Frederick Soddy Nobel Lecture
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O R I G I N S O F T H E C O N C E P T I O N S O F I S O T O P E S 377
thorium subsequently produced, after the separation, from the easily separa- ted mesothorium." (1907, p. 326). The first part of the thorium disintegration series is shown below with the average life of each member and the rays expelled in its change. Thorium Mesothorium I Mesothorium 2 2.10
10 years
9.67 years 8.9 hours Radiothorium Thorium X Emanation, etc. 2.9I years 5.25 days 78 seconds The separation of thorium and radiothorium and of mesothorium I
and thorium X by chemical analysis, now as then, are completely impossible. But each can be obtained alone by a suitable combination of chemical pro- cesses at suitable intervals of time. The exact procedure is dictated by the relative periods of the substances to be separated. In this connection it must be remembered, since the methods of ascertaining the nature of the products separated are purely radioactive, that it is the relative intensity of the radio- activity, or the relative number of α−
or β− particles emitted per second, and not the relative weights, which is of importance. For different radioactive elements, the radioactivity is proportional to the weight divided by the period. A rapidly changing substance reforms and attains its equilibrium value correspondingly quickly, a slowly changing one correspondingly slowly.* The ease with which the mesothorium is separated, and the very long time required for it to reform in substantial amount, is the reason why it was not discovered in 1902 when thorium X was discovered. It would normally * Thorium may be obtained, free from radiothorium, by repeated precipitations with ammonia over an interval sufficient to allow the original radiothorium present to decay. Mesothorium 1
is obtained free from thorium X from the first filtrate in the ammonia precipitation after leaving it for a month for the thorium X to decay. Radio- thorium is obtained free from thorium, by separating the mesothorium, waiting for it partly to change into radiothorium, and then separating the radiothorium from the mesothorium, by adding a trace of aluminium or similar substance to serve as a vehicle in the filtration, and precipitating it with the radiothorium by ammonia. Lastly thorium X is obtained free from mesothorium either from radiothorium or from thorium by the ammonia precipitation, in the latter case from the second and successive precipitations at monthly intervals, the first filtrate containing the mesothorium.
378 1921 F.SODDY be present only in thorium compounds that have remained undisturbed a long time since preparation, or purification. This fact had later an important sequel, as we shall see. The question, whether these non-separable pairs of radio-elements are really chemically identical or not, is not of importance in the argument. What is important is that 15 years ago pairs of radio-elements, actually not separable by chemical analysis, were in fact separated by successive chemical analyses at suitable intervals. If these pairs had been consecutive in the series, instead of being separated by intermediate products of different chemical character, they must have remained unresolved. Such a case, it seemed, was known.
In 1908 Boltwood showed that the relative α− ray activities of the various α− ray giving products of the uranium series, in equilibrium in natural minerals, conformed to the expulsion of one α -particle per atom disintegrating, ex- cept in the case of the actinium series (which so was correctly indicated to be a quite minor branch of the main uranium-radium series), and uranium itself which gives two α -particles per atom disintegrating. This satisfied the atomic weight difference of 12 units between uranium and radium, which corresponds with the expulsion in all of 3 α− particles. Ionium, a long-lived intermediate product giving α
the third α -particle. Thus it appeared that the 2 α− particles from uranium α− ray changes Uranium I Uranium II Uranium X Ionium Radium, etc. and that uranium I and uranium II were consecutive products, completely similar in chemical character, analogous to thorium and radiothorium, except that there was no intermediate product of different chemical character to serve for their separate recognition. The correctness of the inference that the two α -particles are successively and not simultaneously expelled was established by Marsden and Barret in 1911. They observed the scintillations produced by a uranium compound on a zinc-suiphide screen, and found no pairs, or any preponderance of short intervals between scintillations, beyond that required by the theory of O R I G I N S O F T H E C O N C E P T I O N S O F I S O T O P E S 379
probability for a purely random distribution of intervals. Although the other inference, that the two uraniums are consecutive successive products, was theoretically incorrect, because it was later found necessary to place uranium X between the two uraniums, the practical consequences are unaffected. Owing to the relatively short period of uranium X in comparison with its product uranium II, the latter cannot be put into evidence, as radiothorium was, by separating its parent and allowing it to change. The quantity of uranium II so formed (divided by its period) is too small to be detected, even when the uranium X from 20 kilograms of the element uranium is studied. Even if sufficient uranium could be experimented with, the experiment would still be rendered difficult, if not impossible, because of the ionium in the series, which is much shorter-lived than uranium II, and non-separable from uranium X, just as thorium X is from mesothorium I. This then is a case to which the experimental method of separation does not apply owing to the hopelessly unfavourable relation between the periods of the successive products. The chemical identity of different
Examples of chemically non-separable pairs, or groups, of radio-elements now began to accumulate very rapidly. Ionium, discovered by Boltwood, and by Marckwald and Keetman independently, in 1908, which proved to be the direct parent of radium, was shown to be identical in its chemical character with thorium. Boltwood separated it from uranium minerals by adding a little thorium and by separating and purifying the latter. Marck- wald and Keetman obtained it by precipitating the rare earths in the mineral with hydrofluoric acid, and found that any of the regular methods for puri- fying thorium from the rare earths would separate the ionium from the latter and actinium, but not from thorium. Keetman 2 tried twelve good methods, all well known to be effective in the purification of thorium, without effecting the least separation of ionium. He found at the same time that he could not separate uranium X, either, from the ionium-thorium mixture, though here, of course, just as with thorium X and mesothorium, the uranium X, being short-lived, speedily disappears of itself. Auer von Welsbach also, in 1910, carried out a masterly technical separa- tion of the ionium and actinium of the "hydrate" fraction obtained from 30,000 kilograms of Joachimsthal pitchblende in the manufacture of radium, Yüklə 161,11 Kb. Dostları ilə paylaş:
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