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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., Comment: 14 pages and 10 figures

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., 7 pages, 4 figures

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 239Pu(d,pγ)240Pu 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 239Pu(d,pγ)240Pu case.

We have done a systematic study of the low-energy behavior of the γ-ray strength function with full-configuration shell-model calculations. We have performed calculations in several isotonic and isotopic chains, spanning several mass regions. There are systematic trends: The strength function shows a low-energy enhancement that is more pronounced near shell closures and less so in the mid-shell regions. We predict that the low-energy enhancement is omnipresent, but that for high-mass nuclei it has so far been out of experimental reach due to detector thresholds, except for the case of 151, 153Sm.

The Oslo method has been applied to particle-$\ensuremath{\gamma}$ coincidences following the $^{239}\mathrm{Pu}(d,p)$ reaction to obtain the nuclear level density (NLD) and $\ensuremath{\gamma}$-ray strength function ($\ensuremath{\gamma}\mathrm{SF}$) 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 $\ensuremath{\gamma}\mathrm{SF}$. 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 $\ensuremath{\gamma}\mathrm{SF}$ shows a pronounced enhancement between 2--4 MeV that is consistent with the location of the low-energy orbital $M1$ scissors mode.

The nuclear level density and $\ensuremath{\gamma}$-decay strength of $^{192}\mathrm{Os}$ have been extracted using particle-$\ensuremath{\gamma}$ coincidence data from the $^{192}\mathrm{Os}(\ensuremath{\alpha},{\ensuremath{\alpha}}^{\ensuremath{'}}\ensuremath{\gamma})^{192}\mathrm{Os}$ 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 $\ensuremath{\gamma}$-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 $\ensuremath{\gamma}$-strength function in the range ${E}_{\ensuremath{\gamma}}\ensuremath{\approx}1.5--6\phantom{\rule{0.28em}{0ex}}\mathrm{MeV}$. Using the measured nuclear level density and $\ensuremath{\gamma}$-decay strength as input to the nuclear-reaction code talys, we provide the first experimentally constrained Maxwellian-averaged cross section (MACS) for the $^{191}\mathrm{Os}(n,\ensuremath{\gamma})^{192}\mathrm{Os}$ reaction relevant to $s$-process nucleosynthesis. The systematic uncertainties introduced by the normalization procedure of the level density and $\ensuremath{\gamma}$-strength function were investigated and propagated to the calculated Maxwellian-averaged cross section. The obtained result of the Maxwellian-averaged cross section at ${k}_{B}T=30\phantom{\rule{0.28em}{0ex}}\mathrm{keV}$, ${\ensuremath{\langle}\ensuremath{\sigma}\ensuremath{\rangle}}_{n,\ensuremath{\gamma}}=1134\ifmmode\pm\else\textpm\fi{}375\phantom{\rule{0.28em}{0ex}}\mathrm{mb}$, 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.

The Oslo method comprises a set of analysis techniques designed to extract nuclear level density and average $\gamma$-decay strength function from a set of excitation-energy tagged $\gamma$-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 $\gamma$-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., Comment: 20 pages, 6 figures. Updated v5: Eq. (C.6); v4: Major revision of App. C

In this work, we have studied the low-energy behaviour of the \(\gamma \)-ray strength function within the framework of the shell model. We have done calculations on isotopic and isotonic chains, spanning several mass regions. We find systematic trends, where the strength function exhibits a low-energy enhancement that is steeper near shell closures and flatter in the mid-shell regions. Further, we compile strength functions from discrete experimental data and find evidence for a low-energy enhancement.

Numerous scientific fields including astrophysics, nuclear power, and nuclear forensics require a knowledge of basic nuclear properties for large numbers of short-lived, radioactive isotopes far removed from stable nuclei. Neutron-capture cross sections are one such piece of nuclear data where direct measurements are not possible and theoretical predictions can vary by orders of magnitude. A recently developed indirect technique for inferring neutron capture rates, the β -Oslo method, has been introduced but not compared against a known, directly measured neutron capture cross section. To provide this benchmark, two indirect methods based on β decay and charged-particle reactions were used to extract the nuclear level density and γ -ray strength function of 51 Ti . The nuclear level density and γ -ray strength function from the two data sets were found to be equivalent and were used to extract the neutron capture cross section of 50 Ti which agrees with previous direct measurements at high neutron energies. The results demonstrate the validity of the β -Oslo method for extracting neutron capture cross sections of short-lived nuclei and provide a sufficiently small uncertainty to be used in various applications.

Photoneutron cross sections were measured for Ni58,Ni60,Ni61, and Ni64 at energies between the one-neutron and two-neutron thresholds using quasimonochromatic γ-ray beams produced in laser Compton scattering at the NewSUBARU synchrotron radiation facility. These 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 Ni63, 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 quasiparticle–random-phase approximation based on the Gogny D1M interaction for both E1 and M1 components and supplemented with the M1 upbend.

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 $\ensuremath{\gamma}$-emission probability at very low $\ensuremath{\gamma}$ energies. In this work, we present measurements of the $\ensuremath{\gamma}\text{-decay}$ strength of $^{70}\mathrm{Ni}$ over the wide range $1.3\ensuremath{\le}{E}_{\ensuremath{\gamma}}\ensuremath{\le}8$ MeV. A significant enhancement is found in the $\ensuremath{\gamma}\text{-decay}$ strength for transitions with ${E}_{\ensuremath{\gamma}}l3$ 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 ${E}_{\ensuremath{\gamma}}\ensuremath{\simeq}5$ MeV very well. Moreover, large-scale shell-model calculations indicate an $M1$ nature of the low-energy $\ensuremath{\gamma}$ 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 $^{72,74,76}\mathrm{Ni}$.

Statistical $\gamma$-decay from highly excited states is determined by the nuclear level density (NLD) and the $\gamma$-ray strength function ($\gamma$SF). These average quantities have been measured for several nuclei using the Oslo method. For the first time, we exploit the NLD and $\gamma$SF to evaluate the $\gamma$-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., Comment: CGS16, Shanghai 2017, Proceedings, 5 pages, 3 figures

The photoneutron cross sections of Dy162,163 have been measured for the first time in an energy region from the neutron threshold (Sn) up to ≈13MeV. The (γ,n) reaction was induced with quasimonochromatic laser Compton-scattered γ rays, produced at the NewSUBARU laboratory. The corresponding γ-ray strength functions (γSF) have been calculated from the photoneutron cross sections. The data are compared to reanalyzed γSFs of Dy160-164, which are measured below Sn. The excellent agreement with the photoneutron data at Sn confirms the principle of detailed balance. Thus, a complete γSF is established covering in total the energy region of 1≤Eγ≤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′) 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 γSFs 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 γSFs., SCOPUS: ar.j, info:eu-repo/semantics/published

We have made a thorough study of the low-energy behaviour of the $\gamma$-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., Comment: 11 pages, 13 figures

CITATION: Kheswa, B. V., et al. 2017. ¹³⁷,¹³⁸,¹³⁹La(n,γ) cross sections constrained with statistical decay properties of ¹³⁸,¹³⁹,¹⁴⁰La nuclei. Physical Review C, 95(4):1-9, doi:10.1103/PhysRevC.95.045805.

Nuclear level densities and $\gamma$-ray strength functions of $^{56,57}$Fe have been extracted from proton-$\gamma$ coincidences. A low-energy enhancement in the $\gamma$-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 $\gamma$ 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 $\gamma$-ray strength function, in support of the generalized Brink-Axel hypothesis., Comment: 24 pages, 17 figures, accepted for publication in J. Phys. G: Nucl. Phys; Special Issue

The nuclear level densities and gamma-ray strength functions are the key elements for Hauser-Feshbach statistical model calculations to predict reaction cross sections which have many applications including astrophysics. The nuclear level densities and y-ray strength functions have been determined for 145,149,151Nd isotopes below the neutron separation energies using the Oslo method with the 144,148,150Nd(d,p) reactions. The results from the first measurements as well as planned experiments at OCL will be presented.