Development of a scanned electron beam X-ray tomography system for high-speed imaging of technical multiphase flows

In multi-phase flow diagnostics there is a strong interest in ultra fast computed tomography technologies. Such imaging modalities are of ultimate value, for instance in chemical reactor design, optimisation of nuclear fuel element assemblies and mineral oil processing. However, the required time resolution for typical flow measurements is in the range of more than 1000 images per second, thus classical X-ray or gamma ray computed tomography devices with mechanical rotation of object or measurement system are not applicable. Scanned electron beam X-ray tomography, however, is a promising technology. It has been introduced in medicine two decades ago were it is mainly used for cardiovascular diagnostics. However, medical systems are still too slow and too expensive for technical flow diagnostic problems. For this reason we have started to develop a flexible scanned electron beam X-ray apparatus that offers a number of different measurement features including linear scan limited angle tomography, full angle tomography, multi-plane tomography and phase velocity measurement.
The principle setup of a prospective scanned electron beam X-ray CT system for flow measurements is shown in figure 1. Essential components are a fast X-ray detector and an electron beam generator similar to those devices used in electron beam welding. The gun produces an electron beam of at least 150 kV acceleration voltage and a few milliampere beam current. The beam is focussed onto a tungsten or molybdenum target. An x-y-deflection system periodically sweeps the beam on the target along a well defined path, thus producing a focussed moving X-ray source. In front of the target there is the object of investigation, which may be for example a pipeline with a liquid-gas flow or a small chemical reactor. The X-ray arc detector records the X-ray intensity behind the object in synchronisation with the source deflection signal. In that way we obtain radiographic projections from different viewing angles which may then be algorithmically processed to cross-sectional images by means of a computer. Tomography can be done in different ways. Conventional tomography would require moving the X-ray source approximately 210° about the object. Medical EBT scanner work this way. The detector in that case must be annular and arranged with some axial offset to the focal spot path. In many cases such an arrangement is not possible due to constructive constraints. Then still a limited angle linear scan CT may be applicable where the X-ray source is only linearly moved on one side of the object. Multiple targets or a step target thereby allows us to perform tomography in different planes, which is useful for flow velocity measurement.
Primary tests of a limited angle linear scan ultra fast X-ray tomography have been carried out in a conventional electron beam welding box. Therefore we have employed a fast 64 element X-ray detector and operated the equipment within the evacuated chamber of the welding box. We scanned static phantoms as well as a dynamic phantom consisting of moving pearls in an aluminium cup (figure 2). The image rate we achieved was 1000 images per second. Currently we are about to build a fast X-ray tomography scanner which comprises a special electron beam generator with wide angle scanning capability and a 512 element ultra fast CZT X-ray detector. The electron beam unit (figure 3) is operated at acceleration voltages up to 150 kV and 33 mA beam current. The sweeping range is ±15° at 10 kHz large signal deflection frequency in two dimensions. As a target we use a water cooled tungsten block with a multiple step structure. As in a conventional X-ray tube a proper X-ray window made of a 100 µm steel film allows radiation exit to the environment. The whole system will be operated in a lead cabin for radiation protection.