Chap. 12] MISCELLANEOUS GEOPHYSICAL METHODS 871 



tions and combinations the nuclear properties, such as mass and charge, 

 remain unchanged, the radioactive decomposition of the nucleus alters its 

 mass and charge and, therefore, alters the number of electrons in the outer 

 orbit and usually its chemical properties.^ 



Nuclear transformations in radioactive substances are accompanied by a 

 release of energy in the form of corpuscular emission, heat, and other elec- 

 tromagnetic radiation. The corpuscular emission may be positive elec- 

 trically, analogous to "canal" rays. It is then referred to as alpha radia- 

 tion. If it is negative (analogous to "cathode" rays), it is called beta 

 radiation. The electromagnetic radiation may be at the low-frequency 

 end of the spectrum. Then it is noted as heat and will not be further 

 discussed here. It may also be at the very high-frequency end of the 

 spectrum. If so, it is more penetrating than X rays and is spoken of as 

 gamma radiation. As the decomposition of radioactive products proceeds 

 and new elements are formed, the character of these radiations changes. 

 Some of them emit only alpha, others only beta and gamma, and still 

 others all three radiations. As a rule, their velocities (and for the gamma 

 rays, their absorption) are characteristic for the element present. In other 

 words, it is possible to identify radioactive elements by their radiation. 

 Moreover, the radiation intensity is found to be proportional to the quan- 

 tity |of radioactive matter present. As a matter of fact, radioactivity 

 measurements are virtually the only means of quantitative study of radio- 

 active elements, since approach by chemical analysis is not only difficult 

 but frequently impossible. 



Very schematically, the origin and relation of the various radiations is as 

 follows: Within the nucleus there is probably a central nucleus (Ruther- 

 ford) which carries virtually all the mass of the atom. Its number of 

 positive charges is equal to the atomic order number (92 for U, 88 for Ra, 

 82 for lead, and so on). This central nucleus may be assumed to be sur- 

 rounded by neutral helium satellites (possibly on intemuclear quantum 

 orbits) consisting of alpha particles with two electrons. When these elec- 

 trons are lost, a doubly positive alpha particle is expelled with tremen- 

 dous energy by repulsion from the central nucleus. The alpha particles 

 have the greatest individual energy of any particle known to science. 

 They are identical in mass for all types of radioactive elements emitting 

 them and have velocities approaching 10,000 miles per second. Each 

 radioactive substance produces alpha particles of characteristic speed. 

 The shorter the period of transformation of a radioactive element, the 

 greater is the velocity of the alpha particle. One gram of radium emits 



2 This chemical change is most striking in the case of radium, which is a solid 

 resembling barium, then changes into a chemically inert gas (radon) which in turn 

 changes to a solid (Ra A). 



