TABLES 713-730.— NUCLEAR PHYSICS 



651 



Nuclear physics may be divided into three fields : radioactivity, cosmic rays, 

 and artificial disintegration. The third division — artificial disintegration — is 

 today the most active single experimental (and theoretical) problem of the 

 physicist. This new branch of physics has introduced a number of terms, 

 some of which are defined in Table 716. There is hardly a major physical 

 laboratory that does not have at least one of the devices listed in Table 718 for 

 producing high-energy particles of one kind or another. 



The study of nuclear physics started more than 50 years ago with the dis- 

 covery of radioactivity. This was a study of natural disintegration up to about 

 1919 when Rutherford produced and studied artificial disintegration by bom- 

 barding nitrogen with swift a-particles from RaC. However, he had to depend 

 upon nature for the high-speed particles that he used. The value of the speed 

 and energy of the o-rays from natural radioactive materials (Table 732) 

 shows the nature of the particles then available. It was not until about 10 

 years later that a start was made on the development of the various devices 

 for producing the regulated high-speed and high-energy particles listed in 

 Table 718. 



By bombarding different materials with one of the high-speed particles 

 produced by various devices it has been found possible to produce one or more 

 radioactive isotopes of each of the 92 elements and, in addition, to produce 6 

 elements beyond uranium — each with a number of isotopes.* There are now 

 9 or 10 known fundamental particles (Table 720), 5 or 6 of which are used 

 in the bombardment of isotopes for the production of new reactions. Some 

 examples of reactions thus brought about by the use of different ones of these 

 high-speed particles together with the minimum energy of the particles neces- 

 sary to produce the reactions are given in Table 726. 



The relative masses of the isotopes vary from 1.0081374 for H 1 to about 

 242.14152 for Cm 242 . The actual mass in grams for H 1 is 1.67339 x 10' 24 

 grams, and thus the mass, in grams, of any atom may be determined from its 

 atomic weight. The mass of the neutron is 1.67473 X 10 -24 g. The radius of 

 a nucleus, r, is given approximately by 1.4 X 10~ 13 A 1/3 cm, A being the atomic 

 mass number. These values give for the density of the nucleus about 10 14 

 g/cm 3 (see Table 872). The atomic weight, the magnetic moment, and the 

 spin of a number of isotopes are given in Table 719. 



* For reference, see footnote 199, p. 618. 



TABLE 713.— MASS, ENERGY, AND VELOCITY RELATIONS FOR 

 THE ELECTRON 



SMITHSONIAN PHYSICAL TABLES 



