Natural and Artificial Radioactivity of Potassium

Abstract

THE question of the origin of the radioactivity of potassium has been much discussed in recent years¹. The possibility, however, of producing new radioactive isotopes artificially has opened up a new line of attack on this problem. Quite recently Amaldi, D'Agostino, Fermi, Pontecorvo, Rasetti and Segrè² have found that, by bombardment of potassium with neutrons, a new radioactive isotope of potassium is produced having a period of 16 hours. Potassium having two stable isotopes, 39 and 41, it is not possible to draw conclusions from these experiments on the mass of the new isotope. The problem can be settled, however, from the fact that the new isotope of potassium can also be produced by bombarding scandium with neutrons. By experiments in this laboratory in which I have been kindly assisted by Hr. Hoffer-Jensen, we find that scandium can be converted into a radioactive isotope of potassium. As scandium has only one isotope (45) this conversion must take place according to the equation: As the potassium obtained by us has the same period as that found by the Italian workers, we have to conclude that in their experiments it was K⁴¹ which captured a neutron and was converted into K⁴². In our experiments we bombarded scandium oxide with neutrons produced by a mixture of beryllium and radium emanation, thus applying Fermi's beautiful method. The scandium oxide was dissolved in hydrochloric acid and, after the addition of 0.15 gm. of sodium chloride and the same amount of calcium chloride, precipitated with ammonia. The calcium present in the filtrate was removed as oxalate and found to be inactive. The remaining sodium chloride, however, was found to be active and to contain the potassium isotopes looked for. This decayed with a period of about 16 hours, emitting very hard -rays of approximately 1.2 million e.v.