Table 648 e^j 



RADIOACTIVITY 

 GAMMA RAYS 



It is evident from the quantum relation, hv = E + IV, that 7 rays of given frequency 

 will cause the emission of § particles of definite velocities, one for each energy level that 

 can be ionized, from an atom of a given element (including all its isotopes). This has been 

 proved experimentally and has been used in determining the energy of the exciting 7 rays. 

 A discussion of the subject of photoelectron emission by X rays has been given by A. H. 

 Compton. 



When 7 rays pass through thin layers the /3 radiation leaving the layer on the side 

 where the X ray beam emerges is more intense than that on the side where the X ray 

 beam is incident. The asymmetry of this /3 radiation was more marked for light atoms 

 than for heavy atoms ; also for hard than for soft 7 rays. 



Scattering of gamma rays. — When 7 rays are incident on matter 7 rays may be 

 detected on all sides of the piece as if emitted by it. 7 rays so re-radiated were called 

 " secondary " 7 rays. These secondary 7 rays appear to be really a mixture of two types : 

 (1) scattered primary 7 rays; (2) fluorescent or characteristic X rays produced in the 

 atoms of the secondary radiator by high velocity electrons liberated photoelectrically by 

 the primary 7 rays. 



Ishino's values of the mass scattering coefficient for Al, Fe, and Pb are respectively, 

 0.045, 0.042, and 0.034 (cmVgm). The softening of the "secondary" 7 rays is undoubt- 

 edly due to (1) the production of fluorescent radiation which may be in part (Compton) 

 similar to the general " white " radiation emitted by an X-ray tube, and (2) a modification 

 of the true primary scattered radiation. The scattering of 7 rays by thin sheets indicates 

 that the scattering per atom is nearly proportional to the atomic number, and that each 

 electron appears therefore to act as an independent center for scattering whether it is one 

 of a small number of electrons (Al) or one of a larger number (Pb). The scattered 

 radiation on the emergent side is greater in amount than that on the incident side. 



Comparison of gantma-ray sources. — The relative ionizing powers of different types 

 of 7 radiation need to be known if the quantity of any 7-ray emitter is to be determined 

 by comparison with a radium standard. The amount of MsTh 2 in equilibrium with 1 g of 

 Th, e.g., one month after separation of MsThi, gives a 7-ray ionization equivalent to that 

 from 0.524 *X io" 7 g Ra in equilibrium with its 7-ray products. The amount of ThC" in 

 equilibrium with 1 g Th gives a 7-ray ionization equivalent to that from 0.956 X io~ 7 g Ra 

 in equilibrium with its 7-ray products. Since MsThi and Ra are isotopes, chemical 

 separation is impossible, and since the 7 rays compared are of nearly the same quality the 

 detection and estimation of mesothorium impurities in radium by 7-ray measurements 

 (usually used for standardization) is somewhat difficult. Hahn and Bothe have shown 

 how to distinguish between these materials by absorption experiments. Mme. Curie has 

 shown that the ratio of the total heating effect to the 7-ray activity is also characteristic 

 of the proportion of mesothorium in a mixture of the two. 



Smithsonian Tables 



