Patent Number: 
Section: description

The present invention relates to a container for a radiation source. With reference to FIGS. 1 to 10, embodiments of the invention will now be described in further detail as relating specifically to use in an irradiator unit and in a storage and transport container. According to the invention, the container comprises a shielded housing (12) with a central bore (11) extending therethrough. A cylindrically shaped removable sleeve is disposed within the bore. The sleeve may be a sample delivery assembly (14) as depicted in FIGS. 4 and 5, a source delivery assembly (88) as depicted in FIGS. 8 to 10, or a removable plug, depending on the intended use of the container. The container may retain a radioactive source (10) within the housing and located at the periphery of an expanded region (13) of the bore, or within a chamber (20) formed in the removable sleeve. If the radioactive source is to be located within the housing at the periphery of the expanded region (13) of the bore (11), the radioactive source can either be single point source, or a plurality of sources, for example, arranged as an annular ring in the expanded region (13) of the bore (11). For example, the container may be adapted to receive rods comprising cobalt 60 (60Co) or other suitable radioisotope, in an annular ring around the periphery of the expanded region (13) of the bore. An irradiator unit having a housing adapted to receive a radiation source in an annular ring is discussed below in further detail. As an alternative to the annular ring configuration for the radiation source, the housing may be adapted to receive a point source of radiation within the bore, as depicted in FIGS. 8 to 10. In this configuration, a radiation source is typically housed within the removable sleeve, or source delivery assembly, so that the radiation source may be accessed when the sleeve is in the withdrawn position. A storage or transport container having such a configuration is discussed below in further detail with reference to FIGS. 8 to 10. Any radioactive source can be housed in the container according to the invention. Depending on the intended application, the container may be of any acceptable size. The container of the present invention also comprises mating helices which threadedly cooperate. One helix is integrally formed on the outside of the removable sleeve and the corresponding helix is formed on the surface of the bore of the housing. These helices-engage within the gap between the sleeve and the housing to allow removal of the sleeve from the housing through axial rotation, while shielding radiation leaking from the gap. The co-operating helices may be formed of any material suitable to attenuate or scatter radiation. For example but without being construed as limiting, the helices may be formed of metallic, ceramic, or polymeric materials. For example, metals such as lead or cooper alloys, or stainless steels may be used. Irradiator Unit According to one aspect of the invention, the container for a radioactive source may be an irradiator unit. A conventional Gammacell(trademark) 220 irradiator unit is illustrated in prior art FIGS. 1 and 2 in the withdrawn position and inserted position, respectively. The Gammacell(trademark) 220 is adapted to house a radioactive source (10), for example but not limited to 60Co, within a housing (12) which is radiation-shielded. The 60Co source (10) is arranged as rods in an annular ring around the periphery of an expanded region (13) of a central vertical bore (11). The removable sleeve of such a conventional irradiator unit is a sample delivery assembly (14; FIG. 1), typically having a top drawer (16), a bottom drawer (18), and a chamber (20) located between the top and bottom drawers. The top and bottom drawer are comprised of a shielding material to reduce radiation leakage from the bore, however, variations in this arrangement may exist, for example the sample delivery assembly may enter the housing along a horizontal plane within a horizontal bore. The sample delivery assembly (14) has a longitudinal axis (B) and may be withdrawn or inserted within the bore of the housing in the direction of this axis using a suitable longitudinal drive means (22). FIG. 1 depicts the conventional irradiator unit in a withdrawn position, having the chamber (20) accessible from the outside of the housing (12), external of the bore (11), where the chamber can be loaded with a sample. FIG. 2 depicts the unit in an inserted position so that the chamber (20) is located within the housing (12) and is approximately aligned with the radiation source (10), to expose a sample in the chamber to radiation. A lead shielding collar (24) may be opened or closed to allow insertion of a sample into the chamber (20). The drawers (16 and 18), located on either side of the chamber (20) and aligned therewith in respect of the longitudinal axis (B), provide shielding to reduce radiation emanating from the source within the housing (12). When the sample delivery assembly is in the inserted position, and the collar (24) is closed, a significant amount of radiation shielding is realized. However, in this conventional irradiator unit design, in order for the assembly to slide freely in a direction parallel to the longitudinal axis, a gap (26) exists between the sample delivery assembly (14) and the surface of the housing facing into the bore (11), as shown schematically in FIG. 3. Thus, a worker may be exposed to radiation escaping through this gap and exiting the apparatus in the region of the opened shielding collar. An embodiment of the invention is illustrated with respect to an irradiator unit as depicted in FIGS. 4 to 6. In FIGS. 4 to 6, there is shown an irradiator unit having a housing (12) with a bore (11), a removable sleeve, which in this embodiment is a sample delivery assembly (14), comprising a top drawer (16), a bottom drawer (18) and a chamber (20) located between the top and bottom drawers. The sample delivery assembly (14) and the surface of the housing (12) facing into the bore (11) have mating helices (38, 40) disposed thereon which threadedly cooperate when axial rotation about the longitudinal axis (B) is imparted to the sample delivery assembly (14). The profile of the mating helices may be any of a variety of suitable configurations that ensures a mating interaction between these two helices. For example, which are not to be considered limiting in any manner, the profile of the matingly interacting portions of the helices may be crenulate (e.g. xe2x80x9cVxe2x80x9d shaped), curved (e.g. xe2x80x9cUxe2x80x9d shaped), crenellated (e.g. xe2x80x9c[xe2x80x9d shaped) or the like. A first helix (38) is located on the sample delivery assembly (14) and a second helix (40) is located on the surface of the housing (12) facing in toward the bore (11), in regions both above and below the expanded region (13) of the bore adapted to house the radiation source (10). Thus, the helices (38, 40) place shielding material in the path of the radiation emanating from the radiation source (10) to increase the overall radiation scattering effect. The helices may be continuous or discontinuous along the surfaces from which they extend. For example, as depicted in FIGS. 4 and 5, the first helix (38) is located on the sample delivery assembly (14) and may comprise a continuous helix or discrete helical regions above and below the chamber (20). The helical region of the first helix (38) located below the chamber (20) may also be continuous or comprise discrete helical regions. The second helix (40) located on the surface of the housing facing in toward the bore may be continuous or may comprise discrete helical regions, for example a helical region above the radiation source (10) and a helical region below the radiation source. The helices (38, 40) may be continuous or discontinuous, without altering their co-operativity. Thus, both continuous and discontinuous helices, or a combination having a continuous helix and a discontinuous helix are included as embodiments of the invention. The helices may be formed integrally on the surface of the housing facing into the bore (11) and on the sample delivery assembly (14), respectively. In the embodiment represented in FIGS. 4 and 5, mating helices (38, 40) are located both above and below the radioactive source (10). The region of each helix located below the radioactive source could alternatively be omitted, if shielding of radiation emanating from the lower end of the container is not required. For example, if the container housing does not have a lower opening, which may be the case if the container is intended for a shipping or storage. Thus mating helical regions would not need to be located below the radioactive source. The helices may be integrally formed on the respective surfaces, or alternatively may be affixed thereto, permanently, or non-permanently. The helices may be made of any effective radiation blocking material such as but not limited to metals, such as steel, cooper or lead-based alloys, ceramics, or polymeric materials, and each helix may be made of the same or of different materials. In order to avoid galling due to the friction of metal rubbing against metal, the mating helices could be formed of different materials, for example one helix may be formed of stainless steel, or a lead or cooper alloy, and the other of a ceramic or polymeric radiation-blocking material. The helices may be made of the same radiation blocking material as the housing and/or the sleeve, for example, the material forming the top drawer (16) and bottom drawer (18) of the sample delivery assembly (14) in FIGS. 4 and 5. A plurality of helical turns are indicated in the embodiments of the invention depicted in FIGS. 4-6. However, the number of helical turns required may be more or less than the number indicated in this embodiment. At least one, and preferably two or more helical turns along the length of the sleeve, shown in FIGS. 1 and 5 as the sample delivery assembly (14), effectively shield the gap and permit the movement of the sleeve between the withdrawn and inserted positions within the housing. According to the embodiment of the invention depicted in FIGS. 4 and 5, the movement of the sample delivery assembly (14), or sleeve, within the bore (11) is due to a combination of axial rotation and longitudinal movement in the direction of the longitudinal axis (B) of the sample delivery assembly (14). Longitudinal drive means may be used to impart longitudinal movement to the sleeve within the bore. For example, the longitudinal movement of the sleeve within the bore can be imparted using a drive motor and chain system. Any suitable drive means for imparting longitudinal movement to the sleeve within the bore may be used. However, a longitudinal drive means is not necessarily required if the force applied by the rotation imparting means to achieve rotation of the sleeve is adequate to guide the mating helices in longitudinal movement as well as axial rotation. Advantageously, the irradiator unit embodiment of the invention comprises both a longitudinal drive means and rotation imparting means, so as to avoid undue stress on the helices or galling of the surfaces. The drive mechanism of the Gammacell(trademark) 220 may be used as the longitudinal drive means, as depicted in FIGS. 4 and 5. Briefly, this drive mechanism comprises a chain and sprocket system which acts to withdraw or insert the sample delivery assembly (14) into the bore (11). The power may be provided by a suitable motor, for example a 2 hp 220V, 3 phase motor (44), the output speed of which is reduced, for example through a V-belt (46) and pulley connection to a worm gear reducer (48). Further speed reduction may be obtained through a chain and sprocket drive system. For example, a lift sprocket (50) at each end of a drive shaft (52) transmits the shaft rotation to small end head sprockets (51) mounted each on side of the head base (53). The head sprockets rotate less than one revolution each to complete an longitudinal (up or down) movement of the sample delivery assembly (14). Two lift chains (54) are pinned at one end to the lift sprockets (50), and at the other end, to either end of a full width lift bar (56). The lift bar (56) is pin jointed to a bracket on the bottom of the sample delivery assembly (14). With the partial rotation of the head sprockets on the upward movement, the lift chains (54) wrap around the lift sprockets (50) and raise the lift bar (56). Axial rotation of the sample delivery assembly (14) occurs simultaneously with the longitudinal movement. An axial rotation means to impart rotation of the sample delivery assembly (14) about the longitudinal axis (B) may be added to the existing longitudinal drive means of a conventional irradiator unit, which has been modified to also comprise mating helices (38, 40). The helices threadedly guide the sample delivery assembly (14) in axial rotation to either withdraw or insert the sample delivery assembly within the bore (11). Axial rotation may be imparted to the sample delivery assembly using any suitable rotation imparting means, such as the gear and spline assembly depicted in FIGS. 4 and 5. In this embodiment, a 2-way electric motor (60) is used to impart rotation to a spline shaft (62). Two thrust bearings, (64 and 66) allow concurrent rotational movement of the spline shaft (62) and the sample delivery assembly (14), and are connected by a drive chain (72) engaging two axial rotation sprockets (68 and 70). As the lift bar (56), is actuated to a raised or lowered position by means of lift sprockets (50) and chains lift (54), the first axial rotation sprocket (68) moves up and down the spline shaft (62) while rotation is driven by the spline shaft. The first axial rotation sprocket (68) is connected to the sample delivery assembly (14) via drive chain (72) and a second axial rotation sprocket (70), larger in size than the first, is attached to the bottom of the sample delivery assembly (14). However, it is to be understood that other mechanisms to impart axial rotation may also be employed. A number of known mechanical or electrical devices or methods can be used to synchronize the longitudinal and rotational movements. Any of these devices or methods may be employed for use with the present invention. According to an embodiment of the invention, the two drive systems, namely the longitudinal drive mechanism and the axial rotation means, are electrically synchronized in such a way so that the sample delivery assembly (14) travels longitudinally by the distance of one pitch of the mating helices for each axial turn. Use of a synchronized mechanism allows for easy movement of the helices, and is advantageous if the helices are both formed of stainless steel or other materials which would not easily move relative to each other due to frictional forces. The helices do not carry any load when the weight of the sample delivery assembly is supported by the lift bar (56) in combination with the longitudinal drive means, as shown in FIG. 4. Thus, a synchronized mechanism reduces excessive or wear on the helices, and advantageously prevents or reduces galling as a result of friction between metal surfaces, such as two stainless steel surfaces. As shown in FIGS. 6 and 7, the helices (38, 40) protrude into the gap (26) between the sample delivery assembly (14) and the housing (12), providing barriers within the gap around which radiation must pass in order to escape from the gap. Radiation scatters when a barrier is encountered, and thus radiation leakage is attenuated by the helices within the gap. Providing at least one helical turn within the gap reduces radiation emanating from the radioactive source within the container. According to one embodiment of the invention, about 2 helical turns will provide sufficient radiation interference (or xe2x80x9cstep densityxe2x80x9d) to reduce leakage of radiation through the gap (26) to a negligible amount. The number of helical turns (or revolutions), as well as the depth and width of the mating portion of the helices, contribute to the shielding accomplished according to the invention. The clearance between the adjacent portions of the helices may vary, but is large enough to allow facile movement of the adjacent components of the helices. With respect to the embodiment in which the container is an irradiator unit, an air gap of from about 0.01 to about 0.3 inch, and preferably from about 0.03 to about 0.06 inch, is adequate to allow movement of the helices relative to each other. This gap could be greater and still accomplish radiation shielding, depending on the step density, number of helical revolutions, and depth and width of the helices. The basic principle of radiation attenuation by transmission and scatter is illustrated in FIG. 7. According to the invention, a primary radiation beam (76) emanating from a radiation source (10) enters the gap (26) and encounters a xe2x80x9cstepxe2x80x9d of the first helix (38). Part of the beam is deflected around the step through the gap (26), and part of the beam passes through the helix material. Both parts of the beam recombine on the other side of the helix step to a reduced secondary beam (78). The process is repeated at every step of the helices so that the exiting beam is reduced by several orders of magnitude in comparison to the primary radiation beam. Thus, a greatly reduced amount of radiation emanates from the gap (26) between the surface of the housing (12) and the sample delivery assembly (14). Advantageously, the personnel installing, servicing or operating an irradiator will benefit from reduced radiation exposure according to the invention. The process of installing and servicing an irradiator unit requires personnel to reach across the gap (both above and below the unit) and thus expose their hands to the radiation emanating from the gap. In addition, a small amount of scattered (low energy) radiation from the gap may contribute to the overall total dose received by those individuals not exposed directly to the gap. Both gap exposure and overall dose received will be significantly reduced by the invention. Loading and unloading the irradiator unit sample chamber is carried out across the gap (at the top of the irradiator unit). An individual who operates a conventional irradiator unit frequently and for extended periods may receive a significant cumulative dose to the hands. The invention will decrease the cumulative dose received. The reduction of radiation doses leaking from the gaps in a container, for example, but not limited to a Gammacell(trademark) 220, will improve marketability of any container housing a radioactive source while maintaining its desirable performance characteristics. For example, with an irradiator unit, the radiation dose rate and uniformity within the sample chamber are maintained, and the use of numerous accessories with the unit is still possible. Refurbishing of the source within the irradiator requires transportation of the housing containing the source. During transport, the sample delivery assembly remains within the housing and the helices minimize radiation leakage. Further shielding may be fitted to the upper and lower openings of the bore to further reduce any radiation leakage. Storage and Transport Container A further embodiment of the invention wherein the container is a radiation storage and transport container is illustrated in FIGS. 8 to 10. The transport and storage container (80) has a housing (12) with and longitudinal bore (11) therein, and a lid (84). The container also comprises a removable sleeve, which in this embodiment is a source delivery assembly (88), since the assembly is adapted to house a radiation source. A chamber (20) is disposed within the source delivery assembly (88). FIG. 8 depicts a radiation storage and transport container (80) with the source delivery assembly (88) in the inserted position, and FIG. 9 depicts the container (80) with the source delivery assembly (88) in the withdrawn position. FIG. 10 relates to an alternate transport and storage container. First and second helices (38, 40), as described earlier for an irradiator unit, are located on the source delivery assembly and on the surface of the housing (12) facing into the bore (11), respectively. The lid (84) is an optional feature of the storage and transport container (80). In the absence of a lid, the shielding provided by the helices (38, 40) reduces the radiation emanating from the gap (26) between the housing (12) and the source delivery assembly (88). The lid (84) may be formed of a shielding material to ensure further attenuation of radiation, and advantageously prevents entry of dirt or other particulate matter into the gap between the housing (12) and the source delivery assembly (88). Furthermore, the lid (84) is not required to maintain the source delivery assembly (88) in the inserted position, since rotational force must be applied to raise the source delivery assembly, and thus accidental removal of the assembly from the housing would be unlikely. The lid may screw onto the transport container, or be attached through other means known in the art. To move the source delivery assembly (88) between a withdrawn and an inserted position, a rotation imparting means, which in this embodiment is a tab (94), is located at the top of the source delivery assembly and operates to rotate the assembly (88) about longitudinal axis (B). Several high energy applications, for example using 60Co, will require that this operation be carried out remotely, under water, or within a shielded chamber. In the embodiment depicted in FIGS. 8 and 9, the rotation imparting means may be operated manually by a worker. If the rotation imparting means is to be manually operated, it may comprise, for example, a tab, a knob, a button, a bar, a key, or any appropriate form which may be grasped by the worker, or by a tool appropriate for operation by a worker. Alternatively, the rotation imparting means may be combined with or integral to the lid, whereby rotation of the lid withdraws the source delivery assembly from the bore. The rotation imparting means may be lockable and/or removable from the source delivery assembly to allow limited access to the contents of the container. Alternatively, the rotation imparting means may be mechanically automated and associated with a lid, eye-hook, tab or suitable means. When a radiation source is to be stored or transported within the container (80), the radiation source is inserted into the chamber (20) when the source delivery assembly (88) is in the withdrawn position shown in FIG. 9. The source delivery assembly (88) is then moved to the inserted position by applying rotational force to the tab (94), thereby rotating the source delivery assembly (88) into the bore of the housing. In this embodiment, a worker may grasp the tab (94) and apply a twisting force thereto about the longitudinal axis (B), and the helices (38, 40) guide the source delivery assembly into the bore (11) of the housing (12). One or more helical turns are located above the radiation source when in the inserted position, in order to shield the gap (26) between the housing (12) and the source delivery assembly (88). The lid (84) of the container can then be closed. Alternatively, as depicted in FIG. 10, the storage and transport container of the present invention may also comprise a removable sleeve (108) being either a sample delivery assembly or a source delivery assembly inserted within a container (12; e.g., a shielded housing) that is adapted to receive both a plug, generally indicated as 96 and the removable sleeve (108). The internal bore of the housing may comprise a stepped bore as depicted in FIG. 10, wherein the lower portion of the bore (98) is adapted to receive the removable sleeve (108), and the upper bore (100), of an enlarged diameter, to accommodate the plug (96). However, the upper and lower portions of the bore may also be of the same diameter. The plug comprises a eye-book, tab (94) or some other device to permit removal or insertion of the plug in order to gain access to the bore and insert or remove the removable sleeve. A removal or insertion means (generally indicated as 106) imparts rotational movement to the plug during removal and insertion. The plug (FIG. 10) comprises an integral or affixed helix (102) to its outer surface which mates with a corresponding integral or affixed helix (104) formed on the inner surface of the bore. The helix of the plug loosely mates with the helix of the bore, so that air, water or other liquid that the storage and transport container is immersed within during loading or unloading operations, may freely pass between the bore and the outside when the plug is inserted or removed. The plug may also comprise a zigzag vent tube (not shown) in order to further permit venting of air or liquid during the loading or unloading operation. The primary purpose of the mating helices of the plug and bore in this embodiment is to attenuate radiation leakage from the source, and not to seal the source within the housing. Rather the source is sealed within the source delivery assembly (108). The radiation levels of the measured outside fields of a conventional Gammacell(trademark) 220 irradiator, having a gap length of 11 inches (about 28 cm) is up to about 500 to 1000 mR/hour, depending upon the instrumentation used for detection. By applying an attenuation factor to the measured outside fields, the radiation field above and below an irradiator unit having a gap which is shielded according the invention can be determined. For an irradiator unit having a gap of 11 inches in length (about 28 cm), steel helices are inserted, thereby creating a steel barrier in the gap to at least 50% of the length (about 5.5 inches or 14 cm). Applying known principles, the number of logeo reductions of primary gamma radiation (1.17 and 1.33 MeV) is estimated as negative 1.7 or a factor of 0.019. Thus, the measured field range is reduced to 9.5-19.0 mR/hour. These levels are further reduced due to deterioration of primary energy as a result of scatter. The reduced levels of radiation exposure realized in Example 1 can be further decreased by increasing the length of the gap. A gap of approximately 12 inches (about 30 cm) which is shielded according to the invention introduces steel to at least 50% of the gap length (6 inches or 15 cm). The field thus produced is about 8.0-16.0 mR/h. The invention is not limited to application for improved irradiator units, or radiation storage and transport containers, but may extend to any container housing a radioactive source. All publications cited herein are incorporated by reference. Various modifications may be made without departing from the invention. It is understood that the invention has been disclosed herein in connection with certain examples and embodiments. However, such changes, modifications or equivalents as can be used by those skilled in the art are intended to be included. Accordingly, the disclosure is to be construed as exemplary, rather than limiting, and such changes within the principles of the invention as are obvious to one skilled in the art are intended to be included within the scope of the claims.