Patent Number: 
Section: description

With reference to FIG. 1, an electron accelerator 10 produces high energy electrons. In the preferred embodiment, the electron accelerator 10 generates electrons with potentials of 1 to 10 MeV. The accelerator 10 is controlled from a remote control room 12 where an operator manipulates variables such as the potential of the electrons, the destination of the electrons, and the like. The electrons from one accelerator are selectively directed to various treatment areas. The electrons are directed to an x-ray producing device 14 where they are converted into x-rays for use in a product sterilization or other treatment process. The produced x-rays irradiate a region 16, through which a product conveyer 18 conveys packages of product 20 to be sterilized or treated. An entry gate 22 controls the rate of entry of product onto the conveyer 18. This allows the product conveyer 18 to be operated at different speeds relative to other conveyers that bring product to and from the product conveyer 18 depending on the application. For products that need more irradiation, the conveyer 18 is run at a slower speed, if appropriate. Likewise, the conveyer 18 is accelerated, if appropriate, for product that needs less irradiation. In an alternate embodiment, the product conveyer always runs at a constant speed and the radiation intensity, and therefore the dose is changed. This embodiment varies the amount of radiation transmitted into the treatment region 16 as a result of more intense radiation. An exit gate 24 channels irradiated product onto another conveyer for removal from the system. This further allows the product conveyer to be operated independently of its surroundings. For safety purposes most of the conveyer 18 is within a radiation shield 26 which allows no ambient radiation to exit. The gates 22, 24 can be toggled in the preferred embodiment to allow product 20 to be irradiated multiple times if desired. For example, the product can be irradiated once from each side before being discharged and replaced. With reference to FIG. 2 and continuing reference to FIG. 1, a high energy electron beam 28 generated by the accelerator 10 is converted into x-rays 30. These x-rays 30 irradiate the product 20 which is passing on the conveyer 18. In the preferred embodiment, there is an optical or other sensor 32 that senses when the product 20 is in the treatment region 16. The optical senor 32 is coordinated with the electron accelerator control 12 such that the treatment region 16 is only irradiated when there is product 20 present. The optical sensor 32 helps extend the life of a target 34. When the x-ray source 14 is in operation, it is constantly generating heat, and is constantly cooled. By toggling the source 14 on and off, while still cooling it, the target 34 cools down more efficiently. As an option, a shield 36 made of heavy metal, such as lead or iron, is disposed behind the conveyer 18 opposite the x-ray source. This shield terminates most of the radiation that has passed through the product 20 and the conveyer 18, making the surrounding area safer. The shield 36 is preferred when the beam is directed horizontally or the installation is not on the ground floor, to protect the rooms next to or below the x-ray source. With reference to FIG. 3 and continuing reference to FIG. 2, the x-ray source target 34 is made of metal that is capable of producing x-rays when bombarded with high energy electrons. In the preferred embodiment, the target 34 is made of tantalum mounted to a substrate 40 having high thermal conductivity. Aluminum, copper, and their alloys are preferred, but other thermally conductive materials are also contemplated. When electrons cross a vacuum and hit the target 34, much of their energy is converted into heat. The conductive substrate 40 conducts the heat away from the target 34. Coolant fluid, water in the preferred embodiment for simplicity of handling, flows through tubes, bores, or other cavities 42 in the substrate to conduct heat away from the system. Other fluids, such as coolant oil are also contemplated. Preferably, the coolant fluid does not come into direct contact with the target 34. Because of this, the target is protected from oxidation and corrosion as a result of exposure to the coolant. Alternately, the coolant could flow directly over the target 34. Preferably corrosion inhibitors are added to reduce corrosion and extend the life of the target. The x-ray source 14 includes deflection plates 44 located along a periphery of an accelerator horn 46. The deflection plates 44 electrostatically or magnetically manipulate a direction of the electron beam 28 such that the electron beam 28 does not always hit the same spot on the target 34. More specifically, the control 12 controls the deflection plates in accordance with dimensions of the product. Typically, the scan horn is elongated, for example, about a meter long. The electron beam is swept back and forth over a distance commensurate with the corresponding dimension of the passing product. To promote cooling of the target, the electron beam is also moved side to side. For example, the electron beam is swept along one line in a first sweep and along a parallel line on the return sweep. More complex sweep patterns such as following a multiplicity of parallel, shifted sweep paths, sinusoidal or other non-linear sweep paths, oval loops, and other two dimensional paths are also contemplated. In the preferred embodiment, the deflection plates 44 are electrostatic plates which, when negatively charged, repel the electron beam. Positively charged plates to attract the beam are also contemplated. Alternately, they may be magnetic plates. The plates can be located inside or outside of the vacuum. If electrostatic plates are located inside the vacuum, hermetic feedthroughs for electrical leads are provided. With reference to FIG. 4, a detailed view of a preferred target 34 is provided. The target 34 is divided into multiple layers, three in the preferred embodiment. The target layers are sandwiched between by layers of the thermally conductive substrate 40. When the x-ray source 14 of the preferred embodiment is in operation, the electron beam 28 strikes a first layer 34a of tantalum foil. Some of the electrons are converted into x-rays and some pass through the first layer of target. Those electrons which pass through strike a second layer 34b of target, where some are converted and some pass through. The process is again repeated for a third layer 34c.  The target layers in the preferred embodiment are films or coatings of the target material adhered to layers of substrate material. As illustrated in FIG. 4, the target layers 34a, 34b, 34c are progressively thinner. Each layer has a different capability of stopping electrons. Typically, different energies are stopped in different layers. As a result, different x-ray spectra result from each layer. Further, the second and third layers filter out low energy x-rays generated in the upstream target layers. This is an advantage of having multiple layers of target as opposed to one thick layer of target. It is to be understood that the x-rays generated in the preferred embodiment have a direction of propagation that is generally the same as the electron beam. To help focus the x-rays in a forward direction, the substrate is shaped with forward extending side flanges. The greater material thickness at the flanges absorbs more x-rays than the thinner central window portion. Optionally, a layer of filter material, such as stainless steel, is positioned between one or more target layers and the treatment region to absorb low energy x-rays. Typically, the best conventional x-ray targets only convert approximately 15% of the kinetic energy of the incumbent electrons into x-rays. The target 34 of the present invention converts about 80% of the electrons"" energy into x-rays. This is done by supporting a very wide variety of energies in the target. What would not get used in a conventional target, passes through the first layer 34a and interacts with the second, and so on. Since more of the electrons are being used, less are being converted into heat. This makes cooling the target a somewhat easier proposition. In an alternate embodiment, one thick layer of target could be used instead of multiple thinner ones and achieve the same electron stopping power. Because common target materials, such as tantalum and tungsten are relatively poor heat conductors, the heat from the anode target is removed more slowly. The invention has been described with reference to the preferred embodiment. Modifications and alterations will occur to others upon a reading and understanding of the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations insofar as they come within the scope of the appended claims or the equivalents thereof.