Shu A. Hayashi

Calibration of neutron detectors for time-of-flight experiments by making use of a borated graphite standard neutron spectrum pile☆

Abstract In order to obtain a standard field of neutron spectrum in the keV region, a borated graphite pile was designed and used. Theoretical calculations revealed that the 90° (μ=0) angular lethargy spectrum of neutrons at about 22.5 cm from the center, where an isotropic photoneutron source was placed, had an essentially flat shape and little dependence on several factors. By the linac time-of-flight method with this pile, energy dependent neutron detection efficiencies of a 6Li glass scintillation counter bank and a 10Ba-vaseline plug NaI(Tl) counter were determined. As a subsidiary measurement, the spatial neutron distribution was measured by making use of the 58Ni(n,p)58Co and 197Au(n,γ)198Au reactions. It was found that neutrons distributed nearly isotropically around the photoneutron source, which verified the adequacy of the one-dimensional calculation for predicting the neutron spectrum in this pile. Radial attenuations of the reaction rates agreed with the predicted. By making use of these calibrated neutron detectors the neutron spectrum in an aluminum assembly was measured by the time-of-flight method. The values obtained with these two detectors agreed with each other and also with the theoretically predicted value.

Total cross section of lead and contributions from fundamental neutron interactions

Abstract The total cross section of natural lead was measured for neutron energies between 1 eV and 900 eV, in a time-of-flight transmission experiment at the KURRI Electron Linear Accelerator pulsed neutron source. The measured data were used to determine the neutron electrical polarizability, after a careful analysis of the different contributions to the neutron-atom scattering length. The result obtained, α = (2.4 ± 1.5) × 10 −3 fm 3 , is consistent with the most recent and accurate values, while at the same time it emphasizes the need of better statistics and the use of isotopically enriched samples to improve its precision.

Measurement and analysis of the neutron spectrum in a spherical pile of Cu

Abstract In order to assess the neutron cross section for Cu, the angular neutron spectrum at r = 15 cm and μ = 0 in a spherical Cu pile of 60 cm dia has been measured by the linac time-of-flight method. A cylindrical Pb target was placed at the center of the pile to produce photoneutrons. The experimental results have been compared with the theoretical results obtained by 1-D Sn calculations using the group constants from the JENDL-2 and ENDF/B-IV nuclear data files. Better agreement can be seen in the general shape between the ENDF/B-IV calculated angular spectrum for r = 15 cm and μ = 0 and the measured one in three decades of energy range from a few keV to a few MeV than for the JENDL-2 case. But, on detailed examination, several discrepancies in the measured and calculated spectra are observed due to the differences in the JENDL-2 and ENDF/B-IV data used. The JENDL-2 data do not have resonance structure in the energy range 35–500 keV, whereas the ENDF/B-IV data do. From about 10 to 30 keV the ENDF/B-IV total cross section data are 30–60% larger than the data we obtained by a subsidiary measurement. If the ENDF/B-IV total cross section data are decreased in this region, the spectrum calculated with them would approach the measured one.

Application of iron-filtered neutrons to radiography of a copper step within a large iron block and to computer tomography of metallic cylinders

Abstract A time-of-flight technique and iron-filtered neutrons were applied to neutron radiography and to computer tomography. By making use of iron-filtered neutrons, the existence of 1 mm thick copper within an iron block of 20 cm thickness could be clearly distinguished. To investigate the possibility of neutron computer tomography using iron-filtered neutrons, two cylindrical metallic samples made of CuFePb and CuFeAl were used. From the reconstructed image of the samples, we could clearly see the internal structures.

Assessment of nuclear data for copper and silicon by neutron spectra in these piles

Abstract As apart of the series of nuclear data assessment for structural materials for fission and fusion reactors at KURRI, neutron spectra from a few keV to a few MeV in both spherical piles of Cu and Si were measured by the time-of-flight method using an electron linac. The measured spectra were compared with the calculated ones by one dimensional Sn code with two evaluated nuclear data files, ENDF/B-IV and JENDL-2. From these intercomparison, it can be seen that (1) ENDF/B-IV for Cu gives better result in general than JENDL-2. However there still exists a little discrepancy between the calculated spectrum by ENDF/B-IV and the measured, and (2) both nuclear data files for Si afford rather poor prediction of the spectrum in the Si pile.

The Integral Check of Neutron Cross Section Data for Reactor Structural Materials by Measurement and Analysis of Neutron Spectra

In order to check neutron cross section data and group constants for some important structural materials for fast breeder reactors and controlled thermonuclear reactors, the energy spectrum of the neutrons from about 1 keV to a few MeV in a sample pile of assorted materials has been measured by the linac time-of-flight method and the results were compared with that theoretically predicted by one-dimensional transport calculation. To avoid geometrical complexity, we used spherical vessels in which elemental sample powders were packed. Each pile had a lead photoneutron target at the center. To clarify the spherical symmetry of the neutron transport in the pile, the spatial and angular distributions of the 58Ni(n,p)58Co and 197Au(n,y)198Au reactions were measured for each pile. Group constants were produced by SUPERT0G-JR3 with the JENDL and ENDF/B-IV nuclear data files, and then the neutron transport calculations were carried out by DTF-IV or ANISN.