The radiation background is variable around the XMM-Newton operational orbit, depending on the satellite's location with respect to Earth's magnetosphere. For useful XMM-Newton science observations to be conducted, the minimum satellite elevation is 46000 km. At this height, the EPIC, OM and RGS instruments can start to perform scientific observations, but only if the radiation background is low enough. Otherwise this radiation can damage the detectors. Experience after the first years of XMM-Newton operations shows that it is only from very few minutes to about 3 hours after the correct elevation is reached when it is safe for the X-ray instruments to start the observations. Similarly, X-ray instruments have to be closed from very few minutes to about 3 hours before the satellite elevation goes down to the nominal 46000 km at the end of each orbit. The spacecraft spends 143 ks of the 48-hour orbital period above 46000 km, and because of the radiation background, on average about 132 ks can be used to perform scientific observations with the EPIC and RGS instruments. More details about the expected duration of the science window per orbit are available from the Target Visibility Checker web page.
The particle background outside the belts is dominated by the solar particle emission. The incident flux is therefore coupled to the level of solar activity. During intense solar flares the XMM-Newton payload instruments are switched off.
European Space Agency - XMM-Newton Science Operations Centre