Trigger efficiency

  The trigger efficiency of the Super-Kamiokande detector is also important for getting a precise solar neutrino flux. Note that ``trigger efficiency'' means the detection efficiency in each energy region. Of course, because of the energy resolution, the analysis efficiency is 50% at the threshold energy, however, ``trigger efficiency'' means the detection efficiency hereafter.

For the measurement of the trigger efficiency, Nickel calibration is used. In addition to the normal trigger mentioned earlier, the special trigger whose threshold is very low (150 mV) is applied at the same time. This is called the ``super low trigger''. The trigger efficiency is defined by the ratio at the each energy point of events passing the normal trigger threshold to those triggering the super low trigger. Fig 1 shows the trigger efficiency at the center and top position. The Neff at which the trigger efficiency is 50 % at the center position is 38 hits, which corresponds to 5.8MeV.

As mentioned later, the energy threshold for solar neutrino analysis is 6.5MeV. The trigger efficiency above 7MeV is almost 100% at all positions in the fiducial volume both Monte Carlo and Nickel data. However, in the energy region between 6.5MeV and 7MeV, the difference between Monte Carlo and Nickel data is calculated to 1.2%. This difference is corrected for in flux measurements.

trgeff.eps.gif (7699 ?o?C?g) Figure 1: The trigger efficiency at the center (solid line) and top position (dashed line).

 


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