Induced nuclear reactions: processes – systems – and elements – Nuclear transmutation
Reexamination Certificate
1999-07-15
2001-03-27
Jordan, Charles T. (Department: 3641)
Induced nuclear reactions: processes, systems, and elements
Nuclear transmutation
Reexamination Certificate
active
06208704
ABSTRACT:
BACKGROUND OF THE INVENTION
Radioactive isotopes are widely used in industry, medicine and the life sciences. The utility and commercial value of a radioisotope are determined based upon specific activity, with a high specific activity having greater utility and value.
Currently, isotopes are produced by electron beams, ion beams, and nuclear reactors. Electron beams are now generally used to produce short-lived isotopes at locations near the site of use. Ion beams and reactors are generally used to produce longer-lived isotopes at central facilities.
Many isotopes are amenable to production by all three techniques. These include isotopes prepared by either the addition or removal of a neutron from a naturally occurring targeted isotope. Currently, the ion beam has been the method of choice for neutron removal because of its relatively high energy efficiency. However, the ion beam process is disadvantaged by its high initial cost, complexity of operation, and limited ability to be scaled to large production rates. In addition, the relatively heavy mass of the ions makes it very difficult to generate high current density beams. Furthermore, because the ion energy is deposited in a very short distance, thus causing intense local target heating, the beam cannot be sharply focused without destroying the target. This limits the average specific activity achievable by ion beams.
Electron beams have significantly longer stopping distances than do ion beams, however, electron beams must generate photons within the target before the radioisotope can be formed. Further, high electron beam power density, required to generate the photon intensity needed to produce a high specific activity of radioisotope, will typically impose unacceptably high heat loads upon a target material, resulting in target melting.
Fission reactors compete with the beam sources in the production of isotopes through neutron absorption processes and also have a unique role in the production of isotopes separated from fission products.
Fission reactors are the method of choice for neutron addition because of their ability to produce large quantities of product. However, nuclear reactors are extremely expensive, have very high operating costs and are subject to exceedingly stringent siting and operational constraints under Federal regulations.
Therefore, a need exists for a less expensive and less complex means for producing high specific activities of longer-lived radioactive isotopes.
SUMMARY OF THE INVENTION
This invention relates to an apparatus, and method, for producing a high specific activity of a radioisotope in a single increment of target material, or sequentially within in-series increments of target material. In particular, this invention relates to an apparatus and method for producing a high specific activity of molybdenum-99 (Mo
99
) by exposing Mo
100
to a high energy, high intensity photon beam, typically derived from an electron beam with an intensity of about 50 microamps/cm
2
, or more. In producing a high specific activity of Mo
99
, the product of f·R is at least 2.2×10
−8
sec
−1
, where f is the isotopic fraction of Mo
100
in the target and R is the photon path length per unit volume per unit energy, weighted by the photoneutron cross-section integrated over energy. An average specific activity of Mo
99
of at least 1.0 curie/gram can be obtained in molybdenum targets of up to 7.5 cm in thickness. Further, for molybdenum targets of up to 0.5 cm in thickness, an average specific activity of Mo
99
of 10.0 curies/gram can be obtained.
One embodiment of the apparatus of this invention includes an electron accelerator, a convertor for converting an electron beam into a high energy photon beam, and a targeted isotope which is contained in the target material. Optionally, the convertor includes at least two separate convertor plates, wherein the convertor plates have different thicknesses, and coolant channels disposed between adjacent convertor plates for cooling the convertor plates to remove heat generated by the electron beam.
In preferred embodiments of the invention, a concentration of at least one product isotope is sequentially produced within in-series increments of target material. A target assembly contains increments of target material which include the targeted isotope. The increment proximal to the beam source is removable, with radioisotope, from the target assembly, while leaving additional target material for radioisotope production. This apparatus can further include a means for moving increments, in series, toward the photon beam source as the proximal increment is removed from the target assembly. Optionally, this apparatus also includes a means for inserting an additional target material increment into the target assembly distal to the photon beam source.
A target material of the present invention can be a solid mass or selected from the group consisting of a liquid, a slurry or particles. In one embodiment of the apparatus, each increment of target material is separately contained within a container.
The method of invention for producing a high specific activity of a radioisotope, preferably Mo
99
, in a target material containing a targeted isotope, such as Mo
100
, includes exposing the target material to a high energy photon beam to form a high specific activity of within the target material. Typically, the intensity of the electron beam, from which the photon beam is derived is 50 microamps/cm
2
, or more. Further, in producing a high specific activity of Mo
99
, the product of f·R is at least 2.2×10
−8
sec
−1
. In one embodiment the thickness of the target material is about 7.5 centimeters, or less, and convertor is a tungsten convertor, wherein the electron beam power density is about 35,000 watts/cm
3
.
In another embodiment of the method of this invention, the method further includes directing the photon beam from a photon beam source through target material increments, wherein the increments are in-series to said photon beam. This method optionally includes the step of advancing the target material increments in series toward the photon beam source. This method can further include the step of removing a target material increment from the photon beam, wherein the increment is proximal to the photon beam source.
The advantages of this invention include the highly efficient production of radioisotopes using a high energy electron beam to produce a commercially desirable specific activity level of a radioisotope within an increment of a target material. As the desired specific activity is produced in an increment of target material proximal to electron beam source, other increments of target material, in-series to the proximal increment, are sequentially pre-irradiated by the photon beam to commence building up the specific activity level of the radioisotope within each increment. Therefore, the period of time that an increment is irradiated, while proximal to the electron beam source, to produce a desired specific activity level of a radioisotope has been shortened by pre-irradiating the increment.
This invention also has the advantage that each increment of the target material can be removed to harvest radioisotopes without significantly affecting the overall production of the high specific activities in other in-series increments of target material.
An additional benefit of the present invention is that the target material is a source of intense neutron radiation. The neutron radiation can be used for further isotope generation by neutron absorption or other medical or industrial uses, such as imaging. Further, photons not absorbed by the target material can be employed in sterilization and materials processing.
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patent: 3963934 (1976-06-01), Ormrod
patent: 3999096 (1976-12-01), Funk et al.
patent: 4123498 (1978-10-01), Rosenbaum et al.
patent: 4428902 (1984-01-01), Murray
patent: 4598415 (1986-07-01), Luccio et al.
patent: 4701308 (1987-10-01), Koehly et al.
patent: 4839133 (1
Lanza Richard
Lidsky Lawrence M.
Hamilton Brook Smith & Reynolds P.C.
Jordan Charles T.
Massachusetts Institute of Technology
Mun K. Kevin
LandOfFree
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