Trigger

The data acquisition system was ``triggered'' to read in the data, when appropriate conditions were met. This decision making was done in a hardware logic circuit illustrated in Fig. 3.19. Our basic strategy for triggering can be summarized as:

1.

Provide a gate for s after muon arrival, which is sufficiently long compared to the muon lifetime.

2.

Inhibit the triggering if a muon had been already accepted and the gate was opened (BUSY), or the previous data was being processed by the computer (INH).

3.

Identify a pile up, if another muon arrived after the gate was opened.

4.

Collect the signals from all of the detectors, if any one of the detectors fired during the gate.

5.

Clear the TDC and ADC buffers, if no detector fired.

6.

Record various scaler values for data normalization and diagnosis.

 

Figure 3.19: Schematic diagram for the trigger electronics, illustrating the key components. Pulses from the beam counter T1, beam busy signal BUSY, and computer busy INH were combined to provided an event gate EVG, which allowed individual sub-detector systems to accept events and give a sub-trigger TRGn. The circuit diagrams are for illustrative purpose only, and not all of the details are given¹.

The timing diagram is given in Fig. 3.20. A muon, defined by a sufficient energy deposit in T1, opened a pile up gate (PUG), which supplied a BUSY signal for 10 $\mu$s. The BUSY gate, together with the general inhibit signal (INH) due to computer delay, was used to ensure (2.) above. Only when no muon had arrived in the preivous 10 $\mu$s, and the computer was ready, was the coincidence satisfied, opening the event gate (EVG) as well as giving a common start to TDCs. The EVG was sent to all the detector signal processing logic allowing them to accept events.

  

Figure 3.20: Schematic timing diagram for trigger electronics. Lower lines indicate ``on,'' and higher lines ``off,'' corresponding to 0 V and -0.8 V, respectively, in fast-negative NIM logic standard. The horizontal axis represents approximate time flow. The second pulse in T1 illustrates a pile up event, which extends B, and delays EB. The EVCL pulse, not shown in the figure, was given when there was no trigger, and was generated at the same timing as EVTR.

If any of n sub-detection systems described below gave a trigger DETnduring EVG, a trigger gate TRGn was opened, which was subsequently closed with the end of event gate pulse (EEVG). The master event trigger EVTR was provided as a coincidence of TRG and EEVG, requesting the Starburst to collect the ADC and TDC values. On the other hand, when there was no detector giving a trigger, EVCL was given from , clearing the ADCs and TDCs so that they could accept new values in the following event.

Once the master trigger was given, the Starburst provided a computer busy signal (CINH) in a CAMAC output register while processing the signal ( ms). However, since it took a few hundred microseconds for CINH to turn on, an inhibit had to be provided by hardware (HINH) to prevent a pile up during this time. Furthermore, an extended inhibit (IEX) was provided while clearing the ADCs and TDCs, whose duration was determined empirically by monitoring the number of lost events.

If more than one muon arrived during the gating period, the time correlation between the muon stop and the detected reaction would be lost, hence the event had to be discarded. The rejection of pile up events, which amounted to about 5% of total events at our typical beam rate of 5000 s^-1, was achieved in software using a pile-up bit pattern given by the PUG module. Also in the case of pile up, the module extended its BUSY signal for 10 $\mu$s from the time of the second muon entrance, which in turn delayed the-end-of-busy pulse (EB) (see dotted lines in the figure).

 An important scaler GMU (good muons) was derived as a coincidence between EB and EEVG (end of event gate). Since EEVG always occurred 10 $\mu$s after EVG was turned on, regardless of whether another muon came in or not, the coincidence was satisfied only if there was no pile up. Recalling that EVG( ) takes into account the computer dead time, GMU provided the number of incident muons which satisfied all the trigger conditions except detector triggering, hence is to be used for absolute normalization.

Various scalers including GMU were recorded for normalization and system diagnosis purposes. Their values were read into the Starburst typically every 5 s, independent of the muon trigger. Occasionally, some of the scalers were lost due to module malfunction, but there was enough redundancy to recover important scalers. For example, an alternative way of deriving the number of ``good'' incident muons is:

where MON is a scaler for a pulser signal, hence is the measure of live time fraction. The factor, gives the fraction of events which do not have pile up , and GMU is derived by

A further diagnosis and normalization tool was provided by a trigger called 1/N, which was activated every 1024 hits in T1 scintillator, regardless of its deposited energy (i.e., including beam electron hits), and independent of the sub-detector triggers. The latter feature of this trigger allowed yet another way of checking the GMU scaler.