RADIOACTIVITY— ARTIFICIAL AND NATURAL 309 



which this instrument has conferred upon the scientific world, over 

 and above those which radium has already granted. 



Our digression now ends, and we return to the artificial radioactive 

 substances, being now equipped with know^ledge as to how these — or 

 rather, many of them — ^can be made. As I said earlier, many radio- 

 active isotopes differ from existing stable isotopes only in possessing 

 an extra neutron in the nucleus; and this extra neutron can be supplied 

 to the stable nucleus, combining it and converting it into the radio- 

 active type. I have just exhibited one way in which the extra neutron 

 may be, and often is, supplied. In the first-mentioned of the deuteron- 

 deuteron reactions, a neutron is taken away from one of the deuterons 

 by the other, which latter is thus converted from H- into H^ There 

 are many stable isotopes, of many elements, which are able to take 

 away neutrons from impinging deuterons in this manner; a recent list 

 gives no fewer than fifty. The resulting nucleus-types are not in 

 every case radioactive; several are stable, including H-^ itself (at least, 

 no one has yet discovered evidence that H^ is unstable, though there 

 are doubts about it). Most however are radioactive. The reactions 

 in question are known as {d, p) reactions, in allusion to the fact that 

 deuterons enter the target and protons spring out. One might imagine 

 that the deuteron consists of a proton leading along a neutron, which 

 it pushes into the nucleus which it strikes, itself continuing its career 

 as a free particle. 



Neutrons, however, do not have to be escorted into nuclei by 

 protons; those which are already free, such as the ones which are 

 released in the second of the D-D reactions, are quite well able to 

 creep in themselves and make themselves permanently at home. 

 My use of the verb "creep" is not entirely fanciful, for the slower the 

 neutrons are moving as they approach a target, the better their chance 

 of entering its nuclei. Those fresh from their origin in reactions of 

 transmutation are usually moving much too rapidly to be able to come 

 to a halt in a nucleus — or to be liable to capture, whichever way of 

 putting it one may prefer. It is necessary to interpose, between the 

 source and the target, a block of parafifin or a can of water several 

 inches thick. If the source consists of a natural radioactive substance 

 bombarding another element with alpha-particles and thus releasing 

 free neutrons, the two may be mixed with each other and enclosed 

 in a capsule which is then embedded in the centre of a parafifin sphere 

 or immersed in water. As the neutrons make their way out, they 

 collide again and again with the nuclei of atoms in the parafifin or 

 the water, and these recoil from the impacts. It is not, however, 

 their recoiling which is now of importance, but the fact that at every 



