Gamma ray CT – system for multiphase flow imaging

We present a recently developed gamma ray CT system that enables us to visualize rapidly rotating multi-phase distributions, such as in stirred chemical reactors and hydrodynamic machines. Transmission measurements with gamma rays or X-rays are often employed when the flow is confined in dense metal structures and the distribution of gas, liquid or solid suspensions have to be determined. For gamma ray tomography the object of investigation must be placed between an isotopic source and a detector that measures the attenuation of the radiation passing through the object. In the past our group developed a first gamma ray tomography device and supplied it successfully to rapidly rotating systems (Prasser 2003, Hampel 2005). The old system contains a detector made of 64 elements (BGO scintillation crystals and photo multipliers) and a pulse processing unit transferring projection data in 90 µs intervals. As isotopic source Cs137 was used with an activity of about 165 GBq. Our new high resolution gamma ray detector now comprises 320 single detector elements each with an active area of 2 mm by 8 mm. The detectors are made of avalanche photo diodes and lutetium yttrium orthosilicate (LYSO) scintillation crystals whose shape was optimized for a high gamma photon yield. All 320 elements produce weak negative voltage pulses that are shaped and amplified by a pulse processing unit which is built up fully parallel for all 320 channels. The signal processing unit contains also gain adjustable amplifiers, discrimination stages, and binary pulse counters. This allows energy discrimination of every detected gamma photon and thus dividing scattered from non-scattered photons. Data of one projection data set can now be transferred to a PC every 40 µs for 320 detectors periodically via USB2.0. This enables acquisition of 500 radiographic projections per revolution of objects rotating at 3000 rpm. To compound the sinogram a data matrix is initiated to zero. After each revolution the value of every detector element is added to the data matrix with respect to the current angle position in. The detailed procedure is described elsewhere (Prasser 2003, Hampel 2005). Figure 2 shows as an example the reconstructed image of a stirrer of 60 mm outer diameter rotating at 1500 rpm. The result was reconstructed using a standard filtered backprojection algorithm.