NUCLEUS OF THE ATOM — ANDERSON 247 



Tliis difference is sufficient to render the nucleus of the new 

 nitrogen, or radio-nitrogen as it is called, unstable; and it subse- 

 quently disintegrates by the ejection of a positron, after which a 

 stable atom of carbon remains. 



The high-voltage tubes and other devices previously described for 

 speeding up atomic particles have proved very effective for the pro- 

 duction of artificial radioactivity, and a great variety of new radio- 

 active elements have been formed and studied. Plate 2, figure 2 

 shows a photograph of the paths of positrons ejected from carbon 

 after it had been subjected to bombardment by deutons. 



The activity of the artificially stimulated substances decreases with 

 time in a manner similar to that of the natural radioactive products. 

 Most of the artificial radioactive substances so far produced have a 

 half-life — the time required to decrease the activity to half value — 

 which varies from a few seconds to several hours, depending upon the 

 element produced. Recently the emission of negative electrons as 

 well as of positrons has been observed; and, in at least one case, a 

 gamma ray is emitted. 



Although most of the light elements of low atomic weight have 

 yielded to atomic transmutations, the disintegration of the heavy 

 elements by bombardment with electrically charged particles such 

 as protons, deutons, or alpha particles, so far remains unaccom- 

 plished because of the exceedingly strong electrical forces which 

 must be overcome before a charged particle can enter a nucleus. The 

 successful disintegration of the heavy atoms by this means would 

 require the application of exceedingly high voltages, unattainable by 

 present-day technique. 



Within the past few weeks, E. Fermi, working in Rome, has suc- 

 ceeded in inducing radioactivity through bombardment with neu- 

 trons. Working with a large number of elements, he found that the 

 heavy elements yielded as readily as did the lighter ones. Since 

 the neutron has no net electric charge, the intense electric fields sur- 

 rounding the nuclei of the heavy atoms provide no protection what- 

 ever against neutron bombardment. 



So today practically the whole series of the 92 elements has been 

 disintegrated in one manner or another ; and dozens of new isotopes 

 have been formed, the majority of which are radioactive. It now 

 appears quite likely that any element by proper treatment can be 

 prepared in a radioactive condition, and the years to come will un- 

 doubtedly find widespread tasks for them to perform. 



