THE ACTION OF XEUTRONS OX THE RARE EARTH ELEMEXTS 55 



lime of exposure allows us lo calculaie the unknown period oi' decay 

 provided we can assume that all the neutrons absorbed are captured 

 by the nuclei of the absorbing element and that mainly thermal neutrons 

 are involved in both cases. In the oxides investigated, only the nuclei 

 of the rare earth element absorb, for oxygen nuclei capture only a small 

 number of neutrons. Let us consid(M-, for example, the case of scandium. 

 Denote by R^ the observed absorption ralio for equal numbers of scan- 

 dium and dysprosium atoms, and by Rg the ratio of the activities obtained 



after an exposure of N days ; then the half-life of scandium is _L 



Rg 



days. We compared the activity of 66 mgm of scandium and 100 mgm 

 of dysprosium and found after an activation of 24 days an activity 

 ratio of 0.92x10" 2. During this activation time, full saturation of the 

 dysprosium activity was obtained, while the scandium was far from being 

 saturated. For equal numbers of scandium and dysprosium atoms we 

 found an intensity ratio of 0.40 xlO"^. 



To compare the absorbing powers of scandium and dysprosium we 

 inserted in the path of the neutron beam, which had been slowed down in 

 the usual way by a block of paraffin, first, a layer of scandia (590 mgm/cm- 

 Sc) and then a layer of dysprosia (340 mgm/cm^ Dy) and measured the 

 activation of a rhodium foil in the absence and then in the presence of the 

 absorbing layer. The amounts of the absorbing material necessary to 

 reduce the activity of rhodium in each case to 90% of its initial value 

 were calculated to be 300 mgm/cm^ Sc and 43 mgm/cm^ Dy. A more satis- 

 factory way to proceed in comparing the absorbing powers would have 

 been to have used a dysprosium indicator to measure the absorption 

 in dysprosium and a scandium indicator to measure the absorption in 

 scandium, but the small activation of scandium after a few days' expo- 

 sure to neutrons rendered this infeasible. We have, however, applied 

 the last mentioned method to compare the absorption of neutrons in 

 dysprosium, europium, and holmium, as discussed in the next section. 

 The comparison of the absorbing powers of equal numljers of atoms of 

 dysprosium and scandium led to the result that the former absorbed 25 

 times as strongly as the latter. It follows from this result and from 

 the comparison of the activities of the two elements, that the half-life 

 of2iSc is about tw^o months. A similar value was obtained by dtn-ay 

 measurements. 



Strongly absorbing rare earth isotopes forming stable products 



The unusually strong activities of some rare-earth nuclei are to be 

 ascribed to the existence of strong nuclear resonance levels in the nuclei 

 in question, these levels corresponding to energies of slow neutrons 

 abundant in the neutron beam passing through them, and also to the fact 



