Method of producing TC-99M by using nuclear resonance fluorescence

Abstract

Disclosed is a method of producing Tc-99m by using nuclear resonance fluorescence. More specifically, and a method of preparing Tc-99m by using nuclear resonance fluorescence includes irradiating a ground-state Tc-99 nucleus with a photon beam, thereby causing a nuclear transmutation to proceed such that the nucleus excited to high energy and then undergoes a transition to Tc-99m.

Claims

1. A method of producing Tc-99m by using nuclear resonance fluorescence, the method comprising: irradiating a ground-state Tc-99 nucleus with a photon beam, thereby causing a nuclear transmutation to proceed such that the nucleus is excited to high energy and then undergoes a transition to Tc-99m.

2. The method of producing Tc-99m of claim 1, wherein the photon beam is a gamma ray produced by laser Compton scattering.

3. The method of producing Tc-99m of claim 2, wherein the energy of the gamma ray is any one of energy levels of no less than 142 keV of a Tc-99 nucleus.

4. The method of producing Tc-99m of claim 3, wherein the Tc-99 nucleus is repeatedly irradiated with the gamma rays having two energies different from each other among the energy levels of the Tc-99 nucleus.

Description

DESCRIPTION OF DRAWINGS

(1) FIG. 1 schematically shows an NRF reaction;

(2) FIG. 2 is a graph showing an NRF cross section and a BR.sub.total at each of corresponding excitation energy levels enabling generation of Tc-99m;

(3) FIG. 3 is a view conceptually showing an LCS phenomenon;

(4) FIG. 4 is a block diagram showing an LCS gamma ray facility using an ERL system;

(5) FIG. 5 is a graph showing LCS spectra and .sup.99Tc NRF cross sections;

(6) FIGS. 6 and 7 are diagrams showing data related to transition at each energy level of the Tc-99 nucleus; and

(7) FIG. 8 is a view showing a transition process of a Tc-99 nucleus excited by specific energy 1,207 keV to a Tc-99m nucleus.

BEST MODE

(8) Specific structures or functional descriptions presented in the embodiments of the present invention are only illustrated for the purpose of describing the embodiments according to the concept of the present invention, and the embodiments according to the concept of the present invention may be implemented in various forms. In addition, the present invention should not be construed as limited to the embodiments described herein but should be understood to include all modifications, equivalents, and substitutes included in the spirit and scope thereof.

(9) The present invention corresponds to a new method of producing Tc-99m and irradiates Tc-99 with a photon beam generated by laser Compton scattering (LCS) to cause a nuclear resonance fluorescence (NRF) reaction, thereby producing Tc-99m.

(10) That is, it is a method of producing Tc-99m by a nuclear transmutation in which, when a Tc-99 nucleus of the ground state is irradiated with the LCS photon beam of specific energy, the Tc-99 nucleus is excited to high energy by an NRF reaction, and then undergoes a transition to a Tc-99m nucleus.

(11) Tc-99 is a radioactive isotope that undergoes R decay and has a half-life of 211,100 years. Naturally, trace amounts of Tc-99 exist due to spontaneous fission of uranium, but artificially, Tc-99 is produced at a high rate annually as fission products. When Tc-99m is produced by an NRF nuclear method using the LCS, it may be directly produced from Tc-99, which has already abundantly artificially been produced. Meanwhile, the Tc-99 element is stable, there is no problem in transportation, and a Tc-99m production facility is simple. Therefore, transportation and utilization problems are solved, whereby Tc-99m of low-cost may be supplied.

(12) Photoexcitation of the NRF (Nuclear Resonance Fluorescence)

(13) The NRF reaction is a result of absorption of a nucleus and a release of high energy photons arising from the absorption of the nucleus, and FIG. 1 schematically shows the NRF reaction, with E.sub.0 and E.sub.γ representing the energy of the ground and excited states of the nucleus, respectively. The ground-state .sup.99Tc nucleus may be excited by a photon with an energy of approximately E.sub.γ, and an excited-state .sup.99Tc nucleus lasts for a very short time (˜ femto seconds) and emits photons, thereby decaying back to the ground state.

(14) An NRF cross section follows a Breit-Wigner formula and is shown in [Equation 1] below.

(15) $\begin{array}{cc}\sigma \left(E\right)=\frac{1}{4\pi }\frac{2J+1}{2\left(J_0+1\right)}{\left(\frac{hc}{E}\right)}^2\frac{\Gamma {\Gamma }_0}{{\left(E-E_r\right)}^2+{\Gamma }^2/4}& \left[\mathrm{Equation}1\right]\end{array}$

(16) In above equation, J is spin of the excited state, J.sub.0 is spin of the ground state, Γ is total damping width of all damping widths from an excitation energy level, Γ.sub.0 is partial damping width from the excited state to the ground state, and E is incident gamma ray energy, and E.sub.γ is energy level of the excited state. Data for .sup.99Tc was based on the Evaluated Nuclear Structure Data File (ENSDF), and the NRF cross section was calculated by the Particle and Heavy Ion Transport code System (PH ITS).

(17) FIG. 2 is a graph showing an NRF cross section and a BR.sub.total at each of corresponding excitation energy levels enabling generation of Tc-99m. The BR.sub.total means a branching ratio to Tc-99m at a corresponding excitation energy level, and when information on a photon branching ratio for each level deduced from isomer transition decay is missing in the ENSDF file, the BR.sub.total is unable to be calculated for the corresponding level even though a branching ratio to .sup.99mTc exists at the corresponding level. When the ENSDF file does not have a transition path to the ground state for each of the excitation energies, errors may occur for some of the cross sections. In FIG. 2, the BR.sub.total or the cross section in the case described above is not indicated.

(18) The NRF cross section in a state of the nuclear isomer is so small, approximately 10.sup.−13 b, not indicated in FIG. 2, that direct excitation of Tc-99 of the ground state to an energy level of Tc-99m is impossible. Accordingly, indirect excitation using different energy levels should be considered. It is also important to understand that an NRF cross section peak is sharp and narrow. In addition, full width at half maximum (FWHM) occurs at an energy of approximately 2-3 eV. The cross section is proportional to transition strength, so accurate measurement is essential. It should be noted that uncertainty of a current cross section prediction value is considered very high, for example, the actual cross section may be between 1/100 and 100 times a measured value.

(19) LCS (Laser Compton Scattering) Gamma Rays

(20) Photonuclear excitation may occur using high luminance gamma rays generated from LCS interactions. Meanwhile, an LCS phenomenon refers to an increase in energy of a photon (that is, a wavelength is shortened) due to elastic scattering between a low energy laser photon and a high energy electron and may be approximately illustrated as shown in FIG. 3.

(21) Because LCS gamma rays are energy-tunable, are quasi-monochromatic light, and have beam-like properties, the LCS gamma rays may be used for photonuclear excitation. For efficiency and high nuclear excitation rate, intensity of the LCS gamma rays is to be strong enough.

(22) In the present embodiment, an LCS facility using an energy-recovery LINAC (ERL) was used as a gamma ray source. Recently, T. Hayakawa et al. designed a high-flux LCS gamma ray facility using a 350 MeV ERL system, and it is reported the facility generates 10.sup.13 gamma rays per second.

(23) FIG. 4 is a block diagram showing an LCS gamma ray facility using an ERL system of Hayakawa, and the facility employs three loop designs for cost reduction and miniaturization. The electron beam emitted from an injector is accelerated by the superconducting LINAC. After third recycling by passing through the three loops, the electron beam is reinjected into the LINAC with a decelerated status and electron energy is fed back into the high frequency cavity of the superconducting LINAC. The LCS gamma rays are finally generated by collisions of electrons and laser photons at an end part of a third loop. Table 1 below shows the main design parameters. Table 1 below shows the main design parameters.

(24) TABLE-US-00001 TABLE 1 Design parameter Value Electron energy 350 MeV Laser wavelength 1064 nm Electron-beam-induced current 100 mA Average laser output power ~100 W Electron bundle charge 1 nC Pulse energy 1.80 μJ Laser super cavity 3000

(25) Photogeneration of .sup.99mTC

(26) The photonuclear reaction ratio (N.sub.reac) may be calculated using the NRF cross section and an LCS spectrum and is as shown in [Equation 2] below.

(27) $\begin{array}{cc}{N}_{\mathrm{reac}}=n_{\mathrm{target}}{\mathrm{BR}}_{\mathrm{total}}{\int }_{E_t}^{E_h}{\sigma }_{\mathrm{NRF}}{\mathrm{dE}}_{\gamma }\frac{{\mathrm{dN}}_{\gamma }}{{\mathrm{dE}}_{\gamma }}{\int }_0^d{e}^{-{\Sigma }_{\mathrm{NRF}}x}\mathrm{dx}& \left[\mathrm{Equation}2\right]\end{array}$

(28) Here, n.sub.target is number of atoms per unit cubic centimeter of target material, d is thickness of the target material, σ.sub.NRF is an NRF-based resonance cross section, and E.sub.l and E.sub.h are lowest and highest energies of the LCS gamma ray photon spectrum, respectively. Σ.sub.NRF is a macroscopic NRF cross section and is a product of σ.sub.NRF and Nn.sub.target, and dN.sub.γ/dE.sub.γ is a spectral density calculated from [Equation 3] below.

(29) $\begin{array}{cc}\frac{{\mathrm{dN}}_{\gamma }}{{\mathrm{dE}}_{\gamma }}=\frac{N_{\gamma }}{{\sigma }_i}{\int }_{E_0-{\sigma }_E}^{E_0+{\sigma }_E}\frac{d\sigma }{{\mathrm{dE}}_{\gamma }}\frac{1}{{\sqrt{2\pi \delta }}_E}\exp \left[-\frac{{\left(E_e-E_0\right)}^2}{2{\delta }_E^2}\right]{\mathrm{dE}}_e& \left[\mathrm{Equation}3\right]\end{array}$

(30) Here, dN.sub.γ is total gamma ray intensity of a facility in a unit of #/sec, σ.sub.t is a total Compton scattering cross section in mb, dσ/dE.sub.γ is the differential Compton scattering cross section in a unit of mb/MeV, E.sub.0 is central electron beam energy in a unit of MeV, and δ.sup.2.sub.E represents a variance of a central electron beam energy value.

(31) In [Equation 2], gamma ray attenuation in a target region is considered, and some of σ.sub.NRF are so large to greatly affect the gamma ray attenuation. The spectral density is assumed to be constant because it is less than 10 eV in an energy range between E.sub.l and E.sub.h.

(32) Electron energy designed in the ERL facility shown in FIG. 4 is 350 MeV, but as required LCS photon energy is reduced, the electron energy is reduced from 350 MeV to 315 MeV. Here, maximum LCS energy is adjusted to 1.77 MeV, because, in energy no less than 1.77 MeV, the NRF cross section is very small and much information is missing. Intensity of a corresponding LCS photon is 2.1×10.sup.13 γ/s, and by using the LCS gamma ray of such intensity, the spectrum of the LCS photon was optimized as shown in FIG. 5. The photonuclear reaction ratio was calculated to be 6.64×10.sup.10/sec.

(33) Nuclear activity according to the reaction ratio described above is calculated by [Equation 4].

(34) [Equation 4]
A=N.sub.reac(1−e.sup.−in2/T.sup.1/2.sup.×t)

(35) TABLE-US-00002 TABLE 2 Irradiation time (hr) 0.5 1 3 6 12 30 Nuclear activity (mCi) 0.2 0.3 0.9 1.5 2.3 2.9

(36) FIGS. 6 and 7 are diagrams showing data related to transition at each energy level of the Tc-99 nucleus, wherein the Tc-99 nucleus has various energy levels. In FIGS. 6 and 7, the “E.sub.level” represents the quantized energy level of Tc-99, and the “Cross section” represents the nuclear reaction cross section of the NRF reaction in which the Tc-99 nucleus reaches the “E.sub.level” from the ground state. The “I.sub.γ” represents the branching ratio of the transition of nuclear isomers, and the “Final level” represents the energy level to which the excited nucleus undergoes a transition. The “BR” represents the branching ratio from “E.sub.level” to the “Final level”, and the “BR to 143 keV” represents the branching ratio at which the Tc-99 nucleus finally reaches Tc-99m. The “Total BR” is the branching ratio at which the Tc-99 nucleus of “E.sub.level” finally reaches Tc-99m and is equal to the sum of all the “BR to 143 keV” at that “E.sub.level”. When the “Final level” does not have a branching ratio reaching Tc-99m, that is, when the value of the “BR to 143 keV” is 0, a “BR” field is blank and a “BR” field is blank even when the “BR” is unable to be calculated because of no “I.sub.γ” information in the ENSDF file.

(37) FIG. 8 is a view showing a transition process of a Tc-99 nucleus excited by specific energy 1,207 keV to a Tc-99m nucleus and shows an energy level according to each spin of the nucleus when the Tc-99 nucleus excited at 1,207 keV undergoes a transition to Tc-99m. The left side shows spin values representing total angular momenta, and the right side shows the corresponding quantized energy levels. The Tc-99 nucleus is excited to 1,207 keV and then undergoes a transition in two directions of 884 keV and 671 keV with a probability of 29.6% and 26.8%, respectively. Tc-99 undergoes a transition to Tc-99m at an energy level of 884 keV with a probability of 57.3% and undergoes a transition to Tc-99m at an energy level of 671 keV with a probability of 59.9%.

(38) Thus, the Tc-99 nucleus is excited to 1,207 keV and then undergoes a transition to Tc-99m with a total probability of 33%. In other words, when the Tc-99 nucleus is irradiated with a 1,207 keV LCS photon beam, Tc-99m may be produced with a probability of 33% with respect to the excited nucleus. Such efficiency is very high compared to efficiency of current methodologies. Further, by repeatedly irradiating the same target with multiple LCS photon beams each having, that is, 1,176 keV, 1,207 keV, 1,329 keV, and 1,604 keV, the efficiency may be further increased.

(39) As described above, the present invention may produce .sup.99mTc, an isotope for medical use, by recycling .sup.99Tc on the basis of the nuclear resonance fluorescence, and also solve the problem of disposing of nuclear waste by recycling .sup.99Tc which is a radioisotope having a long half-life.

(40) The present invention described above is not limited to the above-described embodiments and the accompanying drawings, and it will be apparent to those skilled in the art that various substitutions, modifications, and changes may be made without departing from the spirit of the present invention.