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Plastic scintillators for the T2K fine-grained detector Bryant, Martyn

Abstract

The T2K Neutrino Oscillation experiment consists of a νμ beam produced at JPARC and two detectors: one placed280m downstream, the near detector, and Super-K, which sits 295km downstream. The goal of the experiment is measure the θ₁₃ mixing angle and the phase δ found in the MNSP neutrino mixing matrix by measuring the disappearance of νμ from the original neutrino beam and appearance of νe in the beam as it reaches Super-K. The near detector contains a FGD (Fine Grained Detector) which is designed to provide target mass for the ν μ and then track the particles which come out once the νμ has reacted. The FGD consist mainly of nominally sized 1 cm x 1 cm x 200 cm plastic scintillator bars, which are read out by wavelength-shifting optical fibers (which are in turn read out by MPPC photosensors) threaded through each bar. The bars are glued into a series of XY layers. In November 2006 we successfully extruded 11900 Scintillator Bars at CELCO Plastics in Surrey, BC. On site QA found only 2.5 % of the bars to fall outside of bar specifications. During the November bar production run we scanned 4-10 of each 100 bars within a day of them coming off the production line. We found a variation in the light yields of the bars to be 5%. We set up an aging test for the bars. Three separate measurements were consistent with a 2% per year aging rate. In December 2006 we put our new bars in the M11 beam line at TRIUMF. The light yield of the bars was measured using MIPS. With the beam hitting the far end of the bar and no reflector on the end of the bar the light yield and was found to be adequate for the FGD. I predicted, using a simulation, the difference in light collected by 2 sizes of MPPC, one 1 mm sq and one 1.3 mm sq with a gap of 0.4 mm between fiber and photosensor to be 25%. This was in good agreement with Kyoto groups direct measurement.

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