INTRODUCTION 



103 



Note: Charge is the number of units of 4.8 x 10~ electrostatic units (esu) of charge. 



Velocity is ;• ; and velocity of light, c, is 3 x 10 cm/sec. (The ratio v/c for protons in cosmic rays and 

 in the Van Allen radiation belt above the earth's surface approaches 0.8 (or larger than that produced arti- 

 ficially). 



The Nucleus 



As has already been seen (in Chapter 4), the size of the nucleus has been 

 measured by means of scattering experiments and found to be 10" 12 cm, or 

 about 10" 4 A. The nucleus carries all the positive charge and most of the 

 weight of the atom. It is thus very dense.* The positive charge carried by 

 such a dense particle is almost unimaginably high — for radium it is 88 times 

 that of a hydrogen ion! — and it is therefore not surprising that the binding 

 forces, whatever they may be, must be orders of magnitude stronger than 

 those of the electron cloud of the atom; and even a minor reorganization or 

 splitting must involve a mass-energy change. It is instructive for one to com- 

 pare again (Table 4-1) the energy of visible light, ~ 1 electron volt/photon, 

 with that of gamma rays, 1,000,000 electron volts/photon, which arise from 

 nuclear rearrangements. 



♦This can be illustrated by a calculation of the weight of a 1-cm cube of nuclei of nickel 

 (Ni) atoms, for instance, it being presumed that the nuclei are close-packed, side by side. Sim e 

 the diameter of each is ~10~ 12 cm, 10 12 nuclei side by side would be 1 cm long; and the 

 cube would contain 10 36 nuclei. Each weighs 65 times as much as hydrogen, or 65 x : ! x 

 10~ 24 g. The weight of the 1-cc cube, then, is about 10 14 g. or approximately 100,000,000 tons! 



