Gamma Rays: A spontaneously released photon from a radioactive material additionally: a photon with greater energy than an X-ray.
Gamma rays have such a strong penetrating force that stopping them may need several inches of a thick substance, like lead, or perhaps a few feet of concrete. The human body can be totally penetrated by gamma rays, which can then generate ionizations that harm DNA and tissue.
Gamma rays are electromagnetic radiation with a very tiny wavelength, hence their penetrating strength is at its highest in this scenario.