170 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1936 



covery by Chadwick in 1933 of great significance. The element 

 beryllium when bombarded by a-particles does not emit protons but 

 a new type of particle of mass about 1 and zero charge called the 

 neutron. This new particle has remarkable properties, since, ow- 

 ing to its absence of charge, it can pass freely through the structure 

 of atoms. Occasionally, it collides elastically with a nucleus, which 

 is set in swift motion, but sometimes it enters a nucleus and is cap- 

 tured by it. The marked efficiency of the neutron in producing 

 transformations in nitrogen, oxygen, and other light elements was 

 early shown by the experiments of Feather and Harkins, and, as 

 we shall see, has led to very wide developments in the last 2 years. 

 Before, however, discussing these advances, I must refer to another 

 discovery of outstanding importance made by M. and Mme. Curie- 

 Joliot in 1933, in which they showed for the first time that veritable 

 radioactive bodies could be artificially created by the bombardment 

 of certain elements by a-particles. Before this observation, it had 

 been supposed that a stable element or elements were always pro- 

 duced as the result of the atomic explosion. They observed that 

 when boron was bombarded by a-particles, an unstable element was 

 produced which broke up with the emission of fast positive elec- 

 trons and behaved exactly like a radioactive body of half period 10 

 minutes. This radioactive body had the chemical properties of 

 nitrogen and the scheme of transformation as given below : 



'IB + tHe -> '^,N -f neutron 

 and '^N -> ^IC -f positron 



Similarily they found that bombardment of aluminum gave rise to 

 radio-phosphorus of half period 3.2 minutes, which also broke up 

 with the emission of positrons. The formation of radioactive bodies 

 in this way is of great interest, and the appearance of the positron in 

 these transformations is very unexpected. 



It was soon shown that artificial radioactive bodies could be pro- 

 duced in various elements not only by bombardment with a-particles 

 but also by protons, neutrons, and deuterons. In particular, Fermi 

 and his collaborators showed that neutrons were exceedingly effective 

 in producing radioactive bodies by bombarding the heavier elements, 

 and more than 50 of these radioactive bodies were soon discovered, 

 each with a characteristic period of decay. In contrast to the radio- 

 active bodies produced by the a-rays in light elements, in the case of 

 the heavier elements, the radioactive body emitted during the trans- 

 formation not positive but negative electrons. In a number of cases, 

 the chemical properties of the radioactive body have been determined 

 and the scheme of transformation made clear, but obviously time 

 will be required to make sure of the process of transformations in 

 elements which consists of a number of isotopes. 



