Johannes Sørby Heines

• Jang, Y.; Moon, B.; Lee, J.; Watanabe, H.; Nishimura, S. & Phong, V. [Show all 43 contributors for this article] (2026). Yrast-state lifetimes in 94Pd at the transition between the isoscalar and isovector coupling regimes. Physical Review C. ISSN 2469-9985. 114(2). doi: 10.1103/smjm-rplr. Full text in Research Archive Show summary

  Half-lives of yrast excited states in 94Pd were measured using isomeric decay spectroscopy. The experiment was performed at Radioactive Isotope Beam Factory, where 94Pd ions were produced via in-flight fragmentation of a 124Xe beam impinging on a 9Be target. These ions were implanted into the GARi active stopper array, and isomeric delayed 𝛾 rays were detected with the IDATEN fast-timing array consisting of 48 LaBr3(Ce) detectors. The half-lives were determined using the generalized centroid difference method, yielding the 𝑇1/2 values of ≤15 ps, 13(11) ps, ≤13 ps, 693(72) ps, and ≤19 ps for the (2+1), (4+1), (6+1), (8+1), and (10+1) states, respectively. Reduced transition probabilities were deduced from the measured half-lives and discussed in the context of shell-model calculations employing the JUN45 effective interaction. The results indicate that 94Pd, with 𝑁=𝑍+2, lies in a transitional regime between the 𝑁=𝑍 isoscalar-pairing domain (92Pd) and the 𝑁=50 isovector-dominated region (96Pd).

• Heines, J. S.; Modamio, Victor; Görgen, A.; Korten, W.; Clément, E. & Dudouet, J. [Show all 29 contributors for this article] (2026). Triaxiality of neutron-rich ruthenium nuclei studied by lifetime measurements. European Physical Journal A. ISSN 1434-6001. 62(3). doi: 10.1140/epja/s10050-025-01782-4. Full text in Research Archive Show summary

  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 $$2_2^+$$ 2 2 + 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 $$\gamma $$ γ rigidity in neutron-rich ruthenium isotopes by measuring lifetimes of excited states in $$^{108-112}$$ 108 - 112 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 ( E 2) 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 $$^{112}$$ 112 Ru. The results are consistent with a transition from $$\gamma $$ γ  soft to $$\gamma $$ γ  rigid motion as the neutron number increases.

• Moon, B.; Lee, J.; Jang, Y.; Watanabe, H.; Nishimura, S. & Phong, V. [Show all 41 contributors for this article] (2026). Commissioning and performance of IDATEN: A large fast-timing γ-ray detector array at the RIBF. Nuclear Instruments and Methods in Physics Research Section A : Accelerators, Spectrometers, Detectors and Associated Equipment. ISSN 0168-9002. 1084. doi: 10.1016/j.nima.2025.171239. Full text in Research Archive
• Heines, Johannes Sørby; Modamio, Victor; Görgen, Andreas; Korten, Wolfram; Ljungvall, Joa & Pasqualato, G. [Show all 45 contributors for this article] (2025). NEW LIFETIME MEASUREMENTS IN THE RUTHENIUM CHAIN: INVESTIGATING THE EVOLUTION OF TRIAXIALITY. Acta Physica Polonica B, Proceedings Supplement. ISSN 1899-2358. 18(2). doi: 10.5506/APhysPolBSupp.18.2-A22. Full text in Research Archive Show summary

  We have used the recoil distance Doppler-shift method to measure lifetimes of excited states in 108Ru, 110Ru, and 112Ru. Excited states in these nuclei were populated by fusion–fission reactions between a 238U beam and a 9Be target in an experiment at GANIL. Fission fragments were identified event by event in the VAMOS++ spectrometer, while γ-rays were detected by the Advanced Gamma Tracking Array (AGATA). The lifetimes in the ground state band are consistent with rotational structures and constant B(E2) values indicate that the deformation does not change significantly across the chain of studied ruthenium nuclei. Preliminary results for lifetimes of states in the γ-band indicate a well deformed triaxial shape with a γ parameter close to 30◦ in 110Ru, with a slight change towards oblate deformation in 112Ru.

• Pasqualato, G; Ansari, S.; Heines, Johannes Sørby; Modamio, Victor; Görgen, Andreas & Beckmann, Kristine Sønstevold [Show all 52 contributors for this article] (2023). Shape evolution in even-mass ^{98 - 104} Zr isotopes via lifetime measurements using the γγ -coincidence technique. European Physical Journal A. ISSN 1434-6001. 59(11). doi: 10.1140/epja/s10050-023-01172-8. Full text in Research Archive Show summary

  The Zirconium (Z = 40) isotopic chain has attracted interest for more than four decades. The abrupt lowering of the energy of the first 2+ state and the increase in the transition strength B(E2; 2+→ 0+) going from 98Zr to 100Zr has been the first example of “quantum phase transition” in nuclear shapes, which has few equivalents in the nuclear chart. Although a multitude of experiments have been performed to measure nuclear properties related to nuclear shapes and collectivity in the region, none of the measured lifetimes were obtained using the Recoil Distance Doppler Shift method in the γγ-coincidence mode where a gate on the direct feeding transition of the state of interest allows a strict control of systematical errors. This work reports the results of lifetime measurements for the first yrast excited states in 98−104Zr carried out to extract reduced transition probabilities. The new lifetime values in γγ-coincidence and γ -single mode are compared with the results of former experiments. Recent predictions of the Interacting Boson Model with Configuration Mixing, the Symmetry Conserving Configuration Mixing model based on theHartree–Fock–Bogoliubov approach and the Monte Carlo Shell Model are presented and compared with the experimental data.

• Pogliano, Francesco; Larsen, Ann-Cecilie; Goriely, Stéphane; Siess, Lionel; Markova, Maria & Görgen, Andreas [Show all 17 contributors for this article] (2023). Experimentally constrained Ho 165,166 (n,γ) rates and implications for the s process. Physical Review C. ISSN 2469-9985. 107(6). doi: 10.1103/PhysRevC.107.064614. Full text in Research Archive

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• Torsvoll, Marius; Wirgenes, Maia Oline Grund; Al-Adili, Ali; Haug, Henrik Sørlie; Heines, Johannes Sørby & Siem, Sunniva [Show all 13 contributors for this article] (2026). Very asymmetric fission yields of 233U(nth, f) at LOHENGRIN. Full text in Research Archive Show summary

  In this work we report on a fission yield experiment done with the LOHENGRIN recoil mass spectrometer. Understanding the dynamics of nuclear fission requires accurate knowledge of the charge and mass distributions of fission fragments, particularly in rare and poorly studied regions. We present a dedicated investigation of the assymetric fission yields in 233U(nth,f), targeting fragment masses below A = 70, a region previously unexplored for this system. A particular focus is on confirming an expected enhancement of the yields of fragments near mass 70, potentially indicative of a super-asymmetric fission mode influenced by nuclear structure effects, such as the Z = 28 major shell closure. The experiment employed the LOHENGRIN recoil mass spectrometer in combination with a ΔE/E Frischgrid ionization chamber. This allows the identification of the mass of light fission fragments. The far asymmetric mass region will serve for benchmarking theoretical models and improving our understanding of shell effects in fission. During the presentation, we discussed the necessary correction steps during the analysis, including digital signal processing, scanning ionic charge and kinetic energy distributions, and target burn-up. Moreover, we discuss a comparison of different foil stacks that can be used as passive attenuators in the next beam-time campaign. Preliminary results from this work suggest a yield enhancement in the studied region, implying that the fissioning system U234* may exhibit a similar effect like the heavier systems. We finish by discussing the outlook and preparations for upcoming beam times at ILL.

• Görgen, Andreas & Heines, Johannes Sørby (2026). Evolution of triaxiality in the mass 𝑨≈𝟏𝟏𝟎 region. doi: https:/workshops.ill.fr/event/549/overview. Full text in Research Archive Show summary

  Strong triaxiality is rare in the nuclear chart, and such nuclei can serve as useful tests of theoretical predictions. The breaking of axial symmetry also enables phenomena which cannot occur in symmetrically deformed nuclei, making it an interesting phenomenon to study. The neutron rich region around mass 110 presents several cases of strong triaxiality, namely in the ruthenium, molybdenum and palladium chains. Ruthenium especially is considered one of the best examples of strong triaxial deformation near the ground state in the entire nuclear chart. This contribution presents new results from an experiment performed at GANIL to measure lifetimes of excited states in this region with the recoil distance Doppler-shift method. The studied nuclei were populated in a fusion-fission reaction, and identified event by event in the Variable Mode Spectrometer (VAMOS++). The Advanced Gamma Tracking Array (AGATA) provided high resolution gamma spectra of the decaying fission fragments. We present new B(E2) values for transitions in 108–112Ru, and compare them with phenomenological triaxial rotor predictions and fully microscopic symmetry conserving configuration mixing calculations. Triaxiality is shown to be increasing from 108Ru to 112Ru, with the latter exhibiting near maximum triaxiality. The results are consistent with a simultaneous transition from γ soft to γ rigid deformation. New results for molybdenum and palladium isotopes will also be presented.

• Torsvoll, Marius; Al-Adili, Ali; Haug, Henrik Sørlie; Heines, Johannes Sørby; Wirgenes, Maia Oline Grund & Siem, Sunniva [Show all 10 contributors for this article] (2025). Very asymmetric fission yields of 233U(nth, f) at LOHENGRIN. Full text in Research Archive Show summary

  Super asymmetric fission represents a rare but informative fission mode, providing sensitive insight into shell effects and scission configurations in the nuclear fission process. In this work, new measurements of neutron induced fission of U233 at thermal neutron energies are presented, with a particular focus on very light fission fragments with mass numbers below A = 80. The experiment was performed at the Institut Laue Langevin using the LOHENGRIN recoil mass spectrometer in combination with a delta E-E Frisch grid ionization chamber. For the first time, fission yields of U233(nth,f)have been measured from A = 90 down to A = 68. The data promise to be important benchmarks for fission model development and are also relevant for applications related to the thorium fuel cycle.

• Görgen, Andreas; Heines, Johannes Sørby & Modamio, Victor (2025). Triaxiality of neutron-rich ruthenium nuclei studied by lifetime measurements. doi: https:/indico.tlabs.ac.za/event/138/. Full text in Research Archive
• Görgen, Andreas; Heines, Johannes Sørby & Modamio, Victor (2025). Triaxiality of neutron-rich ruthenium nuclei studied by lifetime measurements. doi: https:/indico.ibs.re.kr/event/701/. Full text in Research Archive Show summary

  Neutron-rich ruthenium nuclei with mass around A≈110 are considered some of the best examples for nuclei with triaxial shape in their ground state. Quantitative information about the deformation in general and the degree of triaxiality in particular was obtained from lifetime measurements of short-lived excited states in 108Ru, 110Ru, and 112Ru. Lifetimes were measured using the recoil distance Doppler shift (RDDS) technique for states in both the ground-state band and the K=2 gamma band for the Ru isotopes under study. Combining the lifetimes with known branching rations, the measurements provide a multitude of new B(E2) transition strengths. The excited states were populated in fusion-fission reactions between a 238U beam at 6.2 A MeV and a 9Be target in an experiment performed at GANIL. The fission fragments were identified in mass, charge, and atomic number in the magnetic spectrometer VAMOS on an event-by-event basis. The velocity of nuclei exiting the target foil was slowed down in a degrader that was mounted in a plunger device at variable distances from the target. The AGATA gamma-ray tracking array was used to measure picosecond lifetimes with the RDDS method. The experiment produced a wide range of neutron-rich fission fragments, for which lifetimes could be measured under identical experimental conditions. An overview of the results will be presented, with emphasis on the chain of Ru isotopes. The comparison of experimental results with the triaxial rotor model and with beyond-mean field calculations provides quantitative information on the evolution of triaxiality in the chain of Ru isotopes.

• Heines, Johannes Sørby (2025). Triaxiality in neutron rich ruthenium Lifetime measurements in 108–112Ru. Full text in Research Archive Show summary

  The changes of nuclear deformation across the nuclear chart are among the fundamental questions in nuclear structure. While most nuclear properties can be explained by axially symmetric shapes, some nuclei are strongly triaxial. The ruthenium chain is believed to contain one of the strongest cases for highly deformed triaxial shape close to the ground state, but this is difficult to measure directly. Experimental transition probabilities can be used in conjunction with phenomenological models to study triaxiality, and serve as tests for large scale theoretical calculations. Through lifetime measurements with the recoil distance Doppler-shift method, conducted at GANIL in 2017 using the Advanced Gamma Tracking Array AGATA and with event by event particle identification in VAMOS++, we have obtained new transition probabilities in 108–112Ru. The results point to increasing triaxiality in both the odd and even isotopes, and 112Ru is shown to be a particularly good candidate for a triaxial rotor. The values measured in 110Ru and 112Ru are in good agreement with prediction by beyond mean field calculations.

• Heines, Johannes Sørby (2024). New Lifetime Measurements in the Ruthenium Chain Investigating the Evolution of Triaxiality. Full text in Research Archive
• Heines, Johannes Sørby; Görgen, Andreas; Modamio, Victor; Pasqualato, G; Korten, Wolfram & Ljungvall, Joa (2023). Lifetime measurements show high triaxiality in ruthenium. Full text in Research Archive
• Heines, Johannes Sørby; Modamio, Victor & Görgen, Andreas (2023). Evolution of Triaxiality in Neutron-Rich Ruthenium Isotopes Lifetime Measurements of excited states. Full text in Research Archive
• Heines, Johannes Sørby; Görgen, Andreas & Modamio, Victor (2022). Triaxiality in neutron-rich Ruthenium isotopes, New lifetimes measured in Ru-109,110,111. Universitetet i Oslo. Full text in Research Archive Show summary

  This thesis explores the asymmetrical deformation of Ruthenium isotopes, aiming to provide reliable experimental data useful for testing theoretical nuclear structure models. We have obtained reduced transition probabilities B(E2) for a number of transitions in Ru-109,110,111. A fit of the generalised triaxial rotor model indicates Ru-110 is well deformed, with a slightly oblate triaxial shape. The results are compared to various global nuclear structure models. First approximation reveals no evidence of shape changes between Ru-109 and Ru-111, more accurate results will be achieved through triaxial particle rotor calculations. The dataset at the heart of the analysis, obtained with AGATA and VAMOS++ at GANIL in 2017, features much higher statistics for a much larger number of nuclei than have previously been available. This has allowed us to get reliable lifetime measurements for the neutron odd Ru-109,111, and for both the ground- state band and one phonon γ-band in Ru-110. Additionally, it is expected to significantly reduce systematic errors when comparing results from different nuclei. We have developed and applied a new method for extracting lifetimes from the decay curves obtained in recoil distance Doppler shift measurements. By imposing constraints of physicality, it is expected to produce more reliable results than previous methods. A code for simultaneously fitting multiple gamma-spectra with shared parameters has also been developed.

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