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Search Results: 1 - 10 of 600148 matches for " A. C. Hayes "
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Inadequacy of Scaling Arguments for Neutrino Cross Sections
A. C. Hayes
Physics , 2003, DOI: 10.1103/PhysRevC.68.067302
Abstract: 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.
Determining Reactor Flux from Xenon-136 and Cesium-135 in Spent Fuel
A. C. Hayes,Gerard Jungman
Physics , 2012, DOI: 10.1016/j.nima.2012.06.031
Abstract: The ability to infer the reactor flux from spent fuel or seized fissile material would enhance the tools of nuclear forensics and nuclear nonproliferation significantly. We show that reactor flux can be inferred from the ratios of xenon-136 to xenon-134 and cesium-135 to cesium-137. If the average flux of a reactor is known, the flux inferred from measurements of spent fuel could help determine whether that spent fuel was loaded as a blanket or close to the mid-plane of the reactor. The cesium ratio also provides information on reactor shutdowns during the irradiation of fuel, which could prove valuable for identifying the reactor in question through comparisons with satellite reactor heat monitoring data. We derive analytic expressions for these correlations and compare them to experimental data and to detailed reactor burn simulations. The enrichment of the original uranium fuel affects the correlations by up to 3 percent, but only at high flux.
Parity Violation in Neutron Capture Reactions
A. C. Hayes,Luca Zanini
Physics , 2001, DOI: 10.1103/PhysRevC.65.058501
Abstract: 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 $(n,\gamma$) reaction on these same compound nuclear resonances. Symmetry-violating $(n,\gamma$) 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 $(n,\gamma)$ 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.
Structure of the Particle-Hole Amplitudes in No-core Shell Model Wave Functions
A. C. Hayes,A. A. Kwiatkowski
Physics , 2009, DOI: 10.1103/PhysRevC.81.054301
Abstract: We study the structure of the no-core shell model wave functions for $^6$Li and $^{12}$C by investigating the ground state and first excited state electron scattering charge form factors. In both nuclei, large particle-hole ($ph$) amplitudes in the wave functions appear with the opposite sign to that needed to reproduce the shape of the $(e,e')$ form factors, the charge radii, and the B(E2) values for the lowest two states. The difference in sign appears to arise mainly from the monopole $\Delta\hbar\omega=2$ matrix elements of the kinetic and potential energy (T+V) that transform under the harmonic oscillator SU(3) symmetries as $(\lambda,\mu)=(2,0)$. These are difficult to determine self-consistently, but they have a strong effect on the structure of the low-lying states and on the giant monopole and quadrupole resonances. The Lee-Suzuki transformation, used to account for the restricted nature of the space in terms of an effective interaction, introduces large higher-order $\Delta\hbar\omega=n, n>$2, $ph$ amplitudes in the wave functions. The latter $ph$ excitations aggravate the disagreement between the experimental and predicted $(e,e')$ form factors with increasing model spaces, especially at high momentum transfers. For sufficiently large model spaces the situation begins to resolve itself for $^6$Li, but the convergence is slow. A prescription to constrain the $ph$ excitations would likely accelerate convergence of the calculations.
Attachment of Escherichia coli to fine sediment particles within construction sediment basins  [PDF]
Elizabeth A. Kunkel, Charles V. Privette, Calvin B. Sawyer, John C. Hayes
Advances in Bioscience and Biotechnology (ABB) , 2013, DOI: 10.4236/abb.2013.43A054
Abstract:

The conventional best management practice of sediment basins may create a reservoir for pathogenic bacteria. The fine particles that enter these basins have been shown to provide protection for bacteria; the small pores of clays and silts minimize predators and block sunlight. Therefore, while these basins decrease sediment loadings to water bodies downstream, they may introduce harmful levels of pathogenic bacteria into surface waters. In addition to causing human health risks, high bacteria levels alter natural biological makeup of downstream ecosystems. This paper describes the attachment of Escherichia coli to various particle sizes in construction site sediment basins. Five sediment basins, located in Anderson, South Carolina were sampled after rain events to explore trends that exist between various particle sizes and E. coli densities. Results provide evidence to suggest that sediment basins are a reservoir for pathogenic bacteria. Data showed that most E. coli attached to smaller particles with diameters less than 0.004 mm. These particles do not settle out of the water column quickly and are often passed through the basin during intense storms. Consequently, high levels of bacteria are passed to downstream waters. This research provides considerable evidence that the clays and silts within man-made construction basins can cause detrimental effects to South Carolina surface waters. With this knowledge, better stormwater management practices may be developed with the goal of remediating impaired surface waters of South Carolina.

Sensitivity of Nuclear Transition Frequencies to Temporal Variation of the Fine Structure Constant or the Strong Interaction
A. C. Hayes,J. L. Friar
Physics , 2007, DOI: 10.1016/j.physletb.2007.05.021
Abstract: 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 ($\alpha \cong 1/137.036$) or of a strong interaction parameter ($m_q/\Lambda_{QCD}$). 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.
P,T-Violating Nuclear Matrix Elements in the One-Meson Exchange Approximation
I. S. Towner,A. C. Hayes
Physics , 1994, DOI: 10.1103/PhysRevC.49.2391
Abstract: Expressions for the P,T-violating NN potentials are derived for $\pi$, $\rho$ and $\omega$ exchange. The nuclear matrix elements for $\rho$ and $\omega$ exchange are shown to be greatly suppressed, so that, under the assumption of comparable coupling constants, $\pi$ 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.
Theoretical Evaluations of the Fission Cross Section of the 77 eV Isomer of 235-U
J. Eric Lynn,A. C. Hayes
Physics , 2002, DOI: 10.1103/PhysRevC.67.014607
Abstract: 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 $(n,n'\gamma)$ 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.
Shell-model calculations of neutrino scattering from 12C
A. C. Hayes,I. S. Towner
Physics , 1999, DOI: 10.1103/PhysRevC.61.044603
Abstract: Neutrino reaction cross-sections, $(\nu_\mu,\mu^-)$, $(\nu_e,e^-)$, $\mu$-capture and photoabsorption rates on $^{12}$C are computed within a large-basis shell-model framework, which included excitations up to $4\hbar\omega$. 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: $\bar{\sigma} = 13.8 \times 10^{-40}$ cm$^2$ for the $(\nu_{\mu},\mu^{-})$ decay-in-flight flux in agreement with the LSND datum of $(12.4 \pm 1.8) \times 10^{-40}$ cm$^2$; and $\bar{\sigma} = 12.5 \times 10^{-42}$ cm$^2$ for the $(\nu_{e},e^{-})$ decay-at-rest flux, less than the experimental result of $(14.4 \pm 1.2) \times 10^{-42}$ cm$^2$.
Reply to Baur's and Bertulani's Comment
C. J. Benesh,A. C. Hayes,J. L. Friar
Physics , 1997,
Abstract: 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, $\gamma$< 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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