872 MISCELLANEOUS GEOPHYSICAL METHODS [Chap. 12 



alpha particles at the rate of 3.6 x 10^° per second. These particles are 

 assumed to consist of two neutrons and two protrons ( = H nucleus, mass = 

 1 unit,' charge 1 unit positive). Therefore, when an alpha particle is 

 released, the atom numijer decreases two units and the atomic weight four 

 units. 



Despite their great velocities, alpha particles are readily stopped, the 

 fastest (of thorium C, v = 2 x lO' cmsec."^) being absorbed by air of 

 8.62 cm thickness. An aluminum foil of 0.05 mm thickness is sufficient 

 to keep the apha radiation out of an ionization chamber. In different 

 elements the range of the apha particles is proportional to the square root 

 of their atomic weights. Nevertheless, it is probably the most important 

 radiation for the measurement of radioactivity, particularly in connection 

 with emanation measurements. In a chamber of 10 cm side length, the 

 ionizing effects of alpha, beta, and gamma radiations are as 10,000 : 100 : 1 . 



When an alpha particle is released, two electrons become available and 

 may be captured by the central nucleus or shot off as beta particles. Only 

 one of the radioactive elements (radium C) emits beta particles simulta- 

 neously with alpha particles. The others alternate, by themselves or in 

 groups, between emitting alpha and beta particles. The release of a beta 

 particle (charge — e) raises the atomic number by one unit, but it is not 

 believed to affect the atomic weight (since m = 1/2000 unit). The ve- 

 locity of beta rays may be as much as 99.8 per cent of light velocity. The 

 harder components are very penetrating. An aluminum sheet of 0.5 mm 

 thickness absorbs about one-half of the uranium beta rays. This radia- 

 tion possesses much less energy than does the alpha radiation. More than 

 half of the beta radiation incident on a metal plate is reflected and dis- 

 persed; when it passes through matter, X rays are generated. 



Similar in nature is the emission of gamma ("penetrating") rays from 

 the nucleus of the radioactive atom. Since gamma rays never occur by 

 themselves but always in conjunction with alpha or beta radiations, it is 

 probable that they are produced by intemuclear orbit rearrangements 

 following the emission of beta particles Some of the softer radiation 

 possibly originates outside the nucleus by changes in the inner electron 

 orbits. The wave length of gamma rays is of the order of 10~^ to 10~ 

 cm. Their penetrating power is so great that 55 mm of aluminum or 

 12 mm of lead is required to reduce the gamma radiation of radium by 

 one-half. Gamma rays produce a secondary beta radiation (whose pene- 

 trating power is almost as great as that of the primary radiation) not only 

 when they pass through other materials but also in their own atoms by 

 releasing electrons from the inner extranuclear orbits. The absorption of 



' The unit of mass is 1/10 of the weight of the oxygen atom or approximately 

 equal to the atomic weight of hydrogen. 



