@Delta Force: Well, it's possible, but difficult, only if you can change the warhead, because you need a sensor for intercept. In 1984 the Homing Overlay Experiment destroyed an incoming warhead using a kill vehicle mounted on the tip of a Minuteman I. In the 90s a "Minuteman Option" was studied: using Minuteman for mid-course defense, provided that you change the nuclear warheads with kill vehicles. That option was trashed because of strategic and political problems: if you launch ICBM, neutral powers could not know if you are launching a massive nuclear attack or ABMs. But I think it's possible if you have enough time to change the warheads.
If you know different programs, using Minutemen without any warhead modifications, please tell me. Because it could be a very rare and precious info.
About the natural radiation boost, maybe you're referring to 1958 Operation Argus:http://en.wikipedia.org/wiki/Operation_Argus
Phisically it works, but it could be ineffective for intercept: a shielded ICBM could bypass the radiation belt without problems.
The concept was part of the US ABM system.