In-flight calibration of the MONEX high energy solar X-ray monitor (HEM) on the P78-1 satellite

Simultaneous observations of seven medium and large solar X-ray bursts made with the MONEX High Energy Monitor (HEM) on the P78-1 satellite and the X-ray Spectrometer aboard the ISEE-3 (International Sun Earth Explorer-3) spacecraft have been used to examine the detector gain and response characteristics of the HEM instrument. It has been found that the HEM detector gain decreased by approx. 25% after the launch and subsequently decreased slowly with time. The gain was approx. 72% and approx. 56% of its pre-launch value about 5 weeks and 15 months after the launch, respectively. At total counting rates higher than 10,000 counts/sec there is a large decrease in the detector gain resulting in an apparent counting rate saturation. The overall instrument performance appears to be satisfactory and it is expected that the HEM observations will be useful in the study of many small and medium intensity solar X-ray bursts.