arXiv subjects

A. C. Hayes

Publications and source records attributed to A. C. Hayes.

36 records · Page 2Linked to original sources

arXiv2009

Reaction-in-Flight Neutrons as a Signature for Shell Mixing in NIF capsules

We present analytic calculations and results from computational simulations showing that reaction-in-flight (RIF) neutrons act as a robust indicator for mixing of the ablator shell material into the fuel in DT capsules designed for the National Ignition Facility. The sensitivity of RIF neutrons to hydrodynamical mixing arises through the dependence of RIF production on charged-particle stopping lengths in the mixture of DT fuel and ablator material. Since the stopping power in the plasma is a sensitive function of the electron temperature and density, it is also sensitive to mix. RIF production scales approximately inversely with the degree of mixing taking place, and the ratio of RIF to downscattered neutrons provides a measure of the mix fraction and/or the mixing length. For sufficiently high-yield capsules, where spatially resolved RIF images may be possible, neutron imaging could be used to map RIF images into detailed mix images.

physics.plasm-ph↗

arXiv2009

Electric Dipole Polarizabilities of Hydrogen and Helium Isotopes

The electric dipole polarizabilities of $^3$H, $^3$He, and $^4$He are calculated directly using the Schrödinger equation with the latest generation of two- and three-nucleon interactions. These polarizabilities are necessary in order to obtain accurate nuclear-polarization corrections for transitions involving S-waves in one- and two-electron atoms. Our results are compared to previous results, and it is shown that direct calculations of the electric polarizability of $^4$He using modern nuclear potentials are smaller than published values calculated using experimental photoabsorption data. The status of this topic is assessed in the context of precise measurements of transitions in one- and two-electron atoms.

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arXiv2008

Splitting Sensitivity of the Ground and 7.6 eV Isomeric States of 229Th

The lowest-known excited state in nuclei is the 7.6 eV isomer of 229Th. This energy is within the range of laser-based investigations that could allow accurate measurements of possible temporal variation of this energy splitting. This in turn could probe temporal variation of the fine-structure constant or other parameters in the nuclear Hamiltonian. We investigate the sensitivity of this transition energy to these quantities. We find that the two states are predicted to have identical deformations and thus the same Coulomb energies within the accuracy of the model (viz., within roughly 30 keV). We therefore find no enhanced sensitivity to variation of the fine-structure constant. In the case of the strong interaction the energy splitting is found to have a complicated dependence on several parameters of the interaction, which makes an accurate prediction of sensitivity to temporal changes of fundamental constants problematical. Neither the strong- nor Coulomb-interaction contributions to the energy splitting of this doublet can be constrained within an accuracy better than a few tens of keV, so that only upper limits can be set on the possible sensitivity to temporal variations of the fundamental constants.

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arXiv2008

Nuclear Electric Dipole Moment of 3He

A permanent electric dipole moment (EDM) of a physical system requires time-reversal (T) and parity (P) violation. Experimental programs are currently pushing the limits on EDMs in atoms, nuclei, and the neutron to regimes of fundamental theoretical interest. Here we calculate the magnitude of the PT-violating EDM of 3He and the expected sensitivity of such a measurement to the underlying PT-violating interactions. Assuming that the coupling constants are of comparable magnitude for pi-, rho-, and omega-exchanges, we find that the pion-exchange contribution dominates. Our results suggest that a measurement of the 3He EDM is complementary to the planned neutron and deuteron experiments, and could provide a powerful constraint for the theoretical models of the pion-nucleon PT-violating interaction.

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arXiv2007

Sensitivity of Nuclear Transition Frequencies to Temporal Variation of the Fine Structure Constant or the Strong Interaction

There exist in nature a few nuclear isomers with very low (eV) excitation energies, and the combination of low energy and narrow width makes them possible candidates for laser-based investigations. The best candidate is the lowest-energy excited state known in nuclear physics, the 7.6(5) eV isomer of $^{229}$Th. A recent study suggests that a measurement of the temporal variation of the excitation energy of this isomer would have 5-6 orders of magnitude enhanced sensitivity to a variation of the fine structure constant ($α\cong 1/137.036$) or of a strong interaction parameter ($m_q/Λ_{QCD}$). We reexamine the physics involved in these arguments. By invoking the Feynman-Hellmann Theorem we argue that there is no expectation of significantly enhanced sensitivity to a variation in the fine structure constant (beyond that obtained from experimental considerations such as the low energy and narrow width of the isomer). A similar argument applies to the strong interaction, but evaluating the shift due to temporal variations of the underlying parameters of the strong interaction may be beyond current nuclear structure techniques.

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arXiv2004

Electromagnetic excitation rates for nuclear isomers in a hot dense plasma

In high neutron flux environments where isomers can be strongly populated by nucleonic reactions, isotope abundances from reaction network chains can be affected by the population of nuclear isomers. At high temperatures and densities there is the additional possibility of populating these isomers electromagnetically. Here we examine the rates for electromagnetic excitation of the isotopes of several isomers of interest both in astrophysics and applied physics (e.g. 235U, 193Ir, and 87Y,88Y). We consider six possible electromagnetic processes, namely, photo-absorption, inverse internal conversion, inelastic electron scattering, Coulomb excitation, (gamma,gamma) and (e,e'gamma) reactions. We find that for plasma temperatures kT~ 1-10 keV the electromagnetic reactions rates are negligible. Thus, we conclude that reaction network calculations do not need to include for the possibility of electromagnetically exciting nuclear isomers. This is true in both stellar and terrestrial thermonuclear explosions, as well as in plasma conditions expected at the National Ignition Facility.

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arXiv2004

Prompt Beta Spectroscopy as a Diagnostic for Mix in Ignited NIF Capsules

The National Ignition Facility (NIF) technology is designed to drive deuterium-tritium (DT) internal confinement fusion (ICF) targets to ignition using indirect radiation from laser beam energy captured in a hohlraum. Hydrodynamical instabilities at interfaces in the ICF capsule leading to mix between the DT fue l and the ablator shell material are of fundamental physical interest and can affect the performance characteristics of the capsule. In this Letter we describe new radiochemical diagnostics for mix processes in ICF capsules with plastic or Be (0.9%Cu) ablator shells. Reactions of high-energy tritons with shell material produce high-energy $β$-emitters. We show that mix between the DT fuel and the shell material enhances high-energy prompt beta emission from these reactions by more than an order of magnitude over that expected in the absence of mix.

physics.plasm-ph↗

arXiv2004

Detection of Antineutrinos for Non-Proliferation

We discuss the feasibility of using the detection of electron antineutrinos produced in fission to monitor the time dependence of the plutonium content of nuclear power reactors. If practical such a scheme would allow world-wide, automated monitoring of reactors and, thereby, the detection of certain proliferation scenarios. For GW$_e$ power reactors the count rates and the sensitivity of the antineutrino spectrum (to the core burn-up) suggest that monitoring of the gross operational status of the reactor from outside the containment vessel is feasible. As the plutonium content builds up in a given burn cycle the total number of antineutrinos steadily drops and this variation is quite detectable, assuming fixed reactor power. The average antineutrino energy also steadily drops, and a measurement of this variation would be very useful to help off set uncertainties in the total reactor power. However, the expected change in the antineutrino signal from the diversion of a significant quantity (SQ) of plutonium, which would typically require the diversion of as little as a single fuel assemblies in a GW$_e$ reactor, would be very difficult to detect.

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arXiv2003

Inadequacy of Scaling Arguments for Neutrino Cross Sections

The problem with the use of scaling arguments for simultaneous studies of different weak interaction processes is discussed. When different neutrino scattering cross sections involving quite different momentum transfers are being compared it difficult to define a meaningful single scaling factor to renormalize calculated cross sections. It has been suggested that the use of such scaling can be used to estimate high energy neutrino cross sections from low energy neutrino cross sections. This argument has lead to questions on the consistency of the magnitude of the LSND muon neutrino cross sections on $^{12}$C relative to other lower energy weak processes. The issue is revisited here and from inspection of the structure of the form factors involved it is seen that the problem arises from a poor description of the transition form factors at high momentum transfer. When wave functions that reproduce the transverse magnetic inelastic (e,e') scattering form factor for the 15.11 MeV state in $^{12}$C are used there is no longer a need for scaling the axial current, and the different weak interactions rates involving the T=1 1$^+$ triplet in mass 12 are consistent with one another.

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arXiv2003

Neutrino-12C scattering in the ab initio shell model with a realistic three-body interaction

We investigate cross sections for neutrino-12C exclusive scattering and for muon capture on 12C using wave functions obtained in the ab initio no-core shell model. In our parameter-free calculations with basis spaces up to the 6 hbarOmega we show that realistic nucleon-nucleon interactions, like e.g. the CD-Bonn, under predict the experimental cross sections by more than a factor of two. By including a realistic three-body interaction, Tucson-Melbourne TM'(99), the cross sections are enhanced significantly and a much better agreement with experiment is achieved. At the same time,the TM'(99) interaction improves the calculated level ordering in 12C. The comparison between the CD-Bonn and the three-body calculations provides strong confirmation for the need to include a realistic three-body interaction to account for the spin-orbit strength in p-shell nuclei.

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arXiv2002

Theoretical Evaluations of the Fission Cross Section of the 77 eV Isomer of 235-U

We have developed models of the fission barrier (barrier heights and transition state spectra) that reproduce reasonably well the measured fission cross section of $^{235}$U from neutron energy of 1 keV to 2 MeV. From these models we have calculated the fission cross section of the 77 eV isomer of $^{235}$U over the same energy range. We find that the ratio of the isomer cross section to that of the ground state lies between about 0.45 and 0.55 at low neutron energies. The cross sections become approximately equal above 1 MeV. The ratio of the neutron capture cross section to the fission cross section for the isomer is predicted to be about a factor of 3 larger for the isomer than for the ground state of $^{235}$U at keV neutron energies. We have also calculated the cross section for the population of the isomer by inelastic neutron scattering form the $^{235}$U ground state. We find that the isomer is strongly populated, and for $E_n = 1 MeV$ the $(n,n'γ)$ cross section leading to the population of the isomer is of the order of 0.5 barn. Thus, neutron reaction network calculations involving the uranium isotopes in a high neutron fluence are likely to be affected by the 77 eV isomer of $^{235}$U. With these same models the fission cross sections of $^{233}$U and $^{237}$U can be reproduced approximately using only minor adjustments to the barrier heights. With the significant lowering of the outer barrier that is expected for the outer barrier the general behavior of the fission cross section of $^{239}$Pu can also be reproduced.

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arXiv2001

Parity Violation in Neutron Capture Reactions

In the last decade, the scattering of polarized neutrons on compound nucleus resonances proved to be a powerful experimental technique for probing nuclear parity violation. Longitudinal analyzing powers in neutron transmission measurements on p-wave resonances in nuclei such as $^{139}$La and $^{232}$Th were found to be as large as 10%. Here we examine the possibilities of carrying out a parallel program to measure asymmetries in the $(n,γ$) reaction on these same compound nuclear resonances. Symmetry-violating $(n,γ$) studies can also show asymmetries as large as 10%, and have the advantage over transmission experiments of allowing parity-odd asymmetries in several different gamma-decay branches from the same resonance. Thus, studies of parity violation in the $(n,γ)$ reaction using high efficiency germanium detectors at the Los Alamos Lujan facility, for example, could determine the parity-odd nucleon-nucleon matrix elements in complex nuclei with high accuracy. Additionally, simultaneous studies of the E1 and $V_{PNC}$ matrix elements invol ved in these decays could be used to help constrain the statistical theory of parity non-conservation in compound nuclei.

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arXiv1999

Nuclear Octupole Correlations and the Enhancement of Atomic Time-Reversal Violation

We examine the time-reversal-violating nuclear ``Schiff moment'' that induces electric dipole moments in atoms. After presenting a self-contained derivation of the form of the Schiff operator, we show that the distribution of Schiff strength, an important ingredient in the ground-state Schiff moment, is very different from the electric-dipole-strength distribution, with the Schiff moment receiving no strength from the giant dipole resonance in the Goldhaber-Teller model. We then present shell-model calculations in light nuclei that confirm the negligible role of the dipole resonance and show the Schiff strength to be strongly correlated with low-lying octupole strength. Next, we turn to heavy nuclei, examining recent arguments for the strong enhancement of Schiff moments in octupole-deformed nuclei over that of 199Hg, for example. We concur that there is a significant enhancement while pointing to effects neglected in previous work (both in the octupole-deformed nuclides and 199Hg) that may reduce it somewhat, and emphasizing the need for microscopic calculations to resolve the issue. Finally, we show that static octupole deformation is not essential for the development of collective Schiff moments; nuclei with strong octupole vibrations have them as well, and some could be exploited by experiment.

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arXiv1999

Shell-model calculations of neutrino scattering from 12C

Neutrino reaction cross-sections, $(ν_μ,μ^-)$, $(ν_e,e^-)$, $μ$-capture and photoabsorption rates on $^{12}$C are computed within a large-basis shell-model framework, which included excitations up to $4\hbarω$. When ground-state correlations are included with an open $p$-shell the predictions of the calculations are in reasonable agreement with most of the experimental results for these reactions. Woods-Saxon radial wave functions are used, with their asymptotic forms matched to the experimental separation energies for bound states, and matched to a binding energy of 0.01 MeV for unbound states. For comparison purposes, some results are given for harmonic oscillator radial functions. Closest agreement between theory and experiment is achieved with unrestricted shell-model configurations and Woods-Saxon radial functions. We obtain for the neutrino-absorption inclusive cross sections: $\barσ = 13.8 \times 10^{-40}$ cm$^2$ for the $(ν_μ,μ^{-})$ decay-in-flight flux in agreement with the LSND datum of $(12.4 \pm 1.8) \times 10^{-40}$ cm$^2$; and $\barσ = 12.5 \times 10^{-42}$ cm$^2$ for the $(ν_{e},e^{-})$ decay-at-rest flux, less than the experimental result of $(14.4 \pm 1.2) \times 10^{-42}$ cm$^2$.

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arXiv1997

Reply to Baur's and Bertulani's Comment

Shortcomings of a momentum-space treatment of strong absorption, as discussed in the previous Comment [nucl-th/9611025], are only of concern at low projectile energies, $γ$< 1.5. At intermediate and high energies, for which the quantum-mechanical equivalent-photon spectrum is intended, the quantum-mechanical cross sections are reduced relative to the semi-classical results whether one treats strong effects via a momentum-space or an impact-parameter (spatial) cut-off. At these energies the origin of the discrepancy between the predictions of full quantuym-mechanical and the semi-classical calulations cannot be traced to differences in the treatment of strong-interaction effects. Rather, they arise from quantum effects neglected in the semi-classical calculations.

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arXiv1996

Pionic Fusion of Heavy Ions

We report the first experimental observation of the pionic fusion of two heavy ions. The 12C(12C,24Mg)pi0 and 12C(12C,24Na)pi+ cross sections have been measured to be 208 +/- 38 and 182 +/- 84 picobarns, respectively, at E_cm = 137 MeV. This cross section for heavy-ion pion production, at an energy just 6 MeV above the absolute energy-conservation limit, constrains possible production mechanisms to incorporate the kinetic energy of the entire projectile-target system as well as the binding energy gained in fusion.

nucl-ex↗

arXiv1996

A Quantum-Mechanical Equivalent-Photon Spectrum for Heavy-Ion Physics

In a previous paper, we calculated the fully quantum-mechanical cross section for electromagnetic excitation during peripheral heavy-ion collisions. Here, we examine the sensitivity of that cross section to the detailed structure of the projectile and target nuclei. At the transition energies relevant to nuclear physics, we find the cross section to be weakly dependent on the projectile charge radius, and to be sensitive to only the leading momentum-transfer dependence of the target transition form factors. We exploit these facts to derive a quantum-mechanical ``equivalent-photon spectrum'' valid in the long-wavelength limit. This improved spectrum includes the effects of projectile size, the finite longitudinal momentum transfer required by kinematics, and the response of the target nucleus to the off-shell photon.

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arXiv1994

P,T-Violating Nuclear Matrix Elements in the One-Meson Exchange Approximation

Expressions for the P,T-violating NN potentials are derived for $π$, $ρ$ and $ω$ exchange. The nuclear matrix elements for $ρ$ and $ω$ exchange are shown to be greatly suppressed, so that, under the assumption of comparable coupling constants, $π$ exchange would dominate by two orders of magnitude. The ratio of P,T-violating to P-violating matrix elements is found to remain approximately constant across the nuclear mass table, thus establishing the proportionality between time-reversal-violation and parity-violation matrix elements. The calculated values of this ratio suggest a need to obtain an accuracy of order $ 5 \times 10^{-4}$ for the ratio of the PT-violating to P-violating asymmetries in neutron transmission experiments in order to improve on the present limits on the isovector pion coupling constant.

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