At the g1 meeting previous to the most recent one there was
discussion about the photon and event rates we expect during the
upcoming run. Several of us (Reinhard, Elton, Rory, Dan) wanted to
recheck our independent estimates. I think we are all in agreement now
on the numbers, or at least we understand where remaining 20%-ish
differences come from. Under conditions at which we want to run, I find
that we should probably expect a total hadronic trigger rate, including
accidentals, of 500 Hz for the prescaling scheme I suggest below.
The total hadronic cross section were taken from the 1995 "Research at
CEBAF" report, page 2-59. The photon yield came from Jackson, equation
15.35, which is the Bethe-Heitler shape for relativistic bremsstrahlung.
The rates were computed for the Tagger energy ranges given by Dan Sober
in his 1-8-98 memo. The ranges were determined by the present
sub-master setup of the Tagger. I integrated up from the pion threshold,
and there was no energy cut put on the hadronic particles. No
collimation was assumed for the photon beam.
Our beam energy will be 2528 MeV, and our LH2 target will have a
thickness of 1.21 gm/cm2. Let the tagged photon rate be the canonical
1x10^7, before prescaling. The prescaling possibilities are numerous.
Our earlier aggreement was that we would prescale so as to flatten out
the event rate as a function of energy. After looking at it for a while
I recommend the following:
>> Use the four Tagger sub-master (SM) divisions as they now are. <<
>> Prescale by factors of 1, 2, 4, and 8. from SM1 to SM4. <<
This scheme avoids putting discontinuities in the yield near either the
eta or the kaon thresholds.
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CLAS Real Photon Event Rate
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Endpoint energy = 2528.000 MeV
Total Unprescaled tag rate = 1.000E+07 photons/sec
Tagged energy range : 523.296 2409.184 MeV
Target thickness = 1.210 gm/cm2
Range k/E0 k_E0 Eg_lo Eg_hi Prescale Phot/sec Phot/sec Hadr/sec Hadr/sec
Min Max (MeV) (MeV) Factor (x1E6) Prescale Prescale
1) 0.060 0.207 152. 523. -1 12.29 0.00 2263 0
2) 0.207 0.331 523. 837. 8 3.70 0.46 595 74
3) 0.331 0.510 837. 1289. 4 2.89 0.72 438 109
4) 0.510 0.802 1289. 2027. 2 2.55 1.27 306 153
5) 0.802 0.953 2027. 2409. 1 0.83 0.83 83 83
6) 0.953 1.000 2409. 2528. -1 0.17 0.00 16 0
Tagged Photons (prescaled) = 3.29E+06 photons/sec
Total Photons (tagged & untagged) = 2.24E+07 photons/sec
Hadronic Trigger Rate (prescaled) = 420 events/sec
Untagged Hadronic Rate = 2279 events/sec
Total Hadronic Rate (unprescaled) = 3702 events/sec
The total hadronic rate is 3702 Hz, which is roughly comparable to
Elton's scaled estimate of 6 kHz, based on the e1 data of the CLAS
"Level 1" rate using the TOF counters. The prescaled hadronic trigger
rate comes out to be 420 Hz. This seems to be within the range of
present capabilities of the DAQ system, though we would have large
deadtime unless DAQ speeds up considerably. If the DAQ system were
running at the design value of 1.5 kHz we could relax the prescaling.
In that case I would recommend 4,2,1,1 prescaling, which would lead to a
760 Hz trigger rate.
Elton's accidental rate calculation makes sense. Using Elton/Claude's
estimates for a Start counter rate (0.4 MHz), and the expected Tagger
rate (1x10^7Hz) with a 15 nsec coincidence time, gives and accidental
rate of 60 kHz coming out of the Tagger.Start counter coincidence. Given
the 10 kHz cosmic Level 1 rate for the TOF counters, plus my estimate of
about 4kHz for the real hardonic rate, and assuming a 100 nsec
coincidence window in the Trigger Supervisor, results in an accidental
rate of 84 Hz. The ratio of real to accidental events in the data we
write to disk is then 5 to 1, i.e. 17% junk on disk.
I've made some graphs of the event rate as a function of photon energy
that we could examine at our next meeting on Thursday Jan 29. Any
comments, especially about my suggestion for the prescale factors, are
welcome.
Regards, Reinhard.