206 ANNUAL, REPORT SMITHSONIAN INSTITUTION, 19 38 



into two ct-particles shot out in nearly opposite directions. When 

 boron 11 is bombarded by protons, a carbon nucleus of mass 12 is 

 formed which breaks up in most cases into three a-p articles. The 

 deuteron is in some respects even more effective than the proton as a 

 transmuting agent. When deuterons are used to bombard a com- 

 pound of deuterium, previously unknown isotopes of hydrogen and of 

 helium of mass 3 are formed, while fast protons and neutrons are 

 liberated. The bombardment of beryllium by very fast deuterons 

 gives rise to a plentiful supply of neutrons. Lawrence has shown that 

 the bombardment of bismuth by very fast deuterons leads to the pro- 

 duction of a radioactive bismuth isotope which is identical with the 

 well-known natural radioactive product radium E. Many artificial 

 radioactive elements can be produced, often in great intensity. For 

 example, the bombardment of common salt by fast deuterons gives 

 rise to a radioactive isotope of sodium. This breaks up with a half 

 period of 15 hours, emitting not only fast /3-particles but also 7-rays at 

 least as penetrating as those from radium. 



It may well be that in course of time such artificial radioactive 

 elements may prove a useful substitute for radium in therapeutic work. 

 By these methods also, such intense sources of neutrons can be pro- 

 duced that special precautions have to be taken for the safety of the 

 operators of the apparatus. 



Sufficient, I think, has been said to illustrate the variety and interest 

 of the transmutations produced by these bombardment methods. It 

 should, however, be pointed out that transmutation in some cases can 

 be effected by transferring energy to a nucleus by means of 7-rays of 

 high quantum energy instead of by a material particle. For example, 

 the deuteron can be broken up into its components, the proton and 

 neutron, by the action of the 7-rays from radium or thorium. As a 

 result of the bombardment of lithium by protons, 7-rays of extraor- 

 dinarily great quantum energy as high as 17 million volts are strongly 

 emitted. Bothe has recently shown that these high-energy rays are 

 able to transmute a number of atoms, neutrons usually being emitted 

 in the process. 



Some simple laws appear to hold in all individual transformations 

 so far examined. Nuclear charge is always conserved, and where 

 heavy particles are emitted, so also is energy when account is taken 

 of the equivalence of mass and energy. Certain difficulties arise with 

 regard to the conservation of energy in cases where light positive and 

 negative electrons are emitted during transmutation, and there is still 

 much discussion on this important question. 



The study of the transmutation of matter has been extraordinarily 

 fruitful in results of fundamental importance. In addition to the 

 a-particle, it has disclosed to us the existence of those two building 

 units of nuclei, the proton and neutron. It has greatly widened our 



