Comparison of p-terphenyl and 2,2" dimethyl-p-terphenyl as waveshifters for barium fluoride

Abstract

p-Terphenyl (PTP) and 2,2' dimethyl-p-terphenyl (DMT) have been investigated for use as wavelength shifters (WLS) for barium fluoride (BaF<SUB>2</SUB>). These two organic fluors are attractive due to their: high quantum efficiency, fast fluorescence decay time, and emissions above the wavelength cut-off of borosilicate glass. Measurements of these WLS dissolved in cyclohexane are presented. Coincidence time spectra, and fluorescence decay spectra measured with BaF<SUB>2</SUB> as the pump source, are shown. The fluorescence decay time and the ratio of fast to slow intensities were measured by a time correlated single photon technique. The increase in signal resulting from using a WLS between the photomultiplier tube (PMT) and the BaF<SUB>2</SUB> crystal is measured. The motivation behind this work is the development of a gamma-ray spectrometer suitable for high count rate applications. We are investigating the possibility of separating the fast and slow emissions of BaF<SUB>2</SUB> using optical techniques. For our measurements, PTP an DMT do not facilitate spectral separation due to the nature of their absorption and emissions spectra. A discussion of WLS properties needed to spectrally separate BaF<SUB>2</SUB> emissions is presented.