Patent Application
20120177180
X-ray equipment using an X-ray source, typically an X-ray tube, frequently includes the X-ray source inside a housing that is opaque to X-rays. Historically, lead has been used for the housing, frequently supported by a brass or aluminium construction, though brass alone is becoming more prevalent as the material for the housing. The housing has an opening to allow the X-ray beam generated by the X-ray source to pass through the housing for use, and a shutter may be arranged at the opening to close the opening to X-rays except when X-rays are required.
In a first aspect of the invention, there is provided a shutter arrangement, comprising:
The use of a shutter in this form allows the shutter to cover the inner edge of the outlet port behind the shutter in use. This can reduce corrosion as will be explained in more detail below.
According to an second aspect of the invention, there is provided a shutter arrangement having an outlet port in an housing for allowing X-rays to pass through the outlet port from an inner face to an outer face of the X-ray housing, the X-ray housing being substantially opaque to X-rays; and a shutter movable on the inner face of the X-ray housing between a blocking position adjacent to the outlet port where it blocks the outlet port and an open position where it allows X-rays to pass through the outlet port, wherein the shutter is of tantalum, niobium or zirconium, or an alloy containing at least 80% of one of these elements and a further metal/element of atomic number above 26.
The inventors have realised that the arrangement of a shutter of tantalum on the interior face of the housing allows for greatly reduced corrosion.
In another aspect of the invention, there is provided X-ray diffraction apparatus having such shutter arrangements.
For a better understanding of the invention embodiments will be described, purely by way of example, with reference to the accompanying drawings, in which:
The figures are purely schematic and not to scale. The same or like components may be included in more than one figure and the description relating thereto is not necessarily repeated.
Referring to
Referring to
Corrosion of the shutters 10 in such arrangements is a significant problem. X-ray tubes can be used in a variety of locations, some of which can be damp, and therefore increasing corrosion.
Further, the inventors have realised that X-rays ionise air, together with the moist or damp in the air, nitric acid is formed, which can attack the shutter 10 or other shielding. Thus, the whole process of corrosion is not simply a question of conventional corrosion, but a physico-chemical process, especially where the ionisation level is the highest.
One approach to dealing with the corrosion is to cover the shutter in a coating, such as nickel, gold or similar, which resists corrosion. The inventors have realised that a problem with the use of such coatings is that the corrosion is not simply chemical corrosion but can also be caused by electrochemical corrosion when using two different metals with an electrochemical potential having a difference of greater than 0.1V—a battery effect can occur. It is almost impossible to avoid microscopic pin holes with such coatings, and this can then allow both electrochemical and physico-chemical corrosion to occur. The results obtained with such a shutter, namely a tungsten-copper (alloy) shutter coated with silver and gold, are presented below as a comparative example.
An alternative therefore might be to use a non-metallic shutter such as glass or ceramic. Unfortunately, most such materials are not sufficiently X-ray absorbing to function as a shutter.
The choice of a material for the shutter may alternatively be a corrosion resistant alloy such as Incoloy Alloy 825, a nickel-iron-chromium alloy with additions of molybdenum and copper. Incoloy Alloy 825 has proven satisfactory for X-ray corrosion resistance.
Unfortunately the element iron in Incoloy gives undesired iron X-Ray fluorescence which could affect the analytical performance in applications. A commonly used X-ray source uses a copper target which generates copper K-alpha X-ray radiation which has a high enough energy to enable iron fluorescence.
Further, iron has significant lower shielding properties (absorption of X-rays) as compared with tantalum or tungsten.
It should be noted that fluorescence in the region of the shutter contaminates the (pseudo-) monochromatic X-ray beam from the X-ray source and this gives rise to higher background radiation. Thus, in an X-ray diffraction apparatus, the measured diffractogram has a higher background. It is possible that the fluorescence also generates unexpected and unwanted diffraction peaks at other angles and this seriously impedes accurate measurement.
In the arrangement according to an embodiment the invention, the material of the shutter 10 consists of solid tantalum which has proven to give particularly good results and has a high enough atomic number that fluorescence is not a problem. The tantalum may be substantially pure, i.e. the shutter may consist essentially of tantalum. In alternative embodiments, small amounts of impurities may be present, preferably less than 2%, further preferably less than 1%. In particular, the amount of impurity may be sufficiently small that exposure to nitric acid does not cause roughening of the surface.
Alternative, less preferred arrangements use either solid niobium or solid zirconium. Further alternatives include alloys of tantalum, niobium or zirconium with no more than 20%, and preferably relatively small amounts (no more than 10%), of other elements which should have an atomic number of over 26 so that copper K-alpha radiation does not excite fluorescence. Resistance to nitric acid is important. Suitable alternative alloys include tantalum-zirconium, tantalum-niobium or tantalum-tungsten.
These materials give good shielding, and no problems with fluorescence.
The shutter is also arranged in a particular arrangement, as will now be described with reference to
Note that in the open position the solid part of the shutter around the opening 8 completely covers the edges of the opening 8.
Referring to
It will therefore be appreciated that the combination of the solid tantalum material with the shutter arranged in this way protects a particular part of the brass material of the housing from X-rays during operation of the X-ray tube with the shutter in the open position as well as in the closed position.
The inventors have realised that the presence of X-rays greatly increases the amount of ionisation in the air and therefore greatly increases the corrosion. Thus, there is a significant advantage in protecting the brass material of the housing during operation by the shutter. In particular, in embodiments, in the open position, during operation, X-rays do not hit or hit to a lesser extent the material of the housing 6 around the opening 4, and in particular are shielded from the inner edge 15 around the opening 8 by the solid material of the shutter around the through hole 22. In the arrangement of
Further, in a particularly preferred embodiment, the amount of ionisation is reduced by keeping the air gap between the window 4 and the shutter 10 in the closed position as small as possible. Preferably, the air gap is less than 20 mm, further preferably less than 10 mm, and further preferably less than 5 mm. The goal is to keep the air volume as small as possible in the area of the window 4 and shutter 10, since air that is not present will not ionize. When a conventional shutter opens it creates a relatively large volume of air, the volume where the shutter was in the closed position. This is an extra air supply for the physico-chemical process and hence generates extra ions. There is of course air present in the new design but in the close position no X rays are present in this volume in the arrangement of
Thus, the described embodiment of the invention greatly reduces the problem of corrosion by using a tantalum shutter 10, arranged close to the window 4, the shutter having a through hole 22 so that the shutter around the through hole 22 shields the inner edge 15 around the opening 8 in the housing 6 in use.
The shutter arrangements described above are particularly suitable for X-ray diffraction measurements as illustrated in
The shutter mechanism is particularly suitable in such applications because unlike prior approaches to controlling corrosion, such as Incoloy 825, the shutter arrangement combines corrosion resistance with the avoidance of use of materials that impact on X-ray diffraction measurements. X-ray diffraction apparatus can be used in challenging environments that increases the importance of corrosion resistance.
In an alternative embodiment of the shutter mechanism, illustrated in
Although
The skilled person will realise that changes may be made to the embodiments described above. For example, the housing need not be made of brass alone, but other materials such as lead or tantalum may be introduced.
Further, the solid tantalum need not be 100% pure, but impurities may be present.
In some embodiments, niobium or zirconium can replace the tantalum. Alloys with typically at least 80%, preferably 90%, of tantalum, niobium or zirconium may be used.
