N. W. Hill

The time resolution of bismuth germanate scintillation detectors

Abstract The time resolution of a bismuth germanate scintillation detector was measured as a function of γ-ray energy from 1 MeV to 24 MeV. The measured time resolution of the 60 Co cascade is (2.1±0.2)ns. The time resolution improves at higher γ-ray energies and is (890±60) ps at around 20 MeV.

Resonance structure of 33S+n from transmission measurements

Abstract The neutron total cross section for 33 S has been determined for energies from 10keV up to 2 MeV at the Oak Ridge Electron Linear Accelerator (ORELA) by a time-of-flight (TOF) transmission experiment. By combining these results with (n, α) data obtained at the Geel Electron Linear Accelerator (GELINA), resonance parameter ( E λ , J π , and Γ n ) have been determined up to 270 keV. Neutron strength functions for s- and p-wave resonances were derived. The p-wave strength function is much larger than for other nuclides in this mass region ( 30 Si, 32 S, and 34 S). This is discussed within the framework of the spherical optical model.

Fission cross section ratios for sup 233,234,236 U relative to sup 235 U from 0. 5 to 400 MeV

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).

A high-energy gamma-ray detector system for fast neutron-induced reactions

Abstract We have developed a gamma-ray detector system which is used in conjunction with a pulsed spallation neutron source to study fast neutron induced reactions. The detector system consists of five 7.6− × 7.6-cm bismuth germanate crystals that span an angular range from 45° to 140° in the reaction plane. The sample is located 7.2 m from the neutron-production target. With this apparatus it is possible to simultaneously measure the excitation functions and angular distributions of reaction gamma rays in the neutron energy range from 1 to 100 MeV. Results on the 12 C(n,n′ γ = 4.44 MeV) and 40 Ca(n, γ 0 ) reactions show the unique capabilities of this system.

/sup 58/Ni + n transmission, capture and differential elastic scattering data analysis in the resonance region

High-resolution neutron transmission, capture and differential elastic scattering measurements have been made for /sup 58/Ni-enriched targets at the Oak Ridge Electron Linear Accelerator (ORELA) from the eV to the MeV region. The three sets of data were analyzed simultaneously from 10 to 450 keV, and the transmission analysis was extended to 650 keV. For the 52 s-wave resonances observed between 10 and 650 keV, the average level spacing, D/sub 0/, and the strength function, S/sub 0/, are 12.2 +- 1.0 keV and (3.1 +- 0.6) x 10/sup -4/, respectively.

First observation of (s+d)-wave mixing in low-energy elastic neutron scattering

Abstract A large (s+d)-wave mixing is observed in the 256 KeV ( J π = 1 − ) resonance in 207 Pb + n. A possible interpretation in terms of neutron particle-hole states is discussed.

Accurate determination of the parameters of the 292.4-eV resonance of 91Zr and the 301.1-eV resonance of 96Zr

High-resolution transmission measurements of zirconium metal samples have been carried out at ORELA using the 80-m flight path and an improved /sup 6/Li-glass scintillation neutron detector. Four different thicknesses of the pure zirconium metal and one sample of zircaloy were used in four separate experiments. The transmission data for these samples were measured at room temperature, and one sample was cooled to liquid nitrogen temperature. These data were analyzed using the multilevel R-matrix computer code SAMMY. The resonance parameters obtained for the 292.4-eV resonance of /sup 91/Zr (including both statistical and systematic uncertainties) are: J/sup ..pi../=3/sup +/, E/sub 0/=292.40 +- 0.10 eV, GAMMA/sub n/=665 +- 5 MeV, GAMMA/sub ..gamma../=131 +- 10 MeV, and cc(GAMMA/sub n/, GAMMA/sub ..gamma../)=0.14. The parameters obtained for the 301.1-eV resonance in /sup 96/Zr are: J=1/2, E/sub 0/=301.14 +- 0.10 eV, GAMMA/sub n/=223 +- 7 meV, GAMMA/sub ..gamma../=285 +- 38 MeV, and cc(GAMMA/sub n/, GAMMA/sub ..gamma../)=0.80.

The los alamos national laboratory neutron-neutron scattering program

Abstract A theoretical and experimental program is underway to determine the feasibility of a measurement of the neutron-neutron scattering cross section of 10–12% uncertainty using small-angle, low center-of-mass energy, colliding neutron beams derived from a fusion-fission nuclear source, The neutron-neutron scattering length would be inferred from the measured cross sections. The general concept of the experiments and progress are discussed.

Majority-Logic NE-110 Detector for keV Neutrons

An NE-l10 proton-recoil scintillation counter whose efficiency is reproducible and stable has been developed for neutrons in the energy range from 5 keV to 1 MeV. Majority-of-two logic at below the single photoelectron level is used between two or more phototubes viewing the same scintillator. Pulse height distributions as a function of neutron energy have been measured between 5 and 350 keV with two different detector configurations: a thin square slab of NE-110 and a cylinder of NE-l10. The absolute efficiency of the slab detector has also been determined. The measured results are in good agreement with results from Monte Carlo calculations.

A cryogenic ionization chamber for high-resolution fission cross-section measurements☆

Abstract This paper describes a multiple-plate gas ionization chamber designed for cross-section measurements of neutron-induced fission in strongly radioactive nuclei. The requirements of high resolution are discussed, in particular the need for cooling the sample to reduce Doppler broadening, and the selection of the gas mixtures that will enable fast counting at a low temperature. The observed gain in resolution with cooled samples is in good agreement with theoretical predictions.