In-orbit background

The in-flight background in hard X-rays experiments in space is the sum of different contributions, the relative importance of which depends on the experiment energy band, the experiment configuration and the orbit height and inclination. The different contributions can be classified in:

• internal background, due to photons and particles generated inside the detector itself following e.g. the interaction of high Z particles that excite meta-stable atomic states of the crystal (see next section) and the spallation of photons producing short lifetime isotopes;
• local background, due to photons generated by the interaction of particle trapped in the Geomagnetic field or primary cosmic rays with the satellite structures ;
• diffuse cosmic X-ray background, whose spectrum can be described by a thermal bremsstrahlung with kT=$\sim$41 keV at energies between 3 and 60 keV and by a double power-law with photon indexes of 2 and 0.7 at energies from 60 keV to 6 MeV (e.g. [Comastri et al. 1995] and references therein)
• Earth gamma-ray albedo: the spectrum of the gamma-ray radiation radiated as extreme upwards tail of air showers in the atmosphere is approximated by a power-law with photon index 1.7 and a cut-off at energies below 27 keV; it contributes most of the background above $\sim$100 keV;
• charged particles background, constituted by primary cosmic rays, trapped protons, splash albedo electrons. These are detected as high height pulses and in general are not so important in GRBM background due to the SAX 'good' orbit.