MEASUREMENT OF LEAD EQUIVALENT 37 



TABLE 1-7 



Percentage op Energy Transmitted through Various Thicknesses op Lead 

 Tungsten target excited at kilovolt peak indicated 



weighing about 1.5 lb per sq ft, and is equivalent to 0.5 mm of lead. 

 They should be frequently tested for holes and cracks. 



Lead glass is merely a glass into which lead salts have been introduced. 

 A thickness of 15 to 20 mm has an opacity equivalent of about 2.5 mm 

 of lead. Some of the commercial lead glasses are ^ in. thick and have 

 a protective value equal to ^ their thickness in sheet lead. The glass 

 must be entirely free from air holes and other flaws. For proper protec- 

 tion 2.0 mm lead^equivalent is recommended. 



Measurement of Lead Equivalent 



The " lead equivalent " of a given thickness of absorbing material 

 to be used as a protective layer is the ratio of the thickness of lead to 

 the thickness of the material which absorbs a given x-ray beam to the 

 same extent. The protective lead equivalent of gloves, aprons, and 

 other guards is very simple to determine in practice. It can be obtained 

 by means of a lead foil echelon under the actual working conditions to 

 which the operator is subjected. 



An echelon or step wedge is constructed of layers of plane parallel lead 

 foils so arranged that the edges, each overlapping its neighbor, resemble a 

 flight of stairs. Such a series of equally spaced steps may be built up of 

 25 layers, each with a thickness of 0.2 mm. The echelon and sample 

 protective material are then placed on a paper-covered photographic 



