ON-LINE MEASUREMENT OF ABSORBED ELECTRON BEAM DOSAGE IN IRRADIATED PRODUCT

Abstract: A conveyor (32) moves items (30) through an electron beam (22) for irradiation treatment. An array (40a) of inductive detectors detects the energy of the electron beam exiting the item at a number of altitudes. A processor (54) determines the absorbed dose of radiation from the reduction in the beam energy. The dose information is compared by a processor (58) with target doses. Deviations are used to control energy or beam current of the electron beam, the sweep rate, and/or the conveying speed of the items to achieve the target dose. The detectors include a vacuum chamber containing two current transformers (60, 62) on either side of a metal foil layer (64). From the difference in the current induced in the two transformers by a pulsed, collimated electron beam, the energy of the beam is determined.

Description: ON-LINE MEASUREMENT OF ABSORBED ELECTRON BEAM DOSAGE IN IRRADIATED PRODUCT Background of the Invention The present invention relates to the irradiation arts. It finds particular application in conjunction with measuring the absorbed radiation dose in systems for irradiating objects with an electron beam and will be described with particular reference thereto. It is to be ' appreciated, however, that the invention will also find application in conjunction with the monitoring of charged particle beams in coating by a synthesis of powdered material, surface modification of material, destruction of toxic gases, destruction of organic wastes, drying, disinfection of food stuffs, medicine, and medical devices, polymer modification, and the like. Heretofore, electron or e-beam irradiation systems have been developed for treating objects with electron beam radiation. An accelerator generates electrons of a selected energy, typically in the range of 0.2-20 MeV. The electrons are focused into a beam through which containers carrying the items to be treated are passed. The conveying speed and the energy of the electron beam are selected such that each item in the container receives a preselected dose. Traditionally, dose is defined as the product of the kinetic energy of the electrons, the electron beam current, and the time of irradiation divided by the mass of the irradiated product. Various techniques have been developed for precalibrating the beam and measuring beam dose with either calibration phantoms or samples. These precalibration methods include measuring beam current, measuring charge accumulation, conversion of the e-beam to x-rays, heat, or secondary particles for which emitters and detectors are available, and the like. These methods are error prone due to such factors as ionization of surrounding air, shallow penetration of the electron beam, complexity and cost of sensors, and the like. One of the problems with precalibration methods is that they assume that the product in the containers matches the phantom and that it is the same from package to package. They also assume a uniform density of the material in the container. When these expectations are not met, portions of the material may be under-irradiated and other portions over-irradiated. For example, when the material in the container has a variety of densities or electron stopping powers, the material with the high electron stopping power can "shadow" the material on the other side of it from the electron beam source. That is, a high percentage of the electron beam is absorbed by the higher density material, such that less than the expected amount of electrons reach the material downstream. The variation from container to container may result in over and under dosing of some of the materials within the containers. One technique for verifying the radiation is to attach a sheet of photographic film to the backside of the container. The photographic film is typically encased in a light opaque envelope and may include a sheet of material for converting the energy from the electron beam into light with a wavelength that is compatible with the sensitivity of the photographic film. After the container has been irradiated, the photographic film is developed. Light and dark portions of the photographic film are analyzed to determine dose and distribution of dose. One disadvantage of the photographic verification technique resides in the delays in developing and analyzing the film. The present invention provides a new and improved radiation monitoring technique, method of irradiation, and apparatus therefor, which overcomes the above referenced problems and others. giimma γ of the Invention In accordance with one aspect of the present invention, a method of determining an absorbed dose of an electron beam in an irradiated product is provided. The method includes determining a reduction in the kinetic energy of the beam from a final kinetic energy of the electron beam exiting the product and from an initial kinetic energy of the beam before entering the product. The absorbed dose is determined from the reduction in the kinetic energy of the beam. In accordance with another aspect of the present invention, an irradiation apparatus is provided. The apparatus includes a charged particle beam generator for generating and aiming a charged particle beam of a first kinetic energy along a preselected path, and a conveyor which conveys an item to be irradiated through the beam. A beam strength monitor monitors a second kinetic energy of the beam after it has passed through the item. In accordance with another aspect of the present invention, an energy detector for determining the energy of an electron beam is provided. The detector includes a vacuum chamber. First and second inductive coils are disposed in the vacuum chamber in which currents are induced by the electron beam. A foil having known absorption characteristics is disposed between the first and second inductive coils, such that the energy of the beam can be determined from the first and second currents. In accordance with another aspect of the present invention, a method of irradiation is provided. The method includes moving an item through a charged particle beam and determining an energy of the charged particle beam exiting the item. The energy of the charged particle beam exiting the item is subtracted from an energy of the charged particle beam entering the item. One advantage of the present invention resides in the real time measurement of absorbed dose. Another advantage of the present invention resides in more accurate determination of absorbed doses and reducing dosing errors. Another advantage of the present invention resides in the automatic control and modification of an irradiation process on-line to assure prescribed dosing. Still further advantages of the present invention will be apparent to those of ordinary skill in the art upon reading and understanding the following detailed description. 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 a preferred embodiment and are not to be construed as limiting the invention. FIGURE 1 is a perspective view of a e-beam irradiation system in accordance with the present invention; FIGURE 2 is a cross sectional view of one of the detectors of FIGURE 1; and, FIGURE 3 is a graph of K as a function of electron kinetic energy where (1) the thickness of a foil is 300 μm and (2) the thickness of the foil is 500 μm. Detailed Description of the Preferred Embodiments With reference to FIGURE 1, an accelerator 10 is controlled by a beam voltage and current controller 12 to generate a beam of electrons with a preselected energy (MeV) and beam current. In the preferred embodiment, the electrons are generated by a Rhodotron™ brand accelerator in the range of 1-10 MeV. A sweep control circuit 14 controls electromagnets or electrostatic plates of a beam deflection circuit 16 to sweep the electron beam, preferably back and forth in a selected plane. A titanium or aluminum window 18 of a vacuum horn 20 defines the exit from the vacuum system from which the electron beam 22 emerges for the treatment process. An electron absorbing plate 24 collects electrons and channels them to ground. A conveying system conveys items 30 through the e- beam 22. In the illustrated embodiment, the conveyor system includes a horizontal belt conveyor 32 which is driven by a motor 34. A motor speed controller 36 controls the speed of the motor. Of course, other types of conveyor systems are contemplated, including overhead conveyors, pneumatic or hydraulic conveyors, spaced palettes, and the like. In the illustrated belt conveyor system, the items 30 are positioned one after another on the conveyor belt closely packed with a minimal gap in between. Preferably, the items are packages or palettes of fixed size which hold individual items to be irradiated. A plurality of radiation detector arrays 40a, 40b, are positioned in the path of the e-beam 22. The first detector array 40a is in array that measures the strength (energy) of the electron beam after it has exited the item. The optional second detector array 40b detects the energy of the e-beam before it enters the product, if the energy is not otherwise known. The outputs of both the detector arrays 40a, 40b are conveyed to an amplifier section 44 for amplification. In the preferred embodiment, the outputs are digitized 46, serialized 48, converted into optical signals 50, and conveyed to a remote location. The amplifier section 44 is shielded to protect the electronics from stray electrons and static fields that might interfere with the electronic processing. The optical signal is conveyed to a location remote from such stray charges where it is converted to selected electronic format 52 and analyzed by a processor 54, such as a computer. Preferably, the beam control 12 provides the energy of the electrons entering the product. The computer subtracts or otherwise compares the strength of the electron beam before and after it enters each item. The processor 54 further compares the strength of the beam at various distances from the conveyor (heights in the illustrated embodiment) to identify regions in which high density materials may be interfering with complete irradiation of the downstream material. The processor determines the dose received by each region of each item and forwards that dose information to an archival system 56 such as a computer memory, a tape, or a paper printout. In a first alternate embodiment, the processor 54 compares the measured dose information with preselected dose requirements. Based on differences between the selected and actual dosage, a parameter adjustment processor 58 adjusts one or more of the beam energy, the beam sweep, the conveyor speed, and the like. For example, when the detectors detect that near portions of the items are absorbing too much radiation leaving far portions of the items under irradiated, the parameter adjustment processor 58 increases or adjusts the accelerator to increase the MeV or the electron beam current, up to maximum values set for the items being irradiated. Once the maximum dose is reached, the adjustment processor 58 controls the motor speed controller 36 to reduce the speed of the conveyor. When the items have small regions of higher density, the sensing of an increase in the absorbed radiation causes the parameter adjustment processor 58 to increase the energy of the electron beam or decrease the speed of the conveyor until the region of higher density has passed through the beam. Thereafter, the beam power can be reduced or the conveying speed can be increased. Analogously, when the region of higher density is localized vertically, in the illustrated horizontal conveyor embodiment, the parameter adjustment processor 56 causing the sweep control circuit 14 to adjust the sweep such that the electron beam is directed to the higher density region for a longer duration. Preferably, the beam strength and the conveying speed are also adjusted to maintain the appropriate dosing in other regions of the package. Analogously, in response to regions of little absorption of the electron beam, the sweep circuit can be controlled to dwell for a shorter percentage of the time on these regions. In the preferred embodiment, the detectors are inductive detectors that detect the increases and decreases in electron beam energy. That is, although the electron beam may be viewed as a beam that is the full width of the horn 20, more typically the beam of electrons is focused into about a pulsed two centimeter diameter ray. This ray is swept up and down rapidly compared to the speed of the conveyor such that the electron beam is effectively a wall. More specifically to the preferred embodiment, and with reference to FIGURE 2 each detector array includes a first coil or current transformer 60 and a second coil or current transformer 62. Between them, a metal foil 64, aluminum in the preferred embodiment with a selected energy absorption profile, is disposed. Both current transformers 60, 62 and the metal foil 64 are located within a vacuum chamber 66. The pulsed electron beam passes through a collimator 68 equipped with a cooling system and passes through the first current transformer 60. The sweeping electron beam 22 sends electron beam current pulses through the first transformer which induces currents circumferentially therearound in the first transformer which induced current is measured and the measurement held or stored. The beam passes through the metal foil, which is 3xl0 "4 to 6xl0 "4 m thick aluminum in the preferred embodiment. The beam passes through the second current transformer 62, again inducing currents. The second induced current is less than the first induced current by the amount of absorption in the foil which is based on the thickness of the metal foil 64. The currents are compared, and from that information, the energy of the electron beam is determined. The energy of the electron beam can be determined empirically by measuring the current drop between the two coils with electron beams of different known energies. Alternately, the energy can be calculated from the physics of the detector including foil thickness, atomic number of the metal in the foil, number of turns in the transformer coil, and the like. More specifically, the scanning mode of the electron accelerator leads to a pulsed character of the electron beam in cross-section. The primary electron beam has a current I 0 and kinetic energy E Q . After propagation of the electron beam across the irradiated product, the electron beam has a kinetic energy E.,. The number of electrons is the same on both sides of the product, because electrons only lose kinetic energy. In the detector, the measurement of the electron beam current in front and behind the absorption foil 64 by the transformers 60, 62 enables the determination of an absorption factor K of the electron beam within the foil: K = ψ = m (1) where, I, is the beam current in front of the foil and I 2 is the current behind the foil. The charge Q of the beam after the foil is: Q = Q 0 *e -^ d (2) where Q is the charge after the foil and Q 0 is the charge before the foil. M/p is the mass absorption coefficient for the foil and is a function of the energy, f(E), and d is the thickness of the foil. Recognizing that current is charge per unit time, Q=Q 0 *e "(m/p)*d yields: I 2 = /,*e^ ,rf (3) From measurements with a plurality of different foil thicknesses, the dependence of K on the kinetic energy of the electrons can be calibrated. Hence, the kinetic energy of the measured electrons can be determined. Looking to FIGURE 3 , a standard dependency for the coefficient of partial transmission of energy for aluminum foils of 300 and 500μm is illustrated. After the determination of E 1 from these measurements, the energy absorbed in the product E is calculated by: E p = E o ~ E x (4) From the beam current which the accelerator is controlled to put out, the scanning rate and other parameters of the electron beam in the scan horn, and a diameter of the hole in the collimator 68, one can determine the number of electrons N e passing through the detector. The absorbed Joule's energy E j in the product: E. = E p *N e * 1.6 *10- 19 [J] (5) Because the total mass of the product or package is known, the mass of the product along the ray in front of the detector with the diameter of the collimator hole is: M = Q.SD 2 c *L*p (6) where p is the density of the product, L is the thickness of the product, and D c is beam diameter after collimation. Hence, the absorb dose D is: D = EJM (7) The processor 54 calculates this factor. The processor is preferably preprogrammed with lookup tables to which this factor is compared. Based on this comparison, the parameter adjustment processor 58 makes appropriate adjustments to process controls, a human readable display indicative of dosing is produced, data is stored in the archival system 56, or the like. Although illustrated relatively large in comparison to the items, it is to be appreciated that the individual detectors can be very small compared to the items. The array 40a may, for example, include hundreds of individual detectors. The array 40b may, for example, be only a single detector. It is also to be appreciated that the electron beam can be swept in other dimensions. For example, the beam can also be swept parallel to the direction of motion of the conveyor. When the beam is swept in two dimensions, it cuts a large rectangular swath. The electron density entering a unit area of the item per unit time is lower, but the product remains within the beam longer. The side to side movement of the beam allows for the placement of a two dimensional array above or below the items to measure absorbed dose in two dimensions. It is further to be appreciated that this detection system can be used to detect charged beams in numerous other applications. For example, this detector can be used in conjunction with electron beams that are used to create coatings by the synthesis of powdered material, such as diamond like coatings (die) on tools, nanophase silicon nitrite coatings, high purity metal coatings, and the like. It can be used with charged particle beams for surface modification such as cleaning of metals, surface hardening of metals, corrosion resistance, and other high temperature applications. The detector can also be used for electron beams which are used in the destruction of toxic gases such as the cleaning of flue gases for oxides of sulfur and nitrogen, removal of exhaust gases from diesel engines, destruction of fluorine gases, destruction of aromatic hydrocarbons, and the like. The detector may also be used with charged particle beams for treating liquid materials such as for the destruction of organic wastes, the breaking down of potentially toxic hydrocarbons such as tricloroethylenes, propanes, benzenes, phenols, halogenated chemicals, and the like, and for drying liquids, such as ink in printing machines, lacquers, and paints. The detector may also be used to monitor charged particles beams in the food industry such as the disinfection of food stuffs such as sugar, grains, coffee beans, fruits, vegetables, and spices, the pasteurization of milk or other liquid foods, sanitizing meats such as poultry, pork, sausage, and the like, inhibiting sprouting, and extending storage life. It will also find application in conjunction with monitoring electron and other charged particle beams used to form other particles or other types of radiation, such as the generation of ultraviolet irradiation, conversion of the electron beam to x-rays or gamma rays, the production of neutrons, eximer lasers, the production of ozone, and the like. The present system may also be used to monitor charged particles beams in conjunction with polymers and rubbers. The e-beam irradiation can be used for the controlled cross linking of polymers, degrading of polymers, drafting of polymers, modification of plastics, polymerization of epoxy compounds, sterilization of polymer units, vulcanization of rubber, and the like. It is to be appreciated that the determination of dose absorption can also be used to determine the local mass of the product.

Claims: Having thus described the preferred embodiment, the invention is now claimed to be: 1. A method of determining an absorbed dose of an electron beam (22) in an irradiated product (30) , characterized by: determining a reduction in the kinetic energy of the beam from a final kinetic energy of the electron beam exiting the product and from an initial kinetic energy of the beam before entering the product; and determining the absorbed dose from the reduction in the kinetic energy of the beam. 2. The method as set forth in claim 1, further characterized by: determining a charge deposited in the irradiated product by the absorbed electron beam; and determining the absorbed dose from the reduction in the beam kinetic energy, the deposited charge, and a mass of the product. 3. The method as set forth in claim 2 , further characterized by: the step of determining the deposited charge including: measuring a beam current of the electron beam at least after irradiating the product. 4. The method as set forth in any one of preceding claims 1-3, further characterized by: the step of determining a reduction in the kinetic energy of the beam including determining the final kinetic energy of the electron beam exiting the product including: passing the electron beam exiting the product through a foil (64) having a known relationship between the kinetic energy of an electron beam before entering the foil and an absorption factor for the same beam by the foil. 5. The method as set forth in claim 4, further characterized by: measuring a first current induced by the beam exiting the product; measuring a second current induced by the beam exiting the foil; and determining the absorption factor from the first and second currents. 6. The method as set forth in any one of preceding claims 1-3, further characterized by: the step of determining a reduction in the kinetic energy of the beam including: concentrating magnetic flux changes attributable to changes in the kinetic energy of the beam, and with the concentrated magnetic flux changes, inducing electrical currents in windings of a coil (60, 62). 7. The method of any one of preceding claims preceding claims 1-3, further characterized by: the step of determining a reduction in the kinetic energy of the beam including: collimating the electron beam exiting the product to a preselected cross-section; inducing a first electromotive force with the collimated election beam; attenuating the collimated electron beam; inducing a second electromotive force with the attenuated electron beam; and, comparing the first and second electromotive forces. 8. The method as set forth in claim 7, further characterized by: the step of inducing the first electromotive force including pulsing the collimated electron beam through a first annular winding (60) ; the step of attenuating the collimated electron beam including passing the electron beam through a metal layer (64) of preselected thickness; and the step of inducing the second electromotive force including pulsing the collimated electron beam through a second annular winding (60), the second annular winding being disposed closely adjacent the metal layer. 9. The method as set forth in any one of preceding claims 1-8, further characterized by: scanning of the electron beam. 10. The method as set forth in claim 9, further characterized by: the step of determining the kinetic energy including: measuring beam current pulses as the beam current scans past a measurement point. 11. An irradiation apparatus comprising a charged particle beam generator (10) for generating and aiming a charged particle beam (22) of a first kinetic energy along a preselected path, and a conveyor (32) which conveys an item (30) to be irradiated through the beam, characterized by: a beam strength monitor (40a) which monitors a second kinetic energy of the beam after it has passed through the item. 12. The apparatus as set forth in claim 11, further characterized by: a second beam strength monitor (40b) which monitors the first kinetic energy. 13. The apparatus as set forth in either one of claims 11 and 12, further characterized by: a processor (54) for comparing the first and second kinetic energies of the beam and determining a dose of the charged particle beam absorbed by the item. 14. The apparatus as set forth in claim 15, further characterized by: the processor being disposed remote from the monitors; and a transducer (50) associated with the monitors and remote from the processor for converting an output of the monitors into optical signals, the optical signals being input into the processor. 15. The apparatus as set forth in any one of preceding claims 11-14, further characterized by: the beam generator including a beam strength control means (12) for controlling at least one of a voltage and a current of the charged particle beam; and the conveyor including a speed control means (36) for controlling a speed with which the item is moved through the charged particle beam; and a parameter adjustment means (58) which compares the determined absorbed doses with target absorbed doses and selectively adjusts at least one of the beam strength control means and the conveyor speed control means. 16. The apparatus as set forth in any one of preceding claims 11-15, further characterized by: the charged particle beam generator further including a sweep control circuit (14) for sweeping the charged particle beam back and forth across at least one of a planar region and a volumetric region; and the first beam strength monitor (40a) including: a vacuum chamber (66) ; first and second current transformers (60,62) disposed in the vacuum chamber, a current being induced in each of the transformers by the electron beam; an absorbing metal foil (64) disposed in the vacuum chamber between the first and second current transformers whereby the current induced the second current transformer is less than the current induced the first current transformer. 17. The apparatus as set forth in any one of preceding claims 11-16, further characterized by: the charged particle beam generator including an electron accelerator (10) . 18. An energy detector for determining the energy of an electron beam (22), characterized by: a vacuum chamber (66) ; first and second inductive coils (60,62) disposed in the vacuum chamber in which currents are induced by the electron beam; a foil (64) having known absorption characteristics disposed between the first and second inductive coils, such that the energy of the beam can be determined from the first and second currents. 19. The energy detector of claim 18, further characterized by: a beam collimator (68) upstream of the inductive coils for collimating an electron beam before it passes through the first inductive coil, foil, and the second inductive coil. 20. The energy detector as set forth in claim 19, further characterized by: a comparing means (54) for comparing the currents induced in the first and second inductive coils and determining therefrom the energy of the electron beam. 21. A method of irradiation comprising moving an item (30) through a charged particle beam (22) , the method characterized by: determining an energy of the charged particle beam exiting the item; and subtracting the energy of the charged particle beam exiting the item from an energy of the charged particle beam entering the item. 22. The method of claim 21, further characterized by: controlling at least one of a speed with which the product moves through the charged particle beam and the energy of the charged particle beam in accordance with the determined difference between the entering and exiting energies. 23. The method as set forth in either one of preceding claims 21 and 22, further characterized by: the items being conveyed through the charged particle beam in a first direction; and the charged particle beam being swept back and forth in a plane perpendicular to the first direction. 24. The method as set forth in any one of preceding claims 21-23, further characterized by: measuring the charged particle beam current at a plurality of locations along the product. 25. The method as set forth in any one of claims 21-24, further characterized by: determining an absorbed dose for a plurality of regions of the product. 26. The method as set forth in claim 24, further characterized by: determining a difference between a beam current exiting the product and a beam current entering the product; and determining absorbed dose from the difference between the entering and exiting energies, the difference between the entering and existing beam currents, and a mass of the product. 27. The method as set forth in any one of preceding claims 21-26, further characterized by: the charged particle beam being an electron beam.

Inventors: KORENEV SERGEY A, KORENEV, SERGEY, A.
Applicants: STERIS INC, STERIS INC.
Application year: 2001
Main classification: G01V5
IPC codes: B01J3, B01J19, G01N23, G01N27, G01T1, G01V5, G21G5, G21K1, G21K5, H01J37, H05H7