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Results for “NEUTRON-NEUTRON INTERACTIONS”

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 37 records · Page 2
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DOE OSTI2011

Quantum Monte Carlo study of dilute neutron matter at finite temperatures

We report results of fully nonperturbative, path integral Monte Carlo calculations for dilute neutron matter. The neutron-neutron interaction in the s channel is parameterized by the scattering length and the effective range. We calculate the energy and the chemical potential as a function of temperature at density {rho}=0.003 fm{sup -3}. The critical temperature T{sub c} for the superfluid-normal phase transition is estimated from the finite size scaling of the condensate fraction. At low temperatures we extract the spectral weight function A(p,{omega}) from the imaginary time propagator using the methods of maximum entropy and singular value decomposition. We determine the quasiparticle spectrum, which can be accurately parameterized by three parameters: an effective mass m{sup *}, a mean-field potential U, and a gap {Delta}. Large values of {Delta}/T{sub c} indicate that the system is not a BCS-type superfluid at low temperatures.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

The nn quasifree nd breakup cross section: Discrepancies with theory and implications for the {sup 1}S{sub 0} nn force

Large discrepancies between quasifree neutron-neutron (nn) cross section data from neutron-deuteron (nd) breakup and theoretical predictions based on standard nucleon-nucleon (NN) and three-nucleon (3N) forces are pointed out. The nn {sup 1}S{sub 0} interaction is shown to be dominant in that configuration and has to be increased to bring theory and data into agreement. Using the next-to-leading order {sup 1}S{sub 0} interaction of chiral perturbation theory, we demonstrate that the nn quasifree scattering cross section depends only slightly on changes of the nn scattering length but is very sensitive to variations of the effective range parameter. In order to account for the reported discrepancies one must decrease the nn effective range parameter by {approx_equal}12% from its value implied by charge symmetry and charge independence of nuclear forces.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Heavy-particle formalism with Foldy-Wouthuysen representation

Utilizing the Foldy-Wouthuysen representation, we use a bottom-up approach to construct heavy-baryon Lagrangian terms, without employing a relativistic Lagrangian as the starting point. The couplings obtained this way feature a straightforward 1/m expansion, which ensures Lorentz invariance order by order in effective field theories. We illustrate possible applications with two examples in the context of chiral effective field theory: the pion-nucleon coupling, which reproduces the results in the literature, and the pion-nucleon-delta coupling, which does not employ the Rarita-Schwinger field for describing the delta isobar, and hence does not invoke any spurious degrees of freedom. In particular, we point out that one of the subleading {pi}N{Delta} couplings used in the literature is, in fact, redundant and we discuss the implications of this. We also show that this redundant term should be dropped if one wants to use low-energy constants fitted from {pi}N scattering in calculations of NN{yields}NN{pi} reactions.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2011

Counting the number of correlated pairs in a nucleus

We suggest that the number of correlated nucleon pairs in an arbitrary nucleus can be estimated by counting the number of proton-neutron, proton-proton, and neutron-neutron pairs residing in a relative S state. We present numerical calculations of those amounts for the nuclei {sup 4}He, {sup 9}Be, {sup 12}C, {sup 27}Al, {sup 40}Ca, {sup 48}Ca, {sup 56}Fe, {sup 63}Cu, {sup 108}Ag, and {sup 197}Au. The results are used to predict the values of the ratios of the per-nucleon electron-nucleus inelastic scattering cross section to the deuteron in the kinematic regime where correlations dominate.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2012

Charge independence, charge symmetry breaking in the S-wave nucleon-nucleon interaction, and renormalization

We study the interplay between charge symmetry breaking and renormalization in the NN system for S-waves. We find a set of universality relations which disentangle explicitly the known long distance dynamics from low energy parameters and extend them to the Coulomb case. We analyze within such an approach the One-Boson-Exchange potential and the theoretical conditions which allow to relate the proton-neutron, proton-proton and neutron-neutron scattering observables without the introduction of extra new parameters and providing good phenomenological success.

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS↗
DOE OSTI2018

Energy spectrum of neutron-rich helium isotopes: Complex made simple

We demonstrate herein that the intricate energy spectrum of neutron-rich helium isotopes can be straightforwardly described by taking advantage of the low-energy properties of neutron-neutron interaction and the scale separation that is present in diluted drip-line systems. By using arguments based on the halo effective field theory, we further carry out a parameter reduction of the complex-energy configuration interaction framework in the s p d space, including resonant and scattering states. By constraining the core potential to α - n scattering phase shifts and adjusting the strength of the spin-singlet central neutron-neutron interaction, we reproduce experimental energies and widths of 5 – 8 He within tens of keV precision. We predict a parity inversion of narrow resonances in He 9 and show that the ground state of He 10 is an s -wave-dominated configuration that could decay through two-neutron emission. This threshold state can be viewed as a “double-halo” structure in an analogy to the atomic He 3 4 He 2 trimer.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2011

Cold Neutrons Trapped in External Fields

The properties of inhomogeneous neutron matter are crucial to the physics of neutron-rich nuclei and the crust of neutron stars. Advances in computational techniques now allow us to accurately determine the binding energies and densities of many neutrons interacting via realistic microscopic interactions and confined in external fields. We perform calculations for different external fields and across several shells to place important constraints on inhomogeneous neutron matter, and hence the large isospin limit of the nuclear energy density functionals that are used to predict properties of heavy nuclei and neutron star crusts. We find important differences between microscopic calculations and current density functionals; in particular, the isovector gradient terms are significantly more repulsive than in traditional models, and the spin-orbit and pairing forces are comparatively weaker.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2013

Efimov Physics Around the Neutron-Rich Ca 60 Isotope

We calculate the neutron- Ca 60 $S$-wave scattering phase shifts using state of the art coupled-cluster theory combined with modern ab initio interactions derived from chiral effective theory. Effects of three-nucleon forces are included schematically as density dependent nucleon-nucleon interactions. This information is combined with halo effective field theory in order to investigate the Ca 60 -neutron-neutron system. We predict correlations between different three-body observables and the two-neutron separation energy of Ca 62 . This provides evidence of Efimov physics along the calcium isotope chain. Experimental key observables that facilitate a test of our findings are discussed.

71 CLASSICAL AND QUANTUM MECHANICS, GENERAL PHYSIC↗
DOE OSTI2010

Determination of the {sup 1}S{sub 0} neutron-neutron scattering length in the nd breakup reaction at energies in the range E{sub n} = 40-60 MeV

The first results are obtained in a kinematically complete experiment devoted to measuring the n + d {sup {yields}}p + n + n reaction yield at energies in the range E{sub n} = 40-60 MeV and various angles of divergence of two neutrons ({Delta}{theta} = 4{sup o}, 6{sup o}, and 8{sup o}) in the geometry of neutron-neutron final-state interaction. The {sup 1}S{sub 0} neutron-neutron scattering length a{sub nn} is determined by comparing the experimental energy dependence of the reaction yield with the results of a simulation in the Watson-Migdal approximation, which depend on a{sub nn}. For E{sub n} = 40 MeV and {Delta}{theta} = 6{sup o} (the best statistics in the experiment), the value a{sub nn} = -17.9 {+-} 1.0 fm was obtained. A further improvement of the experimental accuracy will make it possible to remove the existing disagreement of the results from different experiments.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2012

A method to measure neutron polarization using P-even asymmetry of {gamma}-quantum emission in the neutron-nuclear interaction

A new method to measure polarization of cold/thermal neutrons using P-even asymmetry in nuclear reactions induced by polarized neutrons is proposed. A scheme profiting from a large correlation of the neutron spin and the circular {gamma}-quantum polarization in the reaction (n, {gamma}) of polarized neutrons with nuclei is analyzed. This method could be used, for instance, to measure the neutron-beam polarization in experiments with frequently varying configuration. We show that high accuracy and reliability of measurements could be expected.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

Low-energy parameters of neutron-neutron interaction in the effective-range approximation

The effect of the mass difference between the charged and neutral pions on the low-energy parameters of nucleon-nucleon interaction in the {sup 1}S{sub 0} state is studied in the effective-range approximation. On the basis of experimental values of the singlet parameters of neutron-proton scattering and the experimental value of the virtual-state energy for the neutron-neutron systemin the {sup 1}S{sub 0} state, the following values were obtained for the neutron-neutron scattering length and effective range: a{sub nn} = -16.59(117) fm and r{sub nn} = 2.83(11) fm. The calculated values agree well with present-day experimental results.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2013

Neutron-deuteron breakup reaction as a tool for studying neutron-neutron interactions

An analysis of the most recent data on the reaction nd {yields} pnn revealed a serious discrepancy between theoretical predictions and cross sections measured for this reaction in various configurations where the role of neutron-neutron interactions is important. In view of this, it seems necessary both to develop theoretical approaches and to obtain new experimental data. For this purpose, a setup for studying the neutron-deuteron breakup reaction was created at the Institute for Nuclear Research on the basis of the neutron beam in the RADEX channel and deuterium targets. This facility makes it possible to perform experiments over a broad region of primary-neutron energies (10-60 MeV) and in various (final-state interaction, quasifree scattering, and spatial-star) configurations. Preliminary results of the respective experiment were obtained for configurations of final-state neutron-neutron interaction and quasifree neutron-neutron scattering.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2017

Medium polarization and pairing in asymmetric nuclear matter

The many-body theory of asymmetric nuclear matter is developed beyond the Brueckner–Hartree–Fock approximation to incorporate the medium polarization effects. The extension is performed within the Babu–Brown induced interaction theory. After deriving the particle–hole interaction in the form of Landau–Migdal parameters, the effects of the induced component on the symmetry energy are investigated along with the screening of {sup 1}S{sub 0} proton–proton and {sup 3}PF{sub 2} neutron–neutron pairing, which are relevant for the neutron-star cooling. The crossover from repulsive (screening) to attractive (anti-screening) interaction going from pure neutron matter to symmetric nuclear matter is discussed.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2018

Detailed Investigations of Neutron–Neutron Angular Correlations in Slow-Neutron-Induced Fission of {sup 233}U, {sup 235}U, and {sup 239}Pu

An experiment devoted to studying neutron–neutron angular correlations in the slow-neutroninduced fission of {sup 233}U, {sup 235}U and {sup 239}Pu nuclei was performed. The experimentally determined angular dependence of the number of neutron–neutron coincidences was compared with the results of a Monte Carlo simulation for various values of the neutron-detection threshold in the range between 490 and 2080 keV. It was found that the experimental angular distributions in question can be described well under the assumption that 2% to 5% of all prompt fission neutrons are emitted isotropically in the laboratory frame. Probably, such neutrons can be interpreted as so-called scission neutrons directly associated with the nuclear-rupture instant. Energy distributions of this component were also obtained from the present analysis.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗
DOE OSTI2018

Low-Energy Parameters of Neutron–Neutron Interaction: Analysis of Data Obtained in nd and dd Breakup Reactions

Data obtained in recent years for the neutron–neutron scattering length in nd and dd breakup reactions are analyzed. Serious discrepancies between experimental data from different studies may be explained by the effect of three-nucleon forces depending on the velocity of separation of a neutron–neutron pair and a charged fragment. It is shown that the data under analysis can be approximated by a smooth curve representing the dependence of the scattering length on the parameter R, which determines the fragment-separation distance after a fixed time. This dependence makes it possible to obtain the scatteringlength value for R→∞—that is, in the absence of the effect of three-nucleon forces.

73 NUCLEAR PHYSICS AND RADIATION PHYSICS↗