Vol.8 No.2 2015

Research paper : Application of laser Compton photon beam to nondestructive tests (H. TOYOKAWA)−91−Synthesiology - English edition Vol.8 No.2 (2015) The word “photon” is the name for quantized electromagnetic waves, and in this paper, it is used to mean X-rays or gamma rays. Radiography is a photographic method using ionizing radiations, and this includes so-called roentgen photography and cross-sectional photography using X-ray CT.In this research, laser Compton scattering (LCS)[3] technology using an electron storage ring is used. An electron storage ring is a circular electron accelerator, in which electrons are enclosed in a donut-shaped ultra-high vacuum chamber by magnetic fields, and are accelerated using electric fields. We used an electron storage ring with a diameter of 10 m and a circumference of about 30 m, which is a medium- to small-scale device. LCS is one of the methods to generate high-energy photons using an electron accelerator. When laser beams are irradiated onto high-energy electron beams, laser photons are scattered with the electrons, receiving part of the electron energy through Compton scattering, and become X-rays and gamma-rays. Compton scattering is the collision of photons and electrons, and the photon energy before and after scattering, scattering angle in the laboratory frame, and relationship with electron energy in LCS are expressed in Equation 1.EγEeE0(1- β cosθ1)1- cos(θ2 - θ1)1- β cosθ2 + E0=The kinematics is shown in Fig. 1. The LCS photon research started in 1985 at the Electrotechnical Laboratory (later became part of AIST), and the photon beam source at the photon range of 1~40 MeV was available for scientific studies since 1990s.[4][5] Since high-energy and highly-oriented photon beams can be obtained, it has been used in much research such as nuclear physics studies, measurement of response function for radiation detectors, measurement of cross sections of photonuclear reactions, and measurement of absorption cross sections of atoms.3 Process of the selection and integration of technologies(1) Development and sophistication of the elemental technologiesFor industrial radiography, a spatial resolution of 1 mm or less is necessary to detect foreign materials. To detect and evaluate the air bubbles in resin, the resolution of approximately 10−2 cm−1 is necessary as the absolute value of linear attenuation coefficient.[6] A photon flux that passes through a substance exponentially attenuates against the distance travelled, which is characterized by the linear attenuation coefficient that has the dimension of inverse length.In X-ray CT systems, the attenuation of photon intensity as a function of the penetration depth is assumed to be exponential. However, as shown in Fig.2, the linear attenuation coefficient is a function of the substance and the photon energy. Because the conventional X-ray CT system uses white-colored X-rays that contain various wavelengths (energy), the CT image is the result of the convolution of various X-rays with different linear attenuation coefficients. As a result, many artifacts appear in the CT images. In medical CT, all substances can be considered more or less equivalent to water since the subject measured is mainly living bodies. So, it is possible to correct the artifacts by using a water phantom in medical CT. Because various substances of wide dynamic range of linear attenuation coefficients will be considered for industrial CT, it is difficult to correct artifacts and to evaluate precisely the density and material distribution. A CT system using monochromatic photons with sufficient transmissivity can 2 (500 MeV) (1064 nm) 1 Figures in ( ) are sample values used in experiment(4.4 MeV) (Laboratory frame coordinate system)After scatteringBefore scatteringLaser Compton photon (energy Eγ)Laser photon (energy E0)Relativistic electron(energy Ee)Fig. 1 Kinematics of laser Compton scatteringFig. 2 Linear attenuation coefficients of iron and titanium at 10 keV ~ 100 MeV[6]Minimum value, constant valueTitaniumIronX-ray energy (MeV)Linear attenuation coefficient (cm-1)10-210-210-1100100102102104101


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