arXiv subjects

A. Görgen

Publications and source records attributed to A. Görgen.

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

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

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

Evidence for coexisting shapes in $^{98}$Zr through lifetime measurements

The lifetimes of first excited 2$^+$, 4$^+$ and 6$^+$ states in $^{98}$Zr were measured with the Recoil-Distance Doppler Shift method in an experiment performed at GANIL. Excited states in $^{98}$Zr were populated using the fission reaction between a 6.2 MeV/u $^{238}$U beam and a $^{9}$Be target. The $γ$ rays were detected with the EXOGAM array in correlation with the fission fragments identified in mass and atomic number in the VAMOS++ spectrometer. Our result shows very small B(E2;2$_1^+$ $\rightarrow$ 0$_1^+$) value in $^{98}$Zr thereby confirming the very sudden onset of collectivity at $N=60$. The experimental results are compared to large-scale Monte Carlo Shell model and beyond mean field calculations. The present results indicate coexistence of two additional deformed shapes in this nucleus along with the spherical ground state.

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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

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

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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arXiv2017

Structure of the neutron mid-shell nuclei $^{111,113}_{\quad \ 47}$Ag$_{64,66}$

$^{111}$Ag and $^{113}$Ag were produced in induced fission reaction, where yrast and near-yrast states were populated. To interpret the new data the Interacting Boson-Fermion model was used. A good agreement with the experimental data is achieved, suggesting that the two Ag nuclei have a well developed collectivity, superimposed on $πg_{9/2}^{-3}$ excitations previously observed throughout the entire isotopic chain.

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arXiv2017

Energy dependence of the prompt $γ$-ray emission from the $(d,p)$-induced fission of $^{234}\mathrm{U}^{}$ and $^{240}\mathrm{Pu}^{}$

Prompt fission $γ$-rays are responsible for approximately 5\% of the total energy released in fission, and therefore important to understand when modelling nuclear reactors. In this work we present prompt $γ$-ray emission characteristics in fission, for the first time as a function of the nuclear excitation energy of the fissioning system. Emitted $γ$-ray spectra were measured, and $γ$-ray multiplicities and average and total $γ$ energies per fission were determined for the $^{233}$U(d,pf) reaction for excitation energies between 4.8 and 10 MeV, and for the $^{239}$Pu(d,pf) reaction between 4.5 and 9 MeV. The spectral characteristics show no significant change as a function of excitation energy above the fission barrier, despite the fact that an extra $\sim$5 MeV of energy is potentially available in the excited fragments for $γ$-decay. The measured results are compared to model calculations made for prompt $γ$-ray emission with the fission model code GEF. Further comparison with previously obtained results from thermal neutron induced fission is made to characterize possible differences arising from using the surrogate (d,p) reaction.

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arXiv2017

Quasicontinuum $γ$-decay of $^{91,92}$Zr: benchmarking indirect ($n,γ$) cross section measurements for the $s$-process

Nuclear level densities (NLDs) and $γ$-ray strength functions ($γ$SFs) have been extracted from particle-$γ$ coincidences of the $^{92}$Zr($p,p' γ$)$^{92}$Zr and $^{92}$Zr($p,d γ$)$^{91}$Zr reactions using the Oslo method. The new $^{91,92}$Zr $γ$SF data, combined with photonuclear cross sections, cover the whole energy range from $E_γ \approx 1.5$~MeV up to the giant dipole resonance at $E_γ \approx 17$~MeV. The wide-range $γ$SF data display structures at $E_γ \approx 9.5$~MeV, compatible with a superposition of the spin-flip $M1$ resonance and a pygmy $E1$ resonance. Furthermore, the $γ$SF shows a minimum at $E_γ \approx 2-3$~MeV and an increase at lower $γ$-ray energies. The experimentally constrained NLDs and $γ$SFs are shown to reproduce known ($n, γ$) and Maxwellian-averaged cross sections for $^{91,92}$Zr using the {\sf TALYS} reaction code, thus serving as a benchmark for this indirect method of estimating ($n, γ$) cross sections for Zr isotopes.

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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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arXiv2017

Is the generalized Brink-Axel hypothesis valid?

Experimental results of the $^{237}$Np($d, p γ)^{238}$Np reaction are presented, which verifies the generalized Brink-Axel (gBA) hypothesis for $γ$ transitions between states in the quasi-continuum. The gBA hypothesis holds not only for specific collective resonances, but for the full dipole strength below the neutron separation energy. We discuss the validity of the gBA hypothesis also for lighter systems like $^{92}$Zr where the concept of a unique $γ$-ray strength function ($γ$SF) is problematic due to large Porter-Thomas fluctuations. Methods for studying the $γ$SF and the fluctuations as function of excitation energy are presented.

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arXiv2016

Study of the 238U(d,p) surrogate reaction via the simultaneous measurement of gamma-decay and fission probabilities

We investigated the 238U(d,p) reaction as a surrogate for the n + 238U reaction. For this purpose we measured for the first time the gamma-decay and fission probabilities of 239U* simultaneously and compared them to the corresponding neutron-induced data. We present the details of the procedure to infer the decay probabilities, as well as a thorough uncertainty analysis, including parameter correlations. Calculations based on the continuum-discretized coupled-channels and distorted-wave Born approximations were used to correct our data from detected protons originating from elastic and inelastic deuteron breakup. In the region where the fission and gamma-decay probabilities compete, the corrected fission probability is in agreement with neutron-induced data, whereas the gamma-decay probability is much higher than the neutron-induced data. The performed statistical-model calculations are not able to explain these results.

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