Radiant energy – Irradiation of objects or material – Ion or electron beam irradiation
Reexamination Certificate
2002-07-16
2004-01-27
Lee, John R. (Department: 2881)
Radiant energy
Irradiation of objects or material
Ion or electron beam irradiation
C250S3960ML, C250S398000, C250S492100, C250S492200
Reexamination Certificate
active
06683319
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an irradiation system, and more particularly to a system and method for irradiating product in a manner that improves the uniformity of the irradiation dose delivered to the product.
Irradiation technology for medical and food sterilization has been scientifically understood for many years dating back to the 1940's. The increasing concern for food safety as well as safe, effective medical sterilization has resulted in growing interest and recently expanded government regulatory approval of irradiation technology for these applications. United States Government regulatory agencies have recently approved the use of irradiation processing of red meat in general and ground meat in particular. Ground meat such as ground beef is of particular concern for risk of food borne illness due to the fact that contaminants introduced during processing may be mixed throughout the product including the extreme product interior which receives the least amount of heat during cooking. Irradiation provides a very effective means of reducing the population of such harmful pathogens.
Various types of radiation sources are approved for the treatment of food products including gamma sources such as radioactive cobalt
60
, accelerated electrons with energy up to 10 MeV, and x-rays from electron accelerators of up to 5 MeV. Electron beam and x-ray machine generated sources are becoming increasingly popular due to their flexibility and a general consumer preference to avoid radioactive materials.
The beneficial effects of irradiation of food are caused by the absorption of ionizing energy that results in the breaking of a small percentage of the molecular bonds of molecules in the product. Most of the molecules in food are relatively small and are therefore unaffected. The DNA in bacteria, however, is a very large molecule and is highly likely to be broken and rendered unable to replicate.
FIG. 1
is a graph of exemplary percentage depth-dose curves showing the reduction of radiation intensity due to absorption of radiation in water (which is a relatively accurate model for radiation absorption in food products). Curve
10
is a percentage depth-dose curve for 1.8 MeV electrons, curve
12
is a percentage depth-dose curve for 4.7 MeV electrons, and curve
14
is a percentage depth-dose curve for 10.6 MeV electrons. For all of the electron energies, the radiation intensity increases to a maximum at a distance somewhat interior to the surface of the product due to scatter emission of radiation from electron collisions with food molecules. After the maximum is achieved, absorption causes the relative intensity to begin to fall off until virtually all of the radiation has been absorbed. At the “tails” of the depth-dose chart the intensity is much less than the maximum, but still results in an incremental amount of beneficial irradiation. Single sided application of radiation that is required to maintain a moderate ratio between maximum and minimum exposure must necessarily waste most of this tail of radiation intensity.
Curve
12
of
FIG. 1
illustrates that the percentage depth-dose for 4.7 MeV electrons is approximately 50% of its maximum value at a penetration depth of about 2.0 centimeters or 0.8 inches. Exposure of food of this thickness would result in a maximum/minimum dose ratio of 1/0.5=2.0. The portion of the beam power that is not absorbed would pass through the material and be wasted. The preferred solution to this inefficient use of the ionizing radiation is to expose the product to the electron beam from two sides.
FIG. 2
is a graph of an exemplary depth-dose curve for two sided 4.7 MeV exposure of product having a 4.0 centimeter or 1.57 inch thickness. The depth exposed is substantially greater than for single sided exposure, and the maximum/minimum ratio is substantially lower, resulting in more precise and consistent product exposure.
While two sided irradiation is preferred for maximum efficiency and most consistent exposure, generation of two sided radiation can be problematic. The typical solutions are to either pass product through the radiation source once per side, which requires twice as long to process and may not be viable for products that cannot be flipped over due to material redistribution, or to create two independent accelerators which is costly and complex.
Electron accelerators of several types are known in the art. A preferred electron accelerator for irradiation applications is the well known linear accelerator or LINAC, which employs a high power microwave source driving a specially constructed waveguide to accelerate electrons by electromagnetic induction. A preferred LINAC operation methodology is pulsed operation, whereby a relatively short, high intensity pulse of accelerated electrons is generated at a selected repetition rate. The timing and magnitude of this pulse of accelerated electrons may be controlled by a computer control system.
The stream of accelerated electrons emerging from a typical LINAC is concentrated into a narrow beam approximately 0.5 centimeters in diameter, which is much too small and intense to apply directly to material to be processed. Prior art systems typically shape and spread the beam by passing it through a quadrupole magnet which spreads the beam in both the vertical and horizontal dimensions in a manner analogous to an optical lens.
FIG. 3
is a diagram illustrating a typical spread beam intensity distribution, which takes the shape of elliptical profile
20
. The intensity profile corresponds generally to bell shaped distributions
24
and
26
centered about the vertical and horizontal axes of symmetry. Line
22
surrounding elliptical profile
20
corresponds to the points where the intensity is at halfpower (or −3 db) from maximum. A two-dimensional bell shaped distribution corresponding to a normalized raised cosine function:
f
(
x,y
)=(1+cos(
x
))*(1+cos(
y
))/4
is represented numerically by the table shown in FIG.
4
.
Prior art irradiation systems, such as the system disclosed in published PCT Application No. WO01/26135 filed by Mitec Incorporated, the same assignee as the present application, apply a series of 50% overlapping pulses of accelerated electrons formed in an intensity profile according to the elliptical pattern shown in
FIGS. 3 and 4
. Various points in
FIG. 4
are shown with a box around them, including the center point with normalized intensity of 1.00, the 25% points (halfway between the center point and the 0.50 intensity points) with a normalized intensity of 0.73, and a set of points forming a generally elliptical shape surrounding the center point. These points represent normalized intensity values between 0.47 and 0.53 (approximately −3 db) and correspond generally to the elliptical shape shown in
FIG. 3. A
50% overlap results in a constant intensity distribution along the axis of symmetry. With 50% overlap in both the vertical and horizontal dimensions, the resultant two dimensional exposure is four times the single pulse peak exposure. This distribution, however, is not exactly constant off the axes of symmetry. The greatest deviation is observed at the 25% points. With 50% overlapping vertical and horizontal exposure, the normalized exposure at these points is:
0.73×4=3.44
which is 14% less than the nominal “on-axis” exposure. When an important performance criterion for irradiation exposure is uniformity of dose, this exposure variation contributes directly to an increased maximum/minimum dose ratio, and is undesirable.
FIG. 5
is a schematic diagram illustrating a single accelerator, two sided irradiation system
30
having a structure similar to that disclosed in published PCT Application No. WO01/26135. Irradiation system
30
includes quadrupole magnet
32
, upper deflection magnet
34
and lower deflection magnet
36
for direction of electrons toward material
38
. The paths that accelerated electrons may be directed by relatively constant currents in deflection magnets
34
Koenck Steven E.
Lyons Stan V.
El-Shammaa Mary
Kinney & Lange , P.A.
Mitec Incorporated
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