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1. Introduction

The aim of this work is to present the new precision experiment to determine the pion beta decay rate taking place at the Paul Scherrer Institute (PSI). As the experiment is still in its early stage[1] the motivation for the experiment, the description of calibration measurements and new developments are the main subjects of this thesis, which is thus divided into 3 parts.

The subject of part I (chapters 2-4) is the proposed pion beta decay experiment (PIBETA experiment) at PSI. The theoretical motivation for a new precision measurement is given chapter 2.

Chapter 3 discusses related rare pion-decay experiments in order to understand the expected experimental difficulties. These experiments used the stopped pion technique, where the decay products were detected by inorganic scintillators. The scintillators had excellent energy resolution but a relatively slow light output. In order to avoid background from pile-up events, pions were stopped at low rates (~104 p /s). It will be shown, that the low energy tail corrections of the scintillator response dominated the systematic uncertainties of these experiments.

In chapter 4 the experimental technique, the design of the detector and its components are presented. Due to the very small branching ratio of the pion beta decay, the new experiment needs to be performed at high beam-stopping rates. The essential part of the detector is an electromagnetic shower calorimeter, consisting of pure CsI crystals. It detects the two photons from the decay of the p 0 and the positron from the decay p +->e+ n e; the photons and the positron from these decays have both an energy of about 70 MeV. The calorimeter requires the detection of these particles with good energy resolution and the capability of handling high event rates. Experimental tests with a particle beam show that the proposed beam rates and contaminations can be handled.

In part II (chapter 5 and 6) the performance of the pure CsI crystals are discussed. The crystals need to meet very stringent requirements. In chapter 5 special methods are described, which serve to determine the properties of the crystals. The obtained results are used as parameters in a Monte-Carlo-Simulation for the calorimeter response to 70 MeV positrons and photons.

A calibration measurement with 70 MeV photons from the reaction p -p-> p 0n was performed with about 10% of the calorimeter modules. The method and the obtained results are presented in chapter 6.

New developments for the pion beta experiment are the subject of part III (chapter 7). A fast analog memory, the Domino Sampling Chip (DSC), has been tested and improved in the last two years. The DSC is capable of digitizing analog signals with frequencies from 100 to 800 MHz. In order to use the DSC for the pion beta experiment, an electronic module hosting several DSCs needs to be designed. The performance of the prototype module, hosting 6 DSCs, is compared to commercial ADCs and TDCs[2].

[1] As of this writing about 25% of the detector components are at PSI

[2] Analog to Digital Converter and Time to Digital Converter

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