********************************************************************
  ****  DBL_N_EM was disabled for 1997 running, and removed from  ****
  ****  the fourth LEVEL1 bit on 30.10.97.                        ****
  ****                                                            ****
  ****  It was replaced by SNG_BCAL (/TAG_SNGC) for fills 4381 -> ****
  ****  4432; LEVEL1 downscaling was factor 2 and LEVEL3 passed   ****
  ****  all events (except fill 4381 - all rejected)              ****
  ********************************************************************

                                                            A.E.Wright
                                                              18/11/97

                         ----------------------

Description : A new gamma-gamma trigger
From : AXAONL::WERNERS
Posted : 18-NOV-1996 13:59:10,    Expires :  3-DEC-1996 13:59:10


                                                             CERN, 18-NOV-1996

      During the last week several studies for a new gamma-gamma trigger,
which will be used to check the efficiency of the TRK_CNT2 trigger, were
performed.
This new trigger, the DBL_N_EM, has been enabled at run 42615, it is situated
on the fourth bit. In its final version it requires a coincidence of two ECal
modules (threshold = 300 MeV per module) and the presence of the ITC_R_PHI_Z 
bit.

The definition, the thresholds and the downscaling were changed several times
during the last runs, and therefore the following lines should clarify the
situation.

  DBL_N_EM:

  Run    DownSc.      Threshold [GeV]                Definition
  42615    40    0.18(EA)/0.21(EB) 0.15(BA)        2 ECW modules
  42627    10    0.18(EA)/0.21(EB) 0.15(BA)           --- " ---
  42643    10    0.4 (EA)/0.4 (EB) 0.35(BA)           --- " ---
  42650     1    0.18(EA)/0.21(EB) 0.15(BA)   2 ECW mods. AND ITC_R_PHI_Z bit
  42654     1    0.3 (EA)/0.3 (EB) 0.3 (BA)           --- " ---

In addition, the requirement for ITC_Back_to_Back bit has been changed:
Instead of 1 vs. 3 sectors (out of 96) it is relaxed to 1 vs. 5 since run
42627.
The only trigger which uses this bit is TRK_CNT2.


                                   Stefan