[BLAST_TOF] Cerenkov status

From: Tancredi Botto (tancredi@mitlns.mit.edu)
Date: Tue Nov 19 2002 - 09:06:46 EST


Hi Ricardo,
I try to summarize the story about the CC counters so far. Tavi is
also analyzing a bunch of new data

        ***** EFFICIENCIES ******

_ Expected number of PE is 3 (from TDR, without central reflector)
  which brings an expected efficiency of > 95% (but of only 80% if the
  threshold cuts the 1 PE peak).

_ Measuraments with field off at nominal and + 10 % voltages showed
  "efficiencies" in the range of 75-85% and 85-95 %, respectively, for
  CC L0,L1,R0,R1. Efficiency is defined as "good tdc" value

_ With field off and cosmics in the hall, I also measured up to 95%
  with a sandwich method, confirming the DTF results.
  At that time, the eff for having > 1 pe (> 1 pmt firing) was only
  50 % (not 80). However in this case one can make a geometrical argument
  for the cosmic measurament, given my configuration.

_ Early field on and off data showed absolutely no change in the ADC
  spectra. We know from the tof that the effect of the field is visible
  in the ADC spectra alone

_ there is a shift in the field on CC adc spectra now

_ With field on (and beam) the efficiency is typically a poor number,
  50-60%. At the very highest HV (above +10 %) and for L0 only I get 75 %.

_ Note: single tubes (beam off) count 10-20 KHz which is mostly tube
  noise. The rate for > 1 PMT is only 30-50 Hz

        ****** MAGNETIC FIELD MEASURAMENTS **********

_ the blast on the field at the CC pmt is typically 100-150 gauss vertical.
  The transverse field, going perp to the CC box, is up to 50 gauss at L0 (which
  the far way from the coils), 100-150 gauss for L1 and 200 gauss for L2
  (which is very close to the coils). I always measure at the surface of
  the can

_ The TDR assumed 50 gauss for this transverse component. Note that what I
  measured (at the top of the can) reverses sign as I move up and down
  along the can

_ With BLAST off, there is a remnant magnetization of the PMT can. Typical values
  are 10 gauss, again radially w.r.t. the can, at the position of the pmt
  photocathode. The field between two pmt cans is 20 gauss.

_ If the can is magnetized, the field inside will "orient" itself in the
  blast field.

_ If I measure a remnant mag field outside the pmt can, than this has
  softend and acts as a "free magnet". Else the field should be
  constrained to stay on the surface of the can (never perp to it).

_ If the above is true, than there may be a large transverse field inside
  the can, with blast on

_ We can shield from outside but can't demagnetize the pmt cans. At the
  very best we could retain the field off efficiency.

- I am looking for spare cans, and to see if we can run a test. The
  question here is whether the shielding failed with time (if at all)
  and so whether we need a "bad" can.

-- 
________________________________________________________________________________
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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