  CMS-HF Calorimeters have been undergoing a major upgrade for the last couple
of years to alleviate the problems encountered during Run I, especially in the
PMT and the readout systems. In this poster, the problems caused by the old
PMTs installed in the detectors and their solutions will be explained.
Initially, regular PMTs with thicker windows, causing large Cherenkov
radiation, were used. Instead of the light coming through the fibers from the
detector, stray muons passing through the PMT itself produce Cherenkov
radiation in the PMT window, resulting in erroneously large signals. Usually,
large signals are the result of very high-energy particles in the calorimeter
and are tagged as important. As a result, these so-called window events
generate false triggers. Four-anode PMTs with thinner windows were selected to
reduce these window events. Additional channels also help eliminate such
remaining events through algorithms comparing the output of different PMT
channels. During the EYETS 16/17 period in the LHC operations, the final
components of the modifications to the readout system, namely the two-channel
front-end electronics cards, are installed. Complete upgrade of the HF
Calorimeter, including the preparations for the Run II will be discussed in
this poster, with possible effects on the eventual data taking.
