Fujio Hiraga

Some experimental studies on time-of-flight radiography using a pulsed neutron source

We are developing a fiber type two-dimentional (2-D) position-sensitive detector having a spatial resolution less than 0.5 mm in order to use it in the time-of-flight (TOF) measurements. The detector performance was examined, and some experiments on the TOF radiography were performed using this detector. The results indicated that the detector could be applied successfully to the TOF measurement and that the TOF radiography was very useful since it gave more informative data than those obtained by the conventional radiography using a broad energy band.

Comparison of coupled liquid hydrogen and solid methane moderators for pulsed neutron sources

Abstract A coupled liquid-hydrogen moderator with a premoderator can provide a much higher cold-neutron beam intensity than a traditional decoupled moderator. A premoderator is being considered also for the solid-methane moderator system, mainly aimed at reducing the radiation damage of the methane. We have studied experimentally the neutronics of two different cryogenic moderators of liquid hydrogen and solid methane, with variable premoderator thickness. The optimal premoderator thicknesses were found for 2 and 5 cm thick cryogenic moderators. The optimal premoderator thickness is thinner for solid methane moderators than for liquid hydrogen moderators, due to the larger hydrogen number density. The pulse characteristics of cold neutrons from these moderators were also measured. It was found that a coupled solid-methane moderator with premoderator cannot exceed a coupled liquid-hydrogen moderator with premoderator in cold neutron intensity. If solid methane is utilized, the thickness of both premoderator and solid methane moderator must be small.

Application of Spectroscopic Radiography Using Pulsed Neutron Time-of-Flight Method for Material Characterization

Neutron imaging using a pulsed neutron time-of-flight method can give an energy dependent transmission image, namely, spectroscopic image. This image includes the structure information if the sample is coherent scatterer. Here, two examples are introduced. First, we obtained the transmission image of a welded sample of SS304 and 308. Change of the crystal structure depending on the position was observed. Furthermore, we measured spatial dependent transmission of SS samples treated in different ways, surface treatment and whole body treatment. There were almost no spatial dependent change, but the cross section change was found between surface and whole body treatment samples. It was suggested that this might be due to the difference of a grain size. These results demonstrated that the spectroscopic imaging using a pulsed neutron source is a useful tool for material characterization.

Measurement of Neutron Capture Cross Section Ratios of 244Cm Resonances Using NNRI

Measurement of Neutron Capture Cross Section Ratios of Cm Resonances Using NNRI Shinji GOKO a , Atsushi KIMURA a , Hideo HARADA a , Masumi OSHIMA a , Masayuki OHTA a , Kazuyoshi FURUTAKA a , Tadahiro KIN a , Fumito KITATANI a , Mitsuo KOIZUMI a , Shoji NAKAMURA a , Yosuke TOH a , Masayuki IGASHIRA b , Tatsuya KATABUCHI b , Motoharu MIZUMOTO c , Yoshiaki KIYANAGI c , Koichi KINO c , Michihiro FURUSAKA c , Fujio HIRAGA c , Takashi KAMIYAMA d , Jun-ichi HORI d , Toshiyuki FUJJI d , Satoshi FUKUTANI d & Koichi TAKAMIYA a a Nuclear Science and Engineering Directorate , Japan Atomic Energy Agency , Tokaimura, Naka-gun, Ibaraki , 319-1195 , Japan b Research Laboratory for Nuclear Reactors , Tokyo Institute of Technology , O-okayama, Meguro-ku, Tokyo , 152-8550 , Japan c Faculty of Engineering , Hokkaido University , Kita 13, Nishi 8, Kita-ku, Sapporo, Hokkaido , 060-8628 , Japan d Research Reactor Institute , Kyoto University , Asashiro Nishi, Kumatori-cho, Sennangun, Osaka , 590-0494 , Japan Published online: 05 Jan 2012.

Effects of Spatial Harmonics on Power Distribution in Accelerator Driven Subcritical Cores

A method for solving the time-energy-dependent diffusion equation was devised to appreciate the effect of spatial harmonics on the power distribution in a subcritical graphite-moderated core with a pulsed fast neutron source. An instantaneous power peaking factor (IPF) was calculated using the thermal neutron flux thus obtained, to characterize the power distribution in the core. In case of a 50-cycles-per-second injections of fast neutron pulses of 3 ms pulse width, it was indicated that the value of IPF increased by 4.1 times with the decrease in the multiplication factor from 0.9951 to 0.9762, accompanying fuel burnup. To appreciate the dependence of the core power distribution on the spectrum hardening of the thermal neutron flux caused by injections of pulsed fast neutrons, comparison was made between the value of IPF and that derived from the time-dependent-one-group diffusion theory where the spectrum hardening of the thermal neutron flux was ignored. It was indicated that the disregard of the spec...

Design of a Collimator System of a Neutron Beam Line for Neutron-Nucleus Reaction Measurements

Abstract A collimator system of a neutron beam line that is used for neutron-nucleus reaction measurements has been designed. Collimators in the middle region are optimized using a Monte Carlo simulation so that the neutron dose in the shielded area is as small as possible. A collimator closest to the experimental target is designed based on several origins of neutron backgrounds studied by the simulation. The simulation data finally obtained show a favorable background level and an expected neutron beam at the target position.

Backscattering moderators for pulsed spallation neutron sources

The effect of the moderator position, relative to the neutron-generating target, on the neutron intensity was studied experimentally for various poisoned and unpoisoned moderators. For all types of moderator, a backscattering geometry is superior to the normal-extraction geometry. In a backscattering position, poisoned moderators give higher intensity than unpoisoned thin moderators, while at a normal-extraction position the latter is better than the former, except in the case of a very thin moderator.

Tailoring of neutron pulse shapes from a coupled liquid-hydrogen moderator for pulsed spallation neutron sources

Abstract In this paper we introduce an idea for improving the broader pulse width and the long-time tail of cold neutrons from a coupled composite moderator system, by decoupling the premoderator from the reflector. We confirmed experimentally that the pulse width from a decoupled system, a 5 cm thick liquid-hydrogen moderator with 3 cm thick light-water premoderator decoupled from a graphite reflector, can be reduced considerably almost at no cost of the peak height and the decay time of the pulse can also be improved significantly, when compared to those from a coupled composite moderator of the same material and dimensions. The present results are compared to those obtained with an alternative method by heterogeneously poisoning the reflector.

Optimal moderator materials at various proton energies considering photon dose rate after irradiation for an accelerator-driven 9Be(p, n) boron neutron capture therapy neutron source

We evaluated the accelerator beam power and the neutron-induced radioactivity of (9)Be(p, n) boron neutron capture therapy (BNCT) neutron sources having a MgF2, CaF2, or AlF3 moderator and driven by protons with energy from 8 MeV to 30 MeV. The optimal moderator materials were found to be MgF2 for proton energies less than 10 MeV because of lower required accelerator beam power and CaF2 for higher proton energies because of lower photon dose rate at the treatment position after neutron irradiation.

Optimum design and criticality safety of a beam-shaping assembly with an accelerator-driven subcritical neutron multiplier for boron neutron capture therapies

The beam-shaping assembly for boron neutron capture therapies with a compact accelerator-driven subcritical neutron multiplier was designed so that an epithermal neutron flux of 1.9×10(9) cm(-2) s(-1) at the treatment position was generated by 5 MeV protons in a beam current of 2 mA. Changes in the atomic density of (135)Xe in the nuclear fuel due to the operation of the beam-shaping assembly were estimated. The criticality safety of the beam-shaping assembly in terms of Xe poisoning is discussed.