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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.

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 0–60° was used to detect recoil protons from neutron interactions with a CH 2 target. Data for seven of these telescopes were obtained with one set of electronics and are presented here. These data, from 108 to 180° 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.

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.

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...

Neutron reactions that produce $\ensuremath{\alpha}$ particles have been investigated experimentally and analyzed by reaction model calculations for incident neutron energies from threshold to 150 MeV on elemental chromium and iron. The cross sections were measured at the Los Alamos Neutron Science Center by direct observation of $\ensuremath{\alpha}$ particles. Previous data on isotopes ${}^{59}$Co and ${}^{58,60}$Ni were also included in the analysis. The model calculations are made for both statistical decay and pre-equilibrium processes. This study particularly focuses on the pre-equilibrium cluster emission, which is described by the clustering exciton model of Iwamoto and Harada. We calculate the $\ensuremath{\alpha}$-particle formation factors numerically without any of the approximations that appeared in the original model. The model parameter $\ensuremath{\Delta}R$, the nuclear surface area where the pickup reaction may occur, is determined by fitting the calculated $\ensuremath{\alpha}$-particle energy spectra to experimental data. The calculated $\ensuremath{\alpha}$-particle-production cross sections agree well with the measured data, except for the Cr case. With a simple sensitivity study for the level density parameters, it is reported that relatively small changes in the level density parameters improve the reproduction of experimental data significantly. Our realistic model calculations for the pre-equilibrium process shed light on uncertainties in the nuclear level densities in statistical decay calculation.

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.

Continuing improvements in electron linear accelerators, and associated targets, detectors, and data acquisition systems, make facilities based on these neutron sources very productive in meeting n...

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.

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.

Neutron-induced fission cross section ratios from 0.5 to 400 MeV for samples of 233,234,236U relative to 235U have been measured at the WNR neutron Source at Los Alamos. The fission reaction rate was determined using a fast parallel plate ionization chamber at a 20-m flight path. Cross sections over most of the energy range were also extracted using the neutron fluence determined with three different proton telescope arrangements. Those data provided the shape of the 235U(n,f) cross section relative to the hydrogen scattering cross section. That shape was then normalized to the very accurately known value for 235U(n,f) at 14.1 MeV which will allow us to obtain cross section section values from the ratio data and our values for 235U(n,f).

To improve the accuracy of the {sup 235}U(n,f) cross section, measurements have been made of this standard cross section at the target 4 facility at Los Alamos National Laboratory (LANL). The data were obtained at the 20-meter flight path of that facility. The fission reaction rate was determined with a fast parallel plate ionization chamber and the neutron fluence was measured with an annular proton recoil telescope. The measurements provide the shape of the {sup 235}U(n,f) cross section relative to the hydrogen scattering cross section for neutron energies from about 3 to 30 MeV neutron energy. The data have been normalized to the very accurately known value near 14 MeV. The results are in good agreement with the ENDF/B-VI evaluation up to about 15 MeV neutron energy. Above this energy differences as large as 5% are observed.

A relative cross section measurement of the 10B(n,α1γ)7Li cross section was made using the ORELA neutron source. The reaction was measured by observing the 478 keV photon using a 30% efficiency germanium detector. The neutron flux was monitored with a high efficiency plastic scintillator. Data analysis required the use of extensive Monte Carlo calculations for corrections to the data. The measured cross section differs as much as 40% from the ENDF/B-VI evaluation for neutron energies greater than 1.5 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.

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).

The mass of 235U contained in a large-volume multideposit fission ionization chamber used for neutron cross section experiments was measured relative to the National Bureau of Standards reference deposit 25S-2-1. The mass ratio utilized the thermal neutron induced fission reaction in a uniform 25 cm diameter neutron beam from the thermal column of the NBS reactor. The mass was independent of the geometrical area and areal density variation of the deposits, absolute neutron flux, thermal neutron energy distribution, and neutron cross sections. The 235U mass in the chamber is 170.9 × 10−3 g with a one standard deviation uncertainty of 1.2%.

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.

Analysis of our measurement of the 235U(n, f) cross section standard at 14 MeV demonstrates that no change in the published results due to random coincidence events is required.

The high-energy $\ensuremath{\gamma}$ rays following low-energy (${E}_{n}l10$ keV) neutron capture in copper, nickel, and manganese have been observed with a NaI(Tl) anticoincidence spectrometer. Large fluctuations in the intensities of ground-state transitions of the two copper isotopes were observed. The relative intensities of the high-energy transitions in both nickel and copper decrease significantly with neutron energy at only \ensuremath{\sim}1 keV. The variations of the twelve highest energy transitions for the three lowest energy resonances in manganese were measured. The 7.04-MeV transition varied by a factor of 25, while the rest showed much less fluctuation. None of the twelve transitions is appreciably correlated with the reduced neutron width.

$\ensuremath{\gamma}$-ray spectra and total capture yields were measured for resonant neutron capture in an enriched $^{111}\mathrm{Cd}$ sample for neutron energies less than 2.3 keV. The experiment utilized the 40-m flight path of the Oak Ridge electron linear accelerator and a nonhydrogenous liquid-scintillator $\ensuremath{\gamma}$-ray detector. A total of 162 resonances were observed in this interval, and the resonance energies and neutron widths deduced. Approximately 14% of the resonances are assigned to $p$-wave capture on the basis of the $\ensuremath{\gamma}$-ray spectral measurements while an additional 23% are assigned to $s$-wave capture on the basis of their large neutron widths. The parity of the remainder is undetermined. A lower limit of 0.9 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$ is observed for the $p$-wave neutron strength function [$\ensuremath{\Sigma}\frac{g\ensuremath{\Gamma}_{n}^{1}}{(2l+1)\ensuremath{\Delta}E}$] while the $s$-wave neutron strength function [$\ensuremath{\Sigma}\frac{g\ensuremath{\Gamma}_{n}^{0}}{\ensuremath{\Delta}E}$] is 0.15 \ifmmode\times\else\texttimes\fi{} ${10}^{\ensuremath{-}4}$.

Capture $\ensuremath{\gamma}$-ray spectra from the $^{92}\mathrm{Mo}(n,\ensuremath{\gamma})^{93}\mathrm{Mo}$ reaction were measured for neutron energies below 100 keV using the 10-m flight path at the Oak Ridge electron linear accelerator. For neutron energies less than 25 keV a total of 23 different $s$- and $p$-wave resonances were resolved. The neutron binding energy is 8067.4\ifmmode\pm\else\textpm\fi{} 1.5 keV. The partial radiation widths for the 12 highest energy $\ensuremath{\gamma}$ rays which populate positive-parity states below 2.5-MeV excitation were deduced and compared to the valency-model predictions of Lane and Lynn. For $\ensuremath{\gamma}$-ray energies less than 6.5 MeV, the average $M1$ partial radiation width is approximately equal to the average $E1$ partial radiation width. The reduced transition strengths as defined by Bartholomew for these transitions are $\overline{k}(E1)=(1.2\ifmmode\pm\else\textpm\fi{}0.2)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$ and $\overline{k}(M1)=(11.5\ifmmode\pm\else\textpm\fi{}2.3)\ifmmode\times\else\texttimes\fi{}{10}^{\ensuremath{-}3}$. For the 7126-keV transition to the ${s}_{\frac{1}{2}}$ first excited state the average reduced strength is enhanced by a factor of 5 and 2 for the $E1$ and $M1$ transitions, respectively. Nearly 30% of this enhancement for the $E1$ multipoles is attributed to the valency-model contribution while the remainder is assigned to other processes such as the giant dipole resonance and doorway-state components. The average $E2$ width for the 8067-keV ground-state $\ensuremath{\gamma}$ rays is approximately 10% of the average $E1$ ground-state width.

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.

The development of improved neutron detection, for energies less than 20 MeV is required for advances in the study of neutron-induced reactions. These neutron detectors must have an accurately known response function as well as fast-timing capabilities for use at pulsed neutron sources. The accuracy in the determination of the response of most neutron detectors is ultimately limited by the accuracy of the neutron standard reaction cross sections. The exceptions to this rule include thick total absorption detectors and associated particle methods. Several examples of recent progress in neutron detection are presented.

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.

A 14 MeV absolute neutron beam has been established at the NBS 3 MV positive-ion Van de Graaff Accelerator Laboratory. The T(d,n)? reaction is used for the production of the 14.1 MeV neutron flux with a source strength of 108 n/sec. The neutron flux is absolutely determined by the measurement of the associated alpha particle rate in a silicon surface barrier detector positioned at 84 degrees with respect to the beam axis. The facility may be used to measure the absolute response of active neutron monitoring devices with high accuracy. The neutron background contributions may be eliminated in the calibration measurements by utilizing the coincidence between the device and the solid state alpha detector. This method allows a neutron flux in a cone of 2 degree half angle to be placed at 90 degrees with respect to the beam axis.

National Bureau of Standards (NBS) participation in an International Bureau of Weights and Measures (BIPM) sponsored neutron flux intercomparison is described. The efficiencies of two transfer instruments, a gas-filled /sup 3/He proportional counter and a BF/sub 3/ counter imbedded in a polyethylene sphere, were determined at neutron energies of 250 keV and 560 keV. The efficiency of the /sup 3/He detector was determined by placing it in the flux monitored by the NBS black detector. Since the polyethylene sphere detector shadows the black detector, a secondary BF/sub 3/ counter was calibrated and it normalized the sphere and black detector runs. The efficiencies are reported and compared to the quantities determined at other participating standards laboratories.