Background

On-ground calibrations background is mostly originated by gamma-rays emitted by means of natural radioactivity by the materials composing the room walls and floor. Occasionally, some specific element lines may be observed. Also, calibration sources of other satellite instruments may generate background lines. The dependency of the background in the GRBM band on $\phi$ at $\theta$=0$\rm ^{\circ}$ is shown for the four LS in Fig. 2.26. As can be seen, there was a variation of the background level, with the maximum reached when the detectors were facing the same wall of the calibration room. This wall was made of different and probably more radioactive material with respect to the others. It can also be seen that the LS4 background level is higher; this is due to contamination from the 692 keV secondary line emitted by the ⁵⁷Co MCS (section 1.2.2), whose parking position is above this shield and whose shielding is not very efficient for these high energy photons.

The spectra of the background in the four detectors are shown in Fig. 2.27. These data have been collected during an entire satellite rotation. In all detectors we observe a continuum which is due to the environmental background photon spectrum. Superimposed to this component, we observe other important features which are due to PDS experiment materials and are thus also observed in in-flight spectra (see section 4.1). At energies of $\sim$ 350 keV there is the photo-peak of the gain calibrator line. We see that the line is not exactly at the same energy for all the detectors, depending mainly on different PMT gains. In the LS2 spectrum the most prominent feature is the photo-peak due to the 88 keV photons emitted by the ¹⁰⁹Cd radioactive sources that are part of the HPGSPC which stands in front of this shield. The 692 keV low energy part of the photo-peak component due to the MCS emitted line is well visible in the LS4 spectrum and marginally in LS2 and LS4.

  

Figure 2.26: ESTEC calibrations background variation with $\phi$

  

Figure 2.27: ESTEC calibrations background spectra