60 ROYAL SOCIETY OF CANADA 



IV. E.cperiments on the Absorption and Rejection of fi Rays by Lead. 



A. Measurements on the transmitted rays. 



Experiments were conducted with lead in an exactly similar 

 manner to those on the transmitted rays through tin foil, in order to 

 find the minimum thickness of lead necessary to prevent the emer- 

 gence of any (i or (i secondary radiations from the far side of a ])late 

 upon which the primary fi rays of radium fell. The radium bromide 

 jvas placed as in Fig. 1, and sets of readings were taken of the ionisation 

 in the chamber, — 1st, with the top of the chamber open, and, 2nd, 

 with it covered by lead foil of varying thicknesses, the bottom of the 

 chamber being always closed by a sheet of aluminium foil .0065 mms. 

 in thickness. As before, these readings were taken as the fi rays 

 were deflected downwards into the ionisation chamber, and upwards 

 and away from it by different magnetic fields. 



The sets of readings taken with the opening at the top of the 

 chamber uncovered, and also covered with lead foil screens .241 mms., 

 .482 mms., .723 mms., and .964 mms., in thickness, respectively, are 

 given in Tables VI and VII. From the values of the ionisations 

 given in columns III, IV and V of Table VI, the curves A, B, and C, 

 Fig. 9, w^ere drawn. The curves A', B', and C also shewn in Fig. 9 

 were plotted from the numbers in columns III, IV and V of Table 

 VII. The curves are of the same type as those for the tinfoil, which 

 were fully discussed in Section II A. 



The curve B drawn for a thickness of .723 mms. of lead indicates 

 that ft rays which were deflected by a field corresponding to about 6 

 amperes penetrated this thickness of lead, while the coincidence of the 

 curves C drawn from the values corresponding both to the upward 

 and downward deflections of the ft rays when .964 mms. was the 

 thickness of the lead screen, shews clearly that the ft and also the ft 

 secondary rays could not pass through this thickness of lead. 



As is fully explained in Section III A, the maximum value of the 

 conductivities in the chamber due to ft and ft secondary rays for 

 the different thicknesses of the screens can readily be deduced from 

 the tables given above. These deduced values are given in row III of 

 Table VIII, and a curve representing them is shewn in Fig. 10. 

 From the curve it is evident that a screen of lead .9 mms. in thickness 

 completely absorbed all of the ft and the ft secondary radiations 

 excited in the lead including the most penetrating. 



B. Measurements on reflected ra3's from lead. 



In this set of measurements the arrangement of the apparatus 

 was the same as when the measurements on the reflected rays from 

 tin were taken, the radium being placed vertically above the ionisation 

 chamber. Different thicknesses of lead were placed over the opening 



