THE ACTION OF NEUTRONS ON THE RARE EARTH ELEMENTS (51 



mentioned (>lement is I'ar the most effective neutron source under 

 Jjombardnient with a-particles or with the y-rays emitted by uranium, 

 thorium, and their disintegration products ; the nuclei of other elements, 

 such as lithium, boron, magnesium, aluminium etc. are much less 

 effective.* Jn minerals containing large amounts of strongly capturing 

 rare earth elements, the neutrons produced in the mineral or in its 

 surroundings are absorbed to a large extent in the element in question. 

 The mineral gadolinite, for example, contains about 50% of rare earths, 

 of which according to Goldschmidt and Thomassen** up to about 

 15% is gadolina ; this mineral often contains, too, other light elements 

 including considerable amounts of beryllium, about 0.3% of thorium, 

 and some uranium as well. 1 gm of thorium and its disintegration pro- 

 ducts produces up to 10^ neutrons per year or in all 10^^ neutrons since 

 the formation of the minerals. If these neutrons are all absorbed in 

 1 kgm of the mineral in question and are absorbed primarily by the gado- 

 linium content, lO^^ gadoKnium atoms will be formed having an atomic 

 weight one unit higher than before the absorption. As 1 kgm gadolinite 

 contains about 10-'^ of gadolinium atoms the equivalent weight of gadoli- 

 nium will increase during that long span of time by but one unit in 

 the fourth decimal place. 



While this result is only a very rough estimate it suffices to demonstrate 

 that some of the rare earth elements which primarily form higher stable 

 isotopes by capturing neutrons, increase in equivalent weight as time 

 proceeds. Dysprosium on the other hand when decaying forms holmium, 

 holmium forms erbium etc.; the process in such cases leads to an increase in 

 theamountsof rare earths of higher atomic number and to a correspond- 

 ing decrease in the amounts of those of lower atomic number. Such 

 behaviour is not confined to the rare earth elements ; during their 

 presence in the earth's crust many elements heavier than zinc will 

 undergo increases, though small ones, of their equivalent weights oi 

 of their abundance relative to the lighter elements. The first named 

 behaviour is shown primarily by even and the last named by odd ele- 

 ments, because elements having an odd atomic number have always a 

 few isotopes only so that the consecutive mass numbers are not filled by 

 stable isotopes and the formation of radioactive isotopes through neutron 

 addition is possible. In the case of several even elements like cadmium, 

 tin, gadolinium, osmium, mercury, lead etc., a long series of consecutive 

 mass numbers are filled by stable isotopes so that the capture of neutrons 



* \\'o compared the activities obtained when dysprosia \\as bombarded with 

 iH'viiions of a bciylhum-radon and a magnesium-radon source in the presence 

 of large amounts of paraffin wax and the figures obtained were as 100 : 1. 



** V. M. Goldschmidt and L. Thomassen, Oslo Viclskcip Scl-skujJets Skrifter I, 

 Xr. 5, S. 44 (1924). 



