In conversations with Gerard and Rory, we have considered accidental rates in
addition to the 3-fold coincidences quoted earlier. These are correlated
background, effectively reducing the 3-fold coincidence to a 2-fold coincidence.
Unfortunately these additional rates are not negligible.
For ease of comparison, I use the same numbers as Reinhard (Note 1-29-98):
Energy = 2528 MeV
Tagged Range = 523 to 2409 MeV
Tagger: Tagged Rate = 10**7/s, Prescaled Tagged Rate = 0.34 * 10**7/s
Start Counter: 0.4 MHz (CLAS-NOTE 95-006)
Hadronic Rate (total) = 3.3 KHz
Hadronic Rate (untagged) = 1.9 KHz
Hadronic Rate (tagged) = 1.4 KHz
Hadronic Rate (tagged, prescaled) = 0.45 KHz
TOF Cosmic Rate = 10 KHz
Level 1 Rate = 13.3 KHz
Tagger.Start Counter coincidence time = 15 ns
(Tagger,Start).(Level 1) coincidence time = 100 ns
I summarize all three accidental sources assuming prescaling in the Trigger Supervisor
(Tagger Rate = 10**7/s):
1. 3-fold accidentals (80 Hz)
Tagger.Start (15 ns) = 60 KHz
(Tagger.Start).(Level 1) (100 ns) = 80 Hz
2. Untagged Hadronic - Time coincidence between TOF and Start (285 Hz)
Tagger.(Untagged Hadronic) (15 ns) = 285 Hz
3. Tagged photons producing electromagnetic background in Start (106 Hz)
20% Start background due to tagged photons = 80 KHz
(20% Start).(Level 1) (100 ns) = 106 Hz
Total Accidental Rate (prescale in T.S.) = 80 + 285 + 106 = 471 Hz
The accidental rates above are reduced by the prescale factor if we
prescale at the tagger. This is the present scheme we are trying to install before g1.
The revised accidental rates (Prescaled Tagger Rate = 0.34 * 10**7/s):
1. 3-fold accidentals (27 Hz)
2. Untagged Hadronic - Time coincidence between TOF and Start (97 Hz)
3. Tagged photons producing electromagnetic background in Start (36 Hz)
Total Accidental Rate (prescale at tagger) = 27 + 97 + 36 = 160 Hz
Summary and Conclusions
=======================
1. Accidental rates are much higher than first considered
2. Prescaling at the T.S. saturates our DAQ rate
3. Prescaling at the tagger results in a rate of 450 (true rate) + 160 (accidentals) =
610 Hz. This will tax our DAQ and may require running at slightly lower photon flux.
4. Contributions 1) and 3) above are proportional to the TOF cosmic-ray rate and could be
reduced significantly with a Level 2 trigger. If eliminated completely, the total
accidental rate will be from contribution 2) which is 97 Hz.
These results demand careful consideration of our trigger and DAQ rates. Elton.