The pion beta decay is one of the most fundamental weak interaction processes. It is analogous to superallowed Fermi transitions in nuclear beta decays. The analysis of nuclear beta decay involves nuclear corrections which are uncertain at the level of a few tenth of a percent. These corrections do not appear in the pion beta process, which, therefore, presents a more stringent test of the weak interaction theory. 

The goal of the Pion-Beta Experiment, is a measurement of the rate of the pion beta decay, pi⁺ -> pi⁰ e⁺ nu, with a precision of better than 0.5%.

 For this experiment a stopped-pion detector system was designed to detect the two gamma's from the pi⁰ decay, as well as the e⁺. Due to the large Michel positron background, the detector must be very efficient in the pi⁰ 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 (http://www.psi.ch/)..