Use of a nonlinear optical medium to drive thorium-229 nuclear transition

Abstract

A device, comprising a 229Th-containing material; a source of coherent radiation configured to illuminate the 229Th-containing material; and a detector configured to detect a 229mTh ↔ 229Th nuclear transition.

Description

UCI-00925 USE OF A NONLINEAR OPTICAL MEDIUM TO DRIVE THORIUM-229 NUCLEAR TRANSITIONRELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No.63 / 670,220, filed on July 12, 2024. The entire teachings of the above application are incorporated herein by reference. GOVERNMENT SUPPORT

[0002] This invention was made with government support under Phys-2013011 awardedby the National Science Foundation and W911NF-11-1-0369 awarded by the Army Research Laboratory – Army Research Office. The government has certain rights in the invention. BACKGROUND OF THE INVENTION

[0003] The thorium-229 (229Th) isomeric nuclear transition allows for the construction ofa nuclear clock and a variety of sensors for electromagnetic fields, gravity, and chemical binding. SUMMARY OF THE INVENTION

[0004] One of the challenges in using this transition is that its operation requires lightaround 148 nm, which is in the vacuum ultraviolet region of the spectrum. Light at this wavelength does not propagate in air, which means that parts of the apparatus must be held under vacuum. This complicates the construction and operation of the aforementioned technology. In certain embodiment, the invention described herein provides a solution to the problem of requiring the use of evacuated conduits for the propagation of the vacuum UV light in parts of the apparatus.

[0005] Accordingly, in an example embodiment, the present invention is a 229Th-containing material, comprising a229Th-doped frequency-conversion material selected from BaMgF4, BaZnF4, γ-Be2BO3F, or KBe2BO3F3 (KBBF).

[0006] In another example embodiment, the present invention is a 229Th-containingmaterial, comprising a first material in optical communication with a second material, wherein the first material comprises a frequency-conversion material selected from BaMgF4, BaZnF4, γ-Be2BO3F, or KBe2BO3F3 (KBBF); and the second material comprises a229Th- doped material.

[0007] In further embodiments, the present invention relates to devices that employ thematerials described herein.UCI-00925

[0008] In an example embodiment, the present invention is a device, comprising a 229Th-containing material as described herein; a source of coherent radiation configured to illuminate a229Th-containing material; and a detector configured to detect a229mTh ↔229Th nuclear transition.

[0009] In yet another example embodiment, the present invention is a method ofdetecting a229mTh ↔229Th nuclear transition, comprising illuminating a229Th-containing material as described herein, thereby inducing a229mTh ↔229Th nuclear transition; and detecting the229mTh ↔229Th nuclear transition. BRIEF DESCRIPTION OF THE DRAWINGS

[0010] The foregoing will be apparent from the following more particular description ofexample embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating embodiments of the present invention.

[0011] FIG. 1 is a schematic diagram of an example embodiment of a 229Th-containingmaterial (e.g., a frequency-conversion material) described herein.

[0012] FIG. 2 is a schematic diagram of an example embodiment of a device describedherein.

[0013] FIG. 3 is a schematic diagram of an example embodiment of a material describedherein.

[0014] FIG. 4 is a schematic diagram of an example embodiment of a material describedherein. DETAILED DESCRIPTION OF THE INVENTION

[0015] A description of example embodiments of the invention follows.

[0016] In certain embodiment, the invention described herein provides a solution to theproblem of requiring the use of evacuated conduits for the propagation of the vacuum UV light in parts of the apparatus.

[0017] The thorium-229 (229Th) isomeric nuclear transition allows for the construction ofa nuclear clock and a variety of sensors for electromagnetic fields, gravity, and chemical binding. The relevant nuclear transition of229Th is at around 148 nm. The electromagnetic radiation (light) that is required to drive such a transition belongs in the so-called vacuum ultraviolet (VUV) region of the spectrum.UCI-00925

[0018] As used herein, the isomeric nuclear transition of 229Th is denoted interchangeablyas “229mTh ↔229Th” or “229Th ↔229mTh.” The designation “229mTh →229Th” refers to the transition from the excited state to the ground state accompanied with an emission of a photon, and the designation “229Th ↔229mTh” refers to the transition from the ground state to the excited state accompanied with an absorption of a photon.

[0019] In various embodiments described herein, nonlinear optical medium is used togenerate light of shorter wavelength (higher frequency) using light of longer wavelengths (lower frequency). In particular embodiments, light required for driving the229Th ↔229mTh nuclear transition can be created in such medium. Thus, the coherent radiation (e.g., one or more laser beams) is directed into the nonlinear medium for production of the electromagnetic radiation at about 148 nm. These lasers can emit at wavelengths that are greater than the VUV spectral range. As such, these lasers do not need to be in vacuum.

[0020] The nonlinear optical medium can comprise materials having optically nonlinearproperties, including, but not limited to crystals or other medium. Materials suitable for practicing the present invention are those that provide a mechanism for frequency multiplication: quasi-phase matching (QPM) materials, in which spatial modulation of nonlinear optical properties is used to increase (e.g., double) the frequency of propagating wave, including but not limited to periodically poled nonlinear crystals; materials in which the second harmonic of the propagating optical wave is generated, including but not limited to Type I second harmonic generation materials; or materials in which frequency summation is achieved.

[0021] Quasi-phase matching (QPM) is a technique used in nonlinear optics to optimizethe efficiency of frequency conversion processes, such as second-harmonic generation or parametric amplification. In standard phase matching, the phase velocities of interacting waves must be matched to ensure constructive interference and maximum conversion efficiency. However, this condition is difficult to achieve in many materials due to dispersion. QPM overcomes this limitation by periodically reversing the sign of the nonlinear coefficient in the material at specific intervals, typically using a periodic poling process. This periodic modulation compensates for the phase mismatch by resetting the phase relationship between the interacting waves, thus maintaining constructive interference over longer distances and enhancing the frequency conversion efficiency. This technique allows for the use of a broader range of materials and wavelengths compared to traditional phase matching methods.UCI-00925

[0022] Second harmonic generation (SHG) and sum frequency generation (SFG) arenonlinear optical processes where new light frequencies are produced from two or more interacting photons. In SHG, two photons of the same frequency combine in a nonlinear medium to form a new photon with twice the energy (frequency) of the original photons.

[0023] In SFG, two photons of different frequencies interact within a nonlinear materialto produce a photon with a frequency equal to the sum of the original frequencies. Thus, in the case of the present invention which requires photons with wavelength around 148 nm for interaction with the thorium-229 nucleus, SHG would use laser light at near 296 nm. SFG would employ two photons with wavelengthand ^^such that ^ ^ ^ ^^^ ^^= ^^+ ^^.

[0024] Exemplary QPM materials include BaMgF4 and BaZnF4, especially periodicallypoled crystals. Exemplary second harmonic generation materials include γ-Be2BO3F. Exemplary frequency summation materials include KBe2BO3F3(KBBF). In various example embodiments, any of these materials can be crystalline materials.

[0025] In an example embodiment, the present invention is a 229Th-containing material(including but not limited to, a frequency-conversion material) comprising an optically nonlinear medium (including but not limited to a crystal) and thorium-229 (229Th).229Th, in one example, can be doped directly in the nonlinear optical medium.

[0026] In another example, illustrated in FIG. 1, 229Th can be doped in a host materialthat is in optical communication with (e.g., optically coupled to) a nonlinear optical medium.

[0027] As used herein, the phrases “optically coupled” or “in optical communication”refer to an operative connection between two materials such that an optical wave propagating through the first material can be directed, with or without the use of optical components, including but not limited to mirrors, prisms, or like materials, into the second material. An example of optical communication is “optical contact,” whereby a face of one material is immediately adjacent to a face of the other material. A further example of an optical communication is an “optical contact-bonding,” wherein polished faces of two materials (e.g. crystals or other medium) are made very flat and pushed together so that intermolecular forces become significant enough to hold the two materials together. In further embodiments, a first and a second materials can be in optical communication with one another via a vacuum gap. In yet further embodiments, a first and a second materials can be in optical communication with one another by being in optical communication each with a third material that is transparent to optical waves, including but not limited to vacuum UV waves (VUV), propagating from a first material into a second material.UCI-00925

[0028] Referring to FIG. 1, an example embodiment of a frequency-conversion material100 is shown. Material 100 includes an optically nonlinear medium 102 (e.g. a crystal) in optical communication with a229Th host material 104 (e.g., a crystal), doped by thorium-229 atoms 106. Laser beam 108 from a source (not shown) is directed in nonlinear medium 102, where it is transformed into a frequency-multiplied beam 110 (e.g., a VUV beam). Beam 110 is then directed to host material 104 thereby driving229Th nuclear transition.

[0029] Accordingly, in a first example embodiment, the present invention is a 229Th-containing material, e.g., a material that comprises a frequency-conversion material that contains229Th.

[0030] In a 1st aspect of the 1st example embodiment, the 229Th-containing materialcomprises a229Th-doped frequency-conversion material. The frequency-conversion material can be selected from BaMgF4, BaZnF4, γ-Be2BO3F, or KBe2BO3F3(KBBF). In any of the example embodiments, any of these materials can be a crystal.

[0031] In a 2nd aspect of the 1st example embodiment, the 229Th-doped material is dopedat a density from 106to 1022atoms per cm3, for example from 1015to 1020atoms per cm3, or from 1016to 1017atoms per cm3. Other features and example features of the 2ndaspect of the 1stexample embodiment are as defined above with respect to the 1staspect.

[0032] In a 3rd aspect of the 1st example embodiment, the 229Th-doped material comprisesBaMgF4or BaZnF4. Other features and example features of the 3rdaspect of the 1stexample embodiment are as defined above with respect to the 1stor 2ndaspects.

[0033] In a 4th aspect of the 1st example embodiment, the 229Th-doped material is aperiodically poled crystal. Other features and example features of the 4thaspect of the 1stexample embodiment are as defined above with respect to the 1stthrough the 3rdaspects.

[0034] In a 2nd example embodiment, the present invention is a 229Th-containing materialthat comprises a first material in optical communication with a second material.

[0035] In a 1st aspect of the 2nd example embodiment, the first material comprises afrequency-conversion material selected from BaMgF4, BaZnF4, γ-Be2BO3F, or KBe2BO3F3 (KBBF), and the second material comprises a229Th-doped material.

[0036] In a 2nd aspect of the 2nd example embodiment, the second material comprises amaterial selected from CaF2, LiSrAlF6 ThF4, NaThF6, YLiF, LiF, MgF2, CaF2, BaF2, SrF2, KF, ZnF2, SiO2, BaMgF4, or BaZnF4. In an example embodiment, the second material can comprise CaF2or LiSrAlF6. Any of these materials can be a crystal. Other features andUCI-00925 example features of the 2ndaspect of the 2ndexample embodiment are as described above with respect to the 1staspect.

[0037] In a 3rd aspect of the 2nd example embodiment, the second material comprises229Th doped at a density from 106to 1022atoms per cm3, for example from 1015to 1020atoms per cm3, or from 1016to 1017atoms per cm3. Other features and example features of the 3rdaspect of the 2ndexample embodiment are as defined above with respect to the 1stor 2ndaspects.

[0038] In a 4th aspect of the 2nd example embodiment, the first material comprisesBaMgF4or BaZnF4. Other features and example features of the 4thaspect of the 2ndexample embodiment are as defined above with respect to the 1stthrough 3rdaspects.

[0039] In a 5th aspect of the 2nd example embodiment, the first material is periodicallypoled crystal. Other features and example features of the 5thaspect of the 2ndexample embodiment are as defined above with respect to the 1stthrough 4thaspects

[0040] In a 6th aspect of the 2nd example embodiment, the optical communication is viaan optical contact. Other features and example features of the 6thaspect of the 2ndexample embodiment are as defined above with respect to the 1stthrough 5thaspects.

[0041] In a 7th aspect of the 2nd example embodiment, the optical contact is an opticalcontact bonding. Other features and example features of the 7thaspect of the 2ndexample embodiment are as defined above with respect to the 1stthrough 6thaspects.

[0042] In an 8th aspect of the 2nd example embodiment, the optical communication is viaa vacuum gap. An example of such a material 300 is shown in FIG.3. As can be seen, in this example, laser beam 308 is directed through the first material 302, where it is frequency- converted into a VUV light, which, in turn, is directed to the second material 304 that includes229Th. The first material 302 and the second material 304 are in optical communication via a vacuum gap 312. Other features and example features of the 6thaspect of the 2ndexample embodiment are as defined above with respect to the 1stthrough 5thaspects.

[0043] In a further aspect of the 2nd example embodiment, the material further comprisesa third material in optical communication with the first material and with the second material. The third material can be transparent in a vacuum ultraviolet wavelength range (VUV), and the optical communication between the first material and the second material can be via the third material. An example of such a material is a material 400 shown in FIG.4. As can be seen, in this example, laser beam 408 is directed through the first material 402, where it isUCI-00925 frequency-converted into a VUV light, which, in turn, is directed to the second material 404 that includes229Th. The first material 402 and the second material 404 are in optical communication via a VUV-transparent material 412. Other features and example features of the 6thaspect of the 2ndexample embodiment are as defined above with respect to the 1stthrough 5thaspects.

[0044] In a 3rd example embodiment, the present invention is a device.

[0045] In a 1st aspect of the 3rd example embodiment, the device comprises the 229Th-containing material of the 1stexample embodiment or the 2ndexample embodiments according to any of their aspects; a source of coherent radiation configured to illuminate the frequency-conversion material; and a detector configured to detect a229mTh ↔229Th nuclear transition.

[0046] In a 2nd aspect of the 3rd example embodiment, the detector is a photon detectorconfigured to detect a photon emitted by the229Th atoms undergoing a229mTh →229Th nuclear transition. Other features and example features of the 2ndaspect of the 3rdexample embodiment are as defined above with respect to the 1staspect.

[0047] In a 3rd aspect of the 3rd example embodiment, the device further comprises ascintillator adapted to absorb a photon emitted by the229Th atoms undergoing a229mTh →229Th nuclear transition and to emit a fluorescent photon. The detector is configured to detect the fluorescent photon. Other features and example features of the 3rdaspect of the 3rdexample embodiment are as defined above with respect to the 1stor 2ndaspects.

[0048] In a 4th aspect of the 3rd example embodiment, the source of coherent radiation isconfigured to vary its emission frequency, and the detector is configured to detect absorption at a frequency corresponding to a229Th →229mTh nuclear transition. For example, the frequency-conversion material absorbs the VUV light at a narrow frequency range that corresponds to frequency corresponding to the229mTh ↔229Th nuclear transition. By varying an emission frequency of a laser source, the frequency at which the peak absorption is detected is found. Other features and example features of the 4thaspect of the 3rdexample embodiment are as defined above with respect to the 1stthrough 3rdaspects.

[0049] In a 5th aspect of the 3rd example embodiment, the source of coherent radiationcomprises at least two laser sources. This embodiment is particularly useful when the frequency-conversion material comprises a sum-frequency-generation material. Other features and example features of the 5thaspect of the 3rdexample embodiment are as defined above with respect to the 1stthrough 4thaspects.UCI-00925

[0050] An example of the device 200 defined by the 3rd example embodiment is shown inFIG.2. In the example shown in FIG.2, a frequency-conversion material 202 that comprises a229Th-doped (204) optically nonlinear crystal is used, according to the 1staspect of the 1stexample embodiment. Device 200 comprises laser source 206A, and can optionally comprise a second laser source 206B. Three types of detectors are shown in FIG.2: emission detector 208, configured to detect a photon emitted by a thorium atom, absorption detector 214 configured to detect an absorption of VUV light propagating through material 202 by the thorium atoms 204, and detector 210 configured to detect fluorescent photons emitted by scintillator 212 when the scintillator absorbs a photon emitted by a thorium atom.

[0051] In a 4th example embodiment, the present invention is a method of detecting a229mTh ↔229Th nuclear transition.

[0052] In a 1st aspect of the 4th example embodiment, the method comprises illuminatingthe229Th-containing material of the 1stexample embodiment or the 2ndexample embodiments according to any of their aspects, thereby inducing a229mTh ↔229Th nuclear transition; and detecting the229mTh ↔229Th nuclear transition.

[0053] In a 2nd aspect of the 4th example embodiment, detecting the 229mTh ↔nuclear transition comprises detecting a photon emitted by the229Th atoms undergoing a229mTh →229Th nuclear transition. Other features and example features of the 2ndaspect of the 4thexample embodiment are as defined above with respect to the 1staspect.

[0054] In a 3rd aspect of the 4th example embodiment, detecting the 229mTh ↔ 229Thnuclear transition comprises causing a scintillator to absorb a photon emitted by the229Th atoms undergoing a229mTh →229Th nuclear transition and to emit a fluorescent photon; and detecting the fluorescent photon. Other features and example features of the 3rdaspect of the 4thexample embodiment are as defined above with respect to the 1stor the 2ndaspects.

[0055] In a 4th aspect of the 4th example embodiment, detecting the 229mTh ↔nuclear transition comprises varying a frequency of the coherent radiation; and detecting absorption at a frequency corresponding to a229Th →229mTh nuclear transition. Other features and example features of the 4thaspect of the 4thexample embodiment are as defined above with respect to the 1stthrough 3rdaspects.UCI-00925 EXEMPLIFICATION

[0056] Frequency-Conversion Materials

[0057] Methods of crystal growth, thorium doping, and periodic poling (for the QPMmaterials) are well known in the art.

[0058] For example, growth and poling of BaMgF4 (BMF) crystals is described inBuchter et al, “Periodically Poled BaMgF4 for Ultraviolet Frequency Generation” Optics Letters Vol.26, No.21, November 1, 2001, p.1693 and in Herr et al. “Fanout Periodic Poling of BaMgF4Crystals” Optical Materials Express, Vol.13, No.8, 1Aug 2023, p.2158. The teachings of both of these publications are incorporated herein by reference. Briefly, BMF growth techniques was previously described in K. Recker, F. Wallerafen, and S. Haussueh, J. Cryst. Growth 26, 97 (1974).

[0059] UV-grade crystalline pieces of BaF2 and MgF2 (available from Korth KristalleGmbH) can be used as the starting materials. They are weighed for the composition with a slight excess of BaF2 from the stoichiometry, and inductively heated and melted in a pyrolytic-carbon-coated graphite crucible. For seeding a BMF crystalline rod oriented to its crystallographic c-axis prepared from another as-grown BMF-crystal can be used. The seed crystal can be pulled along +c-direction at 0.3 mm / h with a rotation speed of 10 RPM. During the entire growth period, nitrogen gas mixed with 5 vol % CF4(99.999% purity) can be continuously purged into the furnace.

[0060] Periodically poled BMF can be fabricated with methods similar to thosecommonly used for LiNbO3 and LiTaO3. The c-cut crystal can have a thickness of 500 µm, and the +c-face can be lithographically patterned with a 19.2-µm-period grating. The grating’s k vector can be parallel to the crystal’s a axis. Sputtered NiCr can be used for the electrode grating. The 9-mm-long, 4-mm-wide metal grating was overcoated with a 2-µm- thick layer of photoresist with openings that permit contact with the conducting LiCl solution. The bottom surface of the BMF can be in direct contact with the solution and was held in place with rubber O rings. Poling can be achieved with a series of 50-ms-long high-voltage pulses applied to a 220-kΩ series resistor, with a diode used to prevent back flow of current. Based on the published value of spontaneous polarization for BMF of 7.7 µC / cm2, a total charge of 5.3 µC is expected for an ideal 50% duty cycle. A total charge of 5.4 µC can be delivered in 19 pulses at voltages from 16 to 20 kV / mm. This transferred charge can be the only way that the poling process was monitored.UCI-00925

[0061] A similar process can be used for BaZnF4 using commercially available powdersof BaF2and ZnF2. For example, commercially available powders of BaF2(^>99.99% purity) and ZnF2 were weighed and mixed in stoichiometric composition. CF4 gas (99.99%) was flowed into the furnace and the powders were melted at approximately 785°C.

[0062] KBe2BO3F3 (KBBF) incongruently melts, so it can be grown by both the flux andhydrothermal method. The flux technique crystallizes KBBF out of a molten KBBF / flux composition when cooled. In the hydrothermal mechanism, the precursor atoms are dissolved in a hot liquid solution, often held under pressure in a sealed tube. One of the tubes is kept colder and the atoms crystalize there.

[0063] For both methods, stoichiometric amounts of BeO, KBF4, and B2O3 are mixed.

[0064] γ-Be2BO3F can be crystallized as follows. Mixture of NaBF4 / BeO / B2O3 / LiF withmolar ratios of 1–1.5:1–2:0.5–1.5:1–3 were fully ground and placed in a platinum crucible. First, the mixture was heated in a computer controlled electric furnace at 800 °C for 2 days until the melts became homogeneous. The saturation temperature was estimated by observing the spontaneous nucleation on the surface of the solution (∼730 °C). Then, it was slowly decreased at a rate of 10 °C / day in the process of crystal growth. Finally, the as-grown crystal was obtained by dealing with hot hydrochloric acid for 2 days.

[0065] The doping techniques are well known by skilled artisans. For example, thedoping is done by placing a compound containing thorium-229 into the materials that are melted for crystal growth. Typically, the thorium-229 is in the form of ThF4, ThO2, or Th(NO3)4, but other forms could be used. The amount of thorium-229 is chosen to achieve the desired density in the crystal. Some crystals may “reject” some of the thorium-229, in which case “extra” thorium-229 is added to the melt to compensate.

[0066] The teachings of all patents, published applications and references cited herein areincorporated by reference in their entirety.

[0067] While this invention has been particularly shown and described with references toexample embodiments thereof, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the scope of the invention encompassed by the appended claims.

Claims

UCI-00925 CLAIMS What is claimed is:

1. A229Th-containing material, comprising a229Th-doped frequency-conversion material selected from BaMgF4, BaZnF4, γ-Be2BO3F, or KBe2BO3F3 (KBBF).

2. The material of Claim 1, wherein the229Th-doped material is doped at a density from 1015to 1020atoms per cm3.

3. The material of Claim 2, wherein the229Th-doped material is doped at a density from 1016to 1017atoms per cm3.

4. The material of one of Claims 1-3, wherein the229Th-doped material comprises BaMgF4 or BaZnF4.

5. The material of Claim 4, wherein the229Th-doped material is a periodically poled crystal.

6. A229Th-containing material, comprising a first material in optical communication with a second material, wherein: the first material comprises a frequency-conversion material selected from BaMgF4, BaZnF4, γ-Be2BO3F, or KBe2BO3F3 (KBBF); and the second material comprises a229Th-doped material.

7. The material of Claim 6, wherein the second material comprises a material selected from CaF2, LiSrAlF6, ThF4, NaThF6, YLiF, LiF, MgF2, CaF2, BaF2, SrF2, KF, ZnF2, SiO2, BaMgF4or BaZnF4.

8. The material of Claim 7, wherein the second material comprises CaF2 or LiSrAlF6.

9. The material of any one of Claims 6-8, wherein the second material comprises229Th doped at a density from 1015to 1020atoms per cm3.UCI-00925 10. The material of Claim 7, wherein the second material comprises229Th doped at a density from 1016to 1017atoms per cm3.

11. The material of any one of Claims 6-10, wherein the first material comprises BaMgF4or BaZnF4.

12. The material of Claim 11, wherein the first material is a periodically poled crystal.

13. The material of any one of Claims 6-12, wherein the optical communication is via an optical contact.

14. The material of Claim 13, wherein the optical contact is an optical contact bonding.

15. The material of any one of Claims 6-12, wherein the optical communication is via a vacuum gap.

16. The material of any one of Claims 6-12, further comprising a third material in optical communication with the first material and with the second material, the third material being transparent in a vacuum ultraviolet wavelength range (VUV), wherein the optical communication between the first material and the second material is via the third material.

17. A device, comprising: the229Th-containing material of any one of Claims 1-14; a source of coherent radiation configured to illuminate the frequency- conversion material; and a detector configured to detect a229mTh ↔229Th nuclear transition.

18. The device of Claim 17, wherein the detector is a photon detector configured to detect a photon emitted by the229Th atoms undergoing a229mTh →229Th nuclear transition.UCI-00925 19. The device of Claim 17, further including a scintillator adapted to absorb a photon emitted by the229Th atoms undergoing a229mTh →229Th nuclear transition and to emit a fluorescent photon, and wherein the detector is configured to detect the fluorescent photon.

20. The device of Claim 17, wherein: the source of coherent radiation is configured to vary its emission frequency; and the detector is configured to detect absorption at a frequency corresponding to a229Th →229mTh nuclear transition.

21. The device of Claim 17, wherein the source of coherent radiation comprises at least two laser sources.

22. A method of detecting a229mTh ↔229Th nuclear transition, comprising: illuminating the229Th-containing material of any one of Claims 1-16 with coherent radiation, thereby inducing a229mTh ↔229Th nuclear transition; and detecting the229mTh ↔229Th nuclear transition.

23. The method of Claim 22, wherein detecting the229mTh ↔229Th nuclear transition comprises detecting a photon emitted by the229Th atoms undergoing a229mTh → 229Th nuclear transition.

24. The method of any one of Claims 22-23, wherein detecting the229mTh ↔229Th nuclear transition comprises: causing a scintillator to absorb a photon emitted by the229Th atoms undergoing a229mTh →229Th nuclear transition and to emit a fluorescent photon; and detecting the fluorescent photon.

25. The method of any one of Claims 22-24, wherein detecting the229mTh ↔229Th nuclear transition comprises: varying a frequency of the coherent radiation; andUCI-00925 detecting absorption by at a frequency corresponding to a 229Th → 229mThnuclear transition.

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