198 ANNUAL KEPORT SMITHSONIAN INSTITUTION, 19 3 9 



of relativity, plays a very important role in nuclear theory. It 

 states that there is a simple relation of proportionality between mass 

 and energy. If the kinetic and radition energies emerging from a 

 nucleus are measured, the change in mass can be deduced. This 

 affords a very accurate method of extending accurate mass measure- 

 ments beyond the region of the lightest atoms into that of the heavier 

 ones where direct comparison methods are relatively inaccurate. 

 There is also the matter of the angular momentum associated with 

 the particles that are ejected by a nucleus. This spin is apparently 

 conserved just as we know it to be in large-scale phenomena, but it 

 still presents many problems to the nuclear physicist. 



As a specific example we may consider lithium, which is one of 

 the simplest and lightest elements we know, but one upon which a 

 good deal of work has already been done. For the discussion of 

 nuclear reactions the elements are generally laid out in a two-dimen- 

 sional chart of which the abscissa is the number of protons or atomic 

 number and the ordinate the number of neutrons in the nucleus. 

 The sum of these coordinates is the total mass number on an arbi- 

 trary scale ((9 = 16). One type of lithium has three protons and 

 three neutrons — this is the so-called isotope of mass 6 — and the 

 other has three protons and four neutrons, the isotope of mass 7. 

 The bombarding particles at our disposal are neutrons, light and 

 heavy hydrogen, and the helium nuclei. We can also shine very 

 short wave length light, which is called gamma radiation, on the 

 nucleus and that may disintegrate it much as ordinary light liberates 

 electrons from a metal plate in the photoelectric effect. When lith- 

 ium of mass 6 is bombarded by high-velocity protons so that the 

 latter enter the nucleus, the charge and mass number are increased by 

 one unit apiece and beryllium of mass 7 results. This is not one of the 

 known stable atomic species, and we find that a helium nucleus splits 

 off, leaving a residue of mass 3 and charge 2, that is, a heliumlike 

 atom. We do not know this as one of the ordinary atomic species 

 either, and it is likely that some subsequent event occurs to it result- 

 ing in heavy hydrogen, though this process has not as yet been ob- 

 served. One possibility is that the helium nucleus captures one of 

 its two external electrons, decreasing its net charge by one, that is, 

 becoming hydrogen of mass 3. As this is not found in nature either, 

 we are led to assume that it is a very fragile structure which splits 

 up into a neutron and hydrogen of mass 2 upon the slightest 

 provocation.^ 



If the lithium 6 is bombarded by helium nuclei, the neutron 



' Since the preparation of this article it has been found that helium of mass 3 is stable 

 and hydrogen of mass 3 is unstable, decaying into this light variety of helium. 



