HEALTH PHYSICS AND RADIATION PROTECTION 85 



enormously higher — of the order of milHons of electron volts. The higher 

 the energy of a given type of radiation, the greater is its penetrating 

 power. This property is of importance in shielding considerations and in 

 the use of detection and measurement instruments. 



Alpha Rays. Radioisotopes emitting alpha rays will not normally be 

 used in biological research. However, because of the health hazards 

 from internally deposited alpha emitters, brief mention of them is made 

 here. Alpha rays are swiftly moving stripped atoms of helium and are 

 emitted chiefly by the heavier elements with initial energies of 2 to 8 Mev. 

 Their outstanding characteristic is the ability to ionize intensely and to 

 penetrate only short distances. Thus sources outside the body constitute 

 no hazard, because the rays are stopped by a few centimeters of air or by 

 the dead layer of skin. Once inside the body, however, alpha emitters 

 constitute a considerable hazard, since the energy of the radiation is dis- 

 sipated in such a small volume of tissue, thus producing an intense local- 

 ized effect. 



Beta Rays. Beta rays are most generally the ones measured in radio- 

 isotope work and are also important as a radiation hazard. They are 

 high-speed electrons emitted from the nucleus of an unstable atom at the 

 instant of disintegration. Unlike alpha rays, beta rays emitted from a 

 given substance have energies ranging from zero up to some maximum, 

 characteristic of the particular radioisotope. The value listed usually 

 refers to the maximum energy. Thus, for example, P'^ is listed as a 

 1.7-Mev beta emitter, which means that this isotope yields beta rays of 

 all energies up to 1.7 Mev, the average being about one-third of the 

 maximum, or 0.6 Mev. 



Beta rays have a lower specific ionization and greater penetrating power 

 than do alpha rays. When beta rays pass through matter, the rapid 

 deceleration causes the production of gamma rays, called bremsstrahlung, 

 which, however, is small in amount compared with the original beta radia- 

 tion. The higher the atomic number of the absorber, the more brems- 

 strahlung is produced. 



Gamma Rays. Unlike alpha and beta rays, which are particulate in 

 nature, gamma rays are electromagnetic radiations (photons). They 

 arise from the atomic nucleus and are similar to light except that they are 

 of much shorter wavelength. Gamma rays are characterized by high 

 penetrating power. X and gamma rays have identical properties and 

 differ only in their mode of origin. 



Absorption of Radiation. Knowledge of the absorption properties of 

 the various ionizing radiations is of importance from the point of view of 

 shielding against external radiation, penetration into tissues, and the 

 characteristics of detection instruments. It is often convenient to express 

 the thickness of absorbing material in terms of mass per unit area instead 



