THE STRUCTURE OF MATTER 73 



The Atomic Nucleus 



Since World War II much research has centered on the forces which hold 

 the nucleus together. The nucleus carries all the positive charge and most 

 of the mass of the atom. As a result of bombardment experiments (Fig- 

 ure 4-2), especially on light nuclei, by 1930 it was known to be composed of 

 two main particles, protons, p, (H + ) or bare hydrogen nuclei, and neutrons, n, 

 particles of the same weight as protons, but with no charge. Moseley 

 showed in the year 1914 the correlation between atomic number and positive 

 charge on the nucleus; and isolation and identification of isotopes (same 

 atomic number, different atomic weight — i.e., more or fewer neutrons) fol- 

 lowed at a fast pace, until today more than 600 isotopes of the 108 elements 

 are known. Some nuclei are stable, but some are unstable, and fly apart 

 spontaneously into fragments. These are the radioactive isotopes. Some un- 

 stable isotopes do not exist in nature, but can be produced artificially by 

 nuclear bombardment (by n, p, etc) techniques. They are called artificially- 

 radioactive isotopes. 



Experimental bombardment of the nucleus and examination of the prod- 

 ucts by cloud chamber, ionization chamber, energy-balance studies, photo- 

 graphic, and other techniques has disclosed about 20 new particles. First 

 came the neutrino and the positive electron, or positron, then a number of new 

 particles, at first all called mesons. Named after the great theoreticians, Bose 

 and Fermi, these are now classified into: 



Bosons (spin = 1) 



(a) pions, or light mesons (t° : 264.2; tt ± : 273.2) 



(b) A;aons, or heavy mesons (k°: 965; k ± : 966.5) 

 Fermions (spin = 1/2) 



(a) leptons, or light particles {n*: 206.77; e*: 1; neutrino) 



(b) barions, or hyperons and nucleons (Xi*: 2585; 2*: 2330; 



A°:2182; p*: 1836; n°: 1837) 



The mass (in multiples of the electron mass) and charge (°, + , or " super- 

 scripts) of these particles (ir, k, Xi, p, etc.) are given in parentheses. The 

 bosons exist in the nucleus and contribute to its phenomenal binding energy. 

 Isolated, all but the electron, proton, and neutrino are unstable. However, 

 the neutron persists for about 20 minutes on the average. The others last 

 only 10- 6 tol0- 10 sec. 



Of some particular interest may be the muon (p*), well established as a 

 cosmic-ray product in the atmosphere in which we live. It is ultimately pro- 

 duced by the impact of a cosmic ray proton and an atomic nucleus in the 

 upper atmosphere. A 7r-meson is first produced, which in turn decays 



