Patent Number: 054616565
Section: summary

FIELD OF THE INVENTION The present invention concerns an X-ray source and method of surface and volume sterilization of such objects as seeds, foods, water, medical supplies and instruments, and numerous other items. More particularly, it concerns the use of an electron cyclotron resonance (ECR) plasma to provide such a source and method. BACKGROUND OF THE INVENTION The classical X-ray sources that have been used for sterilization are generally either radioactive nuclides or high voltage vacuum tubes, both of which have limited applications. The radioactive nuclides typically have comparatively short lives and are expensive and dangerous to handle. In addition, the gamma or X-ray intensity they produce cannot be readily monitored and, once activated, the sources cannot be turned off. The high voltage vacuum tube apparatus requires heavy and cumbersome high voltage supplies, which are also dangerous to handle. Further, this apparatus needs highly qualified personnel to operate and maintain it. To avoid these deficiencies it was recently proposed to use a compact cyclic electron accelerator as a source of X-ray in the range of 150 keV photon energy. See H. R. Gardner, T. Ohkawa, A. M. Howald, A. W. Leonard, L. S. Peranich and J. R. D'Aoust, Rev. Sci. Instruments, 61(2), February 1990, pages 724-727. In this source, the electrons are injected from an electron emitter into a vacuumated resonant cavity, and accelerated by a microwave field along a spiral orbit to a molybdenum target, which produces the X-ray radiation. Although this source advantageously avoids the use of radioactive nuclides and a high voltage supply, it is not a reliable tool for routine use in the industry and agriculture. The reasons are that the emitted X-ray intensity is too weak, and the life of the device is limited by the life of an electron emitter, which cannot readily be replaced without the replacing of the entire vacuumated cavity. Also, since the X-rays are produced by an electron beam striking a molybdenum target, only half of the produced X-rays are reflected from the target, to be useful, while the other half are absorbed in the target and lost. Further, this source cannot produce homogeneous X-ray radiation in all radial directions from a central source, such as is useful, for example, in sterilizing fluids, e.g., water, from a source centrally located within the flow of the fluid as described below. There remains a continuing need for better sources of X-rays, and in particular for economical X-ray sources having sufficient intensity for surface and volume sterilization of goods and food stuffs. SUMMARY OF THE INVENTION The present invention concerns an X-ray source which is free of the above deficiencies and provides nearly the same X-ray intensity and energy as the classical high voltage X-ray sources, although it has a drastically smaller volume, weight, electrical consumption and cost. In addition, the X-ray source of the present invention has no elements, such as microwave coupling elements, electron emitters, targets or the like, within a vacuumated volume. This dramatically increases the stability, reliability, intensity and life of the source. In addition, it does not rely on radioactive nuclides or a high voltage supply, and it is extremely reliable and simple both in operation and maintenance. Advantageously, because of its small size and low cost, the X-ray source of the present invention can be used in small scale operations such as local food processing plants and the like, unlike the known commercial devices which are limited to large sterilization plants. The present invention also concerns using multiple X-ray sources, cascaded in succession along an existing processing or packaging line or assembled in modular units to achieve whatever cumulative amount of X-ray radiation may be desired for a given throughput rate. Moreover, the X-ray sources can be made mobile and easily transported to any location for use. Broadly, the invention is directed to apparatus and methods for producing X-ray radiation by providing a vacuumated chamber that is made of a dielectric material and is filled with a heavy atomic weight gas at low pressure and placing the chamber in a resonant magnetic field that generates an Electron Cyclotron Resonance (ECR) plasma inside the chamber. The hot electrons of the ECR plasma bombard the heavy gas in the chamber and the dielectric material of the chamber which in turn produces X-ray radiation. The vacuumated chamber is preferably a spherical quartz glass chamber and the heavy gas is preferably xenon gas. In one embodiment, the ECR plasma is generated by placing the chamber inside a non-vacuumated microwave resonant cavity which is in its turn disposed in a magnetic field, preferably between opposite poles of two permanent magnets. This configuration, when microwave energy is coupled into the resonant cavity, provides a compact axisymmetric magnetic mirror configuration with an ECR plasma inside of the chamber with respect of the selected microwave frequency. This is described in greater detail below. The resultant X-ray is emitted homogeneously in the equatorial plane of the cavity perpendicular to the magnetic field axis. The lateral wall of the resonant cavity is made of a material which is transparent for the X-ray, but is not transparent for the microwave. For example, a light metal, e.g., aluminum, grid having an appropriate mesh size for trapping microwaves is preferred. The electrical (microwave) supply required for this X-ray source (which is referred below as ECR-X) is similar to those of conventional domestic microwave ovens, but it consumes only half of the electrical power (.about.1 kW). Another aspect of the invention concerns a method of forming a source for use in producing X-rays, which source is a replaceable item of an apparatus for producing X-rays. One method of forming such a source includes the steps of: forming a chamber of a dielectric material; PA1 evacuating the chamber under conditions of temperature and pressure sufficient to out gas impurities from the dielectric material; PA1 filling the chamber with a heavy atomic weight gas or gas mixture; and PA1 sealing the chamber with the pressure inside the chamber being between 10.sup.-4 and 10.sup.-5 Torr. PA1 a plurality of X-ray sources, each X-ray source comprising: PA1 means for advancing the material to be exposed to X-rays emitted by one or more of the plurality of X-ray sources. PA1 (a) forming a flow passageway for passing the water or other material through a defined area; PA1 (b) providing a non radioactive source for producing X-rays inside the flow passageway; PA1 (c) generating X-rays inside the flow passageway at a first intensity; PA1 (d) passing the material in a flow at a first rate through the flow passageway and the X-rays to sterilize the water or other material. Another aspect of the present invention concerns apparatus for irradiating a material, article, or product that is being processed or conveyed with X-ray radiation. One such apparatus includes: a microwave resonant cavity; PA2 a sealed vacuumated chamber filled with a heavy atomic weight gas or gas mixture located inside the cavity; and PA2 means for applying a magnetic field to the sealed chamber for use in producing an electron cyclotron resonance plasma within the chamber; and A cladding, surrounding the advancing means and plurality of X-ray sources, is preferably provided for containing X-rays. The cladding is provided with one or more openings for the entry and exit of materials. Preferably, the apparatus includes a source of microwave energy, which may be one supply or more than one supply, and a waveguide network for feeding microwaves from the source to the resonant cavity of each of the X-ray sources. The waveguide network may be configured so that each microwave energy source feeds microwaves to more than one X-ray source. The apparatus further may be built on a movable platform, such as a truck, a trailer attachable to a vehicle, an airplane or other movable structure. The term "truck bed" refers to a platform on a truck or other automotive vehicle or a trailer attachable to another automotive vehicle. Another aspect of the invention concerns a method for sterilizing water or other flowing material comprising: Preferably, the passageway is clad with a material for containing X-rays. They may be more than one non radioactive X-ray source, each source being interposed in the flowpath and spaced apart along the passageway and generating X-rays at an intensity so that the flowing material is cumulatively exposed to an intensity that corresponds to the first intensity. Preferably, each source is an ECR-X source immersed in the flow. Also, the flow passageway may be bent on opposite sides of the X-ray source(s) and cladding applied to the flow passageway between and about the bends so that X-rays generated by the source(s) inside the flow passageway are confined within the flow passageway.