Compact self-resonant X-ray source

Abstract

An X-ray source, which includes a resonant cavity preferably of a cylindrical shape, is excited in a microwave mode TE.sub.11p and affected by a static and non-homogeneous magnetic field that grows longitudinally. An electron beam is injected longitudinally through one of the lateral walls of the cavity and is continuously accelerated until it reaches an energy sufficient to produce X-rays after the electrons bombard a metallic target located in the plane where they stop their longitudinal movement. The profile of the magnetic field grows in such a way that it maintains the conditions of electron cyclotron resonance along the helical paths of the electrons, The device can be used to obtain radiographic images and even produce hard X-rays.

Claims

1. An X-ray source, comprising: a cylindrical resonant cavity with length, a diameter and a longitudinal axis extending from a first end of the cylindrical resonant cavity to a second end of the cylindrical resonant cavity; an electron gun located at the first end of the cylindrical resonant cavity; a metallic target coupled to the cylindrical resonant cavity adjacent to the second end of the cylindrical resonant cavity; a microwave field energizing system coupled to the cylindrical resonant cavity, the microwave field energizing system comprises two waveguides, each one having an end coupled to the cylindrical resonant cavity and the other end coupled to a microwave source; at least one magnetic field source that generates a magnetic field that increases along the longitudinal axis of the resonant cavity, starting from the first end of the cylindrical resonant cavity to the second end of the cylindrical resonant cavity; and a window transparent to X rays, the window being incorporated into a cylindrical surface of the cylindrical resonant cavity, the window being arranged in a common transverse plane with the target; wherein the length and diameter of the cylindrical resonant cavity meets a relationship according to the following expression:
d=p[(2f/c).sup.2(1.841/r).sup.2].sup.1/2 wherein: d is the length of the cylindrical resonant cavity; p is the subscript of the resonance mode of the cylindrical resonant cavity; f is the frequency of the microwave source; c is the speed of light in vacuum; and r is the diameter of the cylindrical resonant cavity/2.

2. An X-ray source according to claim 1, wherein the magnetic field strength at the electron's point of injection is equal to the value to obtain the classical cyclotron resonance.

3. An X-ray source according to claim 2, wherein the magnetic field has a value of 875 Gauss at the injection point.

4. An X-ray source according to claim 1, wherein the magnetic field is axially symmetric, static and non-homogeneous.

5. An X-ray source according to claim 1, wherein the electron gun is a LaB.sub.6 type electron emitter and injects an electron beam with about 10 keV of energy.

6. An X-ray source according to claim 1, wherein the metallic target has an internal cooling channel.

7. An X-ray source according to claim 1, wherein the metallic target is molybdenum.

8. An X-ray source according to claim 1, wherein the metallic window transparent to X-rays is made of beryllium.

9. An X-ray source according to claim 1, wherein the resonant cavity is made of copper.

10. An X-ray source according to claim 9, wherein the cylindrical resonant cavity resonates in the TE.sub.112 mode.

11. An X-ray source according to claim 10, wherein the cavity length is 21 cm and the diameter is 9 cm.

12. An X-ray source according to claim 1, wherein the magnetic field source is generated by three permanent magnets.

13. An X-ray source according to claim 12, wherein the permanent magnets are made of SmCO.sub.5 or FeNdB magnets.

14. An X-ray source according to claim 1, wherein the waveguides have a rectangular cross section.

15. An X-ray source according to claim 14, wherein the waveguides propagate in a TE.sub.10 mode.

16. An X-ray source according to claim 15, wherein each waveguide comprises: aa ceramic window; and ba ferrite insulator.

17. An X-ray source according to claim 16, wherein the ceramic window is made of Si.sub.2O.sub.3.

18. An X-ray source according to claim 1, wherein the ends of the waveguides coupled to the cylindrical resonant cavity are located at a distance of of the total cavity length, measured from the end where the electron gun is located.

19. An X-ray source according to claim 1, wherein the microwave source is a magnetron.

20. An X-ray source according to claim 19, wherein the magnetron has an operating frequency of 2.45 GHz and excites a microwave field of 7 kV/cm.

21. An X-ray source according to claim 1, wherein the waveguides used for the injection of microwaves into the cavity differ in their lengths by /4, where is the wavelength of the TE.sub.10 mode.

22. An X-ray source, comprising: a rectangular resonant cavity having a length, width and a longitudinal axis extending from a first end of the cavity to a second end of the rectangular resonant cavity; an electron gun located at the first end of the rectangular resonant cavity; a metallic target coupled to the rectangular resonant cavity adjacent to the second end of the rectangular resonant cavity; a microwave field energizing system coupled to the rectangular resonant cavity, the microwave field energizing system comprises a waveguide, the waveguide having a first end coupled to the rectangular resonant cavity and a second end coupled to a microwave source; at least one magnetic field source that generates a magnetic field, the magnetic field increasing along the longitudinal axis of the rectangular cavity, starting from the first end of the rectangular resonant cavity to the second end of the rectangular resonant cavity; and a window transparent to X rays, the window being incorporated into a rectangular surface of the rectangular resonant cavity, the window being arranged in a common transverse plane with the target; wherein the length and width of the rectangular resonant cavity meets a relationship according to the following expression:
d=p[(2f/c).sup.2(1/a).sup.2].sup.1/2 wherein: d is the length of the rectangular resonant cavity; p is the subscript of the resonance mode of the rectangular resonant cavity; f is the frequency of the microwave source; c is the speed of light in vacuum; and a is the cavity width.

23. An X-ray source according to claim 22, wherein the first end of the waveguide is coupled to the rectangular resonant cavity through an iris, and said waveguide propagates a TE.sub.10 mode.

24. An X-ray source according to claim 23, wherein the microwave source is a magnetron located at a distance of /4 from the end coupled to the rectangular resonant cavity, where is the wavelength of the TE.sub.10 mode.

25. An X-ray source according to claim 22, wherein the rectangular resonant cavity resonates in TE.sub.102 mode.

26. An X-ray source according to claim 25, wherein the dimensions of the rectangular resonant cavity are a=7.74 cm, d=20 cm and a height of 3.87 cm.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) For a better understanding of this invention, the following figures are included as examples.

(2) FIG. 1 Preferred embodiment of the X-ray source.

(3) FIG. 2 Front view of the coupling for energizing of the TE112 mode with circular polarization.

(4) FIG. 3 White metallic target with cooling channels.

(5) FIG. 4 Front view of the electron beam.

(6) FIGS. 5A and 5B Description of the external magnetic field including: FIG. 5A showing a system of magnetic rings and the magnetic field lines, and FIG. 5B showing a magnetic field profile along the axis of the cavity of the present invention.

(7) FIG. 6 Side view of the electron beam.

(8) FIG. 7 Alternative embodiment of the X-ray source.

(9) FIG. 8 Top view of the alternative embodiment of the X-ray source (the magnetic field sources are not shown).

(10) FIG. 9 Metallic target and X-ray extraction in the alternative embodiment of the X-ray source.

(11) FIG. 10 Longitudinal view of the electrode-cavity system in the preferred embodiment of the cyclotron radiation source.

DETAILED DESCRIPTION OF THE INVENTION

(12) In FIGS. 1 and 2, the basic components of the preferred embodiment of the compact X-ray source are shown. Referring to FIG. 1, the microwave resonant cavity 1 is coupled with an electron gun 10, a target 11 upon which the electron impact, light metal window 12 and a microwave energizing system. The cavity 1 is affected by a magnetic field generated by three magnetic field sources 13, 13 and 13.

(13) The cavity 1 is of a cylindrical shape and made of metal, preferably of copper to reduce heat losses from the walls thereof. The cavity 1 resonates, in the case of the preferred embodiment, in the cylindrical TE.sub.112 mode, and its length and diameter are 21 cm and 9 cm, respectively, dimensions that maximize the intensity of the electric field within it. These values must have a relationship described by the following expression, d=p[(2f/c).sup.2(1.841/r).sup.2].sup.1/2, where: p=2 (for the TE112 mode), f=frequency of the magnetron, c=310.sup.8 m/s, and r=(cavity diameter)/2. In practice, one of the advantages of using a single resonant cavity is that it reduces the size of the device. In the preferred embodiment a cylindrical cavity is considered. However, the cross section of the cavity may be elliptical, energized with the TE.sub.c11P mode (P=1, 2, 3, . . . ).

(14) The electron gun 10, preferably based on a rare earth electron emitter, preferably of the L.sub.aB.sub.6 type, which is coupled to one end of the cavity 1. The gun 10 injects a quasi mono-energetic electron beam along the axis of symmetry of the cavity 1 with an energy of about 10 keV.

(15) The thermo-resistant and resistant to cracking, preferably molybdenum, nonmagnetic metal target 11, has an internal channel used for cooling by circulating water (as the cooling channel of FIG. 3) or by fan cooling edges.

(16) The light metal window 12, preferably beryllium, must ensure the passage of the emitted X-rays by the impact of electrons with the metal target 11 without damping. That is, it should be transparent for the rays.

(17) The three magnetic field sources 13, 13 and 13 produce an axially symmetric static and homogeneous magnetic field, increasing along the cavity, which in the preferred embodiment is created by a system of permanent magnetic magnets, preferably of ferromagnetic SmCO5 or FeNdB ring shaped. The magnetization, dimensions and spacing of the magnets system is selected so that, preferably: (i) the magnetic field strength at the point of electrons injection is equal to the corresponding value of classical cyclotron resonance, for example 875 Gauss with 2.45 GHz microwave and (ii) the magnetic field strength increases appropriately along the axis of the cavity 1 to hold the ECR by compensating the relativistic effect of the increasing of the mass.

(18) In FIG. 2 it can be seen that the microwave excitation system has two waveguides 2 and 3 coupled to the cavity 1, two ceramic windows 4 and 5, a coupling waveguide 6, two ferrite insulators 7 and 8 and a microwave generator 9. The microwave power is injected into the cavity 1 through the windows 4 and 5, preferably ceramic Si2O3, by means of the waveguides 2 and 3, separated azimuthally by 90 and coupled to the cavity 1 in a plane located at a distance of a quarter of the length of the cavity 1, d/4, distance from the end which is coupled to the electron gun 10. The waveguides 2 and 3 provide microwave energy in a TE.sub.10 from a microwave generator 9, which may be a magnetron of 2.45 GHz (the magnetron has a power source system), though a coupling waveguide 6. The two paths used for the microwave injection have lengths L and L+/4, where is the wavelength of the TE.sub.10 mode, which produces a phase shift of /2 to energize the wave TE.sub.112 with a right polarized circular wave in the cavity 1. Moreover, the microwave generator 9 is coupled to a waveguide coupling 6, which is coupled at each of its ends with ferrite insulators 7 and 8 used to protect the microwave generator 9, which in the preferred embodiment is a magnetron, of the reflected power. The ferrite insulators 7 and 8 are connected to the waveguides 2 and 3 respectively. Ceramic windows 4 and 5, incorporated in the inside of the waveguides 2 and 3 are transparent to microwaves and is used to maintain the vacuum in the cavity 1, which has been hermetically sealed after obtaining vacuum therein.

(19) In order to start the X-ray source, the microwave generator 9 and the electron gun 10 are turned on. The generator 9 transmits the microwave energy at a frequency of 2.45 GHz to the resonant cavity 1 through the waveguides 2 and 3. Due to the location and the magnetization of the magnetic field sources 13, 13 and 13, which in the preferred embodiment are three ring-shaped magnets, a region is created in which the electron cyclotron frequency remains almost constant inside the cavity 1. The microwave energy in the cavity 1 accelerates the electrons by ECR along their helical paths 14 (FIGS. 4 and 6) until impacting the metal target 11, thus producing X-rays, which pass through the window 12. The amplitude of the microwave electric field TE.sub.112 of 7 kV/cm circularly polarized ensures the production of X-rays with energy of the order of 250 keV. In general, cylindrical cavities resonating in modes TE.sub.11p (p=1, 2, 3, . . . ) can be used.

(20) In FIG. 5a, it can be seen a graph illustrating the increased magnetic field along the cavity formed by the magnetic field sources 13, 13, 13, showing the field lines produced in the region of interest. As shown from the separation between the magnetic field lines, this is increased (not monotonically) as the electrons move from the position of the electron gun 10 toward the target 11. FIG. 5b shows an example of the longitudinal profile of the magnetic field adjusted for the microwave TE.sub.112 mode of the preferred embodiment. One can appreciate a local minimum 15 of the magnetic field in the second half of the cavity.

(21) As shown in FIG. 6, the electrons stop their longitudinal movement in a position located between the local minimum 15 (see FIG. 5b) and the rear end of the cavity 1, which determines the position of the target 11. In this position the electrons have increased their radii of rotation, enabling the impact with target 11. Electrons that are able to move beyond the plane where the target is located, are reflected by the static magnetic field that grows in the space behind them, having another chance to hit back in their movement. It can also be seen in FIG. 4 that the length of penetration of the target 11 inside the cavity 1 is defined from the average Larmor radius of the electrons located in this position.

(22) In an alternative embodiment of the X-ray source, the geometry of the resonant cavity 1 is modified, the microwave mode energized in the cavity and the energization mechanism as described below:

(23) In FIGS. 7-9, the basic components of an alternative embodiment of the source are shown. A rectangular resonant microwave cavity 1 which is in vacuum and resonates in a TE.sub.10P mode (P=1, 2, 3 . . . ), a waveguide 2 which is coupled to the cavity 1 through an iris or resonant window 22, a microwave generator 9 connected to the coupling waveguide 6 which is coupled to the waveguide 2 through the ferrite insulators 7, three sources of magnetic field 13, 13 and 13, an electron gun 10 which is coupled to one end of the rectangular cavity 1, and a target 11 coupled to the cavity 1 on which the electrons impact. The positions of the permanent magnets of the magnetic field source 13, 13, 13 shown in FIG. 7 correspond to the case in which a TE.sub.102 mode is energized in the rectangular cavity 1. In FIG. 9 it is shown the cavity dimensions a=7.74 cm, b=3.87 cm and d=20 cm. The dimensions must meet the relationship described by the expression d=p[(2f/c).sup.2(1/a).sup.2], where fmagnetron frequency, and cspeed of light in vacuum. The parameter b is random.

(24) The rectangular cavity 1 is hermetically sealed after obtaining vacuum on it. The microwave power is injected into the rectangular cavity 1 through the iris 22, supplied through the waveguide 2 by a TE.sub.10 mode from a microwave generator 9 located at /4 from the end of the waveguide coupling 6, where is the wavelength of the TE.sub.10 mode. In the rectangular cavity 1, it is energized the TE.sub.10P mode (p=1, 2, 3 . . . ). The ceramic window 4 is transparent to the microwaves and serves to maintain the vacuum in the cavity. The microwave generator 9, preferably a magnetron, is protected from reflected microwave power by means of an ferrite insulator 7. The waveguide 2 by which the direction of propagation of the TE.sub.10 mode is changed, is included in order to avoid any possible impact of the electron beam with the ceramic window 4 at the moment when the X-ray source is turned on, which could happen if the waveguide 6 would be aligned with the cavity 1.

(25) Once the X-ray source is started, the electrons impact the target 11 and are extracted through the window 12 made of a light metal preferably beryllium.

(26) In another alternative embodiment, it may be considered herein as cyclotron radiation source by making some modifications to the cavity. For such purpose, it should be avoided the target 11 on which the electrons impact, and consider a window in a tangential direction to the circular path of the electrons in the plane in which the longitudinal movement stop, and engages to the resonant cavity 1 to a vacuum sample processing chamber. A system of electrodes 23, which are manufactured from a microwave-transparent material preferably graphite, is adapted to the cavity preferably in the nodes planes of the electric field TE.sub.11P as shown in FIG. 10 for the TE.sub.113 mode. The internal radius of the electrodes 23 must obviously be greater than the radius of rotation of the electrons. The insulating layers 24 allow performing different electrical potentials to each section of the cavity 1. The electrical potential along the axis of symmetry of the cavity, growing and non-monotonic, has an associated axially symmetric electrostatic field which opposes the effect of the diamagnetic force that allows electrons of the beam to move along the cavity, thereby controlling the plane where electrons stop their longitudinal movement.

(27) In this alternative embodiment, the other elements remain the same.