[BLAST_ANAWARE] more deadtime

From: Tancredi Botto (tancredi@lns.mit.edu)
Date: Thu Mar 04 2004 - 13:07:16 EST


A brief summary of the present understanding of deadtime sources from the
analysis of recent data:

Deadtime is limiting us in the use of higher beam currents. There are many
components to this: the most significant is "trackless" triggers that pass
the 2nd level trigger thanks to random hits in the wch. The ratio of
these fake 2ndl level triggers (abot 2/3 of total data) is consistent with
the Wch S/N ratios. The ratio of trackless triggers is nearly independent
of trigger type.

Trackless triggers have no known vertex or momentum distribution of course
but they contribute fully to DAQ deadtime. They are very sensitive to Wch
multiplicity and S/N. Possibly this is related also to the collimator design.
Trackless events really have too few wch hits (often < 3 hits in the tdc
range used in the reconstruction of the wch events). We can't use a
momentum cut to truly speak about deadtime..

A second contribution is coming from low-momentum particles that originate
mostly upstream of the target. These events constitute the vast majority
of "tracked" triggers, but a smaller fraction of the overall yield. They
are well characterized in momentum (100-200 MeV/c), charge (positrons for
inbending field, electrons for outbedending - both fire the Cerenkovs) and
location in the detector (tof #'s 10-14).

These events must originate from 300 MeV photons in a EM shower. The shower
having photons (which are not "bent") may contribute again to the Wch S/N.
Note that trackless triggers are instead *uniformly* distributed in the TOF's

We have never experienced such a harsh environment before because we were
not running with an inclusive trigger (requires a cerenkov) prescaled by 6
and because we did not add the LADS to the e,e'n trigger. Having done so
it offers many more opportunities for trackless and low-energy-background
triggers. Indeed trig==2 and trig==7 are the dominant distribution of
trigger types.

To make matters worse, any of these trigger rates will show a dependence
on beam current and as mentioned in the prev email it is important to
operate in a linear region. 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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-- 
________________________________________________________________________________
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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