Thus far, the foundations for a firm understanding of the motivation for the pion beta decay experiment have been laid down. In addition, a description of the Ring accelerator at the Paul Scherrer Institute in relation to the experiment has been presented. The beam studies that have been done to date have also been discussed, particularly the beam particle rates, contaminations, profiles and spot size. After the explanation of the motivation and the description of the production of the pions, a thorough explanation of the decay channels of the pion with the associated kinematical information were supplied since these affect the design of the apparatus which is introduced in this chapter. Here, a general overview of the experiment is given and the description of the salient components of the apparatus is presented.
The pion beta decay experiment is carried out in a stopped-pion mode: the channel of the Paul Scherrer Institute is set to transport particles with a momentum spread of about . The beam is optimized for low positron and muon contaminations relative to the pion rate as discussed in chapter 3. After passing through a degrading material (active degrader), the pions come to rest within a plastic scintillator fiber hodoscope (the active target) where they decay with a mean life time of . The active degrader is designed such that the beam pions stop within the central region of the active target. In addition, the active degrader enables the elimination of the beam muons and positrons by time of flight and pulse height information.
The decay channels of the pion are discussed in chapter 4. Since pion beta decay is the process of interest, the other decay modes constitute the background which must be suppressed. Among the decay products of the pion, one finds positrons, gamma rays, neutrinos and muons which, in turn, decay into the lighter particles with a mean life time of . The decay muons with about , do not have enough energy to exit the fibers; they stop within the target where they decay mostly into Michel positrons some of which have sufficient energy to escape the target. The positrons from with about are also capable of exiting the target. The energy lost by the positrons depends on where in the target they originate from and the amount of material traversed. A small fraction of the positrons (from or Michel decays) annihilates with atomic electrons in the target material and the resulting gamma rays can exit the target.
The gamma rays on the other hand, pass through the plastic scintillators without any appreciable loss of energy. To detect them, a calorimeter is designed and fabricated from pure Cesium Iodide (CsI), one of the inorganic crystals with great stopping power due to their high density ( for CsI) and atomic number. These inorganic crystals also have the highest light output with better energy resolution which make them suitable for the detection of, not only gamma rays, but high energy electrons and positrons as well. Depending on their energy, the positrons which do reach the calorimeter initiate positron-photon showers via bremsstrahlung or suffer energy loss via ionization. The gamma rays also, depending on their energy, initiate showers via pair production or undergo Compton scattering or photo electric effect.
The experimental apparatus comprises the following main components: