A Modified Fast Analog Technique for Determining Luminescence Decay Times of Scintillators
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ABSTRACT We have modified our previous fast analog technique to determine the luminescence decay times of scintillators following an excitation with a Sr 9" /3-source. In the original technique [1], the sample was excited with a nitrogen-pumped dye laser, and the fluorescence pulses (consisting of typically 50 to 1000 photons) were detected by a multi-channel plate photomultiplier tube (MCP-PMT). The output from the MCP-PMT was directed to a fast waveform digitizer triggered externally by the exciting laser. In the modified technique, the digitizer acquires the fluorescence decay in the internal trigger mode, as no corresponding external trigger pulse is available from the Sr 90 source. For efficient light collection from scintillators, an ellipsoidal mirror assembly has been tested. The fluorescence decays are acquired as multi-photon pulses and are subsequently corrected for the temporal instrument response by using a deconvolution technique. The overall time resolution of the technique is about 100 ps. The fluorescence decay time obtained using this technique for a commercial scintillator (SCSN-81) agrees well with literature. We also discuss our results on new epoxy-polymer based scintillators prepared in our laboratory. The primary motivation for this work was development of new scintillators with shorter fluorescence decay times for high collision rate experiments.
INTRODUCTION Scintillating substances have been used in particle detectors since the early days of nuclear and particle physics. Broser and Kallmann [2] in 1947 first used scintillator solutes in aromatic solvents as detectors for nuclear radiation. Since that time, the use of scintillators has much increased. Scintillators have been improved, with respect to their fluorescence decay times, quantum efficiency, and radiation hardness. New scintillators have been made by using different solvents (liquid or plastics) and new fluorescing compounds. The techniques for characterizing such scintillators are important, and our main concern here has been the measurement of their fluorescence decay times and spectral characteristics. There are several methods for determining the decay times: phase-modulation, single-photon-counting, pulse-sampling, and the fast analog technique. We have developed the fast analog technique since 1983 [3, 4] and used it for determining fluorescence lifetimes and the associated time-resolved spectra from a variety of substances. In this paper, we concentrate on scintillators. We believe that our technique and the associated apparatus are simpler than some of the other methods.
EXPERIMENTAL DETAILS The experimental setup for the original and the modified fast analog techniques is shown in Figure 1. In the original technique a PTI model PL 2300 nitrogen laser along with a PTI model PL 201 dye laser was used for fluorescence excitation. The excitation pulses were 500 ps wide and delivered 100-150 pJ per pulse at a rate up to 20 Hz. Typically, the laser was operated at a repetition rate of 10 Hz. A beam splitter was placed in the
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