Highly ionizing alpha particles
The ionizing power of a charged particle is measured by its ‘linear transfer of energy’ : the energy it loses per unit length in the material it goes through. Alpha particles are particularly ionizing, as can be seen by the variation in energy transfer in water as a function of their energy. While the energy transferred by an alpha-induced radioactive decay never goes above 9 million electronvolts (9 MeV), their ionizing power equates to between 60 and 250 MeV per millimetre during the slow down. Beta particles are far less ionizing: the values had to be multiplied by 100 to appear on the same graph.