Physics

The difference is due to the different ways in which the two types of radiation interact with matter. Alpha particles are much more massive than gamma rays, and so they interact with matter more strongly, causing more damage in a given amount of absorbed energy. As a result, alpha particles are much more dangerous, and so it takes twenty times as much alpha radiation to be equivalent to one rad of gamma radiation in terms of its health effects. This is why alpha radiation is given a higher equivalent dose in Rad equivalent man (rem) units.