Patent Number: 
Section: description

With reference to FIG. 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 name 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 FIG. 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 3xc3x9710xe2x88x924 to 6xc3x9710xe2x88x924 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 I0 and kinetic energy E0. After propagation of the electron beam across the irradiated product, the electron beam has a kinetic energy E1. 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 transforms 60, 62 enables the determination of an absorption factor K of the electron beam within the foil: K=I2/I1=f(E)xe2x80x83xe2x80x83(1) where, I1, is the beam current in front of the foil and I2 is the current behind the foil. The charge Q of the beam after the foil is: Q=Q0*exe2x88x92(m/p)*dxe2x80x83xe2x80x83(2) where Q is charged after the foil and Q0 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=Q0*exe2x88x92(m/p)*d yields: I2=I1*exe2x88x92(m/p)*dxe2x80x83xe2x80x83(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 FIG. 3, a standard dependency for the coefficient of partial transmission of energy for aluminum foils of 300 and 500 xcexcm is illustrated. After the determination of E1from these measurements, the energy absorbed in the product Ep is calculated by: Ep=E0xe2x88x92E1xe2x80x83xe2x80x83(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 Ne passing through the detector. The absorbed Joule""s energy Ej, in the product: Ej=Ep*Ne*1.6*10xe2x88x9219 [J]xe2x80x83xe2x80x83(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=0.8D2c*L*pxe2x80x83xe2x80x83(6) where p is the density of the product, L is the thickness of the product, and Dc is beam diameter after collimation. Hence, the absorb dose D is: D=Ej/Mxe2x80x83xe2x80x83(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 (dlc) 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. The invention has been described with reference to the preferred embodiment. Obviously, modifications and alterations will occur to others upon reading and understanding 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.