Alpha Rays

ALPHA RAYS (α-rays) are positively charged material particles ejected at high velocity from the atoms of radioactive elements—radium, thorium, actinium, uranium, etc. The velocity at which alpha particles leave a radioactive atom varies for different elements; its range is from 1.4×10⁹ cm/sec (for uranium I) to 2.1×10⁹ cm/sec (for thorium C¹). Their ability to propagate through air, depending on their velocity, ranges from 2.7 cm to 8.6 cm (at 15° and 760 mm pressure). A screen of aluminum a few hundredths of a millimeter thick is sufficient to stop them; therefore, when using radium preparations enclosed in tubes, the alpha rays do not act. Alpha rays strongly ionize air, act on photographic plates, and cause phosphorescence and fluorescence. The action of individual particles can therefore be studied by electroscope or photographic methods; they can also be observed directly by the eye (see Mesernitsky's alphascop, Crookes' spinthariscope). Possessing catalytic properties, alpha rays may play an important role in biology and medicine. For them to act on the organism, it is necessary to use radioactive emanations or solutions of radioactive preparations. Having lost their velocity and charge when penetrating through various substances, an alpha particle becomes an atom of helium with a specific weight of 4.