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DOE OSTI2011

Towards an improved evaluation of neutron-induced fission cross sections on actinides

Mean-field calculations can now provide all the nuclear ingredients required to describe the fission path from the equilibrium deformation up to the nuclear scission point. The information obtained from microscopic mean-field models has been included in the TALYS reaction code to improve the predictions of neutron-induced fission cross sections. The nuclear inputs concern not only the details of the energy surface along the fission path, but also the coherent estimate of the nuclear level density derived within the combinatorial approach on the basis of the same single-particle properties, in particular at the fission saddle points. The predictive power of such a microscopic approach is tested on the experimental data available for the uranium isotopic chain. It is also shown that the various inputs can be tuned to reproduce, at best, experimental data in one unique coherent framework, so that in a close future it should become possible to make, on the basis of such models, accurate fission-cross-section calculations and the corresponding estimates for nuclei, energy ranges, or reaction channels for which no data exist. Such model uncertainties are usually not taken into account in data evaluations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2011

Gamma-ray strength functions and their relation to astrophysics

The nuclear {gamma}-ray strength function is one of the indispensable inputs needed for reaction-rate calculations, and is particularly important for the neutron-capture cross section. The nuclear physics group at the Oslo Cyclotron Laboratory has developed a method to extract simultaneously nuclear level density and {gamma}-ray strength function from particle-{gamma} coincidence measurements. Data on the strength functions of Sn nuclei as well as for lighter elements are presented. The Sn isotopes all display a resonance-like structure close to the neutron threshold, that could possibly be due to the neutron-skin oscillation mode. This so-called pygmy dipole resonance greatly influences the neutron-capture rates. In the lighter nuclei, an enhancement of the strength function at low {gamma} energies is observed. The possible impact of this increase on Maxwellian-averaged reaction rates has been investigated.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2017

Experimental and Theoretical Understanding of Neutron Capture on Uranium Isotopes

Neutron capture cross sections on uranium isotopes are important quantities needed to model nuclear explosion performance, nuclear reactor design, nuclear test diagnostics, and nuclear forensics. It has been difficult to calculate capture accurately, and factors of 2 or more be- tween calculation and measurements are not uncommon, although normalization to measurements of the average capture width and nuclear level density can improve the result. The calculations of capture for 233,235,237,239 U are further complicated by the need to accurately include the fission channel.

11 NUCLEAR FUEL CYCLE AND FUEL MATERIALS↗

DOE OSTI2017

Modeled Neutron and Charged-Particle Induced Nuclear Reaction Cross Sections and their Uncertainties in the Region 37 < A < 47 (Rb-Ag)

A modeling effort to calculate nuclear reaction cross sections for use in radiochemical di-agnostics is presented. Systematics for the input parameters required by the Hauser-Feshbachstatistical model were developed and used to calculate neutron, proton, and deuteron inducednuclear reaction cross sections for targets ranging from Rubidium (Z= 37) to Silver (Z= 47).This is an expansion of the previous modeling effort described in Hoffman et. al. 2006(UCRL-TR-222275) that allows us to now calculate all new cross sections for radiochemical detector sets Rb, Rh, and Ag. We have also developed new systematics affecting our treatments of the nuclear level density and the gamma-ray strength function. Coupled with a range of the main parameter that affects pre-equilibrium, we develop upper and lower bounds to our recommended cross sections

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2016

On microscopic theory of radiative nuclear reaction characteristics

A survey of some results in the modern microscopic theory of properties of nuclear reactions with gamma rays is given. First of all, we discuss the impact of Phonon Coupling (PC) on the Photon Strength Function (PSF) because it represents the most natural physical source of additional strength found for Sn isotopes in recent experiments that could not be explained within the standard HFB + QRPA approach. The self-consistent version of the Extended Theory of Finite Fermi Systems in the Quasiparticle Time Blocking Approximation is applied. It uses the HFB mean field and includes both the QRPA and PC effects on the basis of the SLy4 Skyrme force. With our microscopic E1 PSFs, the following properties have been calculated for many stable and unstable even–even semi-magic Sn and Ni isotopes as well as for double-magic {sup 132}Sn and {sup 208}Pb using the reaction codes EMPIRE and TALYS with several Nuclear Level Density (NLD) models: (1) the neutron capture cross sections; (2) the corresponding neutron capture gamma spectra; (3) the average radiative widths of neutron resonances. In all the properties considered, the PC contribution turned out to be significant, as compared with the standard QRPA one, and necessary to explain the available experimental data. The results with the phenomenological so-called generalized superfluid NLD model turned out to be worse, on the whole, than those obtained with the microscopic HFB + combinatorial NLD model. The very topical question about the M1 resonance contribution to PSFs is also discussed.Finally, we also discuss the modern microscopic NLD models based on the self-consistent HFB method and show their relevance to explain the experimental data as compared with the phenomenological models. The use of these self-consistent microscopic approaches is of particular relevance for nuclear astrophysics, but also for the study of double-magic nuclei.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2017

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, which is in support of the generalized Brink–Axel hypothesis.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2016

Completing the nuclear reaction puzzle of the nucleosynthesis of Mo 92

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. Here, 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 92 Mo. State-of-the-art p-process calculations systematically underestimate the observed solar abundance of this isotope. Our data provide stringent constraints on the 91 Nb(p,γ) 92 Mo reaction rate, which is the last unmeasured reaction in the nucleosynthesis puzzle of 92 Mo. Based on our results, we conclude that the 92 Mo abundance anomaly is not due to the nuclear physics input to astrophysical model calculations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2011

Evolution of the pygmy dipole resonance in Sn isotopes

Nuclear level density and {gamma}-ray strength functions of {sup 121,122}Sn below the neutron separation energy are extracted with the Oslo method using the ({sup 3}He,{sup 3}He{sup '{gamma}}) and ({sup 3}He,{alpha}{gamma}) reactions. The level densities of {sup 121,122}Sn display steplike structures, interpreted as signatures of neutron pair breaking. An enhancement in both strength functions, compared to standard models for radiative strength, is observed in our measurements for E{sub {gamma}} > or approx. 5.2 MeV. This enhancement is compatible with pygmy resonances centered at {approx_equal}8.4(1) and {approx_equal}8.6(2) MeV, respectively, and with integrated strengths corresponding to {approx_equal}1.8{sub -5}{sup +1}% of the classical Thomas-Reiche-Kuhn sum rule. Similar resonances were also seen in {sup 116-119}Sn. Experimental neutron-capture cross reactions are well reproduced by our pygmy resonance predictions, while standard strength models are less successful. The evolution as a function of neutron number of the pygmy resonance in {sup 116-122}Sn is described as a clear increase of centroid energy from 8.0(1) to 8.6(2) MeV, but with no observable difference in integrated strengths.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2023

Cross-section measurement of the 82 Kr ($p,γ$) 83 Rb reaction in inverse kinematics

The total cross section of the 82 Kr(p,γ) 83 Rb reaction was measured for the first time at effective center-of-mass energies between 2.4 and 3.0 MeV, within the relevant Gamow window for the astrophysical γ process. The experiment took place at the National Superconducting Cyclotron Laboratory at Michigan State University using the ReA facility. A 82 Kr beam was directed onto a hydrogen gas cell located at the center of the Summing NaI(Tl) (SuN) detector. Here, the obtained spectra were analyzed using the γ-summing technique and the extracted cross section was compared to standard statistical model calculations using the non-smoker and talys codes. The comparison indicates that standard statistical model calculations tend to overproduce the cross section of the 82 Kr(p,γ) 83 Rb reaction relative to the experimentally measured values. Furthermore, the experimental data were used to provide additional constraints on the nuclear level density and the γ-ray strength function used in the statistical model calculations.

59 ≤ A ≤ 89↗

DOE OSTI2022

Constraining the nuclear spin distribution using improved 197 Au neutron resonance parameters

New neutron transmission data at resonance energies using a 197 Au sample were measured using an early version of the Device for Indirect Capture Experiments on Radionuclides (DICER), which is under development at the Los Alamos Neutron Science Center (LANSCE). These data were combined with previous neutron transmission and capture data in a simultaneous R-matrix analysis to extract improved neutron resonance parameters for this nuclide. As a result, total radiation widths, Γ γ , were obtained for 33 J=1 and 44 J=2 197 Au+n resonances. Γ γ distributions for these two spins states were compared to distributions calculated according to the nuclear statistical model using published nuclear level density (NLD) and photon strength functions (PSF) measured using the Oslo technique. The calculated distributions were found to be narrower and the average values for the two spins states closer together than the data. The calculation can be brought into agreement with the data by substantial modifications to the spin distribution in 198 Au as a function of excitation energy. As far as we know, the spin distribution currently is otherwise poorly constrained. The modified spin distribution changes the shapes of the NLD and PSF extracted using the Oslo technique and so could have broad implications.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2024

Using DAPPER to extract the photon strength function of 58Fe using the inverse Oslo and shape methods

The photon strength function of 58 Fe has been extracted using both the Oslo and Shape methods from particle-γ coincidence data measured using the Detector Array for Photons, Protons, and Exotic Residues, which probes nuclei utilizing (d,p) reactions in inverse kinematics. Four particle-γ coincidence matrices, each constructed with different treatments of the γ-ray energies, are explored in order to observe the impact on the resulting nuclear level density and photon strength. The final photon strength function reported is found to agree well with previous Oslo measurements of other iron isotopes. Systematic uncertainties are included, using different model parameters and their reported errors to perform the Oslo method normalization. The model-independent Shape method is explored and the functional form of the photon strength function obtained is in agreement with the Oslo method results. A low-energy enhancement is not reported for 58 Fe in this work given possible subtraction issues originating from strongly populated states.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2019

Experimental constraints on the Zn 73 ( n , γ ) Zn 74 reaction rate

Background: The recent observation of a neutron-star merger finally confirmed one astrophysical location of the rapid neutron-capture process (r-process). Evidence of the production of A < 140 nuclei was seen, but there is still little detailed information about how those lighter elements are produced in such an environment. Many of the questions surrounding the A ≈ 80 nuclei are likely to be answered only when the nuclear physics involved in the production of r-process nuclei is well understood. Neutron-capture reactions are an important component of the r-process, and neutron-capture cross sections of r-process nuclei, which are very neutron rich, have large uncertainties. Purpose: Indirectly determine the neutron-capture cross section and reaction rate of 73 Zn(n,γ ) 74 Zn. Methods: The nuclear level density (NLD) and γ-ray strength function (γSF) of 74 Zn were determined following a total absorption spectroscopy (TAS) experiment focused on the β decay of 74 Cu into 74 Zn performed at the National Superconducting Cyclotron Laboratory. The NLD and γSF were used as inputs in a Hauser-Feshbach statistical model to calculate the neutron-capture cross section and reaction rate. Results: The NLD and γSF of 74 Zn were experimentally constrained for the first time using β-delayed γ rays measured with TAS and the β-Oslo method. The NLD and γSF were then used to constrain the neutron-capture cross section and reaction rate for the 73 Zn(n, γ ) 74 Zn reaction. In Conclusion, the uncertainty in the neutron-capture cross section and reaction rate of 73 Zn(n, γ ) 74 Zn calculated in TALYS was reduced to under a factor of 2 from a factor of 5 in the cross section and a factor of 11 in the reaction rate using the experimentally obtained NLD and γSF.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2023

β -delayed neutron emissions from N > 50 gallium isotopes

β-delayed γ-neutron spectroscopy has been performed on the decay of A = 84 to 87 gallium isotopes at the RI-beam Factory at the RIKEN Nishina Center using a high-efficiency array of 3 He neutron counters (BRIKEN). β-2n-γ events were measured in the decays of all of the four isotopes for the first time, which is direct evidence for populating the excited states of two-neutron daughter nuclei. Detailed decay schemes with the γ branching ratios were obtained for these isotopes, and the neutron emission probabilities (P xn ) were updated from the previous study. Hauser-Feshbach statistical model calculations were performed to understand the experimental branching ratios. We found that the P 1n and P 2n values are sensitive to the nuclear level densities of 1n daughter nuclei and showed that the statistical model reproduced the P 2n /P 1n ratio better when experimental levels plus shell-model level densities fit by the Gilbert-Cameron formula were used as the level-density input. We also showed the neutron and γ branching ratios are sensitive to the ground-state spin of the parent nucleus. Our statistical model analysis suggested J ≤ 3 for the unknown ground-state spin of the odd-odd nucleus 86 Ga, from the I γ (4 + → 2 + )/I γ (2 + → 0 + ) ratio of 84 Ga and the P 2n /P 1n ratio. In conclusion, these results show the necessity of detailed understanding of the decay scheme, including data from neutron spectroscopy, in addition to γ measurements of the multineutron emitters.

46 INSTRUMENTATION RELATED TO NUCLEAR SCIENCE AND ↗

DOE OSTI2025

Indirect measurement of the 90 Sr ⁢(𝑛, 𝛾)⁢ 91 Sr reaction cross section and the implications for astrophysical Zr production

Here, the intermediate neutron-capture process (𝑖 process) has gained notable traction within the past decade as a way to describe stellar abundance observations which cannot be explained by the slow and rapid neutron-capture processes. Despite the general success of 𝑖-process models, many open questions remain. Among the observations, Zr stands out, as its elemental abundance is difficult to replicate with available 𝑖-process models, while the reactions that affect its production through the 𝑖 process are close enough to stability to study experimentally. Here, we present the experimental constraint of the nuclear level density and 𝛾-strength function (𝛾 SF) of 91 Sr using the 𝛽-Oslo method, which were then input into the TALYS Hauser-Feshbach code to produce the first experimental constraint of the 90 Sr ⁢(𝑛, 𝛾)⁢ 91 Sr capture reaction. This constraint was used alongside that of 92 Sr ⁢(𝑛, 𝛾)⁢ 93 Sr for a reduction in the uncertainty of [Y/Zr] production in the 𝑖-process relevant environmental neutron density of 10 13.5 and 10 14.5 neutrons/cm 3 .

Physics - Nuclear physics and radiation physics↗

DOE OSTI2025

First Measurement of the Neutron-Emission Probability with a Surrogate Reaction in Inverse Kinematics at a Heavy-Ion Storage Ring

Neutron-induced reaction cross sections of short-lived nuclei are imperative to understand the origin of heavy elements in stellar nucleosynthesis and for societal applications, but their measurement is extremely complicated due to the radioactivity of the targets involved. One way of overcoming this issue is to combine surrogate reactions with the unique possibilities offered by heavy-ion storage rings. In this work, we describe the first surrogate-reaction experiment in inverse kinematics, which we successfully conducted at the Experimental Storage Ring (ESR) of the GSI/FAIR facility, using the 208 Pb⁢(p,p') reaction as a surrogate for neutron capture on 207 Pb. Thanks to the outstanding detection efficiencies possible at the ESR, we were able to measure for the first time the neutron-emission probability as a function of the excitation energy of 208 Pb. We have used this probability to select different descriptions of the γ -ray strength function and nuclear level density, and provide reliable results for the neutron-induced radiative capture cross section of 207 Pb at energies for which no experimental data exist.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗

DOE OSTI2019

Level Densities of 74,76 Ge from compound nuclear reactions

The level densities of 74,76 Ge nuclei are studied with 68,70 Zn( 7 Li , Xp ) reactions. Proton evaporation spectra are measured at backward angles in a wide energy region, from about 2 to 25 MeV. The analysis of spectra allows for the testing of level density models used in modern reaction codes for practical cross-section calculations. Our results show that at excitation energies above the discrete level region, all level density models tested in this work overestimate the level densities that are needed to reproduce proton spectra from these reactions. The Gilbert and Cameron model, which includes the constant-temperature energy dependence of the level density, shows the best agreement with experiment, however, its parameters need to be adjusted to reflect the observed reduction of the level density at higher excitation energies.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2022

Dissipative Reactions with Intermediate-Energy Beams: A Novel Approach to Populate Complex-Structure States in Rare Isotopes

A novel pathway for the formation of multiparticle-multihole excited states in rare isotopes is reported from highly energy- and momentum-dissipative inelastic-scattering events measured in reactions of an intermediate-energy beam of 38 Ca on a Be target. The negative-parity, complex-structure final states in 38 Ca are observed following the in-beam γ-ray spectroscopy of events in the 9 Be( 38 Ca, 38 Ca + γ)X reaction in which the scattered projectile loses longitudinal momentum of order Δp || = 700 MeV/c. Here, the characteristics of the observed final states are discussed and found to be consistent with the formation of excited states involving the rearrangement of multiple nucleons in a single, highly energetic projectile-target collision. Unlike the far-less-dissipative, surface-grazing reactions usually exploited for the in-beam γ-ray spectroscopy of rare isotopes, these more energetic collisions appear to offer a practical pathway to nuclear-structure studies of more complex multiparticle configurations in rare isotopes—final states conventionally thought to be out of reach with high-luminosity fast-beam-induced reactions.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗

DOE OSTI2022

Exploiting dissipative reactions to perform in-beam γ-ray spectroscopy of the neutron-deficient isotopes 38,39 Ca

The neutron-deficient Ca isotopes continue to attract attention due to their importance for testing isospin symmetry and their relevance in capture reactions of interest for nova nucleosynthesis and the shape of light curves in Type I x-ray bursts. To date, spectroscopic information on 38,39 Ca is largely limited to data on lower-spin excited states. Here, we report in-beam γ-ray spectroscopy of complementary higher-spin, complex-structure states in 39 Ca populated in fast-beam-induced, momentum-dissipative processes leading to neutron pickup onto excited configurations of the projectile, 9 Be( 38 Ca*, 39 Ca + γ)X. Such a dissipative reaction was recently characterized for the case of inelastic scattering of 38 Ca off 9 Be, 9 Be( 38 Ca, 38 Ca + γ)X. Additional data and discussion on the nuclear structure of 38 Ca is also presented. Furthermore, an explanation for the more-complex-structure states, populated with small cross sections in one-nucleon knockout reactions, and observed in the tails of their longitudinal momentum distributions, is also offered.

20 ≤ A ≤ 38↗