Low Dose Rate Tests of Plastic Scintillator Plates and Wavelength Shifting Fibers
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plastic scintillator (BC-412 from Bicron Corporation). The plate width varies somewhat but is typically 10 cm. The length of the scintillator varies from about 20 cm out to more than 4 meters. Each scintillator will be read out by an array of twenty 3 mm diameter undoped fibers [Bicron BCF-98]. This method was found to give superior attenuation performance and sufficient light output while maintaining the fast timing properties of the blue scintillator. One of the specifications for the chosen scintillator was that for an integrated dose of 100 Gy (the expected dose over a ten year period), the scintillator should display no more than a 10% decrease in intrinsic light output and a similar limit on the degradation of its attenuation characteristics. During the period when the choice of scintillator and readout technique was under study, an alternative method was also examined. The results of this study are reported elsewhere [3]. Here we point a few key facts relevant to this paper. One was the use of a fast (3.5 ns decay time) green fluor (G2) produced by Bicron that could be used to produce either scintillating plates with good light output and superior attenuation properties (exceeding 5 meters in a 2 meter sample). The other was the possibility of using highly-doped acrylic-based scintillators that were grooved on one side. The scintillator was readout by wavelength shifting fibers doped again with G2. G2 and acrylic-based scintillators are know to be radiation soft for high doses. Such scintillators may prove useful for applications where the material experiences a low total dose.
487 Mat. Res. Soc. Symp. Proc. Vol. 348. 01994 Materials Research Society
EXPERIMENTAL SETUP A key aspect of any irradiation test has been the question of how to relate the results of an accelerated test to what one will observe after the actual elapsed period of time (typically 10 years). It has become apparent that for scintillators exposed to an oxygen-bearing atmosphere during exposure to ionizing radiation, it is important that the accelerated test ensure that oxygen is allowed to diffuse completely into the test sample during irradiation. A variety of evidence [4,5,6,7] has been compiled that indicates that given two irradiations of similar plastic scintillator samples (or undoped samples) to a given dose, but at radically different rates, significantly more optical absorption is induced in those samples irradiated slow enough to allow significant or complete diffusion of oxygen into the sample during the irradiation. For the cases of partial diffusion, the geometric extent of the more discolored region matches the expected diffusion distance of the oxygen into the material during the irradiation [7,8]. From tests with small PVT samples, the dose rate (0.5 Gy/hr) and atmosphere for this test (oxygen) were chosen to ensure that a proper low rate test was performed. In other words, the observed damage should be a realistic estimate of the permanently induced loss in performance. Since one is concerned with subtle ageing effects
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