Sec. 6.11] FISSION 161 



also observed in these experiments that the distribution of alpha-particle 

 tracks tended to bunch about a direction at right angles to that taken by the 

 fragments. The frequency of occurrence of alpha particles appears to be 

 about 1 in 250 fissions of U 235 and 1 in 500 fissions of Pu 239 [11]. 



6.10. Fission Induced by Other Radiations. Neutrons are the most effi- 

 cient means for inducing fission in heavy elements since they are unaffected 

 by the high electrostatic potential barrier, but charged particles and gamma 

 rays will also induce fission when given sufficient energy. Photofission has 

 been investigated with high-energy gamma rays, and the cross sections for 

 uranium and thorium are reported as ou = 0.0035 barn and o-Th = 0.0017 

 barn, respectively [30]. Alpha particles and deuterons are also known to 

 induce fission, but in the case of deuterons it is not yet certain if the entire 

 particle enters the nucleus or is first stripped of its proton so that only the 

 neutron is captured. 



6.11. Fission of Elements below Thorium. Despite the increasing height 

 of the fission barrier in elements with atomic numbers lower than thorium, 

 it was anticipated that fission could be induced in such elements with particles 

 of very high energy. This has been verified by Perlman et al. [31,32] for the 

 elements bismuth, lead, thallium, platinum, and tantalum by bombarding 

 targets of these elements with 400-mev alpha particles, 200-mev protons, and 

 100-mev neutrons produced by the 18-i-in. Berkeley cyclotron. Bismuth, 

 the most carefully investigated element, exhibits marked differences from 

 fission of uranium by low-energy neutrons. Unlike uranium fission the 

 masses of the bismuth fission products are distributed symmetrically in a 

 single peak about mass 98-99. Also a greater proportion of the products are 

 either stable or /3+ active. 



A qualitative explanation of bismuth fission is given [32] which suggests 

 that only after about 12 neutrons have boiled off the compound nucleus 

 (Bi + d) does fission become probable; the fissionability parameter Z 2 /A is 

 then about equal to that of U 236 . Further, if it is assumed that the fragments 

 retain the same neutron-to-proton ratio as the light bismuth nucleus 

 (n/p = 1.36) then, as has been observed, the fragments with masses less 

 than ~ 100 carry off an excess number of neutrons and are (jr emitters, those 

 of intermediate masses are stable, and the heavier fragments are neutron 

 deficient and therefore ^+ or K capture active. 



REFERENCES FOR CHAP. 6 



1. Hahn, O., F. Strassman: Naturwissenschaften, 27, 11 (1939). 



2. Meitner, L., O. R. Frisch: Nature, 143, 239 (1939). 



3. Bohr, N., and F. Kalckar: Kg!. Dansk Vid Selskah. Acad., 1939. 



4. Bohr, N., and J. A. Wheeler: Phys. Rev., 56, 426 (1939). 



5. Breit, G., and E. Wigner: Phys. Rev., 49, 519 (1936). 



6. Bethe, H.: Rev. Mod. Phys., 9, 69 (1937). 



