Energetics
The sterilization of the surface is from UV flux on the surface for geological periods of time, creating a chemically inhospitable environment. UV is easy to shield, i.e. sunblock or clothing.
Neutrons or hard photons (Gammas or X-rays) are not so easily stopped.
There are several other more important issues to deal with regarding the probable sterility of the Martian surface than GCR, particle showers and surface activation. For a human walking on the surface in a suit however, it's something to consider.....