60 ADVENTURES IN RADIOISOTOPE RESEARCH 



ted by the application of neutrons to analytical problems by making 

 use both of the artificial radioactivity and of the great absorbing power 

 of some of the rare earth elements for slow neutrons. 



Qualitative analysis with the aid of artificial radioactivity is based 

 on the determination of periods of decay. All rare earth elements have 

 half-lives varying from a few minutes to a few month, so they can all 

 be measured conveniently. The period of decay of 2.5 h., for example, is 

 completely characteristic of dysprosium and is an unambiguous indication 

 of its presence in the sample investigated ; as little as 0.1 mg can be 

 determined without difficulty. We used the method of artificial radio- 

 activity to determine the dysprosium content of yttrium preparations. 

 The procedure was the following: we mixed 0.1%, 1% etc. of dyspro- 

 sium with neodymium oxide, the latter being chosen because it is one 

 of the cheapest rare earth elements, having a low neutron absorbing 

 power as has yttrium, and determined the intensity obtained. The yttri- 

 um sample to be investigated was then activated under exactly the same 

 conditions, and a comparison of the dysprosium activities obtained 

 gave 1% as the dysprosium content of the yttrium sample. 



Another very beautiful analytical method is based on the very different 

 absorbing powers of the different rare earth elements. A sample, 5 mgm 

 of which spread over 1 cm^ absorbed a quarter of the slow neutrons 

 falling on it, could be identified at once as gadolinium, no other element 

 having so high an absorbing power. 



Unlike the method of artificial radioactivity, the absorption method is 

 limited in its application by the fact that the absorption measure is the 

 sum of the absorptions of the different elements present in the sample. 

 This limitation is, however, largely due to the fact that our knowledge of 

 the absorption of neutrons and still more our devices for producing 

 neutrons of different energies are only in an embryonic state. The absorb- 

 ing powers of different nuclei depend to a high degree on the energy of 

 the neutrons in question and the future development of our knowledge of 

 neutron absorption will presumably make it possible to apply absorption 

 methods of neutron analysis of great simplicity and reliability. This 

 method of analysis, as also that based on periods of decay, gives a direct 

 means of identification of the nuclei involved ; this distinguishes them 

 from all other analytical methods, chemical, spectroscopic, X-ray, and 

 magnetic, which are based on the investigation of the electronic proper- 

 ties of the atom in question. 



Effect of neutrons on minerals containing rare earth elements 



Many of the rare earth minerals, because they are products of residual 

 magmatic crystallisation, contain rare earth elements, thorium, and 

 uranium, along with beryllium and other light elements. The las1 



