arXiv subjects

A. C. Larsen

Publications and source records attributed to A. C. Larsen.

At least 37 records · Page 2Linked to original sources

arXiv2020

The radiative width of the Hoyle state from $γ$-ray spectroscopy

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

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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

The beta-Oslo method: experimentally constrained ($n,γ$) reaction rates relevant to the $r$-process

Unknown neutron-capture reaction rates remain a significant source of uncertainty in state-of-the-art $r$-process nucleosynthesis reaction network calculations. As the $r$-process involves highly neutron-rich nuclei for which direct ($n,γ$) cross-section measurements are virtually impossible, indirect methods are called for to constrain ($n,γ$) cross sections used as input for the $r$-process nuclear network. Here we discuss the newly developed beta-Oslo method, which is capable of providing experimental input for calculating ($n,γ$) rates of neutron-rich nuclei. The beta-Oslo method represents a first step towards constraining neutron-capture rates of importance to the $r$-process.

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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

First experimental constraint on the $^{191}$Os$(n,γ)$ reaction rate relevant to $s$-process nucleosynthesis

The nuclear level density and $γ$-decay strength of $^{192}$Os have been extracted using particle-$γ$ coincidence data from the $^{192}$Os($α,α^\primeγ$)$^{192}$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 $E1$ and $M1$ 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\approx 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 $k_BT=30$ keV, $\langle σ\rangle_{n,γ}=1134\pm 375$ 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.

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arXiv2019

Novel Techniques for Constraining Neutron-Capture Rates Relevant for r-Process Heavy-Element Nucleosynthesis

The rapid-neutron capture process ($r$ process) is identified as the producer of about 50\% of elements heavier than iron. This process requires an astrophysical environment with an extremely high neutron flux over a short amount of time ($\sim$ seconds), creating very neutron-rich nuclei that are subsequently transformed to stable nuclei via $β^-$ decay. One key ingredient to large-scale $r$-process reaction networks is radiative neutron-capture ($n,γ$) rates, for which there exist virtually no data for extremely neutron-rich nuclei involved in the $r$ process. Due to the current status of nuclear-reaction theory and our poor understanding of basic nuclear properties such as level densities and average $γ$-decay strengths, theoretically estimated ($n,γ$) rates may vary by orders of magnitude and represent a major source of uncertainty in any nuclear-reaction network calculation of $r$-process abundances. In this review, we discuss new approaches to provide information on neutron-capture cross sections and reaction rates relevant to the $r$ process. In particular, we focus on indirect, experimental techniques to measure radiative neutron-capture rates. While direct measurements are not available at present, but could possibly be realized in the future, the indirect approaches present a first step towards constraining neutron-capture rates of importance to the $r$ process.

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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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arXiv2019

Impact of restricted spin-ranges in the Oslo Method: The example of (d,p)$^{240}\mathrm{Pu}$

In this paper we present the first systematic analysis of the impact of the populated vs. intrinsic spin distribution on the nuclear level density and $γ$-ray strength function retrieved through the Oslo Method. We illustrate the effect of the spin distribution on the recently performed $^{239}\mathrm{Pu}$(d,p$γ$)$^{240}\mathrm{Pu}$ experiment using a 12 MeV deuteron beam performed at the Oslo Cyclotron Lab. In the analysis we couple state-of-the-art calculations for the populated spin-distributions with the Monte-Carlo nuclear decay code RAINIER to compare Oslo Method results to the known input. We find that good knowledge of the populated spin distribution is crucial and show that the populated distribution has a significant impact on the extracted nuclear level density and $γ$-ray strength function for the $^{239}\mathrm{Pu}$(d,p$γ$)$^{240}\mathrm{Pu}$ case.

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arXiv2019

Consolidating the concept of low-energy magnetic dipole decay radiation

We have made a thorough study of the low-energy behaviour of the $γ$-ray strength function within the framework of the shell model. We have performed large-scale calculations spanning isotopic and isotonic chains over several mass regions, with the purpose of studying the systematic behavior of the low-energy enhancement (LEE) for $M1$ transitions. There are clear trends in the calculations: From being all but absent in the lowest mass region, the LEE becomes steeper and more pronounced as the mass number increases, and for a given mass region it further increases towards shell closures. Moreover, the LEE is found to be steeper in regions near doubly-magic nuclei where proton particles couple to neutron holes. These trends enable us to consolidate several previous works on the LEE into a single, consistent concept. We compare the inferred trends to the available experimental data from the Oslo method, and find suppport for the systematic behaviour. Lastly we have compared the calculations to strength functions compiled from discrete, experimental lifetimes, and find excellent agreement; the discrete data are consistent with a LEE, and indicate that the slope varies as function of mass number.

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arXiv2018

The $γ$-ray strength function for Thallium isotopes relevant to the $^{205}$Pb - $^{205}$Tl chronometry

Photoneutron cross sections were measured for $^{203}$Tl and $^{205}$Tl at energies between the one- and two-neutron thresholds using quasi-monochromatic $γ$-ray beams produced in laser Compton-scattering at the NewSUBARU synchrotron radiation facility. Our new measurement results in cross sections significantly different from the previously reported bremsstrahlung experiment, leading to rather different GDR parameters, in particular to lower GDR peak energies and higher peak cross sections. 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 for the de-excitation mode, we estimate the Maxwellian-averaged cross section for the s-process branching-point nucleus $^{204}$Tl in the context of the $^{205}$Pb - $^{205}$Tl chronometry.

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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

Enhanced low-energy $γ$-decay strength of $^{70}$Ni and its robustness within the shell model

Neutron-capture reactions on very neutron-rich nuclei are essential for heavy-element nucleosynthesis through the rapid neutron-capture process, now shown to take place in neutron-star merger events. For these exotic nuclei, radiative neutron capture is extremely sensitive to their $γ$-emission probability at very low $γ$ energies. In this work, we present measurements of the $γ$-decay strength of $^{70}$Ni over the wide range $1.3 \leq E_γ \leq 8 $ MeV. A significant enhancement is found in the $γ$-decay strength for transitions with $E_γ< 3$ MeV. At present, this is the most neutron-rich nucleus displaying this feature, proving that this phenomenon is not restricted to stable nuclei. We have performed $E1$-strength calculations within the quasiparticle time-blocking approximation, which describe our data above $E_γ\simeq 5$ MeV very well. Moreover, large-scale shell-model calculations indicate an $M1$ nature of the low-energy $γ$ strength. This turns out to be remarkably robust with respect to the choice of interaction, truncation and model space, and we predict its presence in the whole isotopic chain, in particular the neutron-rich $^{72,74,76}\mathrm{Ni}$.

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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

The gamma-ray strength function of $^{89}$Y and $^{90}$Y

In this work, we present new data on the $^{89}$Y($γ$,n) cross section studied with a quasi-monochromatic photon beam produced at the NewSUBARU synchrotron radiation facility in Japan contributing torwards resolving a long standing discrepancy between existing measurements of this cross section. Results for $γ$-ray strength function below threshold obtained by applying the Oslo method to $^{89}$Y($p,p'γ$)$^{89}$Y coincidences combined with the $^{89}$Y($γ$,n) data this providing experimental data for the $γ$-ray strength function of $^{89}$Y for $γ$ energies in the range of $\approx 1.6$ Mev to $\approx$ 20 MeV. A low-energy enhancement is seen for $γ$-rays below $\approx 2.5$ MeV. Shell-model calculations indicate that this feature is caused by strong, low-energy $M1$ transitions at high excitation energies. The nuclear level density and $γ$-ray strength function have been extracted from $^{89}$Y($d,p γ$)$^{90}$Y coincidences using the Oslo method. Using the ($γ,n$) and ($d,pγ$) data as experimental constraints, we have calculated the $^{89}$Y($n,γ$)$^{90}$Y cross section with the TALYS reaction code. Our results have been compared with directly measured (n,$γ$) cross sections and evaluations. The $N=50$ isotope $^{89}$Y is an important bottleneck in the s-process and the magnitude of the $^{89}$Y(n,$γ)$ cross section is key to understanding how s-process stars produce heavy isotopes.

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arXiv2018

Verification of detailed balance for $γ$ absorption and emission in Dy isotopes

The photo-neutron cross sections of $^{162,163}\rm{Dy}$ have been measured for the first time in an energy region from the neutron threshold ($S_n$) up to $\approx$ $13$~MeV. The ($γ$,n) reaction was induced with quasi-monochromatic laser Compton-scattered $γ$ rays, produced at the NewSUBARU laboratory. The corresponding $γ$-ray strength functions ($γ$SF) have been calculated from the photo-neutron cross sections. The data are compared to reanalyzed $γ$SFs of $^{160-164}\rm{Dy}$, which are measured below $S_n$. The excellent agreement with the photo-neutron data at $S_n$ confirms the principle of detailed balance. Thus, a complete $γ$SF is established covering in total the energy region of 1 MeV $\leq$ E$_γ$ $\leq$ 13 MeV. These mid-shell well-deformed dysprosium isotopes all show scissors resonances with very similar structures. We find that our data predict the same integrated scissors strength as ($γ,γ^\prime$) data when integrated over the same energy range, which shows that the scissors mode very likely is consistent with the generalized Brink hypothesis. Finally, using the $γ$SFs as input in the reaction code TALYS, we have deduced radiative neutron-capture cross sections and compared them to direct measurements. We find a very good agreement within the uncertainties, which gives further support to the experimentally determined $γ$SFs.

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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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arXiv2018

Understanding the Low-Energy Enhancement of the $γ$-ray Strength Function of $^{56}$Fe

A model-independent technique was used to determine the $γ$-ray Strength Function ($γ$SF) of $^{56}$Fe down to $γ$-ray energies less than 1 MeV for the first time with GRETINA using the $(p,p')$ reaction at 16 MeV. No difference was observed in the energy dependence of the $γ$SF built on $2^{+}$ and $4^{+}$ final states, supporting the Brink hypothesis. In addition, angular distribution and polarization measurements were performed. The angular distributions are consistent with dipole radiation. The polarization results show a small bias towards magnetic character in the region of the enhancement.

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