4. The Construction of the X-ray Tube
4.7. ASSEMBLY OF THE X-RAY TUBE
In Table 4.3 one can see, that the metal-to-ceramic seals of the tubes were capable of maintaining vacuum pressure levels of 10.5 Torr after they were heated to temperatures of 60°C, 90°C, 120°C, 140°C, 160°C and 180°C, and allowed to cool down to room temperature following each of the aforementioned temperature levels. However, for a temperature of 180°C it was no longer possible to pump the tubes to pressures below 10.3 Torr. By visually inspecting the tube joints under a microscope it was noted, that cracks along the ceramic adjacent to the metal-to-ceramic seals had developed in the tubes after they were exposed to a temperature of 180°C. From these observations it can therefore be concluded, that tubes with vacuum tight seals between the metal Dilver P and ceramic, where the solder alloy CH4 is used to braze the joint, should not be heated above a temperature of approximately 160°C if damage to the seals is to be avoided. It is for this reason that cooling of the ceramic X-ray tube while operating it becomes absolutely essential.
The above results also confirmed the fact, that the formation of stresses along a metal-to-ceramic seal typically occur, if cooling of the seal from a high temperature occurs at too fast a rate. It is therefore essential, that a low rate of cooling be applied to a metal-to-ceramic seal after it has been brazed, so as to minimise the development of stresses between the metal and the ceramic along the joint.
Table 4.4 (cont.): Specifications for WESGO brazing alloys. The data is taken from the WESGO brazing alloy datasheets [51].
Brazing Composition Liquidus, °C Solidus,oC Characteristics Alloy
NIORO Au: 81%-83%; 950 1000 Will "wet" Tungsten and
Ni: 17%-19%; Molybdenum, as well as
Copper, Dilver P, Stainless Steel. Excellent flow.
NICORO Au: 34%-36%; 1030 950 Excellent wetting and flow
Cu: 61%-63%; on Dilver P, Copper,
Ni:2.75%3.25%; Nickel and Steel.
The alloy NICORO was used for all the metal-to-metal brazes except for the joining of tungsten to the Dilver P support of the anode, where a vacuum tight joint was produced with PALCO as the brazing alloy. The brazing of the focusing lens and the cathode was accomplished by placing a NICORO ring (see Figure 4.5) and allowing the welding process to take place at a temperature of 1050°C for a period of ten minutes. The brazing of the tungsten target to its support was effected in the same manner at a vacuum furnace temperature of 1250°C for the same period of time. The copper pumping stem was brazed to the anode using NICORO as the brazing alloy only after the tungsten target was brazed onto its support at the higher temperature of 1250°C necessary for brazing with the alloy PALCO.
Filler/Brazing Alloy
Metal Component
Metal Component
Fig. 4.5: Example of a brazing ring or filler placed at a metal-to-metal joint prior to brazing.
After the c?mpletion of the metal-to-metal joints as well as the metal-to-ceramic seals, the X-ray tube was VIrtually fully assembled with only the X-ray window remaining to be added to the tube
structure. Thus before the tube could be mounted on the vacuum pump station the aluminium window had to be assembled onto the tube. This was accomplished by joining a thin 0.1 mm thick aluminium foil over the window opening with a vacuum tight adhesive, thus producing a so- called "Torr seal", and mechanically fastening it with an aluminium washer or window support (see Figure 3.1.11, section 3.1). The aluminium window could only be mounted after the brazing of the tube was complete, since the melting point of aluminium, which is 660.1°e, is far lower than the active brazing temperature of 960°C. In order to determine the pressure, which a cold cathode X-ray tube should be pumped down to, the mean free path length of the electrons travelling through the tube should be taken into consideration. The mean free path length is the mean distance travelled by a particle between collisions [37]. At relatively high pressures, where molecules collide with each other far more frequently than with the vacuum chamber walls, gas flow is referred to as viscous flow. At low pressures, on the other hand, molecules collide with the vacuum system walls more frequently than with each other. This type of gas flow is known as molecular flow. With molecular flow in the X-ray tube the molecules of the rest gas, air in this case, collide with each other to a far lesser extent while the tube is operated. In the same way it can be stated, that electrons travelling towards the anode of the tube will be far less likely to loose energy by colliding with each other or the rest gas molecules before reaching the target provided that the pressure inside the tube is low enough to allow for molecular flow. From a theoretical point of view this is desirable as a greater number of electrons, which have not lost any energy while propagating towards the target due to collisions with other particles in the tube, will thus be capable oftransferring more energy to the X-radiation produced by the electrons' collisions with the target of the X -ray tube. As shown in chapter 3.1 the electrons have to travel from the cathode to the anode through a distance of 0.056 metres. This means, that for molecular flow to occur the mean free path length should be greater than 0.056 metres. According to Young [31] the mean free path length, which is denoted by It , can be expressed by the following equation:
k-T
It= .
133,3· 4;rr.[2. r2 . p where k
=
Boltzmann's constant (l.38xl0·23 J K\T
=
Temperature (K);P
=
Pressure (Pa);r
=
radius of the rest gas molecules (m);(4.2)
To determine the maximum permissible air pressure inside the X-ray tube for molecular flow to occur It has to be equated to the cathode anode spacing of the tube, which is 0.056 metres. The temperature T can be approximated at 293 K, which is room temperature. The radius r of an air molecule is approximately 2x 10-10 m according to Young [31]. Substituting these values into equation 4.2 a pressure value of 7x 10-4 Torr is obtained. This presents the maximum pressure, that the tube should be operated at. According to Templeman [52], however, who conducted experiments with cold cathode X-ray tubes, the gas pressure of a cold cathode X-ray tube should be in the "soft" vacuum range (10-3 Torr to 10-4 Torr) to permit the passage of electrons from the catho~e to the. X-ray. producing target in a so-called "dark" discharge. Higher pressures would result ill a lumillous dlscharge, as in a neon lamp, with only a small potential drop across the tube, wh~le lower pressures would result in no current flow regardless of the voltage applied [52].
Incldentally the pressure range of 10-2 Torr to 10-4 Torr represents a transition from viscous to molecular flow [37]. This means that the "soft" vacuum range of 10-3 Torr to 10-4 Torr falls into
the pressure range, where both viscous and molecular flow occurs. This observation was verified by the author (see Chapter 6), who found that no current flow took place in a tube with a hard vacuum at a gas pressure of 6x 10-5 Torr whilst high voltage pulses were applied across the X-ray tube. Operating the same type of tube, however, at a higher gas pressure of 7xl04 Torr, which falls within the soft vacuum range mentioned above, resulted in the production of a tube current, and hence X-radiation, during the operation of the X-ray tube.
CyllndrlC\
Jaws of Tool Cold Weld
Copper Tube
\
Fig. 4.6: Technique used to squeeze-off or pinch-off the copper tubulation between the tube and the pumping station [37].
The setup for the vacuum pumping station as illustrated in Figure 4.4 was used to pump down the X-ray tube. After the desired pressure of 7x10-l Torr was attained, it was necessary to separate the tube from the pumping station. The technique used to separate the tube from the pumping station is illustrated in Figure 4.6. As already mentioned in the preceding section the soft copper tubulation was used to make the vacuum connection between the tube and the pumping station.
This tubulation was molecularly clean on its internal surfaces after having been baked along with the tube during the active brazing cycle. By squeezing the tubulation with a special tool, the internal mating surfaces of the copper were forced to flow together to form a cold weld. In Figure 4.7 the different X-ray tube components prior to brazing and assembly are shown.
Fig. 4.7: The components of the end-window cold cathode X-ray tube.