Background

The local radiation environment is mainly produced by two components: the diffuse X-ray background (DXB) and cosmic rays (CR). Most of the latter are rejected on-board with a combination of pulse height, pulse shape, anti-coincidence and ``footprint'' evaluation techniques. These techniques allow a particle rejection efficiency of 99.9% with carefully tuned selection parameters. This high rate of background rejection has ensured that there has been no significant increase in background events in the telemetry despite the steady increase in the CR rate at Solar minimum.

Figure 7 shows an actual background spectrum which is composed of the diffuse X-ray background, instrumental background due to the interactions with cosmic rays and three strong instrumental lines due to the cooper and molybdenum in the collimator (8.04 keV and 17.4 keV) and Xe fluorescence from the detector gas at 29.6 keV.

Figure 7: Empty field background spectrum measured with the nominal detector gain of 1500 (left) compared to the background spectrum with the reduced gain of 500 (right). After these measurements the rejection criteria have been adjusted (2003-02-25), but no blank fields have been observed for a longer period since then. The background has increased with about 10-15% at higher energies and with 20-30% below 10 keV after the adjustment.