Nobuhiro Yamamuro

Systematics of the pygmy resonance in keV neutron capture γ-ray spectra of nuclei with N ≈ 82–126

Abstract We have measured capture gamma-ray spectra of Pr, Tb, Ho, Lu, Ta and Au at neutron energies of 10 to 800 keV with an anti-Compton NaI(Tl) detector, employing a time-of-flight technique. An anomalous bump, so-called the pygmy resonance, was observed in all these spectra. Remarkable features of the pygmy resonance were found to be that the resonance energy and the electric-dipole strength exhausted in the resonance increase with neutron number but these quantities decrease precipitously around the neutron magic number of N = 82. Comparison with theoretical calculations suggests that the pygmy resonance is mainly made up of neutron particle-hole states decoupled from the giant electric-dipole resonance and is excited in a collective mode.

A heavy shield for the gamma-ray detector used in fast neutron experiments

Abstract The 80-20 wt.% mixture of paraffin and boric acid was proposed as a shielding material for the gamma-ray detector used in fast neutron experiments. Its neutron shielding capability was evaluated by neutron and gamma-ray transport calculations and compared with that of other materials. As a result, it was found that the borated paraffin will be an effective and economical shielding material against fast neutrons if it is used with a lead shield. Moreover, a heavy shield for the gamma-ray detector has been constructed, and the typical shield performance was investigated for keV-neutron capture experiments.

Reliability of the weighting function for the pulse height weighting technique

Abstract The pulse height weighting technique is one of the useful methods for neutron capture cross section measurement. The weighting function for this technique was derived using a set of response functions for the C 6 F 6 scintillation detector. When the response function was calculated, only the Compton scattering of gamma-rays with an organic scintillator was taken into account. The parameter for the line broadening of the gamma-ray spectrum was determined by comparing the calculated spectra to the experimental ones. The weighting function thus obtained was W ( E ) = E +0.284 E 2 + 0.00217 E 3 , where E was the energy scale of the pulse height spectrum in MeV. The reliability of the function was experimentally investigated by means of the black resonance method. As a result, it was proven that the weighting function of the C 6 F 6 detector presented here was accurate within 2% error.

( n , α) Reaction on 12 C Induced by 14.1 MeV Neutrons

The ( n , α) reaction on 12 C has been investigated by bombarding a thin polyethylene film with 14.1 MeV neutrons. The emitted α-particles were detected by a CsI(T1) scintillation detector and distinguished from other particles by applying the pulse-shape discrimination method. The angular distribution of the α-particles leaving the residual nucleus 9 Be in the ground state was strongly peaked at about 40° and at the backward hemisphere in the center-of-mass angles, predicting the presence of direct interaction processes. The ground state transition cross section of 76±11 mb was obtained. The forward peaking was compared with the plane wave theory, for both the pick-up and knock-on processes. The reduced width amplitude of the α-cluster at the nuclear surface of 12 C was determined by attempting the theoretical fit to the backward peak of the angular distribution in terms of the heavy-particle stripping model. Comparing this value with those obtained from other reactions on light nuclei, it seems that the...

Neutron total cross-section measurement of 232Th in the off-resonance region below 300eV

Abstract The neutron total cross section of 232 Th at the off-resonance energies from thermal neutron to 300 eV has been measured by the linac time-of-flight method. Neutrons transmitted through a Th sample were detected with a 6 Li glass scintillator, 12.7 cm in diameter and 1.27 cm thick, placed at the 22 m station. Below 20 eV, the present result is in good agreement with the recent measurements at RPI, ORNL and BNL, and is close to BNLs data in the higher energy region. The evaluated nuclear data of ENDF/B-IV are distinctly lower in the energy region from thermal neutron to 300 eV than the present values. The JENDL-2 data are, however, close to the present result.

A miniature fast-neutron spectrometer for scalar spectrum measurement

Abstract A miniature spectrometer has been designed to measure the neutron spectrum inside an assembly. A small spherical container filled with NE213 liquid scintillator is inserted in the assembly and a light guide is used to transmit the scintillation light from the scintillator to a photomultiplier outside the assembly. A light coupler of parabolic shape is inserted between the scintillator and light guide, so that the light in the guide may reflect totally on its side surface. The details of the design, several characteristics and perturbation effects of the spectrometer are mentioned.

Fast Neutron Spectrum in Lithium Fluoride Pile with D-T Neutron Source

A lithium-fluoride pile was constructed by piling the lithium-fluoride ceramic blocks developed for the present study, and scalar neutron spectrum per source neutron in this pile was measured with a miniature NE213 spectrometer. The measured spectrum was unfolded with two window widths, a spectrometer resolution and one modified by one-time FORIST iteration. The measured spectrum was compared with ones calculated using the MORSE-GG Monte Carlo code with a modified point-detector estimator presented by Carter & Cashwell and the GICXFNS group cross-section set. The measured and calculated spectra agreed each other in their error bands, with the exception of the source energy peak.

Direct and collective captures of energetic neutrons by ellipsoidally deformed nuclei

Abstract The direct and collective theories for nucleon radiative capture are applied to the ellipsoidally deformed nucleus within the framework of an adiabatic approximation. A volume-type form factor rf ( r ) is used for the particle-vibration coupling Hamiltonian between a projectile and the E1 vibrational mode of a target nucleus. Numerical application is made for the 238 U(n, γ) 239 U reaction with 5 to 20 MeV neutrons. Contributions to the capture cross section from the rotational bands for all Nilsson levels in 239 U are calculated to produce the excitation curve and capture γ-ray spectra for this reaction. The coupling strength is taken to be 110 MeV and the deformation parameter to be 0.268, with only the Y 2 deformation of the target nucleus being taken into consideration. Good agreement is obtained between the calculated cross sections and the observed data. Discussions are given for some ambiguities in the calculation, i.e. the depth of the central part of an optical potential, the coupling strength, the giant resonance parameters, and the number of final states of a captured nucleon in the residual nucleus. The number of Nilsson levels and the members belonging to each rotational band chosen, strongly influence the results of the calculations.

Measurements of Neutron Slowing Down Time in Graphite by Capture Gamma-Ray Detection

The neutron slowing down time in graphite was measured by means of capture γ-ray detection. Bursts of fast neutrons produced by D-T reaction were injected into a graphite pile. Detection was by Nal(Tl) scintillator covered by metallic foil of either In, Cd or Gd, serving as resonance absorber. The slowing down time was determined from the maximum point of the reaction rate registered by this counter and presented in the form of time spectrum. The values thus determined were 19.2±0.6, 39±3, 43.0±0.7, 46.1±1.0, 47.5±0.8, 55±2 and 35±6 μs, for 1.58, 0.48, 0.44, 0.40, 0.36, 0.30 and 0.17eV respectively. These values agree well with calculated based on crystal model. The time response of the resonance detector also was in reasonable agreement with calculation. The effect of the medium being finite and that of detector position on the observed slowing down time were estimated. From this experiment, it became clear that the process of the neutron slowing down shows some definite influence of the crystal effect o...

Cross-Section Calculations for Neutron-Induced Reactions up to 50 MeV

In the field of accelerator development, medium-energy reaction cross-section data for structural materials of accelerator and shielding components are required, especially for radiation protection purposes. For a d + Li stripping reaction neutron source used in materials research, neutron reaction cross sections up to 50 MeV are necessary for the design study of neutron irradiation facilities. The current version of SINCROS-II is able to calculate neutron and proton-induced reaction cross sections up to {approximately} 50 MeV with some modifications and extensions of the cross-section calculation code. The production of isotopes when structural materials and other materials are bombarded with neutrons or protons is calculated using a revised code in the SINCROS-II system. The parameters used in the cross-section calculations are mainly examined with proton-induced reactions because the experimental data for neutrons above 20 MeV are rare. The status of medium mass nuclide evaluations for aluminum, silicon, chromium, manganese, and copper is presented. These data are useful to estimate the radiation and transmutation of nuclei in the materials.