A Study on Industrial Gamma Ray Ct with a Single Source-detector Pair

With wide industrial application, computerized tomography (CT) is a rapidly developing technique that is especially useful for imaging and measuring multi-component and multi-phase processes. Tomographic measurements can be obtained by using different sensors, such as X-ray, ultrasonic, optical and microwave sensors, as well as electrical impedance electrodes [6]. X-ray, ultrasonic and microwave sensors are widely used for medical purposes, as are various types of gamma emission CT such as single photon emission computed tomography and positron emission tomography [1]. However, medical tomographic systems seldom use transmissionbased gamma-ray CT where the source and detector are located outside of the object. Unlike medical CT, which is optimized for the human body, industrial CT should be applied to systems of various sizes and densities, depending on the local situation. For the resolution and contrast of a small system, X-ray CT or NDT (Non Destructive Test) film is more useful than gamma ray CT. Nonetheless, gamma-ray CT has many advantages over X-ray CT or ultrasonic CT in the diagnosis of large-scale industrial process units. As shown in Fig. 1, the mean free path (1/ ) of a material increases as the photon energy increases [3]. This result means that CT with a higher energy gamma ray can produce a good contrast for a large-scale system. Although the resolution of gamma ray CT is lower than that of X-ray CT, even a low resolution image can Having its roots in medical applications, industrial gamma ray CT has opened up new roads for investigating and modeling industrial processes. Using a line of research related to industrial gamma ray CT, the authors set up a system of single source and detector gamma transmission tomography for wood timber and a packed bed phantom. The hardware of the CT system consists of two servo motors, a data logger, a computer, a radiation source and a radiation detector. One motor simultaneously moves the source and the detector for a parallel beam scanning, whereas the other motor rotates the scan table at a preset projection angle. The image is reconstructed from the measured projections by the filtered back projection method. The phantom was designed to simulate a cross section of a packed bed with a void. The radiation source was 20mCi of Cs-137 and the detector was a 1 inch 1 inch NaI (TI) scintillator shielded by a lead collimator. The experimental gamma ray CT image has sufficient resolution to reveal air holes and the density distribution inside the phantom. The system could possibly be applied to a packed bed column or a pipe flow in a petrochemical plant.