^8 



ADVENTURES IN RADIOISOTOPE RESEARCH 



Table 4. — Absorption of Slow Neutrons in Rare Earth Elements 

 (Amount necessary to reduce the activity of the indicator Vjy ten per 

 cent of that observed after pa.ssage of tlie neutrons througli a "thick" 



layer) 



highest absorbing power ; it is indeed, as has already been shown b}' 

 Dunning, Pegram, Fink, and Mitchell (8), the strongest known 

 absorber of slow neutrons. In view of the very strong absorbing power 

 of gadolinium great care must be taken in interpreting the results of 

 absorption measurements on rare earth preparations which might con- 

 tain traces of gadolinium. The presence of less than 1/2 per cent of 

 gadolinium in erbium, for example, would suffice to explain the wdiole 

 absorption shown by erbium. As europium is often contaminated with 

 gadolinium we used various preparations of europium to compare the 

 absorption in europium and dysprosium. One of the preparations was 

 kindly given us by Prof. Prandtl and was entirely free of gadolinium ; 

 it gave a value only slightly lower than the other specimens investi- 

 gated. 



The high values found by different observers for the absorbing power 

 of yttrium are clearly due to the presence of impurities in the prepar- 

 ations used. According to Amaldi and his collegues (1) the absorbing 

 power of yttrium is 70 per cent of that of cadmium, and Dunning, 

 Pegram, Fink, and Mitchell (8) give 39 per cent ; whereas using very 

 pure preparations as described on page 48 we find that yttrium is a very 



Table 5. — The Relative Activities of the Rare Earth Elements 



