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FISSION OF ELEMENTS FROM Pt TO Bi INDUCED BY FAST NEUTRONS1
As is known, fission under the action of fast particles is energetically possible not only in uranium and thorium, but also in lighter nuclei. Some time ago it was discovered experimentally,2 but quantitative data on the yield were not obtained. In the work cited, experiments are described on the fission of bismuth, lead, thallium, mercury, gold, and platinum under the action of fast neutrons, which led to quite definite quantitative results.
The neutrons were obtained by bombarding a lead plate with deuterons accelerated in the 184-inch synchrocyclotron of the University of California. In this case the average neutron energy \(E_n\), according to Serber,3 is given by
\[ E_n = \frac{1}{2}(E_d - E_b - E_t), \]
where \(E_d\) is the deuteron energy, \(E_b\) is the energy required by the deuteron to overcome the Coulomb barrier for the target nucleus, and \(E_t\) is the average deuteron loss in lead before the reaction. By varying \(E_d\), it was possible to change the neutron energy. The change in \(E_d\) was achieved by having the deuteron beam in the cyclotron intersect the target at different places (i.e., the radius of the deuteron orbit was changed). At a deuteron energy of 190 MeV, \(E_n\), according to the authors’ estimate, was about 84 MeV.
The fission event was established from the ionization produced by the fragments in an argon-filled ionization chamber (in which the test sample was placed). Simultaneously with the element under study, and under the same conditions, a thorium sample was irradiated (the latter was chosen because it is not fissioned by slow neutrons). This made it possible to compare the reaction yield (i.e., the fission probability) in the given element and in thorium. Table I gives the ratios of these quantities at a neutron energy of 84 MeV. By varying \(E_n\), as indicated above, it was possible to measure the dependence of the reaction yield on the neutron energy. The corresponding results are shown in the figure. Along the ordinate axis are plotted the ratios of the yields
Table I
Ratio of the reaction yield in the given element to the reaction yield in thorium at a neutron energy of 84 MeV
| Element | Bi | Pb | Tl | Hg | Au | Pt | background |
|---|---|---|---|---|---|---|---|
| Ratio of yields, calculated per atom | 0.019 | 0.0055 | 0.0032 | 0.0023 | 0.0020 | 0.0009 | \(5 \cdot 10^{-5}\) |
of fission reactions in the given element and in thorium; along the abscissa axis—the radius of the deuteron orbit in the cyclotron, characterizing the neutron energy.
Figure labels visible on the graph: average calculated neutron energy: 25, 34.5, 44, 53.5, 63, 72.5, 82 MeV; ordinate: relative reaction yield; abscissa: radius of the deuteron orbit intersecting the target, cm. Curves are labeled Bi, Pb, Tl, Hg, Au, Pt.
The authors also measured the dependence of the reaction yield in lead on the isotopic composition of the sample. The results are given in Tables II and III.
Table II
Yields of the fission reaction in three samples of lead differing in isotopic composition \((E_n = 84\ \mathrm{MeV})\)
| Sample number | Isotopic composition, % Pb\(^{206}\) | Isotopic composition, % Pb\(^{207}\) | Isotopic composition, % Pb\(^{208}\) | Observed yield (relative to thorium) |
|---|---|---|---|---|
| 1 | 93 | 7 | 0 | \(0.0071 \pm 0.0004\) |
| 2 | 2 | 8 | 90 | \(0.0036 \pm 0.0001\) |
| 3 | 25 | 23 | 52 | \(0.0055 \pm 0.0003\) |
Table III
Calculated yields for pure lead isotopes
| Isotope | Pb²⁰⁶ | Pb²⁰⁷ | Pb²⁰⁸ |
|---|---|---|---|
| Yield (relative to Th) | 0.0070 ± 0.0005 | 0.0101 ± 0.002 | 0.0028 ± 0.0003 |
In conclusion, the authors note that their results can be used for the construction of neutron counters that register only very energetic particles.
V. Averbakh
References
- E. L. Kelly and Clyde Wiegand, Phys. Rev., 73, 1135 (1948).
- Broda and Wright, Nature, 158, 871 (1946); J. Perlman, R. H. Goeckermann, D. H. Templeton, and J. J. Howland, Phys. Rev., 72, 352 (1947). For an abstract of this work see UFN 34, no. 3, 440 (1948).
- R. Serber, Phys. Rev., 72, 1008 (1947).