THE SMALLEST PARTICLES OF MATTER 



19 



Professor William D. Harkins and his collaborators, who were 

 for fifteen years (1913-1928) the only scientists in America en- 

 gaged in work on the structure of the atomic nucleus, advanced 

 the idea that an intermediate compound nucleus of short life was 

 formed by the impact of the alpha particle. 8 He came to this 

 conclusion soon after Rutherford's 1919 experiment, and repre- 

 sents the transmutation of N 14 into O 17 by the following diagram: 



The almost incredible density of atomic nuclei appears from the 

 following quotation from Harkins' paper: "The electron, or elec- 



projectile; 



PRODUCTS 



Figure 1. Formation of oxygen 17 and hydrogen 1 by the disintegration of 

 fluorine 18, the intermediate nucleus formed by the impact of helium 4 with nitro- 

 gen 14. Open circles = neutrons. Circles with plus signs=protons. At higher 

 energies the excited fluorine nucleus disintegrates in two ways: (1) that given in 

 the diagram, and (2) to give a neutron and positive electron in place of a proton 

 (=hydrogen 1). [Courtesy Science, Vol. 103, No. 2671 (1946)] 



trons, may be considered to move around the nucleus, where they form 

 a cloud which is exceedingly tenuous, when considered in comparison 

 with the high density of the nucleus. The mass of the proton is 

 1.672 x 10" 24 gram. If this is contained in the 1.4 x 10~ 38 cc. consid- 

 ered as its volume, the density is 1.2 X 10 14 grams, or 130 million tons 

 per cc. Thus, the whole earth would have a diameter of only 460 

 meters, or less than a third of a mile (0.286 mile) if its whole mass 

 were present as protons and neutrons packed as they are in the nucleus 

 of an atom. The presence of electrons, however, reduces the density 

 from 1.2 x 10 14 g. per cc. to that of the earth (5.522 g. per cc.) which 

 is 20 million million times smaller." 



Neutrons and Positrons 



Though both Rutherford and Harkins independently assumed 

 as early as 1920 that the neutron exists, it was not discovered until 



