598 Mr. J. Chadwick on the 7 Rays 



It will be seen that the amount of excited radiation 

 decreases with the atomic weight of the material. The 

 radiation also becomes softer the lower the atomic weight. 

 The differences between the various elements are, however, 

 not so great as those observed by Gray when the ft rays of 

 radium E are used as the exciting rays. The numbers in 

 the second column represent the energy spent by the excited 

 7 rays in the chamber A, and not the total energy of the 

 rays. Since the softer rays will spend a greater proportion 

 of their energy in A than the harder rays, the differences 

 between the various materials are really greater than the 

 numbers indicate. 



The large difference between the values of fi L and /j, 2 

 indicates that the excited 7 rays differ widely in penetrating- 

 power. The absorption coefficients of the primary 7 rays 

 were measured under identical conditions, and were found 

 to be /x 1 = , 340 cm. -1 and ya 2 = '314 cm. -1 . Thus the excited 

 rays are, on the whole, much softer than the primary rays, 

 although the rapid diminution in the absorption coefficient 

 would show that a small proportion of the excited rays is 

 probably as penetrating as the primary rays. The absorption 

 coefficients could not be found through greater thicknesses as 

 the effect to be measured was too small. 



The above experiments were repeated, using a plate of 

 aluminium 1 cm. thick in place of the lead plate, but the 

 results obtained were practically the same. 



Soddy and Russell* have found that the 7 ray activity of 

 a preparation of uranium X of the same ft ray activity as a 

 preparation of radium C gave only 2 per cent, of the amount 

 of 7 radiation of the latter. Their preparation of UrX was 

 deposited on a piece of platinum. It is shown below that 

 the total amount of 7 radiation excited by the ft rays of RaC 

 when they fall on uranium metal is about 0*3 per cent, of 

 the primary radiation. The amount of excited 7 radiation 

 from platinum will be a little less than this. As the ft rays 

 of uranium X are of about the same average velocity as the 

 average ft ray of radium C, one would expect them to excite 

 about the same amount of 7 radiation. Since in the experi- 

 ments of Soddy and Russell only one half of the ft rays fall 

 on the platinum, it is seen that not more than 10 per cent, 

 of the total 7 radiation of uranium X observed by them can 

 be due to the impact of the ft rays on the platinum. The 

 remainder must arise from the transformation of the atoms 

 of uranium X. It might be expected that the primary 7 rays 

 of uranium X would be of similar penetrating power to the 

 * Soddy & Russell, Phil. Mag. Oct. 1909. 



