530 Table 647 



RADIOACTIVITY 

 GAMMA RAYS 



Nuclear analysis. — Analysis of nuclear 7 rays show evidence for energy levels in 

 the nucleus analogous to those in the extra-nuclear structure as found by X-ray analysis. 

 When instability arises in the nucleus an electron occupying a level of higher energy falls 

 to a level of less energy, the excess energy being emitted as a 7 ray ; since several changes 

 of this kind are possible, 7 rays of several different but definite frequencies may be emitted 

 from this nucleus ; further, different groups of frequencies may be emitted from different 

 individual nuclei. Some of the 7 rays cause (photo-electric) emission of electrons from 

 various extra-nuclear levels, thus producing the /3-ray lines in the /3-ray spectrum, and 

 rearrangement of the extra-nuclear electrons produces the 7 rays which correspond in 

 frequency to characteristic X rays. The nuclear electron finally arrives in a stationary 

 state in which it is not permanently stable and it flies out from the nucleus. The nuclear 

 electrons, one per atom disintegrating, thus leave the atom with different energies and 

 form the continuous /3-ray spectrum. 



The absorption of 7 rays by gases has not been studied at all exhaustively. Chadwick 

 investigated the absorption of the 7 rays from radium, i.e.. from RaC, in air and in CO2 

 by varying the gas pressure, and in air by varying the distance from the source. Hess made 

 measurements in air by varying the distance. Chadwick's value for fi in air, reduced to 

 atmospheric pressure and 15 C, is 6.0 X io~ B /cm and Hess' value is 4.47 X io~ 5 cm. 



Ahmad and Ahmad-Stoner find that the absorption coefficient per atom can be ex- 

 pressed as the sum of two terms, aZ -f- bZ*, which corresponds to a similar expression for 

 the absorption of X rays, aZ +, P\ 3 Z\ the first term representing scattering and the second 

 term true absorption. 



Ionization by gamma rays. — With the development of the theory of atomic structure 

 by study of X rays and 7 rays, of ionizing potentials, and by applications of the quantum 

 theory, views on ionization by 7 rays have become more definite. When an X ray or a 

 7 ray traverses matter its energy hv may be absorbed (hv = E + W), an electron requir- 

 ing energy W to remove it from the atom being ejected with residual kinetic energy E. 

 Such an electron has generally been called a secondary /3 ray. It in turn may react with 

 another atom, losing energy equivalent at least to the ionizing potential of a particular 

 energy level in the atom ionized, repeating the process until its energy is dissipated, and 

 leaving electrons and positive ions in its trail. Each of the ejected tertiary electrons if 

 possessing sufficient energy, loses energy in the same manner. 



Moseley-Robinson's values for the total number of pairs of ions produced per sec. in air 

 at n.p.t. by 7 rays from quantities of RaB and RaC in equilibrium with 1 g of Ra are 

 0.84 X 10" and 11.34 X I0 " respectively. From Chadwick's value for the coefficient of 

 absorption in air the mean " range" is i//t= 1.6 X io 4 cm. This gives as a mean value 

 7.5 X io 10 pairs of ions/sec./cm of path in air for all the penetrating 7 rays from 1 g of 

 radium in equilibrium, and, taking one 7 ray per atom of RaB and RaC disintegrating 

 (Kovarik), this means about one pair of ions per cm of path in air for each penetrating 

 7 ray. 



Smithsonian Tables 



