Neutron Resonance Parameters of Silver-107 and Silver-109

Neutron transmission measurements were carried out on the separated isotopes of silver using the time-of-flight facility at the Japan Atomic Energy Research Institute electron linear accelerator. Neutrons were detected with the ⁶Li-glass detectors at 56 and 191 m. The samples used were metallic powder enriched to 98.2% for ¹⁰⁷Ag and 99.3% for ¹⁰⁹Ag. Transmission data were analyzed with the multi-level Breit-Wigner formula incorporated in a least squares fitting program. Resonance energies and neutron widths were determined for the large number of resolved resonances in the neutron energy region of 400 eV~7 keV. The s-wave strength functions and average level spacings were obtained to be; S₀= (0.43±0.05) × 10^?4, D₀ = 20±2 eV for ¹⁰⁷Ag and S₀= (0.45 ± 0.05) × 10^?4, D₀ = 20 ± 2eV for ¹⁰⁹Ag.     

Average Neutron Capture Cross Sections of 151Eu and 153Eu from 3 to 100 keV

Neutron capture cross sections of europium isotopes ¹⁵¹Eu and ¹⁵³Eu were measured in the neutron energy range of 3~100keV. Experiments were carried out with the time-of-flight facility at the 52 m station of the JAERI Electron Linear Accelerator. Prompt capture γ-rays were detected by a large liquid scintillation detector and the neutron flux shape was determined with a ⁶Li glass scintillation detector. The average capture cross sections were examined in terms of energy independent strength functions for ¹⁵¹Eu and ¹⁵³Eu.     

Neutron Resonance Parameters of Ba-135, Ba-137 and Ba-138

Neutron transmission measurements were carried out on the separated isotopes of Ba at the JAERI electron linac. Resonance energies and neutron widths were determined for a large number of resonances in the neutron energy range from 400 eV to 4.6 keV for ¹³⁵Ba, 15 keV for ¹³⁷Ba and 63 keV for ¹³⁸Ba. Many of these resonances were newly observed in this experiment. The s-wave strength functions obtained are S ₀= (1.33±0.22) x ^10-4 for ¹³⁵Ba, and S ₀= (0.51±0.12) x ^10-4 for ¹³⁷Ba. An apparent energy dependence of the strength function was observed for ¹³⁵Ba. New resonance parameters of ¹³⁸Ba were also obtained for several weak P-wave levels.     

Evaluation of Neutron Nuclear Data for Uranium-233

Neutron nuclear data of ²³³U have been evaluated in the energy range from 10-⁵ eV to 20 MeV. Evaluated quantities are the total, fission, capture, elastic and inelastic scattering, (n,2n) and (n,3n) reaction cross sections, and the average numbers of prompt and delayed neutrons emitted per fission. The thermal and resonance cross sections have been evaluated on the basis of the measured data. The resolved resonance parameters are given up to 100 eV and the unresolved resonance parameters between 100 eV and 30keV. The total and fission cross sections have been evaluated in the higher energy region on the basis of the recently measured data, while the theoretical calculation with the optical, statistical and evaporation models has been used for evaluation of the other cross sections. The presently adopted optical potential parameters have reproduced well the experimental total cross section in the entire energy range as well as the measured data of the s-wave strength function. The structure observed in the vp values below 1 MeV is reproduced by the semi-empirical formula based on the fission fragment kinematics. The presently evaluated fission cross section is considerably lower than that of ENDF/B-IV between 10 and 50keV. This low fission cross section is expected to resolve the ^Keff discrepancy pointed out from the benchmark tests in ²³³U critical assemblies.     

Resonance Parameters of Tantalum-181 in Neutron Energy Range from 100 to 4,300eV

Neutron transmission measurements were performed on natural tantalum (abundance ratio 99.988% for ¹⁸¹Ta) in the energy range of 100–4,300 eV using the Japan Atomic Energy Research Institute linac. The transmissions were measured using 55 and 190 m time-of-flight spectrometers for two and three samples of different thicknesses, respectively. These transmission data were simultaneously analyzed with a least squares fitting program based on a multl-level Breit-Wigner formula, and resonance energies and neutron width were obtained for 696 resonances of ¹⁸¹Ta.

The statistical analysis of these parameters gave the s-wave average level spacing of D=4.10±0.14 eV and s-wave neutron strength functions of (1.67±0.13) × 10^?4, (1.09 ± 0.09) × 10^?4 and (1.42 ± 0.20) × 10^?4 for the energy intervals from 100 ? 1,700 eV, 1,700–3,400 eV and 3,400–4,300 eV, respectively. This significant difference among the neutron strength function for each energy interval is a prominent result of the present experiments and is of great interest.     

Application of BGO Scintillators to Absolute Measurement of Neutron Capture Cross Sections between 0.01 eV and 10 eV

Applying a total energy absorption γ-ray detector composed of 12 bricks (5 × 5 cm², 7.5 cm thick) of BGO scintillators, the absolute measurement of capture cross sections for Au and Sb has been made in an energy region between 0.01 and 10eV using the linac time-of-flight method. Incident thermal neutron flux was absolutely determined by using the BGO detection system with a Sm sample. To extend the neutron flux measurement from the thermal neutron region to higher neutron energies, the ¹⁰B(n, αγ) reaction was applied. Absolute capture yield for the relevant capture sample was obtained by the saturated capture yield at a large resonance of the sample.

Gold was selected to investigate the application of the BGO detection system to the absolute measurement of the capture cross sections, since the ¹⁹⁷Au(n, γ)¹⁹⁸ Au reaction cross section is a well known standard one. The result of the ¹⁹⁷Au(n, γ)¹⁹⁸ Au reaction cross section showed good agreement with the evaluated data in JENDL Dosimetry File and ENDF/B-VI. Then, the detection system was applied to the Sb(n, γ) cross section measurement. Antimony has a large scattering-to-capture cross section ratio comparing to that of gold. The result showed good agreement with the evaluated data in JENDL-3.2 and ENDF/B-VI.     

Neutron Cross Sections of Li-6

Calculation procedures have been developed to evaluate the performance of the multistage counter current extraction of transuranics (TRU) from spent molten salt into liquid metal, taking into account stage efficiency and also the scrub stage. The following results, which supplement previous papers, were derived using these procedures. When Cd is used as the liquid metal and the stage efficiency is assumed to be 100%, at least four stages are necessary to recover 99% of TRU from the salt with a decontamination factor (DF) higher than five. A stage efficiency of the extraction better than 80% is desirable for a practical application. The scrub stage is not very effective in improving the DF when the total number of extractions is less than five. The DF slightly increases with higher TRU concentration in the salt since the accompanying lanthanide FP extracted into the Cd in the later stages works as a mild reducing agent in the earlier stages. Although the extraction process has high separation capability, it is very difficult to separate Np, Am, or Cm from Pu due to their similar separation factors. Therefore, the extraction process has inherent proliferation resistance.     

S-wave Neutron Strength Function of Ta-181

A method for correcting leakage currents is described to predict the radiation-induced threshold voltage shift of sub-micron MOSFETs. A practical model for predicting the leakage current generated by irradiation is also given on the basis of experimental results on 0.8-μm process MOSFETs. The constants in the threshold voltage shift model are determined from the “true” I-V characteristic of the MOSFET, which is obtained by correction of leakage currents due to characteristic change of a parasitic transistor. In this way, the threshold voltage shift of the n-channel MOSFET irradiated at a low dose rate of 2 Gy(Si)/h was also calculated by using data from a high dose rate irradiation experiment (100 Gy (Si)/h, 5h). The calculated result well represented the tendency of measured data on threshold voltage shift. The radiation-induced leakage current was considered to decay approximately in two exponential modes. The constants in this leakage current model were determined from the above high dose rate experiment. The response of leakage current predicted at a low dose rate of 2 Gy(Si)/h approximately agreed with that measured during and after irradiation.     

Neutron Nuclear Data Evaluation for Thorium-232

An evaluation was made on the neutron cross sections, resonance parameters and average neutron yield in fission for ²³²Th in the energy range from thermal energy to 20 MeV. The fission and capture cross sections were evaluated on the basis of the experimental data by converting the relative ratio data into cross section values by making use of recent evaluations for reference cross sections. The total cross section was determined from experimental data in the region from 24 keV to 15 MeV and then extrapolated to lower and higher energies by using the optical model whose parameters had been adjusted as so to reproduce the measured data. The elastic and inelastic scattering, (n, 2n) and (n, 3n) reaction cross sections were calculated by means of the statistical model combined with the optical model. A set of resonance parameters were recommended in the energy range below 3.5 keV and average resonance parameters were deduced in the unresolved resonance region. A value of 7.40 b was chosen for the capture cross section at 0.025 eV, and the picket-fence negative-energy levels were introduced so as to reproduce the non-l/v behavior of the capture cross section in the epithermal region.

The results were incorporated in the Japanese Evaluated Nuclear Data Library, Version 2 (JENDL-2). Comparison was made between the present and other evaluations such as ENDF/B-V and possible reasons for the discrepancy were discussed.     

Total Neutron Cross Sections of Magnesium,Aluminum, Silicon,Zirconium, Niobium and Molybdenum in Energy Range from 0.001 to 0.3 eV

Total neutron cross sections of Mg, Al, Si, Zr, Nb and Mo at room temperature have been measured in the energy range of 0.001–0.3 eV using a chopper and TOF facility of the Musashi Institute of Technology Research Reactor. The experiments were performed for solid and powdered samples. For these samples inelastic and elastic scattering cross sections were respectively calculated with the THRUSH code assuming Debye-type frequency spectrum and with the UNCLE-TOM code using crystal structure and lattice constants of each sample.

For solid samples the measured cross sections agreed well with the calculations below the Bragg cut-off energies. For the powdered samples there were good agreements above the Bragg cut-off, and the measurements were larger than the calculations below the Bragg cut-off.

The experimental cross section, adopting the results of powdered and solid samples for energy ranges above and below the Bragg cut-off respectively, agreed well with the calculation which was performed for the samples in perfect polycrystalline state.