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    User name R. Michaels

    Log entry time 11:30:11 on December13,2006

    Entry number 190649

    keyword=T4 cerenkov threshold

    During the beam-study related access I studied T4.
    The issue was that its rate was very high and appeared
    to not decrease much when Cerenkov was turned "off".

    The cabling and logic looks correct to me. I think the
    problem is the Cerenkov is firing VERY often from noise.
    Even with cosmics one can see lots of randoms piling up
    with scintillators.

    Here is the T4 rate (cosmics) versus Cerenkov threshold

    threshold ......... T4 rate (Hz)

    30 mV ............. 2300
    40 mV (normal) .... 600 .... <--- where we were running
    50 mV ............. 10 to 15 Hz
    60 mV ............. 6.0 to 8.0
    100 mV ............ 4.5 to 6.0
    200 mV ............ 2.5 to 4.0
    250 mV ............ 2.5 to 4.0

    Results were repeatable during 10 min.
    Even with 250 mV threshold a correlation is seen on scope.
    So I think its safe to increase to 50 mV and we'll run this way.