. The American journal of roentgenology, radium therapy and nuclear medicine . ngsten deposit on the bulb. Such adeposit would come only from abuse indiagnostic work and would never happenif the tube were used solely for therapy. J. Loss of Efficiency from the BadPitting of a Target X-rays are produced by the impactupon the target of cathode rays, the pathof which, in the Universal and Radia-tor types of tube, lies essentially in theaxis of the tube. Most work is done withthe central beam of x-rays which is, inthese tubes, essentially perpendicular tothe cathode ray stream. If the focal spot w
. The American journal of roentgenology, radium therapy and nuclear medicine . ngsten deposit on the bulb. Such adeposit would come only from abuse indiagnostic work and would never happenif the tube were used solely for therapy. J. Loss of Efficiency from the BadPitting of a Target X-rays are produced by the impactupon the target of cathode rays, the pathof which, in the Universal and Radia-tor types of tube, lies essentially in theaxis of the tube. Most work is done withthe central beam of x-rays which is, inthese tubes, essentially perpendicular tothe cathode ray stream. If the focal spot were to lie at the bottomof a deep cylindrical pit whose axis wasparallel to the tube axis it is obvious thatthe passage of the cathode rays to thebottom of the pit would be unimpeded butthat the x-rays emerging at right anglesto this direction would have to passthrough a considerable thickness of thetarget material. In the case just considered, the resultantefficiency of x-ray production in the centralbeam, expressed in terms of that of anormal tube, might be extremely Fig. i 3Although no two pitted targets will bealike, it seemed worth while to study theworst case of pitting which could befound. A selection was made from a lotof used tubes which had been returned tothe factory. The tube in question had evi-dently been heavily overloaded in radio-graphic work. The condition of the focalspot may be seen from Figure 13. The maincavity was mm. deep. The tungstenwhich had left the focal spot had beendeposited on the bulb where it would, ofcourse, exert a certain amount of filteringaction. For this reason the target wasremoved from the old tube and put into a fresh bulb. It was, of course, necessary thatthe cathode should be so placed as to makethe new focal spot register with the old the method of trial and error this wasfinally accomplished. The x-ray distribution in different direc-tions from this tube is given in Parts 3 and4 of Section K; its efficiency in two
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