JoLV — On ihe Local Application of Radium in Tiierapeutics. 293 



ordiiiates 0"3 and 1'3 to be about 9'0 units. The loss has, therefore, been 

 rather more than 23 per cent. 



(2). We assume the tumour to be 11 cms. in diameter. The area defining 

 the ionisation from to the radial limit is now 7*2 units. With screening by 

 0'3 cms. of lead the area is 5"2 units. The loss is in this case 29 per cent. 



These figures, we may, of course, also say, define the gcnn, if for the lead 

 screen we substitute one just sufiicient to realize the conditions under which 

 the curve is drawn ; that is if we enclose the radium or emanation in such a 

 tube as will just sufiioe to cut off all those rays of Ra B for which in 

 aluminium the coefficient attains the value 40. 



We have now to consider the beta rays. Such a screen will permit only 

 a small part of the beta rays to pass through. The value of n for these rays 

 in aluminium varies from 90 to 13.' The softer beta rays proceed, as in the 

 case of the gamma rays, from Ra B. Most of tlie beta rays from Ra C are 

 much more penetrating. Tlie velocities of tlie beta radiation from these two 

 substances have lately been analysed by Rutherford and Robinson (" Phil. 

 Mag.," October, 1913). From the coefficients just cited one millimetre of 

 aluminium will transmit 28 per cent, of the fastest beta rays. As these 

 constitute a part only of the total beta radiation, it may be concluded that a 

 screen equivalent to one millimetre of aluminium will suffer only an 

 unimportant percentage of the total beta rays to pass, and will stop, as 

 already seen, practically all those soft gamma rays for wliich [i = 40. 



We have so far considered the mean loss of ionisation within the tissues 

 due to the presence of a lead screen O'S cms. thick. The object of the screen 

 is to reduce the intensity of the radiation near the tube. It is easy to 

 estimate the diminution of intensity. In the absence of a screen other than 

 one which cuts oS the soft gamma and beta rays, the intensity near the tube 

 is given by the ordinate to curve A at 0. This is 100. If 0'3 cms. of lead are 

 interposed, the intensity falls, as shown by the curve, to about 56. 



Now it is evident that a similar reduction of intensity near the tube 

 would be obtained if the quantity of radium in the tube was diminished in 

 the ratio of these figures. We might then dispense with the necessity of 

 screening off some 25 to 30 per cent, of the rays, and at the same time secure 

 immunity from too intense effects near the tube. But if we adopt this 

 expedient, the average intensity of radiation in the morbid tissues becomes 

 insuflacient. We must, then, multiply tlie number of tubes if the illumination 

 is to be sufficient and screening avoided. 



1 " Practical Measurements in Radioactivity " : Makower and Geiger ; Longmans. 1912. 



