Performance of position sensitive scintillator detectors when readout with a low noise photodevice
The technique of position sensitive scintillating bar detectors used as the building blocks of position sensitive planes offers the remarkable advantage of greatly reducing the number of readout electronic chains. A position sensitive bar is usually readout by two photo-detectors and, combining the two signals, energy and position of an interacting particle or gamma-ray can be evaluated. In this kind of detector the performances can suffer from the double readout that introduces the noise and uncertainty of two readout systems. The use of Silicon Drift Chamber as a photo-detector opens new perspectives in the design of scintillating bar detectors. In this case the high quantum efficiency of Si for light detection and the very low noise of the device practically cancel the disadvantages of a double readout system. In this work the results achieved stimulating with gamma rays of different energies scintillating bars with square cross section of 3 mm and various length from 15 cm to few cm, with double side Silicon Drift Chamber readout are presented and discussed. For these test an old scintillating material as CsI(Tl) has been chosen manly for its easy availability in different shapes and the good matching of the wavelength of its scintillation light with Si. The use of different materials is also discussed.
Detectors for high-resolution gamma-ray imaging based on a single CsI(Tl) scintillator coupled to an array of silicon drift detectors
Silicon drift detectors (SDDs) coupled to scintillators have been recently employed successfully in gamma-ray detection. The low value of output capacitance of an SDD allows to reach a lower electronics noise with respect to a conventional silicon photodiode. Detectors based on array of SDDs are of particular interest for gamma-ray imaging. A first prototype conceived for high-resolution imaging is based on a monolithic array of small SDDs (5 mm/sup 2/ each unit) with on-chip junction field-effect transistor (JFET). The first results obtained with a seven-elements array are here reported. For the realization of gamma detectors of larger active areas based on assembled units; SDDs of 30 mm/sup 2/ of area with external JFET have been produced. We present the experimental results obtained in the characterization of a single CsI(Tl)-SDD unit.
Full-scale measurements of SO2 gas phase concentrations and slurry compositions in a wet flue gas desulphurisation spray absorber
Abstract Two measurement campaigns were carried out at ENERGI E2's Asnaes Power plant, unit 5. The unit has a capacity of 620 MW e and is equipped with a wet flue gas desulphurisation (FGD) plant employing a counter-current spray absorber with five spray levels. In the first campaign, the power plant was firing Orimulsion ® with 2.85 wt% S resulting in a flue gas concentration of SO 2 exceeding 2000 ppmv. In the second campaign, the fuel applied was a low-S blended coal and the SO 2 concentration in the raw gas was around 400 ppmv. A novel probe for in situ sampling of gas phase concentrations in wet FGD spray absorbers was developed and applied for measuring axial profiles of the SO 2 gas phase concentrations in the absorber. The expected decrease in SO 2 concentrations along the height of the absorber was found in the spray section (from height 26.5 to 36.2 m) whereas the SO 2 concentration above the holding tank and below the gas inlet was quite low probably due to long local residence times in the region. Horizontal variations, due to somewhat different flow conditions near the column wall were investigated and the SO 2 concentrations were found to be higher near the wall. Measurements at different gross loads showed that the SO 2 gas phase concentration at a given position inside the absorber was roughly linearly related to the L/G ratio in the measuring interval. Turning off one of the lower spray levels, while burning coal with low S content, did not lower the overall removal efficiency of the absorber. However, the SO 2 gas phase concentration inside the lower part of the absorber was increased by a factor of 2–3. Measurements of slurry pH at different positions showed a decrease of approximately 0.5 units from the upper to the lower part of the absorber. The full-scale measurements provide a detailed set of experimental data for validation of mathematical models of a wet FGD spray absorber.