Since we were discussing the possibility of side-penetrators yesterday, I went back and looked at the solid model for the instrument to see how much shielding is on the sides of the detectors.  There are a couple images attached.  The outer wall of the telescope housing is 0.2 cm, and the added shielding was essentially a C-channel with 0.2cm walls that was bolted onto the outside of the telescope housing.  So for normally incident particles there is just the end-caps which are 0.076 cm (which gives us our usual ~10-12 MeV protons required to get into the instrument).  And for side-penetrators to get through a 0.4cm thick wall requires about 27.5 MeV.  That neglects the detector frame, which could add another >0.5 cm.  Of course any particular particle's path through all of this junk is fairly complex, but roughly speaking 30-50 MeV seems like the minimum energy a side-penetrating proton would need to reach the detector.