Radioactivity Induced by Neutron Bombardment

E. Fermi

Editor’s Note

The impact of neutrons could induce radioactivity in many nuclei, as Enrico Fermi here reports. In experiments, he and colleagues had exposed a number of different elements to a source of neutrons, and found that in almost every case the exposed substances became radioactive. They found particularly strong effects with phosphorus, aluminium, silver, iodine and chromium, and measured half-lives varying from a few seconds to several hours. Fermi notes that it seemed likely nuclei were absorbing neutrons, which later transform into protons with ejection of an electron, thereby returning the nucleus to stability. Although very indirectly, these experiments hinted at the possibility of sustained nuclear reactions.　　中文

EXPERIMENTS have been carried out to ascertain whether neutron bombardment can produce an induced radioactivity, giving rise to unstable products which disintegrate with emission of β-particles. Preliminary results have been communicated in a letter to La Ricerca Scientifica, 5, 282; 1934.　　中文

The source of neutrons is a sealed glass tube containing radium emanation and beryllium powder. The amount of radium emanation available varied in the different experiments from 30 to 630 millicuries. We are much indebted to Prof. G. C. Trabacchi, Laboratorio Fisico della Sanità Pubblica, for putting at our disposal such strong sources.　　中文

The elements, or in some cases compounds containing them, were used in the form of small cylinders. After irradiation with the source for a period which varied from a few minutes to several hours, they were put around a Geiger counter with walls of thin aluminium foil (about 0.2 mm. thickness) and the number of impulses per minute was registered.　　中文

So far, we have obtained an effect with the following elements:

Phosphorus—Strong effect. Half-period about 3 hours. The disintegration electrons could be photographed in the Wilson chamber. Chemical separation of the active product showed that the unstable element formed under the bombardment is probably silicon.

Iron—Period about 2 hours. As the result of chemical separation of the active product, this is probably manganese.

Silicon—Very strong effect. Period about 3 minutes. Electrons photographed in the Wilson chamber.

Aluminium—Strong effect. Period about 12 minutes. Electrons photographed in the Wilson chamber.

Chlorine—Gives an effect with a period much longer than that of any element investigated at present.

Vanadium—Period about 5 minutes.

Copper—Effect rather small. Period about 6 minutes.

Arsenic—Period about two days.

Silver—Strong effect. Period about 2 minutes.

Tellurium—Period about 1 hour.

Iodine—Intense effect. Period about 30 minutes.

Chromium—Intense effect. Period about 6 minutes. Electrons photographed in the Wilson chamber.

Barium—Small effect. Period about 2 minutes.

Fluorine—Period about 10 seconds.　　中文

The following elements have also given indication of an effect: sodium, magnesium, titanium, zirconium, zinc, strontium, antimony, selenium and bromine. Some elements give indication of having two or more periods, which may be partly due to several isotopic constituents and partly to successive radioactive transformations. The experiments are being continued in order to verify these results and to extend the research to other elements.　　中文

The nuclear reaction which causes these phenomena may be different in different cases. The chemical separation effected in the cases of iron and phosphorus seems to indicate that, at least in these two cases, the neutron is absorbed and a proton emitted. The unstable product, by the emission of a β-particle, returns to the original element.　　中文

The chemical separations have been carried out by Dr. O. D’Agostino. Dr. E. Amaldi and Dr. E. Segrè have collaborated in the physical research.　　中文

(133, 757; 1934)

Enrico Fermi: Physical Institute, Royal University, Rome, April 10.