NEUTRONS KNOCKED OUT BY $\gamma$-QUANTA FROM BERYLLIUM NUCLEI

When a $\gamma$-quantum of sufficiently great energy is absorbed, an atomic nucleus may undergo a certain transformation; in this process one of the particles composing it—an $\alpha$-particle, proton, or neutron—may be ejected from the nucleus. One case of such nuclear transformation was discovered by Chadwick and Goldhaber^1. Using an ionization chamber connected through a linear amplifier with an oscillograph, they observed the appearance of protons when nuclei of heavy hydrogen were irradiated by the $\gamma$-rays of ThC″.

Szilard and Chalmers^2 developed another method for detecting nuclear transformations accompanied by the ejection of neutrons. They made use of the circumstance that neutrons, on entering a substance, induce in it artificial radioactivity, with a quite definite period characteristic of each element. Using this method, the authors established that beryllium nuclei, when exposed to irradiation by the $\gamma$-quanta of radium, emit neutrons. Their experiments were arranged as follows. 150 mg of radium was sealed in a vessel whose walls stopped all $\alpha$-particles incident on them from the source. Around the vessel was placed beryllium in an amount of 25 g, and all this in turn was surrounded by 100 cm^3 of ethyl iodide, which served as an indicator of the neutron emission of beryllium subjected to the action of the $\gamma$-rays of Ra (we note that the new radioactive element produced from iodine under the influence of neutrons is an isotope of the original element). After exposure, a iodine-containing precipitate was obtained from the ethyl iodide by appropriate treatment; this precipitate exhibited artificial radioactivity with the half-life characteristic of iodine, 30 minutes (in agreement with Fermi’s data). The active precipitate gave about 200 counts per minute in a Geiger–Müller counter, whereas in the absence of beryllium no more than 12 counts per minute were observed. The authors note that the effect observed in iodine is sufficiently large to be detected without chemical separation of the radioactive element formed.

Literature

1. Chadwick and Goldhaber, Nature 134, 237, 1934.
2. Szilard and Chalmers, Nature 134, 494, 1934.

L. Groshev

NUCLEAR REACTIONS ON SEPARATED ISOTOPES OF LITHIUM

In studying nuclear reactions occurring in matter under the influence of a flux of fast, artificially produced particles (protons, deuterons), one usually encounters the difficulty of deciding to which isotope of the element under study the observed decay products should be attributed. The consequence is that sometimes different interpretations are given to one and the same process.

The most radical way out of such a situation would be to study nuclear reactions on separated isotopes, especially since the reactions for individual isotopes of one and the same element are quite different. This is precisely the course taken in their recent work by Oliphant, Shire, and Crowther^1. They applied the mass-spectroscopic method to separate the isotopes of lithium Li^6 and Li^7. The chief drawback of this method is that the currents used here are extremely small and rarely exceed $10^{-8}$ A, while to obtain one quarter of a microgram of Li^7 it would be necessary to pass a current of 1 μA for one hour. However, in the particular case of lithium, as Oliphant and Rutherford^2 had shown earlier, the probability of destruction of lithium nuclei by protons and deuterons is so great that for investigating the decay products it is enough to have only a monomolecular layer of lithium.

To separate the isotopes of lithium, Oliphant, Shire, and Crowther constructed two special mass spectrographs with crossed electric-