WORLD OF SCIENCE

Bay of Plenty Times, Volume LXIV, Issue 11930, 23 January 1936, Page 4

WORLD OF SCIENCE

POTASSIUM RADIO-ACTIVITY AN UNSOLVED MYSTERY OF PHYSICS.

The property of radio-activity, popularly associated with the rare and costly group element radium, belongs also to a group of other chemical elements whose atoms, like those of radium, are among the heaviest in the l:st of atomic species. In fact, the existence of this property was first discovered by Becciuerel in the metal uranium, which of all elements has the highest atomic weight, and of which radium and polonium, discoveied a year or two !ater by the Curies, arc among the score or so of descendant atomic species formed by a succession of atomic disintegrations. Thorium, another very heavy atom, is the progenitor of a second and equally numerous family, all rad-io-active, except the last, and a third family- -the actinum family—is a branch line of the main Uranium-rad-ium series. All of these elements have atomic weights in excess of 200—that is, more than 200 times that of the lightest atom (hydrogen) taken as a unit. (A better specification of an atom than its atomic weight is* its atomic number (A.N.) —that is, the ordinal number attaching to its place in the series of chemical elements ranging from hydrogen (A.N. equals 1) to uranium (A.N. equals 02), for this number determines the physical and chemical properties of the element unambiguously, whereas it is now known that atoms of the same weight may yet be chemically different.) NATURAL INFERENCE. Therefore it appears natural to connect the instability of the atom with a high value for its atomic weight and atomic number, an inference which is greatly strengthened by the fact that the atomic number is likewise an exact measure of the excess positive electrical charge on its central nucleus. Obviously the forces of mutual repulsion between the subatomic particles of which this nucleus is constituted will become continually stronger as their number increases, until at last the structure tends to that spontaneous self-destruction which we know as radioactivity. But in the year 1907 this simple theory was shaken by the discovery of Dr Norman Campbell that potassium and rubidum (A.N.’s 19 and 37 respectively) arc feebly but unmistakably radioactive. The radiation by which this property is evinced was of the type named by Rutherford beta radiation. This consists in the ejection from the disintegrating atoms of a high-speed electron. The number of particles ejected from one gram of potassium has been found to be about 1100 a second only—as compared with millions of millions from the same quantity of some of the elements in the radium family—and the smallest of the proportion that this number bears to the total number of potassium atoms has the implication that the average life of the potassium atoms—which for radium is about 2000 years—is of the order of 15 million million years. SHORT OR LONG LIFE.

Now this length of life in itself creates no theoretical difficulty. In fact a value less than a thousand million years would be inconsistent with the ages which we must assign to the potassium minerals which are contained irr granite and other igneous rocks. But iihere is a serious incompatibility between this too long life and the very high speed of ejection of the beta-rays, for it has been definitely established that energetic expulsion of particles from the disintegrating atoms goes hand in hand with a short life (if a gay one). Unless, therefore, the potassium atoms are to be regarded as exceptions to this rule—and this is made doubly improbable by the recent discovery of artificially produced radioactive elements which obey it—their average life should bo numbered in days, not years—far less in millenia. Here, then,- is a double enigma.

When Acton, of Cambridge, in the course of his famous investigations on isotopes (atoms of the same atomic number but different atomic weight) had proved that potassium consists of a mixture of two kinds of atoms having atomic weights of 39 to 41 (in the proportion of 14/- nearly), the discovery was prematurely hailed as a possible way of escape from the dilemma, for if only-the rare species (A,W. 41) were unstable, the average life would be increased 14 times. This increase is, however, manifestly insufficient to meet the difficulty, and in a critical discussion of the problem last year Otta Klemperer (an expatriated German scientist now working at Cambridge) gave other reasons for rejecting the supposition that either the more abundant species (39) or the less- abundant (41) is the responsible agent. He argued for the existence of a third unknown and rare isotope (of A.W. 40). This view is also taken by Professor von Mevery, of Copenhagen. Quite recently an American physicist, Alfred Nier, of the University of Minnesota, has actually detected this isotope by the use of a highly sensitive mass-spectrograph (Aston’s instrument for analysing a stream of atoms into their various species). But this very confirmation of Klemperer’s suggestion is fatal to his theory, for Isior estimates that the atoms of the new isotope are present to the extent of one in 9000 of the total number—a ratio still far too large to give the required extension of average life. RARE ISOTOPE? It. is apparent, In fact, that if the solution of the puzzle lies —as it probably docs—in the existence of a rare isotope then the atoms of that isotope can exist only in such small proportion as to make their detection by any other effect than their radioactivity extremely difficult and perhaps for ever impossible. In the latest publication on the question Dr Kurt Sitte, of the University of Prague, reaches the conclusion—after a searching critique of Klemperer’s arguments—that the radioactive atom can possess an atomic weight neither of 39, 40, 41 or 42, but that 43 is both a possible and probable value.

He bases this conclusion mainly upon consideration as to the nature of the kind of atom resulting from the

disintegration (which must be a species of calcium atom), but he omits, as I opine, to discuss the ecxually important question of the origin of the comparatively short-lived potassium atoms.

One interesting possibility emerges if we accept Sitte’s well-reasoned conclusion. For, while the separation of a mixture of Isotopes into their several distinct species is not at all an easy matter —in virtue of their practical identity of physical and chemical characteristics —it has been shown to be not an entire impossibility. Not only has the heavy isotope hydrogen been separated out in quantity from ordinary hydrogen, but neon (22) has been extracted pure from neon (20), and minute quantities of lithium. (6) have been separated from the more abundant species (7). (The numbers refer, of course, to the atomic weights.) DIFFICULTY OP SEPARATION. Now the difficulty of separation will depend mainly on the nearness of the atomic weights, and the separation of atoms of weight 43 from those of weight 39 is going to prove a far easier task than would the separation of those of weight 40. I look upon it as certain that attempts will be made in the comparatively near future to effect a concentration of the hypothetical species of radioactive potassium. In view of the fact that potassium minerals are marketed at moderate prices in thousands of tons yearly it may not be an altogether fantastic speculation to contemplate the creation of a new industry, producing a rival substance to radium, in quantities measured by grams rather than by milligrams. . —K.G.