The Pion Beta Decay Experiment and A Remeasurement of the Panofsky Ratio

2.3 The PiBeta Detector

The signal for a pion beta decay are the two photons from the nearly instantaneous (10^-16s) decay of the p ⁰, the positron remains in the target. The two photons are exiting the target nearly back-to-back without degradation, passing the MWPCs and the hodoscope and finally are detected in the calorimeter. The p b -trigger requires two coincident signals with an energy exceeding a threshold of 55 MeV. These signals have to occur in opposing sections of the calorimeter sphere within 20 ns.

A high acceptance for p b as well as for p ⁺->e⁺ n _e can be achieved by a detector that encloses the reaction centre. A detector exclusively filled with sensitive material is capable to detect a major fraction of all photons from the p ⁰. The PiBeta detector consists of several parts which are classified into beam counters, electromagnetic calorimeter, charged particle tracker, background shielding and mechanical support.

Figure 2-1 Cross section of the PiBeta detector. See text for description._.

In the following sections the experimental apparatus will be introduced in more detail starting with the pion beam, followed by the beam counter and target. Then the tracking part and the electromagnetic calorimeter are treated. After the description of mechanical support and outer shielding, the trigger part and data acquisition will be introduced. Background suppression will be discussed continuously throughout the following sections.