US 3840748 A
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llnited States Patent [191 Braunlich  3,840,748 [451 Oct. 8, 1974 ELECTRON AND X-RAY GENERATOR  Inventor: Peter Fritz Braunlich, Bloomfield 3,287,581 11/1966 Rome et a1. 313/59 3,320,467 5/1967 Smith et al.,...; 315/35 Primary Examiner;lames W. Lawrence Assistant Examiner-B. C. Anderson Attorney, Agent, or FirmJames R. lgnatowski [5 7] ABSTRACT Electrons are generated by heating in a vacuum pyroelectric material, such as a crystal of Lithium Niobate (LiNbO3) or Barium Titanate (BaTio to a temperature sufficient to polarize the material and generate an electric field at its surface sufficient for emitting electrons therefrom. A grounded electrode forms a capacitor with the surface of the pyroelectric material. The electrons may be converted to X-Rays by bombarding a suitable target associated with the electrode.
16 Claims, 1 Drawing Figure 2- ELECTRONS 1 ELECTRON AND X-RAY GENERATOR does it use B-ray emitting radioactive isotopes as used in the past.
The generator utilizes thermally induced field emission of electrons below the Curie-temperature from pyroelectric materials, such as a Lithium Niobate (LiN- bO crystal. In one embodiment the temperature of the crystal is changed in vacuum from 25C to about 100C and provides electron currents of Amp/cm? The kinetic energy of the electrons is well over 10 keV and much higher emission current densities and velocities of the electrons are possible. The generator may be used in applications where low density, high energy electrons are required. To generate X-Rays the-electrons are decelerated in a conventional metal target (Bremsstrahlung). Either a large area source or a point source may be provided by selecting the shape of the crystal.
The invention contemplates a heating and/or cooling unit, such as a Peltier-element, for heating the pyroelectric material, such as a Lithium Niobate (LiNbO crystal, enclosed in an evacuated envelope. The envelope may have an electrically grounded electrode, such as a thin metal grid or Lenard window, when the generator is used as a high energy electron source. The device may also be used as an X-Ray generator by bombarding a suitable metallic target with electrons. The electrode, that is, the window, grid or metallic target is placed opposite the emitting surface of the crystal and forms a capacitor therewith. The only power needed is for the heating or cooling unit. The heater preferably is in contact with the material to facilitate replenishing electrons to the pyroelectric material.
The invention also contemplates a method for generating electrons comprising heating pyroelectric material in a vacuum to a temperature to polarize the material and generate an electric field at its surface sufficient for emitting electrons. The method also contemplates positioning the material adjacent an electrically grounded electrode while heating to form a capacitor for receiving the electrons.
One object of the present invention is to provide an electron and/or X-Ray generator using pyroelectric material as a source of electrons.
Another object is to generate electrons or X-Rays by heating pyroelectric material in a vacuum.
Another object is to heat the pyroelectric material sufficiently to polarize the material and generate an electric field at its surface sufficient for emitting electrons therefrom.
Another object is to provide an electron and/or X-Ray generator which is simple in design and inexpensive to manufacture.
These and other objects and advantages of the invention will appear more fully hereinafter from a consideration of the detailed description which follows, taken together with the accompanying drawing wherein one embodiment of the invention is illustrated by way of example. lt is to be understood, however,.that the drawing is for the purpose of illustration only and is not a definition of the limits of the invention, had to the appended claims.
The single FIGURE of the drawing shows an elevation view of an electron and or X-Ray generator constructed according to the invention.
Referring to the drawing the novel generator shown therein and constructed according to the invention comprises an evacuated envelope 1, having a grounded electrode 3 at one end of the envelope. As mentioned above, electrode 3 may be a thin metal grid, a Lenard window, or a metallic target. The grounded grid can be used for conducting an electron current, the Lenard reference being window for passing electrons, and the metallic target for converting electrons to X-Rays. A crystal of pyroelectric material, such as Lithium Niobate (LiNbO 5 is supported by and in contact with a heater 7, which may be of the Peltier type, fixedly mounted in envelope 1. A suitable power source 9'operates the'heater. The crystal of Lithium Niobate (LiNbO is heated to about C or any temperature T below the Curie temperature and electrons are released from the crystal to the electrode. The crystal is mounted such that the direction of polarization is perpendicular to the upper surface 10.
Due to the change intemperature the spontaneous polarization P, of the pyroelectric material is changed by A P so that an electric field E =--411'AP is generated across the crystal.
Assuming A P 0.01 P, where P, 0.7 C/m then the field strength E, at the surface of a-Lithium Niobate (LiNbO crystal may be as high as 7 X 10 V/cm which is sufficient to account for the field .emission of electrons. The kinetic energy of the electrons is determined largely by the voltage V which-develops between the grounded electrode and the charges distributed in the thin surface layer of the crystal, which may be of a thickness S 10 cm. The voltage V'= 41rdAP,, and
is proportional to the distance d the crystal emitting surface is from the grounded electrode. if d 0.1 cm then V= 7 X 10 As mentioned above, this voltage determines the kinetic energy of the emitted electrons as well as the maximum energy of the X-Rays generated as Bremsstrahlung.
An electron and/or X-Ray generator constructed according to'the invention is simple in design and inexpensive to manufacture in that it only requires heating or cooling pyroelectric material in a vacuum to polarize the material and generate an electric fieldat its surface sufficient for emitting electrons therefrom. While Lithium Niobate (LiNbO and Barium Titanate (BaTiO have been given as examples of a pyroelectric material which may be used for generating electrons and/or X- Rays it should be understood that other pyroelectric materials may be usedas well.
What is claimed is:
l. A device for generating electrons comprising an evacuated envelope, pyroelectric material in the envelope, and a heater for heating the pyroelectric material to a temperature to polarize the material and generate an electric field at its surface sufficient for emitting electrons therefrom.
2. A device as described in claim 1 having an electrode associated with the pryoelectric material for collecting emitted electrons.
3. A device as described in claim 2 in which the electrode is metallic and is grounded and cooperates with the crystal to form a capacitor.
4. A device as described in claim 1 having a target plate for converting the electrons to X-Rays.
5. A device as described in claim 4 in which the target plate is grounded and cooperates with the crystal to form a capacitor.
6. A device as described in claim 1 in which the pyroelectric material is Lithium Niobate (LiNbO 7. A device as described in claim 1 in which the pyroelectric material is Barium Titanate (BaTiO;;).
8. A device as described in claim 2 in which the electrode is a window of the Lenard type for passing electrons from the envelope.
9. A device as described in claim 1 in which the heater is of the Peltier type.
10. A device as described in claim 1 including a power source for operating only the heater.
11. A device as described in claim 1 in which the heater is in contact with the pyroelectric material to replenish electrons to the material.
12. A device as described in claim 1 in which the pyroelectric material is heated to a temperature below the Curie temperature.
13. A device as described in claim 1 in which the pyroelectric material is mounted so that the direction of polarization is perpendicular to the surface of the material remote from the heater.
14. A method of generating electrons comprising heating pyroelectric material in a vacuum to a temperature to polarize the material and generate an electric field at its surface sufficient for emitting the electrons therefrom.
15. The method of generating electrons as described in claim 11 in which the material is positioned adjacent an electrically ground electrode to form a capacitor for receiving the electrons.
16. The method as described in claim 11 in which the material is heated while contacting the heater to replenish electrons to the material.