A lead wall (4" thick, 40" wide) which covered the exposed region of the beam pipe immediately
downstream of the tagger vacuum box was installed during the downtime on Friday. TOF scaler rates were
taken along with the E = 1.7 GeV g5 photofission data over the weekend. These rates showed a
substantiall inprovement in the overall TOF rates compared to before shielding (4 GeV data, so
systematics could be a bit different):
Before After
(Run 10592) (Run 10698)
Panel 1 600 315
Panel 2 343 53
Panel 3 605 77
Panel 4 610 93
Total 2175 534
==> Background reduction about 7-8 (panels 3, 4)
Rates are still factor 30 above estimated cosmic + hadronic, so trigger rate studies are a high priority
when we continue commissioning. We note that the rates in Panel 1 may be high due to the running
conditions for photofission. Still to be studied is the threshold dependence of the TOF trigger rates.
Rory estimates that these rates yield a prescaled trigger rate of about 2.5 KHz, dominated by
accidentals. If the tagging rate were decreased by a factor of 1/2 then the trigger rate would be
reduced to 626 Hz, which the DAQ can accommodate.
Elton.