arXiv subjects

Ann-Cecilie Larsen

Publications and source records attributed to Ann-Cecilie Larsen.

11 recordsLinked to original sources

arXiv2026

Microscopic study of the low-energy enhancement in the gamma-decay strength of $^{50}$V

We address the microscopic origin of the low-energy enhancement (LEE) in $^{50}$V with large-scale shell-model calculations to obtain $E1$ and $M1$ transitions within the same theoretical framework. The valence space spans the three major shells $sd$, $pf$ and $sdg$ and is treated with the SDPFSDG-MU interaction using the KSHELL code. With a $1 \hbar ω$ truncation, 3600 energy eigenstates and a basis of $7.02\times10^{6}$ positive and $5.94\times10^{8}$ negative parity states, the calculations yield nearly two million individual dipole transitions. The fourteen lowest experimental levels are reproduced within $0.30$~MeV, the calculated total level density excellently reproduces Oslo-method data up to $E \approx 7.5$~MeV, and the calculated dipole gamma strength function follows the experimental shape -- including the LEE -- for the full gamma-energy range covered by the Oslo experiment. The LEE is shown to be entirely magnetic dipole in origin. Both spin and orbital parts of the $\hat{M}1$ operator are required to reproduce the LEE, with constructive interference between the spin and orbital parts giving an extra enhancement to the LEE. Reduced one-body transition densities identify $0f_{7/2} \rightarrow 0f_{7/2}$ proton transitions as the principal driver of the LEE.

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arXiv2025

Extreme Shape Coexistence Observed in $^{70}$Co

The shape of the atomic nucleus is a property which underpins our understanding of nuclear systems, impacts the limits of nuclear existence, and enables probes of physics beyond the Standard Model. Nuclei can adopt a variety of shapes, including spheres, axially deformed spheroids, and pear shapes. In some regions of the nuclear chart where a spherical nucleus would naively be expected, deformed nuclear states can result from collective action of constituent protons and neutrons. In a small subset of nuclei both spherical and deformed nuclear states have been experimentally observed, a phenomenon termed shape coexistence. We present spectroscopic evidence for the coexistence of $J^π=1+$ spherical and deformed states in $^{70}$Co, separated by less than 275~keV. This close degeneracy of levels with the same $J^π$ and different shapes demonstrates an extreme example of shape coexistence resulting from the interplay of independent particle motion and collective behavior in highly unstable nuclear systems and identifies the Co isotopes as a transition point between deformed ground states observed in the Cr isotopes and spherical configurations observed in the closed-shell Ni isotopes.

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arXiv2023

Impact of level densities and $γ$-strength functions on $r$-process simulations

Studies attempting to quantify the sensitivity of the $r$-process abundances to nuclear input have to cope with the fact that the theoretical models they rely on, rarely come with confidence intervals. This problem has been dealt with by either estimating these intervals and propagating them statistically to the final abundances using reaction networks within simplified astrophysical models, or by running more realistic astrophysical simulations using different nuclear-physics models consistently for all the involved nuclei. Both of these approaches have their strengths and weaknesses. In this work, we run $r$-process calculations for five trajectories using 49 different neutron-capture rate models. Our results shed light on the importance of taking into account shell effects and pairing correlations in the network calculations.

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arXiv2023

Experimentally constrained $^{165,166}\text{Ho}(n,γ)$ rates and implications for the $s$ process

The $γ$-ray strength function and the nuclear level density of $^{167}$Ho have been extracted using the Oslo method from a $^{164}\text{Dy}(α,pγ)^{167}$Ho experiment carried out at the Oslo Cyclotron Laboratory. The level density displays a shape that is compatible with %can be approximated with the constant temperature model in the quasicontinuum, while the strength function shows structures indicating the presence of both a scissors and a pygmy dipole resonance. Using our present results as well as data from a previous $^{163}\text{Dy}(α,pγ)^{166}$Ho experiment, the $^{165}\text{Ho}(n,γ)$ and $^{166}\text{Ho}(n,γ)$ MACS uncertainties have been constrained. The possible influence of the low-lying, long-lived 6~keV isomer $^{166}$Ho in the $s$ process is investigated in the context of a 2~$M_\odot$, [Fe/H]=-0.5 AGB star. We show that the newly obtained $^{165}\text{Ho}(n,γ)$ MACS affects the final $^{165}$Ho abundance, while the $^{166}\text{Ho}(n,γ)$ MACS only impacts the enrichment of $^{166,167}$Er to a limited degree due to the relatively rapid $β$ decay of the thermalized $^{166}$Ho at typical $s$-process temperatures.

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arXiv2023

Comprehensive test of nuclear level density models

For the last two decades, experimental information on nuclear level densities for about 60 different nuclei has been obtained on the basis of the Oslo method. While each of these measurements has been typically compared to one or a few level density models, a global study including all the measurements has been missing. The present study provides a systematic comparison between Oslo data and six global level density models for 42 nuclei for which $s$-wave resonance spacings are also available. We apply a coherent normalization procedure to the Oslo data for each of the six different models, all being treated on the same footing. Our quantitative analysis shows that the constant-temperature model presents the best global description of the Oslo data, closely followed by the mean-field plus combinatorial model and Hartree-Fock plus statistical model. Their accuracies are quite similar, so that it remains difficult to clearly favour one of these models. When considering energies above the threshold where the experimental level scheme is complete, all the six models are shown to lead to rather similar accuracies with respect to Oslo data. The recently proposed shape method can, in principle, improve the situation since it provides an absolute estimate of the excitation-energy dependence of the measured level densities. We show for the specific case of $^{112}$Cd that the shape method could exclude the Hartree-Fock plus statistical model. Such an analysis remains to be performed for the bulk of data for which the shape method can be applied to the Oslo measurements before drawing conclusions on the general quality of a given nuclear level density model.

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arXiv2022

Indirect measurement of the $\pmb{(n,γ)^{127}}$Sb cross section

Nuclei in the $^{135}$I region have been identified as being a possible bottleneck for the \textit{i} process. Here we present an indirect measurement for the Maxwellian-averaged cross section of $^{126}\text{Sb}(n,γ)$. The nuclear level density and the $γ$-ray strength function of $^{127}$Sb have been extracted from $^{124}$Sn$(α,pγ)^{127}$Sb 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 $E_γ\approx$ 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 $^{126}$Sb$(n,γ)^{127}$Sb 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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arXiv2021

A new software implementation of the Oslo method with rigorous statistical uncertainty propagation

The Oslo method comprises a set of analysis techniques designed to extract nuclear level density and average $γ$-decay strength function from a set of excitation-energy tagged $γ$-ray spectra. Here we present a new software implementation of the entire Oslo method, called OMpy. We provide a summary of the theoretical basis and derive the essential equations used in the Oslo method. In addition to the functionality of the original analysis code, the new implementation includes novel components such as a rigorous method to propagate uncertainties throughout all steps of the Oslo method using a Monte Carlo approach. The resulting level density and $γ$-ray strength function have to be normalized to auxiliary data. The normalization is performed simultaneously for both quantities, thus preserving all correlations. The software is verified by the analysis of a synthetic spectrum and compared to the results of the previous implementation, the oslo-method-software.

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arXiv2017

$^{137,138,139}$La($n$, $γ$) cross sections constrained with statistical decay properties of $^{138,139,140}$La nuclei

The nuclear level densities and $γ$-ray strength functions of $^{138,139,140}$La were measured using the $^{139}$La($^{3}$He, $α$), $^{139}$La($^{3}$He, $^{3}$He$^\prime$) and $^{139}$La(d, p) reactions. The particle-$γ$ coincidences were recorded with the silicon particle telescope (SiRi) and NaI(Tl) (CACTUS) arrays. In the context of these experimental results, the low-energy enhancement in the A$\sim$140 region is discussed. The $^{137,138,139}$La($n, γ)$ cross sections were calculated at $s$- and $p$-process temperatures using the experimentally measured nuclear level densities and $γ$-ray strength functions. Good agreement is found between $^{139}$La($n, γ)$ calculated cross sections and previous measurements.

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arXiv2014

Pygmy resonance and low-energy enhancement in the $γ$-ray strength functions of Pd~isotopes

An unexpected enhancement in the $γ$-ray strength function, as compared to the low energy tail of the Giant Dipole Resonance (GDR), has been observed for Sc, Ti, V, Fe and Mo isotopes for $E_γ<4$ MeV. This enhancement was not observed in subsequent analyses on Sn isotopes, but a Pygmy Dipole Resonance (PDR) centered at $E_γ\approx8$ MeV was however detected. The $γ$-ray strength functions measured for Cd isotopes exhibit both features over the range of isotopes, with the low-energy enhancement decreasing- and PDR strength increasing as a function of neutron number. This suggests a transitional region for the onset of low-energy enhancement, and also that the PDR strength depends on the number of neutrons. The $γ$-ray strength functions of $^{105-108}$Pd have been measured in order to further explore the proposed transitional region. Experimental data were obtained at the Oslo Cyclotron Laboratory by using the charged particle reactions ($^{3}$He, $^{3}$He$^{\prime}γ$) and ($^{3}$He, $α$$γ$) on $^{106,108}$Pd target foils. Particle$-γ$ coincidence measurements provided information on initial excitation energies and the corresponding $γ$-ray spectra, which were used to extract the level densities and $γ$-ray strength functions according to the Oslo method. The $γ$-ray strength functions indicate a sudden increase in magnitude for $E_γ>4$ MeV, which is interpreted as a PDR centered at $E_γ\approx8$ MeV. An enhanced $γ$-ray strength at low energies is also observed for $^{105}$Pd, which is the lightest isotope measured in this work. Further, the results correspond and agree very well with the observations from the Cd isotopes, and support the suggested transitional region for the onset of low-energy enhancement with decreasing mass number. The neutron number dependency of the PDR strength is also evident.

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arXiv2014

Level density and gamma-ray strength function in the odd-odd 238Np

The level density and gamma-ray strength function in the quasi-continuum of 238Np has been measured using the Oslo method. The level density function follows closely the constant-temperature level density formula and reaches 43 million levels per MeV at Sn = 5.488 MeV of excitation energy. The gamma-ray strength function displays a two-humped resonance at low-energy as also seen in previous investigations of Th, Pa and U isotopes. The structure is interpreted as the scissors resonance and has an average centroid of wSR = 2.26(5) MeV and a total strength of BSR = 10.8(12)m2N, which is in excellent agreement with sum-rule estimates. The scissors resonance is shown to have an impact on the 237Np(n; g)238Np cross section.

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arXiv2013

A new fission-fragment detector to complement the CACTUS-SiRi setup at the Oslo Cyclotron Laboratory

An array of Parallel Plate Avalanche Counters (PPAC) for the detection of heavy ions has been developed. The new device, NIFF (Nuclear Instrument for Fission Fragments), consists of four individual detectors and covers $60\%$ of 2$π$. It was designed to be used in conjunction with the SiRi array of $ΔE-E$ silicon telescopes for light charged particles and fits into the CACTUS array of 28 large-volume NaI scintillation detectors at the Oslo Cyclotron Laboratory. The low-pressure gas-filled PPACs are sensitive for the detection of fission fragments, but are insensitive to scattered beam particles of light ions or light-ion ejectiles. The PPAC detectors of NIFF have good time resolution and can be used either to select or to veto fission events in in-beam experiments with light-ion beams and actinide targets. The powerful combination of SiRi, CACTUS, and NIFF provides new research opportunities for the study of nuclear structure and nuclear reactions in the actinide region. The new setup is particularly well suited to study the competition of fission and $γ$ decay as a function of excitation energy.

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