Radiant energy – Irradiation of objects or material – Ion or electron beam irradiation
Reexamination Certificate
1999-02-11
2003-06-10
Nguyen, Kiet T. (Department: 2881)
Radiant energy
Irradiation of objects or material
Ion or electron beam irradiation
Reexamination Certificate
active
06576915
ABSTRACT:
TECHNICAL FIELD OF THE INVENTION
This invention relates generally to the field of irradiation systems, and more particularly to a method and system for electronic pasteurization.
BACKGROUND OF THE INVENTION
Irradiation involves exposing a target to an ionizing radiation to change the microbiology of the target. Irradiation is an effective method for killing micro organisms and insects in foods, extending the shelf life of various foods, and sterilizing medical products. Irradiation is particularly suited for treating food, such as meat, to kill food-borne pathogens, such as
E. Coli
, Trichinosis, Salmonella, Yersinia, Campylobacter, Shigella, and the like. Two characteristics determine the effectiveness of an irradiation treatment—the dose, which is the total beam energy delivered per mass of food; and the penetration depth, which is the maximum depth into the food to which the dose is delivered. The penetration depth is a property of the ionizing radiation that is used for irradiation. Food irradiation typically requires a dose of about 300,000 rads to achieve a statistical kill of the pathogenic bacteria.
Conventional irradiation systems utilize one of three methods to produce the ionizing radiation. &ggr;-ray irradiation systems produce &ggr;-rays from radioactive sources, typically Co
60
. X-ray irradiation systems produce X-rays by targeting an electron beam, on the order of 5 MeV, on a metal target which produces X-rays. Conventional electron beam irradiation systems produce a high-energy electron beam, typically on the order of 10 MeV energy, and deliver it directly into the food. The &ggr;-ray irradiation systems and X-ray irradiation systems deliver a deep penetration, on the order of 30 centimeters into food, but require immense shielding assemblies, on the order of 3 meters thick of concrete, for safe operation. Conventional electron beam irradiation systems deliver less penetration, on the order of 7 centimeters in food, but require somewhat less shielding, on the order of 2 meters of concrete.
Conventional electron beam irradiation systems generally include an accelerator, a beam transport system, and a treatment station. Specifically, the accelerator produces an electron beam which is communicated to the treatment station by the beam transport system. Within the treatment station, the electron beam is scanned to deliver a uniform dose as the target passes through the treatment station. The higher the energy of the electron beam, the greater the depth that the electron beam can penetrate the target and deliver the required dose of ionizing radiation.
Conventional electron beam irradiation systems have many disadvantages. For example, conventional electron beam irradiation systems are inefficient, in that the electron beam scans across a specific area within the treatment station, but in many applications, the target covers only a fraction of the scanned area. The utilization efficiency, defined as the fraction of electron beam actually delivered to target, is typically less than 30% in conventional electron beam irradiation systems.
As will be discussed below, conventional electron beam irradiation systems are extremely expensive due to the technical disadvantages of conventional accelerators, beam transport systems, and treatment stations. Conventional electron beam irradiation systems often utilize a radio-frequency linear accelerator (LINACS) to produce an electron beam. LINACS operate by producing a high-intensity electric field within a series of cylindrically symmetric resonant cavity. The electron beam is passed along the axis of the cavities, where it is both accelerated to increase its energy and focussed to confine the beam transversely. A technical disadvantage of LINACS is that only a single electron beam can be produced, because the beam must pass along the axis of the cavities. For food irradiation applications, a typical food processing operation has multiple parallel processing lines, and thus requires multiple parallel treatment stations for irradiation. Since each conventional electron beam irradiation system can produce only one beam, the expense of multiple stations would be extremely high.
Another technical disadvantage of conventional accelerators is that they are expensive to build and do not operate efficiently. Conventional high energy accelerators typically cost on the order of $5,000,000 to $7,000,000, and operate at only 30-70 percent efficiency.
Conventional beam transport systems generally utilize electromagnets to transport the electron beam from the accelerator to the treatment station. The electromagnets generate a magnetic field based on the pattern of electrical currents that flow through the electromagnet. Conventional beam transport systems generally use dipole and quadrupole electromagnets. The dipole electromagnet produces a uniform magnetic field in the region traversed by the beam and thereby bends the electron beam on a constant radius of curvature. The quadrupole electromagnet produces a distribution of magnetic field that increases linearly with distance from the beam axis, and focuses the beam to confine it along the direction of transport. A technical disadvantage of conventional beam transport systems is that the electromagnets require an active electrical power system along the entire length of the beam transport system. The electrical power system adds complexity and cost to conventional beam transport systems, particularly in food irradiation applications where it may be advantageous to locate the accelerator in one location and deliver beams to multiple treatment stations at locations distributed throughout a large facility.
Conventional treatment stations include electro-optics that scan the electron beam transversely to illuminate the scan area, as well as shielding to prevent harmful levels of radiation from escaping the treatment station. Conventional electro-optics direct the electron beam to the outer surfaces of the target. A technical disadvantage of conventional electro-optics is that the internal cavities cannot readily be irradiated with the electron beam unless the beam energy is sufficiently high to penetrate the entire thickness of the target. High energy electron beams necessitate the use of heavy shielding to protect operating personnel.
Conventional irradiation systems must be housed in a structure that shields the intense ionizing radiation so that the system can be operated safely in a food processing plant. Regulatory agencies generally require the dose of ionizing radiation to be reduced to a level commensurate with the radiation dose a person would naturally receive from cosmic rays and natural radioactivity, which is less than 0.0001 rads per year. In order to reduce the dose of ionizing radiation to acceptable levels, the thickness of the shielding in conventional irradiation systems typically exceeds three meters, and is often on the order of 5 meters. In addition, the shielding must include a labyrinth having a similar thickness through which the target is transported in an out of the treatment station. A typical conventional irradiation system occupies an area of about 200 m
2
, which makes it difficult to integrate into the existing process lines of a food processing plant.
The technical disadvantage of heavy shielding is that the treatment station often becomes a separate shielded facility that cannot be integrated into large in-line food processing applications. Another technical disadvantage is that the separate shielded facility creates a processing bottleneck, in that all targets must pass through this one facility. Furthermore, the capital costs associated with constructing the shielded facility often exceed the cost of the accelerator.
As a result of the construction costs and operating expenses, as well as the safety issues associated with conventional irradiation systems, conventional irradiation systems have not generally been commercially implemented in large scale food treatment applications.
SUMMARY OF THE INVENTION
Accordingly, a need has arisen for an improved irradiati
Galasso Raymond M.
Nguyen Kiet T.
Simon Galasso & Frantz, PLC
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