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ANNA PLA-DALMAU AND ALAN D. BROSS Fermi National Accelerator Laboratory, P.O. Box 500, Batavia, IL 60510

ABSTRACT Several 2-(2'-hydroxyphenyl)benzothiazole, -benzoxazole, and -benzimidazole derivatives have been prepared. Transmittance, fluorescence, light yield, and decay time characteristics of these compounds have been studied in a polystyrene matrix and evaluated for use in plastic scintillation detectors. Radiation damage studies utilizing a 60 Co source have also been performed.

INTRODUCTION New applications for plastic scintillation detectors have resulted from recent developments in both scintillating plastic optical fibers and photon detection devices [1]. The renewed attention in plastic scintillators has encouraged research towards the modification and improvement of their fundamental properties - namely light yield, decay time, and radiation resistance. Standard plastic scintillators fluoresce in the blue region of the visible light spectrum and consist of a polymer matrix doped with two fluorescent organic compounds. Polystyrene and poly(vinyltoluene) are common materials utilized as the polymer base. The fluorescent compounds are generally referred to as the primary and secondary dopants. An extensive variety of types and compositions is now commercially available. The new plastic scintillators range in emissions from the blue to the red region of the visible spectrum. In addition, wavelength shifting fibers, in which only a secondary dopant is present, have been developed to couple to scintillating tiles. Furthermore intrinsic plastic scintillators, where a single fluorescent compound functions as both primary and secondary dopant, are being investigated. The use of plastic scintillators emitting at longer wavelengths has been dictated by the results from numerous radiation damage tests. These studies indicate that the optical properties of polystyrene degrade with exposure to ionizing radiation [2, 3]. This effect is observed as an increase of its absorption region towards longer wavelengths and is responsible for losses in the light output of the detector. Since the permanent damage in polystyrene extends up to 500 nm, a dopant fluorescing at wavelengths longer than 500 nm would yield a plastic scintillator less susceptible to radiation [4, 5]. Among the new fluorescent compounds tested, 3-hydroxyflavone (3HF) has been found to be an excellent candidate for green-emitting plastic scintillators. Its absorption and emission peaks are at approximately 350 nm and 530 nm, respectively. The large separation between absorption and emission spectra is due to an excited-state intramolecular

163 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society

proton transfer that occurs when 3HF is excited by UV or ionizing radiation. Many 3HF derivatives have been prepared in an attempt to modify its spectroscopic characteristics to the need of different detectors [6, 7, 8]. Another fluorescent compound which also exhibits excited-state intramolecular proton transfer is 2-(2'-hydroxyphenyl)benzoth