he process of fluidization involves the simultaneous flow of solids in contact with a gas, a liquid, or a gas and a liquid. Methods for T direct local measurement of hydrodynamic properties in operating fluidized beds are necessary. Recently, these techniques have become less invasive, more sophisticated and generally more reliable, particularly when applied to laboratory-scale units. Non-invasive techniques, such as radiation attenuation and tomographic imaging, are now used. Gamma-ray imaging is such a technique that has been used for various applications.In the gamma camera technique that is used in this study, the gamma radiation strikes the face of the collimator mounted to the detector head. With a parallel-hole collimator, only those gamma rays, which are travelling perpendicular to the camera face, can pass through the collimator. Others are stopped by the collimator septa (hole dividers). The scintillation crystals change the energy of each gamma ray into a light energy in direct proportion to the energy dissipated within the crystal. These crystal-yielded photons are gathered in a "light pipe" and are directed to an array of photo-multiplier tubes for detection. Each tube converts the light energy into a pulse of electrical energy, which is directly proportional to the number of photons striking the tube. The crystals are sensitive to a specific keV energy range, thus allowing detection of certain emissions while they are not sensing other energy emissions.By allowing only the perpendicular gamma rays to pass the collimator, the scintillation light produced in the crystal shows the same relative positions corresponding to the origin of the emitted gamma rays. The output of each photo-multiplier tube depends on the energy and proximity to the scintillation counter that produced the output. Besides coordinate signals (x-and z-positioning signals), the camera produces signals representing the scintillation energy for each event.Rotating the head of the camera and recording several frames a t different angles can do three-dimensional viewing of a static source. But, in our experiments, dynamic imaging of a small-moving radioactive particle was performed. This type of acquisition produces a two-dimensional image, only. The advantages of the proposed gamma camera technique as it was used in this study are: 1. It does not depend on the attenuation of the gamma ray. This is because the camera detects location and number of counts of the occurrence of the source but the reconstruction algorithm only acquires the location information. 2. Processing times are very small. *Author to whom conespotirierice rnny be aritiressc~ri. E-mail address: ukntitzns(@ ucalgary.ca A scintillation gamma camera was used for measuring the instantaneous velocity profile and average velocity as well as the trajectories of a radioactive particle in small laboratory scale air-polyethylene-fluidized beds. A large number of frames, with frequencies between 1 and 50 Hz, were analyzed to investigate the effects of bed height, gas velocity a...