Investigations on Detector Non-Linear Response for the Optimization of Laser-Induced Phosphorescence Decay-Time Detection.

Abstract: This thesis investigates the nonlinear detector response to Laser Induced Phosphorescence signal originating from the CdWO4 phosphor. A matrix method is utilized to map the detectors linearity for decay-time determination as function of detector gain and number of photons reaching the detector. The Matrix is created by scanning the energy of the laser beam hitting the phosphor thus changing the phosphorescence intensity while fixing the detector at constant gain. Then detector gain is changed and the same process repeated for the full gain range chosen for each detector. Four different detectors are used in the experiment some of them have the capability to be run in time-gated mode. The detectors used are photomultiplier tube, time-gated photomultiplier tube, micro-channel plate photomultiplier tube, and avalanche photodiode. The experiment was conducted at five temperatures in between 21 and 290 °C and cover the full decay-time range of the CdWO4 (15µs-5ns). The detector nonlinearity changes the computed decay-time of the phosphorescence, which lead to error in the temperature found. Nonlinearity can originate from two saturation effects. The first is optical saturation that is due to the saturation of the detector photocathode under increasing radiation intensity, whereas the second is electrical saturation that is due to saturation in the electron multiplication mechanism. Using the matrix of detector response nonlinear detector operating regions can be found for any detector and actions can be undertaken to correct or avoid saturated signals.