Double-Differential Neutron Emission Cross Sections of 6Li and 7Li at Incident Neutron Energies of 4.2, 5.4, 6.0 and 14.2 MeV

Energy-angle double-differential neutron emission cross sections of lithium isotopes were measured at incident neutron energies of 4.2, 5.4 and 14.2 MeV for ⁶Li and of 5.4, 6.0 and 14.2 MeV for ⁷Li using a time-of-flight spectrometer. Care was taken in background subtraction and in data correction for sample-size effects. Detailed comparison of the present results was made with the evaluated data in JENDL-3PR1. A spectrum fitting method was used to extract the ^6,7Li(n, n'x)α and (n, 2n) reaction cross sections. Neutrons emitted from the (n, 2n) reactions were well described by the conventional evaporation model. A simple calculation with a final-state Coulomb interaction was effectively applied for the ^6,7Li(n, n'x)α reactions. Angle-integrated cross sections of the ⁷Li(n, n't)α reaction were in good agreement with the JENDL-3PR1 data except the data measured at 6.0MeV. The angular distributions of elastically and inelastically scattered neutrons were successfully analyzed with the coupled-channel method at the incident neutron energy of 14.2 MeV.     

Measurements of double-differential neutron emission cross-sections of 238U and 232Th for 2.6, 3.6 and 11.8 MeV neutrons

Double-differential neutron emission cross-sections (DDXs) of ²³⁸U and ²³²Th were measured for 2.6, 3.6 and 11.8 MeV incident neutrons using a time-of-flight (TOF) method at the Tohoku University 4.5 MV Dynamitron accelerator facility. In DDXs for En=2.6 MeV, discrete structures by vibrational level groups are visible clearly around excitation energy (Ex) around 0.75 and 1.15 MeV, while inelastic scattering to continuum-levels become dominant at En=3.6 MeV for both nuclei. For En=11.8 MeV, continuum structures with strong forward peaking are dominant with some discrete structures at around 0.7–4.0 MeV excitation energy for both of ²³⁸U and ²³²Th. Partial cross-sections for the elastic and inelastic scattering processes were also derived from the DDX data.     

Analysis of n + 6Li Reactions Using the Continuum-Discretized Coupled-Channels Method

The n + ⁶Li reactions are important not only from the basic interest but also from the application point of view. We study ⁶Li breakup reactions at incident neutron energies of 11.5, 14.1, and 18.0MeV applying a d + α cluster model to ⁶Li and using the continuum-discretized coupled-channels (CDCC) approach. At the present energies, our calculation well reproduces the experimental data. This successful result indicates the reliability of the CDCC approach based on the cluster model to the nuclear reaction evaluation of light nuclear reactions.     

Evaluation of Photonuclear Reaction Data on Tantalum-181 up to 140MeV

Photonuclear cross sections of ¹⁸¹Ta are evaluated up to an incident photon energy of 140MeV which is the threshold energy for pion production. Re-analyses were performed on the (γ, n) and (γ, 2n) data measured at Saclay and Livermore, and reference data were reconstructed. The absorption cross sections were evaluated with the giant electric dipole resonance (GDR) model below 40MeV. The calculation by the statistical model with preequilibrium correction reproduced the reference data of all the photoneutron cross sections consistently. From 40 to 140 MeV, the quasideuteron model (QDM) was adopted to evaluate photoabsorption cross sections and the results are compared with the measurements. The decaying processes including n, p, d, t, ³He, and α particle emission up to 140MeV were theoretically evaluated by the ALICE-F code.     

Measurement of Gamma-Ray Production Cross Sections of Iron for Incident Neutron Energies between 6 and 33 MeV

Measurement of differential γ-ray production cross sections, i.e. (n, x γ) cross sections, of Fe was made for neutron energies from 6 to 33 MeV. Neutrons used in the experiment were white neutrons produced with (p, n) reactions by 35 MeV protons using a thick Be target. The neutron energy was analyzed by the time-of-flight method and bunched into 3 MeV wide energy bins, for each of which the spectrum of secondary γ-rays produced in an Fe sample was measured by a BGO scintillator at an angle of 144° to the neutron beam direction.

The obtained (n, xγ) cross sections agreed well with other data and the evaluated data file of ENDF/B-IV at neutron energies below 15 MeV where data were existing. The JENDL-3 file overestimated the γ-ray spectra at γ-ray energies of 3 to 7 MeV. The present work newly provided the data in the neutron energy range above 20 MeV. The GNASH calculation made by Young reproduced the measured data fairly well even at these higher energies.     

Geometrical Interpretation of Mutual Relatonship between Ordinary Coherence and Noise Power Contribution Ratio

Angular neutron fluxes leaking from the surface of lithium-oxide and graphite slab assemblies have been measured with irradiation of D-T neutrons. The spectrum measurement was performed using the time-of-flight technique with an NE213 scintillation detector. The thicknesses of the slabs were 0.6 to 5 mean free path for 14.8 MeV neutrons, and the measured leaking angles of the angular fluxes were 0.0°, 12.2°, 24.9°, 41.8° and 66.8°. The experimental results have been compared with the results calculated by the continuous energy Monte Carlo transport code MCNP, using the data in the JENDL-3PR1, ?3PR2, and ENDF/B-V nuclear data files. The comparisons between the experimental and calculated results show that the data of ⁷Li in JENDL-3PR2 is improved for the secondary emission spectra of the 4.63 MeV level and (n, 2n) reactions; the angular distributions of 3rd-and 4th-level inelastic reactions of C in the JENDLs are questionable. The thickness dependences for high energy neutrons also suggest that the total cross section of ⁷Li and the elastic cross sections of C are slightly inadequate.     

Reaction rate analyses of accelerator-driven system experiments with 100 MeV protons at Kyoto University Critical Assembly

At the Kyoto University Critical Assembly, a series of reaction rate experiments is conducted on the accelerator-driven system (ADS) with spallation neutrons generated by the combined use of 100 MeV protons and a lead–bismuth target in the subcritical state. The reaction rates are measured by the foil activation method to obtain neutron spectrum information on ADS. Numerical calculations are performed with MCNP6.1 and JENDL/HE-2007 for high-energy protons and spallation process, JENDL-4.0 for transport and JENDL/D-99 for reaction rates. That the reaction rates depend on subcriticality is revealed by the accuracy of the C/E (calculation/experiment) values. Nonetheless, the accuracy of the reaction rates at high-energy thresholds remains an important issue in the fixed-source calculations. From reaction rate analyses, the indium ratio is newly defined as another spectrum index with the combined use of ¹¹⁵In(n, γ)^116mIn and ¹¹⁵In(n, n′)^115mIn reaction rates, and considered useful in examining the neutron spectrum information on ADS with spallation neutrons.     

Neutron Activation Cross Section of Molybdenum Isotopes at 14.8 MeV

The neutron activation cross sections of Mo isotopes have been measured for the 14.8 MeV neutron. The cross sections have been determined with reference to the known ²⁷A1 (n, α)²⁴Na and the ²⁷Al(n, p)²⁷Mg reactions. The cyclic activation method was employed for the γ-ray measurement of short-lived nuclei. A 55 cm³ Ge(Li) detector was used for the measurement of γ-ray spectra. Cross section data are presented for (n, 2n), (n, p) and (n, a) reactions on Mo isotopes. The cross sections of (n, np) reactions on ⁹⁸Mo are also presented. The exponential dependence on (N-Z)/A of the (n, p) reaction cross sections are discussed.     

Scattering of 14.1 MeV Neutrons by 6Li, 7Li, 9Be, 10B and 11B

Using a time-of-flight spectrometer, the differential cross sections were measured for the elastic and inelastic scattering of 14.1 MeV neutrons by ⁶Li, ⁷Li, ⁹Be, ¹⁰B and ¹¹B. In the case of elastic scattering by ⁷Li and ¹⁰B, correction was applied to subtract the contribution of inelastic scattering from the unresolved first excited state, after which, the elastic scattering data were compared with predictions based on the optical model. The potential parameters derived with a seven-parameter search yielded angular distributions agreeing with the present experimental data. The expressions for these parameters are presented as a function of mass number.

The experimental data on inelastic scattering were analyzed with the distorted wave Born approximation. The deformation parameters were estimated to be nearly equal to or larger than unity for these nuclei.     

Cross Section Measurement for 199Hg(n,n′)199m Hg Reaction from 0.78 to 6.3 MeV

Cross sections for the ¹⁹⁹Hg(n, n′)^199m Hg reaction have been measured at 10 energy points from 0.78 to 6.3 MeV by the activation method. Monoenergetic neutrons below 2 MeV were produced by the ⁷Li (p, n)⁷Be reaction and those above 2 MeV were produced by the D(d, n)³He reaction using a 5.5 MV Van de Graaff accelerator. The neutron flux was determined with a proton recoil telescope counter and In-foils. The measured cross sections were expressed by an empirical formula. The fission spectrum averaged cross section calculated with this formula is 238.3 mb for the Watt-type fission spectrum, and is about 14% smaller than that recently measured by three of the authors.     

Cross Section Assessment by Measurement of Time Dependent Neutron Spectra from Lithium Assemblies

Time dependent neutron spectra from lithium assemblies were measured to assess the neutron cross sections of ⁷Li in ENDF/B-IV, which is important nuclide for the D-T fusion reactor blanket material. Pulsed neutrons produced by D-D or D-T reaction were used to measure leakage neutron spectra from cubical lithium assemblies as a function of time by the use of NE213 liquid scintillator. Calculations of time dependent neutron spectra were carried out by the Monte Carlo code SIMON, which was prepared for this study. The group constants used in these calculations were processed from ENDF/B-IV data. The calculated and the measured neutron spectra were compared for the following three; a stationary spectrum, spectra at each time interval and decay curves for specified energy groups. Discrepancies between the measured and the calculated neutron spectra were found in these comparisons. In order to assure the cause of these discrepancies, some calculations were carried out with recently measured cross sections of inelastic scattering which excite 0.478 and 4.63 MeV level of ⁷Li. It was concluded that some of the neutron cross section data of ⁷Li in ENDF/B-IV should be ameliorated.     

Measurement of Double-Differential Neutron Emission Spectra from Uranium-238

We have performed the measurement of neutron emission spectra from ²³⁸U using a time-of-flight technique, and deduced the following data; (1) the prompt fission neutron spectra for 2 MeV incident neutrons at two emission angles of 90° and 135°, (2) the double-differential neutron emission cross sections at the incident energies of 1.2, 2.0, 4.2, 6.1 and 14.1 MeV. The emission spectra and the cross sections for scattering process were also deduced by subtracting the fission neutrons from the experimental spectra. The experimental results were compared with other experiments and the evaluations of JENDL-3 and ENDF/B-IV.

From the fission spectrum data ranging from 2 to 12 MeV, we have derived the best fit parameters for the Maxwellian and Watt type distribution functions. The experimental spectra are described with the Maxwellian spectrum with temperature of 1.24–1.26 MeV and are softer than both evaluations.

The spectra and cross sections for inelastic-scattering showed substantial disagreement with the evaluations concerning the discrete levels between 0.5 and 1.2 MeV, and continuum neutrons due to evaporation and pre-equilibrium processes. The secondary neutron angular distributions at 14 MeV incident energy were reproduced fairly well with the systematics.     

Evaluation Methods of Resonance Absorption for System with Pellet Surrounded by Fuel Solution

Abstract

The reaction cross sections of ²⁷Al(n, p)²⁷Mg, ²⁷Al(n, a)²⁴Na, ⁵⁶Fe(n, p)⁵⁶Mn, ⁹⁰Zr(n, 2n)^89m+gZr and ⁹³Nb(n, 2n)^92mNb have been measured by the activation method in an energy range of 13.3–14.9 MeV using the intense D-T neutron source, FNS. Absolute flux was determined by the associated α-particle counting method incorporated with neutron spectra obtained from both a Monte Carlo calculation and a time-of-flight measurement. Corrections were extensively performed not only for the neutron flux determination, but also for the low energy neutron contribution to the reaction rates. The present data were compared with comprehensive evaluations as well as recent experimental data. The measured cross sections of ²⁷Al(n, a)²⁴Na, ⁵⁶Fe(n, p)⁵⁶Mn and ⁹⁰Zr(n, 2n)^89m+gZr are generally in good agreement within experimental errors with the values in both the JENDL Dosimetry File and IRDF-90. It is also shown that there are the overestimation of the cross sections for ⁹³Nb(n, 2n)^92mNb in the JENDL Dosimetry File, and the over- estimation and underestimation of the cross section for ²⁷Al(n, p)²⁷Mg in the JENDL Dosimetry File and IRDF-90, respectively.     

Measurement of the 93Nb(n,n')93mNb Reaction Cross Section at 14.5 and 14.9MeV

Abstract

Cross sections at 14.5 and 14.9 MeV for the low threshold energy reaction of ⁹³Nb(n, n')^93mNb were measured by the activation method at the FNS (Fusion Neutronics Source) of JAERI. Although there are multiple experimental data for this reaction cross section from the threshold to 9 MeV, only one data at 14.3 MeV has been reported by Ryves et at. The correction for low energy neutron contribution was found very sensitive on the reaction rate determination. Careful treatments were performed to arrive at final error evaluation considering neutron spectrum calculated with a Monte Carlo Code and cross section curves available. The cross sections measured in the present work are larger by more than 30% than those in both IRDF-90 and JENDL Dosimetry File, which are based on the data of Ryves et al. On the other hand, the present data are lower by 10-15% than the evaluation by Odano et al. It is highly recommended to re-evaluate the cross section by taking the present data into account.     

Calculated neutron cross sections for Cu and Nb up to 32 MeV for neutron damage analysis

Cross sections for neutron interaction with Cu and Nb, with emphasis on spectra of light particles from binary reactions, are calculated for neutron energies from 4 to 32 MeV for estimating recoil probability densities for the analysis of damage experiments with a Be (d, n) neutron source. Nuclear model parameters were adjusted to reproduce the available cross-section data around 14 MeV. Helium production cross sections were also calculated for ⁶³Cu for neutrons below 20 MeV, as an illustration of the Hauser-Feshbach method for calculating tertiary reaction cross sections.     

Evaluation of Neutron Nuclear Data for Sodium-23

Neutron nuclear data of ²³Na have been evaluated in the neutron energy region up to 20 MeV. Evaluated are the elastic and inelastic scattering, capture, (n, 2n), (n, p), (n, α), (n, np), (n, nα) reaction and γ-ray production cross sections, and the angular and energy distributions of neutrons and γ-rays. The evaluation is mainly based on nuclear model calculations. The pre-equilibrium and direct-reaction processes were taken into account in addition to the compound process. The evaluated data have been compiled into the latest version of JENDL, JENDL-3.3.     

Evaluation of neutron nuclear data of praseodymium-141 and -143

For the next version of JENDL general purpose file, neutron nuclear data on ^141,143Pr are evaluated considering cross sections and spectra provided from experiments in the fast neutron energy region up to 20 MeV. Total and elastic cross sections are derived from optical model. Pre-equilibrium and direct-reaction processes are taken into account in addition to the compound process. The statistical model is applied to calculate the cross sections above the resolved resonance region. For unstable ¹⁴³Pr nuclide having less experimental data, we use the same parameters as ¹⁴¹Pr for mass-dependent and -independent terms in the optical potential. The resolved resonance parameter remains unchanged from JENDL-4.0. The present evaluations give consistent results with experimental data, reflecting newly measured one. The evaluated data are compiled in ENDF-6 format for the next version of JENDL.     

Measurement of thick-target gamma-ray production yields of the 7Li(p,p′)7Li and 7Li(p, γ)8Be reactions in the near-threshold energy region for the 7Li(p,n)7Be reaction

The gamma-ray production reactions, ⁷Li(p, p′)⁷Li and ⁷Li(p, γ)⁸Be, occur along with the neutron production reaction ⁷Li(p, n)⁷Be in a p-Li neutron source. These gamma-ray production reactions contribute to a patient's absorbed dose in boron neutron capture therapy (BNCT) when using a neutron beam from the ⁷Li(p, n)⁷Be reaction. The present work experimentally determined the thick-target gamma-ray production yields of the ⁷Li(p, p′)⁷Li and ⁷Li(p, γ)⁸Be reactions at incident proton energies of 1.670 and 1.870 MeV. The present results were compared with previous measurements. The gamma-ray production yield of ⁷Li(p, p′)⁷Li was measured to be 30%–50% smaller than as reported by previous studies. For the ⁷Li(p, γ)⁸Be reaction, the present thick-target yield is 30% smaller than one estimated from cross-section data measured in previous studies. The results must be included in future dose evaluation for BNCT using a p–Li neutron source.     

High Resolution Measurements of Double Differential (n, α) Cross Sections of 58Ni and natNi between 4.2 and 6.5 MeV Neutrons

Double differential (n, α) reaction cross sections of ⁵⁸Ni and ^natNi were measured for 4.2–6.5 MeV neutrons with energy resolution good enough to separate α-particles from the low-lying levels of residual nuclei by using a gridded ionization chamber. Angular distribution and excitation functions were derived for α₀, α₁ and α_i≥2 components (α-particles to the ground level, the 1st level and levels higher than the 2nd level, respectively). The experimental results were compared with these obtained from calculation based on Hauser-Feshbach model employing the optical potential and the level density parameters derived to reproduce the experimental values of total, (n,p) and (n,α)cross sections. The calculation showed fair agreement with the experimental data while it underestimated the (n,α)cross section above 6 MeV.