rate constitutes a
stringent test for the Cabibbo universality through the Conserved
Vector Current (CVC) hypothesis and for the unitarity of the
Cabibbo-Kobayashi-Maskawa (CKM) mixing matrix. The unitarity of the CKM
matrix can be used to constrain certain extensions to the minimal Standard
Model (SM), for instance the existence of a fourth generation of quarks. The
discrepancy between superallowed Fermi decay and neutron beta decay data, and
the level of accuracy of the previous experiments on pion beta decay call for
a new precision measurement of 
rate.
This precision experiment is currently in progress at the Paul Scherrer
Institute in Switzerland. The design of the experimental apparatus and its
calibration are the subject of this work. Positive pions of momentum
of 116 MeV/c were stopped in a scintillator fiber hodoscope.
The beam distributions and contaminations have been investigated and
measured. Monte Carlo simulations aided in the design of the apparatus,
especially the shower calorimeter, and the response of its modules to
monoenergetic positrons and photons have been measured. A cosmic ray
tomography apparatus is being used to determine the light collection
non-uniformity of each of the CsI modules. Finally, the response of the
apparatus to 
events has been measured,
since these events will be used for normalization purposes in determining
the pion beta decay rate.