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The Mass-Spectrum of Uranium Lead and the Atomic Weight of Protactinium

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The Mass-Spectrum of Uranium Lead and the Atomic Weight of Protactinium MARCH 2, 1929] NATURE 313 Letters to the Editor. of Actinium and of the Earth. [The Editor does not hold himself responsible for BY the kindness of Dr. Aston, I h ave had the oppor­ opinions expressed by his correspondents. Neither tunity of inspecting his photographs showing the can he undertake to return, nor to correspond with isotopes of lead obtained from the radioactive mineral the writers of, rejected manuscripts intended for this broggerite. As he concludes, it seems highly pt·obable or any other part of NATURE. No notice is taken that the isotope of mass 207 is mainly due to actinium of anonymous communications.] lead, and that the actinium series has its origin in an isotope of uranium-a suggestion independently put The Mass-Spectrum of Uranium Lead and the forward by several investigators on other evidence. Atomic of Protactinium. Since six a particles are emitted in the successive changes from protactinium to the end product IT will be recalled (NATURE, Aug. 13, 1927) that the actinium lead, the atomic weight of protactinium identification of the isotopes of ordinary lead was should be 231. The direct determination of the made by m eans of a sample of its tetramethide kindly atomic weight of this element number 91 now in supplied to me by Mr. C. S. Piggot, of the Geophysical progress in the laboratory of Prof. Hahn in Berlin Laboratory, Washington. He has since succeeded should afford a crucial test of the accuracy of this in the much more troublesome task of preparing the deduction. similar compound of a rare uranium lead from In the light of this new knowledge and of the Norwegian broggerite. His reasons for this work measurements made by Dr. Aston of the relative have already been published (C. S. Piggot, "Lead intensities of the lead isotopes in the mineral, it may Isotopes and the Problem of Geologic Time," Jour. be of interest to consider its bearing on the origin of Wash. Acad. Sci., May 19, 1928). The first tube of actinium and other problems. W e shall first discm;s uranium lead methide despatched to m e a year ago the probable mass of this new isotope, which for con­ was unfortunately broken in transit, but the second venience will be called actino-uranium. It seems reached Cambridge safely last summer. At that simplest to suppose that its mass is 235, and that it time I was endeavouring to work out a photometric undergoes first an a and then a (:3 ray transformation method of measuring the relative abundance of into protactinium. The (:3 ray body is probably to be isotopes. This work is by no m eans complete, but identified with uranium Y, discovered by Antonoff, has r·ecently reached a stage which justified an which has generally been regarded as the immediate attempt on the mass-spectrum of this v ery precious parent of protactinium. On this view, the successive material. The procedure was the same as with transformations follow the order a(:3a.(:3, where the a and ordinary lead methide, but the general conditions of (:3 changes alternate, and differ in this respect from the the discharge tube, etc., were not so favourable, so main uranium series which follow the order afifl a.. that the spectra obtained are weaker. It is of course possible to assume that actino-uranium The mass-spectrum consists of a strong line at 206, has a mass 239 and number 92, and is converted into a faint one at 207, and a still fainter one at 208. The a mass 235 of number 92 in consequence of an a ray last is barely visible to the eye, but easily distinguish­ change followed by two (:3 ray transformations, but no able on the photometer curves. The impossibility evidence has been obtained of the existence of such of eliminating mercury limits the search for lighter (:3 ray bodies, although a careful search has been made isotopes, but there is not the least indication of 203 for them by Hahn and others. or 205. Unfortunately, the experimental conditions An estimate of the period of transformation of the all conspire to make the determination of the true new isotope of uranium can be deduced on certain relative intensities of the lines from the curve of probable assumptions. The ratio K' of the numhm· photometer wedge readings too complex to be really of atoms of actinium lead to those of uranium lead trustworthy. Calling the intensity of the strong line can be deduced approximately from Aston's measure­ 100, the m ean of the best plates gives 10·7 ± 3 and ments, and we also know the ratio K-about 4·5 ± 2 for 207 and 208 respectively. As the only of the number of atoms delivered in a mineral into the curve availa ble for transforming wedge readings into actinium series compared with the number passing intensities is one derived from krypton, these figures into the radium series. If Ap ;>. 2 are the constant.s of are probably both too high. They correspond to tranf3formation of actino-uranium and the main percentages 86·8: 9·3: 3·9, and as the packing fraction uranium isotope respectively, it can easily be deduced is indistinguishable from that of mercury (0·8 x A eA,t- 1 the mean atomic weight deduced is 206·19, rather that K ' !K = · ? · - --- -- , where t is the age of the ;>. ef•t- higher than that determined chemically for other 1 1 uranium leads. These figures have been communi­ mineral from which the lead is derived. We shall cated to Mr. Piggot, and when combined with the suppose for the purpose of calculation that t is 109 analyses of the mineral should enable its age to be ye!l,rs-an average estimate of the age of old primary fixed with considerable certainty. uranium minerals. Taking as a low estimate that There is, however, another point of view from which K' = 7/100, it can be deduced from the equation that these results are of fundamental interest in connexion ;>. 1 f;>. 2 = 10·6. Since the half-value period of trans­ with the radioactive elements. The line 207 is of formation of uranium is 4·5 x 109 years, it follows that peculiar significance. It cannot be due to the the p eriod of actino-uranium is· 4·2 x 108 years. A presence of lead as an impurity, for in ordinary lead larger value of K ' lower'> the period, while a higher 208 is about twice as strong as 207, neither can it be value for the age of the mineral raises it. the product of radium or thorium. It is difficult to Taking the period as 4·2 x 108 years, it is seen that resist the natural conclusion that it is the end product the amount of actino-uranium is only about 0·28 of the only other known disintegration, namely, that per cent of the main uranium amount of actinium. If this is so it settles the m ass numbers too small to influence appreciably the atomic weight of all the members of this series, that of protactinium of uranium as ordinarily measured. The amount of being 231. Extrapolation of the packing fraction actino-uranium at the time of its formation taken as curve suggests an atomic weight on the oxygen scale 109 years age comes out to be 1·44 per cent. of 231·08. F. W. AsTON. There is another interesting deduction that can be Cavendish Laboratory, made from these estimates. It is natural to suppose Cambridge, Feb. 16. that the uranium in our earth has its origin in the sun, No. 3096, VoL. 123] © 1929 Nature Publishing Group.
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