Trigger Scintillators

The trigger scintillation counters are arranged as shown in Figures 3.3 and 3.4. The dimensions of the counters are given in Table 3.2. These scintillators are used for event triggering by selecting $e^+e^-$ pairs from photon convertion and for event vetoing by rejecting charged particles originating from the target. Each segment is optically isolated and has an independent light guide and photomultiplier tube at one end. Rings $A$, $A'$ have a 4-fold segmentation covering 90$^\circ$ per segment, and are offset at 45$^\circ$ to each other to provide a composite azimuthal 45$^\circ$ angular segmentation in order to cover the seams between the counter segments. This package was used to veto charged particles originating from the target and thereby distinguish the $e^+e^-$ pairs due to photons that result from the decay of the pionic hydrogen atom. The $B$ and $C$ rings each have 12-fold segmentation and are aligned so that each corresponding segment has the same angular coverage. In the present experiment, the $B$ scintillators were redundant and were not used for triggering purposes. The $D$ ring, located outside the drift chamber, has a 16-fold segmentation. As will be described in the following section, our trigger typically required that a pair of real photons originate from the target and convert in the lead wrapped around the $A$,$A'$ scintillators, and fire a certain number of $C$'s and the $D$'s but not the $A$,$A'$ counters.

The detection efficiency of each scintillator layer is $\geq$ 98% for minimum ionizing tracks (26). The length of each subsequent layer of scintillator increases with radius to provide the same solid angle coverage. The resulting pulse from each photo-multiplier tube is fed to independent FASTBUS time-to-digital (TDC) converter and CAMAC analog-to-digital (ADC) converter modules to provide timing and energy information for hits in each segment.