Million-Volt Therapy 243 



X-ray Equipment: Some Technical Considerations 



By 1933, a few experimental high-voltage X-ray equipments 

 had been constructed in the United States, operating at vohages 

 up to one miUion, but they were too unreHable in operation 

 to give biological and clinical results which could be as- 

 sessed. Usually these tubes had at the most only two fixed beam- 

 directions, necessitating tilting of the patient to the tube, in order 

 to accomplish cross-fire techniques. This method is deprecated 

 in Britain, since it is argued that unless the patient is prone, 

 supine or, for a restricted number of sites, sitting up, it is im- 

 possible to know the exact position of the various body organs. 

 Angulation of the tube to the patient is therefore demanded 

 as one of the essential features of an X-ray tube. 



The main ditficultv encountered in sealed tubes in the 

 attainment of higher voltages was that the increased electrical 

 stresses applied to the electrodes and envelopes extracted 

 occluded gas from them, resulting in internal electrical break- 

 down between the electrodes, and often in the puncturing of the 

 glass envelope. In one or two instances, tubes were supplied 

 to withstand 350 kilovolts, but they were never really robust. 



In 1932, a pair of 200 kilovolt steel and porcelain, demount- 

 able X-ray tubes, continuously evacuated by their attached oil 

 dififusion-pumps, were installed in Sheffield Radium Center. 

 The oil dif¥usion-pumps operated on the newly developed low- 

 vapor-pressure Apiezon oils, and did not need the expensive 

 liquid air traps required on mercury-vapor condensation pumps. 

 Continuous evacuation and demountability made possible the 

 cheap replacement of target and ^lament by any mechanically 

 minded member of the X-ray department. Instead of the usual 

 sealed-ofT thermionic rectifiers in the attached high-voltage gen- 

 erator, a pair of continuously-evacuated demountable rectifiers 

 was fitted. 



With the advent of these new oil dififusion-pumps, and the 

 demonstration that continuous evacuation was feasible and re- 

 liable, the development of high-voltage continuously-evacuated 



