On-board Calibration Unit

IBIS contains an on-board collimated radioactive Na source. This allows regular calibration of PICsIT at both the 511keV line (calibration to better than 1% in 4 hours) and 1275 keV (1% in 8 hours). ISGRI can also use the 511keV line, albeit at lower efficiency. Any energy deposits from untagged photons will have an impact of % on the overall continuum sensitivity between 100 keV and 2 MeV.