Patterns of Alpha Decay

The large number of artificially obtained α-radioactive isotopes in recent years makes it possible to derive from the experimental data a number of new regularities of α-decay that had not previously been established empirically.

In the notes under review by Perlman, Ghiorso, and Seaborg\(^{1,2}\), a discussion is given of the regularities that have been found.

Figure 1 shows the dependence of the energy of α-particles on the mass number of the isotope, with the isotopes of each element connected by a solid or dashed line. First of all, one is struck by the almost parallel increase in the energy of α-particles for all elements from curium to bismuth in going from the isotope with the maximum mass number to the lighter ones (the right-hand part of the diagram). Such a course of the curves can be understood from the usual form of the nuclear-energy surface.

However, for elements with small atomic number such a regularity is observed only in the region of heavy isotopes. For them the energy of the α-particles reaches a maximum for certain isotopes, then decreases as the mass number decreases, and, for the lightest isotopes, increases again. To explain such a course of the curves one must suppose that the energy surface is not smooth but has a depression or a bulge (or both). As to the position

and of the magnitude of this distortion of the surface one still cannot say anything on the basis of the available experimental material. The question also remains open whether such a distortion of the energy surface exists in elements with large atomic number. Whereas for Bi and Po the maximum energy of the α-particles corresponds to isotopes lying in the region of β-stability or to heavier ones, which makes it possible to continue the investigation toward the region of lighter isotopes, for elements with large atomic numbers the region of β-stability corresponds to excessively large masses, so that the possible maximum of the α-particle energy lies in the region of β-active isotopes, which are difficult to obtain.

Fig. 1.

Another interesting regularity is found in studying the dependence between the half-life period and the energy of α-particles for isotopes of one and the same elements. For isotopes with even numbers of protons and neutrons this dependence is shown in Fig. 2. The dependence on \(Z\) agrees with that expected theoretically. For isotopes with an even number of protons and an odd number of neutrons and, conversely, with an even-

for nuclei with an odd number of neutrons and an odd number of protons, the values of the half-life periods are, on average, 5 times greater than those calculated by interpolation along the curves of Fig. 2. For isotopes with an odd number of protons and neutrons, the half-life periods are 10–20 times greater than those calculated by interpolation along the curves of Fig. 2. Let us note that some isotopes—At$^{211}$, the Po isotope with mass number less than 212, and bismuth isotopes—do not fall on the constructed curves: their half-life periods are anomalously large. However, according to Fig. 1, it is precisely for these isotopes that the magnitude of the $\alpha$-particle energy is explained by a distortion of the energy surface. Therefore the large half-life period of these isotopes may be explained by a decrease in the nuclear radius (for bismuth a decrease of radius by $\sim 1\%$ is required).

In some cases the observed lengthening of the half-life period can be explained by the above-mentioned decrease in the nuclear radius; sometimes it may be explained by a change in the nuclear spin. However, these factors are insufficient to explain the general observed tendency and, apparently, it should be assumed that the presence of one or several unpaired nucleons in the nucleus hinders the formation and emission of $\alpha$-particles.

Fig. 2.

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CITED LITERATURE

1. I. Perlman, A. Ghiorso and G. T. Seaborg, Phys. Rev. 74, 1730—1732 (1948).
2. I. Perlman, A. Ghiorso and G. T. Seaborg, Phys. Rev. 75, 1097—1098 (1949).