Dye Mixtures for Ultrafast Wavelength Shifters
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ABSTRACT We have produced fast wavelength shifters using mixtures of various Coumarin dyes with DCM in epoxy-polymers (DGEBA+HHPA) and measured the properties of these wavelength shifters. The particular mixtures were chosen because there is a substantial overlap between the emission spectrum of Coumarin and the absorption spectrum of DCM. The continuous wave and time-resolved fluorescence spectra have been studied as a function of component concentration to optimize the decay times, emission peaks and quantum yields. The mean decay times of these mixtures are in the range of 2.5-4.5 ns. The mean decay time increases with an increase in Coumarin concentration at a fixed DCM concentration or with a decrease in DCM concentration at a fixed Coumarin concentration. This indicates that the energy transfer is radiative at lower relative DCM concentrations and becomes non-radiative at higher DCM concentrations.
INTRODUCTION Particle detectors based on scintillation processes have been used since the discovery of radium about 100 years ago. The fast signals that can be obtained with these detectors, although often considered a nice asset, were rarely essential for the success of experiments. However, the new generation of high energy particle accelerators require particle detectors with fast response time. One of the essential components of a variety of particle detectors is plastic scintillator based on a polymer matrix, usually polystyrene, doped with fluorescent compounds, which emits light when exposed to ionizing radiation. The light production in these detectors is a complicated process, that usually takes place in several steps. The excitation of polystyrene leads to ultraviolet light, which gets quickly absorbed. A variety of dyes can be used to shift this light in a fast and efficient way to the blue part of the spectrum (emission peaking typically at 400 nm). Typical commercially available plastic scintillators of this type have decay times of only a few nanoseconds. There are reasons why blue light is less suitable as the main source of information for the mentioned particle physics experiments. For example, plastic is sensitive to radiation damage. It turns yellow under the effect of ionizing radiation and therefore, blue light gets strongly attenuated. Also, in order to transport the light to a place where it can he converted into electrical signals, another wavelength shifting process is often needed because of geometrical reasons. To preserve the hermetic coverage of the particle interaction region, the light has to undergo a 900 change in direction. This is frequently achieved with so-called wavelength shifting fibers, optical fibers doped with some dyes which absorb the blue scintillation light and re-emit at a longer wavelength. The mentioned reasons illustrate the need for dyes emitting at longer wavelengths (greenorange-red) and with the properties required for this type of applications: High quantum yield, small self-absorption and fast decay. It is difficult to satisfy all three requirements simultan
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