Sunniva Siem

• Harris, Caley M.; Spyrou, Artemis; Denissenkov, Pavel; Herwig, Falk; Richard, Andrea L. & Uthayakumaar, Sivahami [Vis alle 23 forfattere av denne artikkelen] (2026). Influence of neutron-capture reactions on the nucleosynthesis of strontium. Communications Physics. 9(1). doi: 10.1038/s42005-026-02713-5. Fulltekst i vitenarkiv Vis sammendrag

  The astrophysical intermediate neutron-capture process (i process) has been reinvoked in recent years to describe stellar observations that cannot be explained by the traditional nucleosynthesis processes. Despite its success, significant uncertainties still remain including the exact astrophysical conditions and site, as well as the nuclear physics input. Available i-process models strongly underproduce the abundance of strontium (Sr) compared to observational data, while successfully describing neighboring elements, like yttrium (Y) and zirconium (Zr). This discrepancy has been attributed to nuclear physics uncertainties, specifically to the unconstrained reaction rate of the neutron-capture on the isotope krypton-88. Here we present an experimental investigation of this reaction and show that our result provides a possible solution to the aforementioned Sr underproduction in i-process models.

• Bell, Lauren Tracy; Siem, Sunniva; Larsen, Ann-Cecilie; Bürger, A.; Markova, Maria & Ingeberg, V. W. [Vis alle 12 forfattere av denne artikkelen] (2026). Extracting the nuclear level density and γ-ray strength function of 90Zr. Physical Review C. ISSN 2469-9985. 114(1). doi: 10.1103/p9y9-xxjf. Fulltekst i vitenarkiv Vis sammendrag

  In this work, we have extracted the nuclear level density (NLD) and 𝛾-ray strength function (𝛾⁢SF) using the Oslo method on particle-𝛾 coincidence data from the 90Zr⁢(𝑝,𝑝′⁢𝛾)90Zr ⁢reaction. We have applied the shape method to the same data set, providing a model-independent 𝛾⁢SF and a cross-check on the slope of the Oslo-method NLD and 𝛾⁢SF. Further, we have combined the 𝛾⁢SF from this work with photonuclear data, thus covering the 2.5<𝐸𝛾<18MeV energy range. We see an increase in strength in the region around 𝐸𝛾≈9MeV, consistent with previous (𝛾,𝛾′) and (𝛾,𝑛) data. On the other hand, surprisingly, we do not observe a strong low-energy increase at low transition energies. The resulting NLD and 𝛾⁢SF were used as inputs to the nuclear reaction code talys to estimate the cross sections and astrophysical reaction rates for the 89Zr(𝑛,𝛾)⁢90Zr 89Y⁡(𝑝,𝛾)90Zr reactions. We find good agreement with available experimental cross sections and JINA calculations for the 89Y⁡(𝑝,𝛾) results, but not for the 89Zr⁢(𝑛,𝛾) reaction rate for stellar temperatures above 0.7 GK. Based on our data, we provide a much more narrow uncertainty band for the 89Zr⁢(𝑛,𝛾) cross section and reaction rate.

• Markova, Maria; Larsen, Ann-Cecilie; Neumann-Cosel, P. von; Litvinova, E.; Goriely, S. & Bell, Lauren Tracy [Vis alle 17 forfattere av denne artikkelen] (2026). Exploring the statistical properties of the neutron-deficient 109In isotope with the Oslo method. Physical Review C. ISSN 2469-9985. 113(4). doi: 10.1103/qrvh-s12k. Fulltekst i vitenarkiv Vis sammendrag

  The nuclear level density and the 𝛾-ray strength function of the neutron-deficient 109In isotope were extracted for the first time with data from the 106Cd⁢(𝛼,𝑝⁢𝛾)⁢109In reaction using a combination of the Oslo and the shape methods. Both quantities are consistent with those of neighboring Cd and Sn nuclei but show substantial discrepancies with currently available model predictions. In contrast to earlier observations in the neighboring isotopic chains, 109In does not exhibit any significant enhancement of the dipole strength near the neutron separation energy. To interpret this feature, random-phase time-blocking approximation calculations have been performed for 109In and the neighboring 110,112Sn nuclei. The experimental data were also employed to estimate cross sections and rates of the radiative neutron- and proton-capture reactions, 108In⁢(𝑛,𝛾)⁢109In and 108Cd⁢(𝑝,𝛾)⁢109In, respectively, with the reaction code talys. Our (𝑝,𝛾) cross section is in excellent agreement with direct measurements over a wide range of proton energies, while the (𝑛,𝛾) cross section demonstrates notable deviations from predictions in the jina reaclib library. The new results on the statistical properties of 109In provide valuable constraints that may help address the problem of large model uncertainties compromising the accuracy of astrophysical 𝑝-process simulations.

• Pellegri, L.; Jivan, H.; Krugmann, A.; Neumann-Cosel, P. Von; Pietralla, N. & Tamii, A. [Vis alle 54 forfattere av denne artikkelen] (2026). Identification of the pygmy dipole resonance in a well deformed nucleus by combined isoscalar and isovector probes. Physics Letters B. ISSN 0370-2693. 876, s. 140360–140360. doi: 10.1016/j.physletb.2026.140360. Fulltekst i vitenarkiv Vis sammendrag

  The low-energy electric dipole response in the axially deformed 154Sm nucleus was investigated for the first time using complementary probes. The isoscalar (IS) response was extracted from an (α, α′γ) coincidence experiment and the isovector (IV) response from polarization observables measured with the (p,p') reaction at 0∘. Both the IS and IV responses exhibit a resonance-like structure at excitation energies 5.5-6 MeV identified as the pygmy dipole resonance. Its evolution with deformation is investigated by comparison to the spherical isotope 144Sm. The low-energy IV strength in 154Sm departs from the K-splitting picture that characterizes the IV giant dipole resonance in this nucleus. This finding is further supported by the absence of significant IS strength above 7 MeV.

• Rykkelid, Anne Marit; Skeie, Hilde Solesvik; Tormodsrud, Vera Helene; Siem, Sunniva; Malinen, Eirik & Edin, Nina Frederike Jeppesen (2026). Cell survival and DNA damage along the distal edge of the proton Bragg peak. International Journal of Radiation Biology. ISSN 0955-3002. s. 1–10. doi: 10.1080/09553002.2026.2666062. Fulltekst i vitenarkiv
• Ekeberg, Hannah Lovise Okstad; Voyles, A.S.; Basunia, M.S.; Batchelder, J.C.; Bernstein, L.A. & Bleuel, D.L. [Vis alle 12 forfattere av denne artikkelen] (2026). Nuclear excitation functions for medical isotope production: Targeted radionuclide therapy via natIr(𝑑,𝑥)193mPt. Applied Radiation and Isotopes. ISSN 0969-8043. 229. doi: 10.1016/j.apradiso.2025.112311. Fulltekst i vitenarkiv
• Heines, J. S.; Modamio, Victor; Görgen, A.; Korten, W.; Clément, E. & Dudouet, J. [Vis alle 29 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Martinsen, Elise Malmer; Voyles, Andrew S.; Li, Kevin Ching Wei; Basunia, M. Shamsuzzoha; Bernstein, Lee A. & Ekeberg, Hannah Lovise Okstad [Vis alle 12 forfattere av denne artikkelen] (2025). Deuteron-induced reactions on natural Zr from threshold to 50 MeV: production of 86gY. Radiochimica Acta. ISSN 0033-8230. 114(4), s. 280–280. doi: 10.1515/ract-2025-0057. Fulltekst i vitenarkiv Vis sammendrag

  Abstract Two stacks of thin Zr foils were irradiated with 30 and 50 MeV deuterons, respectively, using the Lawrence Berkeley National Laboratory 88-Inch Cyclotron, and 19 excitation functions for nat Zr( d , x ) reactions were measured over a beam energy range of 6.3–47.64 MeV, where the independent cross sections for nat Zr( d , x ) 88 Nb and nat Zr( d , x ) 86m,g Y were measured for the first time. The well-characterized nat Fe( d , x ) 56 Co, nat Ni( d , x ) 56 Co, nat Ni( d , x ) 58 Co, nat Ni( d , x ) 61 Cu, nat Ti( d , x ) 46 Sc and nat Ti( d , x ) 48 V monitor reactions were used to determine the deuteron beam current throughout the stacks. All cross sections were determined using High Purity Germanium (HPGe) detector γ -ray spectroscopy. A variance minimization technique was employed to simultaneously constrain the deuteron beam currents with multiple monitor reactions, thus reducing systematic uncertainties. An additional 16 channels are reported for reactions on the nickel, titanium, and iron monitor foils, leading to a total of 35 excitation functions, with seven reaction channels reported for the first time in this work. The measured excitation functions are compared to calculations provided by the reaction modeling codes TALYS – 2.0, ALICE – 2020, CoH – 3.5.3 and EMPIRE – 3.2.3, as well as the TENDL – 2023 data library. The degree of agreement between theory and experiments is discussed. The possible production of the important PET radionuclide 86g Y via the nat Zr( d , x ) route was critically examined. The physical yields for nat Zr( d , x ) 86 Y and other yttrium isotopes produced were calculated and compared to other production pathways. Due to high-level of associated radionuclide impurities, this route cannot deliver 86g Y suitable for medical applications.

• Wiedeking, Mathis; Goriely, Stephane; Guttormsen, Magne; Herwig, Falk; Larsen, Ann-Cecilie & Liddick, Sean N. [Vis alle 10 forfattere av denne artikkelen] (2025). Unlocking i-process nucleosynthesis by bridging stellar and nuclear physics. Nature Reviews Physics. 7(12), s. 696–712. doi: 10.1038/s42254-025-00885-7. Fulltekst i vitenarkiv Vis sammendrag

  The origin of chemical elements and their abundances across the cosmos remain one of the central questions in physics. The formation of elements heavier than iron is traditionally attributed to three main mechanisms: the slow and rapid neutron-capture processes (s and r processes) and the p process (mostly driven by photodisintegration reactions). However, certain astronomical observations reveal elemental abundance patterns inconsistent with these processes. These discrepancies prompted the introduction of the intermediate neutron-capture process (i process), which operates at neutron densities between the s and r processes, and which has emerged as a key area of research. Observations of elemental abundances of stars confirm that the i process does indeed take place. Identifying the required astrophysical conditions and contributions of the i process sensitively depend on neutron-capture reaction rates involving unstable atomic nuclei. Important advancements have been achieved through these new astronomical observations and cutting-edge experimental and analytical techniques in nuclear physics, in addition to models of nuclear physics and nucleosynthesis. State-of-the-art facilities and theoretical models are revolutionizing our ability to explore the i process and offer fresh perspectives on the nuclear behaviour under extreme stellar conditions. This Review underscores the synergy between groundbreaking astronomical and nuclear physics research, bridging nuclear physics and observational astrophysics, and advancing our understanding of i-process nucleosynthesis.

• Ingeberg, Vetle Wegner; Siem, Sunniva; Wiedeking, Mathis; Choplin, A.; Goriely, Stéphane & Siess, Lionel [Vis alle 32 forfattere av denne artikkelen] (2025). Nuclear level density and 𝛾-ray strength function of 67-Ni and the impact on the 𝑖 process. Physical Review C. ISSN 2469-9985. 111(1). doi: 10.1103/PhysRevC.111.015803. Fulltekst i vitenarkiv
• Heines, Johannes Sørby; Modamio, Victor; Görgen, Andreas; Korten, Wolfram; Ljungvall, Joa & Pasqualato, G. [Vis alle 45 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Paulsen, Wanja; Li, Kevin Ching Wei; Siem, Sunniva; Ingeberg, Vetle Wegner; Larsen, Ann-Cecilie & Eriksen, Tomas Kvalheim [Vis alle 15 forfattere av denne artikkelen] (2025). Remeasuring the γ-decay branching ratio of the Hoyle state. Physical Review C. ISSN 2469-9985. 112. doi: 10.1103/2h2s-sbyx. Fulltekst i vitenarkiv
• Torvund, Gulla Louise Serville; Ernoult, Marc; Courtin, Fanny; Doligez, Xavier; Gjestvang, Dorthea & Siem, Sunniva [Vis alle 7 forfattere av denne artikkelen] (2025). Multi-MOX: Facilitating plutonium multi-recycling in the French PWR fleet. Annals of Nuclear Energy. ISSN 0306-4549. 223. doi: 10.1016/j.anucene.2025.111641. Fulltekst i vitenarkiv Vis sammendrag

  This study examines the Multi-MOX (MMOX) strategy for plutonium multi-recycling in PWRs using the CLASS fuel cycle simulation tool with assembly-level calculations. MMOX facilitates the multi-recycling of used nuclear fuel by mixing reprocessed plutonium from various sources to create viable fresh nuclear fuel. The strategy significantly curtails the growth of the plutonium inventory, but does not stabilize it entirely in the long term. Although lower fuel burnup reduces plutonium build-up, it increases the production of minor actinides. Comparing MMOX with non-recycling and mono-recycling scenarios, we find that it reduces plutonium inventory by 35% and 19%, respectively. Despite a higher level of minor actinide production, MMOX decreases overall transuranic element production. Additionally, MMOX reduces the need for interim spent fuel storage by a factor of ten compared to non-recycling and by two-thirds compared to mono-recycling, while substantially lowering the age of stockpiled used nuclear fuel.

• Li, Kevin Ching Wei; Neveling, R.; Adsley, P.; Fujita, H.; Papka, P. & Smit, F. D. [Vis alle 15 forfattere av denne artikkelen] (2024). Understanding the total width of the 3−1 state in 12C. Physical Review C. ISSN 2469-9985. 109(1). doi: 10.1103/PhysRevC.109.015806. Fulltekst i vitenarkiv Vis sammendrag

  Background: Recent measurements indicate that the previously established upper limit for the 𝛾-decay branch of the 3− 1 resonance in 12C at 𝐸𝑥=9.641⁢(5) MeV may be incorrect. As a result, the 3−1 resonance has been suggested as a significant resonance for mediating the triple-𝛼 reaction at high temperatures above 2 GK. Accurate estimations of the 3−1 contribution to the triple-𝛼 reaction rate require accurate knowledge of not only the radiative width, but also the total width. Purpose: In anticipation of future measurements to more accurately determine the 𝛾-decay branch of the 3−1 resonance, the objective of this work is to accurately determine the total width of the 3−1 resonance. Method: An evaluation was performed on all previous results considered in the current ENSDF average of 46(3) keV for the physical total width (FWHM) of the 3−1 resonance in 12C. A new 𝐑-matrix analysis for the 3−1 resonance was performed with a self-consistent, simultaneous fit of several high-resolution 12C excitation-energy spectra populated with direct reactions. Results: The global analysis performed in this work yields a formal total width of Γ⁡(𝐸𝑟)=46⁢(2) keV and an observed total width of Γobs⁡(𝐸𝑟)=38⁢(2) keV for the 3−1 resonance. Conclusions: Significant unaccounted-for uncertainties and a misstated result were discovered in the previous results employed in the ENSDF for the physical (or observed) total width of the 3−1 resonance. These previously reported widths are fundamentally different quantities, leading to an invalid ENSDF average. An observed total width of Γobs⁡(𝐸𝑟)=38⁢(2) keV is recommended for the 3−1 resonance in 12C. This observed total width should be employed for future evaluations of the observed total radiative width for the 3−1 resonance and its contribution to the high-temperature triple-𝛼 reaction rate.

• Markova, Maria; Larsen, Ann-Cecilie; Neumann-Cosel, Peter von; Litvinova, E.; Choplin, A. & Goriely, S. [Vis alle 11 forfattere av denne artikkelen] (2024). Systematic study of the low-lying electric dipole strength in Sn isotopes and its astrophysical implications. Physical Review C. ISSN 2469-9985. 109(5). doi: 10.1103/PhysRevC.109.054311. Fulltekst i vitenarkiv Vis sammendrag

  The 𝛾-ray strength functions (GSFs) and nuclear level densities (NLDs) below the neutron threshold have been extracted for 111–113,116–122,124 Sn from particle-𝛾 coincidence data with the Oslo method. The evolution of bulk properties of the low-lying electric dipole response has been investigated on the basis of the Oslo GSF data and results of a recent systematic study of electric- and magnetic dipole strengths in even-even Sn isotopes with relativistic Coulomb excitation. The obtained GSFs reveal a resonance-like peak on top of the tail of the isovector giant dipole resonance centered at ≈8 MeV and exhausting ≈2% of the classical Thomas-Reiche-Kuhn (TRK) sum. For mass numbers ≥118 the data suggest also a second peak centered at ≈6.5 MeV. It corresponds to 0.1%–0.5% of the TRK sum rule and shows an approximate linear increase with the mass number. In contrast with predictions of the relativistic quasiparticle random-phase and time-blocking approximation calculations, no monotonic increase in the total low-lying 𝐸⁢1 strength was observed in the experimental data from 111 Sn to 124 Sn, demonstrating rather similar strength distributions in these nuclei. The Oslo GSFs and NLDs were further used as inputs to constrain the cross sections and Maxwellian-averaged cross sections of (𝑛,𝛾) reactions in the Sn isotopic chain using talys. The obtained results agree well with other available experimental data and the recommended values from the JINA REACLIB, BRUSLIB, and KADoNiS libraries. Despite relatively small exhausted fractions of the TRK sum rule, the low-lying electric dipole strength makes a noticeable impact on the radiative neutron-capture cross sections in stable Sn isotopes. Moreover, the experimental Oslo inputs for the 121,123 Sn ⁢(𝑛,𝛾) 122,124 Sn reactions were found to affect the production of Sb in the astrophysical 𝑖 process, providing new constraints on the uncertainties of the resulting chemical abundances from multizone low-metallicity asymptotic giant branch stellar models.

• Lewis, R.; Couture, A.; Liddick, S.N.; Spyrou, A.; Bleuel, D.L. & Campo, Lucia Crespo [Vis alle 18 forfattere av denne artikkelen] (2023). Statistical (n, γ) cross section model comparison for short-lived nuclei. European Physical Journal A. ISSN 1434-6001. 59(3). doi: 10.1140/epja/s10050-023-00920-0. Fulltekst i vitenarkiv Vis sammendrag

  Neutron-capture cross sections of neutron-rich nuclei are calculated using a Hauser–Feshbach model when direct experimental cross sections cannot be obtained. A number of codes to perform these calculations exist, and each makes different assumptions about the underlying nuclear physics. We investigated the systematic uncertainty associated with the choice of Hauser-Feshbach code used to calculate the neutron-capture cross section of a short-lived nucleus. The neutron-capture cross section for 73Zn(n,y)74Zn was calculated using three Hauser-Feshbach statistical model codes: TALYS, CoH, and EMPIRE. The calculation was first performed without any changes to the default settings in each code. Then an experimentally obtained nuclear level density (NLD) and y-ray strength function (ySF) were included. Finally, the nuclear structure information was made consistent across the codes. The neutron-capture cross sections obtained from the three codes are in good agreement after including the experimentally obtained NLD and ySF, accounting for differences in the underlying nuclear reaction models, and enforcing consistent approximations for unknown nuclear data. It is possible to use consistent inputs and nuclear physics to reduce the differences in the calculated neutron-capture cross section from different Hauser-Feshbach codes. However, ensuring the treatment of the input of experimental data and other nuclear physics are similar across multiple codes requires a careful investigation. For this reason, more complete documentation of the inputs and physics chosen is important.

• Thisse, D.; Lebois, M.; Verney, D.; Wilson, J.N.; Jovančević, N. & Rudigier, M. [Vis alle 78 forfattere av denne artikkelen] (2023). Study of N= 50 gap evolution around Z= 32 : new structure information for ⁸² Ge. European Physical Journal A. ISSN 1434-6001. 59(7). doi: 10.1140/epja/s10050-023-01051-2. Fulltekst i vitenarkiv
• Markova, Maria; Larsen, Ann-Cecilie; Tveten, Gry Merete; Neumann-Cosel, Peter von; Eriksen, Tomas Kvalheim & Garrote, Frank Leonel Bello [Vis alle 16 forfattere av denne artikkelen] (2023). Nuclear level densities and γ -ray strength functions of Sn 111,112,113 isotopes studied with the Oslo method. Physical Review C. ISSN 2469-9985. 108(1). doi: 10.1103/PhysRevC.108.014315. Fulltekst i vitenarkiv Vis sammendrag

  The 111,112,113Sn isotopes have been studied with (p,dγ), (p,p′γ), and (d,pγ) reactions to extract the nuclear level densities (NLDs) and γ-ray strength functions (GSFs) of these nuclei below the neutron separation energy by means of the Oslo method. The experimental NLDs for all three nuclei demonstrate a trend compatible with the constant-temperature model below the neutron separation energy while also being in good agreement with the NLDs of neighboring Sn isotopes, obtained previously with the Oslo-type and neutron evaporation experiments. The extracted microcanonical entropies yield ≈1.5 kB entropy of a valence neutron in both 111Sn and 113Sn. Moreover, the deduced microcanonical temperatures indeed suggest a clear constant-temperature behavior above ≈3 MeV in 111,113Sn and above ≈4.5 MeV in 112Sn. We observe signatures for the first broken neutron pairs between 2 and 4 MeV in all three nuclei. The GSFs obtained with the Oslo method are found to be in good agreement below the neutron threshold with the strengths of 112,114Sn extracted in the (p,p′) Coulomb excitation experiments.

• Camera, F.; Isaak, J.; Maj, A. & Siem, Sunniva (2023). AGATA: Nuclear structure advancements with high-energy γ rays. European Physical Journal A. ISSN 1434-6001. 59(7). doi: 10.1140/epja/s10050-023-01076-7. Fulltekst i vitenarkiv Vis sammendrag

  High-energy γ rays are emitted either in the decay of nuclear discrete states (generally located in the proximity of the particle-emission threshold) or in the electromagnetic decay of collective states. The branching ratio is rather small, therefore in alternative/coincidence with high-energy γ rays one could have neutron or charged-particle emission. The precise measurement of these branching ratios allows a more comprehensive description of the nucleus and, in particular, the selection of the nuclear models which better describes the interplay between the different acting forces and couplings. In the case of the electromagnetic decay of collective states, the precise identification of the populated low-lying states allows the measurement of their wave-function and/or of their ‘bulk’ properties. This information provides, also in this case, a stringent test for the nuclear models. In both cases, to compensate these small branching ratio, a large number of detectors and an accurate selection of a small region of the phase space are needed. AGATA is considered to constitute an optimal array for this kind of measurements. In fact, AGATA, because of its high granularity and its excellent energy resolution, can detect the high-energy γ rays emitted in a specific decay path or associated to specific reaction channels, shapes, deformations. In the introduction, some details on the measurement of high-energy γ rays using HPGe detector are discussed. The sections of this paper focus on the measurements of high-energy γ rays to obtain the photon strength function, the nuclear level density and to identify extreme shapes in highly excited nuclei.

• Larsen, Ann-Cecilie; Tveten, Gry Merete; Renstrøm, Therese; Utsunomiya, H.; Algin, E. & Ari-Izumi, T. [Vis alle 31 forfattere av denne artikkelen] (2023). New experimental constraint on the W 185 (n,γ) W 186 cross section. Physical Review C. ISSN 2469-9985. 108(2). doi: 10.1103/PhysRevC.108.025804. Fulltekst i vitenarkiv
• Pasqualato, G; Ansari, S.; Heines, Johannes Sørby; Modamio, Victor; Görgen, Andreas & Beckmann, Kristine Sønstevold [Vis alle 52 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Magagula, S.P.E.; Malatji, Kgashane Leroy; Wiedeking, M.; Pellegri, L; Beckmann, Kristine Sønstevold & Garrote, Frank Leonel Bello [Vis alle 18 forfattere av denne artikkelen] (2023). Investigating the strength of the scissors mode in 151Sm. Journal of Physics: Conference Series (JPCS). ISSN 1742-6588. doi: 10.1088/1742-6596/2586/1/012070. Fulltekst i vitenarkiv Vis sammendrag

  Change in nuclei deformation leads to changes in statistical properties such as the nuclear level density (NLD) and γ-ray strength function (γSF). The NLD and γSF of 151Sm were extracted using the Oslo method. The strength of the scissors resonance (SR) and its centroid energy for 151Sm were found to be 2.13 ± 0.60 µ2N and 2.48 ± 0.25 MeV, respectively. These results were used to place the SR of 151Sm and its magnetic dipole strength B(M1)SR into the context of previously measured Sm isotopes.

• Tamii, A.; Pellegri, L.; Söderström, P.-A.; Allard, D.; Goriely, S. & Inakura, T. [Vis alle 82 forfattere av denne artikkelen] (2023). PANDORA Project for the study of photonuclear reactions below A= 60. European Physical Journal A. ISSN 1434-6001. 59(9). doi: 10.1140/epja/s10050-023-01081-w. Fulltekst i vitenarkiv Vis sammendrag

  Photonuclear reactions of light nuclei below a mass of A = 60 are planned to be studied experimentally and theoretically with the PANDORA (Photo-Absorption of Nuclei and Decay Observation for Reactions in Astrophysics) project. Two experimental methods, virtual photon excitation by proton scattering and real photo absorption by a high-brilliance γ-ray beam produced by laser Compton scattering, will be applied to measure the photoabsorption cross sections and decay branching ratio of each decay channel as a function of the photon energy. Several nuclear models, e.g. anti-symmetrized molecular dynamics, mean-field and beyond-mean-field models, a large-scale shell model, and ab initio models, will be employed to predict the photonuclear reactions. The uncertainty in the model predictions will be evaluated based on the discrepancies between the model predictions and experimental data. The data and predictions will be implemented in the general reaction calculation code, TALYS. The results will be applied to the simulation of the photo-disintegration process of ultra-high-energy cosmic rays in inter-galactic propagation.

• Jivan, H.; Pellegri, Luna; Adsley, P; Bahini, A.; Brummer, J.W. & Donaldson, L. [Vis alle 29 forfattere av denne artikkelen] (2023). Pygmy Dipole Response in Samarium isotopes. Journal of Physics: Conference Series (JPCS). ISSN 1742-6588. doi: 10.1088/1742-6596/2586/1/012088. Fulltekst i vitenarkiv Vis sammendrag

  Abstract The influence of nuclear shape deformation on the Pygmy Dipole Response (PDR) was investigated using (α,α′γ) scattering on 144 Sm and 154 Sm. Experiments were conducted at iThemba LABS, coupling for the first time the K600 magnetic spectrometer with the BaGeL (Ball of Germanium and LaBr detectors) array. Preliminary results showcasing the efficacy of the facility and set-up for performing these 0° scattering experiments to investigate the PDR region are presented.

• Gjestvang, Dorthea; Wilson, J.N.; Al-Adili, A.; Siem, Sunniva; Gao, Z. & Randrup, J. [Vis alle 74 forfattere av denne artikkelen] (2023). Examination of how properties of a fissioning system impact isomeric yield ratios of the fragments. Physical Review C. ISSN 2469-9985. 108(6). doi: 10.1103/PhysRevC.108.064602. Fulltekst i vitenarkiv
• Pogliano, Francesco; Larsen, Ann-Cecilie; Goriely, Stéphane; Siess, Lionel; Markova, Maria & Görgen, Andreas [Vis alle 17 forfattere av denne artikkelen] (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. Fulltekst i vitenarkiv
• Mücher, D.; Spyrou, A.; Wiedeking, M.; Guttormsen, Magne Sveen; Larsen, Ann-Cecilie & Zeiser, Fabio Balthasar [Vis alle 25 forfattere av denne artikkelen] (2023). Extracting model-independent nuclear level densities away from stability. Physical Review C. ISSN 2469-9985. 107(1). doi: 10.1103/PhysRevC.107.L011602. Fulltekst i vitenarkiv
• Pogliano, Francesco; Garrote, Frank Leonel Bello; Larsen, Ann-Cecilie; Berg, Hannah Christine; Gjestvang, Dorthea & Görgen, Andreas [Vis alle 19 forfattere av denne artikkelen] (2023). Observation of a candidate for the M1 scissors resonance in odd-odd Ho 166. Physical Review C. ISSN 2469-9985. 107(3). doi: 10.1103/PhysRevC.107.034605. Fulltekst i vitenarkiv Vis sammendrag

  The γ -strength function and the nuclear level density for the odd-odd, rare-earth nucleus 166 Ho have been extracted from 163 Dy(α, pγ ) 166 Ho data using the Oslo method. A structure at ≈3 MeV in the γ -strength function is interpreted as the M1 scissors resonance. By employing three different methods we find that its strength depends rather strongly on the modeling of the E 1 strength, while its centroid does not. The 166 Ho scissors resonance parameters are consistent with previous results on other rare-earth nuclei.

• Pogliano, Francesco; Larsen, Ann-Cecilie; Markova, Maria; Garrote, Frank Leonel Bello; Bjørøen, Marianne Møller & Eriksen, Tomas Kvalheim [Vis alle 17 forfattere av denne artikkelen] (2022). Indirect measurement of the (n,γ) Sb 127 cross section. Physical Review C. ISSN 2469-9985. 106(1). doi: 10.1103/PhysRevC.106.015804. Fulltekst i vitenarkiv Vis sammendrag

  Nuclei in the 135I region have been identified as being a possible bottleneck for the i process. Here we present an indirect measurement for the Maxwellian-averaged cross section of 126Sb(n,γ). The nuclear level density and the γ-ray strength function of 127Sb have been extracted from 124Sn(α,pγ)127Sb data using the Oslo method. The level density in the low-excitation-energy region agrees well with known discrete levels, and the higher-excitation-energy region follows an exponential curve compatible with the constant-temperature model. The strength function between Eγ≈1.5–8.0 MeV presents several features, such as an upbend and a possibly double-peaked pygmy-like structure. None of the theoretical models included in the nuclear reaction code talys seem to reproduce the experimental data. The Maxwellian-averaged cross section for the 126Sb(n,γ)127Sb reaction has been experimentally constrained by using our level-density and strength-function data as input to talys. We observe a good agreement with the jina reaclib, tendl, and bruslib libraries, while the endf/b-viii.0 library predicts a significantly higher rate than our results.

• Naqvi, F.; Karampagia, S.; Spyrou, A.; Liddick, S.N.; Dombos, A.C. & Bleuel, D.l. [Vis alle 20 forfattere av denne artikkelen] (2022). Total absorption spectroscopy measurement on neutron-rich 74,75Cu isotopes. Nuclear Physics A. ISSN 0375-9474. A1018(122359). doi: 10.1016/j.nuclphysa.2021.122359. Fulltekst i vitenarkiv
• Garrote, Frank Leonel Bello; Lopez-Martens, Araceli; Larsen, Ann-Cecilie; Deloncle, I.; Péru, Sophie & Zeiser, Fabio [Vis alle 39 forfattere av denne artikkelen] (2022). Experimental observation of the M1 scissors mode in ²⁵⁴No. Physics Letters B. ISSN 0370-2693. 834. doi: 10.1016/j.physletb.2022.137479. Fulltekst i vitenarkiv Vis sammendrag

  We present the first experimental evidence of the scissors mode in the superheavy nucleus 254No produced in the 208Pb(48Ca, 2nγ))254No reaction. The spectrum of γrays emitted by the excited 254No nuclei shows an enhanced γ-ray yield for transition energies of ≈2.5MeV. By measuring the linear polarization properties of the emitted γrays, we confirm that the transitions in the enhancement region are predominantly of magnetic-dipole character, characteristic for the scissors mode. To further characterize the enhanced γ-ray yield, simulations of the electromagnetic decay of 254No were performed. The observed enhancement is reproduced by including an M1component in the γstrength function with total strength B(M1↑)=11.8(19)μ2N. This is in good agreement with the integrated M1strength from sum-rule estimates and new calculations within the quasi-particle random-phase approximation presented here. Our results provide a stringent test of phenomenological formulae for the scissors mode currently used in stellar nucleosynthesis calculations. We find that those formulae are not satisfactory, and we recommend using sum-rule estimates assuming a rigid-body moment of inertia instead for describing the scissors mode in superheavy nuclei.

• Guttormsen, Magne Sveen; Ay, Kursad Osman; Ozgur, Mustafa; Algin, Emel; Larsen, Ann-Cecilie & Garrote, Frank Leonel Bello [Vis alle 26 forfattere av denne artikkelen] (2022). Evolution of the γ -ray strength function in neodymium isotopes. Physical Review C. ISSN 2469-9985. 106(3). doi: 10.1103/PhysRevC.106.034314. Fulltekst i vitenarkiv Vis sammendrag

  The experimental γ-ray strength functions (γSFs) of 142,144–151Nd have been studied for γ-ray energies up to the neutron separation energy using the Oslo method. The results represent a unique set of γSFs for an isotopic chain with increasing nuclear deformation. The data reveal how the low-energy enhancement, the scissors mode, and the pygmy dipole resonance evolve with nuclear deformation and mass number. This indicates that the mechanisms behind the low-energy enhancement and the scissors mode are decoupled from each other.

• Li, Kevin Ching Wei; Adsley, P; Neveling, R; Papka, P; Smit, F. D & Nikolskii, E [Vis alle 28 forfattere av denne artikkelen] (2022). Multiprobe study of excited states in 12 C : Disentangling the sources of monopole strength between the energy of the Hoyle state and E x = 13 MeV. Physical Review C. ISSN 2469-9985. doi: 10.1103/PhysRevC.105.024308. Fulltekst i vitenarkiv Vis sammendrag

  Background: The Hoyle state is the archetypal α-cluster state which mediates the 3α reaction to produce 12C and is of great interest for both nuclear structure and astrophysics. Recent theoretical calculations predict a breathing-mode excitation of the Hoyle state at Ex≈9 MeV. Its observation is hindered by the presence of multiple broad states and potential interference effects. An analysis with Gaussian line shapes of measurements at the Research Center for Nuclear Physics (Osaka University) with the Grand Raiden spectrometer suggested that additional strength was needed at Ex≈9 MeV to reproduce the data; this analysis did not account for the well-known threshold effects observed in 12C. Nevertheless, various theoretical studies have since concluded that this additional strength corresponds to the predicted breathing-mode excitation of the Hoyle state. To meaningfully identify a new source of monopole strength in this astrophysically significant region, a more appropriate phenomenological analysis which accounts for penetrability and interference effects must be used to determine whether the data can be explained with previously established states. Purpose: We aim to investigate the monopole strength in the astrophysically important excitation-energy region of 12C between Ex=7 and 13 MeV to determine whether the previously established sources of monopole strength are able to reproduce the data. Method: The 12C(α,α′)12C and 14C(p,t)12C reactions, which are expected to exhibit contrasting selectivity towards different monopole excitations, were employed at various detection angles and beam energies to populate states in 12C. The inclusive excitation-energy spectra were simultaneously analyzed with multilevel, multichannel line shapes. Various scenarios with different sources of monopole strength and interference effects were considered to determine whether the ghost of the Hoyle state and the previously established broad 0+3 state at Ex≈10 MeV are able to reproduce the observed monopole strength. Results: Clear evidence was found for excess monopole strength at Ex≈9 MeV, particularly in the 12C(α,α′)12C reaction at 0∘. This additional strength cannot be reproduced by the previously established monopole states between Ex=7 and 13 MeV. Coincident charged-particle decay data suggest that the strength at Ex≈9 MeV is dominantly monopole, with no evidence of a J>0 contribution. Conclusions: The data support a new source of monopole strength at Ex≈9 MeV, which cannot be described with a phenomenological parametrization of all previously established states. An additional 0+ state at Ex≈9 MeV yielded a significantly improved fit of the data and is a clear candidate for the predicted breathing-mode excitation of the Hoyle state. Alternatively, the results may suggest that a more sophisticated, physically motivated parametrization of the astrophysically important monopole strengths in 12C is required.

• Li, Kevin Ching Wei; Smit, F. D.; Adsley, P.; Neveling, R.; Papka, P. & Nikolskii, E. [Vis alle 28 forfattere av denne artikkelen] (2022). Investigating the predicted breathing-mode excitation of the Hoyle state. Physics Letters B. ISSN 0370-2693. doi: 10.1016/j.physletb.2022.136928. Fulltekst i vitenarkiv
• Markova, Maria; Larsen, Ann-Cecilie; Neumann-Cosel, Peter von; Bassauer, Sergej; Görgen, Andreas & Guttormsen, Magne Sveen [Vis alle 23 forfattere av denne artikkelen] (2022). Nuclear level densities and γ -ray strength functions in Sn 120,124 isotopes: Impact of Porter-Thomas fluctuations. Physical Review C. ISSN 2469-9985. 106(3). doi: 10.1103/PhysRevC.106.034322. Fulltekst i vitenarkiv Vis sammendrag

  Nuclear level densities (NLDs) and γ-ray strength functions (GSFs) of 120,124Sn have been extracted with the Oslo method from proton-γ coincidences in the (p,p′γ) reaction. The functional forms of the GSFs and NLDs have been further constrained with the Shape method by studying primary γ-transitions to the ground and first excited states. The NLDs demonstrate good agreement with the NLDs of 116,118,122Sn isotopes measured previously. Moreover, the extracted partial NLD of 1− levels in 124Sn is shown to be in fair agreement with those deduced from spectra of relativistic Coulomb excitation in forward-angle inelastic proton scattering. The experimental NLDs have been applied to estimate the magnitude of the Porter-Thomas (PT) fluctuations. Within the PT fluctuations, we conclude that the GSFs for both isotopes can be considered to be independent of initial and final excitation energies, in accordance with the generalized Brink-Axel hypothesis. Particularly large fluctuations observed in the Shape-method GSFs present a considerable contribution to the uncertainty of the method and may be one of the reasons for deviations from the Oslo-method strength at low γ-ray energies and low values of the NLD (below ≈1×103–2×103MeV−1).

• Ingeberg, Vetle Wegner; Jones, P.; Msebi, L.; Siem, Sunniva; Wiedeking, M. & Avaa, A.A. [Vis alle 12 forfattere av denne artikkelen] (2022). Nuclear level density and γ-ray strength function of Ni 63. Physical Review C. ISSN 2469-9985. 106(5). doi: 10.1103/PhysRevC.106.054315. Fulltekst i vitenarkiv Vis sammendrag

  The nuclear level density (NLD) and γ-ray strength function (γSF) of 63Ni have been investigated using the Oslo method. The extracted NLD is compared with previous measurements using particle evaporation and those found from neutron resonance spacing. The γSF was found to feature a strong low-energy enhancement that could be explained as M1 strength based on large-scale shell model calculations. Comparison of γSFs measured with the Oslo method for various Ni isotopes reveals systematic changes to the strength below 5MeV with increasing mass.

• Persch, C.F.; DeYoung, P.A.; Lyons, S.; Spyrou, Artemis; Liddick, Sean N. & Naqvi, F. [Vis alle 27 forfattere av denne artikkelen] (2021). Beta-decay feeding intensity distributions of 71,73Ni. Physical Review C. ISSN 2469-9985. C103(055808). doi: 10.1103/PhysRevC.103.055808. Fulltekst i vitenarkiv Vis sammendrag

  This paper presents the β-decay feeding intensity distribution and Gamow-Teller transition strength distribution of 71,73Ni. These quantities were measured using the technique of total absorption spectroscopy at the National Superconducting Cyclotron Laboratory with the Summing NaI(Tl) detector. These measurements provide sensitive constraints to theoretical models used to predict β-decay properties far from stability for astrophysical applications. Specifically, for the astrophysical r process, the majority of the involved nuclei are not accessible by current facilities, and the nuclear input is mainly provided by theory. The present work reports on two neutron-rich nickel isotopes in the region where the weak r process is expected to be relevant in stellar nucleosynthesis. The experimental results are compared to two theoretical models, namely the shell model and the quasiparticle random-phase approximation, to help further refine theoretical calculations and aid in future r-process studies.

• Guttormsen, Magne Sveen; Alhassid, Yoram; Ryssens, Wouter; Ay, K.o.; Ozgur, Mustafa & Algin, Emel [Vis alle 22 forfattere av denne artikkelen] (2021). Strong enhancement of level densities in the crossover from spherical to deformed neodymium isotopes. Physics Letters B. ISSN 0370-2693. 816. doi: 10.1016/j.physletb.2021.136206. Fulltekst i vitenarkiv Vis sammendrag

  Understanding the evolution of level densities in the crossover from spherical to well-deformed nuclei has been a long-standing problem in nuclear physics. We measure nuclear level densities for a chain of neodymium isotopes 142,144−151Nd which exhibit such a crossover. These results represent the most complete data set of nuclear level densities to date for an isotopic chain between neutron shell-closure and towards mid-shell. We observe a strong increase of the level densities along the chain with an overall increase by a factor of ≈150 at an excitation energy of 6 MeV and saturation around mass 150. Level densities calculated by the shell model Monte Carlo (SMMC) are in excellent agreement with these experimental results. Based on our experimental and theoretical findings, we offer an explanation of the observed mass dependence of the level densities in terms of the intrinsic single-particle level density and the collective enhancement.

• Wiedeking, Mathis; Guttormsen, Magne Sveen; Larsen, Ann-Cecilie; Zeiser, Fabio; Görgen, Andreas & Liddick, Sean N. [Vis alle 9 forfattere av denne artikkelen] (2021). Independent normalization for gamma-ray strength functions: The shape method. Physical Review C. ISSN 2469-9985. 104(014311). doi: 10.1103/PhysRevC.104.014311. Fulltekst i vitenarkiv Vis sammendrag

  The shape method, a novel approach to obtain the functional form of the γ-ray strength function (γSF), is introduced. In connection with the Oslo method the slope of the nuclear level density (NLD) and γSF can be obtained simultaneously even in the absence of neutron resonance spacing data. The foundation of the shape method lies in the primary γ-ray transitions which preserve information on the functional form of the γSF. The shape method has been applied to 56Fe, 92Zr, and 164Dy, which are representative cases for the variety of situations encountered in typical NLD and γSF studies. The comparisons of results from the shape method to those from the Oslo method demonstrate that the functional form of the γSF is retained regardless of nuclear structure details or Jπ values of the states fed by the primary transitions.

• Malatji, Kgashane Leroy; Beckmann, Kristine Sønstevold; Wiedeking, Mathis; Siem, Sunniva; Goriely, Stephane & Larsen, Ann-Cecilie [Vis alle 21 forfattere av denne artikkelen] (2021). Statistical properties of the well deformed 153,155Sm nuclei and the scissors resonance. Physical Review C. ISSN 2469-9985. 103(1). doi: 10.1103/PhysRevC.103.014309. Fulltekst i vitenarkiv Vis sammendrag

  The nuclear level densities (NLDs) and the γ-ray strength functions (γSFs) of 153,155Sm have been extracted from (d,pγ) coincidences using the Oslo method. The experimental NLD of 153Sm is higher than the NLD of 155Sm, in accordance with microscopic calculations. The γSFs of 153,155Sm are in fair agreement with QRPA calculations based on the D1M Gogny interaction. An enhancement is observed in the γSF for both 153,155Sm nuclei around 3 MeV in excitation energy and is attributed to the M1 scissors resonance (SR). Their integrated strengths were found to be in the range 1.3–2.1 and 4.4–6.4 μ2N for 153Sm and 155Sm, respectively. The strength of the SR for 155Sm is comparable to those for deformed even-even Sm isotopes from nuclear resonance fluorescence measurements, while that of 153Sm is lower than expected.

• Görgen, Andreas; Guttormsen, Magne Sveen; Larsen, Ann-Cecilie; Siem, Sunniva; Adli, Erik & Edin, Nina Frederike J [Vis alle 14 forfattere av denne artikkelen] (2021). The Oslo Cyclotron Laboratory. The European Physical Journal Plus. 136, s. 1–13. doi: 10.1140/epjp/s13360-021-01150-3. Fulltekst i vitenarkiv Vis sammendrag

  Research at the Oslo Cyclotron Laboratory at the University of Oslo is focused on spectroscopy experiments for nuclear structure and nuclear astrophysics using the Oslo Scintillator Array OSCAR. Light-ion beams from the K=35 cyclotron are furthermore used for studies in radiation biology and medical physics, for research and development related to medical isotope production, and for irradiation of materials and electronics components. Here we present an overview of the laboratory and its research infrastructure, give a brief discussion of the respective research programs and methods, and present recent highlights.

• Zeiser, Fabio; Tveten, Gry Merete; Garrote, Frank Leonel Bello; Guttormsen, Magne Sveen; Larsen, Ann-Cecilie & Ingeberg, Vetle Wegner [Vis alle 8 forfattere av denne artikkelen] (2021). The gamma-ray energy response of the Oslo Scintillator Array OSCAR. Nuclear Instruments and Methods in Physics Research Section A : Accelerators, Spectrometers, Detectors and Associated Equipment. ISSN 0168-9002. 985. doi: 10.1016/j.nima.2020.164678. Fulltekst i vitenarkiv Vis sammendrag

  The new Oslo Scintillator Array (OSCAR) has been commissioned at the Oslo Cyclotron Laboratory (OCL). It consists of 30 large volume (⌀ 3.5 × 8 inches) LaBr3(Ce) detectors that are used for 𝛾-ray spectroscopy. The response functions for incident 𝛾 rays up to 20 MeV are simulated with Geant4. In addition, the resolution, and the total and full-energy peak efficiencies are extracted. The results are in very good agreement with measurements from calibration sources and experimentally obtained mono-energetic in-beam 𝛾-ray spectra.

• Markova, Maria; Neumann-Cosel, Neumann-Cosel von; Larsen, A.C.; Bassauer, S.; Görgen, A. & Guttormsen, Magne Sveen [Vis alle 25 forfattere av denne artikkelen] (2021). Comprehensive Test of the Brink-Axel Hypothesis in the Energy Region of the Pygmy Dipole Resonance. Physical Review Letters. ISSN 0031-9007. 127(18). doi: 10.1103/PhysRevLett.127.182501. Fulltekst i vitenarkiv Vis sammendrag

  The validity of the Brink-Axel hypothesis, which is especially important for numerous astrophysical calculations, is addressed for 116,120,124Sn below the neutron separation energy by means of three independent experimental methods. The γ-ray strength functions (GSFs) extracted from primary γ-decay spectra following charged-particle reactions with the Oslo method and with the shape method demonstrate excellent agreement with those deduced from forward-angle inelastic proton scattering at relativistic beam energies. In addition, the GSFs are shown to be independent of excitation energies and spins of the initial and final states. The results provide a critical test of the generalized Brink-Axel hypothesis in heavy nuclei, demonstrating its applicability in the energy region of the pygmy dipole resonance.

• Gjestvang, Dorthea; Siem, Sunniva; Zeiser, Fabio; Randrup, Jørgen; Vogt, Ramona & Garrote, Frank Leonel Bello [Vis alle 20 forfattere av denne artikkelen] (2021). The study of prompt fission γ rays at the Oslo Cyclotron Laboratory. The European Physical Journal Conferences. ISSN 2101-6275. doi: 10.1051/epjconf/202125600005. Fulltekst i vitenarkiv Vis sammendrag

  The study of prompt fission γ rays (PFGs) is crucial for understanding the energy and angular momentum distribution in fission, and over the last decade there has been an revived interest in this aspect of fission. We present the new experimental setup at the Oslo Cyclotron Laboratory for detecting PFGs resulting from charged particle-induced fission. Additionally, PFGs from the reaction 240 Pu(d,pf) were measured in April 2018, and the fission gated proton- γ coincidence spectrum is shown. In order to explore the dependence of the PFG emission on the excitation energy and angular momentum of the compound nucleus, we plan several experiments where charged particle reactions are used to induce fission in various plutonium isotopes. The final results will be compared to predictions made by the Fission Reaction Event Yield Algorithm (FREYA) in an upcoming publication, to benchmark the current modelling of both the PFGs and the fission process.

• Gjestvang, Dorthea; Siem, Sunniva; Zeiser, Fabio; Randrup, Jørgen; Vogt, Ramona & Wilson, Jonathan Nicholas [Vis alle 19 forfattere av denne artikkelen] (2021). Excitation energy dependence of prompt fission γ-ray emission from 241Pu*. Physical Review C. ISSN 2469-9985. 103(3). doi: 10.1103/PhysRevC.103.034609. Fulltekst i vitenarkiv Vis sammendrag

  Prompt fission γ rays (PFGs) resulting from the 240Pu(d,pf) reaction have been measured as a function of fissioning nucleus excitation energy Ex at the Oslo Cyclotron Laboratory. We study the average total PFG multiplicity per fission, the average total PFG energy released per fission, and the average PFG energy. No significant changes in these characteristics are observed over the range 5.75

• Wilson, J. N.; Thisse, D; Lebois, Matthieu; Jovančević, Nikola; Gjestvang, Dorthea & Canavan, R [Vis alle 76 forfattere av denne artikkelen] (2021). Angular momentum generation in nuclear fission. Nature. ISSN 0028-0836. 590, s. 566–570. doi: 10.1038/s41586-021-03304-w. Fulltekst i vitenarkiv
• Kibédi, T.; Alshahrani, B.; Stuchbery, A.E.; Larsen, Ann-Cecilie; Görgen, Andreas & Siem, Sunniva [Vis alle 20 forfattere av denne artikkelen] (2020). Radiative Width of the Hoyle State from γ -Ray Spectroscopy. Physical Review Letters. ISSN 0031-9007. 125(18). doi: 10.1103/PhysRevLett.125.182701. Fulltekst i vitenarkiv Vis sammendrag

  The cascading 3.21 and 4.44 MeV electric quadrupole transitions have been observed from the Hoyle state at 7.65 MeV excitation energy in 12C, excited by the 12C(p,p′) reaction at 10.7 MeV proton energy. From the proton-γ−γ triple coincidence data, a value of Γrad/Γ=6.2(6)×10−4 was obtained for the radiative branching ratio. Using our results, together with ΓE0π/Γ from Eriksen et al. [Phys. Rev. C 102, 024320 (2020)] and the currently adopted Γπ(E0) values, the radiative width of the Hoyle state is determined as Γrad=5.1(6)×10−3  eV. This value is about 34% higher than the currently adopted value and will impact models of stellar evolution and nucleosynthesis.

• Sánchez, R. Pérez; Jurado, Beatriz; Meot, V; Roig, O; Dupuis, M & Denis-Petit, D [Vis alle 21 forfattere av denne artikkelen] (2020). Simultaneous Determination of Neutron-Induced Fission and Radiative Capture Cross Sections from Decay Probabilities Obtained with a Surrogate Reaction. Physical Review Letters. ISSN 0031-9007. 125(12). doi: 10.1103/PhysRevLett.125.122502. Fulltekst i vitenarkiv
• Kawano, T.; Cho, Y.S.; Dimitriou, Paraskevi; Filipescu, D. M.; Iwamoto, N. & Plujko, V. [Vis alle 28 forfattere av denne artikkelen] (2020). IAEA Photonuclear Data Library 2019. Nuclear Data Sheets. ISSN 0090-3752. 163, s. 109–162. doi: 10.1016/j.nds.2019.12.002. Fulltekst i vitenarkiv
• Ingeberg, Vetle Wegner; Siem, Sunniva; Wiedeking, Mathis; Sieja, Kamila; Bleuel, D.l. & Brits, C.P. [Vis alle 29 forfattere av denne artikkelen] (2020). First application of the Oslo method in inverse kinematics. European Physical Journal A. ISSN 1434-6001. 56. doi: 10.1140/epja/s10050-020-00070-7. Fulltekst i vitenarkiv Vis sammendrag

  The γ-ray strength function (SF) and nuclear level density (NLD) have been extracted for the first time from inverse kinematic reactions with the Oslo method. This novel technique allows measurements of these properties across a wide range of previously inaccessible nuclei. Proton–γ coincidence events from the d(⁸⁷Kr,p)⁸⁶Kr reaction were measured at iThemba LABS and the γSF and NLD in ⁸⁶Kr was obtained. The low-energy region of the γSF is compared to shell-model calculations, which suggest this region to be dominated by M1 strength. The γSF and NLD are used as input parameters to Hauser–Feshbach calculations to constrain (n,γ) cross sections of nuclei using the TALYS reaction code. These results are compared to ⁸⁶Kr(n,γ) data from direct measurements.

• Kullmann, Ina Kristine Berentsen; Larsen, Ann-Cecilie; Renstrøm, Therese; Beckmann, Kristine Sønstevold; Garrote, Frank Leonel Bello & Campo, Lucia Crespo [Vis alle 13 forfattere av denne artikkelen] (2019). First experimental constraint on the 191Os(n,γ) reaction rate relevant to s-process nucleosynthesis. Physical Review C. ISSN 2469-9985. 99(6). doi: 10.1103/PhysRevC.99.065806. Fulltekst i vitenarkiv Vis sammendrag

  The nuclear level density and γ -decay strength of 192 Os have been extracted using particle- γ coincidence data from the 192 Os ( α , α ′ γ ) 192 Os reaction by means of the Oslo method. The level density is found to be a rather smooth function of excitation energy, approximately following the constant temperature model. The γ -decay strength is compared to photoneutron cross-section data above the neutron separation energy, and to E 1 and M 1 strengths for nuclei in this mass region derived from primary transitions following neutron capture. Our results are in good agreement with these previous data and draw a consistent picture of the γ -strength function in the range E γ ≈ 1.5 – 6 MeV . Using the measured nuclear level density and γ -decay strength as input to the nuclear-reaction code talys, we provide the first experimentally constrained Maxwellian-averaged cross section (MACS) for the 191 Os ( n , γ ) 192 Os reaction relevant to s -process nucleosynthesis. The systematic uncertainties introduced by the normalization procedure of the level density and γ -strength function were investigated and propagated to the calculated Maxwellian-averaged cross section. The obtained result of the Maxwellian-averaged cross section at k B T = 30 keV , ⟨ σ ⟩ n , γ = 1134 ± 375 mb , is in very good agreement with the theoretical estimate provided by the KADoNiS project, giving experimental support to the adopted KADoNiS value. Good agreement is also found with MACS values obtained from other libraries, such as TENDL-2017, ENDF/B-VII.0, and JEFF.

• Zeiser, Fabio; Tveten, Gry Merete; Potel, Gregory; Larsen, Ann-Cecilie; Guttormsen, Magne Sveen & Laplace, Thibault A. [Vis alle 22 forfattere av denne artikkelen] (2019). Restricted spin-range correction in the Oslo method: The example of nuclear level density and γ-ray strength function from 239Pu(d, pγ) 240Pu. Physical Review C. ISSN 2469-9985. 100:024305(2), s. 1–9. doi: 10.1103/PhysRevC.100.024305. Fulltekst i vitenarkiv
• Lyons, Stephanie; Spyrou, Artemis; Liddick, Sean N.; Naqvi, Farheen; Crider, Benjamin P. & Dombos, Alexander C. [Vis alle 25 forfattere av denne artikkelen] (2019). 69,71Co β-decay strength distributions from total absorption spectroscopy. Physical Review C. ISSN 2469-9985. 100(2), s. 1–10. doi: 10.1103/PhysRevC.100.025806. Fulltekst i vitenarkiv Vis sammendrag

  Background: The rapid neutron capture process is one of the main nucleosynthesis processes of elements heavier than Fe. Uncertainties in nuclear properties, such as masses, half-lives, and β -delayed neutron probabilities can cause orders of magnitude of variation within astrophysical r-process simulations. Presently, theoretical models are used to make global predictions of various nuclear properties for the thousands of nuclei required for these simulations, and measurements are required to benchmark these models, especially far from stability. Purpose: β -decay strength distributions can be used to not only inform astrophysical r-process simulations, but also to provide a stringent test for theoretical calculations. The aim of this work is to provide accurate strength distributions for 69 , 71 Co β decay. Method: The technique of total absorption spectroscopy was used to measure the β decay of 69 , 71 Co for the first time at the National Superconducting Cyclotron Laboratory. The ions were implanted in a double-sided silicon strip detector at the center of the Summing NaI(Tl) detector and identified using standard particle identification methods. The response of the detection system to the β -decay electron and subsequent γ -ray radiation was fit to the observed experimental data using a χ 2 -minimization technique. Results: β -feeding intensities and Gamow-Teller strength distributions were extracted from the fits of the experimental data. The β -decay intensities show that there is a large percentage of feeding to levels above 2 MeV, which have not been observed in previous studies. The resultant β -feeding intensities and Gamow-Teller strength distributions were compared to shell model and quasiparticle random phase approximation (QRPA) calculations. Conclusions: Comparing experimentally determined β -decay strength distributions provides a test of models, which are commonly used for global β -decay properties for astrophysical calculations. This work highlights the importance of performing detailed comparisons of models to experimental data, particularly far from stability and as close to the r-process path as possible.

• Brits, C.P.; Malatji, Kgashane L.; Wiedeking, M.; Kheswa, BV; Goriely, S. & Garrote, Frank Leonel Bello [Vis alle 27 forfattere av denne artikkelen] (2019). Nuclear level densities and γ-ray strength functions of 180,181,182Ta. Physical Review C. ISSN 2469-9985. 99(5). doi: 10.1103/PhysRevC.99.054330. Fulltekst i vitenarkiv Vis sammendrag

  Particle- γ coincidence experiments were performed at the Oslo Cyclotron Laboratory with the 181 Ta ( d , X ) and 181 Ta ( 3 He , X ) reactions to measure the nuclear level densities (NLDs) and γ -ray strength functions ( γ SFs ) of 180 , 181 , 182 Ta using the Oslo method. The back-shifted Fermi-gas, constant temperature plus Fermi gas, and Hartree-Fock-Bogoliubov plus combinatorial models were used for the absolute normalizations of the experimental NLDs at the neutron separation energies. The NLDs and γ SFs are used to calculate the corresponding 181 Ta ( n , γ ) cross sections and these are compared to results from other techniques. The energy region of the scissors resonance strength is investigated and from the data and comparison to prior work it is concluded that the scissors strength splits into two distinct parts. This splitting may allow for the determination of triaxiality and a γ deformation of 14 . 9 ∘ ± 1 . 8 ∘ was determined for 181 Ta .

• Goriely, Stéphane; Dimitriou, Paraskevi; Wiedeking, Mathis; Belgya, Támas; Firestone, Ryan & Kopecky, Jiri [Vis alle 19 forfattere av denne artikkelen] (2019). Reference database for photon strength functions. European Physical Journal A. ISSN 1434-6001. 55:172(10), s. 1–52. doi: 10.1140/epja/i2019-12840-1. Fulltekst i vitenarkiv
• Grigalavicius, Mantas; Mastrangelopoulou, Maria; Berg, Kristian; Arous, Delmon; Ménard, Mathilde & Henriksen-Raabe, Tine Therese [Vis alle 12 forfattere av denne artikkelen] (2019). Proton-dynamic therapy following photosensitiser activation by accelerated protons demonstrated through fluorescence and singlet oxygen production. Nature Communications. 10(1). doi: 10.1038/s41467-019-12042-7. Fulltekst i vitenarkiv Vis sammendrag

  We demonstrate excitation of photosensitisers (PSs) by accelerated protons to produce fluorescence and singlet oxygen. Their fluorescence follows a pattern similar to the proton energy loss in matter, while proton-derived fluorescence spectra match the photon-induced spectra. PSs excited in dry gelatin exhibit enhanced phosphorescence, suggesting an efficient PSs triplet state population. Singlet oxygen measurements, both optically at ~1270 nm and through the photoproduct of protoporphyrin IX (PpIX), demonstrate cytotoxic singlet oxygen generation by proton excitation. The singlet oxygen-specific scavenger 1,4-diazabicyclo[2.2.2]octane (DABCO) abrogates the photoproduct formation under proton excitation, but cannot countermand the overall loss of PpIX fluorescence. Furthermore, in two cell lines, M059K and T98G, we observe differential cell death upon the addition of the PS cercosporin, while in U87 cells we see no effect at any proton irradiation dose. Our results pave the way for a novel treatment combining proton therapy and “proton-dynamic therapy” for more efficient tumour eradication.

• Liddick, Sean N.; Larsen, Ann-Cecilie; Guttormsen, Magne Sveen; Spyrou, Artemis; Crider, Benjamin P. & Naqvi, Farheen [Vis alle 30 forfattere av denne artikkelen] (2019). Benchmarking the extraction of statistical neutron capture cross sections on short-lived nuclei for applications using the β-Oslo method. Physical Review C. ISSN 2469-9985. 100:024624(2), s. 1–12. doi: 10.1103/PhysRevC.100.024624. Fulltekst i vitenarkiv Vis sammendrag

  Numerous scientific fields including astrophysics, nuclear power, and nuclear forensics require a knowledge of basic nuclear properties for large numbers of short-lived, radioactive isotopes far removed from stable nuclei. Neutron-capture cross sections are one such piece of nuclear data where direct measurements are not possible and theoretical predictions can vary by orders of magnitude. A recently developed indirect technique for inferring neutron capture rates, the β -Oslo method, has been introduced but not compared against a known, directly measured neutron capture cross section. To provide this benchmark, two indirect methods based on β decay and charged-particle reactions were used to extract the nuclear level density and γ -ray strength function of 51 Ti . The nuclear level density and γ -ray strength function from the two data sets were found to be equivalent and were used to extract the neutron capture cross section of 50 Ti which agrees with previous direct measurements at high neutron energies. The results demonstrate the validity of the β -Oslo method for extracting neutron capture cross sections of short-lived nuclei and provide a sufficiently small uncertainty to be used in various applications.

• Guttormsen, Magne Sveen; Goriely, Stephane; Larsen, Ann-Cecilie; Görgen, Andreas; Hagen, Trine Wiborg & Renstrøm, Therese [Vis alle 13 forfattere av denne artikkelen] (2019). Indirect(n,γ)91,92Zr Cross SectionMeasurements for the s-Process. I Formicola, Alba; Junker, Matthias; Gialanella, Lucio & Imbriani, Gianluca (Red.), Nuclei in the Cosmos XV. Springer. ISSN 9783030138752. s. 359–362. doi: 10.1007/978-3-030-13876-9_65. Fulltekst i vitenarkiv

Se alle arbeider i NVA

• Siem, Sunniva (2026). Presentation of The Norwegian Nuclear Research Centre (NNRC) and new BSc and MSc program in Nuclear Science. Fulltekst i vitenarkiv
• Siem, Sunniva (2026). Collaboration meeting on Nuclear Technology Education in Norway (Welcome address). Fulltekst i vitenarkiv
• Siem, Sunniva (2026). Norwegian Nuclear Research Centre. doi: https:/indico.global/event/16641/overview. Fulltekst i vitenarkiv
• Siem, Sunniva & Pellegri, Luna (2026). Almost 15 years of fruitful Nuclear Physics collaboration between South Africa and Norway. Fulltekst i vitenarkiv
• Siem, Sunniva (2026). Norwegian Nuclear Research Centre (NNRC) and Norwegian Nuclear Data Needs. Fulltekst i vitenarkiv
• Siem, Sunniva (2026). Collaboration on Research and Education NNRC - IJC-Lab. Fulltekst i vitenarkiv
• Siem, Sunniva (2026). Future fission experiments at the Oslo Cyclotron laboratory. doi: https:/workshops.ill.fr/event/549/overview. Fulltekst i vitenarkiv
• Siem, Sunniva (2026). Norwegian Nuclear Research Centre (NNRC) and New BSc and MSc programs in Nuclear Technology. Fulltekst i vitenarkiv
• Siem, Sunniva (2026). Norwegian Nuclear Research Centre - Nukleærsenteret. Fulltekst i vitenarkiv
• Siem, Sunniva (2026). Kompetanse og utdanningstilbud ved UiO og Nukleærsenteret. doi: https:/www.uio.no/forskning/scene-domus-bibliotheca/arrangementer/2026/kjernekraft-i-norge. Fulltekst i vitenarkiv Vis sammendrag

  Interessen for kjernekraft øker. Er Norge forberedt? Kom og hør hva fagmiljøene sier om kunnskapsstatus og kompetansebehov ved et eventuelt kjernekraftscenario i Norge.

• Siem, Sunniva & Stavik-Døvle, Grete (2026). Studieprogrammene i nukleære fag: Studentenes kompetanse når de begynner på masteroppgaven. Fulltekst i vitenarkiv
• Meir, Michaela Georgine & Siem, Sunniva (2026). NNRC at Norway Life Science 2026. doi: https:/www.linkedin.com/feed/update/urn:li:activity:7429176107171590144. Fulltekst i vitenarkiv Vis sammendrag

  Last week at the Norway Life Science Conference in Oslo, leaders from across the Norwegian health sector and international research communities gathered to discuss how Norway can strengthen knowledge-based innovation and build a competitive life sciences industry.

• Siem, Sunniva & Meir, Michaela Georgine (2026). Norwegian Nuclear Research Centre tildeler Yngve Vogt Forskningsformidlingsprisen 2026. doi: https:/www.linkedin.com/feed/update/urn:li:activity:7511405177401307138. Vis sammendrag

  Under «Yngve Vogts uhøytidelige AFP-avskjedsarrangement» ved Universitetet i Oslo 29. september hadde vi gleden av å tildele Yngve Forskningsformidlingsprisen 2026.

• Meir, Michaela Georgine & Siem, Sunniva (2026). UiO and iThemba LABS: Formalizing a Decade of Scientific Collaboration. doi: https:/www.linkedin.com/feed/update/urn:li:activity:7467550847309348864. Fulltekst i vitenarkiv Vis sammendrag

  The University of Oslo officially renewed the research partnership with iThemba LABS in South Africa. This milestone is the second renewal of a collaboration that first began in 2017. As Deputy Director at iThemba LABS Dr. Rudolph Nchodu, pointed out, this is now one of the largest collaborations we have in terms of the number of people involved. The formal signing by Vice Dean Øystein Prytz and Deputy Director Rudolph Nchodu provides a solid framework for the next five years, giving our joint research and student exchange programs the stability needed.

• Siem, Sunniva & Paulsen, Wanja (2026). Nukleær kompetanse i Norge: Masterveiledning i virksomheter – en investering i framtiden. doi: https:/www.nnrc.uio.no/resultater/rapporter/pdfs/brosjyre-masterstudent-2026.pdf. Fulltekst i vitenarkiv Vis sammendrag

  Veilederen er laget i samarbeid med UiO, NMBU og DSA. Virksomheter innen nukleære fagområder i Norge kan bidra med masteroppgaver og som eksterne veiledere. Heftet gir en grundig informasjon om dette.

• Siem, Sunniva (2026). Innspill fra Norsk Nukleært Forskningssenter. doi: https:/www.regjeringen.no/no/aktuelt/innspillsmoter-om-kjernekraft/id3171050/. Fulltekst i vitenarkiv Vis sammendrag

  Energidepartementet inviterer til to tematiske innspillsmøter knyttet til NOU 2026:4 Kjernekraft i Norge. Begge møtene gjennomføres på Ingeniørenes Hus i Oslo (Kronprinsens gate 17). Det første innspillsmøtet omhandler kraftpolitiske sider ved kjernekraft (kraftmarked, kraftsystem, teknologi, kostnader og produksjon). Foredrag omhandler høringsinnspill fra Norsk Nukleært Forskningssenter, UiO.

• Siem, Sunniva (2026). Beredskap starter før krisen inntreffer. Hvem tar vare på fagmiljøene vi ikke har råd til å miste? doi: https:/www.nnrc.uio.no/resultater/film-podcast/2026-08-12_arendalsuke.html. Fulltekst i vitenarkiv Vis sammendrag

  Fra UiOs arrangement under Arendalsuka 12. august 2026, som handler om hvordan Norge kan bevare små, spesialiserte fagmiljøer som er kritiske for samfunnets beredskap, blant annet innen nukleære fag og pandemiberedskap. Professor og senterleder Sunniva Siem snakker om hvorfor Norge trenger å opprettholde langsiktig kompetanse og sterke fagmiljøer innen nukleære fag – også før en krise oppstår. Hun deltar også i diskusjonen om hvordan kritiske, spesialiserte forskningsmiljøer kan sikres og videreutvikles over tid.

• Torsvoll, Marius; Wirgenes, Maia Oline Grund; Al-Adili, Ali; Haug, Henrik Sørlie; Heines, Johannes Sørby & Siem, Sunniva [Vis alle 13 forfattere av denne artikkelen] (2026). Very asymmetric fission yields of 233U(nth, f) at LOHENGRIN. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Torsvoll, Marius; Wirgenes, Maia Oline Grund; Al-Adili, Ali; Solders, Andreas; Köster, Ulli & Daugas, Jean-Micheal [Vis alle 13 forfattere av denne artikkelen] (2026). Very asymmetric fission yields in U(n,f) at LOHENGRIN. Fulltekst i vitenarkiv Vis sammendrag

  The super assymmetric fission mode and yield enhancement around A = 70 has been observed in actinides as light as U. Modern fission codes modeling the fission process do not predict an enhancement like that found in U233 and heavier actinides. The super asymmetric region has not been measured for U233 and our goal is to explore the yields in the mass region from A = 90 to A = 67. In summary our goal is to extract the unmeasured fission yields in the super asymmetric region of U233(nth, f) with a special emphasis on investigating the possibility of a yield enhancement at atomic mass number A = 70

• Ingeberg, Vetle Wegner; Görgen, Andreas; Inoue, Azusa; Kumar, Neeraj; Larsen, Ann-Cecilie & Li, Kevin Ching Wei [Vis alle 9 forfattere av denne artikkelen] (2026). OSCAR at ISOLDE. Fulltekst i vitenarkiv Vis sammendrag

  For the next run period at ISOLDE, the Nuclear Physics group at the University of Oslo are considering proposing a longer experimental campaign with the Oslo SCintillator ARay (OSCAR). The main aims of such a campaign will be to measure the nuclear level density and -ray strength function of key nuclei relevant for nuclear structure and nuclear astrophysics (e.g. i-process nucleosynthesis). Other cases of interest could be spectroscopy of resonances in light exotic nuclei or rare event searches. In addition, we would also invite other interested groups to propose experiments with the setup while it is present at HIE-ISOLDE. The OSCAR array is the largest LaBr:Ce detector array in the world (at least in terms of detector volume). It offers exceptional efficiency for high-energy -rays without compromising on either energy or time resolution. It consists of 30 large 3.5x8-inch large volume LaBr3:Ce detectors, an icosahedron frame with six rings covering polar angles from to . Detectors can be mounted in a distance of 16.3 to 22 cm from the target, resulting in solid angle coverage of 57%. This translates to about 15% full energy efficiency at 1332 keV and close to 5% at 10 MeV in the closest configuration. In this presentation I will present the physics cases and our preliminary plans for such a campaign.

• Paulsen, Wanja; Li, Kevin Ching Wei; Siem, Sunniva; Ingeberg, Vetle Wegner; Larsen, Ann-Cecilie & Eriksen, Tomas Kvalheim [Vis alle 15 forfattere av denne artikkelen] (2026). CLUSTER'26. Fulltekst i vitenarkiv
• Siem, Sunniva (2025). Future Fission Experiments at the Oslo Cyclotron Laboratory and a short presentation of NNRC. Fulltekst i vitenarkiv
• Siem, Sunniva (2025). Presentation of The Norwegian Nuclear Research Centre (NNRC). doi: https:/indico.cern.ch/event/1604457/. Fulltekst i vitenarkiv
• Siem, Sunniva (2025). Building Competence and Infrastructure on Radionuclides. doi: https:/www.uio.no/english/research/interfaculty-research-areas/growth-house/news-and-events/events/2025/science-impact/. Fulltekst i vitenarkiv
• Siem, Sunniva (2025). Training and education opportunities in the nuclear sector: NNRC and nuclear science BSc and MSc programmes at UiO. Fulltekst i vitenarkiv
• Siem, Sunniva (2025). Almost 15 years of fruitful nuclear physics collaboration between South Africa and Norway. doi: https:/indico.tlabs.ac.za/event/138/overview. Fulltekst i vitenarkiv
• Siem, Sunniva (2025). Nuclear Level Densities and Photon Strength Functions measurements. doi: https:/indico.tlabs.ac.za/event/138/overview. Fulltekst i vitenarkiv
• Torsvoll, Marius; Al-Adili, Ali; Haug, Henrik Sørlie; Heines, Johannes Sørby; Wirgenes, Maia Oline Grund & Siem, Sunniva [Vis alle 10 forfattere av denne artikkelen] (2025). Very asymmetric fission yields of 233U(nth, f) at LOHENGRIN. Fulltekst i vitenarkiv Vis sammendrag

  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.

• Valsdottir, Vala Maria; Hjelmeland, Martin N. & Siem, Sunniva (2025). Ekspertrapport: Etablering av kjernekraft i Norge | Crayon Consulting. doi: https:/crayonconsulting.no/vev/kjernekraftinorge. Fulltekst i vitenarkiv Vis sammendrag

  Hva er mulighetene og utfordringene med en mulig etablering av kjernekraft i Norge? En Phd rapport av doktorgradsstipendiat Vala Maria Valsdottir

• Torvund, Gulla Louise Serville; Martinsen, Elise Malmer; Siem, Sunniva; Görgen, Andreas; Beere, William H. Åstorp & Oughton, Deborah H. (2024). Harde fakta om kjernekraft. Fulltekst i vitenarkiv Vis sammendrag

  Er kjernekraft så fantastisk som forkjemperne mener eller så problematisk som motkjemperne påstår? Kom og hør kjernefysikere snakke om teknologien, stråling og atomavfall. Takket være klimakrisen og det stadig større behovet for strøm, er kjernekraft blitt en del av diskusjonen om nye energiformer i Norge. Har du lyst til å lære mer om kjernekraft? Kom og hør kjernefysikere forklare hvordan teknologien fungerer. Du kan stille spørsmål om det du måtte lure på.

• Siem, Sunniva (2024). Nuclear Level Density and Strength Function Measurements (and presentation of the Norwgian Nuclear Research Centre). Fulltekst i vitenarkiv
• Siem, Sunniva (2024). Kjernekraft i Norge? Fulltekst i vitenarkiv
• Siem, Sunniva (2024). Norges satsing på forskning og udanning inen nukleær teknologi. Fulltekst i vitenarkiv
• Siem, Sunniva (2024). Nuclear Level Densities and Photon Strength Functions: constraining astrophysical reactions. Fulltekst i vitenarkiv
• Paulsen, Wanja; Li, Kevin Ching Wei; Siem, Sunniva; Ingeberg, Vetle Wegner; Larsen, Ann-Cecilie & Eriksen, Tomas Kvalheim [Vis alle 20 forfattere av denne artikkelen] (2023). Remeasuring the gamma-decay branching ratio of the Hoyle state. Fulltekst i vitenarkiv
• Grigalavicius, Mantas; Mastrangelopoulou, Maria; Berg, K.; Arous, Delmon; Ménard, M. & Raabe-Henriksen, T. [Vis alle 12 forfattere av denne artikkelen] (2021). Publisher Correction: Proton-dynamic therapy following photosensitiser activation by accelerated protons demonstrated through fluorescence and singlet oxygen production (Nature Communications, (2019), 10, 1, (3986), 10.1038/s41467-019-12042-7). Nature Communications. 12(1). doi: 10.1038/s41467-021-27175-x. Fulltekst i vitenarkiv
• Paulsen, Wanja; Görgen, Andreas; Siem, Sunniva & Li, Kevin Ching Wei (2021). Studies of shape coexistence in 140Sm. Fulltekst i vitenarkiv Vis sammendrag

  The shape of atomic nuclei can vary vastly, several shapes can even coexist within the same nucleus in certain regions of the nuclear chart. Shape coexistence is believed to be present at higher excitation energies in the N=78 isotones 140Sm and 142Gd [1], where the nucleus of interest is 140Sm in this case. 140Sm has previously been studied in a coulomb excitation experiment performed by M. Klintefjord et. al. [2] at REX-ISOLDE where results showed considerable gamma-softness and the resulting level scheme follows the theoretical prediction of a triaxial nucleus [3], but further investigation is needed to conclude. A new coulomb experiment on 140Sm has been performed using HIE-ISOLDE, yielding data of incredible quality which has also let us vastly improve the level scheme of this nucleus. Preliminary results from this dataset will be presented as well as a deeper look into gamma-softness as well as the possibility of 140Sm having shape coexistence. [1] W. Starzecki et al., Phys. Lett. B 200, 419 (1988). [2] M. Klintefjord et al. Phys. Rev. C 93, 054303 (2016) [3] F. Iachello, Phys. Rev. Lett. 85, 3580 (2000)

• Paulsen, Wanja; Siem, Sunniva & Görgen, Andreas (2020). Assessment of the radiative width of the Hoyle state. Fulltekst i vitenarkiv
• Edin, Nina Frederike J; Rykkelid, Anne Marit; Görgen, Andreas; Baker, Agnes; Siem, Sunniva & Ytre-Hauge, Kristian [Vis alle 7 forfattere av denne artikkelen] (2019). Very high yield of double strand breaks found at the distal end of the proton Bragg peak. Radiotherapy and Oncology. ISSN 0167-8140. 133, s. 594–594. Fulltekst i vitenarkiv
• Rykkelid, Anne Marit; Tormodsrud, Vera; Siem, Sunniva; Görgen, Andreas; Ytre-Hauge, Kristian & Malinen, Eirik [Vis alle 7 forfattere av denne artikkelen] (2019). Very high RBE values found at the distal end of the proton Bragg peak. Fulltekst i vitenarkiv
• Ingeberg, Vetle Wegner; Siem, Sunniva; Wiedeking, M.; Abrahams, Kenzo John; Arnswald, K. & Garrote, Frank Leonel Bello [Vis alle 36 forfattere av denne artikkelen] (2019). Inverse-Oslo Method – a tool for expanding our understanding of the r-process. Fulltekst i vitenarkiv
• Paulsen, Wanja; Siem, Sunniva; Li, Kevin Ching Wei & Eriksen, Tomas Kvalheim (2025). A saga on the γ-decay branching ratio of the Hoyle state at the Oslo Cyclotron Laboratory. Universitetet i Oslo. Fulltekst i vitenarkiv Vis sammendrag

  The triple-α process is a fundamental cornerstone of stellar nucleosynthesis, facilitating the production of 12C, a vital building block for life. Through the triple-α process, three helium nuclei can fuse to ultimately produce a stable 12C nuclide. At medium stellar temperatures between 0.1–2.0 GK, the Hoyle state in 12C mediates the triple-α reaction, making a comprehensive understanding of its properties essential for accurately modeling the stellar production of 12C, as well as the subsequent stellar nucleosynthesis. Numerous measurements of the decay modes of the Hoyle state have been conducted since its discovery, yielding a relatively well-constrained value of for it’s radiative branching ratio, describing the probability of producing 12C through the triple-α process. However, a recent measurement published by Kibédi et al. [Phys. Rev. Lett. 125, 182701 (2020)] reported a highly discrepant value of Γrad/Γ = 6.2(6) × 10−4 for the radiative branching ratio from the Hoyle state. Given the astrophysical significance of the Hoyle state, resolving this conflict is crucial. This study seeks to address this discrepancy through a new measurement of the γ-decay branching ratio of the Hoyle state, together with a comprehensive reanalysis of the data published by Kibédi et al. [Phys. Rev. Lett. 125, 182701 (2020)].

Se alle arbeider i NVA