Patent Application
20140226774
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The present invention relates generally to a device to make medical isotopes. This application discloses methods, geometries and materials for devices suitable for the irradiation of radium-226 for the production of actinium-227. This application discloses a novel production target designed make scarce alpha emitting medical isotopes more widely available. Neutron irradiation of radium-226 produces actinium-227 and thorium-228. Actinium-227 with a half life of almost 22 years and thorium-228 with a half life of almost 2 years are produced in the target disclosed. These two isotopes are parents or precursors of valuable medical isotopes. Their daughter isotopes emit one to five energetic but short ranged alpha particles, helium nuclei that transit only two to ten cell diameters depositing approximately 5 MeV over this short distance. The alpha emitters are: thorium-227, radium-223 and radium-224 and bismuth-212.
Generally radium-226 is irradiated with epi-thermal neutrons in a target containing radium-226 carbonate in a set of sealed sources, the target is opened and it is processed using radiochemical separations techniques. The actinium-227, the thorium-228 and the remaining radium-226 are separated from each other, are purified and stockpiled. Later, pure actinium-227 is milked for recovery of either radium-223 or thorium-227. Also, pure thorium-228 is milked for radium-224 which decays to lead-212 that generates bismuth-212. Radium-226 that is recovered from the irradiated material is recycled and used again in new targets. The half lives of alpha emitting isotopes generated by the decay of actinium-227 and thorium-228 have durations that make them useful as active pharmaceutical ingredients for a class of targetable new drugs for the treatment of cancer and other disorders.
The inventive target uses a tailored neutron spectrum that interacts with radium-226 in a way that suppresses production of less desirable thorium-228. The energy groups of neutrons most suitable by capture by nuclei of radium-226 but not suitable for capture by nuclei of produced actinium-227 are neutrons are in the epithermal energy groups with the most useful neutrons being in the epithermal energy groups less than 1 KeV and greater than 50 eV. Epithermal neutrons in these energy groups have good capture cross sections for nuclei of radium-226 but comparatively poor capture cross sections for nuclei of actinium-227. Thus, in the well tailored epithermal neutron spectrum, the nuclei of produced actinium-227 are less likely to capture neutrons to become nuclei of thorium-228. When the spectrum is shaped so that radium-226 nuclei mostly “see” epi thermal neutrons between one KeV and 50 eV, production of thorium-228 is suppressed and production of actinium-227 is increased. Protecting the produced actinium-227 from exposure to thermal neutrons is desirable since fewer atoms of actinium-227 will be able to capture neutrons to become short lived actinium-228 which quickly decays to thorium-228. Actinium-227's neutron capture cross section for neutrons in the elevated epithermal energy groups is much lower in the 1 KeV to 50 Ev range than for neutrons in the less energetic energy groups in and near the thermal neutron energy groups, 0.025 eV. The difference in actinium-227's neutron capture probabilities between the thermal neutrons and higher energy epithermal ranges on the average exceeds one order of magnitude. Because this difference in neutron capture cross-sections is substantial, significant benefit results from tailoring the neutron spectrum so that neutrons between 1 KeV and 50 eV are made to be those most likely to interact with radium-226 nuclei so that the produced actinium-227 is conserved. This shaping of the neutron spectrum or flux tailoring reduces the co-production of thorium-228 (the first decay product of short lived actinium-228 which is the neutron capture product of actinium-227). By reducing the co-production of thorium-228, during a long irradiation more useful actinium-227 becomes available and scarce radium-226 is conserved. The other factor is to limit irradiation time to minimize production of thorium-228.
Alternatively, if production of thorium-228 becomes more desirable than production of actinium-227, then the neutron spectrum that interacts with the target material can be tailored so that the epithermal neutron population is decreased and the thermal neutron population is increased. Additionally the target material can be configured to enhance self shielding effects to promote double neutron capture. These modifications produce greater production of thorium-228.
The disclosed target device is designed to irradiate compacted radium-226 carbonate. Compacted carbonate is used because voids between the particle grains conveniently provide plenum volume for helium-4 generated by the continuous decay of radium-226 and the continuous decay of most of its decay daughters. The plenum volume between the grains of target material provides adequate space for radon-222 also generated from the decay of radium-226 as well as for helium-4. Additional plenum volume is provided as “head space” by the geometry of each caplet that encloses a disk or other shape: hexagonal octagonal, square, rectangular or trapezoidal of compacted radium-226 carbonate.
Compacted radium-226 carbonate is a convenient form of radium-226 to process after irradiation and is selected because it is insoluble in water, a target material suitable for use in water cooled production reactors. Use of radium-226 carbonate simplifies post-irradiation chemical separations steps in which actinium-227 is separated from the radium-226 and thorium-228. Compacted radium-226 carbonate has the additional advantage of being easily dissolved in several solutions after irradiation. Irradiating dispersed radium-226 in carbonate is advantageous because self shielding effects are reduced because atoms of radium-226 are spread out, and are present in a reduced density compared to the density of atoms of metallic radium-226. Lower density reduces probabilities of a second neutron capture (so that less thorium-228 is produced.) Finally, before compaction, the carbonate powder can be mixed with other powders, powdered silicon dioxide, to reduce self shielding, powdered aluminum nitride or powdered cubic boron arsenide for heat transport, or a powdered lanthanide oxide or any hydride to reduce neutron energy in the target from fast to epithermal. A selected lanthanide oxide powder of a strong neutron absorber can be used for thermal neutron shielding along with a cover foil or tube that captures thermal neutrons. These strong neutron absorber materials can be erbium or europium in oxide form or other chemical form.
One focus of the innovation disclosed is to maximize actinium-227 production by the use of neutrons in the higher epi thermal energy groups for the irradiation of radium-226. For the purpose of this patent application, the thermal energy region is defined as 1/1000 electron volts to 5/10 electron volts, the epi thermal energy region is 5/10 electron volts to 5000 electron volts, the fast energy region as five thousand electron volts (five kilovolts, 5 KeV) to one million electron volts (1 MeV) and the high energy region as above 1 MeV. The optimal energy for transmutation of radium-226 to actinium-227 is in the higher region of the epi thermal spectrum, between one kilovolt (1 KeV) and fifty electron volts (50 Ev.) At lower energy, neutrons below 50 eV, probabilities for neutron capture by actinium-227 increase significantly over probabilities for neutron capture by radium-226 for the most part. These lower neutron energy groups are to be avoided when the object is to make actinium-227 from radium-226.
The target disclosed in this application is more efficient because the rate of production of actinium-227 from radium-226 is increased over other methods which use essentially only thermal neutrons. The novel production device disclosed also reduces the production of less desirable actinium-228 that rapidly decays to thorium-228. This objective is achieved by shaping the neutron spectrum of the neutrons which interact with radium-226 to the higher epithermal range so that the produced actinium-227 nuclei are exposed to as few thermal neutrons as possible. So long as actinium-227 nuclei “see” epithermal neutrons in the right energy groups and not thermal neutrons, production of thorium-228 will be reduced. If, however the production is thorium-228 is desired, neutron spectrum tailoring can be adjusted to maximize production of this isotope by irradiating radium-226 in the thermal spectrum. The density of radium-226 atoms can be increased to increase self shielding effects (using metallic radium-226 beads in the central region of the target instead of powdered, compacted and diluted radium-226 carbonate). The spectrum of the interacting neutrons can be tailored by introducing more hydrogenous moderating material in and around the target to make epithermal neutrons more scarce and thermal neutrons more plentiful. This promotes double neutron capture by radium-226 is to increase production of thorium-228.
By using higher energy epithermal neutrons for the transmutation of radium-226 to actinium-227 and by using strong thermal neutron absorbers to protect the produced actinium-227 to shield actinium-227 from as many thermal neutrons as possible and by irradiating compacted, powdered target materials, the efficiency of actinium-227 production is enhanced. Radium-226 is converted to actinium-227 more quickly because neutron captures in radium-226 nuclei are more likely when the incident energy of the of the incoming neutrons in the radium containing target material is approximately 1 KeV and as neutrons slow down in the target materials from 1 KeV, the probabilities for capture by radium-226 improve until neutrons are slowed below 50 eV and at this threshold probabilities commence to become more elevated for neutron capture by produced actinium-227. One innovation is to tailor the neutron spectrum so that the neutron population the radium-226 atoms “see” in the target is kept above 50 eV and below 1 KeV to make actinium-227.
The other leading innovation disclosed in this application is a new type of sealed capsule. This sealed source is designed to be “radon tight.” Radon-222 is the first decay daughter of radium-226. It is a radioactive gas with a half life of 3.8 days. Radon-222 must be contained in the target if the target is to be approved for irradiation for the 5-10 weeks of irradiation needed for production of actinium-227. The use of compacted powders in the target provides a physical environment, plenum space in the voids between the particle grains to receive atoms of helium-4 and radon-222 which are constantly generated by radium-226's decay. Radon-222 is kept inside the target by the use of a group or set of radium-226 carbonate containing caplets. Sealed source caplets are the first line of defense. Caplets are small, hollow, finely machined metal disks that are fitted together and welded shut, forming a leak tight enclosure, that are qualified as sealed sources. Each caplet is separately welded shut after being loaded with the compacted radium-226 containing disk. A stack of sealed caplets (arranged like a roll of coins) is enclosed in a set of nested set of cylinders each being welded shut or otherwise sealed. Enclosure by nested, redundant metal sealed cylinders with activated charcoal provided in the top and bottom ends of the cylinders provides a secondary plenum for any radon-222 that may leak from or escape from any caplet. The nested cylinders provide redundant enclosures that reduce the risk that radon gas will leak from the capsule during storage, transportation and during irradiation. The caplets and the tubing can be fashioned from aluminum alloy (aluminum-6061), zirconium alloy (zircalloy), qualified stainless steel alloys (HT-9, SS-316) or titanium alloys as well as other alloys used for sealed sources. The alloy used for qualified sealed sources is the most likely enclosure alloy. Further, nested cylinders to contain the caplets can be fashioned from silicon carbide or other ceramic materials that are radon tight including aluminum nitride aluminum titanate.
One additional feature of this invention is that the assembly in which radium-226 is irradiated contains engineered amounts of strong thermal neutron absorbers to capture thermal neutrons so that actinium-227 is exposed to fewer thermal neutrons during periods of lengthy irradiation. Actinium-227 production rates are higher when epithermal neutrons between 1 KeV and 50 eV interact with radium-226. Strong neutron absorbers such as erbium or europium in oxide form or metallic foil form can be used to shape the spectrum so that few thermal neutrons interact with actinium-227.
Production is further enhanced by the use of intermetalic hydrides or deuterides in other plenum spaces around the inventive target. These hydrides in engineered amounts conveniently slow fast neutrons to desirable epithermal energy ranges just above 1 KeV.
Strong thermal neutron absorbers used with selected hydrides efficiently manage and tailor the energy groups of the neutrons interacting with radium-226 so that fast neutrons are slowed to the epithermal range and thermal neutrons are captured by absorbers near the target material. Use of these methods at the same time enhances production of actinium-227.
Other methods of production of actinium-227 discussed in the literature or otherwise commonly known use thermal neutrons for irradiation of radium-226. These methods cause significant numbers of neutron captures by actinium-227 producing more thorium-228. Actinium-227 has a much higher capture cross section for thermal neutrons than radium-226 does for most of the neutron energy groups below 20 Ev. By shielding the interior target volume containing radium-226 from these thermal neutrons, a more efficient method of producing actinium-227 becomes available. The materials and geometry of target components used to do this are the heart of the innovation disclosed in this application.
The general principles governing production of radium-223 from radium-226 are reported in the literature. Roy Larsen et al in “Preparation and Use of Radium-223 to Target Calcified Tissues for Pain Palliation, Bone Cancer Therapy and Bone Surface Conditioning for” issued Oct. 21, 2003 as U.S. Pat. No. 6,635,234, discloses the value of radium-223 for treatment of many types of cancer. Also disclosed by Roy Larsen et al in “Thorium-227 for Use in Radiotherapy of Soft Tissue Disease” issued Feb. 26, 2008 U.S. Pat. No. 7,335,154 is the promise of using thorium-227 for various medical indications. In these disclosures and others dealing with radium-223 and thorium-227 no mention is made of the methods that can be used to produce pure and uniform actinium-227 by use of epithermal neutrons, spectrum shaping materials, by providing trapping media for radon-222 control and a unique caplet and nested tube geometry for the target to accomplish neutron spectrum shaping using strong neutron absorbers and hydrides to generate more epithermal neutrons in the vicinity of the radium-226. The literature does not disclose use of radon trapping media activated charcoal to manage radon gas before during and after irradiation.
The earliest patent discussion in the patent literature concerning the use of neutron absorbers in isotope production targets is found in “Neutron Irradiation Process for Producing Radioisotopes wherein Target Isotope is shielded from Thermal Neutrons” U.S. Pat. No. 3,269,915, issued on Aug. 30, 1966 to Jackson A. Ransohoff. The '915 patent discloses the use of neutron absorbing material in targets used to produce isotopes whose thermal neutron capture cross sections is significantly greater than the parent isotope. There is ample discussion throughout this application on the preparation of actinium-227 from radium-226. What is disclosed by Ransohoff is a type of cladding that is “black” to thermal neutrons. The black cladding is comprised of cadmium, samarium and/or gadolinium. Its thickness is adjusted so that enough neutron absorbing material is present throughout an irradiation. The black cladding disclosed is a cylinder of neutron absorbing material surrounding the target material with a lower neutron capture cross section than the produced material: neptunium-237, radium-226 or gold-197. Further there is discussion on the use of hydrides in the target as a moderator material to convert high energy neutrons to useful neutrons at lower energies. In the instant disclosure use of erbium and europium is made in contrast to the black cladding materials disclosed by Ransohoff. Little further mention of these innovations is found in more recent the literature. In “Neutron Source”, U.S. Pat. No. 4,208,247 to Albert J. Impink, there is disclosure of a neutron source or neutron generator used for the start up on a nuclear reactor. This device unlike other neutron generators remains in the reactor during operations. Advancing the use of the “black cladding”, cladding is opaque to thermal neutrons; this form of generator is shielded from thermal neutrons. It uses plutonium-238 as an alpha source for the product of neutrons by the alpha, n method on beryllium. Without the use of the “black cladding” the plutonium-238 would absorb thermal neutron at a high rate, so that soon plutonium-238 would be depleted and the neutron source would cease adequate neutron production since source of alpha particles to act on beryllium to generate neutrons is transmuted away. This is the background in the literature from published patents.
Radium-223, chelated thorium-227 or chelated lead-212 are administered by intravenous injection to cancer patients. Lead-212 decays to bismuth-212 which emits a single energetic alpha particle. In contrast, thorium-227 and radium-223 emit a cascade of energetic alpha particles. The alphas sever double strands of DNA in the nucleus of the cancer cell. All of the decay energy, in the range of five million electron volts (5 MeV) for each emitted alpha particle is deposited over a very short distance, two to ten cell diameters from the cell where the alpha is emitted. The energetic alpha particles damage cancer cells by ionizing molecules inside the cancer cell. The localized disturbance is generated by the rapidly moving alpha particles as they break chemical bonds in the cells within two to ten cell diameters of their point of origin. Within targeted cancer cells, double strands of DNA in the nucleus of the cancer cells are severed by the effects of the fast moving alpha particles. Once the double strands are severed, the cancer cells' genetic information is lost, causing these cancer cells to lose the ability to divide and to replicate.
Some primary cancer tumors metastasize by establishing secondary tumors within bone tissue. Among these, are prostate cancers, breast cancer, and some forms of lung cancer and kidney and urinary cancer and multiple myeloma. When cancer “spreads” to the bones, a lethal and painful burden is placed on the patient. Generalized radiation, chemotherapy, and surgery are not the most efficacious treatment options.
Thorium-227 and radium-223 have many therapeutic uses for the treatment of many forms of cancer and perhaps even for the treatment of recalcitrant infectious diseases that no longer respond to antibiotics. Radium-223 is used for the treatment of metastatic cancers that “spread” to bone tissue. Radium naturally “seeks” active bone because it follows the metabolic pathways that calcium uses in humans and mammals. Importantly also thorium-227 and radium-223 can be combined with various molecular targeting agents that seek and attach to specific cancer cells in primary tumors.
Thorium-227 decays with an alpha cascade of 5 alpha particles. Radium-223 decays with an alpha cascade of 4 alpha particles. Radium-224 decays with an alpha cascade of 4 alpha particles and bismuth-212 has one alpha decay. The half lives of the various alpha emitters vary: 18.75 days for thorium-227, 11.43 days for radium-223, and 3.66 days for radium-224 and 10 hours for lead-212 the precursor of bismuth-212. The alpha radiation from these isotopes provides a precise method to deliver highly localized radiation doses to cancer cells or to pathogens. Alpha radiation disrupts the ability of targeted cells to replicate. Because the alpha particles do not travel very far, two to ten cell diameters, 100 microns, the patient's healthy tissues are spared in contrast to chemotherapy and other forms of radiation in use.
Radium-223 treatment involves injecting the patient with a very small amount of this isotope that is carried to the bloodstream by a standard sterile saline solution. Radium-223 ions in the bloodstream seek bone because radium mimics calcium in the human and the mammalian body. Four energetic alpha particles cascade from the radium-223 atoms and the atoms of its decay daughters. The energetic helium nuclei, the alphas, attack the targeted cells within two to ten cell diameters from where the radium-223 atoms are incorporated into cancerous bone tissue. Radium-224 will act chemically like radium-223. It too will seek out bone and expose cancer cells that have spread to bone to alpha radiation. Radium-224 has a shorter half-life and may be administered more frequently than radium-223.
Lead-212 and Thorium-227 are used with molecular targeting agents that seek and attach to a unique or over expressed receptor of the targeted cancer cell. Lead-212 and thorium-227 can be chelated to special molecules that anchor to cancerous cells as well as those which target a particular receptor.
The foregoing background discussion reflects the current state of the art of which the present inventors are aware. Reference to, and discussion of, these patents is intended to aid in discharging Applicants' acknowledged duty of candor in disclosing information that may be relevant to the examination of claims to the present invention. However, it is respectfully submitted that none of the above-indicated patents disclose, teach, suggest, show, or otherwise render obvious, either singly or when considered in combination, the invention described and claimed herein. A review of the known art reveals no discussion of the use of compacted granular aluminum nitride or other heat conducting ceramic, or ceramic materials to enclose the caplets within a sealed target using redundant barriers. No mention is made of ceramics used to make useful isotopes to dilute the target material and to transport heat from the target material to the outside of the target using heat transporting ceramic, aluminum nitride, cubic boron arsenide or silicon carbide or graphite to conduct heat from the target's interior to its outer wall.
The novel production device disclosed in this application reveals that commercial scale actinium-227 production is made possible by the use of many features disclosed in this application. The complexity associated with the irradiation of radium-226 is illustrated in
First, Actinium-227 production from radium-226 is enhanced when selected energy groups in the epithermal range between 20 eV and 1 KeV are used. Production is enhanced because the neutron spectrum is tailored so that the neutrons that interact with radium-226 are above 20 eV and below 1 KeV. The target's placement in the core of the production reactor is very important. The target must be in a reactor position that has a high overall population of neutrons in higher or fast energy groups so that tailoring methods shape the neutron spectrum so that it has a high proportion of epithermal neutrons in the range between 1 KeV and 20 eV.
Second, self-shielding effects are reduced by the use of compacted, powdered radium-226 carbonate to disperse radium-226 atoms among neutronically inert atoms, compacted and powdered aluminum nitride, alumina, or silicon dioxide or silica or zirconium oxide, zirconia, for example. The radium-226 containing media is placed in the central region of the production device within inventive caplets. The caplets divide the radium-226 mass to reduce the likelihood that a significant quantity of radon-222 will leak from the target assembly into reactor spaces. Because the atoms of radium-226 are dispersed in radium-226 carbonate and this media can be further diluted with heat transport media such as aluminum nitride or cubic boron arsenide which blend can be further diluted with alumina, silica, or zirconia so that probabilities for double neutron capture are reduced as the density of radium-226 atoms in the target media are reduced.
Third, the preferred geometry is selected. In this geometry the compacted radium-226 carbonate and the other compacted powders are arrayed in compacted disks enclosed by the caplets within three or more nested metallic or ceramic cylinders. Moreover, spectrum shaping materials can be furnished in powder form or metallic foil to surround the caplets arrayed in a column in the central axis of the cylinder. One class of spectrum shaping material is the oxides of strong thermal neutron absorbers erbium oxide and/or europium oxide or each in metal form or in combination as foils. The other class is spectrum shaping materials are the moderating materials to reduce the energy of the fast neutron groups to the productive epi thermal energy groups. These materials are metallic hydrides or deuterides or hydrides or deuterides of the lanthanide group.
The inner target, the subject of this application, are caplets enclosing radium-226 as sealed sources and further enclosed by nested cylinders enclosing the caplets, and jacketed by an outer enclosure that isolates the inner target from the reactor's coolant. The exterior jacket is not the subject of this application for patent. The jacket or rig may contain a cover gas that is circulated in conduits in a closed loop out of the core to shielded and instrumented compartments that monitor the cover gas. The closed loop circulates the cover gas through instrumented radon-222 traps. If radon gas escapes from the sealed inner cylinders, will be contained by the outer jacket and the instrumentation examining the cover gas will provide adequate warning to the reactor operator if troublesome quantities of radon-222 are detected. When radon-222 decays it emits gammas at the 0.511 MeV lines which is the same line as positron emission. Thus, positron emission detection equipment is readily available. The gamma detectors are located proximate to the part of the loop containing activated charcoal traps. If one trap detector shows gamma emissions at this energy, then the other trap receives the cover gas for an interval and the decay rate of the counts in the second trap will reveal if radon-222 is present by its signature decay pattern.
During neutron irradiation of radium-226, the actinium-227 “grows in” at rates which slow over time. Because the half-life of actinium-227 is almost 22 years, the irradiation period can be relatively lengthy but the longer the irradiation period; the more actinium-228 is produced, even with the spectrum tailoring. The production curve begins to fall off because actinium-227 atoms become more common setting the stage for more actinium-228 to be co-produced. The optimal period of irradiation is a function of attributes of production reactors, neutron flux, the flux tailoring of the interacting neutron energy groups and the reactor operator's pre-set schedule for reactor up time and down time.
The neutronic attributes of various existing production reactors are known and the flux tailoring needed for enhanced production varies from production reactor to production reactor. Depending on the volume of the target position and thus the target and the characteristics of the neutron flux at the reactor position where the target is to be irradiated, the radium-226 compounding agents are selected, and the density of the radium-226 atoms is determined, the radium-226 containing compacted carbonate disks and the caplets are sized. The caplets enclose the compacted powder and are joined in the hot cell and welded shut. The caplets are disk shaped and are designed to hold the helium-4, radon-222 radium-226 in the central axis region of the sealed target vessel. The radium-226 containing compound can be placed in a set of four to sixty four caplet disks to reduce the amount of radon-222 that could escape from a defect in the primary enclosure or in the welds that constitute the primary enclosure. The caplets are placed two or four at time in the next metallic or ceramic enclosure and these enclosures are placed in the central axis of the nested cylinders and redundantly enclosed with end caps that are welded shut.
The most favorable ceramic material for heat transport in the radium-226 containing powder is finely powdered passivated aluminum nitride. Heat conducting ceramic aluminum nitride provides advantages in the sealed capsule because heat is spread when the material is present with the material being irradiated.
This design reduces risks that a significant radon leak will occur during irradiation that could require a reactor shut down and removal of the leaking target. Any radon released from the target does not leak into the reactor environment because the target is isolated in the jacket that surrounds the target. This jacket contains a circulating cover gas that is monitored for radon-222. A failing target can be removed from the reactor safely being kept inside the jacket.
This application reveals and discloses the details of the inner sealed target assembly that reduces risks of radon release. The radium-226 target is to be used ideally within an enclosing jacket with an instrumented loop with circulating cover gas to detect a radon leak.
The object of the present invention is to provide a more efficient inner sealed production capsule used with an exterior jacket to isolate the capsule in case a radon leak occurs during irradiation. Actinium-227 will be produced in quantity to make radium-223 and thorium-227 and thorium-228 will be co produced which is a precursor of lead-212.
The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings wherein:
Novel means to efficiently produce radium-223 and thorium-227 from radium-226 are disclosed. As is known by persons familiar with the art, the steps disclosed in the literature that are involved in production of radium-223 and thorium-227 are rather limited and straight forward. The unstable “grandparent isotope”, the “starter”, is radium-226. This is a very rare isotope and must be conserved as much as possible. On a single neutron capture radium-226 is transmuted to radium-227 that promptly decays to actinium-227, becoming the valuable product.
It is well known that, actinium-227 avidly captures thermal neutrons and by this second neutron capture, undesirable actinium-228 is co-produced. The methods disclosed in this application provide for a better conversion of radium-226 to actinium-227 with reduced co-production of unwanted actinium-228 along with ways to manage radon-222 that is constantly generated by the decay of radium-226. Using the various techniques disclosed in this application at the same time in the same target allow more pure product is generated with less consumption of radium-226 and more importantly with lower risks of incidents associated with radium-222 release. So long as one or more grams of radium-226 target can be irradiated simultaneously in the novel nested cylinder target, more economic use of reactor spaces for irradiation is promoted. Provided the higher epithermal energy groups in selected reactor spaces can be fully utilized, the new geometry target will provide efficient service. It is the use of geometry and materials to trap radon-222 with other materials to capture thermal neutrons and other materials that moderate fast neutrons to epithermal energy groups in a novel geometry of four or more enclosures that make this actinium-227 production target both economically competitive and innovative.
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The invention disclosed in this application overcomes the obstacles encountered in producing radium-223 and thorium-227 from radium-226. These obstacles have limited radium-223's and thorium-227's availability for therapeutic uses. This application discloses and reveals a novel production vessel or target enabling the commercial production of uniform radium-223 and thorium-227 from actinium-227.
The new production target addresses important production concerns. The first production concern involves radon-222 gas leaks. Radium-226 constantly generates radon-222 as it decays. This decay product, radon-222, decays to polonium-218 in 3.8 days which promptly decays to lead-210 with a 22 year half-life. Depending on the mass of radium-226 in the target and the gas pressure and temperature in the target, a significant volume of radon-222 will be present in the sealed capsule at all times. The radon-222 gas decays away as it is formed when it is in secular equilibrium. Radon-222 reaches secular equilibrium in approximately 11.4 days.
When the caplets are opened after irradiation for the separation of actininium-227 produced from radium-226 and the other isotopes, inert radon gas could escape and must be captured. Radon contamination raises potential exposure issues. To reduce risks of exposure to radon-222 during irradiation or during the post irradiation separations process, the caplet and the cylinders enclosing radium-226 (target) can contain a selected radon trapping agent silver washed zeolite or activated charcoal.
The new production target addresses important production issues. The first production issue managed is radon. Multiple enclosures and multiple caplets reduce risk of a radon incident. Use of a jacket with circulating cover gas that is monitored for radon that has escaped from the multiple walls manages a radon leak incident so that the isolated target can be removed from the reactor. Radium-226 constantly generates radon-222 as it decays. This decay product, radon-222, decays to polonium-218 in 3.8 days which promptly decays to lead-210 with a 22 year half-life. Depending on the mass of radium-226 in the target and the gas pressure and temperature in the target, a significant volume of radon-222 will be present in the sealed capsule at all times. The radon-222 gas decays away as it is formed when it is in secular equilibrium. Radon-222 reaches secular equilibrium in approximately 11.4 days. The risks are reduced by the use of redundant enclosures and the uncertainty is managed by the use of the instrumented jacket.
The leading and preferred embodiment of the invention successively encloses radium-226 in powdered but compacted “diluted” carbonate form to produce actinium-227 by epithermal neutron capture in a tailored neutron spectrum. This novel type of production device can be considered for the production of other reactor-made isotopes by single or successive neutron capture. The novel techniques are tailoring the neutron spectrum using various materials, reducing self shielding by reduction of the target atom density, and better materials control using stacked caplets in nested cylinder geometry. The features common to all embodiments are multiple, nested metal or ceramic tubular “radon-tight” enclosures to provide redundant barriers to significantly lower the risk of radioactive gases escaping from the caplets during irradiation or during transport of the target. The use of multiple nested tubes, having each end welded shut, enclosing the caplets provides barrier redundancy. The design reflects the use of a small disk shaped metal containers called caplets which are welded shut to enclose the compacted, powdered material to be irradiated. Each caplet is welded shut and placed in a second enclosure. Then several enclosed caplets are placed in a metal tube that is welded shut. This tube is enclosed by the final outer enclosure that is welded shut or by additional tubes. The use of redundant enclosures raises no heat transport issues, as there are no air gaps between the nested metal enclosures. The metals to be used are qualified stainless steel HT-9, SS-316, qualified zirconium alloy, zircalloy, qualified titanium alloy, or aluminum alloy such as aluminum 6061. Further, silicon carbide tubulars or other ceramics can be fashioned to be radon tight. Combinations are possible as well with the caplets being one alloy and the tubing another. In all of the embodiments made possible by this disclosure, the heat spreading material used is passivated aluminum nitride which is mixed with the material to be irradiated radium-226 carbonate which media is then compacted. Further, in many embodiments the fine, granular ceramic heat conduction material is mixed with the granular target material which is compacted before that caplet is filled. This mix transports heat from the irradiated material to the nested metal or ceramic tubular enclosures. The dilutant or dispersant, the powdered or granular heat transporting material, spreads heat following neutron capture. The heat generated by the action of neutrons can be transported and dissipated by powdered passivated aluminum nitride in the caplets. Accordingly, radium-226 can be exposed to neutron fluxes that would cause overheating but for the use of the heat conducting ceramic grains. Importantly, the spaces between the grains of the compacted powder or granular composition provide a plenum volume for gasses released by decay. In this way, internal gas pressures are managed, keeping the gas pressure within the target well within safety margins before, during and after irradiation.
The granular ceramic heat conducting material is mixed with the material selected for irradiation. The ceramic reduces the percentage of or density of the atoms comprising the target material to be irradiated reducing self shielding effects, the ceramic also provides the benefit of heat transport when mixed with target material as needed. The heat conducting material can be aluminum nitride or other chemically inert and neutronically inert material that conducts heat well. An alternate embodiment would use passivated aluminum metal particles that have been treated to have a thin oxidized surface or a thin nitrided surface. Use of passivated metallic particles will increase heat transport over the ceramic. Passivation is important to reduce the pyrophoric properties of finely divided aluminum particles.
Each of the ceramic or metal tubes is capped and welded shut to provide redundant gas-tight enclosures so that risks of leakage of radon-222 from the capsule to the enclosing jacket or the environment are reduced to the negligible level. There are not less than four gas tight compartments that are welded shut enclosing the target material. The first compartment is the inner caplet that encloses the granular mix of compacted material that is to be irradiated and the heat spreading material if needed. The inner caplet is enclosed by the outer caplet. There are a total of eight caplets shown in the drawings positioned end to end but the number of caplets can be increased provided the final geometry is compatible with the reactor position indicated for the irradiation of that target material. Enclosing the caplets are the sealed ceramic silicon carbide or metallic tubes that are welded shut.
The redundant geometry provides leak tight barriers that retain radon and helium, the gasses generated by the decay of radium-226 and its decay daughters. The novel target enclosure has at least four barriers between the reactor spaces on the outside of the target and the material to be irradiated inside the caplets located the central axis of the target. The central axis is where the caplets containing the various target material are located.
The above disclosure is sufficient to enable one of ordinary skill in the art to practice the invention, and provides the best mode of practicing the invention presently contemplated by the inventor. While there is provided herein a full and complete disclosure of the preferred embodiments of this invention, it is not desired to limit the invention to the exact construction, dimensional relationships, and operation shown and described. Various modifications, alternative constructions, changes and equivalents will readily occur to those skilled in the art and may be employed, as suitable, without departing from the true spirit and scope of the invention. Such changes might involve alternative materials, components, structural arrangements, sizes, shapes, forms, functions, operational features or the like.
Therefore, the above description and illustrations should not be construed as limiting the scope of the invention, which is defined by the appended claims.
The present application claims the benefit of U.S. Provisional Patent Application Ser. No. 61/751,208, filed Jan. 10, 2013 (Jan. 10, 2013), and U.S. Provisional Patent Application Ser. No. 61/650,355, filed May 16, 2013 (May 16, 2013).
| Number | Date | Country | |
|---|---|---|---|
| 61751208 | Jan 2013 | US |
