88 



APPLIED RADIOACTIVITY 



atom occasionally succeeded in penetrating this nucleus. When it did 

 this, a proton was ejected with high kinetic energy. 



For the purpose of this discussion an atomic nucleus is sufficiently 

 specified by giving its atomic number, which is numerically equal to 

 the positive charge of the nucleus, in integral multiples of the proton 

 charge, and the mass number in multiples of the proton mass. Actu- 

 ally, the atomic nuclei do not have masses that are exact multiples 



2 He 4 + 7 N 14 > 8 U + ,11'+ Energy 



, • "\ Collides with 



(o o> 



• ' H ' , 



Nuclear proton 



Atomic projectile 



Alpha particle 



2 Protons 

 2 Neutrons 



Target nucleus 



Nitrogen 



atom 



7 Protons 

 7 Neutrons 



Isotope-oxygen atom 



\ n' 7 



\ 8*J 



\ 8 Protons y 

 "9 Neutrons 



Fig. 11-13. Rutherford's classic experiment. Nitrogen bombarded with alpha 

 particles from RaC. 



of the mass of the proton, but the correction factor is not involved here. 

 In the notation used in describing these disintegration phenomena, the 

 upper right-hand corner of the symbol of the element is reserved for 

 the isotopic mass (sO 17 ) and the lower left-hand corner for the atomic 

 number. 



Since the discovery of the neutron by Chadwick in 1932, the accepted 

 conclusion is that the alpha particle is made up of two neutrons and two 

 protons bound tightly together as in the nucleus of the helium atom. 

 The alpha-particle projectile is designated as 2He 4 . This mass, with 

 its equivalent of 4 proton masses (neglect mass of nuclear electrons), 

 joins the 14 protons of the nitrogen nucleus, which then loses 1 nuclear 

 mass unit. This mass unit appears as an ejected high-velocity hydrogen 

 nucleus iH 1 . The decomposition product has gained 3 mass units and 

 forms a nucleus of 17 mass units. This is identified as O 17 , an atom of 

 oxygen whose nucleus is 1 mass unit greater than normal oxygen (O 16 ). 

 Hence it is an oxygen isotope. 



After this first definite example of the artificial transmutation of an 

 element, many other transmutations were accomplished. By 1932 the 

 Curie-Joliots had discovered that charged and uncharged particles were 



