  Full functional and performance tests were performed many times before the
Hard X-ray Modulation Telescope (HXMT) launch. During one of the tests, the
count rate curves of the 18 High Energy Detectors (HED) have been found
increased consistently within an interval of time. A further study on the
correlation between the count rate and rainfall was carried out,and the
increased net spectrum was also analyzed. The analysis results indicate that
the short-lived 222Rn decay products (214Pb and 214Bi) in rainwater were
responsible for the transient changes of the background radiation spectra in
HEDs. The results show that the HXMT/HEDs have a good detection sensitivity on
X/gamma rays, and the detector calibration results are effective.
