https://doi.org/10.1140/epja/s10050-025-01782-4
Regular Article - Experimental Physics
Triaxiality of neutron-rich ruthenium nuclei studied by lifetime measurements
1
Department of Physics, University of Oslo, Oslo, Norway
2
Norwegian Nuclear Research Centre, Oslo, Norway
3
IRFU, CEA, Université Paris-Saclay, Gif-sur-Yvette, France
4
Grand Accélérateur National d’Ions Lourds, CEA/DRF-CNRS/IN2P3, Boulevard Henri Becquerel, 14076, Caen, France
5
Université Claude Bernard Lyon-1, CNRS/IN2P3, UMR 5822, IP2I, Villeurbanne, France
6
Université Paris-Saclay, IJCLab, IN2P3/CNRS, Orasy, France
7
Université de Strasbourg, CNRS, IPHC, UMR 7178, Strasbourg, France
8
Physics Division, Oak Ridge National Laboratory, 37831, Oak Ridge, TN, USA
9
Departamento de Física Atómica, Molecular y Nuclear, Universidad de Sevilla, Sevilla, Spain
10
School of Computing, Engineering and Mathematics, Brighton University, Brighton, UK
11
Department of Physics, University of Surrey, Guildford, UK
12
Institut für Kernphysik, Universität zu Köln, Köln, Germany
13
GSI Helmholtzzentrum für Schwerionenforschung GmbH, Darmstadt, Germany
14
Faculty of Physics, Sofia University ’St. Kliment Ohridski’, 1164, Sofia, Bulgaria
15
Department of Physics, Institute for Nuclear Physics, TU Darmstadt, Darmstadt, Germany
16
Extreme Light Infrastructure (ELI-NP), Horia Hulubei National Institute for R&D in Physics and Nuclear Engineering (IFIN-HH), Str. Reactorului No. 30, 077125, Bucharest-Măgurele, Romania
17
HUN-REN Institute for Nuclear Research (ATOMKI), Debrecen, Hungary
a
This email address is being protected from spambots. You need JavaScript enabled to view it.
Received:
16
September
2025
Accepted:
26
December
2025
Published online:
2
March
2026
Abstract
The breaking of axial symmetry in nuclei enables otherwise precluded behaviours, making it an interesting phenomenon to study. Experimental fingerprints such as very low-lying 
states suggest pronounced triaxial deformation for the neutron-rich ruthenium isotopes. Nevertheless, theoretical calculations differ in the description of the triaxial deformation and its evolution with neutron number, making experimental data crucial to understanding it. We investigated the evolution of the degree of triaxiality and 
rigidity in neutron-rich ruthenium isotopes by measuring lifetimes of excited states in 
Ru with the recoil distance Doppler-shift method. The experiment was carried out at the Grand Accélérateur National d’Ions Lourds using the Advanced Gamma Tracking Array coupled to the Variable Mode Spectrometer. We obtained B(E2) values for 29 transitions in the studied nuclei and compared them with fully microscopic symmetry conserving configuration mixing calculations, and phenomenological generalized triaxial rotor and triaxial particle-rotor models. The models generally reproduce the measured transition strengths, and show an increase in triaxiality with neutron number, reaching near maximum triaxiality in 
Ru. The results are consistent with a transition from soft to rigid motion as the neutron number increases.
Deceased: C. Theisen.
Communicated by Navin Alahari.
© The Author(s) 2026
Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons licence, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons licence, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons licence and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this licence, visit http://creativecommons.org/licenses/by/4.0/.
