914 Mr. F. P. Slater on Excitation of y Madiatio?i 



of the radiation under consideration, the probability of an 

 u particle from the enanation being effective is at the most 

 1 in 6000. 



This is obtained from the ratio of the initial intensity of 

 the lead radiation to the final equilibrium intensity of the 

 7 rays from radium 0. It is assumed that the initial 

 intensity arises from the impact of the emanation a particles 

 only, which is very nearly true as pointed out previously. 

 When a particles fall normally on matter of thickness t and 

 containing n atoms per unit volume, the probability m of 

 entering an atom within a distance p of its centre, dis- 

 regarding the forces present in regions close to the nucleus, 

 is given by 



m = irp 2 nt. 



The expression nt can be obtained from the weight of lead 

 per unit area which will completely stop the a particle from 

 radium emanation. This weight is given by Marsden and 

 Richardson *. Substituting for m the probability obtained 

 above from experimental data, the value cf p obtained is 

 1'OxlO -12 cm. approximately- The upper limit of the 

 radius of the nucleus of a heavy atom like lead is given by 

 the closest distance an a particle can approach, and was 

 found by Marsden and Qeigert to be 3'0 x 10" 12 cm, 

 for gold, and the value for lead will be a little greater. 

 Tims, since the a particle would approach within 1*0 x 

 10 ~ 12 cm. of the centre of an atom, if there were nothing in 

 the way, in order to give the probability found experi- 

 mentally it must be aimed directly at the nucleus. Some 

 information as to the nature of these collisions can be 

 obtained from Rutherford's % formula for the fraction dm of 

 the number of a particles scattered through an angle 



dm = ir/4i nt . b 2 cot 2 <£/2, 



where nt is the same as in the previous consideration and 



2N<?E 



b= ;r, where ~Ne is the central charge of the lead 



mw ° 



nucleus, E the charge on the a particle, m its mass, and u its 

 velocity taken as 1*73 xlO 9 cm. sec. for the emanation 

 a particle. "When the probability 1 in 6000 is substituted 



* Marsden and Kichardson, Phil. Mag. xxv. p. 184 (1913). 



t Phil. Mag. xxv. p. 604 (1913). 



% Kutherford, Phil. Mag. xxi. p. 669 (1911). 



