RADIOGRAPH y 823 



X-ray film emulsion, while it follows the same principle as that of photographic 

 film, is slightly different both as to chemical and physical characteristics, being more 

 sensitive to X rays. Even so, less than 1 per cent of X-ray energy is absorbed by 

 •such an emulsion, and the remaining 99 per cent passes through without performing 

 any useful work. To utilize some of the wasted 99 per cent energy, screens coated 

 with some salt like calcium tungstate are placed in direct contact with the double- 

 sensitized film surfaces. These screens, under X-ray bombardment, fluoresce, i.e., 

 absorb X rays and emit visible light. Thus the X-ray effect on film emulsion is 

 increased many times by the addition of visible light from the screens. This not only 

 reduces exposure time but makes practical the radiography of several thicknesses. 



The grain size of fluorescent coatings on screens is larger than that of film emulsions, 

 and the result is some loss of fine detail in the finished radiograph. The best picture 

 will be made without screens, but many times the use of screens is imperative, and so 

 they may be classed as a necessary evil. 



X-ray Protection. — X rays have a deleterious effect on the human system and every 

 possible precaution should be taken by the operator of an X-ray machine to avoid 

 repeated personal exposure. Before engaging in X-ray work of any kind, it is essen- 

 tial to make a thorough study of X-ray protection under all conditions which might 

 arise. Those technicians who are unequipped with electroscopes or other instru- 

 ments for detecting stray radiation should carry out the dental film test while at their 

 work. This consists in attaching a silver coin to the front of a dental packet loaded 

 with photographic film and carrying it, coin outward, in the vest or watch pocket. If, 

 after several days of X-ray work, the film is developed and shows an image of the coin, 

 it indicates the operator is being exposed to X-rays and should immediately take steps 

 to obtain better shielding. Too much emphasis cannot be laid on this phase of the 

 work, for serious illness, and even death, can ensue from small X-ray doses constantly 

 taken. 



X-ray Equipment. — No attempt will be made to describe in detail the various 

 possible electrical circuits which are capable of producing X rays. For this informa- 

 tion, reference may be made to any of several excellent books on the subject. At the 

 same time, a brief mention of the fundamentals may not be amiss. 



X rays are produced by the impact of electrons on a target of solid hard metal, 

 which is usually tungsten. In modern practice, X rays are produced in a vacuum 

 tube containing a heated filament and the anode or target. The filament is heated to 

 incandescence by means of a transformer operating from the 110- volt line and supply- 

 ing suitable filament voltage and current to the tube. When the filament becomes 

 incandescent, the thermal energy is sufficient to liberate a source of free electrons which 

 are capable of traveling through the vacuous space within the tube. In order that 

 these electrons may travel through the tube rather than cluster around the filament 

 from which they are emitted, it is necessary to accelerate them in a direction toward 

 the target. Since the electrons are negatively charged particles of electricity, they 

 may be made to move in the direction of the target by applying a voltage between the 

 filament and target such that the target is at a positive potential with respect to that 

 of the filament. 



In impinging upon the target, the electrons give up the kinetic energy which was 

 imparted to them by virture of the positive charge on the target, and in so doing they 

 produce those radiations known as X rays. The penetrating power of the X rays 

 depends upon the potential difference between the filament and target. The number 

 of rays depends upon the number of available electrons i.e., the current through the 

 tube, and this in turn depends upon the temperature of the filament which is regulated 

 by the filament voltage and current. Thus, by controlling the filament temperature 

 and the filament-target voltage, the operating characteristics of the tube may be 



