A time-correlated single photon counting (TCSPC) method, which has very high sensitivity and a linear dynamic range, has been increasingly used to determine short luminescence lifetimes. Unlike virtually all other techniques that utilize relatively high emission intensities, single photon counting (SPC) depends on the intensity being low.
The time-correlated single-photon counting (TCSPC) technique has as its basis the timing of individual emission photons relative to a zero-time determined by the onset of the excitation pulse. By using a repetitive laser beam operating at a high frequency it is possible to rapidly record a histogram representing the probability of photon emission as a function of time after excitation.
The key instruments in an SPC system, are an excitation source, a start photomultiplier, a stop photomultiplier, a time-to-amplitude converter (TAC), and a multichannel analyzer (MCA) used in the pulse height analysis mode. The TAC generates an output pulse whose amplitude is directly proportional to the time between the start and the stop pulses. The MCA digitizes the height of the input pulse and increases by one the contents of the memory channel corresponding to this digital value.
The objective of this research is to design, build and characterize a single-photon counting apparatus based on the latest state-of-the-art time resolving technique, and to develop the necessary software for computer control.
The effort is to use this apparatus to measure the luminescence emission and kinetic properties of rare-earth-doped ZnS crystals. The characteristic properties of rare earths in a ZnS lattice will then be related to the experimental conditions of temperature and concentration of rare earth doping in the ZnS.
A single photon counting system has been developed for making spectral and lifetime measurements in luminescent phosphors. The system's most novel feature is its ability to switch rapidly between spectral and kinetic measurement modes.
The time-correlated single-photon counting technique has several advantages over alternative techniques for lifetime measurements, all of which use analog detection. This is because it offers a combination of single-photon sensitivity, the precision of a mainly digital technique, a wide dynamic range, and a time resolution that still surpasses the capabilities of the fastest photomultipliers.
The construction details and performance characteristics for a SPC apparatus are described. The system utilizes a photomultiplier tube for signal detection, a monochromator for scanning, and a multichannel analyzer and computer for data acquistion and analysis.
Monochromator scanning is interfaced to an IBM-PC and Modulynx stepping motor controller. Detailed circuit diagrams and supporting software are presented. Suggestions for adoptation to meet different experimental conditions are given.
To have some general agreement on the method of publishing experimental results, so that it will be of maximum value to others, the recorded spectra are corrected and a reference made to a 'correction curve' for our particular TCSPC system and experimental condition employed. The calibration of the monochromator and detector is accomplished with a tungsten lamp operated at a standard color temperature (2860 K). The monochromator wavelengths were caliberatied by means of standard reference lines of mercury lamp.
Data acquistion is done in a ratiometric mode to compensate the excitation source intensity variations when measuring luminescence emission spectra and to keep the statistical noise of the reference signal constant. The Delayed coincidence method is used for kinetic measurements. This system uses laser excitation that is chopped by an acousto-optic modulator for decay measurements.
Electron transit time is minimized by the application of a high voltage between photocathode and the first dynode of the photomultiplier tube (PMT), as well as some modification of the electronic system. Several operating configurations of the TAC were examined to evaluate its performance in the TCSPC experiment. The inverted configuration has been proved to be optimum.
The resulting data are analyzed to obtain pertinent parameters such as luminescence emission peaks and excited state lifetimes. Tests of the apparatus have demonstrated that luminescence decay times as short as a few picoseconds can be analyzed by the system.
The photoluminescence emission (in MCS mode) and kinetic (in PHA mode) properties of rare-earth doped ZnS crystals (ZnS:Pr 3+and ZnS:Nd 3+) were investigated using the apparatus, and experimental results and analysis are summarized in this thesis.