Hello,
I just wanted to acknowledge with this attachment the status of CODA
livetime vs beam current for a series of runs: 5264, 5280, 5292
respectively.
It is important to keep monitoring this. While intrinsically there is
nothing wrong, a low livetime makes data taking so much more inefficient.
Low deadtime may require better beam (higher lifetime, I_max) or better
Wch S/N or running at a lower current. There is no point to inject to 90
mA if we only take 20 % of the data there. The "real" average beam current
has to be convoluted with this. So 90 mA, 20 % dead is equivalent to 18 mA.
To me livetime is the ratio of the bdcct/dcct scalers, and it is important
to quote at what current it is calculated. Run scaler_ntuple.C to generate
an ntuple, scaler.C to loead the file, scaler->Draw("bdcct/dcct:ldcct")
shows CODA *livetime* ( = 1 - deadtime) vs epics beam current.
Repeated observation shows that rates are linear with beam current up to
a breaking point or critical current. Beyond that point - usually correlated
with a region of shorter lifetime - all rates go up, including hits in
the wch presumably, and deadtime will literally explode. Typically this
problem was found between 80-85 mA (black points, run 5264). Since
wednsday night this point has been shifting to lower currents.
I think that the priority for the momentum is to try to at least stay in a
region where the trigger rates are roughly linear with beam current and
the wch S/N is optimized !! A good parameter to tune against could be
maximum injection current. The higher this is, the better we can operate
in the linear region. Suspicion is that presently the max beam current is
not much more than 100 mA and by running close to that we are running too
close to the unstable region.
Next one could increase the prescale factors (see README for trig==7/phys6)
and the Pb shielding in front of the LADS. Regards
-- tancredi
________________________________________________________________________________
Tancredi Botto, phone: +1-617-253-9204 mobile: +1-978-490-4124
research scientist MIT/Bates, 21 Manning Av Middleton MA, 01949
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