From Current Literature

FISSION OF BISMUTH, LEAD, THALLIUM, PLATINUM, AND TANTALUM BY HIGH-ENERGY PARTICLES

Until quite recently, fission was known only for the last members of the periodic system (uranium, protactinium, thorium). The commissioning of the new giant 184-inch cyclotron with frequency modulation1 made it possible to obtain neutrons, deuterons, and helium ions with energies of 100, 200, and 400 MeV, respectively, and with these particles it proved possible to observe fission of nuclei in the range of atomic numbers from bismuth \((Z = 83)\) to tantalum \((Z = 73)\)2. In particular, the elements listed in the title of this review have the numbers: 83 (Bi), 82 (Pb), 78 (Pt), 73 (Ta). Fission was established by chemical identification of the radioactive products.

The fission of these lighter nuclei by particles of superhigh energies has certain features that distinguish it from the fission of uranium by slow neutrons. Thus, for example, there is no asymmetric fission with a sharp minimum of the yield curve in the middle. Further, in contrast to uranium fission, a good yield of light isotopes of the given element and the formation of a stable isotope as the primary fission product are observed. Thus, for example, in the fission of bismuth by helium ions at 400 MeV and by deuterons at 200 MeV, the formation of Br⁸² is observed in an amount comparable with Br⁸³, whereas in the case of uranium fission by slow neutrons the yields of these isotopes are in the ratio \(1:10^4\). Similarly, in the fission of bismuth and lead by helium ions, Ba¹⁴⁰ (observed with a considerable yield in uranium fission by neutrons) was not detected, but activity was observed which probably belongs to Ba¹³³.

It has not yet been possible to obtain quantitative data on the yield, but it was possible to observe certain qualitative regularities: for a given type of bombarding particle the probability of fission decreases with decreasing atomic number of the target; for a given target the yield decreases with decreasing energy of the incident particles. It is interesting to note that, whereas under bombardment by α-particles fission was observed only at the highest energy (400 MeV), the fission of bismuth by deuterons was observed at energies of 200, 150, 90, 70, and 50 MeV. However, fission of Pb, Tl, Pt, and Ta was observed only at the higher energies of deuterons and neutrons, i.e., 200 and 100 MeV, respectively.

From the standpoint of the conditions for fission, it is significant that in the case of the elements indicated, fission occurs at high excitation energies of the nucleus. The fact that light isotopes arise as products with great probability, in the authors’ opinion, indicates that fission is preceded by the “evaporation” of a large number of neutrons by the excited nuclei. This is also evidenced by the fact that the fission products, as revealed by observation, are considerably poorer in neutrons, appearing to be nuclei whose mass is less than the mass number of the target nucleus. However, the same result could also be obtained if, as a result of fission,

highly excited nuclei arose. Regarding the possibility of the simultaneous evaporation of a large number of neutrons, the authors refer to a report by Hopkins, Perlman, Seaborg, and others, delivered on July 11–12, 1947, but apparently not yet published.

E. Shpolsky

Cited Literature

1. See the abstract by M. Rabinovich, UFN, 32 (issue 3), 396, 1947.
2. I. Perlman, R. H. Hoeckermann, D. H. Templeton, and I. I. Howland, Phys. Rev., 72, 352 (1947).