NUCLEAR FISSION 283 



pose that of four known types of nuclei three (Th^*^, Pa^^^ U^**) are 

 fissurable by fast neutrons only, one (U^^^) by slow neutrons and prob- 

 ably also by fast. The mass-numbers just given are those of the 

 nuclei awaiting the invading neutrons. If one prefers (as many do) 

 to think of the transient composite nuclei formed by the neutron- 

 invasions, one must write Th^^*, Pa^^^, U^^^ and U^^^. 



To speak of "fast" neutrons is vague, but not much vaguer than 

 the state of knowledge, which as yet is rudimentary. Fission has been 

 detected of thorium at neutron -energy of about 2 Mev, of protactinium 

 at about one Mev, of uranium at about 0.5 Mev. It thus appears 

 that the "threshold," or least contribution of energy demanded for 

 fission, declines as the end of the Periodic Table is approached; and 

 this seems natural. (Remember always that even with the fastest 

 neutrons ever used, the contribution is very small compared with the 

 energy released.) Values given in the literature for the "cross-section 

 for fission by fast neutrons" include: O.S-lO^^^cm^ and 0.1 -10"^* for 

 uranium and for thorium bombarded by 2.4-Mev neutrons (Princeton) 

 and 0.1 -10"^^ for uranium bombarded by the "RnBe" neutrons.^* 



If between the source of neutrons and a target of uranium a screen 

 of paraffin or water is inserted, the fissions become more abundant; 

 but if now between the paraffin or water and the uranium a shield of 

 cadmium is placed, the fissions become very rare. Now, paraffin and 

 water convert fast neutrons into slow or "thermal" ones,^^ and cad- 

 mium is a very efficient absorbent for slow neutrons. We recognize, 

 therefore, a specific effect of slow neutrons, peculiar to uranium. 

 "Slow" or "thermal" signify in this usage: having kinetic energies of 

 the very modest magnitudes possessed by molecules of air (or anything 

 else) at ordinary temperatures : fractions of one electron-volt, and rarely 

 more. Clearly then it is not the energy of motion of the neutron 

 which is the insignificant spark setting off the mighty explosion; it is 

 the mere presence of the neutron within the nuclear system. 



A beam of slow neutrons falling upon a thin uranium layer produces 

 many more fissions than does a fast-neutron beam of identical strength. 

 Twenty-to-one was the ratio of yields found by the Columbia school, 

 in the same experiment as gave them the value 0.1 • 10"^* for the cross- 

 section for fission by the fast "RnBe" neutrons. If, however, we put 

 2.10~24 {qi- i^hg cross-section appropriate to the thermal neutrons, we 



" Cross-section for fission, o-/, is so defined that if N neutrons strike a thin layer 

 comprising M nuclei per unit area, MNaj fissions occur. — The "RnBe" neutrons, 

 viz., those released when a-particles from radon and its descendants impinge on 

 beryllium, have a very broad energy-range extending at least from 14 Mev indefinitely 

 downward (cf. Dunning, Phys. Rev. 45, 586; 1934). 



1^ The neutrons lose their great kinetic energies in repeated elastic impacts with 

 hydrogen nuclei. 



