The energy deposition of 70 MeV photons and positrons in various clustering schemes were studied[1]. The efficiency in detecting these particles is defined as the ratio of events with energies above 55 MeV in at least one cluster of events with energies above 55 MeV in the whole calorimeter. The resulting clustering scheme consists of 60 overlapping clusters, each containing six to nine modules. Each module belongs to at most three different clusters. Six clusters around a pentagon and the associated HEXB's form a so called "supercluster". Some of the clusters and one supercluster are visualized in Fig. 4.8 .
The clustering scheme is implemented in the pion beta trigger with the dedicated analog adder and discriminator UVA 125 (ADDER), which is capable of analog adding nine PMT signals; 60 clusters require 12 ADDER modules. The following description refers to the diagram of the trigger electronics in Fig. 4.9. A valid pion stop signal obtained from a coincidence of B1, the active degrader and RF opens the delayed pion gate DPG. The pion gate is delayed by 10 ns in order to avoid prompt interaction processes such as elastic scattering or pion charge exchange reactions. The p b trigger requires the coincidence of two showers exceeding the Michel endpoint (high treshold, HT=55 MeV) within 20 ns inside a valid DPG. Following the delayed pion gate another time window is opened (DPG'). Due to the pion lifetime the DPG' is mostly sensitive to pile-up from Michel events and is used to study background from these events. The crystal PMT's provide two identical signals: One of them is supplied to the trigger branch, where it is split three times and supplied to the ADDERs according to the clustering scheme. The other signal is delayed by 350 ns, split up and then distributed to the Analog-to-Digital-Converters (ADC's), Time-to-Digital-Converters (TDC's) and Domino Sampling Chip modules (DSC's, see chapter 7), which reside in FASTBUS- and NIM-crates, respectively.
The logic output from the ADDERs is supplied to LRS 4564 FAN-IN modules, where the logic OR's for the supercluster signals are generated.
The Memory Look Up unit (MLU) is the central trigger generating unit. It is capable of making Boolean operations on the input signals which include the 10 supercluster signals, the DPG, etc. The p b trigger requires a combination of two superclusters in opposite hemispheres within a valid DPG. On the other hand, the p ->e+ n e trigger requires the presence of a single shower, also within a DPG, occurring in at least one supercluster. As a result, the p ->e+ n e trigger is more sensitive to accidental coincidences of Michel events. In addition to the two triggers described above, there will be a prompt trigger, a cosmic ray trigger, and a more delayed pion gate (DPG') trigger. As outputs, the MLU produces the desired triggers which are stored in pattern registers. The event trigger is the logic OR of the various event triggers. It creates a LAM[10] signal for the computer and supplies the ADC gate and the TDC start signal. A latch mechanism ensures that the event coincidence is closed until the analog- and time-to-digital-conversion processes and the read out phase is completed.
The readout of the FASTBUS and CAMAC crates as well as the control of the scalers, the high voltage system and the temperature probes was done with the data acquisition system HIX (Heterogeneous Information Exchange). It allows the control of several "frontend" PC's, which are connected over the Ethernet-network as well as the online analysis of the acquired data and their storage on disk or magnetic tape. The system has been developed by St. Ritt (UVA) and is described in [Rit 93].
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