74 ADVENTURES IN RADIOISOTOPE RESEARCH 



Comment on papers 4 and 5 



Usually the radioactive indicator must be added to the element the atoms of 

 which are to be traced. It is, however, also possible to produce the radioactive 

 tracer in situ by bombarding the sample with a neutron stream or other energy- 

 rich radiation. 



In contiast to present days very few people disposed of rare-earth elements 

 before \^'orld War II. Among those was my friend Professor Luigi Rolla, 

 33rofessor at the University of Florence. We used to analyse his samples by 

 making use of the method of X-rays analysis described in the comment to 

 papers 3 and 15. After preparing a few kilograms of gadolinium oxide he 

 wished to find out whether or not his samples were free from EujOg, the most 

 likely impurity present in Gd203. At that date we had no X-raj' spectrograph at 

 our disposal and in order not to disappoint Professor Rolla we tried t6 answer 

 the above question by exposing 50 mgm of his sample in a paraffin block to the 

 effect of neutrons emitted by a mixture of 600 mgm radium and beryllium 

 and 300 ]\Ic of radon and beryllium. Neutron sources were placed in the 

 paraffin block to obtain slow neutrons which are strongly captured by europium 

 producing a radioactive europium isotope. They are strongly absorbed by gadoli- 

 nium as well, their absorption leading, however, to the production of stable 

 gadolinium isotopes and not to a radioactive isotope of this element ; the latter can 

 only be produced by more intense neutron streams than applied in our expcr-iment . 

 The presence of an activity in RoUa's exposed samples decaying with a half-time 

 period of 9.2 hr indicated the presence of some europium in his preparations. 

 To carry out a quantitative analysis we added to a known amount of pure Gd203 

 (obtained from the great rare-earth chemist Auer von Welsbach) known amounts 

 of pure EujOg (also obtained from him). The comparison of the activity of RoUa's 

 samples with those of these standard preparations lead to the result that Rolla's 

 purest gadolinium oxide sample contained 0.40, his least pure sample 0.96 per 

 cent of europium oxide. 



We had already previously, as described in paper 4, applied this method of 

 activation analysis in the determination of dysprosimn present inyttrivrm samples. 

 The determination of europiuni in gadolinium is unsurpassed in its simplicity 

 and sensitivity. Europium being the element which can be determined with 

 the greatest sensitivity by activation analysis. We A\ere thus forttrnate to be 

 faced with the task of applying this newly introduced method in a case 

 which proved later to be the most favourable one. The modest neutron flux 

 emitted by our radium-beryllium sources allowed not less than 0.01 per cent 

 of europium to be determined. By making use of the neutron flux of the cyclotrorr 

 Seaborg and Livingwood could determine 6 p. p. m. of gadolinium in iron by 

 activation analysis and after the availability of pile-emitted netrtrons of great 

 density such small amounts of europium could be determined as 10^^* mgm 

 In the determination of gadolinirrm we availed ourselves of the very high absorb- 

 rng power of tliis element for slow neutrons, thus of an absorbtion method. 



References 



Seaborg arrd LivrNGWOOD (1938) J. Awev. C'hcm. Soc. 60, 1784. 



