182 ANNUAL REPORT SMITHSONIAN INSTITUTION, 194 7 



Bethe of Cornell suggests that the source of this energy is atomic and 

 gives us the following "carbon cycle" of nuclear reactions. 





eC^^+ie" 



yN^^+ie" 

 6C^=^+2He^ 



In this cycle enter one isotope of oxygen, three of nitrogen, and two 

 of carbon. The net result of the cycle is to leave the carbon unchanged, 

 emit three y-rays and two positrons, ie° (the counterpart of the electron 

 with a unit positive charge) , and convert four atoms of hydrogen into 

 one atom of helium. Now the mass of four atoms of hydrogen is 4.032 

 units while that of one atom of helium is 4.004. The difference of 

 0.028 units, which amounts to about ^q of 1 percent of the original, has 

 been transformed into energy. The late Sir James Jeans made this 

 statement, "A sun in. which only y^ of 1 percent, was hydrogen could 

 provide the present sun's radiation for 2,000 million years, and it is 

 fairly certain that the sun contains more hydrogen than this." ^ 



ATOMIC ENERGY RELEASED BY MAN— FISSION 



For the splitting of an atom man has at his disposal some very 

 high-power projectiles in the form of neutrons, deuterons, protons, 

 a-particles, y-rays, and rarely heavy particles. It is difficult to hit 

 the nucleus of an atom with a charged particle, since either the shelter- 

 ing cloud of electrons or the repulsive force of the nucleus will turn the 

 missile aside. But neutrons, having no charge, can be deflected only 

 by a direct collision and so are most effective as atom smashers. 

 Using neutrons, men began bombarding all the elements, and it was 

 found that in general the nucleus absorbed the neutron, achieved 

 stability by emitting an electron, and formed a nucleus with atomic 

 number and mass number each one higher than the original. It was 

 natural to investigate what would happen when uranium, the heaviest 

 element, was bombarded with neutrons. Late in 1938 O. Hahn of 

 Germany (who in 1945 was given the Nobel Prize), showed that the 

 heavy uranium nucleus was broken and that one of the fission products 



* For details I commend to you the book "Atomic Artillery and the Atomic Bomb," written 

 by my old friend and classmate. Prof. J. K. Robertson of Queen's University. I am 

 indebted to him for some of these ideas and even some of the phrasing. 



