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NEW HEAVY NUCLEI
The number of artificially produced radioactive nuclei in the region of the heavy elements continues to grow. Recently two papers have been published in which several newly obtained heavy nuclei with insufficient, in comparison with nuclei of the natural sequence, numbers of neutrons are described. The first of the papers reviewed here¹ describes an artificial collateral radioactive series with initial nucleus \(U^{227}\). This series belongs to the actinium family with mass numbers expressed by the formula \(4n+3\), and ultimately leads to the stable lead isotope \(Pb^{207}\). The principal data are presented in Table I.
Table I
Radioactive properties of the collateral radioactive series \(U^{227}\)
| Isotope | Type of transformation | Half-life | Energy of the \(\alpha\)-particle (MeV) | Energy of \(\alpha\)-decay (MeV) |
|---|---|---|---|---|
| \(U^{227}\) | \(\alpha\) | \(1.3 \pm 0.3\) min. | \(6.8 \pm 0.1\) | \(6.9 \pm 0.1\) |
| \(Th^{223}\) | \(\alpha\) | \(\sim 10^{-1}\) sec. (pred.) | \(7.55 \pm 0.10\) | \(7.7 \pm 0.1\) |
| \(Ra^{219}\) | \(\alpha\) | \(\sim 10^{-3}\) sec. (pred.) | \(8.0 \pm 0.1\) | \(7.15 \pm 0.10\) |
| \(Em^{215}\) | \(\alpha\) | \(\sim 10^{-6}\) sec. (pred.) | \(8.6 \pm 0.1\) | \(8.8 \pm 0.1\) |
| \(Po^{211}\) | \(\alpha\) | \(0.52\) sec. | \(7.434\) | — |
| \(Pb^{207}\) | stable | — | — | — |
The production of \(U^{227}\) was carried out by bombarding thorium nitrate powder with helium ions accelerated in an 184-inch cyclotron. The exceptionally short half-life of \(U^{227}\) made it necessary to speed up all operations. In particular, although the distance from the cyclotron to the chemical laboratory was only about 100 m, “pneumatic mail” was constructed—a tube through which the sample was delivered by compressed air, so that 12–15 sec. after the end of irradiation the sample was already in the chemical laboratory. The chemical separation of uranium from the sample was accelerated to such an extent that 1.4 minutes after
after the end of irradiation it was possible to begin analysis of the spectrum of alpha particles. To speed up the counting of alpha particles, the dial of a mechanical counter registering alpha particles in the spectroscope was photographed on motion-picture film. The curves were compiled later, from the photographs taken.
Since it was not possible to obtain a sufficient amount of the final decay product, the isotopes were identified by alpha-decay energies and half-lives, using alpha systematics (see 2). The half-lives of the short-lived isotopes thorium, radium, and emanation could not be determined experimentally. The magnitude of their half-lives was estimated from the alpha-decay energies, on the basis of alpha systematics (see 2).
The second of the papers reviewed 3 describes the production of neutron-deficient nuclei of emanation, francium, and radium with the special aim of obtaining nuclei of these elements with 126 or fewer neutrons. These nuclei make it possible to study the effect of formation in nuclei of a closed neutron shell of 126 neutrons. The greatest success was achieved in the production of neutron-deficient isotopes of emanation. Table II gives the data for the newly obtained and studied nuclei of the new isotopes of emanation and francium.
Table II
Data on the new isotopes of emanation and francium
| Isotope | Branching ratio ($E/\alpha$) | Half-life | Energy of the $\alpha$ particle (MeV) | Energy of $\alpha$ decay (MeV) |
|---|---|---|---|---|
| Em$^{209}$ . . . . | 4—6 | 31 min. | $6.02 \pm 0.02$ | $6.14 \pm 0.02$ |
| Em$^{210}$ . . . . | $\sim 0.1$ | 2.7 hours | $6.02 \pm 0.02$ | $6.14 \pm 0.02$ |
| Em$^{211}$ . . . . | 2.8 | 16 hours | $5.82 \pm 0.02$ | $5.93 \pm 0.02$ |
| Em$^{212}$ . . . . | $<0.01$ | 23 min. | $6.23 \pm 0.02$ | $6.35 \pm 0.02$ |
| Fr$^{212}$ . . . . | $6.36 \pm 0.02$ | $6.48 \pm 0.02$ |
The isotopes of emanation were obtained by bombarding thorium with protons accelerated in a cyclotron to an energy of 340 MeV. The atoms of emanation were ionized in a glow discharge; the ions were accelerated in an electric field to several hundred volts and directed onto a platinum plate, which absorbed them. Such platinum samples with absorbed emanation could be studied as ordinary solid samples.
The isotope Fr$^{212}$ was identified with the aid of a new type of mass spectroscope, based on comparison of the flight times of ions in a magnetic field.
Attempts were also made to obtain radium isotopes with mass numbers below 214. According to alpha systematics 2, it may be assumed that, for example, Ra$^{213}$ will have a half-life of about 2 min.; therefore this isotope has so far not been observed. To obtain neutron-deficient radium isotopes, a method was also used for bombarding bismuth with sixfold ionized carbon atoms. This method appears to be more convenient for obtaining light radium isotopes, but so far it too has yielded no results.
The influence of the shell of 126 neutrons on the magnitude of the alpha-decay energy is shown in the graph depicting the dependence of the alpha-decay energy on the number of neutrons in the nucleus. The sharp jump in the alpha-decay energy at \(N > 126\) is clearly visible. It should only be noted that this jump gradually decreases with increasing atomic number \(Z\). The jump is greatest for the polonium isotopes and decreases successively toward emanation. The shape of the curve remains the same for all elements.
V. K.
Cited Literature
- W. Meinke, A. Ghiorso and G. Seaborg, Phys. Rev. 85, 429 (1952).
- I. Perlman, A. Ghiorso and G. Seaborg, UFN 42, 220 (1950).
- F. Momyer, E. Hyde, A. Ghiorso and W. Glenn, Phys. Rev. 86, 805 (1952).