Patent Number: 055770908
Section: summary

FIELD OF THE INVENTION The present invention relates to systems and methods for product irradiation and particularly to x-radiation of foods and water and sterilization of medical wastes. BACKGROUND Radiation processing of foods is an effective means of preservation, and of controlling insect infestation, pathogens, spoilage and deterioration. The process eliminates harmful bacteria, such as Salmonella in poultry and E. coli in beef, and insect infestation in grain, fruit and spices. The attributes of enhanced shelf life of disease and insect free food products, afforded by irradiation, promotes wider commercial trade between developing countries and industrialized nations without the dangers associated with the importation of foreign agricultural products. The efficacy of food irradiation processing is well substantiated by the results of research and testing performed over the past forty years throughout the world. Today, there are twenty-seven countries using irradiation for processing food in commercial ventures in their own domestic market or in developing foreign markets for their food products. The major growth in the commercial use of irradiation for food preservation has occurred in developing countries; however, irradiated fruits, vegetables, spices, and poultry are also accepted in the United States. At the present time, the U.S. Food and Drug Administration (FDA) is under petition to permit the commercial irradiation of hamburger patties. FDA acceptance of the petition is anticipated, and after passage, a very large market for irradiated meat products is expected to develop. In addition to radiation processing of foods, there is a growing need for water and medical waste sterilization systems. Radiation Sources Food irradiation facilities use three types of ionizing radiation: 1) Gamma (.gamma.) rays from radioisotopes, 2) X-rays generated by energetic electron bombardment on hard metal targets, and 3) Direct energetic electron impact. This background discussion is limited to .gamma.- and x-ray radiation as their frequency and energy are similar to radiation produced by the device of the present invention. Low energy Gamma rays and x-rays of the same energy differ only in the manner in which the radiation is generated. Both are electromagnetic waves and physically the same. The former is generated by nuclear processes within a radioactive nucleus, while the later arises from acceleration of energetic electrons by electric (Coulomb) forces from atomic targets. Isotopic Sources of Gamma Radiation Most current operating irradiation facilities employ large quantities of radioactive cobalt-60 (.sub.27 Co.sup.60) as a source of gamma-rays. The energies of the .gamma.-ray emitted by Co.sup.60 are mainly at 1.332 and 1.173 MeV. Also, the cesium-137 (.sub.55 Cs.sup.137) isotope, which emits gamma rays at energies of 0.662 MeV, is used in some food irradiation facilities. Radioactive cobalt is produced artificially in nuclear reactors by bombarding pencil-like rods of stable, naturally-occurring Co.sup.59 with slow neutrons. The transformation occurs with the absorption of a slow-neutron by a stable Co.sup.59 nucleus followed by emission of a .gamma.-ray from the unstable product nucleus Co.sup.60. This form of nuclear reaction is called an n,.gamma. or neutron-gamma ray reaction. The "pencils" of Co.sup.59 are left in the reactor for one or more years, after which time about 10% of the Co.sup.59 is transformed into Co.sup.60. Industrial irradiation facilities require that the radioactive cobalt rods are encapsulated in stainless steel sheaths with welded end enclosures, which in turn are covered with an aluminum sheath with welded end enclosures. Encapsulation of the radioactive material in this manner insures containment of the radioactive materials and prevents contaminating the products undergoing irradiation. In a typical food irradiation facility, the products are moved automatically into a thick walled, shielded chamber in which a large amount of the encapsulated radioactive isotope Co.sup.60 or Cs.sup.137 rods are arrayed on racks to provide proper product irradiation. The total .gamma. radiation dosage received by the food products is determined by exposure time, location of the product within the chamber, and the linear attenuation coefficient .mu. of the absorber, which in this case is the food product receiving the radiation. The activity of an isotope source is measured in curies. Typically, a Co.sup.60 food irradiation facility has isotope source activities of .apprxeq.2 to 5 million curies, costing about $1.00 to $1.25 per curie at current prices. As the emission of .gamma.-rays from radioactive materials cannot be turned off, the isotopes are submerged in a deep pool of water for safe storage when the irradiator is not in use. The contention of opponents of using isotopic radiators for food preservation is the possibility that the metal encapsulation of the radioactive material may fail, contaminating the food or the local environment. The probability of this occurrence is small, and it is further reduced by the stringent monitoring requirements for these facilities that are mandated by law. However, the public's fear of radioactive isotopes still persists. Electrically Powered X-Ray Sources Electrically powered x-ray devices cannot contaminate food undergoing processing with radioactive substances, for no radioactive materials are used in the process. Furthermore, x-ray machines can be turned off since they are driven electrically, so they do not have to be stored in deep pools of water when not in use. The ability to turn-off the electrically powered device permits transporting the apparatus without enclosing it in a massive radiation shield as is required for transporting radioactive isotope irradiators. Since transportation is not problematical, an electrical x-ray machine can be brought directly to the crop harvesting area, with a water filled bladder used as a radiation shield. Crop irradiation can be performed in situ. Thus, the "off" property directly reduces capital and operating costs, and also, provides flexibility and mobility in locating the food irradiation facilities. The electrical process of producing x-rays has remained relatively unchanged the since Wilhelm Roentgen at the University of Wurzburg discovered them in 1895 up until the recent invention of the x-ray laser at the Lawrence Livermore National Laboratory. Since, the use of an x-ray laser for food irradiation is not economically feasible, only the classical method of x-ray production, i.e., energetic electron bombardment on a heavy metal target is addressed here. The impact of energetic electrons produces x-rays through two atomic collision processes: 1) bremsstrahlung radiation is emitted by decelerating energetic electrons during collisions with atoms in the target; and 2) characteristic x-ray emission is radiation emitted by outer bound electrons of the atom upon replacing k or l inner-shell electrons that have been knocked out by incident energetic electrons. Bremsstrahlung emission exhibits a continuous energy spectra up to the energy of the electrons incident on the target, while characteristic radiation appears only at particular or discrete energies (frequencies) determined by the target material. Characteristic x-rays have energies.apprxeq.100 keV. The energy of bremsstrahlung x-rays is directly related to the energy of the incident electrons. However, the energy of characteristic x-rays from a given target material is independent of the incident electron energy, provided the incident electron energy exceeds the characteristic x-ray energy. Also, as the electron current incident on the target increases, the intensity of x-ray emission will increase proportionally. High voltages, produced by electrostatic or inductive generators, accelerate electrons to energies E.apprxeq.1-5 MeV. After acceleration, the electrons are directed onto a high-Z (atomic number) metal target, e.g., tungsten, to produce bremsstrahlung x-rays. There are several types of electron accelerators, such as Van der Graff, betatrons, sychrotrons, and linacs, that are useful for food irradiation. Linear accelerators are large, complex, and costly experimental devices, requiring highly skilled personnel to operate and maintain, while providing limited beam access and small irradiation volumes. Thick target bremsstrahlung production by an impacting accelerator beam suffers from the fundamental disadvantage that the beam electrons penetrate only a very shallow depth into solid material. Thus, x-rays appear to be emanating from a point or, at most, a small area source. This circumstance causes the x-ray intensity to fall off inversely with the square of the distance from the point of electron impact, and leads to an uneven distribution of dosage within the volume of the food product being irradiated. If the product is irradiated by a broad parallel beam of x-rays, the x-rays are exponentially attenuated to produce a dose distribution in which the front of the product will receive a higher dose than the back of the product. Thus, a trade-off between exposure time versus irradiated volume ensues. The distribution can be made somewhat more uniform by beam-target curvature tending to converge the x-rays to a focus in back of the product. To increase the x-ray intensity, and thus reduce the stand-off distance for a given volume of food products, one could accelerate more electrons, i.e., increase electron beam current. However, with high current electron beam accelerators come concomitant increases in operating electrical power and cost, target destruction becomes problematical, and accelerator capital cost become unmanageable. The present invention overcomes the disadvantages of the prior art food irradiation systems. It is an object of the present invention to provide an electrically powered x-ray device that is suitable and practicable for product irradiation generally, and specifically for food irradiation. A further object is to provide steady irradiation at intense radiation levels, a large irradiation volume, and uniform dose distribution. Another object of the present invention is to provide a system that is electrically efficient, reliable, simple to operate and of reasonable cost. SUMMARY OF THE INVENTION Ionization is the process in which one or more electrons are detached from an atom, resulting in the formation of a positive ion and one or more free electrons. Plasma, the fourth state of matter, is a heated gas in which a large number of gas atoms are ionized, and the resulting ions and free electrons remain in close proximity to each other. In the device of the present invention, an annular hot-electron plasma is created and confined in a simple magnetic mirror machine by resonant microwave breakdown of the working gas. A simple mirror machine consists of two circular electromagnet coils, centered on a single axis, as depicted in FIG. 1 showing the coil arrangement and magnetic field configuration. Experiments at Oak Ridge National Laboratory (ORNL) and the Plasma Physics Institute at the University of Nagoya over two decades have provided indisputable evidence that an annular hot electron plasma can be maintained, indefinitely, by a continuous wave (cw) source of microwave power. See, for example, the following publications which are incorporated herein by reference: R. A. Dandl, H. O. Eason, P. H. Edmonds, and A. C. England, "Electron-Cyclotron Heating by 8-mm Microwave Power in the Magnetic Facility ELMO," Relativistic Plasmas, Edited by O. Buneman and W. B. Prardo, W. A. Benjamin, Inc., New York, 1968; PA0 R. A. Dandl, et al, "Electron Cyclotron Heated "Target" Plasma Experiments", Proc. Plasma Phys. and Controlled Thermonuclear Res., Vol. II, Novosibersk, IAEA, August 1968; PA0 M. Hosokawa and H. Ikegami, "Characteristics of Hot Electron Ring in a Simple Mirror Field," Res. Report. IPPJ-497, Nagoya University, 1980; PA0 R. A. Dandl, "Review of Ring Experiments," Proc. of the EBT Ring Physics Workshop, Dec. 3-5, 1979, ORNL-Conf. Proc. #791228, Oak Ridge, Tenn.; and PA0 G. R. Haste, "Hot Electron Rings: Diagnostic Review and Summary of Measurements," Proc. of the EBT Ring Physics Workshop, Dec. 3-5, 1979, ORNL-Conf. Proc. #791228, Oak Ridge, Tenn. The microwave frequency is chosen to be resonant with the second harmonic of the electron cyclotron frequency of particular regions of the mirror field. Heating electrons in this manner primarily increases their perpendicular energy (energy related to the velocity component perpendicular to the magnetic field) at the resonant field position. This perpendicular heating process is referred to as "electron cyclotron heating" (ECH). As electrons gain energy, their collision cross section (probability of colliding with plasma ions and gas atoms) decreases, and the electrons "runaway", i.e., they continually gain energy from the microwave field and accelerate to higher and higher energies. With sufficient microwave power, a very large number of electrons is heated to relativistic energies, and, confined by a magnetic mirror field, they gyrate about field lines while the centers of gyration drift about the magnetic axis of the mirror field. It is these electronic motions that give rise to an annular plasma structure. In the present invention, the annular plasma is generated in a magnetic mirror preferably having a mirror ratio R=2, i.e., the maximum magnetic field on axis at the center of one field coil is twice the magnitude of the minimum field on axis at the mid plane between the two coils. FIG. 2 shows the drift motion of an electron at the mid-plane of a magnetic mirror field, viewed along the magnetic axis. A large number of energetic electrons, undergoing this cyclonic drift motion in the mirror field, make up a hot electron plasma annulus. The density of energetic electrons in the ECH generated plasma annuli depends on the value of the magnetic field, frequency and power of the microwave radiation, and fill gas density. In the device of the present invention, the required annular plasma density range is preferably about 10.sup.17 -10.sup.19 electrons/m.sup.3. The background plasma density ranges from 10.sup.18 -10.sup.20 electrons/m.sup.3. Continuous emission of bremsstrahlung results from collisions between the highly energetic electrons in the annulus and the background plasma ions and fill gas atoms. Quantitatively, the power density w radiated by electrons in a plasma due to encounters with only the plasma ions is given by Equation 1: EQU w=4.8.times.10.sup.-37 Z.sup.2 n.sub.i n.sub.e T.sub.e.sup.1/2 watts/m.sup.3 where Z is the atomic number of the gas species, n.sub.e, n.sub.i is the density of electrons in the annulus and density of background plasma ions, respectively, and T.sub.e is the electron temperature (in keV) in the plasma. See for example, D. J. Rose and M. Clark, Jr., "Plasmas and Controlled Fusion," pg. 233, The MIT Press, and J. Wiley & Sons, Inc., New York, 1961. The use of electron temperature in Equation 1 reveals the tacit assumption of a Maxwellian electron energy distribution in the plasma. Past ELMO experiments, using hydrogen gas, Z=1, at Oak Ridge National Laboratory (ORNL) and the Institute of Plasma Physics (IPP) of the University of Nagoya established the Maxwellian nature of hot electrons in the plasma annulus, as discussed in the above-referenced publications. The bremsstrahlung x-ray spectrum from the ELMO device experiments shows that the electron temperature of the plasma annulus may lie in the MeV energy range. The electron energy distribution plotted in FIG. 3, unfolded from bremsstrahlung data exhibits a high average electron energy and a truncated high energy tail. Truncation of the high energy tail arises from a loss of adiabatic electron confinement at extreme energies. Thus, with an assertion that an electron temperature can be defined for the annular plasma, Equation 1 is used to estimate the radiated bremsstrahlung power from an annular, hot electron plasma confined in a simple mirror field. Before calculating the radiated bremsstrahlung power from the annulus, an additional bremsstrahlung production process that occurs in the ELMO device is first considered. These x-rays arise from energetic ring electrons impacting the walls of the vacuum chamber in a manner similar to bremsstrahlung production by electron beams from linear accelerators. The velocity vector of some energetic electrons in the plasma annulus is modified by collisions with plasma particles and background gas atoms, i.e., the directed velocities of these electrons are scattered. As a result of these collisions, if the altered velocity vector of a scattered electron is aligned, or nearly so, along the magnetic field lines, the electron cannot experience a magnetic force, nor is it confined by the mirror field. The scattered electron follows the magnetic field lines until it impacts the vacuum chamber wall. Scattered energetic electrons predominately impact the area at the intersection of field lines with the chamber walls, where the sidewalls narrow down to accommodate the mirror field coils. Experimental measurements of radiant power produced at chamber walls agrees well with classical calculations of expected bremsstrahlung power produced by scattered ring electrons striking the walls. See, for example "Hot Electron Rings, etc.", cited above. The impact of these high-energy electrons on the walls results in thick target x-ray emission in the same manner as electron beams striking a tungsten target. In K. Z. Morgan and J. E. Turner, "Health Physics," American Institute of Physics Handbook, 3rd Edition, D. E. Gray, Editor, page 8-305, McGraw-Hill Book Co., New York, 1972 (Reissue 1982), it is reported that bremsstrahlung power P.sub.S radiated from the walls is proportional to the product of the atomic number of the wall material Z.sub.W, electron density in the ring n.sub.e, background plasma density n.sub.i, square root of the electron temperature T.sub.e in the ring, and the volume V of the annulus, i.e., EQU P.sub.S .varies.Z.sub.W n.sub.e n.sub.i T.sub.e.sup.1/2 V. Equation 2 Thus, the energetic electrons scattered from the rings enhance the rate and intensity of radiation from the device. The proportionality, described by Equation 2, was established by x-ray power experiments on the ELMO Bumpy Torus (EBT), and a series of measurements performed on toroidally-linked magnetic mirror machines. However, the reported radiation levels are only relative measurements and cannot be used for scaling purposes. Therefore, estimates of thick-target x-rays radiation levels are not included in the radiation level calculations for the present invention. Such calculations are based solely on estimates of bremsstrahlung radiation from the annulus electrons and the well documented experimental and operational database of the ELMO studies at ORNL to establish the attractiveness of the present invention for application to radiation preservation of foods or irradiation of products, generally. In summary, the ELMO experiments at ORNL established the physical basis and understanding of microwave driven, annular hot-electron plasmas in simple mirror machines. From that work, the present invention takes advantage of the following important properties of plasma annuli: continuous stable operation; plasma density scales with microwave power; continuous high-level x-ray emission; radiation level scales with the product of annulus and background plasma density, and hence, microwave power; thick target radiation power from electrons scattered into the chamber walls agrees with classical calculations; operational simplicity; and constructional simplicity.