THE SMALLEST PARTICLES OF MATTER 23 



of light, and m the mass. This means that if one pound of matter 

 could be entirely converted into energy, it would yield 1 X (186,000 

 x5,280) 2 foot-pounds of energy, or nearly 11£ billion kilowatt 

 hours of energy, an amount nearly equal to the total monthly 

 output of the electric power industry in the United States as of 

 1939. In the fission of the U 235 nucleus it is estimated that only 

 one-tenth of one per cent of the total mass-energy is released, 

 i.e., 11-J million kilowatt hours per pound. 



Experimental Proof of the Equivalence of Mass and Energy 



Proof that Einstein's equation satisfactorily expresses the relation 

 between mass and energy is concisely stated by Professor H. D. Smyth 12 

 about as follows: Gradual improvement in high-voltage technique 

 made it possible to substitute artificially produced high-speed ions of 

 hydrogen or helium for natural alpha particles. J. D. Cockcroft and 

 E. T. S. Walton in Rutherford's laboratory were the first to succeed 

 in producing nuclear changes by such methods. In 1932 they bom- 

 barded a target of lithium with protons of 700 kilovolts energy and 

 found that alpha particles were ejected from the target as a result of 

 the bombardment. The nuclear reaction which occurred can be 

 written symbolically as 



3 Li 7 + 1 H 1 -^ 2 He < + 2 He* 

 where the subscript represents the positive charge on the nucleus 

 (atomic number, generally represented by Z, the number of nuclear 

 protons), and the superscript is the number of massive particles in the 

 nucleus (??iass number, which is the sum of the number of protons and 

 neutrons, but which omits the tiny masses of any electrons, positrons, 

 or other particles which may be present). 



The above equation, like most chemical equations, does not include 

 anything relative to mass or energy, but merely indicates a redistribu- 

 tion, in nuclei, of protons and neutrons. Since both mass and energy 

 are indestructible, their sums should be the same before and after the 

 reaction. The masses of Li 7 and H 1 , as determined from mass spectra, 

 total 8.0241, while the mass of 2 He 4 is 8.0056. Therefore 0.0185 unit 

 of mass "disappeared" in the reaction. 



The experimentally determined energies of the alpha particles ( 2 He) 

 was approximately 8.5 million electron-volts each — a figure so great 

 that the kinetic energy of the incident proton which caused the change 

 may be neglected. That is, 0.0185 unit of mass had disappeared and 

 17 mev of energy had appeared. To convert these figures into quanti- 

 ties we can use in the Einstein equation, we must remember that in 

 atomic and nuclear physics the unit of mass is one-sixteenth of the 

 mass of the predominant oxygen isotope, O 16 , which is equal to 



