Production of thorium-229 from thorium-230

Abstract

A method for producing 229Th uses 230Th as a target material. 229Th can be formed from 230Th using energetic particles, including neutron, gamma ray, proton or deuteron bombardment.

Description

FIELD OF THE INVENTION

The invention relates to methods for producing thorium-229.

BACKGROUND OF THE INVENTION

The goal in the treatment of cancerous tumors and micrometastases has long been to kill the cancerous cells without killing healthy cells. Today, in the development of new short-range, site-specific therapies, there is increasing interest in using radioisotopes which decay with the emission of alpha particles. Indeed, recent clinical trials have shown the effectiveness of the alpha-emitter bismuth-213 in killing cancer cells in patients with acute myeloid leukemia. In addition, lung tumors in mice have been effectively treated for the first time by using an antibody radiolabeled with bismuth-213, targeting the lung vascular endothelial cells.

Alpha-particles are of interest in site-specific therapy because of their short range. Bismuth-213emits an 8 MeV alpha particle which penetrates only 6 to 10 cell layers nearby, killing the cells in its short path (˜80 μm), including cancer cells. In addition to bismuth-213, there are only eight other known alpha-emitters with potential for this type of application, namely, astatine-211, bismuth-212, lead-212, radium-223, radium-224, radium-225, actinium-225, and fermium-255.

There are a number of factors that need to be considered in using any radioisotope in humans, especially those radioisotopes emitting alpha particles. These factors include availability, cost, nuclear characteristics, chemistry, and in vitro and in vivo stability of the biomolecules labeled with alpha-emitters. The first two alpha-emitters to be used in human trials are bismuth-213 and astatine-211; the other seven radioisotopes mentioned above are under more preliminary investigations. Bismuth-213 is currently being used in human trials at Memorial Sloan-Kettering Cancer Center (New York) and is generated in-house from the decay of actinium-225. This radioisotope is produced from the decay of radium-225, which is the daughter of thorium-229, which, in turn is the alpha decay daughter of uranium-233.

Currently, uranium-233 is the only viable source for high purity thorium-229. However, the anticipated growth in demand for actinium-225 may soon exceed the levels of thorium-229 present in the aged uranium-233 stockpile (in fact, there have been occasions that supply has not been able to keep up with the current demand). It is estimated that only ˜45 g or ˜9 curies of thorium-229 (²²⁹Th specific activity is 0.2 mCi/mg) can be extracted from entire uranium-233 stockpile at the Oak Ridge National Laboratory (hereinafter “ORNL”). The uranium-233 stockpile at ORNL is about 50% of the high quality uranium-233 available in the world which provides reasonably low quantities of both Th-228 and Th-232. This stockpile is only about eighty times the current thorium stock. Large quantities of Th-228 or Th-232 can make the use of a uranium-233 stockpile impractical. Considering the rather low annual production rate of thorium-229 from uranium-233 (0.92 mCi/kg) and the increasing difficulties associated with uranium-233 safeguards, large-scale routine processing of uranium-233 is, at a minimum, problematic.

A number of approaches have been identified as alternative routes for the production of ²²⁹Th (t_1/2=7340 y), or for direct production of ²²⁵Ra (t_1/2=15 d), and ²²⁵Ac (t_1/2=10 d). These approaches include a) production of ²²⁹Th in a nuclear reactor by thermal neutron transmutation of ²²⁶Ra targets, b) direct production of ²²⁵Ac from proton and deuteron irradiation of ²²⁶Ra targets via the [p,2n] and [d,3n] reactions, respectively, at accelerators, and c) indirect production of ²²⁵Ac from the decay of ²²⁵Ra which in turn is produced by high energy γ-ray irradiation of a ²²⁶Ra target, [γ,n] reactions. The alternate route (a) noted above produces a low yield of ²²⁹Th.

SUMMARY OF THE INVENTION

A method for producing ²²⁹Th includes the steps of providing ²³⁰Th as a target material, and bombarding the target material with energetic particles to form ²²⁹Th. The energetic particles can comprise neutrons sufficient to result in a ²³⁰Th[n,2n]²²⁹Th reaction to form ²²⁹Th. The energetic particles can comprise gamma rays having energies sufficient to result in ²³⁰Th[γ,n]²²⁹Th reaction to form ²²⁹Th, such as having an energy of from 8 MeV to about 12 MeV. The energetic particles can comprise protons or deuterons having energies sufficient to result in ²²⁹Pa, the ²²⁹Pa decaying or transmuting into ²²⁹Th. When protons are used, the energy of the protons can be from 8 MeV to about 16 MeV. When deuterons are used, the energy of the deuterons can be from 16 MeV to about 28 MeV.

DETAILED DESCRIPTION OF THE INVENTION

The invention provides methods for the production of thorium-229 using thorium-230. The methods have good yields and generally lower contamination levels as compared to known methods for production of thorium-229 other than by decay of U-233.

In one embodiment, Thorium-229 is produced via neutron bombardment of a thorium-230 target. Thorium-230 with a half-life of 7.5×10⁴ y, is a part of the uranium-238 decay chain, and depending on the geological location the amount of ²³⁰Th in uranium mines is about 16 g per ton of uranium.

The ²³⁰Th[n,2n]²²⁹Th reaction has a threshold energy of 6.8 MeV and a cross section of 1.34 barns at 14 MeV. These assumptions translate to a production rate of ˜2.5 nCi of ²²⁹Th per day per gram of ²³⁰Th at neutron flux of 10¹¹ n s⁻¹·cm⁻² with an energy of 14 MeV. The 14 MeV neutrons can be produced in a cyclotron through a number of nuclear irradiations, the most common being the irradiation of a Be target with deuterons having an energy of ˜30 MeV, generating a neutron flux of ˜3×10¹⁰n.s⁻¹μA⁻¹ at 0-20° solid angle. For a 10 μA deuteron beam, the total neutron flux in the forward direction would be ˜3×10¹¹, distributed over an area ˜2 cm². By controlling the incident deuteron energy below ˜35 MeV, production of higher energy neutrons (>20 MeV) will be substantially minimized, and hence the production of unwanted ²²⁸Th which is produced via ²³⁰Th[n,3n]²²⁸Th can be substantially reduced. It is noted that the threshold for the ²³⁰Th[n,3n]²²⁸Th reaction is about 12 MeV. As noted above, high energy neutrons can be obtained from reactors such as High Flux Isotope Reactors. Alternatively, 14 MeV neutrons can be readily obtained from D-T fusion reactions. Also, high energy neutrons can be produced in charged particle accelerators via fission, fusion, pick-up, spallation, and other reactions.

Alternatively, high-energy neutrons available from a nuclear reactor can be used, where the flux of neutrons with energy >7 MeV is on the order of 5×10¹³ n s⁻¹·cm⁻². The fission averaged cross section of ²³⁰Th[n,2n]²²⁹Th reaction is 10.66 mb. The yield of ²²⁹Th from reactor irradiation of ²³⁰Th would be on the order of 10 nCi per gram of target per day or 3.7 μCi per gram per year of irradiation. The main disadvantage of the ²³⁰Th[n,2n]²²⁹Th reaction would be the generation of fission products as the fission averaged cross section of ²³⁰Th is rather significant (163 mb).

In another embodiment, thorium-229 is produced via gamma ray bombardment of a thorium-230 target via the ²³⁰Th, [γ,n] reaction. No excitation function for the ²³⁰Th[γ,n]²²⁹Th reaction is currently known. However, from the reported excitation functions for ²³²Th[γ,n] reactions, a threshold energy of ˜6 MeV can be expected, and a maximum cross section of ˜440 millibarns at ˜11.5 MeV. The maximum incident energy of the incident gamma ray for this reaction is about 12 MeV in order to minimize the production of unwanted ²²⁸Th by the ²³⁰Th[γ,2n]²²⁹Th reaction. Production of ²³¹Th via the ²³²Th[γ,n] reaction is known to be 22 mCi/h/g of ²³²Th in a 10 kW electron accelerator producing 25 MeV electrons. If ²²⁹Th is produced from ²³⁰Th at the same rate, the product activity of ²²⁹Th will be 0.21 μCi/d/g of ²³⁰Th.

In another embodiment, thorium-229 is produced via proton and deuteron irradiation of thorium-230 targets, such as in an accelerator. Both reactions are believed to actually proceed through production of relatively short-lived protactinum-229 having a half life of only 1.5 day, ²³⁰Th [p,2n]²²⁹Pa (EC, t_1/2=1.5 d) ²²⁹Th and ²³⁰Th [d,3n]²²⁹Pa (EC, t_1/2=1.5 d)²²⁹Th reactions, respectively. No excitation functions for these reactions are reported. In the case of the proton-induced reaction, from the reported excitation function for a similar reaction using a thorium-232 target, ²³²Th[p,2n] reaction, a threshold energy of ˜10 MeV can be expected, a maximum cross section of ˜400 millibarns at ˜15 MeV, and a cross section of ˜200 mb at 20 MeV. However, in order to minimize the production of unwanted ²²⁸Th by the ²³⁰Th[p,3n]²²⁸Pa (EC, t_1/2=22 h)²²⁸Th reaction, the maximum energy of the incident proton used for this reaction is limited to about 16 MeV. Assuming an average cross section of 200 mb, bombarding a foil of ²³⁰Th with a thickness of 0.5 mm (˜0.55 g/cm², range of protons 16→10 MeV) translates to a production rate of ˜0.6 μCi of ²²⁹Th per day at a 100 μA current of protons with an incident energy of 16 MeV.

In the case of the deuteron-induced reaction, from the reported excitation functions of for a similar reaction using a bismuth-209 target, ²⁰⁹Bi[d,3n] reaction, a deuteron threshold energy of ˜16 MeV can be extrapolated. Above the threshold, the cross section sharply increases to a maximum of ˜1.5 barn, then drops off rapidly to ˜500 mb at 32 MeV. The maximum energy of the incident deuteron for this reaction is about 28 MeV, in order to reduce the probability of the evaporation of an additional neutron which results in the production of unwanted ²²⁸Th by the ²³⁰Th[d,4n]²²⁸Pa (EC, t_1/2=22 h) ²²⁸Th reaction. In this case, assuming an average cross section of 700 mb, bombarding a foil of ²³⁰Th with a thickness of 0.7 mm (˜0.78 g/cm², range of deuterons 28→16 MeV) results in a production rate of ˜3 μCi of ²²⁹Th per day at a 50 μA current of deuterons with an incident energy of 28 MeV.

The necessary fast turn-around for processing of the Ra target in the direct production of ²²⁵Ra and ²²⁵Ac (a few days post-irradiation) is the main disadvantage for proton, deuteron and gamma ray irradiation of a radium target via ²²⁶Ra[p,2n]²²⁵Ac, ²²⁶Ra[p,pn]²²⁵Ra (t_1/2=15 days, β⁻)²²⁵Ac, or ²²⁶Ra[γ,n]²²⁵Ra (t_1/2=15 days, β⁻)²²⁵Ac reactions. The main drawback in the reactor approach for the production of ²²⁹Th using a ²²⁶Ra target is the significant contamination of ²²⁹Th with ²²⁸Th (t_1/2=2.8y) that generally results. The approaches for production of ²²⁹Th via alpha or ³He bombardment of a radium-226 target described above generally provides thorium-229 with significantly reduced levels of ²²⁸Th contamination. The fast neutron irradiation of a thorium-230 target, or neutron capture by ²²⁸Ra generally also provides thorium-229 with significantly reduced levels of ²²⁸Th contamination as compared to the reactor approach using a ²²⁶Ra target.

The thorium-229 generated using the invention must be separated from the target material and other by-products generated for most uses. Radiochemical procedures can be used for the separation of thorium-229 from target materials and by-products. The chemical processing of U, Th, Ac and Ra has been studied extensively in the past 70 years and is well known. In summary, after irradiation, the target can be dissolved and Th selectively retained on anion exchange resin (e.g. MP1 resin, BioRad Inc.) from 7.5 M HNO₃ as the Th (NO₃)₆²⁻ complex, while U (VI), Ac (III), Fe (III), Al (III), Ra (II) and Pb (II) and a number of fission products are eluted. Subsequent to the elution of Th from the column with O.1 M HNO₃, the Th is further purified by hydroxide precipitation in the presence of the Fe⁺³ carrier to eliminate Tc and I. Thorium is then separated from the Fe⁺³ carrier by retaining FeCl₄⁻ on an anion exchange column in 10 M HCl. After allowing ²²⁵Ra and ²²⁵Ac to reach their equilibrium values (˜45 days), they are separated from Th using anion exchange resin and 7.5 M HNO₃ as described above. Separation of Ac from Ra is accomplished by one of two methods, both based on cation exchange resin from nitric acid media. In the first method, using 1.2 M HNO₃ as eluent, Ra⁺² is eluted ahead of Ac⁺³ with a small overlap. When the eluent is changed to 0.15 M NH₄Cl and 0.1 M NaEDTA, pH ˜5, (the second method), Ac is eluted quantitatively whereas Ra remains adsorbed on the resin (reverse phase chromatography). Both methods have been tested extensively for the separation of carrier-free ²²⁵Ac from ²²⁴Ra and ²²⁵Ra and they work well.

Thorium-229 produced using the invention is expected to be used for a variety of medical applications, such as for killing cancer cells. With appropriate biological targeting molecules, bismuth-213 can be used not only in cancer therapy but also for autoimmune diseases, organ transplantations, bone marrow ablations, and vasculature irradiation following restenosis.

For example, the invention can be used for cell-directed radiation therapy. In this method, millions of cancer seeking antibodies guide radiation to the cancer. Energetic radioactive isotopes (radioisotopes which are capable of depositing a significant amount of energy in a short distance in the tissue) according to the invention are attached to the antibodies. As the cancer hunting antibodies flow though the blood stream, the radioactive isotopes ride. along. The antibodies target cell surface binding sites specific to the cancer cells. When the antibodies reach a cancer cell, they attach. Radiation from these bound radioisotopes then destroys the cancer cells that make up the malignant tumor.

This invention can be embodied in other forms without departing from the spirit or essential attributes thereof and, accordingly, reference should be had to the following claims rather than the foregoing specification as indicating the scope of the invention.

Claims

1. A method for producing 229Th, comprising the steps of: providing 230Th as a target material, and bombarding said target material with energetic particles to form 229Th.

2. The method of claim 1, wherein said energetic particles comprise neutrons sufficient to result in a 230Th[n,2n]229Th reaction to form 229Th.

3. The method of claim 1, wherein said energetic particles comprise gamma rays having energies sufficient to result in 230Th[γ,n]229Th reaction to form 229Th.

4. The method of claim 3, wherein an energy of said gamma rays is from 8 MeV to 12 MeV.

5. The method of claim 1, wherein said energetic particles comprise protons having energies sufficient to result in 229Pa, said 229Pa decaying or transmuting into 229Th.

6. The method of claim 5, wherein an energy of said protons is from 8 MeV to 16 MeV.

7. The method of claim 5, wherein said deuterons are used, an energy of said deuterons being from 16 MeV to 28 MeV.