O. A. Wasson

Measurement of the 235U Neutron-Induced Fission Cross Section at 14.1 MeV

The /sup 235/U neutron-induced fission cross section was measured at a neutron energy of 14.1 MeV using the time-correlated associated-particle technique with the /sup 3/H(d,..cap alpha..)n reaction at the National Bureau of Standards 3-MV Van de Graaff Laboratory. The areal density and total mass of the /sup 235/U deposits were measured relative to the standard /sup 235/U reference deposit (Los Alamos National Laboratory Spare Number 1) using thermal-neutron-induced fission counting. The total mass was also determined from the alpha-particle decay rate. The measured /sup 235/U cross section at 14.1 + or - 0.1 MeV is 2.080 + or - 0.030 b where the uncertainty i one standard deviation. This value agrees within 1% with other recent measurements using this technique and with the ENDF/B-V evaluation.

The 59Co(n,xα) Reaction from 5 to 50 MeV

Alpha-particle production cross sections and spectra produced by neutron bombardment of 59Co are measured at 30, 60, 90, and 135 deg over the neutron energy range from 5 to 50 MeV. A source of neut...

Design and calibration of an absolute flux detector for 1–15 MeV neutrons

Abstract An absolute neutron flux monitor having fast timing and a calculable response has been developed for use in a collimated beam of 1–15 MeV neutrons. The detector consists of dual thin plastic scintillators in which the proton recoil spectrum distortion caused by the escape of protons from the first scintillator is eliminated experimentally. The absolute detector efficiency was measured at 2.45 and 1.40 MeV neutron energies using the associated-particle technique at the NBS Positive-Ion Van de Graaff facility. The efficiency and pulse height distributions were calculated using a Monte Carlo based program in order to extent the efficiency throughout the 1–15 MeV interval. The uncertainty in the efficiency is 1–2% (standard deviation).

Absolute Measurement of the Uranium-235 Fission Cross Section from 0.2 to 1.2 MeV

The absolute /sup 235/U neutron-induced fission cross section has been measured at the U.S. National Bureau of Standards (NBS) 3-MV Van de Graaff Laboratory from 0.2- to 1.2-MeV neutron energy. The mass of the /sup 235/U contained in a large volume multiplated fission ionization chamber was measured relative to the NBS fissionable isotope mass standards. Pulsed beam time-of-flight techniques were used with neutrons from the /sup 7/Li(p,n)/sup 7/Be reaction while the neutron flux was monitored with a large plastic scintillator whose efficiency was both calculated and measured with the associated-particle technique. The cross sections, which were measured with a typical uncertainty of 2.3%, are about2% lower than the ENDF/B-V evaluation.

Calibration of the NBS black neutron detector at 2.3 MeV using the time-correlated associated-particle method

Abstract A time-correlated associated-particle measurement capability using the D(d, n)3He source reaction has been developed at the National Bureau of Standards 3-MV positive-ion Van de Graaff Accelerator Laboratory. The facility has been used to measure the efficiency of the NBS Black Neutron Detector at a neutron energy of 2.3 MeV. The associated 3He particles are detected at an angle of 45° with respect to the beam axis which is a more forward angle than conventionally employed. The kinematically more energetic 3He particles detected at the forward angle are readily separated from scattered deuterons at an incident beam energy of 250 keV. The time-correlated coincidence requirement on events detected in the Black Neutron and associated-particle detectors virtually eliminates the need for background corrections to the Black Neutron Detector rate. A result for the efficiency of the Black Neutron Detector at 2.3 MeV has been obtained with an accuracy of about ± 1% and agrees well with a Monte Carlo calculated value. The measurement extends the usefulness of the Black Neutron Detector as an absolute neutron flux monitor to the higher energy region.

An Absolute Neutron Flux Detector for the 1-20 MeV Energy Region

A neutron flux detector for use in the 1–20 MeV energy range is described. A dual thin scintillator configuration yields a proton recoil spectrum which approaches the ideal thin scintillator response. Corrections due to multiple neutron scattering and escape of recoil protons from the scintillator are small. The detector efficiency was experimentally calibrated at 2.47 and 14.1 MeV using the associated particle technique and was extended to other energies by means of a Monte Carlo calculation. The efficiency uncertainty is estimated to be 1–2% in the 1–20 MeV region.

An update on (n,charged particle) research at WNR

Neutron-induced reactions producing light charged particles continue to be investigated at the spallation fast-neutron source at the Los Alamos Neutron Science Center (LANSCE). New data on the cross sections for alpha-particle production for neutrons on {sup 58}Ni and {sup 60}Ni are presented from threshold to 50 MeV. Recent changes in the experiment now allow protons, deuterons, tritons, {sup 3}He and alpha particles to be identified.

Erratum: Measurement of the n-p elastic scattering angular distribution at E{sub n}=10 MeV [Phys. Rev. C 65, 014004 (2001)]

The reported data are given for the mean angles measured rather than for the central angles. The data are normalized to the most recent Evaluated Nuclear Data File evaluated angle-integrated elastic-scattering cross section and refitted with a Legendre polynomial expansion.

Measurements of the H(n,n)H Angular Distribution at 10 MeV Neutron Energy

Relative measurements of the cross section for scattering of neutrons by protons have been made at 10 MeV neutron energy for center-of-mass neutron scattering angles from 60° to 180°. The measurements were made using the Ohio University Accelerator Laboratory’s tandem Van de Graaff accelerator with the D(d,n) reaction as the neutron source. The data are in good agreement with predictions from the phase shift analyses of Arndt, the groups of Nijmegen and Bonn, and the ENDF/B-V evaluation. The ENDF/B-Vl evaluation does not appear to have the same angular dependence as the data.

Measurement of the angular distribution of neutron-proton scattering at 10 MeV

The relative angular distribution of neutrons scattered from protons was measured at an incident neutron energy of 10 MeV at the Ohio University Accelerator Laboratory. An array of 11 detector telescopes at laboratory angles of 0 to 60 degrees was used to detect recoil protons from neutron interactions with a CH{sub 2} (polypropylene) target. Data for 7 of these telescopes were obtained with one set of electronics and are presented here. These data, from 108 to 180 degrees for the center-of-mass scattering angles, have a small slope which agrees better with angular distributions predicted by the Arndt phase shifts than with the ENDF/B-VI angular distribution.