Background spectral properties

As can be seen in Fig. 4.3, the in-orbit background spectrum presents characteristics very similar to that measured during on-ground calibrations, i.e. a continuum with superimposed the gain calibrator line. Actually this kind of calibrator, however, is rather sensitive to temperature variations, and can therefore be used only for comparing situations with similar experiment temperatures.
In addition, the prominent 88 keV line is clearly visible in the LS2 with an energy resolution of about 36% FWHM.

The spectral properties of the background are normally stable during an orbit. Significative variations occur when exiting the SAGA anomaly, during the pre-SAGA and in presence of spikes.
As discussed above describing the ratemeters background, CsI activation effects during the SAGA passage, the pre-SAGA crossing and spikes contribute all to soften the measured spectrum. In these cases, a very careful background subtraction has to be done in order to extract weak/short transient source spectra (see next section) without being affected by these effects.

  

Figure 4.3: In-flight background energy spectra of the four detectors of the GRBM experiment. The 88 keV line due to the HPGSPC calibration radioactive source is clearly visible in the spectrum of shield LS2.