Performance

The energy sum spectra for Bhabha events show a correlation between the forward and backward EFC detectors. A clear peak at 8 GeV with an rms resolution of 7.3 % is seen for the forward EFC, while a clear peak at 3.5 GeV with an rms resolution of 5.8 % is seen in the backward EFC. These results are compatible with the beam test results [17] and are slightly worse than those obtained by a GEANT Monte Carlo simulation. The discrepancies are due to dead channels and crystal-to-crystal nonuniformity. An expected counting rate for Bhabha events is a few kHz at an ultimate luminosity of 10$^{34}$ cm$^{-2}$s$^{-1}$.
EFC has provided a fast online feedback about the luminosity and background rates to the KEKB operations group. Fig. shows an operational history of a typical beam fill at KEKB. The coincidence and accidental rates of the forward and the backward EFC are shown as a function of time in seconds together with $e^+$ (LER) and $e^-$ (HER) beam currents. During the injection period from 0 to 1800 s accidental Bhabha rates are quite high, but after the injection the accidental rate is very low. Singles rates of EFC can provide very useful information for diagnosing of background sources.

Figure: An operational history of a typical beam fill of KEKB. The top figures show e$^+$ and e$^-$ beam current, and the bottom figures the coincidence rate of the forward and backward detectors (left) and their accidental rates (right) of EFC.