[BLAST_SHIFTS] L1 cerenkov efficiency hits 99 % (but..)

From: Tancredi Botto (tancredi@mitlns.mit.edu)
Date: Mon Oct 07 2002 - 16:51:51 EDT


This afternoon I tested LC1 using a cosmics set up with the start counter.

The trigger was SC (on LC1 inner side) and Ltof #5 (on the back). The C analog
sum was done with the new patch panels (passive resistor chains). This set up
also allowed to trigger on the coincidence of top and bottom tubes of the
C-detector (or any two combinations of top and bottom tubes).

All threshold were 30 mV and the "cosmic" coinc window was 40 ns. The
cerenkov pulse width for overlap was 60 ns.

At nominal voltages I measured a 60-70 % efficiency only when triggerring
on one half of LC1. A similar value for this box has been measured with e-p
elastics (field off). When requiring a coinc "top" and "bottom" in LC1 the
efficiency was 20-30 %. So i tried various combinations of thresholds, SC
voltage, inputs. Note that in the second half of LC1 there is a dead tube.

When I finally increased the LC1 voltages by ca. 20 % I measured cosmic
efficiencies of 85 % (using again the "OR" between two halves) and 50 %
(using the AND) respectively.

By increasing the voltage further I obtained 99 % on the OR. Finally I only
got 65 % on the "AND" when all voltages were almost at their max value (2680)

Conclusions:

_ the cosmic set up works (since you can measure 100 %.. which is an
  issue if the sandwich overlap is not optimal). It will work fine for the
  other two big boxes, but the overlap can be an issue for box #0

_ increasing the C-voltages does reasonable things to the efficiency.

_ since the 30 mV threshold can't be further decreased, tomorrow I'll
  throw in the analog pmt amplifiers from lecroy, with the hope to obtain
  similar results at a lower operating voltage.

_ at nominal voltage the cosmic coinc rate (top and bottom) on LC1 was 15 Hz
  which seemed reasonable. However, the efficiency for such C trigger is low.
  It may have to do with the bad pmt and hopefully we can find out moving
  on the next box

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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