Initial Tests Of The Pion Beta Decay Detector

3.3.2 Targets

Two different targets will be used during the acquisition of data for the pion beta decay experiment. The first is depicted in Figure 3.8 and is intended for measuring stopping distributions of the pions (i.e. how stopped pions are distributed throughout the target) as well as providing some tracking ability for decay particles leaving the target. There are 69 3 × 3 mm² scintillating fibers each mated with a fiber optic line leading to a phototube outside the detector on the downstream side. These 69 pieces are enclosed by eight additional larger scintillator pieces formed so as to give the entire target package a cylindrical shape.

The target constructed and used in the 1994 beam time will not be used for pion beta decay data runs because the granularity makes accurate energy reconstruction

Figure 3.7: Diagram of the pion beta decay detector [Brö 96]. 1) Central beam trajectory 2) vacuum pipe 3) active degrader 4) active target 5) fiber optic light guides 6) cylindrical MWPC's 7) segmented plastic veto detector 8) 1" PMTs 9)CsI shower vetos 10) CsI shower calorimeter 11) 3'' PMTs

Figure 3.8: The granular target constructed in 1994 and used in the 1994 beam time.

difficult for the escaping positron and therefore reduces the overall resolution of the detector.

The second target is shaped identically to the first except that it is a solid piece of plastic scintillator. This target was used in the 1996 and 1997 beam times and is discussed in more detail below. An schematic view of the array showing the location of that target is shown in Figure 3.9.