X-ray or gamma ray systems or devices – Source – Target
Reexamination Certificate
1999-02-02
2001-02-06
Porta, David P. (Department: 2876)
X-ray or gamma ray systems or devices
Source
Target
C378S124000, C378S140000, C378S121000
Reexamination Certificate
active
06185276
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention deals generally with x-ray tubes and more specifically with an x-ray tube which generates highly collimated radiation.
X-ray tubes function on the basis of an electron beam being generated by a cathode within the tube, and the electron beam bombarding a very small spot on an anode which is also within the tube. The bombardment of the anode, which is constructed of a suitable x-ray generating material, creates the x-rays along with a great deal of heat.
Until now most x-ray tubes have generated radiation which is poorly focused and have required secondary structures or devices to focus the beam on an object to be studied. Typical focusing structures external to the x-ray source have been spherical mirrors (U.S. Pat. No. 5,604,782 by Cash), curved crystals (U.S. Pat. 5,008,910 by Van Egeraat), capillary tubes (U.S. Pat. No. 5,001,737 by Lewis et al), and bent crystals on the inside surface of tubular structures (U.S. Pat. No. 3,898,455 by Furnas, Jr.).
A few efforts have also been made to generate a more focussed beam within the x-ray tube itself. In U.S. Pat. No. 4,352,021 by Boyd et al, multiple curvelinear anodes are disclosed, but they are also followed by a collimator structure to improve the focus. In U.S. Pat. No. 3,821,574, Burns discloses a single crystal anode of elongated channel shape which is used to generate a more intense x-ray beam because the beam is diffracted from the single crystal structure many times as it travels along the channel.
Despite this prior art, a simple structure for an x-ray tube which produces a collimated beam is not available. It would be very beneficial for both industrial and medical applications to have available an x-ray tube which is essentially interchangeable with x-ray tubes in common use but which produces a highly collimated beam which requires minimal external focusing devices.
SUMMARY OF THE INVENTION
The present invention is an x-ray tube which generates a highly collimated beam within the x-ray tube itself. To accomplish this a single crystal or a highly oriented coating is used for the x-ray generating anode (or target) of the tube. To generate a focused beam, this single crystal structure is attached to a spherical or parabolic surface. Thus, x-ray photons which leave the structure on a path perpendicular to the surface are focused at a specific focal point determined by the curvature of the single crystal.
For some applications it may be desirable to produce a collimated beam which is not focused, that is, a beam which actually is comprised of multiple parallel individual beams. Such a beam, which can, for instance, be used in large area illumination of photolithographic masks, can be generated by the use of a single crystal attached to or comprising a flat anode surface.
The x-ray photons are generated in a conventional manner by bombarding the anode with electrons from an electron source within the x-ray tube. The electrons emitted from the source are accelerated to a high velocity before striking the anode by the use of a voltage gradient between the electron source and the anode. The voltage gradient is established by the application of appropriate voltages to the electrodes from an external power supply.
The electron beam can also be scanned by a magnetic deflection coil, similar to that used in television picture tubes. Such scanning permits the generation of x-rays from multiple points on a large surface as opposed to the more traditional manner of directing the electron beam to a single location on the anode, and, in some x-ray tubes, rotating the anode so that no single location on the anode overheats.
The benefit derived from the single crystal structure is the limited number of paths followed by photons generated within the crystal lattice and the parallelism of all the photons emitted in any one of the limited directions. Photons which try to leave the crystal lattice in directions other than the several preferred paths are refracted into the preferred paths or absorbed by the crystal lattice and re-emitted in one of the preferred paths. Thus, if the anode surface is perfectly flat, although photons are emitted at several specific angles to the surface, all the photons leaving the surface at each of the specific beam angles will be parallel to all the other beams of photons departing from the surface, even though the photons are generated at multiple locations within the crystal lattice.
In more familiar terms, the emission of x-rays from each spot on a single crystal anode structure is similar to the illumination from the narrow beams of several spotlights positioned at a single location, so that they form a limited number of narrow beams of light from that location. Furthermore, all other locations on the anode generate only light beams which are parallel to those from the first location.
In a similar example, each x-ray generating spot of a typical prior art x-ray anode can be represented by a single simple incandescent light bulb which sends out photons in a full semi-spherical pattern. Just as we regularly do with flashlights and search lights, the x-rays from conventional anodes must then be focused with reflectors and lenses.
However, the focus of x-rays from a single crystal structure can be determined, not by external focusing devices, but by the curvature of the anode surface itself. When the surface is parabolic, the x-rays will be focused at the focal point of the parabola, and if the surface is perfectly flat the x-rays will simply generate a shaft of parallel collimated x-ray beams.
This pattern of collimated beams is particularly useful in the photolithography process used in the semiconductor industry. The number of circuit elements which can be squeezed into a specific area is now approaching a new limit, the resolution available with the light used for illuminating the photolithography mask. The minimum spacing between individual elements is limited by the wavelength and collimation of the light used for transferring the image from the mask to the semiconductor material. An x-ray beam generated by a single crystal can take this process to the next level because the wavelengths of x-rays are not only much shorter than those of visible light but they are also collimated.
Thus, the present invention can not only furnish better focused x-rays for use in conventional medical and industrial uses, but can also yield shorter wavelength collimated beams for improving the integrated circuit manufacturing process.
REFERENCES:
patent: 3564251 (1971-02-01), Youmans
patent: 3821579 (1974-06-01), Burns
patent: 3898455 (1975-08-01), Furnas, Jr.
patent: 4065211 (1977-12-01), Vig
patent: 4126805 (1978-11-01), Randall
patent: 4266138 (1981-05-01), Nelson et al.
patent: 4352021 (1982-09-01), Boyd et al.
patent: 4405876 (1983-09-01), Iversen
patent: 4573186 (1986-02-01), Reinhold
patent: 5001737 (1991-03-01), Lewis et al.
patent: 5008910 (1991-04-01), Van Egeraat
patent: 5148462 (1992-09-01), Spitsyn et al.
patent: 5199058 (1993-03-01), Tani et al.
patent: 5550889 (1996-08-01), Gard et al.
patent: 5604782 (1997-02-01), Cash
Fruitman Martin
Hobden Pamela R.
Porta David P.
Thermal Corp.
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