produced by Partly-absorbed Gamma Rays. 437 



gives the absorption coefficients, /x, of the 7 rays in lead as 

 measured by the falling-off of the ionization due to the 

 secondary ft rays projected from the lead radiator, which 

 ionization follows the law I=Ce~^, where x cm. is the 

 distance the 7 rays travel through lead. The first column 

 gives the thickness in millimetres of the aluminium screen, 

 Aft, over the electroscope through which the secondary 

 ft rays pass. The second column gives the range of x from 

 the point where the logarithm-graph becomes straight to the 

 point where measurements cease to be reliable. 



It will be noticed that the values of ja agree well among 

 themselves, and also agree roughly with that found directly 

 by Rutherford and Richardson * and by Richardson f for the 

 hardest 7 rays of RaC. 



The most important deduction to be made from this result 

 is that there is no trace of 7 rays more penetrating than 

 those whose absorption-coefficients and frequencies are 

 already known, even in the production of the hardest ft rays. 

 Indeed, the values obtained for /jl may be said to amount to 

 a. proof that a secondary ft particle derives its energy, not 

 from 7 radiation whose frequency is so great that one 

 quantum suffices to account for this energy, but from 

 7 rays of considerably lower frequency, which must there- 

 fore impart several of their quanta to each ft particle. For 

 instance, if we consider the ft rays that pass through '8$ mm. 

 of aluminium, which are produced by y rays having an 

 absorption-coefficient, //,= *67 cm. -1 in lead, we find, "using 

 Varder's % results, that these ft rays must have energy at 

 least equal to 6'2 x 10 13 £, where e is the charge of an electron 

 in electromagnetic units ; again, the absorption-coefficient 

 *67 cm. -1 in lead corresponds to a frequency for which 

 }iv=l'2 x 10 13 <?. This number is deduced by Bragg' s law, 

 i/GCyL6" 2/5 , from the absorption-coefficient in lead, 1*5 cm. -1 , 

 ascribed with much probability by Rutherford § to the 7 rays 

 of RaB whose wave-length is T37 x 10~ 9 . Thus no less 

 than 5 quanta are needed to account for the energy of these 

 ft rays. 



It should also be remarked that the results support 

 Rutherford's conclusion that the preponderance of the 

 harder ft radiation is due to a single 7 radiation. This is 

 shown by the fact that the exponential law begins to hold 

 good for smaller thicknesses of lead than when the absorption 



* Rutherford and H. Richardson, Phil. Mag. xxv. p. 722 (1913). 

 t H. Richardson, Roy. Soc. Proc. A. xci. p. 398 (19L5). 

 t Varder, Phil. Mag. xxix. p. 725 (1915). 

 § Rutherford, Phil. Mag. xxx. p. 356 (1915). 



