  Three high energy particle monitors (HPMs) employed onboard the Hard X-ray
Modulation Telescope Insight-HXMT) can detect the charged particles from South
Atlantic Anomaly (SAA) and hence provide the alert trigger for switch-on/off of
the main detectors. Here a typical design of HPM with high stability and
reliability is adopted by taking a plastic scintillator coupled with a small
photomultiplier tube (PMT). The window threshold of HPM is designed as 1 MeV
and 20 MeV for the incident electron and proton, respectively. Before the
launch of Insight-HXMT, we performed in details the ground calibration of HPM.
The measured energy response and its dependence on temperature are taken as
essential input of Geant4 simulation for estimating the HPM count rate given
with an incident particle energy spectrum. This serves as a guidance for
choosing a reasonable working range of the PMT high voltage once the real SAA
count rate is measured by HPM in orbit. So far the three HPMs have been working
in orbit for more than two years. Apart from providing reliable alert trigger,
the HPMs data are used as well to map the SAA region.
