N. W. Hill

Oak Ridge National Laboratory, Oak Ridge, FL, United States

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Publications (126)175.84 Total impact

  • [Show abstract] [Hide abstract]
    ABSTRACT: We have measured the neutron total and elastic scattering cross sections for the 204Pb+n reaction over the neutron energy range 35–1000 keV. Neutron resonance widths and energies have been deduced for 1357 resonances from an R-matrix analysis of the total cross section data. Of this number 317, 424, and 617 are due to s-, p-, and d-wave interactions, respectively, with the parity assignments based mainly upon resonance asymmetries observed in the scattering data. The level densities and strengths of all partial waves are uniform up to a neutron energy of 500 keV. The s-, p-, and d-wave strength functions for this energy region are (in units of 10-4) 1.3±0.2, 0.19±0.02, and 1.0±0.1, respectively. If the strengths are calculated over the entire energy range the values for s and d waves are reduced by 35% and 12%, respectively, while that for p waves is increased by 40%. The s-, p-, and d-wave level densities (in keV-1) are 0.33±0.03, 0.44±0.02, and 0.68±0.01, respectively.
    Physical Review C 02/2003; 67(2). · 3.72 Impact Factor
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    ABSTRACT: In support of the US Department of Energy need for new data for criticality safety applications, new high-resolution fission cross-section measurements of {sup 233}U have been made at the Oak Ridge Electron Linear Accelerator. The measurements were carried out over a 0.4-eV to 700-keV energy range at the 80-m flight station using a fission chamber. Corrections were made for experimental effects to obtain the average fission cross section in this energy range. Results are compared to previous measurements.
    Nuclear Science and Engineering. 05/2000; 135(2).
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    ABSTRACT: In support of the US Department of Energy need for new data for criticality safety applications, new high-resolution fission cross-section measurements of ²³³U have been made at the Oak Ridge Electron Linear Accelerator. The measurements were carried out over a 0.4-eV to 700-keV energy range at the 80-m flight station using a fission chamber. Corrections were made for experimental effects to obtain the average fission cross section in this energy range. Results are compared to previous measurements.
    Nuclear Science and Engineering - NUCL SCI ENG. 01/2000; 135(2):141-149.
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    ABSTRACT: In support of the Nuclear Criticality Predictability Program established in response to the Defense Nuclear Facility Safety Board Recommendation 93-2, time-of-flight (TOF) measurements of the fission cross sections of {sup 233}U in the neutron energy range from 0.36 eV to several hundred keV have been initiated at the Oak Ridge Electron Linear Accelerator (ORELA). Also total and capture cross sections of Al, Cl, and K in the energy range from about 100 eV to several hundred keV have been measured or are under way. The goal is to derive accurate cross section representations for the materials involved in criticality calculations of fuel storage, transportation, etc., configurations. Additional high-resolution measurements of the total cross sections of {sup 233}U below a few keV neutron energy are being planned for 1998, as well as for the other involved material. Evaluated data files in ENDF-6 format will be processed into formats for use in criticality analysis and utilized in benchmark data testing. Finally the data will be submitted for inclusion in ENDF/B.
    07/1998
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    ABSTRACT: We have made improved measurements of the neutron capture and total cross sections for {sup 137}Ba over a sufficiently wide range of energies so that the reaction rate at s-process temperatures (kT=6{endash}23 keV) can be determined solely from the data. These rates are crucial for the interpretation of recently discovered anomalies of Ba isotopes in silicon carbide grains from the Murchison meteorite. Recent stellar models of the s process are in agreement with the meteoric anomaly data for Ba only if the {sup 137}Ba(n,{gamma}) reaction rate is 20{percent} larger than the previously accepted rate. Our reaction rates at s-process temperatures are in agreement with the extrapolated reaction rate from the most recent previous measurement. Hence, our results uphold, and place on much firmer footing, the discrepancy between recent stellar models of the s-process and the meteoric anomaly data. {copyright} {ital 1998} {ital The American Physical Society}
    Physical Review C 04/1998; 57(4). · 3.72 Impact Factor
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    ABSTRACT: Recent models of the s process occurring in low-mass Asymptotic Giant Branch (AGB) stars have succeeded in reproducing the isotopic anomalies measured in certain meteorites for all the barium isotopes except ^137Ba. The disagreement for ^137Ba may be due to a problem with the ^137Ba(n,gamma ) reaction rate used in the calculations because the measurements on which this rate is based had a lower energy limit (E_n=10 keV) which was too high to determine reliably the reaction rate at the low temperatures (kT=6-8 keV) needed by the new AGB s-process models. We have addressed this problem by making new high resolution (n,gamma ) and transmission measurements on an isotopically enriched sample of ^137Ba at the ORELA for a range of energies (100 eV to 280 keV) which is broad enough to allow an accurate determination of the reaction rate at the temperatures needed by the new AGB s-process models. We will discuss the impact of our new data on the interpretation of the meteoric barium anomalies.
    Physical Review C 01/1998; 57(4). · 3.72 Impact Factor
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    ABSTRACT: The ORELA experiment on the neutron charge radius has been reevaluated. The neutron transmission of liquid thorogenic 208Pb with the neutron time-of-flight method in the neutron energy range from 0.08 to 800 eV was measured. Additional studies on condensed matter effects for liquid Bi and liquid Pb have provided more accurate corrections resulting in smaller systematic uncertainties for the neutron-electron scattering length bne. We have also reevaluated the transmission data on liquid Bi of Melkonian and co-workers. We have obtained values of bne=(-1.33±0.027±0.03)×10-3fm for 208Pb and bne=(-1.44±0.033±0.06)×10-3fm for 209Bi where for both results the two uncertainties denote the statistical and the systematic uncertainty, respectively. The results for 208Pb and 209Bi were found to be in good agreement.
    Physical Review C 10/1997; 56(4). · 3.72 Impact Factor
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    ABSTRACT: Improved astrophysical reaction rates for 116,120Sn(n,gamma) are of interest because nucleosynthesis models have not been able to reproduce the observed abundances in this mass region. The authors have made high-resolution reaction rate and 116Sn-transmission measurements on isotopically enriched samples at the Oak Ridge Electron Linear Accelerator in the energy range from 100 eV to 500 keV.
    04/1997;
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    ABSTRACT: We have made high-resolution neutron capture and transmission measurements on isotopically enriched samples of 134Ba and 136Ba at the Oak Ridge Electron Linear Accelerator (ORELA) in the energy range from 20 eV to 500 keV. Previous measurements had a lower energy limit of 3 – 5 keV, which is too high to determine accurately the Maxwellian-averaged capture cross section at the low temperatures (kT ≈ 6 – 12 keV) favored by the most recent stellar models of the s-process. Our results for the astrophysical reaction rates are in good agreement with the most recent previous measurement at the classical s-process temperature, kT = 30 keV, but show significant differences at lower temperatures. We discuss the astrophysical implications of these differences.
    Nuclear Physics A 01/1997; 621(1):258-261. · 1.53 Impact Factor
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    ABSTRACT: Because of the internal structure of the neutron, i.e. consisting of one up and two down quarks surrounded by a pi-meson cloud, the neutron should be polarizable in a strong electric field like that of a heavy nuclide. The total neutron-nucleus potential scattering cross section can then be parametrized( J. Schmiedmayer, P. Riehs, J.A. Harvey and N. W. Hill, Phys. Rev Lett. 66, 1015 (1991)) by sigma_s(k)= sigma_s(0)+ak+bk^2+O(k^4). The parameter a depends only on the electric polarizability of the neutron. We have recently completed transmission measurements on thorogenic ^208Pb at the ORELA. High resolution transmission data for resonance analysis were obtained at 80 m in the energy range from 10 keV to 1 MeV in order to obtain more information on the neutron-nucleus potential. With a measurement at a much shorter time-of-flight distance of 18 m from the neutron target, we covered the low energy range from 2 eV to 50 keV and we will obtain a new value for the polarizability.
    10/1996;
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    ABSTRACT: We have made high-resolution neutron capture and transmission measurements on isotopically enriched samples of 134Ba and 136Ba at the Oak Ridge Electron Linear Accelerator (ORELA) in the energy range from 20 eV to 500 keV. Previous measurements had a lower energy limit of 3–5 keV, which is too high to determine accurately the Maxwellian-averaged capture cross section at the low temperatures (kT≊8-12 keV) favored by the most recent stellar models of the s process. By fitting the data with a multilevel R-matrix code, we determined parameters for 86 resonances in 134Ba below 11 keV and 92 resonances in 136Ba below 35 keV. Astrophysical reaction rates were calculated using these parameters together with our cross section data for the unresolved resonance region. Our results for the astrophysical reaction rates are in good agreement with the most recent previous measurement at the classical s-process temperature kT=30 keV, but show significant differences at lower temperatures. We determined that these differences were due to the effect of resonances below the energy range of previous experiments and to the use of incorrect neutron widths in a previous resonance analysis. Our data show that the ratio of reaction rates for these two isotopes depends more strongly on temperature than previous measurements indicated. One result of this temperature dependence is that the mean s-process temperature we derived from a classical analysis of the branching at 134Cs is too low to be consistent with the temperature derived from other branching points. This inconsistency is evidence for the need for more sophisticated models of the s process beyond the classical model. We used a reaction network code to explore the changes in the calculated isotopic abundances resulting from our new reaction rates for an s-process scenario based on a stellar model. These calculations indicate that the previously observed 20% discrepancy with respect to the solar barium abundance is reduced but not resolved by our new reaction rates. © 1996 The American Physical Society.
    Physical Review C 09/1996; 54(3). · 3.72 Impact Factor
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    ABSTRACT: The neutron total cross section of 124Sn has been measured over the energy range 0.014–0.315 MeV. An R-matrix analysis has been performed to obtain resonance and average parameters which provide for a complete representation of the neutron entrance channels for the s1/2, p1/2, and p3/2 contributions. The s- and p-wave neutron strength functions have been determined (for a channel radius of 7.23 fm) to be 0.12±0.03 and 1.8±0.2, respectively (in units of 10-4). Limits are placed on the average level spacings and strength functions for the individual partial-wave components. The number of definite p1/2 levels exceeds the p3/2 levels by the factor 2 and their average strengths differ by the factor 4. s- and p-wave potential scattering radii have been determined to be 6.4±0.2 fm and 10.5±0.3 fm, respectively. Average scattering functions, deduced from the average parameters, have been used to determine the real well depth of an optical-model potential which reproduces these functions. There is evidence of an angular momentum dependence of the real-well potential depth and the level spacings. © 1996 The American Physical Society.
    Physical Review C 01/1996; 54(5). · 3.72 Impact Factor
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    ABSTRACT: The neutron transmission through a thorogenic liquid 208Pb sample 2 in. thick has been measured in the neutron energy range between 0.1 and 360 eV at the ORNL neutron source ORELA. Analyzing the shape of the transmission spectra as a function of neutron energy, agreement was found with the predictions by the atomic form factor. With a sensitivity for the mean squared charge radius of the neutron as high as 3%, a very reliable and also accurate result of = -0.113+/-0.003+/-0.004 fm2 was extracted. For the neutron-electron scattering length we obtained bne = \(-1.31+/-0.03+/-0.04\)×10-3 fm.
    Physical Review Letters 04/1995; 74(13):2427-2430. · 7.73 Impact Factor
  • P.E Koehler, J.A Harvey, N.W Hill
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    ABSTRACT: We have developed two detectors for (n,p) and (n,α) cross section measurements at white neutron sources which make possible large increases in the sample size and geometric efficiency while at the same time reducing the potentially large background associated with the “gamma flash” to manageable levels. We present measurements of two (n,α) cross sections made with these detectors to energies as high as a few MeV.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 01/1995; · 1.14 Impact Factor
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    ABSTRACT: The neutron total cross section of 122Sn has been measured over the energy range 0.013 to 0.310 MeV. An R-matrix analysis has been performed to obtain resonance and average parameters which provide for a complete representation of the neutron entrance channels for the s1/2, p1/2, and p3/2 contributions. The conventional s- and p-neutron strength functions have been determined to be 0.123±0.023 and 2.0±0.2, respectively (in units of 10-4). Limits are placed on the average level spacings and strength functions for the individual partial wave components. The s-wave potential scattering radius has been determined to be 6.3±0.1 fm. Average scattering functions, deduced from the average parameters, have been used to determine the real well depth of an optical model potential which reproduces these functions. We find, as have others, that the real well depth is parity dependent.
    Physical Review C 01/1995; 52(3). · 3.72 Impact Factor
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    ABSTRACT: We have developed two detectors for (n,p) and (n,alpha) cross section measurements at white neutron sources which make possible large increases in the sample size and geometric efficiency while at the same time reducing the potentially large background associated with the ``gamma flash'' to manageable levels. We present measurements of two (n,alpha) cross sections made with these detectors to energies as high as a few MeV.
    Nuclear Instruments & Methods in Physics Research Section A-accelerators Spectrometers Detectors and Associated Equipment - NUCL INSTRUM METH PHYS RES A. 01/1995; 361:270-276.
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    ABSTRACT: We find a normal temperature dependence for the Doppler broadening of the 230-eV resonance in the neutron cross section of 65Cu atoms in YBa2Cu3O7 between 10 and 300 K. No abrupt changes in the copper momentum spectrum corresponding to vibrations in the ab plane were observed in either the superconducting YBa2Cu3O7 or nonsuperconducting YBa2Cu3O6.2 compounds. This result differs from ion-channeling experiments in which sharp changes in the channeling minimum yield and width of the rocking curve indicated irregular vibrational behavior near the superconducting critical temperature.
    Physical review. B, Condensed matter 01/1995; 50(21):16129-16132. · 3.77 Impact Factor
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    ABSTRACT: Neutron-induced fission of [sup 236]U has been measured at the Los Alamos Neutron Scattering Center with a white neutron source using a fast parallel plate ionization chamber at a flight path of [similar to]56 m. In the resonance and the intermediate resonance region, very little of the previously reported structure was detected. Only five resonance structures were observed. Additionally, the width of the 5.45 eV resonance is approximately 100 times smaller than previously reported. An explanation for the discrepancies between old data and data reported here is discussed. New fission widths for resonances from 5.45 eV to 10.4 keV are reported. The new data are in agreement with theoretical estimates.
    Physical Review C 03/1994; 49(2):672-677. · 3.72 Impact Factor
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    ABSTRACT: The results of a study of the compound nucleus 114Cd by neutron time-of-flight spectroscopy methods are presented. Targets of both natural cadmium and cadmium enriched in the 113 isotope were used. The neutron total capture and neutron transmission were both measured. A total of 275 new resonances were located. In addition, 102 other resonances which were previously known but not assigned to a particular cadmium isotope were definitively assigned to 113Cd. Resonance parameters E0 and gΓn were obtained for both newly identified and previously known resonances. Of the 437 resonances now known in 113Cd, we identify 104 of them as l=1 based on their small widths. Strength functions and level spacings are obtained for both l=0 and l=1 resonances. Comparisons of the data with Porter-Thomas reduced width distributions, Wigner nearest neighbor spacing distributions, and the Dyson-Metha Δ3 statistic are given. The linear correlation coefficient between adjacent spacings is also discussed. The spectroscopic information obtained is of importance for planning and interpretation of parity violation measurements on the p-wave resonances of 113Cd.
    Physical Review C 01/1994; 50(6):2774-2785. · 3.72 Impact Factor
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    ABSTRACT: We find a normal temperature dependence for the Doppler broadening of the 230-eV resonance in the neutron cross section of [sup 65]Cu atoms in YBa[sub 2]Cu[sub 3]O[sub 7] between 10 and 300 K. No abrupt changes in the copper momentum spectrum corresponding to vibrations in the [ital ab] plane were observed in either the superconducting YBa[sub 2]Cu[sub 3]O[sub 7] or nonsuperconducting YBa[sub 2]Cu[sub 3]O[sub 6.2] compounds. This result differs from ion-channeling experiments in which sharp changes in the channeling minimum yield and width of the rocking curve indicated irregular vibrational behavior near the superconducting critical temperature.
    Physical review. B, Condensed matter 01/1994; 50(21):16129-16132. · 3.77 Impact Factor

Publication Stats

673 Citations
175.84 Total Impact Points

Institutions

  • 1973–2003
    • Oak Ridge National Laboratory
      • Physics Division
      Oak Ridge, FL, United States
  • 1983–1996
    • Denison University
      • Department of Physics and Astronomy
      Granville, Ohio, United States
  • 1995
    • Los Alamos National Laboratory
      Los Alamos, California, United States