Tetsuya Kai

Particle and Heavy Ion Transport code System, PHITS, version 2.52

An upgraded version of the Particle and Heavy Ion Transport code System, PHITS2.52, was developed and released to the public. The new version has been greatly improved from the previously released version, PHITS2.24, in terms of not only the code itself but also the contents of its package, such as the attached data libraries. In the new version, a higher accuracy of simulation was achieved by implementing several latest nuclear reaction models. The reliability of the simulation was improved by modifying both the algorithms for the electron-, positron-, and photon-transport simulations and the procedure for calculating the statistical uncertainties of the tally results. Estimation of the time evolution of radioactivity became feasible by incorporating the activation calculation program DCHAIN-SP into the new package. The efficiency of the simulation was also improved as a result of the implementation of shared-memory parallelization and the optimization of several time-consuming algorithms. Furthermore, a number of new user-support tools and functions that help users to intuitively and effectively perform PHITS simulations were developed and incorporated. Due to these improvements, PHITS is now a more powerful tool for particle transport simulation applicable to various research and development fields, such as nuclear technology, accelerator design, medical physics, and cosmic-ray research.

Optimization of Coupled and Decoupled Hydrogen Moderators for a Short-pulse Spallation Source

Neutronic performance of coupled and decoupled hydrogen moderators for a short pulse spallation source was extensively studied by calculations to seek the optimal parameters, especially the para/ortho hydrogen ratio. A decoupled 100% para-hydrogen moderator gives excellent pulse characteristics at lower energies. Gadolinium (Gd) poisoning is useful to obtain narrower pulses up to about 70 meV at a reasonable intensity penalty. At higher energies, where the pulse tail decay characteristics become most important, the choice of the reflector material and the decoupling energy (E d) play an important role. One important technical issue is the radiation damage of a Gd-poison and a boron carbide decoupler. We discuss the possibility of using a composite decoupler material such as Cd+In (indium)+Eu (europium). In a coupled moderator a 100% para-hydrogen moderator of a larger volume gives an additional intensity gain over a normal one but the gain is not as large as in a decoupled one.

Commissioning start of Energy-Resolved Neutron Imaging System, RADEN in J-PARC

Neutron News Volume 26 • Number 2 • 2015 1

Counting-type neutron imaging detectors of the energy-resolved neutron imaging system RADEN at the J-PARC/MLF

The recently commissioned Energy-Resolved Neutron Imaging System, RADEN, located at the J-PARC Materials and Life Science Experimental Facility (MLF), is the worlds first dedicated high-intensity, pulsed neutron imaging instrument. In addition to conventional radiography and tomography, the wide bandwidth and accurate measurement of neutron energy by time-of-flight is utilized to perform energy-resolved neutron imaging. Such techniques allow direct imaging of the macroscopic distribution of microscopic properties of materials in situ, including crystallographic structure and internal strain, nuclide-specific density and temperature distributions, and internal/external magnetic fields. To carry out such measurements in the high-rate, high-background environment at RADEN, we use cutting-edge detector systems, recently developed in Japan, employing micro-pattern detectors or fast Li-glass scintillators with high-speed, Field Programmable Gate Array-based data acquisition. These counting-type detectors offer sub-μs time resolution, high neutron count rates, and event-by-event gamma rejection. The available detectors offer a range of spatial resolutions from 0.3 to 3 mm and counting rates from 0.6 to 8 Mcps. In the present paper, we show the performance of these detectors as measured at RADEN. We also consider planned improvements to the detector systems that will allow us to achieve finer spatial resolutions by several factors and order-of-magnitude higher count rates.

Performance of Optical Devices for Energy-Selective Neutron Imaging in NOBORU at J-PARC

The NeutrOn Beam-line for Observation and Research Use (NOBORU) is a day-1 neutron instrument serving as a test beam port at the Materials and Life Science Experimental Facility of the Japan Proton Accelerator Research Complex. Energy-selective neutron imaging is one of the more important research activities performed with NOBORU. To obtain a high spatial resolution with low background environment in the imaging experiment, improved neutron optics is necessary. Therefore, a rotary collimator to control the spatial resolution with high neutron intensity and a neutron filter device to control the neutron spectral intensity and gamma ray intensity are designed and installed on the middle of the beam-line. It is found from the results of a neutron radiography test that neutron transmission images with high spatial resolution (~50 m) can be obtained using the smallest hole in the rotary collimator. It is also confirmed that the remote-controlled filter device introduced in front of the rotary collimator can control the intensity of neutrons and gamma rays with only a small increase of the background. In particular, as bulk lead plates and bismuth single crystal plates attenuate the prompt gamma rays while only slightly sacrificing neutron intensity, neutrons in the epithermal neutron region can be easily measured.

Development of the next-generation micro pixel chamber-based neutron imaging detector (μNID) for energy-resolved neutron imaging at the J-PARC/MLF

The Energy-Resolved Neutron Imaging System RADEN, located at the J-PARC Materials and Life Science Experimental Facility in Japan, is the worlds first dedicated high-intensity, short-pulsed neutron imaging beam line. To carry out energy-resolved neutron imging at RADEN, we use cutting-edge detector systems employing micropattern detectors and data acquisition systems based on Field Programmable Gate Arrays to provide the necessary sub-μs time resolution, high counting rates, and event-by-event background rejection. One such detector, the Micro Pixel Chamber-based Neutron Imaging Detector (μNID), provides a spatial resolution of 120 μm (s), time resolution of 0.6 μs, 18% detection efficiency for thermal neutrons, and effective gamma sensitivity of less than 10−12. We have recently increased the rate capacity of the μNID from 600 kcps to more than 8 Mcps via an upgrade of the readout electronics and the introduction of a new gas mixture optimized for higher count rate, better spatial resolution, and higher detection efficiency. We are also developing new detection elements with strip pitches of 280 μm and 215 μm, down from 400 μm, with a corresponding improvement in the spatial resolution expected. Here, we present the progress of the ongoing development of the μNID, including the results of recent on-beam tests performed at RADEN.

Induced-radioactivity in J-PARC Spallation Neutron Source

Radioactivity estimation was performed on a MW-class spallation neutron source by using the DCHAIN-SP code system to design facility from both safety and maintenance points of view. Remote handling procedures and shielding parameters were considered based on the estimation. The results are reflected on the design of J-PARC neutron source facility.

Off-gas processing system operations for mercury target vessel replacement at J-PARC

An off-gas processing system was installed in the J-PARC spallation neutron source to reduce radioactivity of xenon-127 and tritium contained in a helium cover gas in a surge tank of a mercury circulation system to obey the regulation by law. In addition to this role it has been utilized to a purging process before the target vessel replacement and an air-flow control procedure to minimize uncontrollable radioactivity release during the replacement. An standard and urgent model plans of the off-gas processing system operation were established indicating that 31 days were required at least to replace the target vessel.

Measurements of neutronic characteristics of rectangular and cylindrical coupled hydrogen moderators

ABSTRACT Extensive simulation calculations were performed in the design studies of the coupled hydrogen moderator for the pulsed spallation neutron source of the Japan Proton Accelerator Research Facility (J-PARC). It was indicated that a para-hydrogen moderator had an intensity-enhanced region at the fringe part, and that pulse shapes emitted from a cylindrical para-hydrogen moderator gave higher pulse-peak intensities with narrower pulse widths than those from a rectangular one without penalizing the time-integrated intensities. To validate the peculiar distribution and advantages in pulse shapes experimentally, some measurements were performed at the neutron source of the Hokkaido University electron linear accelerator facility. It was observed that the neutron intensity was enhanced at edges of the para-hydrogen moderators, whereas it decreased at the same part of the ortho-rich-hydrogen moderator, where the dimension of those moderators was 50 mm in thickness and 120 mm in width and height. The spatial distribution and pulse shapes were also measured for a cylindrical coupled para-hydrogen moderator that has the same dimensions as for the coupled moderator employed for J-PARC. The measured results from the cylindrical moderator were consistent with the results obtained in the design studies for the moderator for J-PARC.

Development of AC Magnetic Field Imaging Technique Using Polarized Pulsed Neutrons at J-PARC

We have been developing a quantitative magnetic field imaging technique at J-PARC. As was previously reported [1], we successfully quantified strength and direction of a static magnetic field by analyzing the wavelength dependence of polarization position by position for images, which were obtained using a time-of-flight (TOF) method of pulsed neutrons. Applying this method to observe a magnetic field in industrial products, such as voltage converters and motors, it is necessary to extend this technique to the AC magnetic field driving at a frequency of commercial power supply (50~60Hz). In this study, we attempted to measure an AC magnetic field quantitatively with the TOF method. Magnetic field imaging experiments were performed at the beam line of BL10 “NOBORU” in the Materials and Life science experimental Facility (MLF) of J-PARC. The experimental setup was the same as the previous experiment [1]. An AC magnetic field was produced by applying an AC electric current to a small solenoid coil with the diameter of 5 mm and length of about 50 mm. The frequency of applied field was set to 50.5 Hz, which is slightly higher than that of a two-fold repetition of the pulsed neutrons of J-PARC. Polarization images were obtained under applying the AC field in the coil and wavelength dependence of polarization in a selected area was analyzed, in which polarization changes due to the neutron spin rotation were observed. By fitting the results with a model assuming that only the magnetic field inside the coil contributed to the neutron spin rotation, the amplitude of applied AC field was estimated to be 3.22±0.14×10 A/m, which was corresponded to the designed value of 3.3×10 A/m. This work was supported by Photon and Quantum Basic Research Coordinated Development Program from the Ministry of Education, Culture, Sports, Science and Technology, Japan.