NEW HEAVY NUCLEI.

PRODUCTION OF ISOTOPES OF THE 99TH AND 100TH ELEMENTS

Although popular literature has long been discussing the artificial production of isotopes of the 99th and 100th elements, the first, still preliminary, reports in scientific journals were published only in 1954.[^1][^2][^3][^4] The production of the new heavy nuclei was carried out by two methods, applied for the first time.

1. By developing the technique of accelerating multiply charged heavy ions in the 60-inch cyclotron, methods were worked out for accelerating sixfold charged nitrogen ions—\( \mathrm{N}^{14}(+6) \). The practice of accelerating carbon ions—\( \mathrm{C}^{12}(+6) \)—was used, and a current of nitrogen ions—\( \mathrm{N}^{14}(+6) \)—with an intensity reaching \(0.1\) microampere, at an energy of the nitrogen ions greater than \(100\) MeV, was obtained. With this current of nitrogen ions, \( \mathrm{U}^{238} \) was bombarded.

The products obtained as a result of the bombardment were identified by atomic numbers by precipitation with fluorosilicic lanthanum in an ion-exchange column. As a result, isotopes of transcurium elements—berkelium, californium, and the 99th element—were found; they are given in Table I.

Table I

Nuclei obtained from bombardment of \(U^{238}\) with \(N^{14}\) ions

The amount of transcurium isotopes obtained was very small, despite the comparatively large current of nitrogen ions. In three separate experiments out of 40 alpha-radioactive atoms, each time the 7.3-minute isotope of the 99th element was on the ion-exchange column immediately before californium, i.e. in the eka-holmium position. Thus the atomic number of the 99th element may be regarded as accurately determined. Its mass number was estimated from the systematics of alpha-radioactive isotopes to be 6.

After bombardment of \(U^{238}\) with nitrogen ions, a large number of fission products was found, in an amount considerably greater than after bombardment with carbon ions.

From this one may conclude that the nuclei obtained as a result of the reaction of \(U^{238}\) with the isotope \(N^{14}\) are more capable of undergoing fission than the nuclei obtained in the reaction of \(U^{238}\) with \(C^{12}\).

1. The second method\(^{4,5}\) for obtaining transcurium isotopes consisted in bombarding the isotope \(Pu^{239}\) with neutrons. The bombardment was carried out in a materials-testing reactor. By this method it was possible to obtain exclusively isotopes containing an excess of neutrons in comparison with beta-stable isotopes. The results of the reaction

with neutrons were identified by the place of their deposition in the ion-exchange column and are given in Table II.

The determination of the ordinal numbers of all the isotopes obtained, given in Table II, should be considered quite reliable, including the 99th and 100th elements.

Table II

Nuclei obtained from Pu\(^{239}\) and Cf\(^{252}\) by bombardment with neutrons in a reactor

Mass numbers are determined from the proposed course of nuclear reactions and from the systematics of alpha-radioactive nuclei\(^6\). The process of obtaining the transcurium isotopes is a successive capture of neutrons and negative decay. It may be assumed that the chain of reactions leading to the production of \(\mathrm{Bk}^{249}\) from \(\mathrm{Pu}^{239}\) has the following form:

\[ \begin{aligned} &\mathrm{Pu}^{239}(n,\gamma)\ \mathrm{Pu}^{240}(n,\gamma)\ \mathrm{Pu}^{241}(n,\gamma)\ \mathrm{Pu}^{242}(n,\gamma)\ \mathrm{Pu}^{243}\\ &\qquad\downarrow \qquad\qquad\qquad\ \uparrow \qquad\qquad\downarrow\\ &\qquad\mathrm{Am}^{241}(n,\gamma)\ \mathrm{Am}^{242}(n,\gamma)\ \mathrm{Am}^{243}(n,\gamma)\ \mathrm{Am}^{244}\\ &\qquad\qquad\qquad\downarrow \qquad\qquad\qquad\qquad\downarrow\\ &\qquad\qquad\mathrm{Cm}^{242}(n,\gamma)\ \mathrm{Cm}^{243}(n,\gamma)\ \mathrm{Cm}^{244}(n,\gamma)\ \mathrm{Cm}^{245}\\ \\ &\mathrm{Cm}^{245}(n,\gamma)\ \mathrm{Cm}^{246}(n,\gamma)\ \mathrm{Cm}^{247}(n,\gamma)\ \mathrm{Cm}^{248}(n,\gamma)\ \mathrm{Cm}^{249}\\ &\qquad\qquad\qquad\qquad\qquad\qquad\qquad\uparrow\\ &\qquad\qquad\qquad\qquad\qquad\qquad\qquad\mathrm{Bk}^{249} \end{aligned} \]

With reference to an unpublished work, the communication states that \(\mathrm{Cm}^{247}\) is \(\beta\)-stable, whereas among the Bk isotopes there are no entirely \(\beta\)-stable ones. Thus, apparently, the chain of reactions leading to the production of the isotope \(99^{253}\) and the isotope \(100^{254}\) may have the form:

\[ \begin{aligned} &\mathrm{Bk}^{249}(n,\gamma)\ \mathrm{Bk}^{250}\\ &\downarrow \qquad\qquad\downarrow\\ &\mathrm{Cf}^{249}(n,\gamma)\ \mathrm{Cf}^{250}(n,\gamma)\ \mathrm{Cf}^{251}(n,\gamma)\ \mathrm{Cf}^{252}(n,\gamma)\ \mathrm{Cf}^{253}\\ &\qquad\qquad\qquad\qquad\qquad\qquad\downarrow\\ &\qquad\qquad\qquad\qquad\qquad 99^{253}(n,\gamma)\ 99^{254}\\ &\qquad\qquad\qquad\qquad\qquad\qquad\downarrow\\ &\qquad\qquad\qquad\qquad\qquad\qquad 100^{254} \end{aligned} \]

The half-life periods for most of the isotopes listed in Table II were found approximately from the systematics of alpha radioactivity,^6 sometimes with an accuracy to within a factor of 10.

In the notes to all the papers cited, the existence of many unpublished works carried out in other laboratories and also concerning the production of isotopes of the 99th and 100th elements is indicated.

The most recent reports^7,8 give data on isotopes of the 99th and 100th elements obtained in other laboratories. The isotopes were obtained by irradiating plutonium with thermal neutrons in a reactor for testing materials for reactions caused by higher elements. The elements were separated in an ion-exchange column. The fraction containing the 99th element was subjected to an additional four-day irradiation. In this way the following were obtained: isotope \(99^{253}\) with a half-life of \(19.3 \pm 0.3\) days, with alpha particles of energy \(6.61 \pm 0.01\) MeV, and isotope \(100^{254}\) with a half-life of \(3.3 \pm 0.2\) hours, with alpha particles of energy \(7.17 \pm 0.01\) MeV, in agreement with the preceding works. In papers^7,8 rough measurements were also made of the spontaneous-fission period of the newly discovered isotopes, and it turned out that isotope \(99^{253}\) has a spontaneous-fission period \(>10^5\) years, while \(100^{254}\) has \(220 \pm 40\) days. This shows that, in accordance with existing theories, the probability of spontaneous fission increases rapidly with atomic number.

V. K.

Cited Literature

1. A. Ghiorso, G. Rossi, B. Harvey and S. Thompson, Phys. Rev. 93, 257 (1954).
2. Nature 173, 290 (1954).
3. S. Thompson, A. Ghiorso, B. Harvey and G. Choppin, Phys. Rev. 93, 908 (1954).
4. B. Harvey, S. Thompson, A. Ghiorso and G. Choppin, Phys. Rev. 93, 1129 (1954).
5. G. Rossi, W. Jones, J. Hollander and J. Hamilton, Phys. Rev. 93, 256 (1954).
6. I. Perlman, A. Ghiorso and G. Seaborg, UFN 47, 220 (1954).
7. M. Studier, P. Fields, H. Diamond, J. Mech, H. Friedman, P. Sellers, G. Pyle, C. Stevens, L. Magnusson and J. Huizenga, Phys. Rev. 93, 1428 (1954).
8. P. Fields, M. Studier, J. Mech, H. Diamond, H. Friedman, L. Magnusson and J. Huizenga, Phys. Rev. 94, 209 (1954).