Patent Number: 061809519
Section: description

DETAILED DESCRIPTION The present invention relates to a method of electron beam irradiation which produces a substantially constant dose of electrons throughout the thickness of an irradiated target material. FIG. 2 shows a perspective view of an apparatus for performing electron beam irradiation configuration in accordance with one embodiment of the present invention. Electron beam 200 is emitted from scan horn 202, with a direction of sweep 203 along the Y-axis as indicated. Because of intrinsic physical properties of the irradiation apparatus, emitted electrons at periphery 200a of the beam sweep have less energy than emitted electrons present at center 200b of the beam sweep. Cylindrical reel 204 is positioned within electron beam 200, and is rotated around center axis 206. Center axis 206 is oriented along the X-axis, perpendicular to the direction of the beam sweep of scan horn 202. As a result of this orthogonal orientation of beam sweep relative to axis of rotation 206, frontside of reel 204 receives only emitted electrons at center 200b of the beam sweep. Target material 208 is disposed around reel 204. Core 210 of reel 204 possesses sufficient density that electron beam 200 does not pass through. FIG. 3 shows a depth/dose profile of electron beam irradiation of two thicknesses (0.5" and 1") of polyethylene material disposed around a rotating reel as shown in FIG. 2. Inspection of FIG. 3 reveals that for both material thicknesses, a linear depth/dose profile is produced, with surface regions receiving a lesser dose than subsurface regions. The linear depth/dose profile shown in FIG. 3 contrasts markedly with the non-linear depth/dose profile shown in FIG. 1 resulting from conventional irradiation techniques. It has also been discovered that where the dense core of the reel is replaced with a less-dense core which permits electrons of the beam to pass and thereby irradiate target material on the backside of the reel, a constant depth/dose profile may be achieved. FIG. 4 compares the depth/dose profiles resulting from irradiation of polyethylene material disposed around reels having a solid core and a core of lower density. Inspection of FIG. 4 reveals that for reels having either types of core, a substantially constant depth/dose profile was observed. Moreover, with the less dense (porous) core, a substantially constant depth/dose profile was observed. Thus, surface regions received approximately the same dose as subsurface regions. This result is central to the present invention, and is now examined in detail. FIG. 5 shows a cross-sectional view of a reel 500 positioned in beam 502 of electron radiation. Target material 504 is disposed around reel core 506 having a diameter. Electron beam 502 is emitted from scan horn 506. The relative size of scan horn 506 and reel 500 are not shown to scale in FIG. 5. Unlike the reel shown in FIG. 2, core 506 of reel 500 is of a sufficiently low density that the electrons from beam 502 pass through target material 504 disposed on the frontside of reel 500, pass through core 506, and then further irradiate target material 504 disposed on the backside of reel 500. Dosimeters 508 are positioned at four depths of target material 504 (at the surface, 2/3 off of the core, 1/3 off of the core, and at the core) at each of sites 1-31. Measurement of the dose resulting from this irradiation reveals four general regions of dosing. These regions, labeled A-D, are listed below in order of decreasing electron dose received: TABLE 1 REGIONS OF DOSING OF TARGET MATERIAL POSITIONED ON STATIONARY REEL REGION SITE NOS. Region A 1, 2, 31 Region B 3-7; 26-30 Region C 8-12; 21-25 Region D 13-20 FIGS. 6A-6D plot the effect upon the depth/dose profile of material of different thicknesses positioned on a stationary reel as shown in FIG. 5. The depth/dose profiles plotted in FIGS. 6A-6D generally confirm the conventional dopant profile shown in FIG. 1. For example, the electron dose received in frontside surface portions directly in the beam path (FIG. 6A, Region A-sites 1, 2, and 31) is generally lower than the electron dose received in subsurface portions in the same region (FIGS. 6B-6D, Region A-sites 1, 2, and 31). Moreover, the highest doses in Region A appear at intermediate depths (FIGS. 6B-6C, Region A-sites 1, 2, and 31). Where the irradiated material curves away from the beam, a spike in dosage in surface portions is observed. (FIG. 6A, Region B-sites 7 and 26). This dosing behavior likely attributable to intervening target material causing the "surface" regions to actually receive "subsurface" type doses. As stated above, irradiation of target material on the backside of the reel is critical to achieving a constant depth/dose profile in accordance with the present invention. For target material positioned on the backside of the reel, surface portions (FIG. 6A, Region D-sites 12-21) receive a lower dose than portions at the core (FIGS. 6B-6D, Region D-sites 12-21). This is likely attributable to the shadowing effect of target material intervening between the beam and the surface of target material on the backside of the reel. The increased dose observed at the backside surface with a thinner target material further supports this view, as there is significantly less intervening target material. (Compare FIG. 6A, Region D-sites 12-21, for 0.507" thick material versus 1.014" thick material and 1.482" thick material). Further consistent with this theory, the shadowing effect diminished with material closer to the core on the reel backside, due to the presence of less intervening target material. (Compare FIGS. 6A-6B, Region D-sites 12-21, with FIG. 6D, Region D-sites 12-21). Thus, from FIGS. 6A-6D it is seen that the thickness of the target material can significantly affect the depth/dose profile. FIGS. 7A-7D plot the effect upon the depth/dose profile for target material disposed about stationary reels having three different core diameters. FIGS. 7A-7D also shows that the size of the core diameter affects the dosage received at various regions of the target material. An additional parameter affecting the depth/dose profile is the density of the irradiated material. FIGS. 8A-8D plot the effect upon dose for target materials of different densities disposed around the stationary reel of FIG. 5. FIGS. 8A-8D reveal that the density of the target material will also affect the dose of radiation received. Where a reel having a low density core is rotated within the electron beam, a substantially linear depth/dose profile will result. FIG. 9A plots the depth/dose profile for three thicknesses of polyethylene material positioned on a rotating reel having a 10" diameter core. All three samples show a substantially linear depth/dose relationship. Moreover, the sample of intermediate thickness (1") evidences a substantially constant depth/dose relationship. FIG. 9B plots the slope of the linear depth/dose profiles shown in FIG. 9A, versus depth into the target material. FIG. 9B indicates that polyethylene material having a thickness of about 1" disposed around a 10" diameter core should exhibit a constant (slope=0) depth/dose profile. The reproducibility of this result was confirmed by performing the same experiment using a reel having a different diameter core. FIG. 10A plots the depth/dose profile for three samples of polyethylene material of varying thickness positioned on a rotating 8" reel. FIG. 10B plots the dose slope versus material thickness for the samples shown in FIG. 10A. Again, all three samples exhibit a substantially linear depth/dose profile. Moreover, based upon the slopes of the depth/dose curves of the 0.5", 1", and 1.5" thick samples, FIG. 10B predicted that a constant depth/dose should be obtained by a polyethylene material having a thickness between 0.5" and 1.0". This was confirmed by experimentation, as FIG. 10C shows that polyethylene material having a thickness of approximately 0.780" produced a substantially constant depth/dose profile having a slope of -2.2 kGy/inch. To explore the effect of target material density upon irradiation in accordance with the present invention, the experiments described above in FIGS. 9A-9B were repeated using target material made of cork having a significantly lower density (0.390 g/cm.sup.3) than polyethylene material (0.643 g/cm.sup.3). FIG. 11A plots the depth/dose profile for three thicknesses of cork material positioned on a rotating 10" reel. All three samples show a substantially linear depth/dose profile. Moreover, the sample of least (0.5") thickness evidences a substantially constant depth/dose relationship. FIG. 11B plots the dose slope versus target material depth of the linear depth/dose curves shown in FIG. 11A. FIG. 11B indicates that polyethylene material having a thickness of about 0.79" disposed around a 10" reel will exhibit a constant (slope=0) depth/dose profile. The reproducibility of this result was confirmed by performing the same experiment using a reel with a different diameter core. FIG. 12A plots the depth/dose profile for three thicknesses of cork material positioned on a rotating 8" reel. FIG. 12B plots dose slope versus target material depth for the cork samples shown in FIG. 12A. Again, all three samples exhibit a substantially linear depth/dose relationship. Moreover, based upon the slopes of the depth/dose curves of the 0.5", 1", and 1.5" samples, FIG. 12B predicted that a constant depth/dose should be obtained by a cork material having a thickness between 0.5" and 1" disposed around an 8" core. This was also confirmed by experimentation, as FIG. 12C shows that cork material having a thickness of approximately 0.78" produced a substantially constant depth/dose profile having a slope of 1.1 kGy/inch. To further explore the effect of target material density upon irradiation in accordance with the present invention, the experiments described above in FIGS. 9A-9B and 11A-11B were repeated using target material made of nylon strap material having a significantly higher density (0.746 g/cm.sup.3) than either polyethylene (0.643 g/cm.sup.3) or cork (0.390 g/cm.sup.3). FIG. 13A plots the depth/dose profile for three thicknesses of nylon strap material positioned on a rotating reel having a 10" core. All three samples show a substantially linear depth/dose relationship. Moreover, the sample of least (0.5") thickness evidenced a constant depth/dose relationship. FIG. 13B plots the dose slope versus material thickness for the three nylon strap samples shown in FIG. 13A. FIG. 13B indicates that nylon strap material having a thickness of about 0.5" that is disposed around a 10" core will exhibit a constant (slope=0) depth/dose relationship. The reproducibility of this result was confirmed by performing the same experiment using a reel having a different diameter. FIG. 14A plots the depth/dose profile versus depth for three thicknesses of nylon strap material positioned on a rotating real having an 8" core. FIG. 14B plots the dose slope versus material thickness for the nylon strap samples shown in FIG. 14A. Again, all three samples exhibit a substantially linear depth/dose relationship. Moreover, based upon the slopes of the depth/dose curves of the 0.5", 1.0", and 1.5" samples, FIG. 14B predicted that a constant depth/dose should be obtained by a polyethylene material having a thickness of between 0.5" and 1.0" disposed around an 8" core. This was also confirmed by experimentation, as FIG. 14C shows that nylon strap material having a thickness of approximately 0.816" produced a substantially constant depth/dose profile having a slope of 0.84 kGy/inch. Orientation of direction of rotation of the reel relative to the direction of beam sweep plays a critical role in performing the process for irradiation in accordance with the present invention. In order for the present method to function, the axis of rotation of the reel must be substantially perpendicular to the direction of beam sweep. This is illustrated in FIG. 15, which shows the result of irradiating 75 ft of polyethylene material wrapped around a 22" rotating core, with the polyethylene material having dosimeters positioned every 5 ft. Irradiation of the reel having an axis of rotation perpendicular to the beam sweep yielded relatively constant dosing throughout the sample: the maximum dose differed from the surface dose by about 12.3% (73-65=8; 8/65.times.100=12.3%). By contrast, irradiation of the reel under the same conditions, except with the axis of rotation parallel to the beam sweep, yielded a much wider range of dosing throughout the sample (106-84=22; 22/84.times.100=26.2%). This variation is probably attributable to the fact that where the axis of rotation of the reel is parallel to the beam sweep, target material located at the periphery of the beam sweep receives a lower dose of radiation than target material located at the center of the beam sweep. Thus, the lack of constant dosing evidenced by the triangles in FIG. 15 is likely the result of the orientation of the beam sweep relative to the axis of rotation. Irradiation of target material in accordance with the present invention offers a number of important advantages over conventional methods. Most importantly, irradiation in accordance with the present invention results in the target material having a substantially constant dose of radiation extending into a depth of the material. The permissible amount of variation in dose will vary with the particular application. In general however, irradiation in accordance with the present invention achieves a depth/dose profile whose maximum subsurface dose varies by 10% or less from the surface dose. Irradiation in accordance with the present invention is particularly suited for sterilization applications in which traditional processes of irradiation could generate unwanted heat. Thus, where heat-sensitive material such as plastic is being exposed to radiation under tension between two spools, conventional irradiation could cause heating of the plastic, resulting in stretching or even fracture of the tubing. The constant dosing provided by the present invention eliminates this problem. Other advantages of the present invention include reduced power consumption, and, in cross-linking applications, a greater degree of control over the polymerization reaction throughout the thickness of the target material. Although the invention has been described in connection with one specific preferred embodiment, it must be understood that the invention as claimed should not be limited to such specific embodiments. Various other modifications and alterations in the method of operation of this invention will be apparent to those skilled in the art without departing from the scope of the present invention. For example, the experimental examples provided above describe the result of electron beam irradiation in which 1) target material thickness, 2) reel core diameter, and 3) target material density were varied, with the energy of the electron beam maintained constant (at 6 MeV). However, it is also possible to vary other irradiation parameters in order to affect the depth/dose profile. For example, it may be possible to vary the energy of the electron beam in order to ensure constant a constant depth/dose profile. Variation of this parameter is particularly important where cumulative radiation exposures will be employed to avoid the heat associated with a single heavy exposure. Moreover, it may also be possible to vary the speed of rotation of the target material within the radiation beam in order to ensure constant dosing. The speed of rotation of the reel must create sufficient exposure at different points on the reel during the irradiation process, in order to harmonize or normalize the dose received by the target material. Certain practical realities may dictate which irradiation parameters can be varied to produce the desired constant depth/dose profile. For example, in many electron beam irradiation devices, the energy of the beam is fixed, and a change of the beam's energy requires calibration and adjustment. Moreover, the density of the target will be dictated by the target material chosen for irradiation. Finally, the core diameter may be determined by the reel apparatus employed in a particular laboratory or industrial setting. Therefore, one likely procedure for producing a constant depth/dose profile in an irradiated target material would be to maintain a constant core diameter and electron energy, while varying the thickness of the target material. While the above discussion includes experimental examples involving exposing a target material to electron beam irradiation, the present invention is not limited to this form of irradiation. Other forms of radiation, such as X-ray and gamma radiation, could also be utilized in the present method to produce a constant depth/dose profile. The physical mechanism giving rise to the constant depth/dose profile of the present invention is not yet completely understood. It is possible that rotating the target in front of the beam continuously shifts the position of each point of the irradiated material relative to the beam, thereby distributing electron dose throughout the various depths of the target material. For example, with reference to FIG. 5, if the reel is rotated relative to the beam, at a first point in time the surface dosimeter at site 1 will receive a typical surface dose. However, after rotation of the reel 1/4 turn, this same dosimeter will be positioned at a different, "subsurface" location relative to the electron beam. Moreover, by reducing the density of the core, it is possible to ensure further homogenization of dosing. Thus, again considering the reel shown in FIG. 5 rotating in the electron beam, at a first point in time the surface dosimeter at site 1 will receive a "surface" type dose. However, once the reel has rotated 1/2 turn, this dosimeter will be positioned at a polar opposite position (site 16) relative to the beam, such that the "surface" of the target material will receive a "core" type dose. This is shown in FIG. 16, where target material 1600 disposed around core 1602 having diameter D is rotated in the path of electron beam 1604. Averaging the total dose received by the target material over time would produce a constant depth/dose profile. Given the specific embodiments of the present invention described above, it is intended that the following claims define the scope of the present invention, and that the methods and structures within the scope of these claims and their equivalents be covered hereby.