I'd guess that it's a function of distance and extremely low-level radiation over a very long period of time, as opposed to the high-energy short-burst theories that have been bandied about over the years. If I recall correctly, the entire cloth has been affected to one degree or another, the image areas having a greater number of affected fibrils than the non-image areas.
It is irrelevant whether a radiation source is low-level or high-energy. The physics of it is that it radiates in all directions at a rate that averages the same regardless of the vector. Why, if radiation is the modality, does the radiation work at 180º to change the coating, but not at 175º, 170º, or 185º?
If I recall correctly, the entire cloth has been affected to one degree or another, the image areas having a greater number of affected fibrils than the non-image areas.
No, the non-image area fibrils' coatings have not been changed to caramel. That happens only in the image areas.