R. O. Nelson

Los Alamos National Laboratory, Los Alamos, California, United States

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Publications (219)214.2 Total impact

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    ABSTRACT: Timepix pixel detectors have been used to study the response of silicon hybrid pixel detectors to fast neutrons from a pulsed neutron beam at WNR FP30R, a 14 m long flight path, in the Los Alamos Neutron Science Center. Neutrons with kinetic energies up to 600 MeV were available. In order to enhance the conversion of neutrons to energetic charged particles, several converter foils and filters were attached to the 300 μm thick silicon sensor, i.e. polyethylene, polyethylene with aluminum, 6LiF, 6LiF with aluminum, aluminum. The Time-of-Arrival mode of the Timepix detectors has permitted the application of the Time-of-Flight (TOF) technique for the assignment of the detected interactions in the form of clusters (groups of adjacent pixels) in the pixel matrix, to the kinetic energies of the incident neutrons.
    Journal of Instrumentation 05/2014; 9(05):C05048. · 1.66 Impact Factor
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    ABSTRACT: A new neutron counter for research experiments on nuclear fission has been developed. This instrument is designed for the detection of prompt fission neutrons within relatively high levels of gamma and neutron background. It is composed of a set of 3-He proportional counters arranged within a block of polyethylene which serves as moderator. The detection properties have been studied by means of Monte Carlo simulations and experiments with radioactive sources. These properties are confirmed by an experiment on neutron-induced fission of 238-U at the WNR facility of the Los Alamos Neutron Science Center during which the mean prompt fission neutron multiplicity, or <ν> has been measured from 1 to 20 MeV of incident neutron energy.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 05/2014; 745:99-105. · 1.14 Impact Factor
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    ABSTRACT: In order to investigate the discrepancy in the available sets of data on 239Pu, Chi-Nu is a program to measure prompt-fission-neutron spectra at the Los Alamos Neutron Science Center (LANSCE). To meet the required accuracy based on the compilations among different theoretical models and sets of data, two different types of neutron detectors, liquid scintillators and 6Li-glass scintillators, are used in conjunction to the actinide Parallel Plate Avalanche Counter by measuring a time of flight. Monte Carlo simulations are used to study detector responses. In this manuscript, we discuss the status of the Chi-Nu project including the commissioning in 2012 summer.
    Fission and Properties of Neutron-Rich Nuclei: Proceedings of the Fifth International Conference on ICFN5; 01/2014
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    ABSTRACT: New half-life measurements have been made of the millisecond isomers 71mGe, 114m2I, 208mBi, 88m1Y, 88m2Y, and 75mAs populated in neutron-induced reactions. These measurements were made using the unique time structure of the LANSCE/WNR neutron source, by observing the γ-ray decays of the isomers during the time between the LANSCE proton macropulses. Two different LANSCE proton beam time structures were used. The GEANIE array of HPGe detectors was used to detect the γ-ray decays.
    Nuclear Data Sheets. 01/2014; 120:48–51.
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    ABSTRACT: An ionization chamber for measuring the energy loss and kinetic energy of fragments produced through neutron-induced fission at the Los Alamos Neutron Science Center (LANSCE) has been designed as a component of the the new SPIDER detector. Design criteria included energy resolutions of <1% for high energy resolution and increased charge resolution. The ionization chamber will be combined with a high resolution time-of-flight detector to achieve fragment yield measurements with mass and nuclear charge resolutions of 1 amu and Z=1. The present status of the ionization chamber will be presented.
    Fission and Properties of Neutron-Rich Nuclei: Proceedings of the Fifth International Conference on ICFN5; 01/2014
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    ABSTRACT: Prompt fission neutron spectra in the neutron-induced fission of 239Pu have been measured for incident neutron energies from 1 to 200 MeV at the Los Alamos Neutron Science Center. Mean energies deduced from the prompt fission neutron spectra (PFNS) lead to the observation of the opening of the second chance fission at 7 MeV and to indications for the openings of fission channels of third and fourth chances. Moreover, the general trend of the measured PFNS is well reproduced by the different models. The comparison between data and models presents, however, two discrepancies. First, the prompt neutron mean energy seems constant for neutron energy, at least up to 7 MeV, whereas in the theoretical calculations it is continuously increasing. Second, data disagree with models on the shape of the high energy part of the PFNS, where our data suggest a softer spectrum than the predictions.
    12/2013; 89(1).
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    European Physical Journal A 09/2013; 49(9):114. · 2.04 Impact Factor
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    ABSTRACT: Prompt K X-ray emission yields in the fission induced by neutrons on 238U have been measured for the first time for incident energies ranging from below 1MeV up to 400MeV. Results are used to investigate the evolution with incident neutron energy of the fragment elemental distribution and the X-ray emission probability per element. The progressive increase of the symmetric fission probability with neutron energy is observed in qualitative agreement with Wahl systematics for the primary fission fragment charge yields.
    European Physical Journal A 09/2013; · 2.04 Impact Factor
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    ABSTRACT: Experiments designed to study rare processes, such as neutrinoless double beta decay ($0\nu\beta\beta$), are crucial tests for physics beyond the standard model. These experiments rely on reducing the intrinsic radioactive background to unprecedented levels, while adequately shielding the detectors from external sources of radioactivity. An understanding of the potential for neutron excitation of the shielding and detector materials is important for obtaining this level of sensitivity. Using the broad-spectrum neutron beam at LANSCE, we have measured inelastic neutron scattering on $^{nat}$Cu. The goal of this work is focused on understanding the background rates from neutrons interacting in these materials in regions around the Q-values of many candidate $0\nu\beta\beta$ decay isotopes, as well as providing data for benchmarking Monte Carlo simulations of background events. Results: We extracted the level cross sections from the $\gamma$ production cross section for 46 energy levels in $^{nat}$Cu . These level cross sections were compared with the available experimental data, as well as the ENDF/B-VII evaluation for discrete levels. We also examined the potential implications of our measurements on $0\nu\beta\beta$ measurements and found that many of the commonly studied $0\nu\beta\beta$ isotopes had Q-values below the cutoff for ENDF/B-VII evaluated discrete levels in either Cu nucleus.
    Physical Review C 06/2013; 87(6):064607. · 3.72 Impact Factor
  • N Fotiades, M Devlin, R O Nelson, T Granier
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    ABSTRACT: Background: 86Kr50 is a neutron-rich nucleus amenable to shell-model calculations due to the shell closure at N=50. It is also produced as a fragment in the fissioning of actinides.Purpose: The level structure of 86Kr at low excitation energies needs additional investigation for detailed comparison with calculations from theoretical models. By determining the cross sections for transitions that feed directly the ground state of 86Kr, a large fraction of the total cross section for the 86Kr(n,n′)86Kr reaction channel can be obtained.Methods: Low-spin states of 86Kr were populated in the 86Kr(n,n′γ) reaction and were studied with the Germanium Array for Neutron-Induced Excitations (GEANIE) spectrometer. The broad-spectrum pulsed neutron beam of the Los Alamos Neutron Science Center's (LANSCE) WNR facility provided neutrons in the energy range from 1 to above 600 MeV. The time-of-flight technique was used to determine the incident-neutron energies.Results: Partial γ-ray cross sections were measured for 21 γ rays of 86Kr and for neutron energies 1 MeV <En<20 MeV. A large part of the total cross section for the 86Kr(n,n′)86Kr reaction is observed. Ten new transitions are observed and placed on the level scheme at excitation energies below 3.7 MeV, doubling the number of placed transitions up to this excitation energy. These transitions de-excite five previously known levels, among them the second and third 0+ states, and one new level. The excitation energy of these levels was more accurately determined and the relative intensities of their decay paths were measured. All previously known levels up to 3.7-MeV excitation energy were identified, and the new level was added at 2917-keV excitation energy. Predictions from shell-model calculations are compared with the data.
    Physical Review C 04/2013; 87(4):044336. · 3.72 Impact Factor
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    ABSTRACT: The SPectrometer for Ion DEtermination in �fission Research (SPIDER) measures both position and time-of-flight (TOF) of charged particles using a system of thin carbon foils, electrostatic mirrors, microchannel plates, delay-line anodes, and a fast TDC. Tests have been conducted using 229Th and the alpha emitters in its decay chain. To date, timing resolution of 200 ps (FWHM) has been achieved corresponding to roughly 0.5% uncertainty in velocity measurements of fi�ssion fragments over a flight path of 52.1 cm. This velocity resolution, in combination with demonstrated fragment energy resolution is sufficient for 1 amu resolution of light mass fragments.
    International Conference on Nuclear Data for Science and Technology, New York, NY; 04/2013
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    ABSTRACT: Depending on the energy of the neutron, fast neutron-induced reactions can populate the same nucleus via α emission at low energies and via emission of lighter-than-α particles (indicated here as 2p2n emission) at higher energies as was observed with the GEANIE array at the Los Alamos Neutron Science Center's (LANSCE) WNR facility in neutron reactions on stable ^48Ti and ^92Mo. In ^48Ti the α emission was stronger while for ^92Mo stronger population via 2p2n emission was observed in accordance with predictions from the GNASH reaction model. On the other hand, in lighter-Z nuclei the α emission is dominant, as observed in reactions on ^16O. In order to locate experimentally a nuclear charge region where the population of the same nucleus in fast neutron reactions along the stability line via α emission at low energies is comparable to the population via 2p2n emission at higher energies, fast neutron-induced reactions on ^60Ni were studied with GEANIE at LANSCE/WNR. The results indicate that in the Ni region α- and 2p2n-emission are comparable, thus establishing experimentally a trend of decreasing transmission for the α particle inside the increasing charge of the compound nucleus.
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    ABSTRACT: The outgoing neutron energy spectra from neutron-induced fission of various actinides are important for basic understanding of the fission process near the scission point as well as playing a large role in neutron transport codes, which are heavily relied upon in the design of advanced nuclear reactors and simulations of critical assemblies. The reliability of the results of neutron transport models is a strong function of the quality of the nuclear data used as input. Currently, the world’s experimental database of fission neutron spectra is severely incomplete (especially for higher incident neutron energies) with large uncertainties in key portions of the outgoing energy spectra. Many transport codes use evaluated data libraries, which are based on the approach of the Los Alamos model. Other theoretical models have been developed, but the available data cannot distinguish the results of different models (as is the case for 239Pu). Better measurements are needed for all incident and outgoing neutron energies, but most urgently in the low-energy (below 1 MeV) and high-energy (above 6 MeV) portions of the outgoing spectra where theoretical model results differ greatly. We present the design considerations (and some characterization results) of the two Chi-Nu neutron detector arrays: one array of Li-glass detectors and one array of liquid-scintillator detectors. These detector arrays are being constructed to meet the challenge of measuring the prompt fission neutron spectra (for a few common actinides) to a higher accuracy and precision than achieved previously and over a larger incident energy range than has been covered by previous experimenters. We see a significant reduction in neutron-scattering backgrounds with our new array designs.
    IEEE Transactions on Nuclear Science 04/2013; 60(2):879-884. · 1.22 Impact Factor
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    ABSTRACT: Prompt-fission-neutron spectra for neutron-induced fission reactions on uranium and plutonium isotopes are important for nuclear applications. We have used 6Li-glass scintillation detectors to measure outgoing neutron energies in the range from 10 keV to 1 MeV, where there is currently large uncertainty in nuclear data. To better understand the response of 6Li-glass detectors in this energy range, measurements of well-known spontaneous-fission neutrons from a 252Cf source were done in the neutron-beam flight path. Results were compared with Monte Carlo simulations and they show good agreement. Similar measurements with a 7Li-glass detector were used to assess gamma-ray background yields.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 03/2013; 703:213-219. · 1.14 Impact Factor
  • Bulletin of the American Physical Society. 01/2013; 58.
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    ABSTRACT: A time-of-flight fission fragment detector capable of determining the velocity and total energy of various nuclear species to high precision has been designed for beam experiments at LANL and tested with a ^252Cf source. A system of thin carbon foils, electron reflectors, microchannel plates, and delay-line anodes are presently being optimized to measure the path length and velocity of fission fragments to high precision. Future incorporation of an ionization chamber will complete one leg of the SPIDER detector and pave the way for 1 amu resolution measurements of neutron-induced fission fragments. The present capabilities of SPIDER will be discussed.
    Bulletin of the Amer.Phys.Soc. 10/2012; 57(9):22.
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    ABSTRACT: Background: Neutron-induced reactions are a significant concern for experiments that require extremely low levels of radioactive backgrounds. Measurements of gamma-ray production cross sections over a wide energy range will help to predict and identify neutron backgrounds in these experiments. Purpose: Determine partial gamma-ray production cross sections for neutron-induced reactions in natural neon. Methods: The broad-spectrum neutron beam at the Los Alamos Neutron Science Center (LANSCE) was used for the measurement. Gamma rays from neutron-induced reactions were detected using the GErmanium Array for Neutron Induced Excitations (GEANIE). Results: Partial gamma-ray cross sections were measured for the first excited-state transitions in Ne-20 and Ne-22. The measured cross sections were compared to the TALYS and CoH3 nuclear reaction codes. Conclusions: These are the first experimental data for (n,n') reactions in neon. In addition to providing data to aid in the prediction and identification of neutron backgrounds in low-background experiments, these new measurements will help refine cross-section predictions in a mass region where models are not well constrained.
    Physical Review C 10/2012; 86(6):067601. · 3.72 Impact Factor
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    ABSTRACT: The distributions of prompt γ rays from the spontaneous fission of 252Cf and neutron-induced fission of 235U were measured up to ∼4MeV using a liquid scintillator array. The unfolding of measured fission γ rays is presented using the Single Value Decomposition and iterative Bayesian methods. General agreement was found with comparisons made with previous measurements. The energy dependence of the prompt γ-ray distributions for the spontaneous fission of 252Cf and the neutron-induced fission of 235U from bombarding energies of 1-2, 5-10, and 10-20 MeV were found to be almost identical in the γ-ray energy region 1-4 MeV.
    Nuclear Instruments and Methods in Physics Research Section A Accelerators Spectrometers Detectors and Associated Equipment 10/2012; 688(1):55-61. · 1.14 Impact Factor
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    ABSTRACT: Background: Neutron-induced backgrounds are a significant concern for experiments that require extremely low levels of radioactive backgrounds such as direct dark matter searches and neutrinoless double-β decay experiments. Unmeasured neutron scattering cross sections are often accounted for incorrectly in Monte Carlo simulations.Purpose: The purpose of this article is to determine partial γ-ray production cross sections for (n, xnγ) reactions in natural argon for incident neutron energies between 1 and 30 MeV.Methods: The broad-spectrum neutron beam at the Los Alamos Neutron Science Center (LANSCE) was used used for the measurement. Neutron energies were determined using time of flight, and resulting γ rays from neutron-induced reactions were detected using the GErmanium Array for Neutron Induced Excitations (GEANIE).Results: Partial γ-ray cross sections were measured for six excited states in 40Ar and two excited states in 39Ar. Measured (n, xnγ) cross sections were compared to the talys and CoH3 nuclear reaction codes.Conclusions: These new measurements will help to identify potential backgrounds in neutrinoless double-β decay and dark matter experiments that use argon as a detection medium or shielding. The measurements will also aid in the identification of neutron interactions in these experiments through the detection of γ rays produced by (n, xnγ) reactions.
    Physical Review C 06/2012; 85(6):064614. · 3.72 Impact Factor
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    Chocs Focus. 06/2012; 3:8-9.

Publication Stats

462 Citations
214.20 Total Impact Points


  • 1984–2014
    • Los Alamos National Laboratory
      • • Los Alamos Neutron Science Center
      • • Physics Division
      Los Alamos, California, United States
  • 2005–2012
    • Lawrence Livermore National Laboratory
      • Physical & Life Sciences Directorate
      Livermore, California, United States
    • Atomic Energy and Alternative Energies Commission
      • Service de Physique Nucléaire (SPhN)
      Gif-sur-Yvette, Ile-de-France, France
  • 2008
    • University of South Dakota
      • Department of Physics
      Vermillion, SD, United States
  • 2007
    • University of Guelph
      • Department of Physics
      Guelph, Ontario, Canada
  • 2004
    • Eskisehir Osmangazi University
      • Department of Physics
      Eskişehir, Eskisehir, Turkey
  • 1972–1998
    • North Carolina State University
      • Department of Physics
      Raleigh, North Carolina, United States
  • 1973–1985
    • Duke University
      • Department of Physics
      Durham, North Carolina, United States