Kyohei Shimahashi

Low-Temperature Operation of a Bulk HTSC Staggered Array Undulator

A use of bulk high-temperature superconductors (HTSs) for an undulator is attractive since a high magnetic field can be generated at low-temperatures. While potential for generation of the high magnetic field is high, in-situ magnetization of the bulk HTSs for periodic field generation is challenging issue. Recently, we proposed a new type of undulator using bulk high-Tc superconductors (HTS) and a solenoid magnet. The undulator, named Bulk HTSC staggered array undulator (Bulk HTSC SAU), consists of a stacked array of bulk HTSs and copper insulators and a solenoid magnet. A proof of principle experiment at 77 K using liquid nitrogen has been carried out. The estimated performance at about 30 K was estimated using results of property measurements for the HTS used for the Bulk HTSC SAU. The expected undulator peak field reaches to 1.08 T for undulator period length of 9.9 mm for the undulator gap of 4.0 mm. This performance is about 2 times higher than that of existing technologies.

Analysis of SNIP Algorithm for Background Estimation in Spectra Measured with LaBr3: Ce Detectors

LaBr3:Ce scintillating detectors exhibit excellent properties for γ-ray spectroscopy such as high energy resolution and operation under room temperature as well as MHz counting rates. On the other hand, sever background radiations exist due to the internal contamination of radioactive materials that are very difficult to be avoided during the manufacture. To decrease the effect of these background levels, some analytical techniques, e.g. background subtraction, should be applied. In the present work, we investigate the efficiency of the sensitive nonlinear iterative clipping peak (SNIP) method for background estimation and subtraction. Optimization of the clipping window is discussed for range of energy up to 3 MeV. Enhancement of energy resolution up to 50% was obtained.

Current Status of the Non-destructive Assay for 235U and 239Pu Toward More Secure Nuclear Power

Nuclear energy still represents one important option for energy sources that do not emit CO2 even after the Fukushima accident. Right now, it constitutes about 16–17% of the world’s total power supply, and is expected to increase and spread significantly by the coming three decades. Special nuclear materials (SNM), mainly 235U and 239Pu, used as fuels for nuclear power plants are required to be fully controlled and accounted by the IAEA. Non-destructive assay (NDA) for screening SNM is an important for countering terrorism as well as accountability purposes, because it is difficult to detect hidden SNM. NDA methods are mainly classified into two categories in terms of radiation detection; neutron-spectrometry methods and gamma-spectroscopy methods. This paper is devoted to discuss the current status of the NDA different techniques and assess usage of nuclear resonance fluorescence (NRF) technique in combination with quasi-monochromatic gamma-rays generated by laser Compton backscattering as a promising one.

Assessment of LaBr 3 (Ce) scintillators system for measuring nuclear resonance fluorescence excitations near 2 MeV

Nuclear resonance florescence (NRF) is a powerful tool for non-destructive assaying of nuclear materials. Detection system is a key issue for investigating nuclear materials using this technique. For a long time, Ge detectors were used because they have an excellent energy resolution but also have drawbacks of high cost and slow response time. On the other hand, LaBr3(Ce) detectors have high response speed and high detection efficiency as well as easy for assembly. However, a drawback of LaBr3(Ce) crystals comes from the unavoidable internal background levels near 2 MeV. This regime has great importance because it is very rich with 235U and 239Pu levels. We tested two different volume scintillators with 2.13 MeV quasi-monochromatic gamma-ray beam from the laser Compton backscattering (LCS) in National Institute of Advanced Industrial Science and Technology (AIST). We could clearly detect the NRF excitation level from 11B at 2.13 MeV. For this level, the energy resolution for the small LaBr3 (Ce) detector was 1.8% (FWHM), and 2.4% (FWHM) for the large LaBr3(Ce) detector. Potential of this scintillator for non-destructive assay of nuclear material identification is discussed in the light of our results.

Monte Carlo Calculations of γ-Rays Angular Distribution Scattering from 11B in (γ, γ) Interaction

An investigation of angular distribution of scattered gamma-rays is important to get information about an efficient arrangement of γ-ray detectors and it is necessary to design a real detection system of the inspection system by using a (γ, γ) interaction. Angular distribution of scattered gamma radiation from 11B at 4,440 keV from transition J π (5/2− → 3/2−) has been simulated by extended GEANT4. In the simulation, seven LaBr3:Ce detectors were recording the scattered photons from nuclear resonance fluorescence (NRF) process in a plane perpendicular to the incident polarized γ-ray beam. The γ-ray beam was assumed to be monoenergetic and linearly polarized with energy spread of 5%. All the LaBr3:Ce detectors were similar in the diameter of 1.5 in. and length of 3 in., positioned at seven different directions. Angular distribution of the scattered γ-rays is discussed in terms of the detectors’ positions with respect to the target and incident γ-ray beam. The result, which indicates the largest count rate from NRF signals is backward (135° and 225°) and forward (45° and 315°) directions with respect to the incident gamma-ray, is useful when using the NRF process in inspection of the special nuclear materials (SNM) like 235U and 239Pu.

Optimization of the New Designed FEL Beam Transport Line

A mid-infrared free electron laser (MIR-FEL) (target wavelength: 5 ~ 20 μm) facility named KU-FEL (Kyoto University Free Electron Laser) was constructed to aid various energy science researches at the Institute of Advanced Energy, Kyoto University. In December 2011, KU-FEL was upgraded by replacing its undulator and optical cavity mirrors. By this upgrade, the tunable range of KU-FEL was improved to 5.5 ~ 15 μm. According to replacing the cavity mirrors, size and divergence of FEL beam at the emitting point was changed. Therefore, we designed and constructed a new MIR-FEL transport line. By using calculation code Zemax (http://www.zemax.com), the condition for keeping the FEL beam radius less than 25 mm during the transportation length of 24 m was determined. In addition, the FEL intensity profile was measured after passing through the constructed transport line. The beam waist at the out-coupling hole was calculated from the measured FEL intensity profile. By using the beam waist calculated from the measurement, we confirmed the validity of the calculated optimum condition.

Development of a field measurement system for the Bulk HTSC SAU

To realize a short-period strong-field undulator, we proposed a high temperature superconducting bulk staggered array undulator (Bulk HTSC SAU) and proceeded proof of principle experiments and numerical studies. We have succeeded to generate periodic transverse magnetic fields whose strength was controlled by an external solenoid field. At the same time, we revealed a problem; at both ends of undulator, field distribution is substantially distorted. We proposed several approaches of field correction. To verify the effectiveness of these field correction methods, it is necessary to measure the magnetic field distribution precisely, not only inside of the undulator but also both ends. For this purpose, we developed a rotary measurement system to measure the magnetic field distribution at the end of the undulator. Multiple Hall sensors are placed on a circuit board at equal intervals from the centre of the board. By rotating and moving the board, the probe can measure axial field in 3D space on the undulator ends. In this paper, we deliver specifics of the system.

Design Study for Direction Variable Compton Scattering Gamma Ray

A monochromatic gamma ray beam is attractive for isotope-specific material/medical imaging or non-destructive inspection. A laser Compton scattering (LCS) gamma ray source which is based on the backward Compton scattering of laser light on high-energy electrons can generate energy variable quasi-monochromatic gamma ray. Due to the principle of the LCS gamma ray, the direction of the gamma beam is limited to the direction of the high-energy electrons. Then the target object is placed on the beam axis, and is usually moved if spatial scanning is required. In this work, we proposed an electron beam transport system consisting of four bending magnets which can stick the collision point and control the electron beam direction, and a laser system consisting of a spheroidal mirror and a parabolic mirror which can also stick the collision point. Then the collision point can be placed on one focus of the spheroid. Thus gamma ray direction and collision angle between the electron beam and the laser beam can be easily controlled. As the results, travelling direction of the LCS gamma ray can be controlled under the limitation of the beam transport system, energy of the gamma ray can be controlled by controlling incident angle of the colliding beams, and energy spread can be controlled by changing the divergence of the laser beam.

Simulation of Electron Trajectory in Bulk HTSC Staggered Array Undulator

To realize short-period high-magnetic-field undulator, we have proposed an undulator using bulk high temperature superconductor in a staggered array structure. To investigate the effect of the longitudinal solenoid field on the electron beam trajectory, the magnetic field near the center of this undulator was modeled and the trajectory of the single electron and the undulator radiation was calculated. As a result, we found that the stronger solenoid field had a good effect on the electron beam confinement. However, we found that the radiation wavelength became longer and the peaks of the spectrum became smaller at stronger solenoid field.