THE ACTION" OF NEITROXS OX THE KARE EAHTH ELK.MEXTS 59 



l)oor absorber, its absorbing power being only 4 per cent of that of 

 cadmium and 0.3 per cent of that of gadolinium, if Ihe absorption of 

 equal numbers of atoms of the different elements are compared. In 

 Table 6 are given the relative intensities of the activities produced in the 

 rare earth elements by neutrons that have been slowed down by large 

 amounts of paraffin wax. We are still investigating the intensiti(>s 

 obtained under the action of fast and semi-fast neutrons and the possible 

 existence of resonance levels. 



Comparison belween ihe effect of neutrons on rare earlh elements and 

 other elements 



As is shown in this paper numerous radioactive isotopes of the elements 

 of the rare earth group are formed under the action of neutrons, a result 

 which was to be expected from the known existence of a large number of 

 stable isotopes of these elements. Thus the reactions of neutrons with 

 the rare earth elements show the same typical features as their reactions 

 with elements of lower and higher atomic number. The most remarkable 

 feature is perhaps the comparatively frequent occurrence of pronounced 

 resonance phenomena, which phenomena are much commoner among 

 the rare earth elements than in any other part of the periodic system. 

 This fact may be considered as a simple consequence of Bohr's 

 theoretical considerations on neutron capture, since it would be 

 expected that the distribution of resonance levels would be an especially 

 close one in this region. In fact the product of the number of nuclear 

 particles multiplied by the binding energy of a neutron in the nucleus 

 reaches a maximum in the domain of the rare earth nuclei on accouni of 

 the circumstance that Ihe Ijinding energy for higher particle numbers 

 decreases considerably until — for the natural radioactive bodies — it 

 has fallen to about half its maximum value. The more frequent occurrence 

 of resonance capture in processes leading to the formation of stable 

 isotopes, than in those giving radioactive isotopes is also in conformity 

 with general experience and is easily explained by the theoretical con- 

 siderations mentioned above since the distribution of levels will 

 ])e much closer in the former case on account of the fad that the binding 

 "nergy is considerably larger in processes of this kind 1han in those 

 leading to the production of unstable isotopes. 



The use of neutrons in analytical chemistry 



The usual chemical methods of analysis fail, as is well-known, for mosi 

 <)[ the rare earth elements and have to })e replaced l)y spectroscopic, 

 X-ray, or magnetic methods. These methods can now be supplemen- 



