The pion beta (
) decay, 
, is
one of the most fundamental weak interaction processes. It is analogous to
superallowed Fermi transitions in nuclear 
decays. The analysis of
nuclear 
decay involves nuclear corrections which are uncertain at the
level of a few tenth of a percent. These corrections do not appear in the 
process, which, therefore, presents a more stringent test of the weak
interaction theory.
The difficulty in measuring the 
decay is due to the small branching
ratio of 
. The most recent and precise determination of
the 
decay rate was carried out by McFarlane et al. [1], and is
in good agreement with theory. However, the measurement uncertainty, 
, is an order of magnitude higher than the theoretical uncertainties [2].
The goal of this experiment is a measurement of the 
decay rate
with a precision of 0.5 %. We have designed a stopped-pion detector system to
detect the two 
's from the 
decay, as well as the 
. Due to
the large Michel positron background, the detector must be very efficient in
the 
detection and have excellent background suppression. This is
achieved by using a pure CsI calorimeter with a large solid angle coverage and
good energy resolution together with an active target and two cylindrical wire
chambers for charged particle tracking (Figure 1).