Academic interests
Courses taught
- Writing Science as course coordinator.
- HON1000 as teaching assistant.
- FYS1120 as teaching assistant (Fall 2020).
- FYS4505 as teaching assistant and laboratory assistant (Fall 2019/Fall 2021).
Background
Publications
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Markova, Maria; Larsen, Ann-Cecilie; Neumann-Cosel, P. von; Litvinova, E.; Goriely, S. & Bell, Lauren Tracy
[Show all 17 contributors for this article]
(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.
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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,𝑝𝛾)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.
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Li, Kevin Ching Wei; Neveling, R.; Adsley, P.; Fujita, H.; Papka, P. & Smit, F. D.
[Show all 15 contributors for this article]
(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.
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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.
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Pogliano, Francesco; Larsen, Ann-Cecilie; Markova, Maria; Garrote, Frank Leonel Bello; Bjørøen, Marianne Møller & Eriksen, Tomas Kvalheim
[Show all 17 contributors for this article]
(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.
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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.
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Guttormsen, Magne Sveen; Ay, Kursad Osman; Ozgur, Mustafa; Algin, Emel; Larsen, Ann-Cecilie & Garrote, Frank Leonel Bello
[Show all 26 contributors for this article]
(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.
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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.
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Markova, Maria; Larsen, Ann-Cecilie; Neumann-Cosel, Peter von; Bassauer, Sergej; Görgen, Andreas & Guttormsen, Magne Sveen
[Show all 23 contributors for this article]
(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.
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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).
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Häfner, G.; Lozeva, R; Naïdja, H.; Lebois, M.; Jovančević, N. & Thisse, D.
[Show all 72 contributors for this article]
(2021).
First lifetime investigations of iodine isotopes: The quest for collectivity.
Physical Review C.
ISSN 2469-9985.
104(1).
doi:
10.1103/PhysRevC.104.014316.
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We report on spectroscopic information and lifetime measurements in the neutron-rich 135,137,139I isotopes. This is the first lifetime data on iodine isotopes beyond N=82. Excited states were populated in fast neutron-induced fission of 238U at the ALTO facility of IJCLab with the LICORNE neutron source and detected using the hybrid ν-ball spectrometer. The level schemes of the 135,137,139I isotopes are revised in terms of excited states with up to maximum spin-parity of (33/2+), populated for the first time in fast neutron-induced fission. We provide first results on the lifetimes of the (9/2+1) and (13/2+1) states in 137I and 139I, and the (17/2+1) state in 137I. In addition, we give upper lifetime limits for the (11/2+1) states in 135−139I, the (15/2+1) state in 137I, the (17/2+1) state in 139I, and reexamine the (29/2+1) state in 137I. The isomeric data in 135I are reinvestigated, such as the previously known (15/2+1) and (23/2−1) isomers with T1/2 of 1.64(14) and 4.6(7) ns, respectively, as obtained in this work. The new spectroscopic information is compared to that from spontaneous or thermal-neutron induced fission and discussed in the context of large scale shell-model (LSSM) calculations for the region beyond 132Sn, indicating the behavior of collectivity for the three valence-proton iodine chain with N=82,84,86.
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Markova, Maria; Neumann-Cosel, Neumann-Cosel von; Larsen, A.C.; Bassauer, S.; Görgen, A. & Guttormsen, Magne Sveen
[Show all 25 contributors for this article]
(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.
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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.
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Häfner, G.; Naïdja, H.; Lebois, M.; Jovancevic, N.; Thisse, D. & Etasse, D
[Show all 72 contributors for this article]
(2021).
Spectroscopy and lifetime measurements in 134,136,138Te isotopes and implications for the nuclear structure beyond N = 82.
Physical Review C.
ISSN 2469-9985.
doi:
10.1103/PhysRevC.103.034317.
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We report on spectroscopic information and lifetime measurements of even-even neutron-rich Te isotopes. Excited states were populated in fast-neutron induced fission of 238U at the ALTO facility of IJCLab with the LICORNE neutron source and detected using the hybrid ν-ball spectrometer. We provide first results on lifetimes of the 6+1 state in 136Te and the (6+1), (4+1), and (2+1) states in 138Te and discuss the results in the context of large-scale shell-model calculations. The level schemes of 136Te and 138Te are revised in terms of lifetimes of their 2+1,4+1 states and updated information on the (4+2) state in 136Te is presented. In addition, previously reported data on spectroscopy and lifetimes in 134Te are reexamined. This work provides new insights into the evolution of collectivity for Te isotopes with N=82,84,86.
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Kibédi, T.; Alshahrani, B.; Stuchbery, A.E.; Larsen, Ann-Cecilie; Görgen, Andreas & Siem, Sunniva
[Show all 20 contributors for this article]
(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.
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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.
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Gerst, RB; Blazhev, A.; Warr, N.; Wilson, J. N.; Lebois, M & Jovančević, Nikola
[Show all 50 contributors for this article]
(2020).
Prompt and delayed γ spectroscopy of neutron-rich Kr 94 and observation of a new isomer.
Physical Review C.
ISSN 2469-9985.
102(6).
doi:
10.1103/PhysRevC.102.064323.
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Prompt and delayed γ-ray spectroscopy of the neutron-rich 94Kr was performed, as part of the fission campaign at the ALTO facility of the IPN Orsay, using the fast-neutron-induced fission reaction 238U(n,f) in combination with the ν-Ball array, a novel hybrid γ spectrometer for energy and lifetime measurements. Several new yrast and nonyrast transitions were observed for the first time, extending the previously known level scheme. Additionally, we report on the observation of a new short-lived isomer at 3444 keV with a half-life of 32(3) ns. The analysis of the Nilsson orbitals obtained from Gogny cranked Hartree-Fock-Bogoliubov calculations suggests a (9−) spin and an oblate deformation for this isomer corresponding to a two-quasineutron state, indicating an isomeric structure very similar to that of the neighboring isotones 96Sr and 92Se.
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Paulsen, Wanja & Meir, Michaela Georgine
(2026).
𝐒𝐭𝐫𝐞𝐧𝐠𝐭𝐡𝐞𝐧𝐢𝐧𝐠 𝐍𝐨𝐫𝐰𝐚𝐲’𝐬 𝐍𝐮𝐜𝐥𝐞𝐚𝐫 𝐄𝐱𝐩𝐞𝐫𝐭𝐢𝐬𝐞: 𝐀 𝐉𝐨𝐢𝐧𝐭 𝐈𝐧𝐢𝐭𝐢𝐚𝐭𝐢𝐯𝐞 𝐛𝐲 𝐔𝐢𝐎, 𝐍𝐌𝐁𝐔, 𝐚𝐧𝐝 𝐃𝐒𝐀.
doi:
https:/www.linkedin.com/feed/update/urn:li:activity:7449427917899522048.
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On April 9th, the Norwegian Radiation and Nuclear Safety Authority, Direktoratet for strålevern og atomsikkerhet (DSA), hosted in collaboration with Universitetet i Oslo (UiO) (UiO) and NMBU - Norwegian University of Life Sciences a strategic workshop "Sammen om å øke den nasjonale kompetansen i nukleære fag" (Together to strengthen the national competence in nuclear subjects) at Radisson Blu Plaza Hotel focused on master’s student supervision in businesses in the nuclear sector.
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Paulsen, Wanja & Meir, Michaela Georgine
(2026).
Upcoming experiment with scientists from the University of Naples Federico II and the Università di Catania in Italy at OCL.
doi:
https:/lnkd.in/p/eTXjq6E9.
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Last week, several scientists from the University of Naples Federico II and the Università di Catania in Italy visited the Norwegian Nuclear Research Centre and the Oslo Cyclotron Laboratory at the Universitetet i Oslo (UiO) | University of Oslo to prepare for an upcoming experiment. Through this collaboration, we will perform an alpha-scattering experiment on a magnesium target to investigate carbon-carbon fusion inside stars. When carbon fuses with carbon inside stars, magnesium becomes a gateway to elements such as neon and sodium. It is therefore very important to have a detailed understanding of the nuclear properties of magnesium. This gives us a better understanding of how the elements we and our Earth consist of were created
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Paulsen, Wanja; Görgen, Andreas; Siem, Sunniva & Li, Kevin Ching Wei
(2021).
Studies of shape coexistence in 140Sm.
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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)
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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.
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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)].
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Published
Sep. 17, 2020 10:11 AM
- Last modified
May 5, 2026 2:53 PM
