TARGET FOR PRODUCTION OF X-RAYS

Abstract: A source of electrons (10) generates a beam of free electrons which are accelerated through a vacuum chamber and collide with a target (34). The target has multiple layers of a high Z material, such as tungsten or tantalum, for producing x-ray radiation when bombarded with high energy electrons. The target layers are located in sequence such that electrons that are not terminated in one layer pass to a subsequent layer. The target layers are sandwiched between layers of a thermally conductive, low Z metal substrate (40), such as aluminum or copper. Hollow passages (42) are bored in the substrate (40) to allow a coolant fluid, such as water, to flow within them and carry unwanted heat away from the target. The passages are within the substrate, keeping the water form target material. This reduces oxidation and corrosion of the target and results in extended life.

Description: TARGET FOR PRODUCTION OP X-RAYS Background of the invention The present invention relates to the irradiation arts. It finds particular application in the field of product sterilization, disinfection, and radiation treatment and will be described with particular reference thereto. However, the present invention is applicable to a wide variety of other applications including, but not limited to, food and spice treatment, plastics modification, x-ray imaging, genetic modification, and other fields in which controlled doses of radiation are advantageous. Products are typically irradiated by being conveyed past a radiation source, such as cobalt rods, electron beam accelerators, or x-ray sources. Cobalt rods are effective, but cannot be turned off for maintenance in the treatment vault. Rather, they are mechanically immersed in heavy water. Spent cobalt rods are changed and stored deep in the heavy water. Accelerated electron beams are easy to control, but have limited penetration power relative to x-ray or γ-ray radiation. X-rays are high energy photons that are produced as a result of accelerated electrons interacting with a target. Typically, metals such as tungsten or tantalum are used. To produce x-rays, free electrons are generated, such as by being boiled off of a filament. The electrons are accelerated in a vacuum through a potential to a desired kinetic energy toward the metal target. The accelerated electrons interact with the electrons naturally present in the target metal. As the electrons interact, some of the kinetic energy of the incoming electrons is transferred into the electrons of the target metal perturbing them into higher energy states. Over time these electrons decay back to their lower energy states releasing energy in the form of x-rays . X-rays have been found to be very useful in the sterilization of products. This type of high energy radiation, in sufficient doses, kills most all types .of living organisms. This includes parasitic bacteria and viruses which have the potential of making people ill. This is useful for sterilizing food meant for consumption, as well as other products such as medical instruments. Of course there is no chance of residual radiation with x-rays, so the product is safe afterwards, and will not harm the consumer as a result of being irradiated. One of the biggest problems with x-ray production is that not all of the energy of the incoming electrons is converted into x-rays in this manner. The majority of the energy is lost to non-useful collisions and converted into heat. Typically, the best systems convert approximately 15% of the kinetic energy of the incoming electrons into x-rays, i.e. approximately 85% of the energy is converted into heat. This amount of heat is sufficient to destroy or damage the target. In order to conserve the integrity of the target, and thus, the system, sufficient heat is removed to maintain the target below a preselected maximum temperature. Different types of cooling systems are employed. Relative movement between the electron beam and the target permits heated spots of the target to cool between electron beam irradiations. In high energy applications, the electron beam returns before cooling is complete and heat builds to target damaging levels. Some x-ray systems have a fluid coolant that flows over the target, transferring the produced heat away from the target. Problems with this type of system are low efficiency of the cooling system and short life of the target. Typically, the fluid used is water which flows over the metal target. Over time and extreme stress, the target corrodes. The present invention presents a new method and apparatus that overcomes the above-referenced problems and others . summary of the Invention In accordance with one aspect of the present invention, a product irradiation device is provided. A conveyor conveys products past a scan horn. An electron accelerator accelerates electrons. An evacuated path conveys the accelerated electrons from the accelerator to the scan horn. An electron sweeping system sweeps the accelerated electrons across the scan horn. A face plate on the scan horn is of a thermally conductive material. An anode target is mounted to the face plate to convert the accelerated electrons into x-rays. Coolant fluid channels are defined in the face plate. In accordance with another aspect of the present invention, a a product irradiation device is provided. A conveyor conveys products past a scan horn. An electron accelerator accelerates electrons. An evacuated path conveys the accelerated electrons from the accelerator to the scan horn. An electron sweeping system sweeps the accelerated electrons across the scan horn. A face plate on the scan horn is of a thermally conductive material. An anode target is mounted to the face plate to convert the accelerated electrons into x-rays. The anode target includes a plurality of layers of a high Z target material interleaved with thermally conductive material. In accordance with another aspect of the present invention, a method of x-ray production is provided. The method includes generating and accelerating an electron beam and striking a target with the electron beam to generate x- rays. A first layer of the target is struck with the electron beam and a first portion of the electrons is converted into x-rays. A second portion of the electrons passes through the first target layer and strikes a second layer of the target. The second portion of the target is spaced from the first portion of the target by a thermally conductive layer. A portion of the electrons striking the second layer of target is converted into x-rays. In accordance with another aspect of the present invention, an x-ray target for closing an evacuated chamber through which high energy electrons travel is provided. The target includes multiple layers of high Z target material and multiple layers of thermally conductive low Z substrate interleaved between the target layers. One advantage of the present invention is that it produces x-rays efficiently. Another advantage of the present invention is that anode life is extended. Another advantage of the present invention is that coolant corrosion of the target is eliminated. Yet another advantage of the present invention resides in reduced heating. Still further benefits and advantages of the present invention will become apparent to those skilled in the art upon a reading and understanding of the preferred embodiments . Brief Description of the Drawings The invention may take form in various components and arrangements of components, and in various steps and arrangements of steps. The drawings are only for purposes of illustrating preferred embodiments and are not to be construed as limiting the invention. FIGURE 1 is an overhead view of a product treatment system in accordance with the present invention; FIGURE 2 is a more detailed view in partial section of a radiation generation region of the system of FIGURE 1; FIGURE 3 is a side sectional view of a scan horn and an x-ray generating apparatus in accordance with the present invention; and FIGURE 4 is a detailed view of a target of the x- ray producing apparatus of FIGURE 3. Detailed Description of the Preferred Embodiment With reference to FIGURE 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 12 in a room 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 10 are selectively directed to various treatment areas. The electrons are directed to an x-ray producing device 14, via an evacuated path 15, 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 conveyor 18 conveys packages of product 20 to be sterilized or treated. An entry gate 22 controls the rate of entry of product onto the conveyor 18. This allows the product conveyor 18 to be operated at different speeds relative to other conveyors that bring product to and from the product conveyor 18 depending on the application. For products that need more irradiation, the conveyor 18 is run at a slower speed, if appropriate. Likewise, the conveyor 18 is accelerated, if appropriate, for product that needs less irradiation. In an alternate embodiment, the product conveyor 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 conveyor for removal from the system. This further allows the product conveyor to be operated independently of its surroundings. For safety purposes most of the conveyor 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 FIGURE 2 and continuing reference to FIGURE 1, a high energy electron beam 28 generated by the accelerator 10 is converted into x-rays 30 in an evacuated chamber 31. These x-rays 30 irradiate the product 20 which is passing on the conveyor 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 sensor 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, positioned in the evacuated chamber 31, which converts the accelerated electrons to x-rays. 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 conveyor 18 opposite the x-ray source. This shield terminates most of the radiation that has passed through the product 20 and the conveyor 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 FIGURE 3 and continuing reference to FIGURE 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 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 channels, such as 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 an electron sweeping system, such as deflection plates 44. These are located along a periphery of an accelerator horn 46 which defines the evacuated chamber 31. 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 FIGURE 4 , a detailed view of a preferred target 34 is provided. The target 34 is divided into multiple layers 34a, 34b, 34c, three in the preferred embodiment. The target layers are sandwiched between layers 40a, 40b, 40c 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 or tungsten 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 (which are High-Z , i.e., tend to absorb radiation) adhered to layers of substrate material (Low-Z, i.e., permit radiation to pass through readily) . As illustrated in FIGURE 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 40 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, which are often high-Z materials, such as tantalum and tungsten are relatively poor heat conductors, the heat from the anode target is removed more slowly.

Claims: Having thus described the preferred embodiments, the invention is now claimed to be: 1. A product irradiation system comprising a conveyor (18) which conveys products past a scan horn (46) , an electron accelerator (10) which accelerates electrons, an evacuated path which conveys the accelerated electrons from the accelerator to the scan horn, an electron sweeping system (44) which sweeps the accelerated electrons across the scan horn, a face plate (40) on the scan horn of a thermally conductive material, an anode target (34) to convert the accelerated electrons into x-rays, characterized by: the anode target being mounted to the face plate; the face plate (40) being of a thermally conductive material ; and coolant fluid channels (42) being defined in the face plate. 2. The product irradiation system as set forth in claim 1, further characterized by: the target including a plurality of layers (34a, 34b, 34c) , the layers being spaced from each other by a layer of a thermally conductive material (40a, 40b) . 3. The product irradiation system as set forth in claim 2, further characterized by: the layers being in thermal contact with the coolant fluid channels (42) . 4. The product irradiation system as set forth in claim 1, further characterized by: the target including layers (34a, 34b, 34c) of target material interleaved with layers (40a, 40b, 40c) of a thermally conductive material. 5. The product irradiation system as set forth in claim 4, further characterized by: the layers (40a, 40b, 40c) of a second material being in thermal contact with each other and with the coolant fluid channels (42) . 6. The product irradiation system as set forth in claim 5, further characterized by: the target layers (34) being mounted to the layers (40a, 40b, 40C) . 7. The product irradiation system as set forth in any one of preceding claims 1-6, further characterized by: the target including three layers (34a, 34b, 34c) . 8. The product irradiation system as set forth in any one of preceding claims 1-7 , further characterized by: the face plate (40) including three layers (40a, 40b, 40c) . 9. The product irradiation system as set forth in any one of preceding claims 1-8 , further characterized by: the electron sweeping system sweeping the electrons transversely and longitudinally across the target. 10. The product irradiation system as set forth in any one of preceding claims 1-9 , further characterized by: a radiation shield (26, 36) that protects surrounding areas from stray radiation. 11. The product irradiation system as set forth in any one of preceding claims 1-10, further characterized by: a coolant system which pumps a coolant fluid from a remote location to the channels. 12. The product irradiation system as set forth in claim 11, further characterized by: an operator accessible control system (12) that coordinates the operation of the electron accelerator, the scan horn, the product conveyor, and the coolant system. 13. The product irradiation device as set forth in any one of claims 2-6 and 12, further characterized by: the target layers (34a, 34b, 34σ) each including a coating of target material upon an adjacent layer (40a, 40b, 40c) of the thermally conductive material. 14. The product irradiation device as set forth in any one of preceding claims 2-6 and 12, further characterized by: the target layers (34a, 34b, 34c) including tantalum or tungsten foil. 15. The product irradiation device as set forth in any one of preceding claims 1-6 and 12, further characterized by: a coolant fluid which flows through the coolant channels (42) to draw heat away from the target. 16. The product irradiation device as set forth in claim 15, further characterized by: the coolant fluid being water. 17. The product irradiation device as set forth in any one of preceding claims 15 and 16, further characterized by: the channels being remote from the target layers such that the coolant fluid flows do not physically contact the target. 18. The product irradiation device as set forth in any one of preceding claims 1-17, further characterized by: an optical sensing device (32) that senses when a product is in a sterilization region and directs the electron accelerator to emit electrons only when there is a product in the sterilization region. 19. A product irradiation system comprising a conveyor (18) which conveys products past a scan horn (46) , an electron accelerator (10) which accelerates electrons, an evacuated path which conveys the accelerated electrons from the accelerator to the scan horn, an electron sweeping system (44) which sweeps the accelerated electrons across the scan horn, a face plate (40) on the scan horn of a thermally conductive material, an anode target (34) to convert the accelerated electrons into x-rays, characterized by: the anode target including a plurality of layers (34a, 34b, 34c) of a high Z target material interleaved with thermally conductive material (40a, 40b, 40c) . 20. A method of x-ray production comprising generating and accelerating an electron beam and striking a target (34) with the electron beam to generate x-rays, the method characterized by the step of striking the target including: striking a first layer (34a) of the target with the electron beam; converting a first portion of the electrons in the beam into x-rays, a second portion of the electrons passing through the first target layer; striking with the second portion of electrons a second layer (34b) of target, the second portion of the target being spaced from the first portion of the target by a thermally conductive layer (40a) ; converting a portion of the electrons striking the second layer of the target into x-rays. 21. The method as set forth in claim 20, further characterized by: striking at least one additional target layer with electrons that passed through the second target layer and producing x-rays. 22. The method as set forth in either one of preceding claims 20 and 21, further characterized by: dissipating heat generated in the target by contacting a thermally conductive material (40) with a cooling fluid, the thermally conductive material being thermally connected with the thermally conductive layer (40a) . 23. An x-ray target (34) for closing an evacuated chamber (31) through which high energy electrons travel, the target characterized by: multiple layers (34a, 34b, 34σ) of high Z target material ; and multiple layers (40a, 40b) of thermally conductive low Z substrate interleaved between the target layers. 24. The x-ray target as set forth in claim 23, further characterized by: channels (42) remote from the target layers through which a coolant fluid flows to draw heat from the low Z substrate layers, without physically contacting the target.

Inventors: KORENEV SERGEY A, KORENEV, SERGEY, A.
Applicants: STERIS INC, STERIS INC.
Application year: 2001
Main classification: H05H6
IPC codes: A23L3, A61L2, B01J19, G21K5, H01J35, H05G2, H05H6