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:
and ensuring high degree suppression of random coincidences
of Michel events.
decays and the positron from
.
