Artificial Production of a New Kind of Radio-element

F. Joliot and I. Curie

Editor’s Note

Scientists were still exploring the kinds of nuclear transmutations that could be induced by bombarding stable elements with particles such as alpha particles. Here Irène Curie and Frédéric Joliot report the formation of new, unstable isotopes of nitrogen, silicon and phosphorus made by alpha-irradiation of aluminium, boron and magnesium. The new “radio-elements” are evident from their decay over periods of several minutes, with emission of positrons. Uniquely, the two French scientists use chemical methods to separate the new isotopes and thereby identify their chemical nature. Short-lived positron-emitting isotopes, including the nitrogen-13 reported here, are now used in the medical imaging technique of positron emission tomography. (Note that the third paragraph seems to mistakenly mention “beryllium” in place of “boron”.)ft　　中文

SOME months ago we discovered that certain light elements emit positrons under the action of α-particles1. Our latest experiments have shown a very striking fact: when an aluminium foil is irradiated on a polonium preparation, the emission of positrons does not cease immediately, when the active preparation is removed. The foil remains radioactive and the emission of radiation decays exponentially as for an ordinary radio-element. We observed the same phenomenon with boron and magnesium2. The half life period of the activity is 14 min. for boron, 2 min. 30 sec. for magnesium, 3 min. 15 sec. for aluminium.ft　　中文

We have observed no similar effect with hydrogen, lithium, beryllium, carbon, nitrogen, oxygen, fluorine, sodium, silicon, or phosphorus. Perhaps in some cases the life period is too short for easy observation.ft　　中文

The transmutation of beryllium, magnesium, and aluminium α-particles has given birth to new radio-elements emitting positrons. These radio-elements may be regarded as a known nucleus formed in a particular state of excitation; but it is much more probable that they are unknown isotopes which are always unstable.ft　　中文

For example, we propose for boron the following nuclear reaction:

5B10 + 2He4 = 7N13 + 0n1

7N13 being the radioactive nucleus that disintegrates with emission of positrons, giving a stable nucleus 6C13. In the case of aluminium and magnesium, the radioactive nuclei would be 15P30 and 14Si27 respectively.ft　　中文

The positrons of aluminium seem to form a continuous spectrum similar to the β-ray spectrum. The maximum energy is about 3 × 106 e.v. As in the case of the continuous spectrum of β-rays, it will be perhaps necessary to admit the simultaneous emission of a neutrino (or of an antineutrino of Louis de Broglie) in order to satisfy the principle of the conservation of energy and of the conservation of the spin in the transmutation.ft　　中文

The transmutations that give birth to the new radio-elements are produced in the proportion of 10–7 or 10–6 of the number of α-particles, as for other transmutations. With a strong polonium preparation of 100 millicuries, one gets only about 100,000 atoms of the radioactive elements. Yet it is possible to determine their chemical properties, detecting their radiation with a counter or an ionisation chamber. Of course, the chemical reactions must be completed in a few minutes, before the activity has disappeared.ft　　中文

We have irradiated the compound boron nitride (BN). By heating boron nitride with caustic soda, gaseous ammonia is produced. The activity separates from the boron and is carried away with the ammonia. This agrees very well with the hypothesis that the radioactive nucleus is in this case an isotope of nitrogen.ft　　中文

When irradiated aluminium is dissolved in hydrochloric acid, the activity is carried away with the hydrogen in the gaseous state, and can be collected in a tube. The chemical reaction must be the formation of phosphine (PH3) or silicon hydride (SiH4). The precipitation of the activity with zirconium phosphate in acid solution seems to indicate that the radio-element is an isotope of phosphorus.ft　　中文

These experiments give the first chemical proof of artificial transmutation, and also the proof of the capture of the α-particle in these reactions3.ft　　中文

We propose for the new radio-elements formed by transmutation of boron, magnesium and aluminium, the names radionitrogen, radiosilicon, radiophsphorus.ft　　中文

These elements and similar ones may possibly be formed in different nuclear reactions with other bombarding particles: protons, deutrons, neutrons. For example, 7N13 could perhaps be formed by the capture of a deutron in 6C12, followed by the emission of a neutron.ft　　中文

(133, 201-202; 1934)

F. Joliot and I. Curie: Institut du Radium, Paris.

References:

  1. Irène Curie and F. Joliot, J. Phys. et. Rad., 4, 494; 1933.

  2. Irène Curie and F. Joliot, C.R., 198; 1934.

  3. Irène Curie et F. Joliot, C.R., meeting of Feb. 29, 1934.