arXiv subjects

V. W. Ingeberg

Publications and source records attributed to V. W. Ingeberg.

18 recordsLinked to original sources

arXiv2026

Statistical and non-statistical $γ$-decay properties of $^{64}$Zn

We present a study on the $γ$-decay properties of $^{64}$Zn using the Oslo method on $^{64}$Zn($p,p^\prime γ$) data combined with $^{64}$Zn$(γ,n)$ cross-section measurements at the NewSUBARU facility. With the Oslo method, we have measured the $γ$-ray strength function ($γ$SF) and the nuclear level density (NLD) below the neutron threshold. We observe that the NLD trend in the quasi-continuum region of $^{64}$Zn is best characterized by a constant-temperature-like model. %with temperature parameter $T_{\rm CT}=1.21(5)$ MeV. Surprisingly, we find that $γ$-ray transitions from the quasi-continuum decaying directly to the $0^+$ ground state seem to be strongly hindered with a hindrance factor of $κ\approx 0.5$, which could be an indication of non-statistical effects in the ground-state decay due to, \textit{e.g.}, differences in nuclear shapes. For $γ$ energies above the neutron separation energy, the NewSUBARU ($γ, n$) data set probes a significant part of the giant dipole resonance. Furthermore, we find that the Oslo-method $γ$SF shows a rather smooth behavior, with a clear low-energy enhancement (LEE) for $E_γ < 4$ MeV.

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arXiv2026

Statistical properties of $^{133}$Xe and the $^{132}$Xe$(n,γ)$ cross section

$^{133}$Xe is an interesting case for plasma physics to explore nuclear excitation by electron capture, as the process can be studied using statistical properties of $^{133}$Xe. In this work we present results on $^{133}$Xe from the inverse-Oslo method where we extract the nuclear level density and the $γ$-strength function, which is used to calculate the (n,$γ$) cross section on $^{132}$Xe. The $γ$-strength function of $^{133}$Xe can constrain the estimated decay rate from nuclear excitation by electron capture. The $\mathrm{d}(^{132}\mathrm{Xe},\mathrm{p})^{132}\mathrm{Xe}$ reaction was used to create the compound nucleus $^{133}$Xe, which was recorded with an annular particle telescope and a scintillator array consisting of \la and BGO-shielded HPGe Clover detectors. With the inverse-Oslo method, it is possible to study nuclei that are impossible or unable to manufacture targets from, short lived isotopes, or as in this work, noble gases. We present the extracted nuclear level density, and $γ$-strength function for $^{133}$Xe, along with shell-model calculations of the statistical properties of $^{133}$Xe. These are the first statistical properties extracted below 6 MeV for any xenon isotope. We constrain the $^{132}$Xe(n,$γ$) $^{133}$Xe cross section and reaction rate using the TALYS reaction code.

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arXiv2025

A saga on the $γ$-decay branching ratio of the Hoyle state

The radiative branching ratio of the Hoyle state is crucial to estimate the triple-$α$ reaction rate in stellar environments at medium temperatures of $T=0.1$ to 2 GK. Knowledge of the $γ$-decay channel is critical as this is the dominant radiative decay channel for the Hoyle state. A recent study by Kibédi et al. [Phys. Rev. Lett. 125, 182701 (2020)] has challenged our understanding of this astrophysically significant branching ratio and its constraints. The main purpose was to perform a new measurement of the $γ$-decay branching ratio of the Hoyle state to deduce the radiative branching ratio of the Hoyle state, an additional objective was to independently verify aspects of the measurement conducted by Kibédi et al. For the primary experiment of this work the Hoyle state was populated by the $^{12}\textrm{C}(p,p')$ reaction at 10.8 MeV at the Oslo Cyclotron Laboratory. The $γ$-decay branching ratio was deduced through triple-coincidence events between a proton populating the Hoyle state and the subsequent $γ$-ray cascade. An independent analysis of the 2014 data published by Kibédi et al. has been carried out. From the main experiment of this work, a $γ$-decay branching ratio of the Hoyle state was determined as $Γ_γ^{7.65}/Γ^{7.65}=4.0(3)\times 10^{-4}$, yielding a radiative branching ratio of $Γ_{\textrm{rad}}/Γ=4.1(4) \times 10^{-4}$. The reanalysis of the 2014 experiment in this work yielded $Γ_γ^{7.65}/Γ^{7.65}=4.5(6)\times 10^{-4}$, with a radiative branching ratio of $Γ_{\textrm{rad}}/Γ=4.6(6) \times 10^{-4}$. The measurements of the radiative branching ratio of the Hoyle state in this work is in excellent agreement with several recent studies, as well as the previously adopted ENSDF average of $Γ_{\textrm{rad}}/Γ=4.16(11)\times 10^{-4}$.

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arXiv2024

Nuclear Level Density and $γ$-ray Strength Function of $^{67}\mathrm{Ni}$ and the impact on the i-process

Proton-$γ$ coincidences from $(\mathrm{d},\mathrm{p})$ reactions between a $^{66}\mathrm{Ni}$ beam and a deuterated polyethylene target have been analyzed with the inverse-Oslo method to find the nuclear level density (NLD) and $γ$-ray strength function ($γ$SF) of $^{67}\mathrm{Ni}$. The $^{66}\mathrm{Ni}(n,γ)$ capture cross section has been calculated using the Hauser-Feshbach model in TALYS using the measured NLD and $γ$SF as constraints. The results confirm that the $^{66}\mathrm{Ni}(n,γ)$ reaction acts as a bottleneck when relying on one-zone nucleosynthesis calculations. However, the impact of this reaction is strongly dampened in multi-zone models of low-metallicity AGB stars experiencing i-process nucleosynthesis.

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arXiv2024

Photoneutron cross section measurements on $^{208}$Pb in the Giant Dipole Resonance region

Photoneutron reactions on $^{208}$Pb in the Giant Dipole Resonance energy region have been investigated at the $γ$-ray beam line of the NewSUBARU facility in Japan. The measurements made use of quasi-monochromatic laser Compton backscattering $γ$-ray beams in a broad energy range, from the neutron threshold up to 38 MeV, and of a flat-efficiency moderated $^3$He neutron detection system along with associated neutron-multiplicity sorting methods. We report absolute cross sections and mean photoneutron energies for the $^{208}$Pb$(γ,\,inX)$ reactions with $i$~=~1 to 4. The fine structure present in the $^{208}$Pb$(γ,\,n)$ cross sections at incident energies lower than 13~MeV has been observed. The photoabsorption cross section has been obtained as the sum of the $(γ,\,inX)$ reaction cross sections. By reproducing the measured ring-ratio values at excitation energies below the two neutron separation energy, we were able to extract estimations on the $^{208}$Pb$(γ,\,n)$ photoneutron energy spectra and on the partial photoneutron cross sections for leaving the residual $^{207}$Pb in its ground and first two excited states. The present results are compared with data from the literature and statistical model calculations.

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arXiv2023

New experimental constraint on the $^{185}$W($n,γ$)$^{186}$W cross section

In this work, we present new data on the $^{182,183,184}$W($γ,n$) cross sections, utilizing a quasi-monochromatic photon beam produced at the NewSUBARU synchrotron radiation facility. Further, we have extracted the nuclear level density and $γ$-ray strength function of $^{186}$W from data on the $^{186}$W($α,α^\primeγ$)$^{186}$W reaction measured at the Oslo Cyclotron Laboratory. Combining previous measurements on the $^{186}$W($γ,n$) cross section with our new $^{182,183,184}$W($γ,n$) and ($α,α^\primeγ$)$^{186}$W data sets, we have deduced the $^{186}$W $γ$-ray strength function in the range of $1 < E_γ< 6$ MeV and $7 < E_γ< 14$ MeV. Our data are used to extract the level density and $γ$-ray strength functions needed as input to the nuclear-reaction code \textsf{TALYS}, providing an indirect, experimental constraint for the $^{185}$W($n,γ$)$^{186}$W cross section and reaction rate. Compared to the recommended Maxwellian-averaged cross section (MACS) in the KADoNiS-1.0 data base, our results are on average lower for the relevant energy range $k_B T \in [5,100]$ keV, and we provide a smaller uncertainty for the MACS. The theoretical values of Bao \textit{et al.} and the cross section experimentally constrained on photoneutron data of Sonnabend \textit{et al.} are significantly higher than our result. The lower value by Mohr \textit{et al.} is in very good agreement with our deduced MACS. Our new results could have implications for the $s$-process and in particular the predicted $s$-process production of $^{186,187}$Os nuclei.

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arXiv2022

Evolution of the $γ$-ray strength function in neodymium isotopes

The experimental gamma-ray strength functions (gamma-SFs) of 142,144-151Nd have been studied for gamma-ray energies up to the neutron separation energy. The results represent a unique set of gamma-SFs 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. The data indicate that the mechanisms behind the low-energy enhancement and the scissors mode are decoupled from each other.

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arXiv2021

Statistical properties of the well deformed $^{153,155}$Sm nuclei and the scissors resonance

The Nuclear Level Densities (NLDs) and the $γ$-ray Strength Functions ($γ$SFs) of $^{153,155}$Sm have been extracted from (d,p$γ$) coincidences using the Oslo method. The experimental NLD of $^{153}$Sm is higher than the NLD of $^{155}$Sm, in accordance with microscopic calculations. The $γ$SFs of $^{153,155}$Sm are in fair agreement with QRPA calculations based on the D1M Gogny interaction. An enhancement is observed in the $γ$SF for both $^{153,155}$Sm 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 $μ^{2}_{N}$ for $^{153}$Sm and $^{155}$Sm, respectively. The strength of the SR for $^{155}$Sm is comparable to those for deformed even-even Sm isotopes from nuclear resonance fluorescence measurements, while that of $^{153}$Sm is lower than expected.

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arXiv2020

Strong enhancement of level densities in the crossover from spherical to deformed neodymium isotopes

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 $^{142,144-151}$Nd which exhibit such a crossover. These results represent to date the most complete data set of nuclear level densities 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 $\approx 170$ at an excitation energy of 7.5 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.

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arXiv2020

The energy response of the Oslo Scintillator Array OSCAR

The new Oslo Scintillator Array (OSCAR) has been commissioned at the Oslo Cyclotron Laboratory (OCL). It consists of 30 large volume (diameter 3.5 x 8 inches) LaBr$_3$(Ce) detectors that are used for $γ$-ray spectroscopy. The response functions for incident $γ$-rays up to 20 MeV are simulated with $\texttt{Geant4}$. 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.

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arXiv2019

Nuclear level densities and $γ$-ray strength functions of $^{87}\mathrm{Kr}$ -- First application of the Oslo Method in inverse kinematics

The $γ$-ray strength function ($γ$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 $\mathrm{d}(^{86}\mathrm{Kr}, \mathrm{p}γ)^{87}\mathrm{Kr}$ reaction were measured at iThemba LABS and the $γ$SF and NLD in $^{87}\mathrm{Kr}$ obtained. The low-energy region of the $γ$SF is compared to Shell Model calculations which suggest this region to be dominated by M1 strength. The $γ$SF and NLD are used as input parameters to Hauser-Feshbach calculations to constrain $(\mathrm{n},γ)$ cross sections of nuclei using the TALYS reaction code. These results are compared to $^{86}\mathrm{Kr}(n,γ)$ data from direct measurements.

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arXiv2019

$γ$-ray Strength Function for Barium Isotopes

Photoneutron cross sections were measured for $^{137}$Ba and $^{138}$Ba at energies below two-neutron threshold using quasi-monochromatic $γ$-ray beams produced in laser Compton-scattering at the NewSUBARU synchrotron radiation facility. The photoneutron data are used to constrain the $γ$-ray strength function on the basis of the Hartree-Fock-Bogolyubov plus quasi-particle random phase approximation using the Gogny D1M interaction. Supplementing the experimentally constrained $γ$-ray strength function with the zero-limit E1 and M1 contributions which are unique to the deexcitation mode, we discuss radiative neutron capture cross sections relevant to the s-process nucleosynthesis of barium isotopes in the vicinity of the neutron magic number 82.

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arXiv2019

Restricted spin-range correction in the Oslo Method: The example of nuclear level density and $γ$-ray strength function from $^{239}\mathrm{Pu}(\mathrm{d,p}γ)^{240}\mathrm{Pu}$

The Oslo Method has been applied to particle-$γ$ coincidences following the $^{239}\mathrm{Pu}$(d,p) reaction to obtain the nuclear level density (NLD) and $γ$-ray strength function ($γ$SF) of $^{240}\mathrm{Pu}$. The experiment was conducted with a 12 MeV deuteron beam at the Oslo Cyclotron Laboratory. The low spin transfer of this reaction leads to a spin-parity mismatch between populated and intrinsic levels. This is a challenge for the Oslo Method as it can have a significant impact on the extracted NLD and $γ$SF. We have developed an iterative approach to ensure consistent results even for cases with a large spin-parity mismatch, in which we couple Green's Function Transfer calculations of the spin-parity dependent population cross-section to the nuclear decay code RAINIER. The resulting $γ$SF shows a pronounced enhancement between 2-4 MeV that is consistent with the location of the low-energy orbital $M1$ scissors mode.

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arXiv2019

Nuclear level densities and gamma-ray strength functions of $^{180,181,182}$Ta

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 where used for the absolute normalisations 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 $14.9^{\circ} \pm 1.8^{\circ}$ was determined for $^{181}$Ta.

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arXiv2018

Photoneutron cross sections for Ni isotopes: Toward understanding $(n,γ)$ cross sections relevant to the weak s-process nucleosynthesis

Photoneutron cross sections were measured for $^{58}$Ni, $^{60}$Ni, $^{61}$Ni, and $^{64}$Ni at energies between the one-neutron and two-neutron thresholds using quasi-monochromatic $γ$-ray beams produced in laser Compton-scattering at the NewSUBARU synchrotron radiation facility. The new photoneutron data are used to extract the $γ$-ray strength function above the neutron threshold complementing the information obtained by the Oslo method below the threshold. We discuss radiative neutron capture cross sections and the Maxwellian-averaged cross sections for Ni isotopes including $^{63}$Ni, a branching point nucleus along the weak s-process path. The cross sections are calculated with the experimentally constrained $γ$-ray strength functions from the Hartree-Fock-Bogolyubov plus quasi-particle-random phase approximation based on the Gogny D1M interaction for both $E1$ and $M1$ components and supplemented with the $M1$ upbend.

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arXiv2018

Experimental $γ$-decay strength in $^{59, 60}$Ni compared with microscopic calculations

Nuclear level densities and $γ$-ray strength functions have been extracted for $^{59, 60}\rm{Ni}$, using the Oslo method on data sets from the $^{60}$Ni($^{3}$He,$^{3}$He$^{\prime}γ$)$^{60}$Ni and $^{60}$Ni($^{3}$He,$αγ$)$^{59}$Ni reactions. Above the neutron separation energy, S$_n$, we have measured the $γ$-ray strength functions for $^{61}$Ni and $^{60}$Ni in photoneutron experiments. The low-energy part of the $^{59,60}$Ni $γ$-ray strength functions show an increase for decreasing $γ$ energies. The experimental $γ$-ray strength functions are compared with $M1$ $γ$-ray strength functions calculated within the shell model. The $E1$ $γ$-ray strength function of $^{60}$Ni has been calculated using the QTBA framework. The QTBA calculations describe the data above $E_γ\approx$ 7 MeV, while the shell-model calculations agree qualitatively with the low energy part of the $γ$-ray strength function. Hence, we give a plausible explanation of the observed shape of the $γ$-decay strength.

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arXiv2018

Gamma-widths, lifetimes and fluctuations in the nuclear quasi-continuum

Statistical $γ$-decay from highly excited states is determined by the nuclear level density (NLD) and the $γ$-ray strength function ($γ$SF). These average quantities have been measured for several nuclei using the Oslo method. For the first time, we exploit the NLD and $γ$SF to evaluate the $γ$-width in the energy region below the neutron binding energy, often called the quasi-continuum region. The lifetimes of states in the quasi-continuum are important benchmarks for a theoretical description of nuclear structure and dynamics at high temperature. The lifetimes may also have impact on reaction rates for the rapid neutron-capture process, now demonstrated to take place in neutron star mergers.

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arXiv2017

Low-energy enhancement and fluctuations of $γ$-ray strength functions in $^{56,57}$Fe: test of the Brink-Axel hypothesis

Nuclear level densities and $γ$-ray strength functions of $^{56,57}$Fe have been extracted from proton-$γ$ coincidences. A low-energy enhancement in the $γ$-ray strength functions up to a factor of 30 over common theoretical E1 models is confirmed. Angular distributions of the low-energy enhancement in $^{57}$Fe indicate its dipole nature, in agreement with findings for $^{56}$Fe. The high statistics and the excellent energy resolution of the large-volume LaBr$_{3}$(Ce) detectors allowed for a thorough analysis of $γ$ strength as function of excitation energy. Taking into account the presence of strong Porter-Thomas fluctuations, there is no indication of any significant excitation-energy dependence in the $γ$-ray strength function, in support of the generalized Brink-Axel hypothesis.

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