The Collaborative International Dictionary of English v.0.48:

Alpha rays \Alpha rays\ (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]