A magnetic field (Van Allen belt, or analog on other bodies) deflects lighter charged particles better than heavier ones (with a greater mass-to-charge ration).

A magnetic field (Van Allen belt, or analog on other bodies) deflects lighter charged particles better than heavier ones (with a greater mass-to-charge ration). E.g. - helium ions (alpha particles) vs iron nuclei as mentioned in the article. Otherwise, pure mass over your head is the best defense.