X-ray generation from miniaturized vacuum tubes with diameters ranging from 1 to 5 mm

Several prototype catheters, ranging from 1 mm to 5 mm in diameter, have been fabricated using fused silica and borosilicate tubes. The design of one of the gated field emitter X-ray device is presented. In addition to the field emitters, great emphasis was placed on sealing the miniaturized tubes and encapsulation of the system into a catheter. For the fused silica tubes, Mo was used as the X-ray generating anode since it seals well with the fused silica once a thin layer of Mo oxide is generated. For the borosilicate tubes, a graded sealing approach consisting of a uranium glass intermediary and a tungsten anode was used. Since these small X-ray tubes operate at voltages from 15-40 kV, breakdown events and flash-overs have to be avoided. To avoid these problems along the surface of the tubes, the tubes were encapsulated in PDMS that was made conductive by doping it with carbon black. The resistance of the mixture was adjusted so that only a few microamperes of current are conducted in the PDMS shell during catheter operation.