474 Scientific Proceedings, Royal Dublin Society. 



(2) The emanation is assumed to be confined to a plane parallel to the 

 face of the box in contact with the skin. 



(3) The activity of a parallel beam of rays which penetrate the box 

 is assumed to be a linear function of the extra distance traversed. This will 

 not introduce a large error for distances up to 10 cms. of tissue if a suitable 

 absorption coefficient is chosen. 



(4) Scattering is neglected [as in previous calculation]. 



Let r be the radius of the box, t its thickness, h the distance from the 

 inner face of the box to the radioactive layer, and / the surface-density in 

 millicuries per sq. mm. ; then obviously the activity at an axial point a mm. 

 below the surface is equal to 



[ '■ [K- {kit + kiCi) sec 6] 2irlxdx 

 J„ (b + t + ay sed'e ' 



where 



tan 



b + t + a' 



and 7v', kj, and ^2 are constants defining the absorption in brass and tissue. 

 Hence 



Activity = 27r/ [K - (ht + ha) sec 6] tan Odd 

 ■J 



= 2TrI [K logeSec <p - (k^t + ha) (sec </> - 1)], 



where 



tan d) = :; ; . 



^ + t + a 



This expression can be evaluated rapidly and with sufficient accuracy by 

 means of a slide-rule, provided that (p — the angle subtended by the radius of 

 the radioactive disc at the given point — is not too small. For small values 

 of (jt it is best to substitute 



tan'^ tan*^ 



2 4 



and 



tan" (j) tan" ^ 



for logc sec (p, 



2 g for (sec </. - 1). 



For very small values the expression reduces to 



7r/tau- (j> [K - (kit + ha)]. 



From inspection of the curves in fig. 2, and consideration of the known 

 rate of absorption of the complex y rays from HaB and BaC, we find 

 IC = 10o6, h = 032, and h = 0-04 (assuming the mass coefficient of 



