The experimental approach relies on the coincident detection of the two photons which
originate from the subsequent decay of the neutral pion,
. The process 
is used for the normalization of the pion beta decay events. The experiment is
therefore a relative experiment with a stopped pion technique. The relative
measurement of the pion beta decay rate is possible because of the similarity between
the detector acceptances and responses to 
and
events. As a result, the differences in the
systematic uncertainties are very small. Monte Carlo calculations have shown that the
processes which affect the detection efficiency of pion beta events and constitute the
main sources of systematic uncertainties occur at the level of few percent.
Positive pions of 116 MeV/c are transported by the secondary beam channel (
)
of the Ring accelerator to the experimental apparatus which consists of: beam counters
and an active target to identify and stop the pions, two cylindrical MWPC's for
charged particle tracking and for additional suppression of the background, a
segmented, cylindrical plastic veto detector for the identification of charged
particle induced events, a highly segmented shower calorimeter made from pure CsI for
the detection of gamma rays and high energy electrons and positrons, thermal
insulation around the detectors for the control and stabilization of the temperature
and the humidity, a lead shield to avoid self-vetoing due to shower leakage,
cosmic veto detectors and the read out electronics. The segmentation of the calorimeter was obtained
from a class II geodesic triangulation of an icosahedron with the requirements of
uniform coverage of solid angle by the resulting modules, minimal number of
the different modular shapes, high rate capability and good energy resolution.
The choice of CsI as the material of the calorimeter was due to the properties
of this inorganic scintillator. The development run of 1989 has shown that CsI
can provide a good timing resolution and has a much longer attenuation length
to its radiation than previously reported. In addition, compared to some other
organic scintillators, the higher refractive index of CsI makes it easier to
achieve better uniform light collection for the tapered truncated pyramidal
shapes of the calorimeter. The thickness of the calorimeter was chosen as a
compromise between the energy resolution and the cost, both of which increase
with the thickness. Monte Carlo studies of the spreadings of
the showers produced by photons from pion beta decay and positrons from
aided in the choice of the granularity
and the design of the trigger elements as overlapping clusters of 6-9
calorimeter modules. The current clustering scheme is 
efficient in
detecting valid 
and
events with energy depositions above the
threshold. The expected angular resolution of the calorimeter is 
FWHM as
shown in figure 
.
