The idea that low levels of radiation can be beneficial is not such a strange idea. It is well documented that radiation at 270nm to 290nm is very beneficial at low doses though high doses gives you cancer(*). I could believe that the same thing could apply to other wavelengths including gamma radiation. There are some plausible arguments as to why it would be good for you. I am not sure I am convinced by these arguments but I would not rule them out. Alpha radiation (produced by Radon) on the other hand does a lot more damage so I would find it harder to believe that it was possible for it to be beneficial.
(*) I am of course talking about UV light and the generation of vitamin D in the skin.
(*) I am of course talking about UV light and the generation of vitamin D in the skin.