  The radiation damage to a silicon photomultiplier (SiPM) set on a satellite
orbit increases energy threshold for scintillator detectors. We confirmed that
1 krad of radiation increases the energy threshold by approximately a factor of
10, which is worst for our system. Using one or two SiPMs damaged by proton
irradiation and a plastic scintillator, we performed the following three
experiments in our attempt to lower the energy threshold of radiation-damaged
SiPMs to the greatest extent: (1) measurements using a current waveform
amplifier rather than a charge-sensitive amplifier, (2) coincidence
measurements with two radiation-damaged SiPMs attached to one scintillator and
summing up their signals, and (3) measurements at a low temperature.
  Our findings confirmed that the use of a current waveform amplifier, as
opposed to a charge-sensitive amplifier and a shaping amplifier, could lower
the energy threshold to approximately 65% (from 198 keV to 128 keV).
Furthermore, if we set the coincidence width appropriately and sum up the
signals of the two SiPMs in the coincidence measurement, the energy threshold
could be lowered to approximately 70% (from 132 keV to 93 keV) with little loss
of the acquired signal, compared to that of use of only one scintillator.
Finally, if we perform our measurements at a temperature of -20 {\deg}C, we
could lower the energy threshold to approximately 34% (from 128 keV to 43 keV)
compared to that of at 20 {\deg}C. Accordingly, we conclude that the energy
threshold can be lowered to approximately 15% by using a combination of these
three methods.

Instrumentation and Detectors;Instrumentation and Methods for Astrophysics