<P> Alpha particles are helium - 4 nuclei (two protons and two neutrons). They interact with matter strongly due to their charges and combined mass, and at their usual velocities only penetrate a few centimeters of air, or a few millimeters of low density material (such as the thin mica material which is specially placed in some Geiger counter tubes to allow alpha particles in). This means that alpha particles from ordinary alpha decay do not penetrate the outer layers of dead skin cells and cause no damage to the live tissues below . Some very high energy alpha particles compose about 10% of cosmic rays, and these are capable of penetrating the body and even thin metal plates . However, they are of danger only to astronauts, since they are deflected by the Earth's magnetic field and then stopped by its atmosphere . </P> <P> Alpha radiation is dangerous when alpha - emitting radioisotopes are ingested or inhaled (breathed or swallowed). This brings the radioisotope close enough to sensitive live tissue for the alpha radiation to damage cells . Per unit of energy, alpha particles are at least 20 times more effective at cell - damage as gamma rays and X-rays . See relative biological effectiveness for a discussion of this . Examples of highly poisonous alpha - emitters are all isotopes of radium, radon, and polonium, due to the amount of decay that occur in these short half - life materials . </P> <P> Beta - minus (β) radiation consists of an energetic electron . It is more penetrating than alpha radiation, but less than gamma . Beta radiation from radioactive decay can be stopped with a few centimeters of plastic or a few millimeters of metal . It occurs when a neutron decays into a proton in a nucleus, releasing the beta particle and an antineutrino . Beta radiation from linac accelerators is far more energetic and penetrating than natural beta radiation . It is sometimes used therapeutically in radiotherapy to treat superficial tumors . </P> <P> Beta - plus (β) radiation is the emission of positrons, which are the antimatter form of electrons . When a positron slows to speeds similar to those of electrons in the material, the positron will annihilate an electron, releasing two gamma photons of 511 keV in the process . Those two gamma photons will be traveling in (approximately) opposite direction . The gamma radiation from positron annihilation consists of high energy photons, and is also ionizing . </P>

Which one of the following types of radiation is considered short wave radiation