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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 $^{66}\mathrm{Ni}(\mathrm{n},γ)$ capture cross section has been calculated using the Hauser-Feshbach model in TALYS using the measured NLD and $γ$SF as constraints. We confirm that $^{66}\mathrm{Ni}(\mathrm{n},γ)$ acts as a bottleneck when relying on one-zone nucleosynthesis calculations. However, we find that the impact of this reaction is strongly damped in multi-zone low-metallicity AGB stellar models experiencing i-process nucleosynthesis., Submitted to Phys. Rev. C

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., 17 pages, 15 figures; to be submitted to Phys. Rev. C

Nuclear level densities (NLDs) and $γ$-ray strength functions (GSFs) of $^{120,124}$Sn have been extracted with the Oslo method from proton-$γ$ coincidences in the ($p,p^{\prime}γ)$ reaction. The functional forms of the GSFs and NLDs have been further constrained with the Shape method by studying primary $γ$-transitions to the ground and first excited states.The NLDs demonstrate good agreement with the NLDs of $^{116,118,122}$Sn isotopes measured previously. Moreover, the extracted partial NLD of 1$^{-}$ levels in $^{124}$Sn is shown to be in fair agreement with those deduced from spectra of relativistic Coulomb excitation in forward-angle inelastic proton scattering. The experimental NLDs have been applied to estimate the magnitude of the Porter-Thomas (PT) fluctuations. Within the PT fluctuations, we conclude that the GSFs for both isotopes can be considered to be independent of initial and final excitation energies, in accordance with the generalized Brink-Axel hypothesis. Particularly large fluctuations observed in the Shape-method GSFs present a considerable contribution to the uncertainty of the method, and may be one of the reasons for deviations from the Oslo-method strength at low $γ$-ray energies and low values of the NLD (below $\approx1\cdot10^{3}-2\cdot10^{3}$ MeV$^{-1}$)., 17 pages, 12 figures

Recent measurements indicate that the previously established upper limit for the $\gamma$-decay branch of the $3_{1}^{-}$ resonance in $^{12}\textrm{C}$ at $E_{x} = 9.641(5)$ MeV may be incorrect. The $3_{1}^{-}$ resonance has thus been touted as a significant resonance for mediating the triple-$\alpha$ reaction at high temperatures above 2 GK. Accurate estimations of the $3_{1}^{-}$ contribution to the triple-$\alpha$ reaction rate require accurate knowledge of not only the radiative width, but also the total width. In anticipation of future measurements to more accurately determine the $\gamma$-decay branch of the $3_{1}^{-}$ state, the objective of this work is to accurately determine the total width of the $3_{1}^{-}$ state. An evaluation was performed on all previous results considered in the current ENSDF average for the $3_{1}^{-}$ total width of 46(3) keV. A new R-matrix analysis for the $3_{1}^{-}$ state was performed with a self-consistent, simultaneous fit of several high-resolution $\mathrm{^{12}C}$ excitation-energy spectra populated with direct reactions. The global analysis performed in this work yields a formal total width of $\Gamma(E_{r}) = 46(2)$ keV and an observed total width of $\Gamma_{\textrm{obs}}(E_{r}) = 38(2)$ keV for the $3_{1}^{-}$ state. Significant unaccounted-for uncertainties and/or formalism errors were discovered in the previous results employed in the ENSDF for the observed total width of the $3_{1}^{-}$ state. These previously reported widths are fundamentally different quantities, leading to an invalid ENSDF average. An observed total width of $\Gamma_{\textrm{obs}}(E_{r}) = 38(2)$ keV is recommended for the $3_{1}^{-}$ state in $\mathrm{^{12}C}$. This total width should be employed for future evaluations of the radiative width for the $3_{1}^{-}$ state and its contribution to the high-temperature triple-$\alpha$ reaction rate.

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., 14 pages and 10 figures

International audience; Photo-nuclear reactions of light nuclei below a mass of $A=60$ are studied experimentally and theoretically by the PANDORA (Photo-Absorption of Nuclei and Decay Observation for Reactions in Astrophysics) project. Two experimental methods, virtual-photon excitation by proton scattering and real-photo absorption by a high-brilliance gamma-ray beam produced by laser Compton scattering, will be applied to measure the photo-absorption cross sections and the decay branching ratio of each decay channel as a function of the photon energy. Several nuclear models, e.g. anti-symmetrized molecular dynamics, mean-field type models, a large-scale shell model, and ab initio models, will be employed to predict the photo-nuclear reactions. The uncertainty in the model predictions will be evaluated from the discrepancies between the model predictions and the experimental data. The data and the predictions will be implemented in a general reaction calculation code TALYS . The results will be applied to the simulation of the photo-disintegration process of ultra-high-energy cosmic rays in inter-galactic propagation.

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 performed239Pu(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 the239Pu(d,pγ)240Pu case.

The Nuclear Level Densities (NLDs) and the $\gamma$-ray Strength Functions ($\gamma$SFs) of $^{153,155}$Sm have been extracted from (d,p$\gamma$) 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 $\gamma$SFs of $^{153,155}$Sm are in fair agreement with QRPA calculations based on the D1M Gogny interaction. An enhancement is observed in the $\gamma$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 $\mu^{2}_{N}$ 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.

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 Nd-142,Nd-144-151 which exhibit such a crossover. These results represent the most complete data set of nuclear level densities to date 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 approximate to 150at an excitation energy of 6 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. (C) 2021 The Author(s). Published by Elsevier B.V.

The level density of quantum states in statistical mesoscopic systems is a critical input for various fields of physics, including nuclear physics, nuclear astrophysics, atomic physics and their applications. In atomic nuclei, the level density is a fundamental measure of their complex structure at relatively high energies. Here we present the first model-independent measurement of the absolute partial nuclear level density for a short-lived unstable nucleus. For this purpose, we introduce the ``Shape method'' to extract the shape of the $\gamma$-ray strength function. Combining the Shape method with the existing $\beta$-Oslo technique allows the extraction of the nuclear level density without the need for theoretical input. We benchmark the Shape method using results for the stable $^{76}$Ge nucleus, finding an excellent agreement to previous experimental results. We apply the Shape method to new experimental data on the short-lived $^{88}$Kr nucleus. Our method opens the door for measurements of the nuclear level density and $\gamma$-ray strength function far away from stability, a pivotal input required to understand the role of exotic nuclei in forming the cosmos.

The Shape method, a novel approach to obtain the functional form of the $��$-ray strength function ($��$SF) in the absence of neutron resonance spacing data, is introduced. When used in connection with the Oslo method the slope of the Nuclear Level Density (NLD) is obtained simultaneously. The foundation of the Shape method lies in the primary $��$-ray transitions which preserve information on the functional form of the $��$SF. The Shape method has been applied to $^{56}$Fe, $^{92}$Zr, $^{164}$Dy, and $^{240}$Pu, which are representative cases for the variety of situations encountered in typical NLD and $��$SF studies. The comparisons of results from the Shape method to those from the Oslo method demonstrate that the functional form of the $��$SF is retained regardless of nuclear structure details or $J^��$ values of the states fed by the primary transitions., 13 pages, 8 figures

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-$��$-$��$ triple coincidence data, a value of ${��_{\rm rad}}/��=6.2(6) \times 10^{-4}$ was obtained for the radiative branching ratio. Using our results, together with ${��_��^{E0}}/��$ from Eriksen et al., Phys. Rev. C 102, 024320 and the currently adopted $��_��(E0)$ values, the radiative width of the Hoyle state is determined as $��_{\rm rad}=5.1(6) \times 10^{-3}$ eV. This value is about 34% higher than the currently adopted value and will impact on models of stellar evolution and nucleosynthesis., 6 pages, 6 figures

This paper was published online on 29 January 2016 with an error in the author list. The ninth author's name should read as "F. L. Bello Garrote." The author's name has been corrected as of 5 September 2019. The author's name is incorrect in the printed version of the journal.

The Oslo method has been applied to particle-γ coincidences following the Pu239(d,p) reaction to obtain the nuclear level density (NLD) and γ-ray strength function (γSF) of Pu240. 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 γSF. 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 γSF shows a pronounced enhancement between 2-4 MeV that is consistent with the location of the low-energy orbital M1 scissors mode.

Photo-induced reaction cross section data are of importance for a variety of current or emerging applications, such as radiation shielding design and radiation transport analyses, calculations of absorbed dose in the human body during radiotherapy, physics and technology of fission reactors (influence of photo-reactions on neutron balance) and fusion reactors (plasma diagnostics and shielding), activation analyses, safeguards and inspection technologies, nuclear waste transmutation, medical isotope production and astrophysical applications. To address these data needs the IAEA Photonuclear Data library was produced in 2000, containing evaluated photo-induced cross sections and neutron spectra for 164 nuclides which were deemed relevant for the applications. Since the release of the IAEA Photonuclear Data Library however, new experimental data as well as new methods to assess the reliability of experimental cross sections have become available. Theoretical models and input parameters used to evaluate photo-induced reactions have improved significantly over the years. In addition, new measurements of partial photoneutron cross sections using mono-energetic photon beams and advanced neutron detection systems have been performed allowing for the validation of the evaluations and assessments of the experimental data. Furthermore, technological advances have led to the construction of new and more powerful gamma-beam facilities, therefore new data needs are emerging. We report our coordinated efforts to address these data needs and present the results of the new evaluations of more than 200 nuclides included in the new updated IAEA Photonuclear Data Library, where the photon energy goes up to 200 MeV. We discuss the new assessment method and make recommendations to the user community in cases where the experimental data are discrepant and the assessments disagree. In addition, in the absence of experimental data, we present model predictions for photo-induced reaction cross section on nuclides of potential interest to medical radioisotope production.

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

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 $\gamma$ 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.

The lifetimes of the 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 by mass and atomic number in the VAMOS++ spectrometer. Our result shows a very small B(E2;2_{1}^{+}→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 the coexistence of two additional deformed shapes in this nucleus along with the spherical ground state. ispartof: PHYSICAL REVIEW LETTERS vol:121 issue:19 pages:192501-192501 ispartof: location:United States status: published

A model-independent technique was used to determine the γ-ray strength function (γSF) of Fe56 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 γSF 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.

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 $��$-emission probability at very low $��$ energies. In this work, we present measurements of the $��$-decay strength of $^{70}$Ni over the wide range $1.3 \leq E_�� \leq 8 $ MeV. A significant enhancement is found in the $��$-decay strength for transitions with $E_��< 3$ 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 $E_��\simeq 5$ MeV very well. Moreover, large-scale shell-model calculations indicate an $M1$ nature of the low-energy $��$ 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 $^{72,74,76}\mathrm{Ni}$., 9 pages, 9 figures

A model-independent technique was used to determine the $��$-ray Strength Function ($��$SF) of $^{56}$Fe down to $��$-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 $��$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.

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

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 $��$-ray beams produced in laser Compton-scattering at the NewSUBARU synchrotron radiation facility. The new photoneutron data are used to extract the $��$-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 $��$-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., 9 pages, 7 figures, re-submitted to PRC

In this work, we present new data on the $^{89}$Y($\gamma$,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 $\gamma$-ray strength function below threshold obtained by applying the Oslo method to $^{89}$Y($p,p'\gamma$)$^{89}$Y coincidences combined with the $^{89}$Y($\gamma$,n) data this providing experimental data for the $\gamma$-ray strength function of $^{89}$Y for $\gamma$ energies in the range of $\approx 1.6$ Mev to $\approx$ 20 MeV. A low-energy enhancement is seen for $\gamma$-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 $\gamma$-ray strength function have been extracted from $^{89}$Y($d,p \gamma$)$^{90}$Y coincidences using the Oslo method. Using the ($\gamma,n$) and ($d,p\gamma$) data as experimental constraints, we have calculated the $^{89}$Y($n,\gamma$)$^{90}$Y cross section with the TALYS reaction code. Our results have been compared with directly measured (n,$\gamma$) 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,$\gamma)$ cross section is key to understanding how s-process stars produce heavy isotopes., Comment: 18 pages (including references) and 16 figures, to be submitted to PRC

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

The nuclear level density and $��$-decay strength of $^{192}$Os have been extracted using particle-$��$ coincidence data from the $^{192}$Os($��,��^\prime��$)$^{192}$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 $��$-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 $��$-strength function in the range $E_��\approx 1.5-6 $ MeV. Using the measured nuclear level density and $��$-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($n,��$)$^{192}$Os reaction relevant to $s$-process nucleosynthesis. The systematic uncertainties introduced by the normalization procedure of the level density and $��$-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, $\langle ��\rangle_{n,��}=1134\pm 375$ 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., 14 pages, 11 figures, accepted to Phys. Rev C

© 2017 American Physical Society. Prompt-fission γ rays are responsible for approximately 5% of the total energy released in fission, and therefore important to understand when modeling nuclear reactors. In this work we present prompt γ-ray emission characteristics in fission 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 U(d,pf)233 reaction for excitation energies between 4.8 and 10 MeV, and for the Pu(d,pf)239 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 ∼5 MeV of energy is potentially available in the excited fragments for γ decay. The measured results are compared with 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.

Prompt-fission γ rays are responsible for approximately 5% of the total energy released in fission, and therefore important to understand when modeling nuclear reactors. In this work we present prompt γ-ray emission characteristics in fission 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 U(d,pf)233 reaction for excitation energies between 4.8 and 10 MeV, and for the Pu(d,pf)239 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 ∼5 MeV of energy is potentially available in the excited fragments for γ decay. The measured results are compared with 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.

A program to study the γ-ray decay from the region of high-level density has been established at iThemba LABS, where a high-resolution gamma-ray detector array is used in conjunction with silicon particle-telescopes. Results from two recent projects are presented: 1) The 74Ge(α,α′γ) reaction was used to investigate the Pygmy Dipole Resonance. The results were compared to (γ,γ′) data and indicate that the dipole states split into mixed isospin and relatively pure isovector excitations. 2) Data from the 95Mo(d,p) reaction were used to develop a novel method for the determination of spins for low-lying discrete levels utilizing statistical γ-ray decay in the vicinity of the neutron separation energy. These results provide insight into the competition of (γ,n) and (γ,γ′) reactions and highlights the need to correct for angular momentum barrier effects.

One of the greatest questions for modern physics to address is how elements heavier than iron are created in extreme astrophysical environments. A particularly challenging part of that question is the creation of the so-called p-nuclei, which are believed to be mainly produced in some types of supernovae. The lack of needed nuclear data presents an obstacle in nailing down the precise site and astrophysical conditions. In this work, we present for the first time measurements on the nuclear level density and average γ strength function of Mo92. State-of-the-art p-process calculations systematically underestimate the observed solar abundance of this isotope. Our data provide stringent constraints on the Nb91(p,γ)Mo92 reaction rate, which is the last unmeasured reaction in the nucleosynthesis puzzle of Mo92. Based on our results, we conclude that the Mo92 abundance anomaly is not due to the nuclear physics input to astrophysical model calculations.

The γ-ray strength functions and level densities of Ge73,74 have been extracted up to the neutron-separation energy Sn from particle-γ coincidence data using the Oslo method. Moreover, the γ-ray strength function of Ge74 above Sn has been determined from photoneutron measurements; hence these two experiments cover the range of Eγ≈1-13 MeV for Ge74. The obtained data show that both Ge73,74 display an increase in strength at low γ energies. The experimental γ-ray strength functions are compared with M1 strength functions deduced from average B(M1) values calculated within the shell model for a large number of transitions. The observed low-energy enhancements in Ge73,74 are adopted in the calculations of the Ge72,73(n,γ) cross sections, where there are no direct experimental data. Calculated reaction rates for more neutron-rich germanium isotopes are shown to be strongly dependent on the presence of the low-energy enhancement.

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

The level density and γ-ray strength function (γSF) of Pu243 have been measured in the quasicontinuum using the Oslo method. Excited states in Pu243 were populated using the Pu242(d,p) reaction. The level density closely follows the constant-temperature level density formula for excitation energies above the pairing gap. The γSF displays a double-humped resonance at low energy as also seen in previous investigations of actinide isotopes. The structure is interpreted as the scissors resonance and has a centroid of ωSR=2.42(5) MeV and a total strength of BSR=10.1(15)μN2, which is in excellent agreement with sum-rule estimates. The measured level density and γSF were used to calculate the Pu242(n,γ) cross section in a neutron energy range for which there were previously no measured data.

The γ -ray strength functions and level densities of 73,74Ge have been extracted up to the neutron-separation energy Sn from particle-γ coincidence data using the Oslo method. Moreover, the γ -ray strength function of 74Ge above Sn has been determined from photoneutron measurements; hence these two experiments cover the range of Eγ ≈ 1–13 MeV for 74Ge. The obtained data show that both 73,74Ge display an increase in strength at low γ energies. The experimental γ -ray strength functions are compared with M1 strength functions deduced from average B(M1) values calculated within the shell model for a large number of transitions. The observed low-energy enhancements in 73,74Ge are adopted in the calculations of the 72,73Ge(n,γ ) cross sections, where there are no direct experimental data. Calculated reaction rates for more neutron-rich germanium isotopes are shown to be strongly dependent on the presence of the low-energy enhancement.

Neutron-rich 96;98Sr isotopes have been investigated by safe Coulomb excitation of radioactive beams at the REX-ISOLDE facility. Reduced transition probabilities and spectroscopic quadrupole moments have been extracted from the differential Coulomb excitation cross sections. These results allow, for the first time, the drawing of definite conclusions about the shape coexistence of highly deformed prolate and spherical configurations. In particular, a very small mixing between the coexisting states is observed, contrary to other mass regions where strong mixing is present. Experimental results have been compared to beyond-mean-field calculations using the Gogny D1S interaction in a five-dimensional collective Hamiltonian formalism, which reproduce the shape change at N ¼ 60.

© 2016 American Physical Society. The level density and γ-ray strength function (γSF) of Pu243 have been measured in the quasicontinuum using the Oslo method. Excited states in Pu243 were populated using the Pu242(d,p) reaction. The level density closely follows the constant-temperature level density formula for excitation energies above the pairing gap. The γSF displays a double-humped resonance at low energy as also seen in previous investigations of actinide isotopes. The structure is interpreted as the scissors resonance and has a centroid of ωSR=2.42(5) MeV and a total strength of BSR=10.1(15)μN2, which is in excellent agreement with sum-rule estimates. The measured level density and γSF were used to calculate the Pu242(n,γ) cross section in a neutron energy range for which there were previously no measured data.

Neutron-rich {96,98}Sr isotopes have been investigated by safe Coulomb excitation of radioactive beams at the REX-ISOLDE facility. Reduced transition probabilities and spectroscopic quadrupole moments have been extracted from the differential Coulomb excitation cross sections. These results allow, for the first time, the drawing of definite conclusions about the shape coexistence of highly deformed prolate and spherical configurations. In particular, a very small mixing between the coexisting states is observed, contrary to other mass regions where strong mixing is present. Experimental results have been compared to beyond-mean-field calculations using the Gogny D1S interaction in a five-dimensional collective Hamiltonian formalism, which reproduce the shape change at N=60.

The nuclear level density and the $��$-ray strength function have been extracted for $^{89}$Y, using the Oslo Method on $^{89}$Y($p,p' ��$)$^{89}$Y coincidence data. The $��$-ray strength function displays a low-energy enhancement consistent with previous observations in this mass region ($^{93-98}$Mo). Shell-model calculations give support that the observed enhancement is due to strong, low-energy $M1$ transitions at high excitation energies. The data were further used as input for calculations of the $^{88}$Sr($p,��$)$^{89}$Y and $^{88}$Y($n,��$)$^{89}$Y cross sections with the TALYS reaction code. Comparison with cross-section data, where available, as well as with values from the BRUSLIB library, shows a satisfying agreement., 12 pages, 10 figures, version accepted for publication in Phys. Rev. C

The level density and gamma-ray strength function (gammaSF) of 243Pu have been measured in the quasi-continuum using the Oslo method. Excited states in 243Pu were populated using the 242Pu(d,p) reaction. The level density closely follows the constant-temperature level density formula for excitation energies above the pairing gap. The gammaSF displays a double-humped resonance at low energy as also seen in previous investigations of actinide isotopes. The structure is interpreted as the scissors resonance and has a centroid of omega_{SR}=2.42(5)MeV and a total strength of B_{SR}=10.1(15)mu_N^2, which is in excellent agreement with sum-rule estimates. The measured level density and gammaSF were used to calculate the 242Pu(n,gamma) cross section in a neutron energy range for which there were previously no measured data., 9 pages, 8 figures

The $��$-ray strength functions and level densities of $^{73,74}$Ge have been extracted up to the neutron separation energy S$_n$ from particle-$��$ coincidence data using the Oslo method. Moreover, the $��$-ray strength function of $^{74}$Ge above S$_n$ has been determined from photo-neutron measurements, hence these two experiments cover the range of E$_��\approx$ 1-13 MeV for $^{74}$Ge. The obtained data show that both $^{73,74}$Ge display an increase in strength at low $��$ energies. The experimental $��$-ray strength functions are compared with $M1$ strength functions deduced from average $B(M1)$ values calculated within the shell model for a large number of transitions. The observed low-energy enhancements in $^{73,74}$Ge are adopted in the calculations of the $^{72,73}$Ge(n,$��$) cross sections, where there are no direct experimental data. Calculated reaction rates for more neutron-rich germanium isotopes are shown to be strongly dependent on the presence of the low-energy enhancement.

Our measurements of $^{195,196}$Pt $\gamma$-strength functions show a double-humped enhancement in the $E_{\gamma}= 4-8$ MeV region. For the first time, the detailed shape of these resonances is revealed for excitation energies in the quasicontinuum. We demonstrate that the corresponding neutron-capture cross sections and astrophysical reaction rates are increased by up to a factor of 2 when these newly observed pygmy resonances are included. These results lend credence to theoretical predictions of enhanced reaction rates due to such pygmy resonances and hence are important for a better understanding of r-process nucleosynthesis., Comment: 5 pages, 4 figures

The level densities and $��$-ray strength functions of $^{105,106,111,112}$Cd have been extracted from particle-$��$ coincidence data using the Oslo method. The level densities are in very good agreement with known levels at low excitation energy. The $��$-ray strength functions display no strong enhancement for low $��$ energies. However, more low-energy strength is apparent for $^{105,106}$Cd than for $^{111,112}$Cd. For $��$ energies above $\approx$ 4 MeV, there is evidence for some extra strength, similar to what has been previously observed for the Sn isotopes. The origin of this extra strength is unclear; it might be due to $E1$ and $M1$ transitions originating from neutron skin oscillations or the spin-flip resonance, respectively., 10 pages, 11 figures

The $��$-ray strength function of $^{56}$Fe has been measured from proton-$��$ coincidences for excitation energies up to $\approx 11$ MeV. The low-energy enhancement in the $��$-ray strength function, which was first discovered in the ($^3$He,$����$)$^{56}$Fe reaction, is confirmed with the ($p,p^\prime��$)$^{56}$Fe experiment reported here. Angular distributions of the $��$ rays give for the first time evidence that the enhancement is dominated by dipole transitions., 5 pages, 4 figures. Version accepted for publication in Phys. Rev. Letters

Primary gamma-ray spectra for a wide excitation-energy range have been extracted for 44Ti from particle-gamma coincidence data of the 46Ti(p,t gamma)44Ti reaction. These spectra reveal information on the gamma-decay pattern of the nucleus, and may be used to extract the level density and radiative strength function applying the Oslo method. Models of the level density and radiative strength function are used as input for cross-section calculations of the 40Ca(alpha,gamma)44Ti reaction. Acceptable models should reproduce data on the 40Ca(alpha,gamma)44Ti reaction cross section as well as the measured primary gamma-ray spectra. This is only achieved when a coherent normalization of the slope of the level density and radiative strength function is performed. Thus, the overall shape of the experimental primary gamma-ray spectra puts a constraint on the input models for the rate calculations., Comment: 14 pages, 13 figures