The apparatus is equipped with two MWPC's whose importance is dictated by the
need for double track resolution and the 
trigger.
Figure: Cylindrical multi-wire
proportional chamber for the pion beta experiment. Shown here is the outer chamber.
The inner chamber is geometrically the same but smaller with fewer anode wires and
cathode strips.
The multiple shower resolution of the calorimeter is limited to
(
of 
in solid angle). However, for a
high degree suppression of the accidental Michel events, a higher double event
resolution is required. Additional suppression factors of the order of
from the MWPC's suffice to significantly reduce the Michel
background under the 
peak in high pion stopping
rate: 
. In addition to the double track
resolution capability, the MWPC's have low mass --- 
radiation
length of CsI
--- which
is necessary to reduce the probability of conversion of pion beta 
's prior to
their arrival in the calorimeter and to detect low energy positrons.
They are also
expected to handle high rates with an efficiency of the order of 
.
Table: Some parameters of
the multi-wire proportional chambers of the pion beta experiment.
The MWPC 's have a cylindrical geometry (as shown in figure 
)
with one anode wire plane in the longitudinal direction and two cathode strip
planes in stereoscopic geometry. The MWPC's were fabricated in Dubna (Russia)
and tested at the Paul Scherrer Institute. A cathode strip resolution of
full width half maximum has been measured.
The main characteristics of the MWPC's are
compiled in table .