Patent Number: 060842431
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The subject invention relates to methods and apparatus for containing radioactive sources and, more particularly, to containing low level radioactive sources for shipment and subsequent handling. 2. Description of the Invention Background In various industrial and commercial operations, there is a need to measure the density or specific gravity of process liquids, suspensions or bulk materials flowing in pipes or contained in tanks or vessels. There is also the need to detect the presence or absence of process material in tanks, pipes, hoppers and chutes, as well as to continuously monitor the level of material therein. A prior technique of measuring density and/or level within tanks, pipes, hoppers and chutes (collectively referred herein as "conduits") employed a technique known as radiation absorption gauging. Another application of the use of radiation absorption gauging techniques is in measuring the level of molten metal in a continuous casting apparatus as shown in Cahill et al., U.S. Pat. No. 5,564,487, the disclosure of which is hereby incorporated by reference. The radiation absorption gauging technique includes the provision of a radioactive source and a detector spaced therefrom. The radioactive source comprises a material that is continuously disintegrating and which emits particles and energy in the form of alpha, beta and gamma rays in transitioning to a lighter elemental material. The detector is responsive to the impingement of these particles/energy to provide a given signal level which is inversely proportional to the square of the distance between the source and the detector. When an object is disposed between the source and detector, or the aggregate density of medium between the source and detector is otherwise increased, the source emissions will be absorbed by the increased mass of the intervening medium and the output signal from the detector will be proportionately reduced. This principle may be used to detect the density of material within a conduit by disposing a radioactive source on one side of, or within, the conduit and a detector on the opposite side. As such, the radiation field created by the source passes through the conduit and the material therein before impinging on the detector. By proper calibration techniques, the material passing through the conduit absorbs radiation thereby diminishing the output signal from the detector by a measurable amount. The same principles are involved in gauging the level of material in a vessel. In this situation, the radioactive source is disposed on one side of a conduit and the detector is disposed on the opposite side of the conduit. There is thus created a radiation field that spans a portion of the height of the conduit. The intensity of the radiation impinging on the detector and the output signal therefrom is inversely proportional to the degree to which the material in the conduit absorbs radiation which, in turn, is a function of the level of the material in the conduit. In each of the above described measuring techniques, the radioactive source has historically comprised material such as cesium-137 which emits radiation in the range of 0.001 to over 5 curies. Radioactive sources of such intensity are regulated by federal and state regulations throughout the world due to concerns associated with radiation sources of such levels. Such concerns include those associated with the handling of such radioactive sources to insure that workers were not exposed to dangerously high levels of radiation. Similarly, the shipment of such radiation sources in interstate commerce has been heavily regulated to ensure the safe and error free handling of radioactive sources. Further, the disposal of radiation sources of such intensity is regulated to insure that radiation sources do not contaminate landfills, water supplies or other natural resources. In response to the several concerns relating the use of strongly radioactive sources, the applicant and others have developed technology whereby lower level radioactive sources may be employed. Such sources have radioactive material, such as cesium-137, colbalt-60 or americium-241 in units measured in the micro-curie (10.sup.-6 curie) range. For example, a cesium-137 radioactive source may be provided in units of less than 10 micro-curies substantially exempt from regulation. By means of comparison, common household smoke detectors may contain americium radioactive sources which contain radioactive sources in the range of 1 micro-curie. Such low level radioactive sources are usually not regulated to nearly the extent as stronger radioactive sources. For present purposes, the regulation of concern is that radioactive sources not be shipped or installed in packages containing more than 100 micro-curies of radioactive material. Further, some of the shipping requirements of low level radioactive sources require that less than a predetermined amount, such as 0.5 milliRem per hour, be emitted at the surface of the container. Indeed, some jurisdictions have imposed standards at the level of 0.1 milliRem per hour. However, even low-level radioactive sources mounted on small disks, spheres, pellets, rods, etc. are shipped loose in containers and require further handling and assembly into the radiation absorption gauging system on site. Heretofore, there has not been a preassembled package of low-level radioactive sources for immediate insertion into a measuring system. Furthermore, there has not been a suitable, reusable shipping container for a plurality of low-level radioactive sources. Thus, there is a need for a preassembled holder for a plurality of low level radioactive sources. There is also a need for a shipping container for low level radioactive sources which will allow for the safe shipment of such sources without regulation. There is still another need for an assembly of low-level radioactive sources which may be repeatedly used or taken off-line. The instant invention is directed toward a holder and a container for low level adioactive material which can be used in connection with density, level and/or belt weighing gauge systems for radiation absorption gauging. SUMMARY OF THE INVENTION In accordance with a particularly preferred form of the present invention, there is provided a holder for a plurality of radioactive sources. The holder is formed from lead or lead alloy, tungsten alloy or steel alloy or other high density material as a short cylindrical housing body having a central cylindrical cavity therein and opposed planar faces having openings which extend into the cavity. The internal diameter of the cavity is approximately the same size as the disk-shaped diameter of the radioactive sources. Inert end-caps may be provided on opposite ends of a stack of, for example, ten (10) radioactive sources and the resulting stack is inserted into the cavity. Such stack is then permanently retained within the housing by means of mylar or metal tape or epoxy. Radiation will thus pass from the holder in a beam path facing in the direction of the cavity's opening. In this configuration, the level of radiation on the surfaces of the holder normal to the beam path may be maintained below a predetermined amount, such as 0.5 milliRem per hour. This invention also includes the provision of a shipping container for an assembly of the holder with a stack of radioactive sources retained therein. The shipping container comprises a pair of cylindrical container bodies having approximately the same outside diameter as that of the housing body. The respective container bodies have planar faces which engage the respective faces of the housing body. Bores are provided through the first and second bodies and the housing body in order that attachment bolts may pass therethrough and be retained therein by corresponding nuts. The shipping container will contain the radioactive sources such that the level of radiation on its surface will be less than a predetermined amount, such as 0.5 milliRem per hour. In operation, the bolts are removed and the holder with the radiation sources previously mounted therein may be readily inserted in to a radiation absorption gauging apparatus. In such apparatus, the holder with the radioactive sources mounted therein is provided on one side of a conduit by mounting the same on a mounting plate. A rear shield which is formed as a cylindrical lead body similar to the first and second container bodies is mounted on the side of the housing opposite the conduit. The detector is mounted on the opposite side of the conduit from the radiation source. A microprocessor unit is coupled to the scintillation detector to calculate the appropriate density or level measurements. In an alternative embodiment of the invention, the holder for radioactive sources is formed as a two-piece column with a series of recesses therealong. The radioactive sources are mounted in the cavities. For shipment, the pieces of the column form the shipping container. In that case, the pieces of the column are separated and deposed in confronting relationship. End members are provided so that the level of radiation on the surface of the container is less than a predetermined amount, such as 0.5 milliRem per hour. Accordingly, it is a feature of the present invention to provide a holder assembly for radioactive sources. It is another feature of the present invention to provide a shipping container for radioactive sources. It is yet another feature that the radioactive source assembly, or even the system itself, may serve as the shipping container whose surface radiation is less than a predetermined amount. Accordingly, the present invention provides solutions to the shortcomings of prior radiation absorption gauging systems by providing apreassembled, permanent holder for radioactive sources, a shipping container therefor and a radioactive source assembly for use in such system. Those of ordinary skill in the art will readily appreciate, however, that these and other details, features and advantages will become further apparent as the following detailed description of the preferred embodiments proceeds.