302 BELL SYSTEM TECHNICAL JOURNAL 



versity of mass and charge are the artificial radioactive bodies now out- 

 standing by far. 



These artificial examples forming now so large and important a 

 group among the radioactive nuclei, I make a digression to speak of 

 some of the transmutations from which they are derived. The art 

 of transmutation is already so huge a subject that the digression must 

 be severely limited if ever we are to come back to radioactivity. I 

 must therefore make only a passing allusion to the fact that the first 

 of the new radioactive nuclei were made by bombarding various light 

 elements with very energetic alpha-particles. Here the second Curie 

 generation must be introduced, for the daughter and son-in-law — 

 Irene Curie and Frederic Joliot — of the first Curie pair were the ones 

 who made this discovery. (It was not their entry into the field of 

 radioactivity, they having already studied natural radioactive bodies 

 for a number of years). 



Returning to Figs. 2 and 3, notice that many of the radioactive 

 isotopes lie just one step to the right of stable isotopes: Li^, Be^", B^^^ 

 Qu^ N^^, O^^, F^", Ne^^ are the examples found in these two pictures 

 alone. It seems as though they might differ from their neighbors 

 on the left — Li'', Be^ and so forth — by possessing an extra particle of 

 mass (approximately) 1 and charge zero. If only one could find such 

 particles roaming freely about in Nature, might one perhaps succeed 

 in adding them to the stable nuclei of lithium and beryllium and 

 boron and the other elements, and so produce these radioactive nuclei.-* 



Such particles may indeed be found roaming about in Nature, but 

 not of their own volition. These "neutrons" — for such is their 

 name — must themselves be set free by the art of transmutation. Free 

 neutrons were first produced by bombarding certain elements with 

 alpha- particles; the discovery was an international one, and its story 

 is interesting, but to keep this digression within bounds I must again 

 content myself with giving the names — Bothe and Becker in Germany, 

 Curie and Joliot in France, Chadwick in England — ^of those who carried 

 it through its consecutive stages from first intimation to triumph. 

 More than a hundred different ways of freeing the neutron are already 

 kno\Yn, but of all this diversity I will take one only, which consists in 

 projecting deuterons against deuterons. 



The "deuteron-deuteron reactions "—D-D reactions for short — are 

 produced by applying high voltage to deuterons (emerging from a 

 discharge-tube containing heavy hydrogen, in which some of the atoms 

 are divested of their electrons and the nuclei are left bare) and then 

 directing them across a vacuum against a target containing other 

 deuterons. (The target may be some solid compound of heavy 



