FISSION OF BISMUTH AND LEAD BY HIGH-ENERGY PARTICLES*

The authors irradiated the substances being studied for fission with a beam of deuterons of energy 190 MeV in the 184-inch cyclotron. The irradiated samples were subjected to chemical treatment in order to isolate the fission products. To describe the experimental results a graphical method was used. In Fig. 1 the yield curve of fission fragments of Bi is presented as a function of atomic number. For comparison, an analogous curve for the fission of uranium by slow neutrons is given, which, by its asymmetric form, differs strongly from the symmetric curve for the fission of Bi. For fission fragments of uranium by slow neutrons an excess of neutrons is characteristic, and therefore they are \(\beta^{-}\)-active. The fission fragments of Bi corresponding to points on the distribution curve to the left of the maximum are \(\beta^{-}\)-active, while those to the right are \(\beta^{+}\)-active or undergo \(K\)-capture. To explain these differences and the fact that the maximum of the curve lies in the region \(A \sim 106\) (\(A\) is the mass number), the authors assume that initially capture of the deuteron occurs and 10–12 neutrons are emitted according to the reaction

\[ \mathrm{Bi}^{209} + d \to \mathrm{Po}^{199} + 12\,n. \]

Fig. 1.

The excited nucleus \(\mathrm{Po}^{199}\) undergoes fission, in such a way that for the fragments formed the ratio \(n/p\) (the number of neutrons \(n\) to the number of protons \(p\)) remains the same as in \(\mathrm{Po}^{199}\). The fission in this case proceeds predominantly symmetrically. Accepting these assumptions, the authors (taking \(A = n + p\)) predict the most probable fission product and the type of activity (\(\beta^{-}\) or \(\beta^{+}\)). The fragments predicted in this way correspond to points lying on the smooth experimental curve. Points falling outside the smooth experimental curve refer to cases in which the fission fragments do not satisfy conservation of the ratio \(n/p\) as in \(\mathrm{Po}^{199}\). If

* R. H. Goeckermann and I. Perlman, Phys. Rev. 76, 629 (1949).

take into account the simplifications adopted in this fission model, as well as the difficulties of measuring fragment activities, good agreement is obtained between experiment and the proposed fission mechanism.

To confirm that neutrons are indeed emitted before fission occurs, the authors irradiated lead of various isotopic composition. On the assumption that, for the different Pb isotopes, essentially one and the same nucleus is responsible for fission, and that the cross section increases with increasing excitation energy, a greater fission yield should have been shown by Pb with a smaller average $A$. This is found to be in agreement with the measurements carried out (Fig. 2).

Fig. 2

The authors believe that the mechanism they have considered is probably operative in all cases of fission of heavy nuclei by particles of high energy (direct fission under the action of $\gamma$-rays is excluded from consideration). In this case fission proceeds through the formation of a compound nucleus, i.e., the nucleus captures the bombarding particle, whose energy is distributed among all particles of the nucleus. The result is a calculation from which it follows that in 75% of collisions of a deuteron of energy 190 MeV with a heavy nucleus, the latter receives an energy of $\sim 150$ MeV. These cases lead to fission of the nuclei and, under the assumption of preliminary emission of 10–12 neutrons, agreement is obtained with the measured fission cross section of Bi by deuterons. From their observations the authors conclude that if the fission threshold is of the order of 12 MeV, then, at excitation energies near or considerably above the fission threshold, neutron emission mainly occurs. If, as a consequence of this, the fission threshold is lowered, then the latter competes with neutron emission. The fission cross section then grows with increasing excitation energy, which agrees with the experimental data they present.

The charge distribution occurs in such a way that the $n/p$ ratio of the fragments remains the same as that of the fissioning nucleus, which does not occur in the fission of uranium by slow neutrons.

A characteristic property of fission of nuclei by high-energy particles is the predominance of symmetric fissions, which is found

in sharp contrast to the asymmetric fission of uranium by slow neutrons.

The paper thus attempts to combine all fission reactions of heavy elements by high-energy particles and gives the characteristic features of these fissions.

A. B.