A number of radioisotopes are currently being utilized as markers and for other purposes in various medical, scientific, industrial and other applications. Since such radioisotopes frequently have a relatively short half-life, from a few hours on down to a few minutes, it is generally desirable that such radioisotopes be either produced at the site where they are going to be utilized, or at a site relatively close thereto.
However, the equipment for generating radioisotopes is currently relatively large and expensive, normally involving the use of a cyclotron, and the equipment for some radioisotopes, including .sup.18 F, also suffer from a lack of uniform results and an inability to achieve high yields. The lack of high yields, coupled with the short half life of the radioisotopes, limits the procedures in which such radioisotopes can be used to procedures requiring small radioisotope quantities, and also limits the number of procedures which can be performed. The cost and bulk of the equipment also makes it impractical to have such equipment at anything other than major hospital centers or research facilities, and thus limits the locations where procedures such as positron emission tomography (PET), or other procedures requiring such radioisotopes, can be performed to such facilities or ones situated in close proximity thereto. However, the usefulness of procedures utilizing radioisotopes in medical diagnosis and other applications render the wider availability of such radioisotopes desirable. In particular, Fluorine-18 (.sup.18 F), primarily because of its relatively long half-life (110 minutes), has emerged as the most widely used radioisotope in PET procedures, and a need exists for a procedure to permit on site generation of the radioisotope.
Current radioisotope generators normally operate by bombarding a selected target material with a high energy particle beam from a cyclotron or other particle accelerator. This results in a nuclear reaction leaving the desired radioisotope at the target.
One of the reasons for the relatively low yield obtained with such radioisotope generators for radioisotopes such as .sup.18 F which are generated from a water based target is that there is a lack of proportionality between increases in the current of the high energy beam and the radioisotope yield. This lack of proportionality is particularly true for high beam currents (i.e. currents in excess in 15 microamps). This loss of yield stems from a number of sources, including bubbles formed from vapor produced in the target by local boiling, and radiolysis which reduces the effective thickness of the target layer. Radiolysis is the breaking of the chemical bonds of the target substance. For example, with a water target, various forms of water often being used as targets, radiolysis would result in the water breaking into hydrogen and oxygen gas which would be dissipated. Thus, radiolysis can result in a reduction in the effective thickness of the target layer which in extreme cases can result in a substantial percentage of the target material being lost.
Since factors such as vapor production and radiolysis appear not to occur uniformly for a given beam current, yields of certain radioisotopes may vary substantially from batch to batch. In some situations, a substantial percentage, approaching 30%, of batches produce as little as 50% of the average yield. Since the time required to generate a batch of radioisotopes may be as long or longer than the half life of the radioisotope, unreliability in yield is a substantial limitation in utilizing such radioisotopes in a clinical setting since the yield from a given batch may not be adequate to meet a scheduled patient need. The inability to increase yield by increasing currents for the reasons indicated above also limits the usefulness of such procedures because of limited isotope availability. Still another problem with existing technology is the high cost of target materials such as enriched .sup.18 O water (i.e., $100/ml). Targets have, therefore, been designed with small volumes to reduce the cost of producing the radioisotopes. This has also held down the yields available, and means that the loss of target material due to vapor, radiolysis and the like discussed above can substantially add to radioisotope production costs.
Radiolysis also results in an increase in pressure at the target. Since the high energy beam must be generated in a vacuum, if vacuum cannot be maintained at the target, then a window transparent to the high energy particles must be provided between the high energy particle source and the chamber containing the target. Such windows, which are generally in the form of a thin foil, absorb energy from the beam passing therethrough and, particularly for high energy beams, must be cooled in order to avoid their burning out. The pressure differential across such windows, with vacuum on one side and target pressure on the other, which pressure differential can at times be substantial, particularly for fluid or gaseous targets (fluid or gaseous being sometimes collectively referred to hereinafter as "liquid") also results in stresses on the window which lead to window failure. Therefore, the existence of such windows in a radioisotope generating system presents a severe maintenance problem which reduces the time which the equipment can be used for generating radioisotopes, and thus reduces the yield of radioisotope available from a given machine. The overhead required for cooling the window also adds to the complexity in the design and use of the equipment. The ability to either eliminate the need for a window, or as a minimum to reduce the stresses on the window is, therefore, another important factor in reducing cost for generating radioisotopes and in increasing the yield available from a given radioisotope generating device.
While the problems discussed above are more common for radioisotopes, some of the problems, such as those caused by the need for a window to isolate target pressure, may also be present where stable isotopes, such as .sup.15 N or .sup.5 Li, are being generated.
It is, therefore, desirable to provide an improved method and apparatus for generating isotopes in general, and radioisotopes in particular, which can be smaller and less expensive than prior art generators so as to be usable at a greater number of facilities. It is also desirable to reduce the losses of target material due to radiolysis and the like and to thus increase the yields available from a given quantity of target material. The improved method and apparatus should also permit vacuum or near vacuum pressure to be maintained in the chamber containing the target so that windowless operation may be achieved, or as a minimum, that pressure differentials across the window be minimized. The above would permit higher yields of radioisotopes to be obtained at lower cost.