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Produits de
transformation lente du radium
Ernest Rutherford,�Produits de transformation lente du radium. Lettre publi�e originalement en anglais en 1905 dans la revue Nature, dans laquelle Rutherford mentionne notamment l'observation d'un nouveau produit de d�sint�gration du radium qu'il nomme � D1 �. Lettre traduite en fran�ais par Simon Villeneuve en f�vrier 2013 et relue par Cantons-de-l'Est.
[341] Dans une parution r�cente
de Philosophical Magazine (novembre,
1904), j�ai montr� que le radium, apr�s �tre pass� par quatre changements
rapides, se d�sint�gre en deux produits qui se transforment lentement, que j�ai
nomm� radium D et radium E.
Ces deux produits peuvent �tre s�par�s en utilisant les m�thodes physiques et chimiques appropri�es. Le radium D, parent du E, n��met que des rayons β, alors que le E n��met que des rayons α. Il a �t� calcul� que le D devrait �tre transform� � moiti�[1] en quarante ans, et le E en � peu pr�s un an. D�autres exp�riences ont d�montr� que le radium D est le constituant actif du plomb radioactif de Hofmann, et que le radium E est la substance active du polonium de Mme Curie et du radio-tellure de Marckwald.
Des travaux ult�rieurs ont confirm� ces conclusions. J�ai examin� les taux de diminution de l�activit� du radium E et du radio-tellure et j�ai trouv� qu�ils sont identiques. Chacun perd la moiti� de son activit� en environ 150 jours, plut�t que la p�riode d�un an pr�alablement calcul�e. L��chantillon de radio-tellure a �t� obtenu de Sthamer, de Hambourg, sous la forme d�un film mince d�pos� sur une tige de bismuth polie. J�ai constat� des r�sultats semblables de d�sint�gration et d�activit� du radio-tellure chez Meyer and Schweidler (Akad. D. Wiss. Wien., 1er d�cembre 1904). Malheureusement, je n�ai pas r�ussi � d�terminer pr�cis�ment la baisse d�activit� du polonium. Un sp�cimen de polonium (bismuth radioactif) a �t� en ma possession pendant 3 ans et a pendant ce temps perdu [342] une bonne proportion de son activit� initiale. En le testant, son activit� pr�sente une valeur faible et presque constante. Cependant, en r�alisant des observations r�guli�res et rigoureuses, j�ai d�couvert que le taux de d�sint�gration de ce polonium n�est certainement pas tr�s diff�rent de celui du radium E. D�autres exp�riences seront n�cessaires pour trancher d�finitivement la question, mais j�ai peu de doutes que le taux de d�sint�gration observ� sera le m�me.
Le polonium, le radio-tellure et le radium E pr�sentent des propri�t�s radioactives et chimiques tr�s similaires. Chacun n��met que des rayons α et est d�pos� sur une plaque de bismuth plong�e dans la solution active. Conjugu� avec leur radiation similaire et leur propri�t�s chimiques, leur taux de d�sint�gration probablement identique montre que les propri�t�s radioactives des constituants sont les m�mes dans chaque cas. Nous en concluons donc que la substance active pr�sente dans le polonium et le radio-tellure est un produit de la d�composition du radium, le sixi�me (ou, comme nous le verrons plus loin, probablement le septi�me) membre de la famille du radium.
Dans le pass�, la principale objection contre l�identit� du polonium et du radio-tellure se basait sur l�affirmation de Marckwald, soulignant qu�une solution tr�s active de cette substance ne perd pas son activit� de mani�re significative en six mois. � moins d�utiliser des m�thodes peu orthodoxes, il serait difficile de d�terminer avec pr�cision la variation de l�activit� de ce type de mat�riau radioactif. L'�chantillon de radio-tellure obtenu � la fois par Meyer et Schweidler ainsi que par moi-m�me perd sans l'ombre d'un doute son activit� tr�s rapidement.
R�cemment, j�ai observ� plus pr�cis�ment le radium D et j�ai obtenu des preuves solides qu�il n�est pas un produit unique et contient deux substances distinctes. Ainsi, le radium D n��met pas de rayonnement. Il se transforme en une substance qui n��met que des rayons β et qui est � moiti� transform� en six jours. � moins d�observer le radium D peu apr�s sa s�paration, ce changement rapide passe inaper�u. Bien que les travaux sur le sujet ne soient pas termin�s, les observations actuelles permettent de constater que le d�p�t actif, apr�s �tre pass� par trois �tapes rapides donnant le radium A, B et C, se transforme en un produit D qui n��met pas de rayonnement et qui se transforme extr�mement lentement. D produit une autre substance - que nous nommerons pour le moment D1 - qui se transforme en quelques semaines en n��mettant que des rayons β. Le D1 se transforme ainsi en E (polonium).
Puisque l�activit� du D1 atteint une valeur maximale quelques semaines apr�s sa production, puis se d�sint�gre ensuite au m�me taux que le D, cela ne remet pas en question la conclusion pr�c�dente, � savoir que D se transforme � moiti� � peu pr�s en quarante ans. L�id�e que le radium D est le constituant actif du soi-disant radio-plomb de Hofmann est appuy�e fortement par certains r�sultats exp�rimentaux obtenus par Hofmann, Gonder et W�lfl (Annal. Der Physik, vol. xv, 3, 1904).
Ils ont ainsi trouv� que des pr�parations de radio-plomb produisent de mani�re continue un produit �mettant du rayonnement α, qui peut �tre isol� sur une plaque de bismuth. Ce produit actif est probablement du radium E puisqu�ils ont observ� qu�il perd une bonne partie de son activit� en un an. De plus, ils ont trouv� que l�on peut, par certains proc�d�s chimiques, isoler un autre produit n��mettant que du rayonnement β et perdant la plus grande partie de son activit� en six semaines. Cette substance est probablement le nouveau produit du radium D1 que j�ai identifi�.
Derni�rement, Debierne a conclu que le radio-plomb et le polonium sont identiques. Il a sugg�r� que le nom � radio-plomb � soit abandonn� au profit de celui de polonium. � la lumi�re des r�sultats pr�c�dents, cette affirmation n�est pas d�fendable. Il n�y a aucun doute que la pr�paration de radio-plomb en ma possession, ainsi que celle analys�e par Hofmann, contient une substance distincte qui, �tant la substance m�re du polonium, m�rite autant un nom que sa descendance. La substance radioactive dans le � radio-plomb � n�a pas plus de lien avec le plomb que le � radio-tellure � de Marckwald n�a de lien avec le tellure. Ces noms ont �t� adopt�s parce que la mati�re active a �t� initialement trouv�e associ�e � ces �l�ments.
Afin d��viter toute confusion, j�ai nomm� le nouveau produit du radium � radium D1 �. �Si aucun autre produit interm�diaire du radium n�est d�couvert, il serait plus simple de l�appeler radium E et de renommer radium F le produit �mettant des rayons α (polonium).
E. Rutherford.
Universit� McGill, Montr�al, 24 janvier.
VERSION ORIGINALE EN ANGLAIS
Letters to the editor
Slow Transformation
Products of Radium.
[341] In a recent number of the Philosophical Magazine (November, 1904), I have shown that radium, after passing through four rapid changes, finally gives rise to two slow transformation products, which, on the scheme of changes there outlined, were called radium D and radium E.
These two products can be separated from each other by suitable physical and chemical methods. Radium D, which is the parent of E, gives out only b rays, while E gives out only a rays. It was calculated that D should be half transformed in forty years, and E in about one year. Evidence was also shown that radium D was the active constituent in the radio-active lead of Hofmann, and that radium E was the active substance present in both the polonium of Mme Curie and the radio-tellurium of Marckwald.
Later work has confirmed these conclusions. I have examined the rates of decay of the activity of radium E and of radio-tellurium, and have found them to be identical. Each loses half its activity in about 150 days, instead of the calculated period of one year. The specimen of radio-tellurium was obtained from Sthamer, of Hamburg, in the form of a thin film deposited on a polished bismuth rod. I find that the same value for the decay and activity of radio-tellurium has recently been obtained by Meyer and Schweidler (Akad. D. Wiss. Wien., December I, 1904).
I was, unfortunately, unable at the same time to determine accurately the decay of the activity of polonium. A specimen of polonium (ratio-active(sic) bismuth) had been in my possession for three years, and had during that time lost a [342] large proportion of its original activity. On testing it, the activity was found to have reached a small and nearly constant value. Rough observations, however, wich I had made from time to time indicated that the rate of decay of this polonium was certainly not very different from that of radium E. More accurate experiments will be required to settle the question definitely,� but I think there is little doubt but that their rates of decay will be found to be the same.
Polonium, radio-tellurium, and radium E have very similar radio-active and chemical properties. Each gives out only a rays, and each is deposited on a bismuth plate placed in the active solution. The probable identity of their rates of decay, taken into conjunction with the similarity of their radiations and chemical properties, shows that the radio-active constituent present is in each case the same. We may thus conclude that the active substance present in polonium and radio-tellurium is a decomposition product of radium and is the sixth (or, as we shall see later, probably the seventh) member of the radium family.
The main objection, in the past, against the identity of polonium and radio-tellurium has rested on the statement of Marckwald that a very active preparation of his substance did not lose its activity to an appreciable extent in six month. Unless very special methods were employed, it would be difficult to determine with accuracy the variation of the activity for such very active material. The specimen of radio-tellurium obtained both by Meyer and Schweidler and by myself undoubtedly does lose its activity fairly rapidly.
I have recently examined more carefully the product radium D, and have found strong evidence that it is not a single product, but contains two distinct substances. The parent product, radium D, does not give out rays at all, but changes into a substance which gives out only b rays, and is half transformed in about six days. Unless observations are made on the product radium D shortly after its separation, this rapid change is likely to escape detection. The work on this subject is still in progress, but the evidence at present obtained indicates that the active deposit from the emanation, after passing through the three rapid stages, represented by radium A, B, and C, is transformed into a ��rayless�� product D, which changes extremely slowly. D continuously produces from itself another substance � which may for the time be termed D1 � which is transformed in the course of a few weeks and emits only b rays. This product D1 gives rise to E (polonium).
Since the activity of D1 reaches a maximum value a few weeks after the production of D1 and will then decay at the same rate as D, the conclusion, previously arrived at, viz., that D is half transformed in about forty years, still holds good. The view that radium D is the active constituent present in the so-called radio-lead of Hofmann has been very strongly supported by some experimental results recently obtained by Hofmann, Gonder and W�lfl (Annal. Der Physik, vol. xv., 3, 1904).
They found that preparations of radio-lead continuously produced an a ray product, which could be separated on a bismuth plate. This active product is probably radium E, for they found it lost a large proportion of its activity in one year. They found, in addition, that by certain chemical methods another distinct product could be separated which gave out only b rays, and lost much of its activity in six weeks. This substance is probably the new radium product D1 already refered to.
Debierne recently concluded that radio-lead and polonium were identical, and proposed that the name radio-lead should be dropped in favour of polonium. In the light of the above results, this position is not tenable. There is no doubt that the preparation of radio-lead in my possession, and also that experimented on by Hofmann, contains a dinstinct substance which, as the parent of polonium, has certainly as much right to a name as its offspring. The radio-active substance in ��radio-lead�� has no more connection with lead than Marckwald�s active matter ��radio-tellurium�� has with tellurium. The names both arose because the active matter was initially found associated with these substances.
In order to avoid confusion, I have called the new radium product ��radium D1.�� If no further intermediate products of radium are brought to light, it would be simpler to call it radium E and to call the a ray product (polonium) radium F.
E. Rutherford.
McGill University, Montreal, January 24.