Understanding and calibrating the DS-ECal for T2K's ND280

Davies, Gavin and Kormos, Laura (2011) Understanding and calibrating the DS-ECal for T2K's ND280. PhD thesis, Lancaster University.

Abstract

The T2K (Tokai to Kamioka) experiment is an off-axis long-baseline neutrino oscillation experiment with a baseline of 295 km to the Super-Kamiokande far detector. The near detector, ND280, measures the flux and energy spectra of electron and muon neutrinos 280m from the muon neutrino beam source. Electromagnetic calorimeters constructed from lead and plastic scintillator bars cover the largest active area in ND280. The most downstream calorimeter is the Downstream Electromagnetic Calorimeter (DS-ECal). This thesis details aspects of the construction and calibration of the DS-ECal. Construction of the DS-ECal and quality assurance of the scintillator bars was performed at Lancaster University. All scintillator bars that were selected for optical quality assurance tests were accepted for use in the construction of the DS-ECal. Each bar that was tested was shown to have a light yield within ∼2σ of the mean light yield of 35.9 ± 4.9 photoelectrons per minimum-ionising particle, measured at 66 cm from the scintillator bar end with three wavelength-shifting fibres coupled to a photomultiplier readout. Cosmic ray data collected at CERN, upon the successful construction of the DS-ECal, was used to develop a simple track-selection algorithm that reconstructed the position of charge deposits in the DS-ECal. The resulting attenuation profiles of all bars have been demonstrated to be consistent with one another within uncertainties using a developed quadruple-exponential function that describes the attenuation of the detected light along the length of the DS-ECal scintillator bars both quantitatively and qualitatively. The attenuation length of light traveling along the length of scintillator bars within the calorimeter has been measured to be 341 ± 90 cm. This is consistent with expected values of the order of 350 cm for Y11 WLS fibres. The Tokai (in − situ) and CERN cosmic ray data are also shown to be consistent with within approximately 1% as calculated from the mean most probable value of a convoluted Landau-Gaussian distribution of charge hits at several positions along the bars for all channels in the detector combined. The DS-ECal is currently collecting neutrino data for inclusion in the T2K 2010a physics run and is in good working order with only 11 dead channels from a total of 3400.