MATERIAL RECOGNITION SYSTEM USING 950 KEV X-BAND LINAC WITH DUAL ENERGY X-RAY SCINTILLATOR ARRAY (2010)

Dual energy X-ray system using high energy X-ray from linear accelerator (Linac) applies two times X-ray irradiation which has different energy spectrum each other in many cases. Two different X-rays yield two tomography images which are analyzed through numerical calculation with pixel values for material recognition of an object. However if the X-ray generation is not stable, the results of numerical calculation shows irregular tendency during the inspection. We propose the two-fold scintillator array in detection part, because two tomography images are obtained by just one irradiation. That leads to the time saving during inspection and the cost down for additional facilities. The optimal condition is researched to increase the ability of material recognition in interesting materials designing the detector with CsI and CdWO4 scintillators. We focus on the discrimination between heavy materials and light materials with the system in the research. X-ray source is 950 keV X-band Linac we developed for industrial application, which produce pulsed X-ray, 10 pps with around 400 mA beam current.. INTRODUCTION According to the development of linear accelerator which produces high energy X-ray, the new possibility in the field of X-ray imaging is greater than before because, the higher energy X-ray can penetrate the thicker target material. Practically, the inspection system for containers in a port using Linac as X-ray source, 4 ~ 9 MeV accelerating energy, have been introduced in several countries [1, 2]. They show the discrimination between several materials which has atomic number range from 5 to 82 effectively. That was impossible with lower energy X-ray source below 500 keV, which is popular for baggage custom inspection in an airport. Dual energy X-ray concept is applied with Linac for material recognition. Through the numerical calculation of contrast in each pixel in two images of a target material, colorized image which has atomic number information is reconstructed. Two images are obtained by dual energy Xray which has different energy spectrum from Linac normally. Monochromatic dual energy X-ray provides definite separation all of materials theoretically. However monochromatic X-ray cannot be applied easily, because it is produced from a big ring accelerator, synchrotron radiation or from a big Linac, undulator radiation. Therefore we suggest a new idea for dual energy X-ray with two-fold scintillator detector (Fig. 1). Figure 1: Dual energy X-ray concept with two-fold scintillator detector. We can obtain two images, low energy X-ray image and high energy X-ray image, from two scintillators during X-ray irradiation once. In this way, we do not need two times irradiations with dual energy X-ray. That leads to inspection time-saving and prevention of discordance between two images. In the research, we set our goal to distinguish light metal such as Fe from heavy metal such as Pb. They cannot distinguish these metals up to atomic number 82 with established inspection system because they use Xray tube as X-ray source which can generate X-ray energy up to 500 keV. X-rays below 500 keV cannot penetrate material target which is composed of high atomic number element in certain thickness range. Therefore we tried to overcome the restriction with higher X-ray in a range which cannot be distinguished with X-ray tube such as up to Pb; atomic number 82. EXPERIMENT We set the size of entire system to 50 × 50 × 50 cm to place in an airport for custom inspection. It is realized by X-band Linac which is equipped with small size of accelerator and magnetron (Fig. 2, Table 1). 950 keV X-band Linac Table 1: Specifications of 950 keV X-ray source Resonant frequency X-band 9.4 GHz RF source 250 kW magnetron Cavity type On-axis coupling Shunt impedance ~ 70 MW/m Gun type Thermionic, Diode, 20 keV Tube length ~ 30 cm ____________________________________________ kiwoo@nuclear.jp MOPEA030 Proceedings of IPAC’10, Kyoto, Japan 130 08 Applications of Accelerators, Technology Transfer and Industrial Relations U02 Materials Analysis and Modification One of the special features is the 250 kW magnetron; size is 0.1×0.2×0.2 m. The small size of power supply system is provided to supply electric power. Another feature is 950 keV accelerating energy. We do not concern to provide radiation safety manager on-site within 1 MeV X-ray energy [3]. Figure 2: Schematic view of 950 keV X-band Linac. Two-fold scintillator detector We performed Monte carlo simulation to design twofold scintillator, accelerating the electron bunch to generate bremsstrahlung X-ray from tungsten target. Accumulated energy in each scintillator was analyzed to decide the thickness of two scintillators, CsI and CdWO4. Low energy X-ray is absorbed in CsI, thin scintillator, because high energy X-ray penetrates thin scintillator easily. High energy X-ray behind thin scintillator reaches thick scintillator and is absorbed in it. 500 μm for CsI, 15 mm for CdWO4 are proposed to increase the ability of material recognition. Target Iron and lead, representing light and heavy metal, are introduced as targets (Fig. 3 a). These are placed in containers to conceal the target materials changing the thickness while we put small concealment in bigger one. Container materials are polyethylene and polyvinyl chloride which are normal material around us. Its thickness is 5 mm. Every time we put the small one in the bigger one, thickness of container is increased 10 mm each from 10 mm to 50 mm (Fig. 3 b).