Vol.8 No.2 2015

Research paper : Application of laser Compton photon beam to nondestructive tests (H. TOYOKAWA)−92−Synthesiology - English edition Vol.8 No.2 (2015) overcome this difficulty. As shown in Fig. 2, the linear attenuation coefficients for industrial materials remain almost constant for photon energy of 5~20 MeV. For example, the linear attenuation coefficient for iron shows a shift of 0.6 % at photon energy of 10 ± 3 MeV. This value is equivalent to the energy spread of several eV per 1 keV of photons. That is, the CT measurement using photons at MeV range of little over 10 % becomes equivalent to the photon CT measurement with energy spread of 1 % or less at keV range, and therefore it can be regarded as using monochromatic photons. The linear attenuation coefficient becomes the minimum in the MeV range for many substances. Therefore, photons readily pass through substances in the MeV range. That is, the photon beams at MeV range are the optimal tool for testing thick samples at high density resolution.High spatial resolution, high density resolution, and high time resolution (short measurement time) are required for a good industrial radiography system. To achieve these concurrently, it is necessary to scan samples with a pencil-like thin beam at a small pitch, or with a cone-like beam and an X-ray camera with a small pixel with high detection efficiency. Models that realize these properties are shown in Fig. 3. The diagram in (A) is the CT method where scanning is done using a thin beam, which is called the first-generation CT. The sample is moved perpendicularly to the beam axis, moved up and down, and rotated 360 degrees to measure the transmission image. The diagram in (B) shows the third-generation CT method where the transmission image is obtained without moving the sample by using the cone-beam and the X-ray camera placed behind the sample. While the third-generation CT system has an excellent time resolution, the spatial resolution cannot be improved better than a few mm, because of the scattered X-rays within the camera. Because the technical goal of the present research is to develop a prototype industrial CT system using MeV photons, and to demonstrate high numerical performance for density and spatial resolutions with fine CT images, we built a first-generation CT system in this study. The outline of the first-generation CT system developed in this research is shown in Fig. 4.(2) Open user researchWe have successfully developed the elemental technologies and the CT system. The next step was to enhance the utility of the system by supplying the CT system for open use, and to continue discussions with the users on improvement points and requirements. The system was actually released to open user experiments to hear their voices. We conducted joint research with companies in the automobile and electricity industries in this research phase, and found their demands were to have spatial resolution of at least 1 mm or less, and to detect inhomogeneity in density distribution of 1 % or less. The measurement results of transmission images of industrial products using the present radiography system are shown in Fig. 5 (left). We succeeded in demonstrating the principles but failed to obtain satisfactory images in the photography tests for metal bolts that were conducted in the early stages of this R&D.Various improvements were conducted to enhance the performance.[7] For example, we improved the alignment precision of the collimator, the spatial-and-temporal collision precision of the laser and electron beams, intensity and stability of the LCS photons, and so on. We made an electron Fig. 3 Explanation of CT generations. (A) First-generation and (B) third-generation.Fig. 4 LCS photon CT system using the electron accelerator(B)(A)X-ray camera (low efficiency)Large detector (high efficiency)Pencil beamThird-generation CT (rotation only)・Low detection efficiency・Pixel is determined by size of camera element(Image blur is large)First-generation CT (In tandem ↔ rotate ↔ up-down)・High detection efficiency・Pixel is determined by photon beam diameter(High image quality)Up-downRotateConical beamRotateMove in tandemPhoton detectorSamplePhoton beamLaser deviceElectron storage ringElectron linear accelerator (injector)


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