Patent Number: 048274933
Section: summary

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates in general to a radiographic source and an associated method of making a radiographic source. The radiographic source and the associated method of making thereof are described herein in connection with the manufacture of a cobalt-60 radiographic source. The present invention also relates to a new technique for the accurate measurement of focal spot size for in particular cobalt-60 radiographic sources. 2. Background Discussion Radiographic sources are presently known and are constructed with the use of such radioactive materials as cobalt-60 and iridium-192. A cobalt-60 radiographic source presently manufactured by the assignee herein is constructed of inner and outer stainless steel capsules with the inner capsule containing a plurality of cobalt-60 pellets; the number of cobalt-60 pellets being the function of the source size. The inner capsule is seal welded by a plug or cover also of stainless steel. In the past attempts have been made to compact the cobalt-60 pellets. However, the compaction has not been totally effective. The techniques used to date have caused deformation of the capsule to the extent that the inner capsule could not be inserted into the outer capsule. Accordingly, one object of the present invention is to provide an improved method of making a radiographic source in which the source radioactive material is effectively compacted so as to provide an actual effective density as high as 90% of the density of the cobalt metal. It is to be noted that this improved radiographic material compression provides a considerably higher activity in a smaller focal spot size, resulting in substantial money savings in radiography exposure time alone. With the presently employed stainless steel inner capsule there has always been some inherent factors that yielded uncertainty over the exact focal spot size provided. There was a statistical error relating to the cobalt pellet orientation at the edges of the focal spot (edge effects). There is also distortion of the focal spot due to ineffective compression. There was uncertainty over void volume due to the ineffective compression. There was also a statistical error from the cobalt-60 specific activity which is measured before encapsulation. While the contribution from each of these factors may be only a few percent, they are additive when calculating the overall uncertainty. The smallest focal spot size has been in demand by the industry in general for many years. However, actual verification of focal spot size has been virtually impossible due to the fact that cobalt and steel have very similar atomic numbers (cobalt=27, steel=26) and a very similar density (cobalt=8.9 gm/cc, steel=7.9 gm/cc ). This means that it is extremely difficult to separate these materials by x-ray analysis. Accordingly, it is another object of the present invention to provide an improved radiography source and associated method of manufacture with the inner capsule construction of a material that enables adequate film contrast by x-ray radiography between the inner capsule and the radioactive material. A further object of the present invention is to provide an improved technique for in particular accurately measuring the focal spot size for a radiographic source and for in particular a cobalt-60 radiographic source. Still another object of the present invention is to provide an improved apparatus for compacting radioactive pellets and for particularly compacting cobalt-60 pellets in a capsule. Still a further object of the present invention is to provide the improved apparatus as recited in the preceding object and in which the capsule is constructed of a material that has a sufficiently different atomic number in comparison to the radioactive material so that adequate film contrast by x-ray radiography can be achieve while at the same time constructing the capsule of a material with sufficient tensile strength so as to assist in preventing deformation thereof during the compacting step. SUMMARY OF THE INVENTION To accomplish the foregoing and other objects, features and advantages of the invention, there is provided a method of making a radiography source of a radioactive material, described herein in a preferred embodiment in connection with the manufacture of a cobalt-60 radiographic source. This method comprises the steps of providing a plurality of radioactive pellets and furthermore providing an open capsule of a rigid metal having sufficient tensile strength to resist substantial deformation under pressure and selected from a group including elements of the periodic table displaced in density on the amount at least on the order of 2.0 gm/cc in comparison to the density of the radioactive pellet material. Generally speaking, this difference in density may also be expressed in terms of atomic number in which case the capsule is formed of an element displaced in atomic number by at least two from the atomic number of the radioactive pellet material. The radioactive pellets are disposed in the capsule and are compacted to reduce the source of focal spot size. A rigid metal plug is inserted in the open end of the capsule and the capsule is then welded closed. The compacting is provided by means of a ram that is dimensioned for close tolerance fit in the capsule. The pellets are compacted by inserting the ram into the capsule and applying a predetermined pressure to the ram. The pressure that is applied is a direct function of the area being compacted. In connection with this compacting step, there is also provided a die in which the capsule is disposed. The capsule is disposed in the die, which is preferably a split die, in a manner to hold the capsule so as to prevent any substantial deformation of the capsule due to pellet compaction. In accordance with the invention, the capsule, in a preferred embodiment, is constructed of titanium which has an atomic number of 22 in comparison to the cobalt atomic number of 27, and also has a density of 4.5 gm/cc in comparison to the aforementioned density of cobalt which is 8.9 gm/cc. In association with the pellets, there may also be provided one or more inserts of the same material as the capsule material. These inserts are disposed in the capsule after compacting the pellets and before inserting the plug. In accordance with a further feature of the present invention there is provided a radiography source of a radioactive material which in the further embodiment is radioactive cobalt-60. This source comprises a plurality of radioactive pellets disposed in an open capsule. The capsule is of a rigid metal having sufficient tensile strength to resist substantial deformation under pressure and selected from a group including elements of the periodic table displaced in atomic number by at least two from the atomic number of the radioactive pellet material. The pellets are disposed in the capsule and compacted therein to reduce the source focal spot size. A plug is disposed in the open capsule and sealed therewith. The aforementioned capsule is then contained in an outer capsule that may be constructed of stainless steel. In accordance with the invention, the preferred construction of the inner capsule is of titanium as this material has an atomic number displaced by five from that of the radioactive material in the case of radioactive cobalt-60. In accordance with still a further aspect of the present invention there is provided an improved press apparatus for providing pellet compaction and employed in the manufacture of a radiography source in which the source is of a radioactive material such as the aforementioned cobalt-60. The source includes a plurality of radioactive pellets contained in a capsule of a rigid metal having sufficient tensile strength to resist substantial deformation under pressure and selected from a group including elements of the periodic table displaced in atomic number by at least two from the atomic number of the radioactive pellet material. The press apparatus of the invention includes a ram means dimensioned from a close tolerance fit in the capsule and means for applying pressure for contacting the ram means, once positioned in the capsule. This latter means includes means for applying a predetemrined pressure to the ram means of an order of magnitude that is a direct function of the number of pellets to be compacted. The ram means preferably fits within the capsule with a close tolerance fit. The press means described herein includes a housing for supporting a press member. The press member preferably has a diameter greater than the diameter of the ram means and is supported so that the ram means progresses into the capsule during compaction without any cocking between the ram and the capsule. In this connection, there is provided a die having a hole therein for receiving the capsule. The die is preferably a split die comprised of separate die parts that can be opened and closed. The die parts are supported for sliding movement along a track from an initial position at which the die parts are opened for receipt of the capsule to a compacting position at which the die parts are locked closed during contact of the press member with the ram. The die, as well as the selected tensile strength of the capsule material together prevent any substantial deformation, particularly radial deformation of the capsule. The ram is preferably of cylindrical shape adapted to be disposed to about one-half its length into the capsule before pellet compaction and about three-fourths its length into the capsule after pellet compaction. The ram preferably has right angle corners at its bottom where the ram contacts the pellets.