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 .