Naohiro Hirakawa

Development of monoenergetic neutron calibration fields between 8 keV and 15 MeV

For characterization and calibration of neutron dosimeters and spectrometers, monoenergetic neutron calibration fields have been developed at eight energy points between 8 keV and 15 MeV (8 and 27 keV, 0.25, 0.55, 1.0, 2.0, 5.0 and 15 MeV). Monoenergetic neutrons are obtained by the Sc(p, n) reactions for 8 and 27 keV, and by the 7Li(p, n), T(p, n), D(d, n) and T(d, n) reactions between 0.25 and 15 MeV. Care was taken to reduce background neutrons by parasitic reactions and the scattering of primary neutrons at the target. The neutron spectrum was characterized by the time-of-flight technique, and the neutron fluence was measured with two independent methods within ±5%. The field has been applied successfully for calibration and characterization of various neutron dosimeters and spectometers.

Large solid angle spectrometer for the measurements of differential (n, charged-particle) cross sections

Abstract A charged-particle spectrometer with a large solid angle close to 4π has been developed for studies of fast neutron induced charged-particle emission reactions. On the basis of a high-pressure gridded ionization chamber employing high-Z structural elements, the spectrometer permits the measurements of energy-angular distribution of secondary charged-particles with very high geometrical efficiency and a good signal-to-noise ratio. It has been applied successfully for the measurements of double-differential cross sections of (n, xα) reactions for incident neutrons up to 14 MeV, and of (n, p) reactions up to 6 MeV by use of the data reduction and particle selection methods developed in this study. The present spectrometer proved to be an efficient means for studies of differential (n, charged-particle) cross sections.

Neutron Economy of Transmutation of TRU in Thermal and Fast Neutron Fields

Neutron economy of the transmutation of TRU was examined in well thermalized, thermal and fast neutron fields. Burn-up chains of 237Np, 241Am and 243Am, which are the main TRU nuclides in the high level waste, were calculated in the flux region from 1014 to 1017 n/cm2.s. Numbers of neutrons absorbed and produced of each chain were calculated using JENDL-3. The net number of neutron produced n net, which was obtained by the difference of the two numbers, largely varied with the neutron fields, the nuclides and the flux levels. The n net value in the fast neutron field was positive (0.0–1.0) for 237Np, 241Am, 243Am and TRU with the nuclide composition in the high-level waste generated by the conventional PWR. The transmutation of TRU by fission can be performed with producing neutrons in the fast neutron field. On the other hand, the n net value was negative for the well thermalized and thermal neutron fields. For TRU in the high-level waste, the values in those fields were —1.0 at 1014 n/cm2.s and 0.0 at 1...

Improvement of reactivity coefficients of metallic fuel LMFBR by adding moderating material

Abstract For a metallic fuel liquid metal fast breeder reactor, we studied a core concept for improving the Doppler coefficient and the sodium void reactivity without much sacrificing the breeding ratio and the burnup reactivity loss. In the concept, several ordinary fuel pins in all fuel assemblies of a core are substituted by pins containing only zirconium hydride (ZrH). A parametric survey for the ZrH fraction from about 1 to about 5% was performed in this study to investigate the reactivity coefficients and the associated demerits in order to search the optimum fraction of ZrH. The metallic fuel core containing about 3% of ZrH showed the good results for all parameters. Following the parametric study, the effect of hydrogenous material in a metallic fuel core was experimentally confirmed. Doppler reactivity, sodium void reactivity and sample reactivity worths of plutonium and B 4 C were measured in a series of critical experiment at FCA of JAERI. The experimental results showed that the hydrogenous material significantly improved the Doppler and the sodium void reactivities. Analysis of experimental results was performed to check the applicability of the present design codes for a fast reactor with hydrogenous materials.

Measurements of (n, xp), (n, xd) double differential cross sections of carbon and aluminum for 65 and 75 meV neutrons

Double differential (n, xp), (n, xd) cross sections of carbon and aluminum were measured for 65 and 75 MeV neutrons at angles between 12° and 70° using ΔE-E telescopes at a 7 Li(p, n) neutron source facility at TIARA. The data at 12° were obtained by employing an annular geometry. The telescopes consisting of SSD and Nal(Tl) scintillator, experimental methods and the data reduction procedures are presented as well as the results. The carbon data are compared favorably with existing experimental data. The (n, xp) spectra of both carbon and aluminum agreed fairly well with theoretical calculations based on the intra-nuclear cascade model and the multi-step Hauser-Feshbach model including preequilibrium effects, but the (n, xd) spectra differ significantly from those calculations.

Correction of double-differential neutron emission data for sample-dependent effects

Abstract A data correction method was developed to correct double-differential neutron emission data for sample-size effects and the sample-dependent background due to contaminant source neutrons. The latter correction proved to be very important to obtain consistent results. The correction is based on the following: i) Monte Carlo simulation of neutron scattering considering the kinematics and finite experimental geometry, ii) derivation of the energy-angular neutron emission data that provide simulation results consistent with the experimental data, and iii) correction for sample-size effects and the sample-dependent background by, in a unified manner, using the correction factor deduced by the simulation. The present method was applied successfully for a wide range of target masses. The correction procedure, examples of the correction and the effect of the input data on the correction are presented.

Developments of a long liquid scintillation detector for fast neutron time-of-flight experiments

Abstract We have developed a long NE213 liquid scintillation detector (LLSD, 80 cm × 6.5 cm × 10 cm) which is useful as a position sensitive detector and a high-efficiency neutron detector for measurements of neutron emission cross sections. LLSD is a long rectangular shaped NE213 scintillator coupled with two photomultipliers at both ends of the scintillator cell. Position information along the long cell axis is obtained by the time difference method. The time compensated signal needed for time-of-flight (TOF) experiments is provided by a mean timer. Data are acquired for TOF, position and pulse-height signals gated by n-γ discriminator signals using a three-parameter data acquisition system. The updated data acquisition system and a position-dependent biasing analysis could achieve a remarkably lower bias level down to ∼1 MeV neutrons with good n-γ discrimination compared to the similar type NE213 detectors and remove position dependence of detection efficiency. LLSD with the sophisticated data taking and processing techniques proved to be very effective for neutron emission cross section measurements for 10–18 MeV incident neutrons as a wide-angle scattering detector and a high-efficiency single detector.

A compact post-acceleration beam chopper for a 4.5 MV Dynamitron pulsed neutron generator

Abstract A post-acceleration beam chopper (PACS) has been installed for a 4.5 MV Dynamitron accelerator to improve the energy resolution of neutron time-of-flight (TOF) experiments by shortening the duration of the ion beam pulses. The PACS sweeps the accelerated ion pulses across a chopping slit and eliminates the tails of the beam pulses. It operates sinusoidally at a frequency of 8 MHz with a maximum voltage of 10 kV peak to peak in synchronization with the accelerated ion pulses. The high voltage generator of the PACS was constructed of commercially available amplifiers and components, which realized easy maintenance and low cost. The PACS proved to be very effective to improve the pulse shape and has been applied for double-differential neutron emission cross section measurements.

Measurement of fast neutron induced fission cross sections of 232Th, 233U and 234U relative to 235U

Abstract Neutron induced fission cross section ratios relative to 235U have been measured for 233Th, 233U and 234U with a electrostatic accelerator in the energy range from 0.5 to 7.0 MeV. The estimated errors in the present measurement were about 2%.

Measurements of Elastic Scattering Cross Sections of Carbon, Silicon, Iron, Zirconium and Lead for 75 MeV Neutrons

We performed measurements of neutron elastic scattering cross sections of carbon, silicon, iron, zirconium and lead at 75 MeV using a7Li(p,n) quasi-monoenergetic neutron source at TIARA of JAERI. Neutron spectra at 25 laboratory angles between 2.6°and 53.0° were measured by a time-of-flight method (TOF) using five liquid scintillation detectors. The data were corrected for the inelastic scattering neutrons and sample size effects. The experimental data were compared with the neutron cross section libraries (DLC119/HIL086, LA 150) and systematics used in cascade/transport codes (HETC-KFA2, NMTC/JAERI). The DLC119 data and sytematics show large discrepancy from the present data, while the LA 150 data are in fair agreement.