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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 19 records
All sourcesNASA NTRSDOE OSTI
DOE OSTI2025

Electric dipole excitations near the neutron separation energies in 96 Mo

Electric dipole strength near the neutron separation energy significantly impacts nuclear structure properties and astrophysical scenarios. These excitations are complex in nature and may involve the so-called pygmy dipole resonance (PDR). Transition densities play a crucial role in understanding the nature of nuclear excited states, including collective excitations, as well as in constructing transition potentials in DWBA or coupled-channels equations. In this work, we focus on electric dipole excitations in spherical molybdenum isotopes, particularly 96 Mo, employing fully consistent Hartree-Fock-Bogoliubov (HFB) and Quasiparticle Random Phase Approximation (QRPA) methods. We analyze the dipole strength near the neutron separation energy, which represents the threshold for neutron capture processes, and examine the isospin characteristics of PDR states through transition density calculations. Examination of proton and neutron transition densities reveals distinctive features of each dipole state, indicating their isoscalar and isovector nature. We observe that the primary component in the enhanced low-energy region exhibits isovector character. The PDR displays a mixture of isoscalar and isovector nature, distinguishing it from the isovector giant dipole resonance (IVGDR). These findings lay the groundwork for future investigations into the role of transition densities in reaction models and for their application to inelastic scattering calculations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2015

Radiative thermal neutron-capture cross sections for the 180 W($n,\gamma$) reaction and determination of the neutron-separation energy

Prompt thermal neutron-capture partial $\gamma$-ray production cross sections were measured for the first time for the 180 W($n; \gamma$) reaction using a cold guided-neutron beam at the Budapest Research Reactor. Absolute 181 W $\gamma$-ray cross sections were internally standardized using well-known comparator $\gamma$-ray cross sections belonging to the other tungsten isotopes present in the 11.35%-enriched 180 W sample. Transitions were assigned to levels in 181 W based largely upon information available in the literature. The total radiative thermal neutron-capture cross section $\sigma$0 was determined from the sum of direct prompt $\gamma$-ray cross sections populating the ground state and a modeled contribution accounting for ground-state feeding from the quasi continuum. In this work, we find $\sigma$0 = 21.67(77) b. A new measurement of the cross section for the 5/2 - metastable isomer at 365.6 keV, $\sigma_{5/2-}(^{181}W^m; 14.6 \mu$s) = 19.96(55) b, is also determined. Additionally, primary $\gamma$ rays, observed for the first time in the 180 W($n,\gamma$) reaction, provide the most precise determination for the 181 W neutron-separation energy, $S_n$ = 6669.02(16) keV.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2019

Most theoretical approaches used in nuclear astrophysics to model the nucleosynthesis of heavy elements incorporate the so-called statistical model in order to describe the excitation and decay properties of atomic nuclei. One of the basic assumptions of this model is the validity of the Brink–Axel hypothesis and the related concept of so-called photon strength functions to describe γ-ray transition probabilities. We present a novel experimental approach that allows for the first time to experimentally determine the photon strength function simultaneously in two independent ways by a unique combination of quasi-monochromatic photon beams and a newly implemented γ–γ coincidence setup. This technique does not assume a priori the validity of the Brink–Axel hypothesis and sets a benchmark in terms of the detection sensitivity for measuring decay properties of photo-excited states below the neutron separation energy. The data for the spherical off-shell nucleus 128 Te were obtained for γ-ray beam-energy settings between 3 MeV and 9 MeV in steps of 130 keV for the lower beam energies and in steps of up to 280 keV for the highest beam settings. We present a quantitative analysis on the consistency of the derived photon strength function with the Brink–Axel hypothesis. The data clearly demonstrate a discrepancy of up to a factor of two between the photon strength functions extracted from the photoabsorption and photon emission process, respectively. In addition, we observe that the photon strength functions are not independent of the excitation energy, as usually assumed. Thus, we conclude, that the Brink–Axel hypothesis is not strictly fulfilled in the excitation-energy region below the neutron separation threshold (S n = 8.78 MeV) for the studied case of 128 Te.

Not Available

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2010

Dipole strength in {sup 139}La below the neutron-separation energy

The {gamma}-ray strength function is an important input quantity for the determination of the photoreaction rate and the neutron capture rate for astrophysics as well as for nuclear technologies. To test model predictions, the photoabsorption cross section of {sup 139}La up to the neutron-separation energy was measured using bremsstrahlung produced at the electron accelerator ELBE of Forschungszentrum Dresden-Rossendorf with an electron beam of 11.5 MeV kinetic energy. The experimental data were analyzed by applying Monte Carlo simulations of {gamma}-ray cascades to obtain the intensities of the ground-state transitions and their branching ratios. We found a significant enhancement of electric dipole strength in the energy range from 6 to 10 MeV that may be related with a pygmy dipole resonance. The present data are combined with photoneutron cross sections for {sup 139}La and compared with results of calculations on the basis of a quasiparticle-random-phase approximation using an instantaneous-shape sampling.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2016

Dipole response of the odd-proton nucleus 205 Tl up to the neutron-separation energy

The low-lying electromagnetic dipole strength of the odd-proton nuclide 205 Tl has been investigated up to the neutron separation energy exploiting the method of nuclear resonance fluorescence. In total, 61 levels of 205 Tl have been identified. Lastly, the measured strength distribution of 205 Tl were discussed and compared to those of even–even and even–odd mass nuclei in the same mass region as well as to calculations that have been performed within the quasi-particle phonon model.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Event-by-Event Evaluation of the Prompt Fission Neutron Spectrum From 239 Pu(n,f)

We have developed an improved evaluation method for the spectrum of neutrons emitted in fission of 239 Pu induced by incident neutrons with energies up to 20 MeV. The <v> covariance data, including incident energy correlations introduced by the <v> evaluation method, were used to fix the input parameters in our event-by-event model of fission, FREYA, by applying formal statistical methods. Formal estimates of uncertainties in the evaluation were developed by randomly sampling model inputs and calculating likelihood functions based on agreement with the evaluated <v>. Our approach is able to employ a greater variety of fission measurements than the relatively coarse spectral data alone. It also allows the study of numerous fission observables for more accurate model validation. The combination of an event-by-event Monte Carlo fission model with a statistical-likelihood analysis is thus a powerful tool for evaluation of fission-neutron data. Our empirical model FREYA follows the complete fission event from birth of the excited fragments through their decay via neutron emission until the fragment excitation energy is below the neutron separation energy when neutron emission can no longer occur. The most recent version of FREYA incorporates pre-equilibrium neutron emission, the emission of the first neutron before equilibrium is reached in the compound nucleus, and multi-chance fission, neutron evaporation prior to fission when the incident neutron energy is above the neutron separation energy. Energy, momentum, charge and mass number are conserved throughout the fission process. The best available values of fragment masses and total kinetic energies are used as inputs to FREYA. We fit three parameters that are not well under control from previous measurements: the shift in the total fragment kinetic energy; the energy scale of the asymptotic level density parameter, controlling the fragment 'temperature' for neutron evaporation; and the relative excitation of the light and heavy fragments, governing the number and energy of neutrons emitted from each fragment. The latter two parameters are assumed to be independent of the incident neutron energy while the first varies with incident energy. We describe our method and the subsequent spectral evaluation and present the results of several standard validation calculations that test our new evaluation. These benchmarks include critical assemblies, sensitive to criticality in fast systems; pulsed sphere measurements testing the spectra at incident neutron energies of 14 MeV; and other tests.

22 GENERAL STUDIES OF NUCLEAR REACTORS↗
DOE OSTI2021

First β-decay spectroscopy of 135 In and new β-decay branches of 134 In

The β decay of the neutron-rich 134 In and 135 In was investigated experimentally in order to provide new insights into the nuclear structure of the tin isotopes with magic proton number Z=50 above the N=82 shell. The β-delayed γ-ray spectroscopy measurement was performed at the ISOLDE facility at CERN, where indium isotopes were selectively laser-ionized and on-line mass separated. Three β-decay branches of 134 In were established, two of which were observed for the first time. Population of neutron-unbound states decaying via γ rays was identified in the two daughter nuclei of 134 In, 134 Sn and 133 Sn, at excitation energies exceeding the neutron separation energy by 1 MeV. The β-delayed one- and two-neutron emission branching ratios of 134 In were determined and compared with theoretical calculations. The β-delayed one-neutron decay was observed to be dominant β-decay branch of 134 In even though the Gamow-Teller resonance is located substantially above the two-neutron separation energy of 134 Sn. Transitions following the β decay of 135 In are reported for the first time, including γ rays tentatively attributed to 135 Sn. In total, six new levels were identified in 134 Sn on the basis of the βγγ coincidences observed in the 134 In and 135 Inβ decays. A transition that might be a candidate for deexciting the missing neutron single-particle 13/2 + state in 133 Sn was observed in both β decays and its assignment is discussed. Experimental level schemes of 134 Sn and 135 Sn are compared with shell-model predictions. Using the fast timing technique, half-lives of the 2 + , 4 + , and 6 + levels in 134 Sn were determined. From the lifetime of the 4 + state measured for the first time, an unexpectedly large B(E2;4 + → 2 + ) transition strength was deduced, which is not reproduced by the shell-model calculations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

(n,{gamma}) Experiments on tin isotopes

Neutron capture experiments on highly enriched {sup 117,119}Sn isotopes were performed with the DANCE detector array located at the Los Alamos Neutron Science Center. The DANCE detector provides detailed information about the multi-step {gamma}-ray cascade following neutron capture. Analysis of the experimental data provides important information to improve understanding of the neutron capture reaction, including a test of the statistical model, the assignment of spins and parities of neutron resonances, and information concerning the Photon Strength Function (PSF) and Level Density (LD) below the neutron separation energy. Preliminary results for the (n,{gamma}) reaction on {sup 117,119}Sn are presented. Resonance spins of the odd-A tin isotopes were almost completely unknown. Resonance spins and parities have been assigned via analysis of the multi-step {gamma}-ray spectra and directional correlations.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2010

Photoexcitation of astrophysically important states in {sup 26}Mg. II. Ground-state-transition partial widths

The level structure of {sup 26}Mg near the neutron-separation energy, which is of interest for s-process nucleosynthesis, was studied at the High Intensity Gamma-Ray Source of the Triangle Universities Nuclear Laboratory using the method of nuclear resonance fluorescence. A nearly monoenergetic and linearly polarized {gamma}-ray beam was used to scan the excitation energy range from 10.5 to 11.7 MeV. For the five states observed, the total widths and partial widths are determined. Precise measurement of these widths is necessary for the prediction of neutron production for the s-process.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Neutron-unbound states in {sup 25,26}F

Neutron-unbound states in {sup 25}F and {sup 26}F were populated via the reactions {sup 9}Be({sup 26}Ne,{sup 24}F + n) and {sup 9}Be({sup 26}Ne,{sup 25}F + n), respectively. A resonance close to the neutron separation energy in {sup 25}F was identified with a decay energy of 28{+-}4 keV. This resonance corresponds to an excited state in {sup 25}F at 4249{+-}116 keV assuming it decays to the ground state of {sup 24}F. Guided by shell-model calculations, a spin and parity of 1/2{sup -} can be assigned to this state. In the spectrum of {sup 26}F, which was produced in a nucleon-exchange reaction, there are indications for an excited state with a decay energy of {approx}270 keV.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Halo structure of 17 C

17 C has three states below the 16 C + n threshold with quantum numbers \(J^{P}=3/2^{+}, 1/2^{+}, 5/2^{+}\) . These states have relatively small neutron separation energies compared to the neutron separation and excitation energies of 16 C. This separation of scales motivates our investigation of 17 C in a halo effective field theory (Halo EFT) with a 16 C core and a valence neutron as degrees of freedom. In this paper, we discuss various properties of the three states such as electric radii, magnetic moments, electromagnetic transition rates and capture cross sections. In particular, we give predictions for the charge radius and the magnetic moment of the \( 1/2^{+}\) state and for neutron capture on 16 C into this state. Furthermore, we discuss the predictive power of the Halo EFT approach for the \( 3/2^{+}\) and \( 5/2^{+}\) states which are described by a neutron in a D -wave relative to the core.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2024

Mass measurements of neutron-rich nuclei near $N = 70$

The astrophysical origin for the chemical elements between the first and second r-process peaks is a matter of intense debate, with a number of nucleosynthesis processes at explosive stellar environments possibly contributing to their production. Reliable data on the trends of neutron separation energies of neutron-rich isotopes are required to model neutron-capture processes that would produce these elements. Masses of 104 Y, 106 Zr, 112 Mo, and 115 Tc have been measured with the time-of-flight-magnetic-rigidity (ToF–Bρ) technique at the National Superconducting Cyclotron Laboratory at Michigan State University. The experiment is the first application of the ToF–Bρ technique at the S800 spectrograph that reached the mass region relevant to heavy-element nucleosynthesis. Finally, the two-neutron separation energy deduced from the measured masses exhibits a smooth trend consistent with the theoretical predictions within the range of experimental uncertainty, indicating that there is no sudden shape transition in these isotopes as hinted at by previous data.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Signatures of structural changes in neutron-rich Sr, Zr and Mo isotopes

The evolution of the ground-state nuclear shapes in neutron-rich Sr, Zr, and Mo isotopes, including both even-even and odd-A nuclei, is discussed within the mean-field approximation based on the Gogny-D1S Gogny Energy Density Functional. Calculations for neutron separation energies and charge radii are presented and compared with available data. A correlation between a shape transition and a discontinuity in those observables is found within our microscopic framework. It is shown that, while in Sr and Zr isotopes the steep behavior observed in the isotopic dependence of the charge radii is a consequence of a sharp prolate-oblate transition, the smooth behavior found in Mo isotopes has its origin in an emergent region of triaxiality.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2026

From closed shells to open shells: Coupled-cluster calculations of atomic nuclei

Coupled-cluster theory is a powerful tool for first-principles calculations of atomic nuclei, enabling accurate predictions of nuclear observables across the Segrè chart. While coupled-cluster computations are especially efficient at shell closures, extensions have been developed to tackle open-shell nuclei, by exploiting the equation-of-motion method or by expanding the coupled-cluster wave function on top of a symmetry-breaking (either deformed or superfluid) reference state. In this study, we provide a comprehensive comparison of these different formulations applied to the calcium and nickel isotopes using nuclear two-and three-body interactions from chiral effective field theory. Here, based on ground-state energies, two-neutron separation energies, and two-neutron shell gaps, different coupled-cluster computations—based on symmetry-broken reference states and equationof-motion techniques— offer consistent descriptions of bulk properties across medium-mass isotopic chains.

Marino, Francesco [Johannes Gutenberg-Universität ↗