ELEMENTS WITH ATOMIC NUMBERS GREATER THAN 92

As has already been reported,* Fermi and his collaborators established that, when uranium is irradiated with neutrons, several new radioactive elements are formed (half-lives 10, 40, 13, and 90 min), the atomic numbers of which, in all probability, are greater than 92. In studying the chemical nature of these elements the authors limited themselves to the element with the 13-minute decay period. Concerning the 90-minute product they established only that it is possibly an isotope of the 13-minute one.

In identifying the new element with the previously known ones, the greatest difficulties were encountered for \(Z = 91\), owing to the short half-life of the protactinium isotope \(UX_2\) available to the authors. Subsequently a number of notes appeared devoted to clarifying this question. Gross and Agruss, on the basis of their investigations of the chemical properties of protactinium, came to the conclusion that the new element with a half-life of 13 min is an isotope of element 91. However, subsequent, more detailed investigations by Meitner and Hahn** confirm the assumptions put forward by Fermi.

Meitner and Hahn endeavored first of all to show that the 13- and 90-minute elements arising from uranium when it is irradiated with neutrons have an atomic number greater than 92. In these experiments they proceeded as follows. If a solution containing neutron-irradiated uranium is treated with alkali, then under definite conditions one can achieve the result that the elements with atomic numbers greater than 92 remain in solution, whereas elements 90, 91, 92 precipitate together with the precipitate. In their experiments several grams of uranium (in the form of a uranium salt), previously freed of \(UX\), were irradiated with neutrons from a source consisting of 250 \(mg\) Ra Em—Be. The uranium salt was then dissolved and, after potassium perrhenate (a rhenium compound) and chloroplatinic acid had been added, precipitation was effected with NaOH. The precipitate was separated, and the solution was weakly acidified with hydrochloric acid, and platinum was precipitated from it with the aid of \(H_2S\). After this, by additional introduction of HCl and \(H_2S\), rhenium was precipitated from the solution. Investigation of the radioactivity of the precipitates by means of a Geiger-Müller counter showed that a considerable part of the 13- and 90-minute elements was precipitated with the platinum, while the rhenium remained inactive. However, if only rhenium is added to the solution and its precipitation is carried out by the same method, the substances under investigation (in somewhat smaller amounts) are precipitated together with it. These experiments exclude the possibility of identifying the new elements with elements of atomic numbers 90, 91, and 92 and confirm the assumption expressed by Fermi concerning the existence of elements with atomic numbers greater than 92.

Since in Fermi’s experiments the question of identifying the new elements with protactinium remained in doubt, Meitner and Hahn carried out one more experiment, attempting to precipitate the new elements with \(UZ\), which is an isotope of protactinium and has a half-life of 6.7 h. This experiment showed, however, that the elements with 13- and 90-minute decay periods can be chemically separated from \(UZ\), i.e. that they are not isotopes of protactinium.

In subsequent experiments by the same authors it was established that the new elements with decay periods of 13 and 90 min, which had previously precipitated together all the time, are in fact not isotopes and can be separated chemically. This separation is carried out by a method analogous to that described above. Osmium (in the form of osmium ammonium chloride) and rhenium (in the form of sodium perrhenate) are added to the solution containing uranium irradiated with neutrons. Then osmium is precipitated from the solution with NaOH, while rhenium remains in solution. The precipitate obtained is dissolved in a hydrochloric-acid solution, from which osmium sulfide is precipitated. Together with this sulfide there precipitates a radioactive element with a 90-minute decay period—

* Uspekhi fiz. nauk 14, 829, 933, 1934.
** Naturwiss. 23, 37, 230, 1935.

fallout. From the solution containing rhenium, subsequent treatment precipitates rhenium sulfide, together with which the radioactive element with a 13-minute half-life is precipitated. In this way separation of the new elements is achieved.

In studying the separated active products it was established that the substance with a 90-minute half-life is not an element, but consists in turn of a mixture of two elements with half-lives of 50–70 min. and 2–3 days. These latter radioactive elements have not yet been successfully separated by chemical methods. Possibly isotopes are involved here.

In the experiments described above one might have feared that, upon prolonged irradiation of uranium with neutrons, UX₁ (half-life 24 days) and UZ (6.7 hours) could be formed in appreciable amounts. However, the atomic-control experiments carried out showed that this introduces no noticeable distortions into the results obtained.

Thus the experiments of Fermi and co-workers, as well as those of Meitner and Hahn, speak in favor of the probability that elements with atomic numbers greater than 92 exist. True, as yet there are still too few data to assign a definite atomic number to each of the new radioactive elements.

L. Groshev

ARTIFICIAL RADIOACTIVITY OF GASEOUS NITROGEN INDUCED BY DEUTRONS*

Artificial radioactivity usually induced by fast particles is observed on plates of a solid substance. In some cases, however, as Krenn and Lauritsen have shown, radioactive centers can be in the gaseous state, diffusing into the gas from the surface of the irradiated plate. In this case tracks beginning inside the gas are obtained if the gas is introduced into a Wilson chamber.

Recently Livingston and McMillan* observed artificial radioactivity induced by a stream of deuterons directly in a gas. These authors established that certain substances—for example, platinum annealed to white heat in an oxygen flame, pure platinum, and copper freshly annealed and oxidized—show artificial radioactivity with one and the same half-life, 126 ± 5 sec., if they are subjected to bombardment by fast deuterons (energy about 2 MeV) in a vessel with air. Placing the plates in various gases and irradiating them with deuterons, the authors established that in the case of O₂, H₂, A the radioactivity practically disappears, but it remains almost the same as for air in the case of N₂ and CO₂. True, for the latter gas, instead of a period of 126 sec., the period characteristic of C, 10.4 sec., is observed. These experiments show that the radioactivity of platinum irradiated in air by deuterons is caused by the radioactivity of gaseous nitrogen.

Further experiments showed that the radioactive atoms arising from nitrogen are very firmly bound to platinum. The authors consider that the firm “sticking” of the radioactive atoms to the platinum occurs as a consequence of the momentum which the atoms receive in the reaction with the deuteron. Proof of the correctness of such a mechanism of “sticking” of the radioactive atoms is the fact that platinum exhibits radioactivity with a half-life of 156 sec. if it is irradiated in a vacuum while deuterons are passed through a dense aluminum foil covering the entrance aperture. A period of 156 sec. is characteristic of the artificial radioactivity of aluminum. In this case the radioactive atoms, evidently, can get from the aluminum foil onto the platinum only by virtue of the momentum acquired by them in nuclear reactions.

For a more direct investigation of the radioactivity of gases the authors

* Mc. Millan a. Livingston, Phys. Rev. 47, 452, 1935.