52 BIOPHYSICALLY ACTIVE X-RAYS 



operator from x-radiations. A fine-focus x-ray tube driven at 88 to 

 105 kv peak and 3 to 5 ma is usually recommended to give sharp images. 



Intensifying Screens 



The blue fluorescing screens are used for roentgenographic work. 

 The x-ray film is pressed in close contact between the active faces of 

 two such screens. X-ray photographic films are coated on both sides 

 (duplitized), thus permitting the photographically active blue fluores- 

 cent emissions from the activated surface of each screen to irradiate the 

 film. Calcium tungstate is nearly universally used in the production of 

 intensifying screens because its region of maximum fluorescence is be- 



o 



tween 3570 and 5100 A. As seen in Fig. 1-21 this emission band lies 

 within the spectral photographic-sensitivity region of the x-ray film. 



The older forms of intensifying screens were merely sheets of card- 

 board coated with the fluorescent material. In the present type of 

 intensifying screens the calcium tungstate is introduced into the cellu- 

 loid binding material itself. This type of screen is washable and flexi- 

 ble, and it can be closely pressed to the photographic material so as to 

 reduce distortion of the photographic image. 



In the manufacture of intensifying screens the following factors are 

 kept in mind for high-quality work. 



1. Speed refers to the relative amount of x-radiation required to 

 produce a developable photographic image when films are used with or 

 without screens. If a given product, distance X milliampere X kilo- 

 volt-peak X time, produces a desired photographic density with no 

 screens, and the same density can be obtained with the aid of screens in 

 one fifth the time, using the same x-ray intensity, then the screens are 

 said to have a speed factor of 5 to 1 at that intensity. A cassette con- 

 taining a good double screen should possess a speed factor from 5 or 6 

 to 1. 



The major factor used in controlling the speed of screens is the size 

 of the calcium tungstate crystals. Ordinarily, the larger the crystals 

 (other factors being constant), the faster the screens. The screen speed 

 also varies with the x-ray tube's peak voltage excitation, i.e., effective 

 wavelength; in general, the higher the kilovolt-peak, the faster are the 

 screens. 



2. Grain is usually caused by the use of too large crystals of calcium 

 tungstate or the presence of impurities. Screens of very high speed 

 must of necessity be grainy, owing to the large crystals used. A com- 

 promise is always made by the manufacturer in the size of the crystals 

 in order that maximum speed may be obtained with minimum of grain. 

 Too much grain will materially mar the diagnostic value of a film in 



