. (Physics & Chem.) Rays of relatively low penetrating power emitted by radium and other radioactive substances, and shown to consist of positively charged alpha particles (helium nuclei) having enormous velocities but small masses. They are slightly deflected by a strong magnetic or electric field. Compare beta ray and gamma ray. Because the rays penetrate only a small distance in dense matter such as living tissue, small pellets of radioactive substances emitting alpha rays have been used to cause local destruction of tissue, as in cancer therapy. [ Webster 1913 Suppl. +PJC ]