Aufbau eines Bleifluorid-Kalorimeters zur Messung der Paritätsverletzung

The structure of the nucleon is at present the subject of intensive research.Although the proton does not possess strangeness, pion scattering and deepinelastic muon scattering experiments indicate that pairs of strange quarks andantiquarks contribute substantially to the properties of the nucleon. For thisreason the collaboration A4 at the Mainz Mikrotron (MAMI) examines thepossible contribution of strangeness to the vector form factors of the nucleon. Inthis intermediate energy experiment the parity-violating asymmetry in elasticelectron proton scattering will be measured with an accuracy of 5%.In the context of thls thesis the highly transparent Cherenkov material leadfluoride (PbF$_{2}$) has been examined for its suitability as a calorimetric detector.These studies were dominated by systematic quality controls of PbF$_{2}$ monocrystalsconcerning their light yield and radiation resistance as well as thelrtransmittance and its homogeneity.The internat transmittance of crystals with small concentrations of impurities isclearly better than those of comparable Cherenkov materials. Furthermore,crystals with surface damages or higher defect concentrations have beenreplaced by the Chinese manufacturer SICCAS. The effective light yield N$_{p.e.}$ ofabout 1.7 photoelectrons per MeV was measured with a hybrid photomultipliertube. The material has a non-uniform radiation resistance, with about a seventhof the examined crystals showeng absorption bands or reduced transmissionspectra. An annihilation of the induced colour centres was achieved both bymeans of fibres and by direct illumination.Besides, a simulation of the intensvvy and the spatial distribution of Cherenkovphotons in PbF$_{2}$ as well as model parameterisations are presented. With theirhelp the detectors were optimised for their application in a calorimeter.The homogeneous electromagnetic calorimeter has been constructed in thefollowing years. In thls thesis the design features and the completed set-up areshown. Each of the 640 detectors consists of a PbF$_{2}$ crystal wrapped inreflective material, a photomultiplier with its base and a coupled fibre.Substantial difficulties for the construction resulted from fluctuations in themechanical dimensions of the crystals of up to $\pm$ 500 $\mu$m. In order to be able toreject crystals with unsatisfactory geometry, all crystals were measured with acomputer-controlled milling machine to an accuracy of 12 $\mu$m. The determinedcharacteristics of the crystals are presented in thls thesis.In 2000 the first 511 channels of the calorimeter were taken into operation. Toachieve stable pulse heights the successfully applied method of optlcalbleaching was implemented in the calorimeter. The mean energy resolution$\Delta$EIE of the central channels amounted to 4.27%, corresponding to a statisticalterm $\sigma$/ E of (3.1 $\pm$ 0.3)% / E [GeV]. After the separation of inelastic scatteringevents and of pile up a raw asymmetry A = (+3.0 $\pm$ 3.0) x 10$^{-6}$, averaged overall central detectors, was determined from the first measurements