An alpha ray is a stream of alpha particles. An alpha particle consists of two protons and two neutrons bound together into a particle identical to a helium nucleus; it is produced in the radioactive process called alpha decay. Alpha particles, like helium nuclei, have a net spin of zero. The energy of alpha particles varies, depending upon the specific decay reaction, with higher-energy alpha particles being emitted from larger nuclei, but most alpha particles have energies of between 3 and 7 MeV, corresponding to extremely long to extremely short half-lives of alpha-emitting nuclides. They are a highly ionizing form of particle radiation that when resulting from radioactive alpha decay have low penetration depth. Helium nuclei, which form 10–12 % of cosmic rays, are usually of much higher energy than those produced by radioactive decay.