Following Tancredi's mail, I think we should significantly prescale the
inclusive and put more lead shielding in front of the forward LADS. Ernie
is working up a modification of the collimator which has the potential to
improve the deadtime situation for the inclusive trigger. Until we can
implement that, we should optimize running conditions for (e,e'd), (e,e'p)
and (e,e'n) both vector and tensor.
Richard
On Thu, 4 Mar 2004, Tancredi Botto wrote:
>
> 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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