The other electrons pass the inactive trap safely. By confining the electrons to a narrow buried channel during transfer, fewer traps are encountered and the charge-loss is further reduced. This precaution is not yet necessary, but will, when damage becomes severe, recover most of the consequent CTE loss as shown in Fig. Presently the CCDs are operated at 170 K, to minimise condensation build-up on the CCDs.
More energetic protons can penetrate the shielding, but their flux and cross section are much lower.
The recovery of radiation damage on CCD 22 with cooling. The damage was induced by 10 MeV protons at the Birmingham University Cyclotron. The large effective area of the XMM-Newton mirrors makes this focused soft proton flux very high, and the potential for damage very serious. Fortunately the closed position of the filter wheel places 1.05 mm of aluminium in the focused beam and completely stops the protons. For all perigee passages the filter wheel is rotated to the closed position before the spacecraft encounters the outer radiation belts.