$\pi ^o$ trigger

The $\pi ^o$ trigger will be the primary trigger for physics events. The electronics basically use information from HYCAL to create a trigger when two clusters exist in the calorimeter. This is done by essentially fanning in strips of detectors 5 or 6 high which span the calorimeter. The strip signals are discriminated on. By having strips which span the entire calorimeter, showers are guaranteed to be at least 50% contained in a single strip. Because the strips are at least 10cm apart, events in which two non-adjacent strips fire would signal two clusters in the calorimeter. This explanation is somewhat simplified since shapes, amplitudes and timing of signals from the lead-glass and PbWO4 differ, preventing one from mixing the two detector types in a single fan-in. Another consideration is that two clusters could occur on opposite sides of the same strip, causing only that strip to fire. This is addressed by replicating the logic, but for strips rotated 90 degrees with respect to the first.

Figure 2: The detector groups formed by the first stage fan-ins.

This algorithm is implemented in the electronics in the following way: UVA120A linear fan-in modules housed in crates mounted under the HYCAL will fan-in between 25 and 36 dynode signals from the detectors (see fig. 2). The UVA120A's have two inverted outputs. High speed (0.87c), low-impedance coaxial cables are then used to carry the inverted ¹signals up to the UVA125A modules in the racks on level 2 of the space frame. The UVA125A modules perform a second stage of fan-in and apply a discriminator. The discriminator will see the sum of the entire strip. Discriminator thresholds are set via a front panel DC input. These are in turn, driven by a CAMAC controlled DAC so that the discriminator thresholds can be adjusted from the counting house.

Th $\pi ^o$ trigger will actually be dominated by accidentals from Compton scattering and scattering from the beam pipe upstream of the PS dipole (see section 1.2.6). The rate can be adjusted by either raising the threshold, or lowering the beam current. These will be adjusted so that the $\pi ^o$ trigger rate is about 2kHz.