RADIOACTIVITY— ARTIFICIAL AND NATURAL 303 



hydrogen, such as ice in which plenty of the hydrogen atoms belong 

 to the isotope H-; or it may be gaseous heavy hydrogen). The high 

 voltage is required, so that the impinging deuterons may override the 

 electrostatic repulsion between the positive charges which they bear 

 and the positive charges of the deuterons waiting in the target, and 

 come into contact with these last. Generally in transmutation, 

 "high voltage" signifies volts by the millions. These particular 

 reactions are, however, among the easiest to produce, and with less 

 than a hundred thousand volts it is quite possible to liberate neutrons 

 at such a rate that their peculiar qualities can be well studied. (One 

 reaction indeed has been detectably produced at 8000 volts, a figure 

 so low that it arouses speculation as to what the course of physics might 

 have been if the second isotope of hydrogen had been discovered say 

 thirty years ago.) 



@ PROTON Q NEUTRON @0 DEUTERON 



®o + ©o — ®oo + © 

 ©o + ®o — - ©©o + o 



Fig. 5 — Scheme of the deuteron-deuteron reactions. 



To explain what is actually observed to happen, I ask the listener 

 to imagine the deuteron as a composite of a proton and a neutron, as it 

 is exhibited in Fig. 5. With this image in mind, one might well expect 

 that when deuterons are hurled with great energy and speed against 

 a plate of matter containing massive nuclei — -lead, for instance— they 

 would be broken in two. This has been sought for but apparently 

 does not happen, showing that we must keep our imaginations under 

 continual check by experiment. What does happen is displayed, for 

 the impacts of deuteron against deuteron, in Fig. 5. It seems that 

 one deuteron is after all broken in two, but only under the condition 

 that either its component proton or its component neutron adheres 

 to the other. Another metaphor: one deuteron snatches either the 

 proton or the neutron away from the other, leaving the abandoned 

 neutron or proton to go free. Both of these descriptions are too 

 figurative, but what is certain is this: from the scene of such impacts, 

 particles of all the four kinds shown to the right of the arrows in Fig. 

 5 are observed to be proceeding. The labels show (what should al- 

 ready be obvious) that the newborn particles of mass 3 are isotopes 



