Protective Methods in Radiology 277 



daily tolerance dose of 0.2 r would, therefore, be received in just 

 over 3 minutes, which is about the time taken on one patient only. 

 Hence the need for a protective screen on the side of the couch. 

 In the case of X-ray therapy, the intensity of the scattered radia- 

 tion at 1 meter to the side of a patient, who is exposed to 200 

 kilovolt X-rays from a tube run at 30 milliamperes and having 

 a filtration of 0.5 millimeter copper, is about 250 X 10'^ r per 

 second, corresponding to a dose of 0.2 r in 80 seconds. 



The absorption of direct and scattered X-rays and gamma 

 rays in various materials has been determined experimentally by 

 workers in many countries. For direct X-rays excited at volt- 

 ages up to 5 million volts and for radium gamma rays, theoretical 

 values have also been obtained ^^ for absorption in lead and for 

 the lead equivalents of barium concrete. 



From a knowledge of the outputs of X-ray tubes, working 

 under various conditions of excitation, and from a knowledge of 

 the degree of absorption of the rays in lead, it is a simple step 

 to calculate the thicknesses of lead required to reduce the radia- 

 tion at any point to the tolerance amount. Binks - has prepared a 

 simple nomogram, relating kilovoltage, milliamperage, distance, 

 and the amount of lead protection. By means of this, it is possible 

 to find the amount of lead required to give adequate protection 

 for any tube voltage between 200 kilovolts and 3 million volts, 

 for any tube current between 0.5 and 30 milliamperes, and for 

 any distance from the tube between 0.5 and 10 meters. A similar 

 nomogram has been prepared - for the determination of lead 

 protection against radium gamma rays. The corresponding pro- 

 tective thicknesses of other materials, such as brick, concrete 

 and barium concrete, are also known. ^^ 



During the war, there was a rapid increase in the number of 

 workers engaged in luminizing instrument dials and in the aver- 

 age quantity of radioactive luminous compound handled by each 

 worker. As previously mentioned, the ]\linistry of Labor and 

 National Service issued an Order in April, 1942, giving fairly 

 detailed instructions to employers and employees regarding the 

 protective arrangements w'hich are to be adopted in luminizing 

 departments. The main features are: 



