. Pietralla and O.M. Gorbachenko, bridges the gap between the X(5) critical point and the rigid-rotor limit in the region where the R_4/2 = E(4+)/E(2+) ratio lies between 2.904 and 3.333. In the present work, the CBS framework is employed to calculate ground-state band energies, associated B(E2) transition rates, and beta-band excitations of even-even nuclei in the rare-earth region. The theoretical results are systematically compared with available experimental data, and predictions are provided for nuclear observables that have not yet been measured, offering guidance for future experimental investigations.

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arXiv2026

Parameter-free deformation variables of the proxy-SU(3) symmetry in even-even atomic nuclei with Z=28-82, N=28-126

The proxy-SU(3) approximation to the shell model, which restores the SU(3) symmetry of the 3-dimensional harmonic oscillator beyond the sd shell, predicts the collective deformation variables beta and gamma of even-even atomic nuclei in a parameter-free way, based on the most symmetric irreducible representation (irrep) of SU(3) allowed by the Pauli principle and the short-range nature of the nucleon-nucleon interaction, which in group theoretical language is the highest weight (hw) irrep. In the few cases in which the hw irrep turns out to be completely symmetric, thus being able to accommodate only the ground state band, the next hw (nhw) irrep becomes indispensable. In the present article complete tables of the hw and nhw irreps are given for all atomic nuclei ranging from Z=28, N=28 to Z=82, N=126, along with the corresponding parameter-free predictions for the deformation variables beta and gamma. A few examples using the tabulated results for providing microscopic insight for specific effects in various regions of the nuclear chart are also given.

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arXiv2025

Triaxial shapes in even-even nuclei: A theoretical overview

Triaxial shapes in even-even nuclei have been considered since the early days of the nuclear collective model. Although many theoretical approaches have been used over the years for their description, no effort appears to have been made for grouping them together and identifying regions on the nuclear chart where the appearance of triaxiality might be favored. In addition, over the last few years, discussion has started on the appearance of small triaxiality in nuclei considered so far as purely axial rotors. In the present work we collect the predictions made by various theoretical approaches and show that pronounced triaxiality appears to be favored within specific stripes on the nuclear chart, with low triaxiality being present in the regions between these stripes, in agreement with parameter-free predictions made by the proxy-SU(3) approximation to the shell model, based on the Pauli principle and the short-range nature of the nucleon-nucleon interaction. The robustness of triaxiality within these stripes is supported by global calculations made in the framework of the Finite-Range Droplet Model (FRDM), which is based on completely different assumptions and possesses parameters fitted in order to reproduce fundamental nuclear properties.

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arXiv2024

Robustness of the proxy-SU(3) symmetry in atomic nuclei and the role of the next highest weight irreducible representation

The proxy-SU(3) symmetry predicts, in a parameter-free way, the collective deformation variables beta and gamma in even-even atomic nuclei away from closed shells based on the highest weight irreducible representations (irreps) of SU(3) in the relevant proton and neutron shells, which are the most symmetric irreps allowed by the Pauli principle and the short-range nature of the nucleon-nucleon interactions. The special cases in which the use of the next highest weight irrep of SU(3) becomes necessary are pointed out and numerical results are given for several regions of the nuclear chart, which can be used as input for irrep-mixing calculations.

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arXiv2024

Preponderance of triaxial shapes in atomic nuclei predicted by the proxy-SU(3) symmetry

The proxy-SU(3) symmetry predicts, in a parameter-free way, based only on the Pauli principle and the short-range nature of the nucleon-nucleon interaction, non-vanishing values of the collective variable gamma almost everywhere across the nuclear chart. Substantial triaxiality with gamma between 15 and 45 degrees is proved to be expected along horizontal and vertical stripes on the nuclear chart, covering the nucleon numbers 22-26, 34-48, 74-80, 116-124, 172-182. Empirical support for these stripes is found by collecting all even-even nuclei for which the first two excited 2+ states are known, along with the B(E2)s connecting them, as well as the second 2+ state to the ground state. The stripes are related to regions in which oblate SU(3) irreducible representations appear, bearing similarity to the appearance of triaxiality within the SU(3)* dynamical symmetry of the interacting boson model-2. Detailed comparisons of the proxy-SU(3) predictions to the data and to predictions by state-of-the-art Monte Carlo shell model calculations for deformed N=94, 96, 98 isotones in the rare earth region show good overall agreement, with the exception of Z=70 and N=94, which correspond to fully symmetric proxy-SU(3) irreps, suggesting that the latter are an artifact of the method which can be amended by considering the influence of the neighboring irreps.

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arXiv2022

On the multipole mixing ratio of the $1066$ keV transition from the 0.52 $μs$ isomer of $^{180}$Hf

The nucleus $^{180}$Hf is one of the most primary of examples of an axially symmetric prolate rotor. Combined with the presence of high-$K$ isomers, spectroscopic studies can provide important information on the nature of its single-particle levels. Precise measurements are essential for constraining nuclear models and interpreting the nature of such isomeric states. In this work, the nucleus $^{180}$Hf was populated using the proton pick-up reaction $^{181}$Ta($^{11}$B,$^{12}$C)$^{180}$Hf at beam energy of 47 MeV at Horia Hulubei National Institute of Nuclear Physics and Engineering (IFIN-HH). The spin of the 1374 keV state and the mixing ratio of the $1066$ keV transition have been measured, the latter with an increased precision compared to the previous value from literature. The presently measured spin of the 1374 keV state, currently assigned a tentative value of $(4^-_1)$, favors one of the two different values reported in the literature. The particular state constitutes the band-head of a rotational band in $^{180}$Hf. The measured multipolarity mixing ratio of the inter-band transition $1374 \rightarrow 309$ keV can provide important information for the testing and constraining of theoretical nuclear models used for the study of the intrinsic properties of $^{180}$Hf as well as its neighboring isotopes.

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arXiv2021

An Investigation of radiative proton-capture reactions in the Cd-In mass region

The reaction network in the neutron-deficient part of the nuclear chart around $A \sim 100$ contains several nuclei of importance to astrophysical processes, such as the p-process. This work reports on the results from recent experimental studies of the radiative proton-capture reactions $^{112,114}\mathrm{Cd}(p,γ)^{113,115}\mathrm{In}$. Experimental cross sections for the reactions have been measured for proton beam energies residing inside the respective Gamow windows for each reaction, using isotopically enriched $^{112}\mathrm{Cd}$ and $^{114}\mathrm{Cd}$ targets. Two different techniques, the in-beam $γ$-ray spectroscopy and the activation method have been employed, with the latter considered necessary to account for the presence of low-lying isomers in $^{113}\mathrm{In}$ ($E_γ \approx 392$~keV, $t_{1/2} \approx 100$~min), and $^{115}\mathrm{In}$ ($E_γ \approx 336$~keV, $t_{1/2} \approx 4.5$~h). Following the measurement of the total reaction cross sections, the astrophysical S factors have been additionally deduced. The experimental results are compared with Hauser-Feshbach theoretical calculations carried out with the most recent version of TALYS. The results are discussed in terms of their significance to the various parameters entering the models.

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