Takao Tsuruta

Dose-equivalent response CR-39 track detector for personnel neutron dosimetry

Abstract A dose-equivalent response detector based on CR-39 has been designed to be applied for personnel neutron dosimetry. The intrinsic detection efficiency of bare CR-39 was first evaluated from irradiation experiments with monoenergetic neutrons and theoretical calculations. In the second step, the radiator effect was investigated for the purpose of sensitization to fast neutrons. A two-layer radiator consisting of deuterized dotriacontane (C32D66) and polyethylene (CH2) was designed. Finally, we made the CR-39 detector sensitive to thermal neutrons by doping with orthocarborane (B10H122C2), and also estimated the contribution of albedo neutrons. It was found that the new detector — boron-doped CR-39 with the two-layer radiator — would have a flat response with an error of about 70% in a wide energy region, ranging from thermal to 15 MeV.

Diallyl phthalate resin and its copolymers containing allyl diglycol carbonate as nuclear track detectors

Abstract Diallyl phthalate (DAP) and allyl diglycol carbonate (CR-39), and mixtures of DAP and CR-39 were cast into plates under the same polymerizing conditions. The plates were irradiated with alpha-particles, fission fragments or fast neutrons. After etching, enlarged tracks on the plates were observed and counted using an optical microscope. Pure DAP plate was found to have high detection efficiency for fission fragments, but to be insensitive to alpha-particles and fast neutrons. This characteristic was suitable for detecting fission fragments. On the other hand, pure CR-39 plate was convenient for the detection of alpha-particles and fast neutrons because of high detection efficiency and sensitivity, and short etching time. The copolymers of DAP and CR-39 showed intermediate characteristics. The fabrication of the copolymers made it possible to control the discrimination level for the detection of heavy charged particles.

Fission-fragment registration and etching properties of diallyl phthalate with reference to its use as an external detector in fission-track dating

Abstract The properties of diallyl phthalate (DAP) resin were studied with reference to its use as an external detector in fission-track (FT) dating. The etching characteristics and bulk etch rates of DAP in 30% KOH in water at 90 °C (PW-etch) and 15% KOH, 65% ethanol and 20% H 2 O at 60 °C (PEW-etch) are different. In the case of the PW-etch, high background densities of recoil tracks were observed, that are produced by collisions of fast neutrons with C and O in the detector during irradiation. The PEW-etch does not reveal the background tracks as a result of the high bulk etch rate. The PEW-etch is recommended for use in FT dating because of the absence of a recoil-track background and the shorter etching time. The optimum etching time for both solutions was determined in step-etching experiments. For both etching solutions, the track density in DAP is 4–5% higher than in muscovite mica, as a result of the lower track registration threshold. Automated track counts confirm that image analysis enables accurate determinations of the track density in DAP etched in the PEW-etch.

Characteristics of diallyl phthalate resin as a fission track detector

Diallyl phthalate (DAP) resin plates were irradiated with fission fragments, and then etched in aqueous solution of KOH. Etched tracks were observed and counted by using an optical microscope. The detection efficiency of fission fragments was about 100% for both perpendicular and random incidence. DAP plates were insensitive to alpha particles and fast neutrons. These characteristics are suitable for detecting selected fission fragments, which coexist with alpha particles or fast neutrons. DAP plates are valuable for quantitative analysis of fissionable materials and neutron dosimetry. DAP and allyl diglycol carbonate (CR-39) were formed into copolymers in various ratios. The copolymers showed intermediate characteristics between DAP and CR-39. The fabrication of the copolymers made it possible to control the discrimination level for detection of heavy charged particles.

Reduction in etching time for fission tracks in diallyl phthalate resin

Diallyl phthalate (DAP) resin plates were irradiated with fission fragments, and then etched in a conventional aqueous solution of KOH or a PEW solution containing ethanol, water and KOH. Etched tracks were observed and counted by using an optical microscope. The PEW solution made the fission tracks etch-pits clearer in an incomparably shorter etching time. The large and round etch-pits developed on a smooth surface were convenient for optical automatic counting. These etching characteristics of DAP in a PEW solution are suitable for quantitative analysis of fissionable materials and neutron dosimetry.

Polymerizing condition of DAP resin as fission track detector

Three kinds of diallyl phthalate resin plates were cast under different polymerizing conditions. The plates were irradiated with fission fragments, α-particles, fast and thermal neutrons. After irradiation, the plates were etched with an aqueous solution or a PEW solution. After etching, enlarged nuclear tracks on the plates were observed and counted using an optical microscope. All plates were found to have high detection efficiency for fission fragments, but to be insensitive to α-particles, fast and thermal neutrons. The etching speed of fission tracks on under-cured plates was faster than that on full- and over-cured plates whichever etching solution was used. In the case of adopting the aqueous solution, surface roughness occurs remarkably on the full- and over-cured plates in comparison with the under-cured plates. From these experimental results the conclusion was drawn that the under-cured plate was superior to the full- and over-cured plates as fission track detector.

Fast neutron radiography tests at the YAYOI-reactor, university of Tokyo

Fast neutron radiography (FNR) has been jointly studied by the members of neutron radiography laboratories in Japan by using the CR-39 nuclear track detector at the fast neutron source reactor “YAYOI” of the University of Tokyo. Three beam holes installed at the reactor are utilized, whose collimator ratios, LD, are up to 135, and the fast neutron fluxes are more than 106 ncm−2 s−1. Some well-defined FNR images of various types of objects are obtained with a fast neutron fluence of about 1010 n/cm2. Small holes of 0.5 mm in diameter and 10 mm in depth in an acrylic plate are well imaged through 60 mm thickness of iron plate. Hydrocarbon specimens placed behind thick iron plates (30 mm) are clearly imaged. For the ASTM image quality indicators (IQIs for thermal neutron radiography), very thin A1 spacers with 0.05 mm thickness in the SI indicator have been discernible. The scattering component of fast neutrons from object materials does not seriously degrade the resolution of the FNR image.

Effects of heat-treatment and gamma-ray irradiation on the etch-pit formation in allyl diglycol carbonate resin

Allyl diglycol carbonate resin plates possessing alpha tracks were etched after heat-treatment, or after gamma-ray irradiation. The strong heat-treatment and irradiation brought similar effects to two indicators: etch-pit diameter and bulk etching rate. The heat-treatment above 120°C lasting for 1h and irradiation above 4.0x102 Gy increased the indicators and caused the etch-pits to disappear faster. On the other hand, mild heat-treatment brought the opposite effects to under-cured resin. A reduction of the etch-pit diameter was observed in the resin exposed to heat-treatments between 80°C and 120°C lasting for 1h. These phenomena were observed in the study of plates in which alpha tracks were made after the above mentioned heat-treatment or irradiation. These results suggest not only the conditions under which the resin may be used, but also the possibility of enhancement or suppression of etching speed, and of estimation of thermal history or measurement of gamma-ray dosage.

Measurements and analysis of neutron flux distribution in UTR-KINKI

The neutron flux distributions in UTR-KINKI are comprehensively analyzed in terms of the Au activation reaction rate and compared with those calculated by SRAC-CITATION and MCNP4C. The vertical distribution at the central stringer is found to be a cosine-like one with a smaller FWHM by about 15%, which is in good agreement with both simulation results within an RMS deviation of 2.4% and 2.1%, respectively. However, the CITATION fails to predict the horizontal distribution and humps in the vertical distribution in the fuel tank (FT) just outside the fuel meat due to low energy neutrons moderated in the H2O reflector region. It is suggested that the method of homogenization of the fuel assembly should be optimized to have satisfactory agreement with the measured distributions. On the contrary, the MCNP reproduces well the vertical distribution within the experimental uncertainty of ±3% in the FT and the horizontal distribution within the error of ±10%. From a little asymmetry of the distributions measured in the FT, it is implied that perturbation of the low energy neutron flux due to control rods or voids for them could be appreciable, and should be taken into account in the calculations in order to have even better agreement with the measured distributions.

Current Status and Future Directions of Nuclear Education in Elementary and Secondary Education -Several Measures for Revitalization-

It has been a long time since a necessity to deepen education concerning energy, in particular, nuclear-related education in the elementary and secondary education curriculums in Japan was pointed out. To attain this objective, the nuclear industry and the education industry should work in close cooperation. As the Ministry of Educations Course of Study substantially regulates the direction of school education in Japan, nuclear energy experts should be involved in its development from an early stage in an appropriate manner. At least, training for the teaching profession for science teachers should include experiments related to nuclear energy and radiation. It is considered quite effective to provide incumbent teachers with various training opportunities by nuclear organizations in order to solve the problem in question.