Untersuchung der optischen Eigenschaften großflächiger Plastikszintillatoren für den KARMEN-Upgrade

The present work was performed in the framework of the KARMEN-experiment, run by a German-British collaboration at the neutron-spallation-source ISIS. In the year 1996 a major experimental upgrade has been taken place. The intention has been that the KARMEN-experiment is able to confirm or exclude the evidence on neutrino-oscillation in the channel v11 ---t Ve announced by LSND afterfurther 3 years of data taking. So far, the sensitivity of KARMEN in this channel has been limited by untagged cosmic muons producing high-energetic neutrons in the 7000 t passive steel-shielding. These neutrons penetrate into the detector unseen by the then existing anti-counters. Inside the detector their reaction-signatures simulate an oscillation-event. Therefore the upgrade compromised the extension of the detector-system by a new veto-counter which tags these cosmic muons inside the steel-shielding. This reduces the cosmic induced oscillation-background by a required factor of 40. The new veto-counter consists of 136 single BICRON BC-412 plastic-scintillators with a length up to 4 m, a width of 65 cm and a thickness of 5 cm. Each of these modules has at its both ends a 180°-lightguide and is read-out by 4 photomultipliers (PM) of the type Philips Valvo XP2262/PA. The aim of this work has been the investigation of the optical properties of large-area plastic-scintillators with the intention of finding the optimized configuration for the modules of the new veto-counter. At the beginning of this work the manufacturer of plasticscintillator for assembling the modules had tobe selected. The choice was between BC-412 from BICRON corporation and NE-110A from nuclear enterprises. The intrinsic optical properties, i.e. the absolute Iightoutput and the spectral attenuation-length, have been measured first. NE-llOA showed a higher absolute lightoutput, while the measurement of the spectral attenuation-length with a dual-beam-laser-photometer showed a better spectral transparency for BC-412. With the aid of a module-specific figure of merit the decision was made for BC-412. The measured spectral attenuation-length and the absolute Iightoutput have been used in addition as important input-parameters for the 3-dimensional Monte-Cario-simulation of the spatial-dependent Iightoutput in a veto-module. With regard to the geometry, the spectrum of emission of the scintillator, the spectral quantum-efficiency of the PM's, as weil as the refractive-indices, the roughness of surfaces and the absorptive-coefficients of the materials, it has been possible to simulate precisely the spatial-dependent Iightoutput and to verify the experimentalresults. On the basis offurther simulations the configuration of the veto-modules was optimized specifically with respect to the effective attenuation-length, the Iightoutput and its homogeneity. The final configuration of the veto-modules yields 61% more integral Iightoutput than the "minimum" configuration. The following components were essential in improving the lightoutput: a) the slightly shrinkled aluminiumfoil in which the scintillator is wrapped-in, b) the high-reflective aluminium-mirror-sheets assembled at the criticalplaces between the PM's at the lightguide and c) the glueing ofthe PM's with optical cement. The veto-modules in the final, optimized configuration show an excellent f.L-7-separation. Due to this property, it was further possible to decrease the muonleakage-rate, which is additionally dependent on the electronic threshold of the modules, so that the required muon-detection-efficiency of 99.4% of the whole veto-counter-system is achieved. Consequently the background-reduction for the v11 ---t Ve -oscillation-reaction meets the requirements.