310 BELL SYSTEM TECHNICAL JOURNAL 



such impact the neutron loses some of its energy. The point about 

 choosing water or paraffin is that they are rich in hydrogen and 

 consequently full of protons, and the elastic impacts of the neutrons 

 against these entail a greater average loss of neutron-energy per 

 collision than do impacts against any other nuclei.^ There is good 

 reason to believe that most of the emerging neutrons have energies 

 no greater than those which the atoms of the water or the paraffin 

 possess by virtue of their thermal agitation. These are the neutrons 

 which are most effective in converting stable into radioactive nuclei 

 by letting themselves be captured. 



Even yet I have not mentioned all of the ways in which radioactive 

 substances can be and are being made. Time does not suffice for 

 commenting on the others, but some are exhibited in Fig. 11, which 



24 25 26 27 28 29 30 31 

 '2 Mg O O^O 



13 Al 



14 Si 



15 P 'O 

 Fig. 11 — Various ways of making the radioactive nucleus Al^^ 



displays all of the stable but only one of the unstable isotopes of the 

 four elements numbered 12 to 15. This one radioactive type, which 

 I have tried to make more conspicuous by leaving out the rest, is the 

 isotope 28 of aluminium — one of the few elements, which, very con- 

 veniently for physicists, has one stable isotope only. The arrows 

 converging onto the star show the different ways in which AP^ is made. 

 The two coming from the left signify that it is made by adding a 

 neutron to the stable isotope Al^^; there are two of them, because the 

 Al^^ nucleus can either absorb a slow free neutron or annex the neutron 

 from an impinging deuteron, whichever it has the opportunity of 

 doing. The arrow slanting downward from the left signifies that Al^^ 

 can be made by bombarding magnesium with alpha-particles; some 

 of these are absorbed by nuclei of the isotope Mg^^ which thereupon 

 at once emit protons. The arrow slanting upward from the right 

 signifies a process in which phosphorus is bombarded by neutrons 

 (fast ones, in this case!) and some are absorbed by the nuclei P^^ 



* Energy-transfer between an initially-moving and an initially-stationary elastic 

 sphere is greatest when the latter is of the same mass as the former, and for protons 

 and neutrons this equality of mass is realized within 0.1 per cent. 



