WO1986002771A1

WO1986002771A1 - Irradiation processing system with dynamic source scan

Info

Publication number: WO1986002771A1
Application number: PCT/US1985/002155
Authority: WO
Inventors: Martin H. Stein; Russell N. Stein
Original assignee: Precision Materials Corporation
Priority date: 1984-10-31
Filing date: 1985-10-30
Publication date: 1986-05-09
Also published as: AU5201586A; EP0200781A1

Abstract

In a radiation chamber (19), the target to be irradiated is arranged on both sides of the opening of a pool of water (20) in which a radioactive source (22) in the form of a plaque is immersed. Under timed control, the radioactive source (22) is drawn incrementally through the chamber (19) so as to irradiate segments of the target in accordance with a predetermined scan coder.

Description

Irradiation Processing System With Dynamic Source Scan

Technical Field This invention relates to radiation processing and, more particularly, to apparatus for improving the flux distribution of the electromagnetic radiation emitted by a radioactive source.

Background Art Irradiation processing is the use of ionizing radiation to effect beneficial chemical, physical or biological changes in a material or substance. Typ¬ ically, isotopes of such elements as cobalt and cesium are used as the radioactive source as these isotopes disintegrate spontaneously to emit gamma rays.

Ionizing radiation is used to crosslink various polymers, most commonly polyethylene. The cross- linking provides memory to the material, increases its thermal stability and improves its stress crack resis- tance. Various glass and crystalline materials can be made to change color by treating them with ionizing radiation. The radiation produces f-centers which are caused by physical dislocation of electrons which are then trapped in vacancies in the matrix. Finally, in food processing applications, ionizing radiation is used to sterilize or reduce populations of bacteria, molds and insect eggs. Many applications of this process are employed in the production of pharmaceu¬ ticals, medical devices, cosmetic devices, cosmetic materials and food.

Each of the foregoing examples of radiation pro¬ cessing applications is dependent to a large degree on the amount of ionizing radiation absorbed in the target, viz., materials or product being treated. Control and distribution of this energy within the material are critical to the successful design of an irradiator or irradiation facility. The distribution of energy throughout the target is directly dependent on the nature of the field of radiation into which the target is placed. It might seem obvious that the radiation will be absorbed uni- formly throughout the target if the target is placed in a uniform flux of radiation. This is rarely the case because the target alters the uniformity of the flux as it absorbs energy. Fortunately, perfectly uniform radiation dose distribution is seldom required.

However, it is necessary that every part of the target being treated absorb the minimum amount of radiation to accomplish the objective of the process. This is defined as the minimum absorbed dose. The absorbed radiation dose should not exceed a certain amount because of the production of unacceptable side effects. This amount of absorbed radiation is called the maximum dose allowed for a specific target. The relationship of the maximum allowed and minimum dos- ages is called the max/min ratio. An irradiator must be designed in such a way that it can deliver a max/ min ratio within that specified for the target under treatment.

Another term of art in the field of radiation processing is utilization efficiency. The utiliza¬ tion efficiency of an irradiator is defined as the total useful amount of radiation absorbed in the target (minimum absorbed dose) divided by the total amount of radiation emitted by the radioactive source. There are two basic configurations of irradiator designs: source overlap and target overlap. With both systems, the target is made to move into a posi¬ tion adjacent the source of radiation. Most sources consist of pencils arranged in a planar rack, often referred to as a plaque.

When source overlap is employed, the source plaque extends beyond the ends of the target, usually in the vertical direction. If the plaque were the same height as the target, the radiation dose delivered to the target will be much lower at the top and bottom of the target than it is at the center. In achieving minimum irradiation of the target at the ends thereof, the center of the target may receive too large a dose of radiation.

Source overlap is an inefficient process of irra¬ diation because some of the radiation emitted by the source is lost. The amount of overlap required depends on several variables such as the distance of the target from the source, target thickness, and den¬ sity and the required max/min ratio.

In target overlap irradiators, the target must be stored in superposed containers, or totes, which extend beyond the source. This method produces a high efficiency but requires an interchange in the posi¬ tions of the containers and as many as four passes of irradiation to achieve a uniform dose throughout the target. This increases the complexity and expense of the conveyor system. This arrangement also restricts the size of the target containers. Additional expense results from the fact that this arrangement further requires large irradiation chambers.

As between the two techniques, source overlap has many inherent advantages in product handling. Accord¬ ingly, many attempts have been made to improve the efficiency of this design approach. For example, assuming that target thickness, distance from the source, and density are constant, the radiation losses from the overlap are the same regardless of the total height of the plaque. Therefore, the greater the height of the plaque the less effect the losses from the overlap will have on overall efficiency. This method, however, has a disadvantage in that the depth of the source storage pool and the height of the irradiation chamber have to be increased proportion¬ ally. Also, the required totes are quite tall and present loading and unloading problems. Another attempt to improve on the source overlap system is to reduce the amount of source overlap and absorb some of the radiation from the plaque by plac¬ ing material such as lead in the center portion of the plaque. This will reduce the dose rate to the center of the target to compensate for the lower dose rate at the top and bottom of the product stack. The max/min ratios are improved, but the loss in efficiency is increased. A more successful approach to the problem is to employ source shaping techniques. All the aforemen¬ tioned approaches make use of a radiation source plaque that is uniformly loaded, viz., the pencils of radioactive material are uniformly spaced within the plaque. The source shaping approach places the pen¬ cils closer together near the top and bottom of the plaque so that the source plaque is hotter on the ends. This helps to flatten the radiation flux. This approach, combined with some overlap is presently in use in several irradiation facilities.

Disclosure of Invention

Accordingly, it is an object of the invention to provide an irradiator design having an improved utili¬ zation efficiency. It is also an object of the present invention to provide an irradiator design which controls flux dis¬ tribution to provide improved max/min ratios with min¬ imal loss in efficiency.

These and other objects of the present invention are accomplished by providing an irradiator having a source of ionizing radiation smaller in dimension than the target to be irradiated and means for causing relative movement between the source and product so as to permit the segmental irradiation of the product. Preferably, the target is retained in a station¬ ary position and the source of radiation is moved across it. Movement of the source is timed so that each segment of the target receives the minimum required radiation dose. Also, the target is stacked in two arrays opposite from each other and on opposite sides of the radiation source. In this manner, the source irradiates both arrays at the same time.

Brief Description of Drawings

Figure 1 is a plan view of the housing in which the irradiation processing of the present invention occurs; Figure 3 is a flow chart of the preferred control system for the irradiator of the present invention; and

Figures 2(a)-2(f) illustrate schematically the movement of the radiation source through the housing of Figure 1 under the control of the Figure 3 control system.

Best Mode for Carrying Out the Invention

Figure 1 illustrates an enclosed housing 10 in which the irradiator of the present invention is contained. The housing 10 includes an outer concrete wall 12 having an access opening 14 in one side. A second housing 16 is located within the housing 10. It, too, has an access opening 18 formed in one wall. The walls of the second housing 16 define an interior chamber 19 within which the irradiation processing of the present invention takes place.

Inside the chamber 19 of housing 16 a deep gen¬ erally rectangularly shaped walled opening or pool 20 is formed in the floor that supports both housings. The opening 20 is filled with water within which a radioactive source 22, in the form of an array or plaque of pencils of radioactive material, is immersed when the system is at rest.

A means (not shown) for conveying the target, viz. products to be irradiated, into and out of the chamber 19 is also provided. In one form of the present invention which will be reduced to practice, the conveyor means will take the form of rolling carts stacked with product. The carts will be pushed and/or pulled from outside the housing 10, through opening 14 and the passageway defined by the walls of the hous¬ ings 16 and 18, and into the chamber 19. There they will be arranged on both sides of the pool 20.

Typically, the products to be irradiated will be stacked one on top of the other and the carts will be placed next to each other to form an array. The arrays are about 11 feet (3.4 meters) long and can have any height from about one foot to six feet (0.3 to 1.8 meters). The total height of product will be irradiated in five sectors, the height of each sector being approximately one-fifth the product height.

Referring now to Figures 2(a)-2(f), the radio¬ active source 22 is secured to a cable 23 which, in turn, is secured to and driven by a hoist motor (not shown) located on and supported by the roof of the housing 10. The radioactive source 22 is arranged as a horizontal source plaque. In a typical arrangement, the plaque is 12 feet (3,7 meters) long, one foot (0.3 meters) high and two inches (5 cm) thick. The source 22 remains under water in the pool 20 in the same way that the plaque is positioned in conventional irradia- tors until the chamber 19 is loaded with the target to be irradiated.

To start the radiation process, the source 22 is raised to the first (lowest) irradiation position (Fig. 2(b)). It remains in this position for a fixed and predetermined period of time. The source 22 is then raised again into the next highest position. Each one of these positions is called a sector. The plaque remains at each sector position for a predeter- mined period of time until it has completed the desig¬ nated time in highest sector. The plaque then returns to the pool 20 (Fig. 2(c)). The target arrays are then turned around inside the chamber 10. The result, shown in Figure 2(d), is that the sides of the array that had faced away from the source 22 is again raised in stages, irradiating each container of the array (Figures 2(e), 2(f)).

Although it is possible to control the timing and positions of the source 22 in each sector position by hand controlled electric switches, the preferred sys¬ tem incorporates ccmputer-controlled movement. A suitably programmed personal computer such as the APPLE II or IBM PC will suffice. The computer is interfaced with the hoist motor in such a way as to control the plaque positions. A simplified flow chart of a suitable control system is illustrated in Figure 3.

It will be understood that the present invention permits wide flexibility in the configurations of both the target and the source. For example, the height of the target is limited only by the practical considera- tions relating to housing design. The source plaque may in fact be very short. It is believed that the most effective distribution of flux can be achieved with a very short actual plaque height. To this end, the source can be arranged as a line source, or even a point source with the target located around the source, rather than on the sides of the source.

The source plaque may also be programmed to move continuously but variably through the cycle. The plaque can be made to traverse the sectors from top to bottom rather than from bottom to top as illustrated. The timing intervals and irradiation sequence required for each target must be calculated or measured. The portion of the total irradiation time spent at each sector can be expressed with a scan code. A typical scan code for a five sector system would be 9-3-4-3-9. As an example, if the radiation cycle time for treating one side of the target is 28 minutes, 9 minutes would be spent in sector I, 3 minutes in sector II, 4 minutes in sector III, 3 minutes in sector IV and 9 minutes in sector V. The program for the scan (the scan code) will vary for each target and set of conditions. Determining factors are density of the target, target height, target thickness, distance of the target from the source, the minimum dose required and the max/min ratio specified.

There are several obvious advantages to be gained by utilizing the present invention. Because the source rack (plaque) is a good deal shorter than the target it scans, the pool depth can be corre¬ spondingly smaller. This results in a considerable cost savings. A typical source height as employed in the present invention is one foot. This will allow the use of a dry storage cask rather than a water pool. Water filtration, deionization and monitoring systems are eliminated vastly simplifying the entire irradiator and lowering cost. The need for separate individual product carriers or totes is eliminated since the product does not have to be moved during irradiation. Inefficiencies resulting from gaps between totes are also eliminated. The product can be stacked any height up to the design limit of the irradiator since the scan is adjusted to product height for each cycle. Efficient use of the unit can be made for the irradiation of small lots of material since product height can be adjusted. The scan of the present process is set for only the height required.

The irradiator of the invention can be employed for a batch or a batch-continuous system. Corre¬ spondingly, non-uniform dose distribution can be delivered if desired or required. The exact amount of overlap and scan-code can be adjusted for each target and set of conditions. Finally, the number of pencils required to fill a large plaque is not as critical with the present irradiator since all of the pencils fill a smaller plaque. This is particularly important when the irradiator is initially loaded with only a small fraction of its source capacity and there are not enough pencils to fill a large plaque.

Although the invention has been described, many modifications will become readily apparent to those skilled in the art. All such modifications are intended to be included within the scope of the claims of this invention.

Claims

1. In a radiation processing system wherein a target is exposed to ionizing radiation for a predeter¬ mined time period to effect changes therein, the improvement comprising a source of radiation smaller in size than the target and means for positioning the source adjacent to the target, said positioning means including means for driving the source of radiation to positions contiguous with substantially the entire surface area of the target to thereby subject all of the target to the effects of ionizing radiation.

2. In a radiation processing system wherein a target is exposed to ionizing radiation for a predeter- mined time period to effect changes therein, according to claim 1, wherein the positioning means further comprises control means coupled to the driving means for activating the driving means in accordance with the amount of radiation absorbed in the target,

3. In a radiation processing system wherein a target is exposed to ionizing radiation for a predeter¬ mined time period to effect changes therein, according to claim 2, wherein the control means includes means for periodically energizing the driving means for a fixed period of time, said periodic energization of the driving means being dependent upon the amount of radiation absorbed in the target as a result of the flux distribution of the ionizing radiation.

4. In a radiation processing system wherein a target is exposed to ionizing radiation for a predeter¬ mined time period to effect changes therein, according to claim 3, wherein the source of radiation is composed of an array of pencils of radioactive material which extend lengthwise beyond the terminal ends of the target and which extend vertically a fraction of the height of the target.

5. In a radiation processing system wherein a target is exposed to ionizing radiation for a predeter¬ mined time period to effect changes therein, according to claim 4, wherein the target is arranged in sectors on a carrier on both sides of the source of radiation and wherein the fixed period of time during which the driving means is energized corresponds to the movement by the radiation source from a position adjacent one sector to a position adjacent the very next sector on the carrier.