Patent Number: 047633441
Section: summary

TECHNICAL FIELD This invention relates to an apparatus for the production of X-rays for technological, scientific and medical purposes. BACKGROUND OF THE INVENTION For nearly a century X-rays for medical and technological use have been generated using bremsstrahlung and characteristic line emission. The intensity of this radiation is relatively weak for many commercial and medical applications. This is especially true for moving mechanical systems (e.g. gear trains) and biological tissue (e.g. arteries of the heart). In the past twenty years a brighter more collimated X-ray source from synchrotron emission has been used to generate both hard X-rays and soft X-rays for scientific and technological research. For example, very recent work using X-ray synchrotron emission from electron storage rigs offers the prospect of a new method of non-invasive coronary angiography (medical imaging of the arteries of the heart, see Hughes et al., "The application of synchrotron radiation to non-invasive angiography," Nuc. Instrum. Meth., vol. 208, p. 665, 1983). The high intensity and collimation of the synchrotron radiation permit the X-rays to be Bragg-diffracted so that only a narrow band of energies remain. The selected energy of the X-rays are subject to fine adjustment by small changes in the Bragg angle allowing digital subtraction of the X-ray images acquired at energies slightly above and below that of the iodine k-shell-photoabsorption edge at 33.16 keV, the iodine having been injected into the bloodstream intraveniously. This digital subtraction, called dichromography, substantially eliminates all image contrast due to other body structures and thereby achieves maximum contrast between the iodinated arteries and the surrounding tissue. Furthermore, when using the scanning method, the intensity of the synchronotron X-ray beams is such that the pairs of one-dimensional images, above and below the k-edge, can be recorded in a very short time. In this way, the prospect of visualizing the coronary arteries without motion artifacts is achieved. A conventional X-ray tube is generally not bright enough or collimated enough to achieve this kind of imaging in such a short time. Unfortunately, the large storage rings with periodic magnetic fields for the generation of synchrotron radiation are presently extremely expensive. Estimated costs for such facilities are between 10 and 25 million dollars. A cheaper source is clearly needed. Another source of X-rays is transition radiation from thin foils using electrons from high-current linear accelerators. Transition radiation occurs when charged particles encounter a sudden change in dielectric constant at the interface between dissimilar media (e.g. between a vacuum and a solid). Conservation of energy and momentum requires that a cone of X-rays be emitted. In the prior art transition radiation has only been applied to high-energy-particle detection. Previously only low-density foils were used (densities&lt;2.25 gm/cm.sub.3), and, in order to raise the output photon frequency, the electron-beam energy was raised. For example, electron energies of 2 GeV or more were used with low-density foils such as mylar, lithium and beryllium. (see M. L. Cherry et al. "Transition radiation from relativistic electrons in periodic radiators," Phys. Rev. D vol. 10, pp. 3594-3607, December 1974.) Transition radiation has also been considered as a source of soft X-rays (photon energy&lt;2 keV) using low density (.rho.&lt;3 gm/cm.sup.3 ) foils for lithography (see M. A. Piestrup et al. "Measurement of transition radiation from medium energy electrons", Phys. Rev. A, vol. 32. pp. 917-927, August 1985). SUMMARY OF THE INVENTION In accordance with the preferred embodiments of the invention, an intense, well-collimated-X-ray source is provided which uses thin high-density foils and in some applications relatively moderate electron-beam energies to generate X-ray radiation. The radiation is achieved through transition radiation. The source produces X-rays having an energy greater than 2 keV corresponding to a frequency of maximum photon flux, hereafter the peak frequency .omega., and uses a number of foils M arranged as a succession of parallel elements to form a stack. The foils are constructed of a material having an atomic weight A, a atomic number Z, and a density .rho..gtoreq.3 gm/cm.sup.3, with each foil having a minimum thickness l.sub.2. The foils are held together by a holding device which maintains a spacing l.sub.1 between adjacent foils in the stack. An electron accelerator directs an electron beam towards the stack to create transition radiation, the electron beam having an energy ##EQU1## but less than 500 MeV, where E.sub.o is the electron rest energy, m.sub.e is the mass of the electron, N.sub.o is Avogadro's number, and e is the electron charge. All units are in the cgs system. A housing provides a controlled environment for the electron beam and the foil stack. To produce the desired characteristics of the transition radiation, the number of foils M.ltoreq.(0.5)2/.mu.l.sub.2, where .mu. is the absorption coefficient of the foil material at the frequency .omega.. Also, ##EQU2## where .lambda. is the wavelength of the X-rays at the peak frequency .omega., and where .gamma.=(1-.beta..sup.2).sup.1/2 where .beta. is the velocity of the electrons in the electron beam relative to the speed of light, and .omega..sub.p is the plasma frequency of the foil material. The spacing between the foils l.sub.1 is ##EQU3## if the housing provides a vacuum environment; and ##EQU4## if the housing provides a gas environment, where .omega..sub.pg is the plasma frequency of the gas. An objective of the invention is to make an economical X-ray source, as compared to a synchrotron emitter, in order to produce photon energies greater than 2 keV. To minimize the cost of construction and operation, the electron-beam energy is kept as low as possible. This is achieved by increasing the density of the foils. The photon emission falls off at the "cutoff" frequency, .omega..sub.c =E.omega..sub.p /E.sub.o (where E.sub.o is the electron rest mass, 0.511 MeV, .omega..sub.p is the plasma frequency of the foil material, and E is the energy of the electron beam). To keep .omega..sub.c as large as possible, while not increasing E, .omega..sub.p should be increased relative to the prior art values by going to high density materials since .omega..sub.p is proportional to the square root of the density. However, selection of higher density materials typically results in materials of higher atomic number Z. Since bremsstrahllung is also emitted by the foils and is proportional to the square of the atomic number, bremsstrahlung can be large if Z is chosen to be too high. Hence, in some embodiments it is important to minimize the bremsstrahlung since it has a flat spectrum from very long wavelengths to photon energies equal to that of the electron-beam energy. Otherwise, extremely hard X-rays would be produced at high Z which are not desired and are detrimental to the X-ray optics and other experimental apparatus directly in line with the X-ray flux. Thus for some applications it is important to select foil materials with thicknesses and densities that minimize the bremsstrahlung and maximize the transition radiation. Selection of materials of high density and moderate Z is therefore desirable in these situations. For example, iron (stainless steel) and copper foils are excellent candidates since they have comparatively high densities and moderate atomic numbers. High density foils which also have high Z such as gold or tungsten can be used in other embodiments if it is desirable to lower the electron beam energy further and if extremely hard bremsstrahlung contamination of the transition radiation spectrum does not matter. This would depend upon the X-ray optics and other experimental apparatus that might be effected by the extremely hard-X-ray emission. Also the photon flux from the transition radiation source can be further increased by designing on the low-frequency side of the k-shell-absorption edge of the foil material. In this frequency band, there is a dramatic decrease in absorption of the X-rays in the foils themselves, thereby allowing the passage of the X-rays through a greater number of foils. This is accomplished by choosing the thickness of the foils l.sub.2 to be: ##EQU5## where .omega..sub.k is the k-shell photoabsorption-edge frequency of the foil material.