CHAPTER 6 



FISSION 



6.1. Mechanism of Fission. The discovery by Hahn and Strassmann 

 [1] that elements of medium atomic weight were formed from uranium bom- 

 barded with neutrons came as the culmination of an unexplained phenomenon 

 encountered in the investigations of Fermi, Meitner, Hahn, Strassmann, 

 Curie, and Savitch. Meitner and Frisch [2] named the process "fission" 

 from its similarity to the rupture of a sphere of charged incompressible fluid 

 as a result of deformations induced by some external influence. The analogy 

 of the "liquid drop " had already been applied in principle to heavy nuclei by 

 Bohr and Kalckar [3], and the general theory of liquid-sphere dynamics had 

 been developed in detail for its application to stellar dynamics. Following 

 the discovery of fission, Bohr and Wheeler [4] extended the liquid-drop 

 analogy into a theory of fission with marked success in its detailed agreement 

 with observations of fission processes. 



The aggregate of elementary particles in heavy nuclei, according to this 

 model, forms the equivalent of a spherical drop of incompressible liquid with 

 a volume proportional to the number of particles or atomic weight and an 

 effective surface tension arising from the short-range attractive fields between 

 the particles. The surface tension is compensated for in part by the repulsive 

 electrostatic field of the uniform distribution of proton charge eZ throughout 

 the nucleus. Under the influence of an external force, the nucleus may be 

 excited into one of many possible modes of vibration, producing deformations 

 in the spherical form similar to the corresponding vibrations of a liquid 

 sphere. The stability of the drop to these small deformations depends on the 

 relative strengths of the surface tension and the counteracting electrostatic 

 field. As the total number of particles in stable nuclei increases, a limiting 

 size is attained for which the compensation of the cohesive nuclear force by 

 the electrostatic field is complete and the drop is then unstable to spon- 

 taneous fission. Nuclei only slightly smaller than the limiting size, although 

 exceedingly stable against spontaneous fission, will divide when sufficiently 

 excited. A small deformation accompanying excitation allows a redistribu- 

 tion of charge to the surface regions of smaller curvature, thus giving the 

 repulsive field an advantage over the surface tension in regions of smaller 

 curvature. A sufficiently large deformation therefore leads to complete 

 division, and the separated fragments recoil, because of their similar charges, 



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