Abstract
Photomultiplier tube (PMT) nonlinearities relevant for single shot high-speed lifetime phosphor thermometry were investigated by simulating decay curves with a light emitting diode (LED) at repetition rates between 1Hz and 10kHz. The PMT gain, LED decay time, and background radiant flux were also varied to investigate their impact on the measured decay time error. Errors in the measured decay time due to nonlinear PMT performance lead to temperature measurement errors; therefore, having the measured decay time sensitive to only phosphor temperature is highly valuable for more reliable temperature measurements. Photocathode bleaching had a significant impact on the signal level linearity for PMTs with excitation frequency in the kHz regime but had a smaller impact on the decay time error. Space charge effects were most noticeable at high radiant flux levels and high repetition rates. Strong background radiant flux may lead to decay time errors, and a gateable photocathode could be an effective method to reduce decay time errors. The best decay time measurement configuration to maximize precision without sacrificing accuracy is to use PMT gain in the recommended range and the highest radiant flux where the PMT response is still linear. The degree of nonlinearity in the PMT response is partly detector dependent; therefore, the results in this work may differ among detectors; however, the analysis presented in this work provides guidelines for improving the temperature accuracy of kHz lifetime phosphor thermometry measurements.
Highlights
Temperature has an intrinsic impact on the reliability and performance of thermodynamic systems; precise and accurate temperature measurement methods are highly valuable
The trend for the saturation behavior of the Photomultiplier tube (PMT) after exposure to the light emitting diode (LED) radiant flux over the consecutively recorded decay curves collected by the PMTs varied based on the detector, detector gain, radiant flux, and LED repetition rate
With higher radiant flux decay curves, fast LED repetition rates, and low gain, the sharp increase in the peak signal level after the shutter opened was followed by a sharp decline in the peak signal level most likely due to photocathode bleaching
Summary
Temperature has an intrinsic impact on the reliability and performance of thermodynamic systems; precise and accurate temperature measurement methods are highly valuable. The luminescence from phosphors, which is usually induced by laser excitation and detected remotely, is used to measure temperature. There are two phosphor luminescence characteristics that are most often leveraged for temperature measurements: one is the change in decay time with temperature and the other is the variation in the intensity of different spectral regions with temperature. These methods are called the lifetime and intensity ratio method, respectively. The lifetime method is the focus here because the method generally has better temperature precision at elevated temperatures than the intensity method.. The lifetime method is the focus here because the method generally has better temperature precision at elevated temperatures than the intensity method. PMTs are used as they have excellent time response and light sensitivity and as a result is the most common point detector for phosphor thermometry.
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