Which one of the following has the maximum ionization
potential?
When a radioactive atom releases an α or a β particle, the atom is transmuted into another element. α particles are helium nuclei  consisting of two protons and two neutrons. They result from the radioactive decay of many large atomic number elements. Because of their double positive charge and heavy mass, α particles densely ionize matter through which they pass. They quickly give up their energy and  penetrate only a few micrometers of body tissue. (An ordinary sheet of paper absorbs them.) After stopping, α  particles acquire two electrons and become neutral helium atoms.
Ref: Oral Radiology, Principles and Interpretation / Stuart C. White, Michael J. Pharoah - 7th ed - pg - 3
Electron
Proton
Helium ion
Gamma - Photon