[Olympus] rates in TOFs

[Alexander Kiselev]

  Good mornig colleagues,

>   Installation of say two more crates would increase the effective readout 
> speed by a factor of almost 2, as we discussed today. This may be pretty 

  this factor of 2 is way too optimistic I admit. Let's estimate correct 
numbers useful for further discussion. Sorry if the below calculations
look trivial.

  Indeed numbers quoted during the Monday meeting (max readout rate 1300Hz
at 2% live time, "typical readout fastbus") have nothing to do with the
real running conditions. Let's assume ~750us average "event cycle (readout
time + waiting for next trigger)" for simplicity. 2% live time would mean
that next trigger waiting time was only ~750us*0.02=15us, so the original 
trigger load was ~67kHz. Clearly we will never run under conditions like 
this. 

  I'd say with so large readout times we should never go beyond ~1kHz 
original trigger load (~1ms next trigger waiting time in the above 
calculations). Assuming now ~750us average readout time for simplicity, 
this would correspond to the on-tape (accepted) trigger rate of ~1/1750us 
or ~570Hz at a ~43% dead time, obviously. 

  If we manage to decrease the readout time from ~750us to say ~400us, 
average event cycle will become 1400us for the same 1kHz trigger load, 
which would mean ~715Hz accepted trigger rate at a ~29% dead time. So 
gain would be significant, but no way close to a factor of 2.

  It should also be mentioned that useful (ep-elastic) trigger rate at 
nominal luminosity is ~15Hz in BLAST acceptance and about the same in 
12-degree Lumi acceptance. Even if we increase average luminosity by 
a factor of 2, trigger budget will (should!) be always dominated by  
calibration (inclusive) triggers and random coincidences. Under these
circumstances with a relatively slow readout hardware it does not make 
sense to run at higher dead times, since this causes 1) smaller 
*fraction* of ep-elastic events on tape, obviously; 2) in fact smaller 
*rate* of ep-elastic events on tape, less obvious; 3) worse signal
to background ratio if random coincidences start dominating. Derivation 
of the "best figure of merit" running strategy in terms of DAQ rates
as a function of luminosity, readout time, random coincidence scaling 
and allowed fraction of calibration triggers is a rather straightforward 
excersice, which "everybody else can do" :-)

  Best regards,
    Alexander.