Patent Grant
8170179
The inventive improvements disclosed herein generally relate to electron-impact x-ray sources. More particularly, the disclosure is directed to the reduction of debris and improvement of x-ray brightness in electron-impact x-ray sources having a liquid-jet anode.
X-rays have been used for imaging ever since the discovery thereof by Roentgen at the turn of the 19th century. Since available x-ray optics are severely limited, x-ray imaging is still mostly based on absorption shadow-graphs. This is basically true even for modern Computer Tomography (CT) imaging and, as a consequence, the brightness of the x-ray source is a figure of merit limiting both the exposure time and the attainable resolution in many applications.
Today x-ray imaging is a widespread and standard method in science, medicine and industry. Although well established, there are numerous applications that would greatly benefit from an increased brightness. Among these are applications in medicine requiring high spatial resolution, such as mammography and angiography, and emerging techniques requiring monochromatic radiation which currently can not be achieved with reasonable exposure times. Also, certain protein crystallography, today only possible at synchrotron radiation facilities, may be feasible with a compact source. Furthermore, a significant increase in the brightness of compact x-ray sources could enable phase imaging with reasonable exposure times. This is important since the phase contrast is often much higher than the absorption contrast. In addition, phase contrast imaging could reduce the absorbed dose during imaging.
The basic physics relied upon for x-ray production in compact electron-impact sources has been the same since the days of Roentgen. As the electrons impact the target they lose energy in one of two ways: either they can be decelerated in the electric field close to an atomic nucleus and emit continuous bremsstrahlung radiation, or they can knock out an inner-shell electron, resulting in the emission of a characteristic x-ray photon when the vacancy is filled. The efficiency of x-ray production by electron impact is very poor, typically below 1%, and the bulk of the energy carried by the electron beam is converted to heat.
The brightness of current state-of-the-art compact electron-impact x-ray sources is limited by thermal effects in the anode. The x-ray spectral brightness [i.e. photons/(mm2·sr·s·BW), where BW stands for bandwidth] is proportional to the effective electron-beam power density at the anode, which must be limited not to melt or otherwise damage the anode. Since the first cathode-ray tubes only two fundamental techniques, the line focus and the rotating anode, have been introduced to improve the power load capacity of the anode.
The line focus principle, introduced in the 1920s, utilizes the fact that the x-ray emission is non-Lambertian to increase the effective power load capacity by extending the targeted area but keeping the apparent source area almost constant by viewing the anode at an angle. Ignoring the Heel-effect and field of view, this trick increases the attainable power load capability by up to ˜10×. The rotating anode was introduced in the 1930s to further extend the effective electron-beam-heated area by rotating a cone-shaped anode to continuously provide a cool target surface.
After these improvements, progress with respect to brightness has been rather slow for compact electron-impact sources and has only been due to engineering perfection in terms of target material, heat conduction, heat storage, speed of rotation etc. Current state-of-the-art sources now allow for 100-150 kW/mm2 effective electron-beam power density. Typical high-end implementations are, e.g., 10 kW, 0.3×0.3 mm2 effective x-ray spot size angiography systems and 1.5 kW, 0.1×0.1 mm2 effective x-ray spot size fine-focus mammography systems. Low-power micro-focus sources (4 W, 5 μm effective x-ray spot diameter) have similar effective power densities (200 kW/mm2) and are also limited by thermal effects.
The power load limit of a modern rotating anode can be calculated by
where Aeffective is the apparent x-ray source area, R is the anode radius, l is the spot height, 2δ is the spot width, Tmax is the maximum permissible temperature before breakdown, ΔTmargin is a safety margin, Tbase is the anode starting temperature, λ is the thermal conductivity, ρ is the density, cp is the specific heat capacity, f is the rotation frequency, t is the load period, and k is a correction factor taking into account radial heat conduction, heat loss by radiation and anode thickness. As can be seen from Eq. 1, the only way to increase the power load limit is to increase the spot speed, i.e., f and R. Unfortunately even a quite unrealistic set of parameters (1 m diameter anode and 1 kHz rotation) would only increase the output flux ˜6×. It therefore seems unlikely that conventional x-ray source technology can be developed much further, even with significant engineering efforts.
A way to increase the brightness in compact electron-impact based hard-x-ray sources would be a fundamentally different anode configuration allowing a higher electron-beam power density. To this end, there has previously been reported a new liquid-metal-jet anode concept. This anode configuration could allow a significantly higher (>100×) thermal load per area than current state of the art due to fundamentally different thermal limitations, as explained below. Liquid-jet systems have been extensively used as targets in negligible-debris laser-produced plasma soft x-ray and EUV sources. A liquid-gallium jet has also been used as target in hard x-ray production in femto-second laser-plasma experiments. Furthermore, an electron beam has been combined with a water jet for low power soft x-ray generation via fluorescence. X-ray tubes with liquid anodes, either stationary or flowing over surfaces, have previously been reported but their advantages for high-brightness operation are limited due to the intrinsically low flow speed and cooling capacity of such systems. Recent work also includes a liquid anode flowing behind a thin window.
The much higher power-density capacity of liquid-metal-jet systems compared to conventional anodes (2-3 orders of magnitude or more) is, in brief, due to three main reasons: (i) different thermal properties of the liquid-jet anode compared to a solid anode, (ii) the potential for higher jet speeds than what is possible for a rotating anode, and (iii) the regenerative nature of the liquid jet, which makes the requirement of keeping the anode intact more relaxed.
However, when attempting to increase the power for such systems, emission of debris is a potential practical difficulty. Hence, improvements are called for to reduce the debris issue for liquid-jet anode x-ray sources.
In short, it is proposed herein a method for generating x-ray radiation, which is characterized in that the full width at half maximum of the electron beam in the transverse direction of the target jet is about 50% of the target jet transverse dimension or less. It has now been discovered that this results in a considerable shielding effect of the very hot electron-beam impact area on the target jet, thus advantageously reducing the amount of debris produced. In addition, the further technical effect is obtained that the effective power density is increased when the x-ray spot is viewed from the side. This latter is in analogy with the line focus principle described in the introduction.
Hence, the inventive principles disclosed herein have the attractive advantage that reduction of debris can be obtained without significantly increasing the target-jet propagation speed, but rather by employing an electron beam having, at impact on the target, a full width at half maximum (FWHM) which is about half the transverse dimension of the target jet or less. By employing an electron beam which is considerably smaller than the transverse dimension of the target jet, the target jet will give rise to a shielding effect which limits the amount of produced debris in an advantageous manner.
The inventive principles also extend to a system for generating x-ray radiation, said system comprising means for carrying out the method.
It should be understood that the size (FWHM) of the electron beam at impact upon the target jet could be slightly larger than 50% of the target jet transverse dimension and still produce the inventive shielding effect.
Suitably, the generated x-ray radiation could be used in applications such as imaging, medical applications, crystallography, x-ray microscopy, proximity or projection lithography, photoelectron spectroscopy or x-ray fluorescence, to name a few.
Experiments were carried out in order to evaluate the inventive principle of producing x-rays. Debris deposition rates for several different system parameters were studied: an e-beam power between 38 W and 86 W, a jet speed of 22 or 40 m/s, a 30 or 50 μm jet diameter, and an e-beam focus of 15 or 26 μm. The witness plates 12 were exposed to tin vapor for 6-24 minutes and analyzed with a surface profilometer (KLA Tencor P-15).
The debris rates will naturally increase when higher-brightness operation is attempted by increasing the e-beam power and power density. We note that for sub-kW e-beam guns, the technological e-beam power density limit due to the cathode emissivity is a few tens of MW/mm2, i.e. two orders of magnitude above the highest power density of the metal-jet anode reported here. A significant improvement of the power density capacity of the jet anode may be achieved by having a much faster jet, and it has, in fact, been shown that it should be possible to produce stable tin jets at speeds up to at least ˜500 m/s. On the other hand, this may not necessarily be the only way to modify the jet for reduced debris production. As is indicated by the results in
It should be noted that the spot of the electron beam on the target jet may be circular, elliptical or a line focus as desired. For example, and as shown in
However, when an elongated electron beam spot is used according to the above, it is not required that the extension thereof is transverse to the target jet. Any general orientation of the elliptic or line focused electron beam spot is conceivable, and an effective increase of the x-ray brightness may be obtained by viewing (collecting) the generated x-ray from an appropriate angle. For example, if an electron beam spot is used having a line focus extending generally along the target jet, increased x-ray brightness may be obtained by viewing the spot from a slanting angle along the target jet.
Moreover, it should be pointed out that the line focus principle may be used also when a circular electron beam spot is utilized. The reason is the following. When the electron beam impacts on the target jet, x-ray radiation will typically be generated within the first few microns of target material as the electrons penetrate the target jet. As a non-limiting example, the electrons may typically penetrate about 4 microns into the target material. This is schematically shown in the enlarged side view of
The principle of using a reduced-size electron beam in order to reduce debris may advantageously be combined with prior-art techniques for reducing debris, such as increased jet-propagation speed, debris mitigation systems, etc.
The target jet may be electrically conductive or non-conductive. For example, the target jet may comprise a metal (e.g. tin or gallium), a metal alloy or a low melting-point alloy, a cryogenic gas or any other liquid substance suitable as a target for electron-impact x-ray sources.
It should also be understood that the target jet may have any cross-sectional shape, for example circular, rectangular or elliptical.
Typical diameters for the target jet are from about 10 μm to about 100 μm, such as 30 μm or 50 μm. However, in some applications even larger target jet cross-sections are conceivable. The propagation speed of the target jet in the area of interaction can be up to about 500 m/s, and typical values are from about 20 m/s to about 60 m/s. As will be understood, an increase in propagation speed for the target jet will lead to an improved power density capacity of the jet anode.
It will be understood that the examples given above are only for illustrative and enabling purposes, not intended to limit the scope of the invention. The scope of the invention is defined by the appended claims.
| Number | Date | Country | Kind |
|---|---|---|---|
| 0601048 | May 2006 | SE | national |
| Filing Document | Filing Date | Country | Kind | 371c Date |
|---|---|---|---|---|
| PCT/SE2007/000448 | 5/8/2007 | WO | 00 | 2/12/2009 |
| Publishing Document | Publishing Date | Country | Kind |
|---|---|---|---|
| WO2007/133144 | 11/22/2007 | WO | A |
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| Number | Date | Country | |
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