On-line measurement of absorbed electron beam dosage in irradiated product

[0002] 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.

[0003] 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.

[0004] 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.

[0005] 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.

[0006] 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.

[0007] One disadvantage of the photographic verification technique resides in the delays in developing and analyzing the film.

[0008] The present invention provides a new and improved radiation monitoring technique which overcomes the above referenced problems and others.

[0009] In accordance with the present invention, a method of irradiation is provided. Items are moved through a charged particle beam. Energy of the charged particle beam entering the item is determined and the energy of the charged particle beam exiting the item is measured.

[0010] In accordance with a more limited aspect of the present invention, the difference between the entering and exiting energies is used to determine absorbed dosage.

[0011] In accordance with another more limited aspect of the present invention, the difference between the entering and exiting beam energies is used to control at least one of the entering charged beam energy, and a speed of moving the items through the charged particle beam.

[0012] In accordance with another aspect of the present invention, an irradiation apparatus is provided. A charged beam generator generates and aims a charged particle beam along a preselected path. A conveyor conveys items to be irradiated through the beam. A first beam strength calculator determines a strength of the beam before entering the item. A beam strength monitor monitors a strength of the beam after it is passed through the item.

[0013] In accordance with yet another aspect of the present invention, the beam strength calculator and the beam strength monitor include energy detectors. The detectors include first and second current transformers disposed across a metal foil from each other.

[0014] One advantage of the present invention resides in the real time measurement of absorbed dose.

[0015] Another advantage of the present invention resides in more accurate determination of absorbed doses and reducing dosing errors.

[0016] Another advantage of the present invention resides in the automatic control and modification of an irradiation process on-line to assure prescribed dosing.

[0017] 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.

[0018] 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.

[0019] FIG. 1 is a perspective view of a e-beam irradiation system in accordance with the present invention;

[0020] FIG. 2 is a cross sectional view of one of the detectors of FIG. 1; and,

[0021] FIG. 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.

[0022] 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.

[0023] 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.

[0024] A plurality of radiation detector arrays 40a, 40b, are positioned in the path of the e-beam 22. The first detector array 40ais in array that measures the strength (energy) of the electron beam after it has exited the item. The optional second detector array 40bdetects 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, 40bare 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.

[0025] 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.

[0026] 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.

[0027] 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.

[0028] 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 3×10⁻⁴to 6×10⁻⁴m 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.

[0029] 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 I₀and 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:

[0030] K=I₂/I₁=f(E)  (1)

[0031] where, I₁, is the beam current in front of the foil and I₂is the current behind the foil. The charge Q of the beam after the foil is:

[0032] Q=Q₀*e^−(m/p)*d  (2)

[0033] where Q is charged after the foil and Q₀is 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*e^−(m/p)*dyields:

[0034] I₂=I₁*e^−(m/p)*d  (3)

[0035] 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.

[0036] Looking to FIG. 3, a standard dependency for the coefficient of partial transmission of energy for aluminum foils of 300 and 500 μm is illustrated. After the determination of E₁from these measurements, the energy absorbed in the product E_pis calculated by:

[0037] E_p=E0−E₁  (4)

[0038] 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_epassing through the detector. The absorbed Joule's energy E_j, in the product:

[0039] E_j=E_p*N_e*1.6*10⁻¹⁹[J]  (5)

[0040] 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:

[0041] M=0.8D²_c*L*p  (6)

[0042] where p is the density of the product, L is the thickness of the product, and D_cis beam diameter after collimation. Hence, the absorb dose D is:

[0043] D=E_j/M  (7)

[0044] 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.

[0045] 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 40amay, for example, include hundreds of individual detectors. The array 40bmay, for example, be only a single detector.

[0046] 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.

[0047] 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.

[0048] 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.

[0049] It is to be appreciated that the determination of dose absorption can also be used to determine the local mass of the product.

[0050] 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.