Koji Yoshii

Development of imaging techniques for fast neutron radiography in Japan

Abstract Neutron radiography with fast neutron beams (FNR) has been studied at the fast neutron source reactor “YAYOI” of the University of Tokyo since 1986. Imaging techniques for FNR have been developed for CR-39 track-etch detector, electronic imaging system (television method), direct film method, imaging plate and also fast and thermal neutron concurrent imaging method. The review of FNR imaging techniques and some applications are reported in this paper.

Application of fast neutron radiography to three-dimensional visualization of steady two-phase flow in a rod bundle

Abstract Three-dimensional void fraction distribution of air–water two-phase flow in a 4×4 rod-bundle near a spacer was visualized by fast neutron radiography using a CT method. One-dimensional cross sectional averaged void fraction distribution was also calculated. The behaviors of low void fraction (thick water) two-phase flow in the rod bundle around the spacer were clearly visualized. It was shown that the void fraction distributions were visualized with a quality similar to those by thermal neutron radiography for low void fraction two-phase flow which is difficult to visualize by thermal neutron radiography. It is concluded that the fast neutron radiography is efficiently applicable to two-phase flow studies.

A study on the mechanism model of luminescence in fast neutron television converter

Abstract For a real time imaging technique using a fast neutron radiography (FNR), a fast neutron television system (FNR-TV) was studied using the fast neutron source reactor YAYOI at the University of Tokyo. In order to obtain a good quality image with the FNR-TV, it is essential to develop a high sensitive converter to detect fast neutrons. A sensitive prototype converter made of polypropylene (PP) resin and ZnS(Ag) mixture was recently developed. It was called a PP converter. In this paper, the mechanism models of luminescence for fast neutrons and gamma-rays in the PP converter are presented as well as the comparison with the experimental results. The analytical results agree well with the experimental results.

CORRECTION OF FAST NEUTRON SCATTERED COMPONENTS FROM FAST NEUTRON RADIOGRAPHY IMAGES

Abstract Fast Neutron Radiography (FNR) results have been successfully obtained by the use of the fast neutron source reactor YAYOI of the University of Tokyo. However, an effect of contrast distortion in the FNR images was observed. The intensity of scattered fast neutrons at the center of an object image was analyzed with a simple model of an iron cylindrical sample. Analytical results agreed well with experimental results.

A study on the development of a fast neutron television converter

Abstract Experiments on fast neutron radiography (FNR) were carried out using the fast neutron source reactor YAYOI of the University of Tokyo. For a real-time imaging technique using FNR, for a fast neutron television system (FNR-TV), it is essential to develop a highly sensitive luminescent converter in order to obtain good quality images. A new converter was recently developed and tested. It showed an about six times higher sensitivity compared with the first prototype converter. In this paper, the characteristics of the converter are discussed as well as the clearer images of a FNR-TV when using the converter. The FNR-TV images were obtained with 500 frames integrated. A small hole (1.0 mm in diameter and 10 mm in depth) in an acrylic plate was well imaged through a 50 mm thick iron block. A 0.15 mm thick aluminum spacer in the calibrated fuel pin was also discernible.

Femtosecond electron beam generation and measurement for laser synchrotron radiation

One of the S-band twin linacs (18L linac) of Nuclear Engineering Research Laboratory of University of Tokyo is modified in order to produce femtosecond electron single bunch for femtosecond X-ray generation via Thomson backward scattering, namely laser synchrotron radiation. Laser photocathode RF gun and chicane-type magnetic pulse compressor are installed at the S-band linac. 10 ps (FWHM) laser pulse generates 5 MeV, 10 ps (FWHM), 1 nC electron single bunch, which is accelerated up to 20 MeV in the S-band accelerating tube and compressed to 200 fs (FWHM) by the chicane. Design study has been performed by using the code of PARMELA and the installation has been finished. For precise and reliable measurement of the compressed pulse length, the comparison of measurement between the femtosecond streak camera and coherent transition radiation interferometry was carried out. Good agreement between them for 1—10 ps (FWHM) pulses was achieved. A new Michelson interferometer for the 200 fs pulse is now under construction. ( 1998 Elsevier Science B.V. All rights reserved.

Preliminary examination of the applicability of imaging plates to fast neutron radiography

Abstract Fast neutron radiography is an attractive non-destructive inspection technique because of the excellent penetration characteristics of fast neutrons in matter. However, the difficulty of detecting fast neutrons reduces this attractive feature. As an experiment to overcome the difficulty, imaging plates were applied to fast neutron radiography. A simple combination of two sheets of imaging plates and a sheet of polyethylene as a proton emitter was examined with the (fast neutron, thermal neutron and gamma ray) FTG discriminator proposed by Yoneda et al. . The experimental results showed that the method could be applicable to fast neutron radiography with effective discrimination of γ-rays

CHARACTERIZATION OF THE YAYOI FAST NEUTRON RADIOGRAPHY FIELD

Fast Neutron Radiography (FNR) has been studied with the use of the fast neutron source reactor YAYOI of the University of Tokyo. Three kinds of imaging techniques have been applied to the FNR. A CR39 nuclear track detector called the track-etch method has been successfully used in the first FNR experiments at YAYOI. The etched out images on the CR39 sheet usually have a low optical density, and took about 24 hours to get images. In order to perform an imaging technique with a high resolving power and high contrast, a thin type of luminescent converter and X-ray film method (film method) has been developed for the FNR. The converter is made of a mixture of PMMA and ZnS(Ag). the thickness of which is 40 μm. The system could observe a gap of 100 μm, and took about 3 hours to get results. To make the required imaging time shorter, a television system of fast neutron radiography (FNR-TV) was proposed. In the system, the same technique as a current thermal neutron television system was applied while using a different luminescent converter. The prototype converter was made of a mixture of polypropylene resin and ZnS(Ag). A method of determining L and D was proposed in this paper. Experimental result of L agreed with the geometric dimension of L, while the value of D will be discussed in another paper.

Development of a fast neutron radiography converter using wavelength-shifting fibers

A fluorescent converter for fast neutron radiography (FNR) comprising a scintillator and hydrogen-rich resin has been developed and applied to electronic imaging. The rate of the reaction between fast neutrons and the converter is increased by thickening the converter, but its opaqueness attenuates emitted light photons before they reach its surface. To improve the luminosity of a fluorescent converter for FNR, a novel type of converter was designed in which wavelength-shifting fibers were adopted to transport radiated light to the observation end face. The performance of the converter was compared with that of a polypropylene-based fluorescent converter in an experiment conducted at the fast-neutron-source reactor YAYOI in the University of Tokyo.

Experimental verification of laser photocathode RF gun as an injector for a laser plasma accelerator

The feasibility of the laser photocathode RF gun, BNL/GUN-IV, as an injector for a laser plasma accelerator was investigated at the subpicosecond S-band twin linac system of the Nuclear Engineering Research Laboratory, University of Tokyo. Electron beam energy of 16 MeV, emittance of 6/spl pi/ mm mrad, bunch length of 240 fs (FWHM), and charge per bunch of 350 pC were confirmed at 10 Hz. As for diagnosis of the femtosecond electron bunch, the quantitative comparison of performance of the femtosecond streak camera, the coherent transition radiation (CTR) Michelson interferometer, and the far-infrared polychromator was carried out. We concluded that the streak camera is the most reliable up to 200 fs and that the polychromator is the best for the shorter electron bunch. The 3.5-ps (rms) resolved synchronization between the YLF laser driver for the gun and the electron bunch was achieved. Based on the above experiences, we have designed and installed a much better laser-electron synchronization system using the Kerr-lens mode-locked Ti:Sapphire laser with the min harmonics synchrolocker and the stable 15-MW klystron. The timing jitter is expected to be suppressed down to 320 fs (rms).