RADIOACTIVITY— RUTHERFORD 171 



Fermi made another observation of great interest when he found 

 that in the case of some elements, slow neutrons were much more 

 effective in producing transformations than fast ones. Slow neutrons 

 can be readily produced by passing the fast neutrons formed in a 

 transformation through a considerable thickness of hydrogen- 

 containing material, for example, water or paraffin. This aspect of 

 the problem is now under intensive investigation throughout the 

 world, and certain general conclusions have been reached. For fast 

 neutrons, the cross-section of the atom for capture of the neutron 

 varies somewhat from element to element, but is of the order 10"" cm.^ 

 For slow neutrons, however, the cross-section for capture for some 

 elements may be from 100 to 10,000 times greater than for fast neu- 

 trons. For example, cadmium and boron readily absorb slow 

 neutrons and a still stronger absorption is shown by europium and 

 gadolinium. So marked is the effect of the latter that a layer a small 

 fraction of a millimeter thick is almost a complete absorber for slow 

 neutrons. It is natural to suppose that absorption of the neutrons in 

 an element is a sign of its transformation, even though it may not be 

 easy to obtain proof of the exact nature of the transformation. There 

 is now clear evidence, as shown by Moon and others, that some of the 

 slow neutrons which are effective have thermal velocities. Still more 

 remarkable, absorption in some elements seems to take place over a 

 small range of velocities, a result which may indicate that there are 

 very low energy resonance levels in some nuclei. 



The ease with which slow neutrons are able to enter the nucleus of 

 even the heaviest elements has proved of great service. The use of 

 the neutron as a projectile has disclosed the existence of 50 or more 

 ephemeral elements of the radioactive type, representing unstable 

 varieties of isotopes of the elements, and has thus much extended our 

 knowledge of atomic species. Most of these unstable atoms appear 

 to be transformed directly into a stable atom, but in the case of the 

 heavier elements it is quite likely we may be able to produce radio- 

 active atoms which may break up in a succession of stages like the 

 atoms of uranium and thorium. Professor Hahn and Fraulein 

 Meitner, and also M. and Mme. Joliot-Curie conclude that a radio- 

 active element of this type is formed by the action of slow neutrons 

 on thorium, but the evidence is yet not complete. It will be a matter 

 of great interest if we are able to create in this way new radioactive 

 families for study. 



My time is too limited to discuss with any detail the large number 

 of new types of transformation which can be brought about by using 

 fast protons and deuterons as projectiles. In some cases, the element 

 formed by the capture of the incident particle breaks up into frag- 

 ments; in others, a new stable element is formed, and in others a 

 radioactive element. The use of deuterons as projectiles has disclosed 



