56 APPLIED RADIOACTIVITY 



Subsequently Villard (1900) discovered that a third and very pene- 

 trating type of radiation was also associated with beta-ray emissions 

 from uranium minerals, and these were designated by the third letter in 

 the alphabet, namely, " gamma rays." 



By 1913 Rutherford and Soddy had coordinated the various radio- 

 active processes and proposed an acceptable theory of spontaneous 

 disintegration of their " nuclear atom model " to account for the radio- 

 activity of the atom. They suggested that the nuclear disintegration 

 was explosive and was accompanied by the ejection of an alpha particle 

 or if the explosion resulted in the ejection of a beta particle then the 

 disintegration was accompanied by the emission of a gamma ray. The 

 ejected alpha particle was shown to be a particle of matter comparable 

 to a helium atom which had lost its two planetary electrons, i.e., a 

 helium nucleus. The beta particles were found to be high-speed nega- 

 tive electrons, and the accompanying gamma-ray emission was dis- 

 covered to have the properties of x-radiation of exceptionally short 

 wavelength. 



As the result of the explosive emission of an alpha particle, any atom 

 drops in the scale of atomic numbers by two units and there is a loss 

 in atomic weight equal to the atomic weight of a helium nucleus. Since 

 the helium nucleus is equal to four mass units, its loss will reduce the 

 mass of the parent atom by four units. This residue is a new, but less 

 massive, atom having physical and chemical properties different from 

 its parent atom. 



Disintegration of Radium 



Radioactive changes are accompanied by the emission of alpha or 

 beta particles. They are never emitted simultaneously. All radium salts 

 actively emit alpha and beta particles with the accompanying gamma 

 radiations. Usually gamma radiations accompany beta-particle ejec- 

 tions. Sometimes weak gamma radiations also accompany alpha- 

 particle emissions. 



The radium atom is an unstable complex structure. Its atomic 

 weight is 226, and its atomic number is 88. Its compact unstable mas- 

 sive nucleus can be represented pictorially as surrounded by 88 planetary 

 electrons. We can imagine this nucleus undergoing a sudden explosive 

 readjustment with the emission of an alpha particle of characteristic 

 speed. The residue is the radon atom of atomic weight 222. It is a 

 radioactive inert gas sometimes called radium emanation. 



This and the subsequent transformations are shown schematically in 

 Fig. II— 1; all but the last atoms contain radioactive nuclei. A radio- 



