Separation of isotopes or atomic species using selective ionization

Abstract

Separation of isotopes (or atomic species) can use laser pumping in combination with an optical resonator. Neutral atoms propagating as a beam are exposed to one or more pumping lasers that selectively excite a target isotope (or target atomic species) into an excited state. An optical resonator is locked in transmission to a separate laser that is tuned to a wavelength that selectively ionizes the excited isotope (or atomic species). The resulting ions can be collected, e.g., using an electrically-charged plate or other collector.

Description

Attorney Docket No.093331-1493899-8273RAIWO SEPARATION OF ISOTOPES OR ATOMIC SPECIES USING SELECTIVE IONIZATION CROSS-REFERENCE TO RELATED APPLICATION

[0001] This application claims priority to U.S. Provisional Application No.63 / 569,569, filed on March 25, 2024, the disclosure of which is incorporated by reference herein. TECHNICAL FIELD

[0002] This disclosure relates in general to separation of isotopes and in particular to separation of isotopes or atomic species using selective ionization. BACKGROUND

[0003] Atomic species, or chemical elements, are distinguished by the number of protons in their nuclei. An element can have multiple isotopes, i.e., atoms with same number of protons but different numbers of neutrons. Isotopes occur in nature with varying abundance, and a given isotope may be stable or unstable.

[0004] Various applications in medicine and industry mostly rely on particular isotopes whose natural abundance is low. Accordingly, isotope separation processes, which can extract a low-abundance isotope of interest (also referred to as a “target” isotope) from a larger sample, are of considerable interest. In general, isotope separation is not perfect, and the result of processes referred to as “isotope separation” can be a sample of the element in which the abundance of the target isotope is enriched or increased above the natural or initial abundance. In other cases, an unwanted isotope can be depleted or removed from an element that has multiple isotopes.

[0005] Isotope separation poses many challenges. The chemical and physical processes that are typically used to separate atomic species generally do not differentiate strongly between isotopes of the same atomic species. Various approaches have been tried. Examples include gaseous diffusion and centrifuge separation, which rely on the small mass difference between different isotopes. More recently, lasers have been used in isotope separation, relying on isotopic shifts in electron energy levels. For instance, in an approach known as atomic vapor laser isotope separation (or “AVLIS”), a set of finely tuned lasers are directed at a vapor containing a mixture of isotopes. The lasers are tuned to selectively ionize only thetarget isotope. More specifically, one laser (or in some cases two lasers) is tuned to pump the target isotope into an excited state (without affecting other isotopes, which remain in the ground state) while a second (or third) laser is tuned to ionize the excited state (but not the ground state). An electric field can then be applied to attract the ionized isotope. However, AVLIS has proven to be inefficient in practice, due to factors such as the low repetition rate of the pulsed lasers that are typically employed and the last ionization step, which is non- resonant and requires very high peak intensity.

[0006] In another approach, pumping lasers have been used to excite atoms of a target isotope into a high-lying Rydberg state that is very close to the ionization limit, after which passage through a strong DC electric field is used to ionize the Rydberg state.

[0007] In still another approach, a laser can be tuned to selectively drive only the target isotope into a state that has different paramagnetic properties than the other isotopes. Magnetic fields can then be used to separate the target isotope from other isotopes.

[0008] Existing methods of isotope separation generally require large apparatus. This makes them less than ideal for applications that involve unstable isotopes and any other application where the enriched sample is best prepared at a point close (in time and space) to where it will be used. SUMMARY

[0009] Some embodiments disclosed herein relate to separation of isotopes or atomic species using laser pumping in combination with an optical resonator. Neutral atoms propagating as a beam are exposed to one or more pumping lasers that selectively excite atoms of a target type (e.g., a target isotope or target atomic species) into an excited state. An optical resonator is locked in transmission to a separate laser that is tuned to a wavelength that selectively ionizes the excited atoms of the target type. The resulting ions can be collected, e.g., using an electrically-charged plate or other collector. In some embodiments, the optical resonator can significantly increase the probability of ionizing excited atoms of the target type. As a result, efficiency of isotope separation (or separation of atomic species) can be enhanced while producing samples with high purity (e.g., abundance of the target isotope above 99%).

[0010] Certain embodiments relate to methods of separating atoms. Such methods can include preparing a sample of atoms, the sample including atoms of a target type and atomsof one or more other types, where the target type has an initial abundance in the sample of atoms; producing an atom beam from the sample of atoms; irradiating the atom beam with a pumping laser operating at a first wavelength, the first wavelength being capable of selectively exciting atoms of the target type to an excited state; passing the atom beam through an optical resonator driven by an ionization laser operating at a second wavelength, the second wavelength being capable of selectively ionizing atoms of the target type from the excited state; applying an electric field to the atomic beam downstream of the optical resonator to divert ionized atoms from a beam path of the atomic beam; and collecting the ionized atoms. An abundance of the target type among the collected ionized atoms can be higher than the initial abundance. In some embodiments, the abundance of the target type among the collected ionized atoms can be at least 99%.

[0011] Certain embodiments relate to systems for separating atoms. Such systems can include: an atom beam source to produce an atom beam that includes atoms of a target type and atoms of one or more other types, wherein the target type has an initial abundance in the atom beam; a pumping laser having a beam path that intersects a path of the atom beam, the pumping laser having a first wavelength that is capable of selectively exciting atoms of the target type within the atom beam to a metastable excited state; an ionization laser having a second wavelength that is capable of selectively ionizing atoms of the target type from the excited state; an optical resonator positioned to intersect the path of the atom beam and having a resonant wavelength equal to the second wavelength, wherein the ionization laser is locked in transmission to the optical resonator; and an ion collector positioned along the path of the atom beam downstream of the optical resonator.

[0012] In these and other embodiments, irradiating the atom beam with the pumping laser can be performed prior to passing the atom beam through the optical resonator. For example, the pumping laser and the optical resonator can be arranged such that the beam path of the pumping laser intersects the atom beam upstream of the optical resonator.

[0013] In these and other embodiments, irradiating the atom beam with the pumping laser is performed while the atom beam is passing through the optical resonator. For example, the pumping laser and the optical resonator can be arranged such that the beam path of the pumping laser intersects the atom beam within the optical resonator.

[0014] In these and other embodiments, the pumping laser can be a continuous-wave laser. Multiple pumping lasers (e.g., operating at different wavelengths) can be used, and thepumping lasers can be arranged such that light from different pumping lasers intersects the atom beam in the same region (e.g., within the optical resonator) or in different regions (e.g., light from one pumping laser can intersect the atom beam upstream of the optical resonator while light from another pumping laser can intersect the atom beam within the optical resonator).

[0015] In these and other embodiments, the optical resonator can be formed using opposing reflective surfaces defining a resonant cavity. Each of the opposing reflective surfaces can have a large radius of curvature (e.g., at least 30 km, at least 100 km), or each of the opposing reflective surfaces can be a flat surface. The reflective surfaces can have high reflectivity (e.g., at least 99% or at least 99.9%).

[0016] In these and other embodiments, the atom beam can be produced by heating the sample of atoms in an oven having an aperture, with the atom beam being formed by vapor pressure in the oven.

[0017] In these and other embodiments, the target type and the one or more other types can be different isotopes of an element, such as an alkali, lanthanide or alkaline earth element. In some embodiments, the element can be strontium, ytterbium, mercury, lutetium, or lithium.

[0018] In these and other embodiments, the target type can be a target atomic species and the one or more other types can be one or more other atomic species. For example the target atomic species can be a precious metal such as gold, and the one or more other atomic species include one or more elements in the platinum group of metals.

[0019] In these and other embodiments, the ion collector can include a charged plate to attract positive ions.

[0020] In these and other embodiments, neutral atoms can be collected from the atom beam downstream of the electric field, and the neutral atoms can exhibit depletion of the target type.

[0021] In these and other embodiments, the excited state can be a metastable excited state having a lifetime of at least 10 microseconds, at least 1 millisecond, or the like.

[0022] The following detailed description, together with the accompanying drawings, will provide a better understanding of the nature and advantages of the claimed invention.BRIEF DESCRIPTION OF THE DRAWINGS

[0023] FIGs.1A-1C are atomic state diagrams illustrating an operating principle applied in some embodiments.

[0024] FIG.2 shows a simplified schematic diagram of an isotope separation system according to some embodiments.

[0025] FIG.3 shows a resonant cavity that can be used to implement an optical resonator in an isotope separation system according to some embodiments.

[0026] FIG.4 shows a resonant cavity that can be used to implement an optical resonator in an isotope separation system according to some embodiments.

[0027] FIGs.5A and 5B show graphs of representative results of numerical simulations of resonant cavity behavior for different beam spot sizes and for a plane wave.

[0028] FIG.6 shows a simplified schematic diagram of a compact isotope separation system according to some embodiments.

[0029] FIG.7 is a simplified schematic diagram further illustrating an arrangement of pumping lasers and an optical resonator according to some embodiments.

[0030] FIG.8 shows a simplified three-dimensional view further illustrating the arrangement of lasers and optical resonators in a system according to some embodiments.

[0031] FIG.9 shows a flow chart of a process for isotope separation according to some embodiments.

[0032] FIG.10 shows a Grotrian diagram of relevant energy levels in atomic strontium. DETAILED DESCRIPTION

[0033] The following description of exemplary embodiments of the invention is presented for the purpose of illustration and description. It is not intended to be exhaustive or to limit the claimed invention to the precise form described, and persons skilled in the art will appreciate that many modifications and variations are possible. The embodiments have been chosen and described in order to best explain the principles of the invention and its practical applications to thereby enable others skilled in the art to best make and use the invention in various embodiments and with various modifications as are suited to the particular use contemplated.

[0034] Some embodiments disclosed herein relate to separation of isotopes or atomic species using laser pumping in combination with an optical resonator. Neutral atoms propagating as a beam are exposed to one or more pumping lasers that selectively excite atoms of a target type (e.g., a target isotope or target atomic species) into an excited state. An optical resonator is locked in transmission to a separate laser that is tuned to a wavelength that selectively ionizes the excited atoms of the target type. The resulting ions can be collected, e.g., using an electrically-charged plate or other collector. In some embodiments, the optical resonator can significantly increase the probability of ionizing excited atoms of the target type. As a result, efficiency of isotope separation (or separation of atomic species) can be enhanced while producing samples with high purity (e.g., abundance of the target isotope above 99%). Operating Principles

[0035] FIGs.1A-1C are atomic state diagrams illustrating an operating principle applied in some embodiments. In the state diagrams of FIGs.1A-1C, the discrete energy levels or states of an electron bound to an atom are represented as horizontal lines (with higher energy states at higher positions on the page), and ionization energy is represented as a continuum 150. A ground state, denoted herein as |^^^, corresponds to a lowest-energy state of the electron. An electron can be removed (thereby ionizing the atom) by absorbing a photon having at least the minimum ionization energy. As shown in FIGs.1A-1C, a given atom generally has one or more excited states (e.g., states |^^^and |^^^^ at discrete energy levels between the ground state and the minimum ionization energy. Subject to various quantum constraints, a bound electron can transition between different energy levels by absorbing or emitting a photon having energy equal to the difference between the energy levels. It is also well-known that different isotopes of an atom typically have slight differences in at least some of their energy levels, a phenomenon referred to herein as “isotopic shift.”

[0036] FIG.1A illustrates a simple example, in which a first isotope of an atom has an excited state |^^^ while a second isotope of the atom has a slightly different excited state |^^′^. The difference between the energy levels of states |^^^ and |^^′^ is the isotopic shift (denotedas ∆^^ in FIG. 1A). A photon having wavelength ^^^ (and energy ^^ ൌ ℎ^^ / ^^^, where ℎ isPlanck’s constant) can excite the first isotope into state |^^^ but cannot excite the second isotope into state |^^′^. While the first isotope is in state |^^^, a second photon having wavelength ^^^can ionize the first isotope without also ionizing the second isotope. That is,the ionizing wavelength ^^^can be chosen to provide enough energy to ionize an atom in state |^^^ but not enough to ionize an atom in ground state |^^^. In a sample of atoms, if the first isotope is ionized while the second isotope remains neutral, separation of isotopes becomes possible using an electric field.

[0037] To achieve ionization using two photons of wavelengths ^^^andthe arrival of the photons needs to be timed such that the isotope is in the excited state |^^^when the second photon arrives. Thus, where there may be a time gap between arrival of the two photons, it can be desirable for excited state |^^^to be a metastable state that persists long enough to allow the second photon to arrive and be absorbed. Metastable states typically occur where a direct transition to the ground state is forbidden by quantum constraints, and the simple two- step ionization path shown in FIG.1A can be extended to involve multiple excited states. FIGs.1B and 1C show examples involving multiple excited states.

[0038] In FIG.1B, a first photon of wavelength ^^^^excites the electron in a first isotope from the ground state |^^^to a first excited state |^^^having lower energy than metastable state|^^^, and a second photon of wavelength ^^^ଶfurther excites the electron to the metastable state |^^^, which can be ionized by a photon of wavelength ^^^. In this example, another isotope has shifted energy levels |^^′^and |^^′^, and the wavelengths ^^^^, ^^^ଶ, and ^^^are selected such that electrons in the other isotope are not excited out of the ground state or ionized.

[0039] In FIG.1C, a first photon of wavelength ^^^excites the electron in a first isotope from the ground state |^^^to a first excited state |^^^having higher energy than metastablestate |^^^. In this example, excited state |^^^quickly decays to metastable state |^^^. Thistransition emits a photon of wavelength ^^ௗ. Atoms in metastable state |^^^can then be ionized by a photon of wavelength ^^^In this example, another isotope has shifted energy levels |^^′^and |^^′^, and the wavelengths ^^^, ^^ௗ, andare selected such that electrons in the other isotope are not excited out of the ground state or ionized.

[0040] In some embodiments, a cyclic transition between states |^^^, |^^^, and |^^^can beestablished, in which the first isotope is pumped from the ground state |^^^via a state |^^^into a metastable state |^^^that spontaneously decays to the ground state |^^^; the cycle can repeat as long as pumping radiation (e.g., laser light of appropriate wavelength(s)) continues to be applied. Such cyclic transitions can increase the amount of time spent in the metastable state and therefore the likelihood of ionization. Further, the state |^^^ need not be a metastablestate. As long as the excitation wavelength(s) ^^^and ionization wavelength ^^^are chosen to selectively excite and ionize a target isotope, it is in principle possible to ionize the target isotope without also ionizing other isotopes. Then the target isotope can be separated from the other isotopes using static (dc) electric fields. Example Systems

[0041] Systems and methods described herein exploit the operating principles illustrated in FIGs.1A-1C by using lasers of appropriate wavelengths to selectively excite and ionize atoms of a target type (e.g., a target isotope or a target atomic species) within an atomic beam, while leaving atoms of other type(s) in a neutral (and preferably non-excited) state. A first laser (or lasers) excites atoms of the target type into an excited state, which can be a metastable state (without also exciting atoms of other types), after which the second laser is used to ionize the excited atoms of the target type. An electric field can then be applied to separate ionized atoms from the neutral atoms in the beam. To enhance the efficiency of the ionization, ionizing radiation can be provided using an optical resonator of macroscopic dimensions and high mode-width, examples of which are described below. Provided that the laser wavelengths are selected such that only atoms of the target type are excited and ionized, atoms of a target type can be separated from other atoms with high efficiency and purity.

[0042] FIG.2 shows a simplified schematic diagram of an isotope separation system 200 according to some embodiments. System 200 includes an atom source 202 that produces an atom beam 204. Atom beam 204 propagates in the “z” direction, as indicated by directional arrow 201. Atom beam 204 contains a mixture of different isotopes, including a target isotope that is to be separated from the rest. Pumping laser(s) 206 produces laser beams 207 having one or more excitation wavelengths (^^^), and laser beams 207 intersect the path of atomic beam 204, providing selective optical pumping of the target isotope into an excited state, which can be a metastable state, without exciting other isotopes in atom beam 204 (e.g., as described above with reference to FIGs.1A-1C). Atom beam 204, including excited atoms of the target isotope, enters an optical resonator 208 that is locked in transmission to an “ionization” laser 210. Ionization laser 210 produces a laser beam with a wavelength ^^^that provides sufficient energy to ionize excited atoms of the target isotope but not to ionize atoms in the ground state (e.g., as described above with reference to FIGs.1A-1C). Optical resonator 208 is tuned to resonate at wavelength As described below, optical resonator 208 can be designed to provide high mode-volume and high intracavity power factor, suchthat the probability of an excited atom of the target isotope being ionized approaches 1. Downstream of optical resonator 208, an ion collector 212, which can be, e.g., an electrically charged plate, is positioned off the beam axis to attract and collect ions from beam 204 (shown as triangles). As shown, ion collector 212 can be negatively charged to attract positive ions from beam 204. Neutral atoms in beam 204 (shown as circles) can continue to propagate along the z direction (e.g., into a beam stop or another collection surface).

[0043] To avoid undesirable collisions between atoms of atom beam 204 and atmospheric gases, system 200 (or components thereof) are advantageously implemented in vacuum, e.g., within a vacuum chamber at a pressure of approximately 10−7or 10−8torr. For instance, atom source 202 and the path of beam 204, including optical resonator 208 and ion collector 212, can be placed inside the vacuum chamber. Laser light sources (pumping laser(s) 206 and ionization laser 210) can be inside or outside the vacuum chamber, provided that the laser light has a path into the vacuum chamber to intersect with atom beam 204.

[0044] Atom source 202 can include an oven or crucible or the like, in which a sample 214 can be placed. Sample 214 can initially be in solid or fluid form and can include a mixture of different isotopes including the target isotope. For instance, the composition of sample 214 can match a naturally-occurring abundance ratio of the different isotopes. Atom source 202, or the oven or crucible therein, can be constructed of a material that can be heated to a high temperature and that does not react with the atoms of sample 214. Suitable materials include tungsten, tantalum, graphite, molybdenum, and stainless steel. Atom source 202 can heat sample 214 to release atoms as a vapor (e.g., with vapor pressure of at least 1 Pa). Heating can use contact methods (e.g., resistive heating) or non-contact methods (e.g., RF induction heating or electron-beam heating) or any other method that heats sample 214 to an appropriate temperature for vapor production (e.g., 1000 to 3000 K, depending on the particular content of sample 214). Atom source 202 can include an aperture 216 via which the vapor can be emitted, thereby forming atom beam 204. Aperture 216 can define a divergence angle and shape of the beam; for instance, the beam cross-section can be circular or elliptical (e.g., with an aspect ratio of 10:1 or any other desired aspect ratio) with angular divergence of around 1 degree. Depending on particular design details, atom beam 204 can have a density of at least 1012or 1013atoms / cm3and a flux of at least 1018atoms / second. The particular density or flux depends on design choices. Those skilled in the art with the benefit of this disclosure will appreciate that higher density and / or higher flux can increase the yield (e.g., number of atoms of target isotope collected per unit time), up to the point where theradiation from pumping laser(s) 206 and / or optical resonator 208 is insufficient to ionize all of the atoms of the target isotope, or where collisions between neutral atoms and ions limits the purity of the extracted isotopes.

[0045] Pumping laser(s) 206 can include one or more lasers operating at one or more excitation wavelengths. The excitation wavelength(s) can be chosen to selectively drive the target isotope into a metastable state (e.g., as described above with reference to FIGs.1A- 1C). The metastable state can be any excited state with a lifetime long enough that the atom can propagate through optical resonator 208 as part of atom beam 204. The minimum lifetime depends on the particular dimensions of system 200 and velocity of atom beam 204; in various embodiments, a lifetime of about 10 microseconds or longer, or 1 millisecond or longer, may be sufficient. In addition, the metastable state (or an intermediate excited state |^^^as described above) should be selected such that the isotopic shift exceeds the Doppler shift of the atom beam. Specifically, divergence of the atom beam induces a Doppler shift infrequency (^^^) given by ^^^ ൌ ^^^ / ^^, where ^^^ is the velocity component of the atom beam inthe direction of the laser beam (transverse to the z-direction in FIG.2). If the isotopic shift is comparable to or less than the Doppler shift, isotopes other than the target isotope could be excited. Depending on the particular divergence of atom beam 204 (which depends on aperture 216), isotopic shifts of 100 MHz to 1 GHz are sufficiently large to enable selective excitation of a target isotope.

[0046] Pumping laser(s) 206 can be implemented as continuous wave lasers or pulsed lasers that irradiate atom beam 204 to provide optical pumping of the target isotope into the excited state. In some embodiments, continuous wave lasers may provide advantages over pulsed lasers. For instance, if atom beam 204 provides a continuous flux of atoms, continuous irradiation of atom beam 204 can excite atoms of the target isotope in all portions of the beam. In addition, continuous wave lasers can have narrower spectral widths than pulsed lasers, which can help to avoid excitation of isotopes other than the target isotope. Pumping laser(s) 206 can be arranged to irradiate the entire cross-section of atom beam 204. If the excitation scheme uses multiple laser wavelengths (e.g., as shown in FIG.1B), lasers of different wavelengths can be intermixed or arranged such that the first excitation wavelength is applied to atom beam 204 first, followed by the second excitation wavelength.

[0047] In some embodiments, pumping lasers 206 can be operated to excite the target isotope using stimulated Raman adiabatic passage (STIRAP). STIRAP is a well-establishedtechnique for transferring a population of atoms from an initial state |^^^ to a metastable state |^^^ without also populating a third state |^^^. Lasers are used to create two coherent radiation fields, referred to as the “pump” field and the “Stokes” field. The pump field couples state|^^^to state |^^^, while the Stokes field couples state |^^^to state |^^^. The lasers are cycled:initially, only the Stokes field is applied; then the Stokes field is gradually decreased while the pump field is gradually increased, with the time rate of change being slow compared to the energy splitting between non-dark states. STIRAP can be implemented, e.g., by using two pulsed lasers with a slight delay between activation times such that the pulses have some temporal overlap. (For a further description of STIRAP, see, e.g., N.V. Vitanov et al., “Stimulated Raman adiabatic passage in physics, chemistry, and beyond,” Reviews of Modern Physics 89 (1): 015006 (2017)). As in other examples described herein, the target isotope can be selectively excited as long as the relevant isotopic shift exceeds the Doppler shift of the atomic beam.

[0048] Ionization laser 210 can be implemented as a continuous-wave or pulsed laser that drives optical resonator 208 with a resonance wavelength ^^^that provides sufficient energy to ionize excited atoms (e.g., atoms of the target isotope) but not to ionize atoms in the ground state. Optical resonator 208 is used to increase interaction time and effective laser power, thereby driving the probability of ionizing an excited atom of the target isotope toward unity. Accordingly, optical resonator 208 preferably provides high-mode volume, which increases interaction time.

[0049] Various implementations of optical resonator 208 can be used. FIG.3 shows a resonant cavity 300 that can be used to implement optical resonator 208 according to some embodiments. Resonant cavity 300 has flat end walls 302 and is sometimes referred to as an etalon. The inner surfaces 304 of end walls 302 are coated with a highly reflective coating and separated from each other by a distance ^^. Light is admitted through one end wall 302and reflects back and forth between end walls 302. Light with wavelength ^^ ൌ 2^^ / ^^ (forinteger ^^ ^ 1) constructively interferes with its own reflection, while light of otherwavelengths experiences destructive interference. Thus, etalon 300 can selectively amplify coherent light at the resonant wavelength. In general, intracavity power (^^^^௩) is given by^^^^௩ ൌ ^^^^ / ^^, where ^^^^ is the input power (e.g., the power of the laser light source) and ^^ isthe transmissivity coefficient of end walls 302.

[0050] To implement optical resonator 208 using etalon 300, the length ^^ can be chosen tobe ^^ ൌ ^^^^^ / 2. Integer ^^ can be chosen such that ^^ is longer than the width or diameter ofatomic beam 204; in some embodiments, ^^~10 cm is sufficient. A high reflectivitycoefficient ^^ (or, equivalently, a low transmissivity coefficient, since ^^ ^ ^^ ൌ 1) provides adesirable high intracavity power boost. Reflectivity of 99.9% or higher can be achievedusing multilayer dielectric coatings; examples are known in the art. ^^ ൌ 0.999 providesintracavity power boost on the order of 1000x.

[0051] Etalon 300 is most effective if the reflective surfaces 304 of end walls 302 are highly parallel to each other, as deviation from parallelism can lead to light loss at the edges of end walls 304. In addition, if the incident laser beam has divergence, the divergence can eventually lead to light loss.

[0052] In some embodiments, light loss may be reduced by introducing curvature to the end walls of the optical resonator. FIG.4 shows a resonant cavity 400 that can be used to implement optical resonator 208 according to some embodiments. Resonant cavity 400 has curved end walls 402 and is also known as a Fabry-Perot resonator. Similarly to resonant cavity 300, the inner surfaces 404 of end walls 402 are coated with a highly reflective coating and separated from each other by a distance ^^ (the cavity length) along the central axis. Inner surfaces 404 can be spherically curved surfaces with a radius of curvature (^^). As shown in FIG.4, the curvature of inner surfaces 404 acts to refocus the light beam as it reflects back and forth within the cavity, thereby reducing light loss. However, the refocusing effect creates a waist 405 where the beam width can be significantly smaller than the corresponding beam in etalon 300 of FIG.3. More specifically, waist 405 has a width ^^^that scales as ^^^~^^^^^ / 4^^ / ସwhere ^^ is the radius of curvature of the mirror surface. Thus waist width increases slowly with increasing ^^. For efficient ionization of the excited atoms, ^^^should be comparable to the width of the atomic beam, which (for reasonable atomic beam shapes) suggests a very large radius ^^. Reflective surfaces with a large radius of curvature, e.g., 30- 100 km or even higher, can be achieved using thin-film coatings. (Examples of such surfaces are described in S. Miyoki et al., “Manufacture of a 10-km-scale radius-of-curvature surface by use of a thin-film coating technique,” Optics Letters 30, 1399 (2005).)

[0053] In resonant cavity 400, resonance occurs as long as there is a uniform phase front to the beam and the overall cavity length is an integer multiple of half-wavelengths of the light input light. This can be seen by the resonant frequencies of transverse modes, which areshifted from the fundamental resonance by integer multiples of cosି^൫1 െ ^^^ / ^^^൯. In thelimit of infinite ^^, these frequency shifts are zero, so mode-matching is not a concern for etalon 300, although it may be a concern for resonant cavity 400. The Rayleigh length of a laser beam is the distance over which the transverse radius increases by √2 and is proportional to the square of the minimum spot size divided by the wavelength. For example, a laser at a wavelength of 1 μm with a spot size of 6 cm has a Rayleigh length of about 3 km. If the cavity length is 10 cm, then 1,000 round trips is only 100 m, and the beam does not expand significantly. For mirror reflectivity of 99.9%, a laser of power 10 W would produce intracavity power of about 10 kW. (For comparison, LIGO, the Laser Interferometer Gravitational-Wave Observatory, routinely has a circulating power of that magnitude or higher in its resonators.) To minimize absorption losses and heating of the coatings, near- infrared wavelengths longer than 800 nm may be optimum; however, shorter wavelengths can be used.

[0054] Numerical simulations modeling resonant behavior in an etalon have been performed using MATLAB and ray tracing techniques. Cavity length (mirror spacing) was set at 50 mm, with incident beam spot sizes modeled in a range from 20 mm to 1 mm (with Gaussian distribution). Results were computed for 99% and 99.9% mirror reflectivity. FIGs. 5A and 5B show graphs of representative results of numerical simulations for different beam spot size (20 mm, 10 mm, 5 mm, and 1 mm) and for a plane wave. In Fig 5A (5B), mirror reflectivity was set at 99% (99.9%). In each graph, transmission coefficient is plotted as a function of wavelength, with resonances occurring where the cavity length is an integer multiple of half the wavelength. Alternatively, one could hold the wavelength constant and change the cavity length, in which case resonances would occur when cavity length changed by half a wavelength. In the case of mirrors with 99% reflectivity (FIG.5A), results match the plane wave for spot sizes above 10 mm. In the case of mirrors with 99.9% reflectivity (FIG.5B), spot sizes larger than 20 mm would be desired, in agreement with the considerations above.

[0055] An ionizing laser in a resonant cavity may induce an optical Stark shift on another transition (e.g., a transition to an excited state) being driven simultaneously with the ionizing laser, due to the high intracavity power. In some embodiments, the Stark shift can be mitigated by using a “magic-wavelength” laser as is employed in some optical lattice clocks. In embodiments where other transition is dipole-allowed, the Stark shift would not be of concern.

[0056] Referring again to FIG.2, either etalon 300 or Fabry-Perot resonator 400 can be used to implement optical resonator 208. Regardless of the particular design, it is desirable to maintain high energy density within optical resonator 208 while atom beam 204 is passing through. It is noted that thermal expansion of resonator 208 may result in shifting the resonant wavelength(s). To compensate, optical resonator 208 and / or ionization laser 210 can be made tunable. For instance, where optical resonator 208 is tunable, servo motors driven by a feedback loop can move the end reflectors to preserve a constant length ^^ (such that the wavelength of laser 210 is resonant). Alternatively, since a precise wavelength is not required for ionization, laser 210 can be tunable (e.g., using servo motors driven by a feedback loop) so that the wavelength of the laser light adapts to thermal shifts in the length of the cavity. It is noted that tuning of laser 210 can be done with smaller components as compared to adjusting the cavity length, providing faster servo response. Suitable implementations of servo loops are known in the art.

[0057] The size of the laser beam emitted by ionization laser 210 can be chosen to match the size and aspect ratio of atomic beam 204 as it passes through optical resonator 208. For instance, the laser beam spot size can be around 1-2 cm. The laser power can be around 1-5 W, as the intra-cavity power boost of optical resonator 208 can be ~1000x, providing sufficient energy to ionize all or most of the excited atoms in atomic beam 204.

[0058] Ion collector 212 can include, for example, a charged plate, which can be negatively charged to attract positive ions. The size, shape, and position of ion collector 212 can be tuned according to the characteristics of atom beam 204, including the average velocity (and distribution of velocities) and angular divergence of atom beam 204. Ion collector 212 can be large enough that ions do not pass the edges.

[0059] In some embodiments, a system such as system 200 can be implemented with a compact form factor. FIG.6 shows a simplified schematic diagram of a compact isotope separation system 600 according to some embodiments. Isotope separation system 600 is similar to system 200 described above and includes an atom source 602 (corresponding to atom source 202) that produces an atom beam 604 (corresponding to atom beam 204), as well as an ion collector 612 (corresponding to ion collector 212) positioned off-axis to attract and collect ions. In system 600, the pumping laser light and optical resonator intersect atm beam 604 in the same excitation and ionization region 618. For example, pumping lasers 606 (corresponding to pumping lasers 606) can produce beams 607 propagating in the “x”direction (into the plane of the drawing, as indicated by coordinate axes 601), while optical resonator 608 (corresponding to optical resonator 208), driven by ionization laser 610 (corresponding to ionization laser 210), is arranged with its principal propagation direction in the “y” direction.

[0060] FIG.7 is a simplified schematic diagram further illustrating the intersection of atom beam 604 with pumping lasers 606 and optical resonator 608 in system 600 according to some embodiments. The view in FIG.7 is along the z direction, as indicated by coordinate axes 601. Atomic beam 604 is shown as having a circular cross-section in the xy plane; it should be understood that the cross-section can be elliptical or have other shapes as described above. Pumping lasers 606 emit laser beams 607 that propagate along the x direction, transverse to the principal propagation direction of optical resonator 608. (It should be noted that optical resonator 608 need not have side walls, so that laser beams 607 are not obstructed.)

[0061] FIG.8 shows a simplified three-dimensional view further illustrating an arrangement of lasers and optical resonators for system 600 according to some embodiments. Atom source 602 can be an atomic oven in which a sample of atoms is heated, e.g., using a heating coil 802. Atom beam 604 exits in the z direction, as indicated by coordinate axes 801. Ion collector 612 can be implemented using anode 811 and cathode 813 to generate an electric field that separates neutral atoms (path 604-a) and ions (path 604-b), allowing ions to be collected at an ion collector 812 disposed along path 604-b. (Neutral atoms along path 604-a can be separately collected if desired.) The pumping laser light and optical resonator intersect in excitation and ionization region 618, through which atom beam 604 also passes. For example, pumping lasers 806-a and 806-b (corresponding to pumping lasers 606) can produce beams 807-a and 807-b that are combined at a dichroic plate 809 to produce beams 607, which propagate in the x direction (as indicated by coordinate axes 801) through region 618. Optical resonator 608, driven by ionization laser 610, is arranged with its principal propagation direction in the y direction. As shown, atom source 602, optical resonator 608, and ion collector 812 can be disposed within a vacuum chamber 830. Pumping lasers 806-a, 806-b and ionization laser 610 can be outside vacuum chamber 830, with light being transmitted through optically transparent regions (e.g., glass windows) in the walls of vacuum chamber 830. In this example, acousto-optic modulators 832-a, 832-b, and 834 are provided to control the power of pumping lasers 806-a, 806-b and ionization laser 610.

[0062] System 600 can provide a compact form factor. For instance, in some embodiments, all components of system 600 can fit into a volume of about 15 cm × 30 cm × 30 cm. The compact form factor may be particularly useful in applications where it is desirable to produce the isotope in close spatial and temporal proximity to where it is to be used, such as when working with radioactive isotopes in nuclear medicine, industrial testing, or the like.

[0063] In addition, providing the excitation and ionizing radiation in the same volume (as in system 600) allows a variety of excited states to be used, including excited states that are not metastable states. In some embodiments, this arrangement can be used to establish a cyclic transition between the ground state and one or more excited states, which can increase the probability of ionization, particularly if one or more of the excited states is short-lived.

[0064] It will be appreciated that systems 200 and 600 are illustrative and that variations and modifications are possible. The number and arrangement of pumping lasers can be selected based on the target isotope and dimensions of the atomic beam, and the pumping lasers can apply a combination of different wavelengths concurrently or sequentially. For example, one or more pumping lasers can produce beams that intersect the atom beam at a location upstream of the optical resonator while one or more other pumping lasers produce beams that intersect the atom beam within the optical resonator. Similarly, the optical resonator can be driven by one or more lasers, including lasers at two or more wavelengths, provided that the cavity dimensions are selected such that each of the two or more wavelengths is a resonant wavelength of the cavity. Further, while systems 200 and 600 are described in the context of separating isotopes of the same atomic species, similar systems can be used to perform separation of atoms of different types, including different isotopes of the same species or atoms of different atomic species.

[0065] In some embodiments, a sample of atoms can be placed within the atom source prior to evacuating the vacuum chamber that contains the beam path, and the system can run until the sample is exhausted. In some embodiments, the atom source can be arranged such that additional atoms can be placed within the atom source without having to open the vacuum chamber. If desired, neutral atoms from the atom beam can be collected downstream of the optical resonator and returned to the atom source or used for other purposes. (For instance, it may be desirable to obtain a sample with reduced abundance of a particular isotope, and the neutral atoms can provide such a sample.)Example Methods

[0066] In operation, system 200 (or system 600) can be used to produce high-purity samples of a target isotope. FIG.9 shows a flow chart of a process 900 for isotope separation according to some embodiments. Process 900 can be implemented, e.g., using system 200 or system 600 or other systems.

[0067] At block 902, a sample of atoms is prepared. The sample can include atoms of the target isotope and one or more other isotopes. The target isotope has an initial abundance in the sample of atoms, which depends on how the sample is prepared. In some embodiments, the initial abundance of the target isotope can correspond to its naturally-occurring abundance.

[0068] At block 904, a neutral atom beam is produced from the sample. For example, as described above, the sample can be heated in atom beam source 202 to create vapor pressure, and the vapor can escape through aperture 216, forming atom beam 204.

[0069] At block 906, the atom beam is irradiated using one or more pumping lasers directed at the atom beam (e.g., along a direction transverse to the beam as shown in FIGs.2 and 6). The pumping lasers (e.g., pumping lasers 206 or pumping lasers 606) can operate at one or more excitation wavelengths selected to selectively excite the target isotope while leaving other isotopes in the ground state, as described above. Examples of excitation wavelengths for specific isotopes are described below.

[0070] At block 908, the atom beam is passed through an optical resonator (e.g., optical resonator 208 or resonator 608), which can be a Fabry-Perot resonator or etalon as described above) tuned to an ionization wavelength that is selected to ionize the target isotope from the excited state while leaving ground-state atoms in the ground state, as described above. Examples of ionization wavelengths for specific isotopes are described below.

[0071] At block 910, an electric field is applied in a direction transverse to the atom beam downstream of the optical resonator to divert ions from the beam path. For instance, a charged plate can create an electric field that attracts positive ions. Other arrangements of charged plates or other electric field sources can also be used.

[0072] At block 912, the diverted ions are collected, e.g., on a surface of ion collector 212 (or ion collector 612) or another surface introduced into the path of the ions. The collected ions provide a purified sample of atoms, in which the abundance of the target isotope ishigher than in the original sample. Accordingly, process 900 can be used to purify a sample to contain an increased abundance of the target isotope. Depending on particulars, the abundance of the target isotope among the ions collected by ion collector 212 may be above 60%, above 90%, 99% or higher.

[0073] It should be understood that process 900 is illustrative and that variations and modifications are possible. Steps described sequentially may be performed concurrently or in a different order. For instance, the pumping lasers may be applied while the atoms are within the optical resonator. In some embodiments, neutral atoms exiting the beam may be collected in addition to or instead of collecting the target isotope. If desired, the neutral atoms can be recirculated into the atom source to allow further separation of the target isotope. Further, while process 900 is described in the context of separating isotopes of the same atomic species, similar processes can be used to perform separation of atoms of different types, including different isotopes of the same species or atoms of different atomic species. Example Implementations

[0074] Embodiments of system 200 (or system 600) and process 900 can be adapted for separation of specific target isotopes by selecting an appropriate combination of excitation and ionization wavelengths. Proposed embodiments of system parameters for specific target isotopes will now be described. It should be understood that systems and methods of the kind described herein are not limited to these specific examples.

[0075] Example 1: Separation of strontium (Sr) isotopes. Sr-84 is a rare isotope that has medical application as a tracer for bone absorption, e.g., to detect the onset of osteoporosis. Sr-84 can also be used to produce the radioisotope Sr-85, which may have applications in tests of quantum mechanics. FIG.10 shows a Grotrian diagram of relevant energy levels in atomic strontium. The ionization limit of Sr is an energy of 5.695 eV. In one proposed embodiment, the ground1S0 state would be excited by a first laser at a wavelength of approximately 689.3 nm to the3P1state, which has a decay rate of 4.69×104s−1. A second laser at a wavelength of approximately 687.8 nm would excite atoms from the3P1 state to the3S1 state, which has a decay rate of 2.7×107s−1. Atoms would scatter the second laser until the upper state decays to a long-lived metastable state3P2, while emitting a photon at 707 nm. Approximately 80% of the atoms would be optically pumped into this metastable state, which can be improved to 100% by adding a repump laser from the3P0 state at approximately 679.1 nm. Atoms prepared in the3P2state would propagate to the optical resonator, where theywould be excited by a laser near 403 nm to the3D3 state, which has a decay rate of approximately 2×107s-1. While the atoms are cycling on this transition at 403 nm, they would be ionized in the cavity by an ionization laser that is tuned to a wavelength to the blue of (i.e., shorter than) 1612 nm. The ionization laser can be tuned just above the ionization limit to optimize ionization with an autoionizing state, or any high-power laser that is above the ionization threshold can be used. In another proposed embodiment, a laser near 461 nm would excite Sr-84 to an excited state, and a laser tuned slightly to the blue of 405 nm would ionize the atoms. (While the latter implementation involves fewer lasers, for reasons noted above, a resonator at 405 nm may provide less power buildup than a resonator at infrared wavelengths.)

[0076] Example 2: Separation of ytterbium (Yb) isotopes. Yb-176 is a stable isotope that is used to produce radioactive lutetium-177 (Lu-177) for use in medical applications such as cancer treatment. In one proposed embodiment, Yb-176 would be separated from a sample of ytterbium using pumping lasers at wavelengths of approximately 556 nm and 458 nm to selectively excite Yb-176 to a metastable state. Ionization would be completed by resonant atomic excitation with a laser near 493 nm and a second laser tuned to the blue (i.e., toward shorter wavelengths) of 950 nm in a resonant cavity.

[0077] Example 3: Separation of mercury (Hg) isotopes. Mercury has numerous uses, including in UV fluorescent lamps that have applications in water and air sterilization. Hg- 196 is an isotope with a natural abundance of less than 1%. It has been observed that enriching the Hg-196 content of the mercury in UV fluorescent lamps to around 3% can improve their light energy efficiency. In one proposed embodiment, Hg-196 would be separated from other mercury isotopes by pumping with a laser having a wavelength near 254 nm, followed by a second laser near 435 nm to populate a metastable state. Ionization would be completed by resonant atomic excitation with a laser near 365 nm and a second laser tuned to the blue of 784 nm in a resonant cavity. The resulting purified samples of Hg-196 may have abundance significantly higher than 3%; for UV lamps, the purified Hg-196 can be mixed with ordinary mercury to provide the desired abundance. Performance can be further enhanced by optimized mixtures of all stable isotopes of mercury.

[0078] Example 4: Separation of lutetium (Lu) isotopes. As noted above, Lu-177 is a radioisotope that is used in medical applications such as cancer therapy. In one proposed embodiment, the ground state of Lu-176 (an isotope that is effectively stable) would beexcited with a laser near 452 nm. A second laser tuned to the blue of 460 nm in a resonant cavity would complete ionization. In this case, due to the low ionization-energy of lutetium, the ground state may serve as the metastable state. This method would enable enrichment of Lu-176, which can be irradiated with neutrons to produce Lu-177.

[0079] Example 5: Separation of lithium (Li) isotopes. Lithium has two stable isotopes, Li-7 which is 92.4% abundant, and Li-6 which is 7.6% abundant. Li-7, in the form of LiOH, is used in pressurized water nuclear reactors to prevent corrosion. Pure Li-7 (at least 99.95%) is needed since Li-6 is a strong neutron-absorber that would spoil the nuclear reaction. The highly enriched Li-7 is called “Highly Depleted Lithium” or HDLi for short. The current method to enrich Li-6 or Li-7 involves a chemical amalgamation reaction that uses large quantities of mercury and is banned everywhere except for China and Russia. In one proposed embodiment, Li-7 would be excited from the ground state with a laser having a wavelength near 274 nm, and ionization with would be completed with a laser in the resonator tuned to the blue of 1424 nm.

[0080] Example 6: Separation of elements. According to some embodiments, system 200 or system 600 can be used for separation of different elements, e.g., within an alloy or other mixture of different elements. In general, different atomic species (elements) have different excited states at different energy levels. Accordingly, selective excitation and ionization of a target atomic species can be used to separate different species that are initially mixed. One example use-case pertains to separation of gold and the platinum group of metals (PGMs). PGMs include ruthenium, rhodium, palladium, rhenium, osmium, iridium, and platinum. These elements have very similar chemistry, making their separation challenging. Current techniques use complex and highly toxic chemicals to achieve satisfactory purity. In one proposed embodiment, Doré bars (which are gold / silver alloys conventionally created at mining sites for further offsite refinement) would be heated and vaporized to create an atomic beam. Laser excitation and ionization as described herein would be applied to separate the target species, e.g., gold, from other metals. In embodiments where different atomic species are to be separated, it is not necessary for the excitation to be selective for a specific isotope of the target atomic species, provided that the excitation is selective against other atomic species expected to be present in the initial sample.

[0081] As the foregoing examples illustrate, systems and methods described herein are expected to be applicable to a variety of isotopes or elements, including but not limited tolanthanides and alkaline earths. The particular excitation wavelengths for a given target isotope or atomic species depend on the particular atomic spectrum (or energy levels) of the species in question, including isotopic shift. Atomic spectra for many atomic species and particular isotopes have been experimentally determined, e.g., using spectroscopic techniques, and persons skilled in the art will be able to determine atomic spectra for additional species and / or isotopes that may be of interest.

[0082] In general, the target isotope (or element) should have an ionization energy that can be provided using available lasers (currently up to around 6.5 eV) and an excited state that differs from the excited state of non-target isotopes (or elements) by at least the Doppler shift of the atomic beams. It is noted that alkali elements generally do not have a metastable excited state, with the exception of lithium as described above. Similarly, actinides have a more complex state structure than lighter elements (including lanthanides), which makes it unlikely that they would have cyclic transitions that can be excited without also exciting other isotopes. Techniques described herein also assume that a neutral atom beam can be formed, which would require further development for elements having extremely high vaporization temperatures.

[0083] Purity of the purified sample (or abundance of the target isotope (or element) in the purified sample) depends on various design choices and on the particular isotope being purified. In some embodiments, purity may be 99% or better.

[0084] Output rate of the target isotope (or element) can be measured, e.g., in number of ions or weight of ions collected per unit time. Output rate depends on various system parameters, including shape and density of the atom beam, beam velocity, and the efficiency with which the target isotope (or element) is excited and / or ionized. As described above, efficiency of ionization can be increased by providing an optical resonator with high mode- volume and high intracavity power. Additional Embodiments

[0085] While the invention has been described with reference to specific embodiments, those skilled in the art will appreciate that variations and modifications are possible. Features described in connection with one or some embodiments may be incorporated into other embodiments (to the extent logical consistency allows), and features described in connection with one or some embodiments need not be present in every embodiment.

[0086] As described above, various embodiments can be used to separate atoms of different types, including isotopes of the same atomic species and / or atoms of different atomic species, provided that an appropriate set of state transitions can be identified and that lasers are available to provide radiation at the appropriate energy (or wavelength) for selective excitation and ionization of atoms of the target type.

[0087] Target isotopes that have been separated using techniques of the kind described herein can have a variety of applications, which may involve further processing of the target isotope. For instance, as indicated above, radioactive isotopes are used in medical applications such as imaging and / or therapy. In some embodiments, a radioactive isotope of interest can be produced from a stable (or long-lived) isotope. The stable isotope can be isolated using techniques described herein, after which irradiation or other techniques can be used to convert the stable isotope to the radioactive isotope. Stable isotopes can also be used directly for some applications.

[0088] Systems described herein (or components thereof) can be employed in a variety of contexts. For instance, systems of the kind described herein can be used for production of radioisotopes (as an alternative to radiochemistry) for medical applications; since the systems can have a small footprint, they can be fit within existing hot cells.

[0089] As another example, an assay to determine isotopic composition of a sample can be performed by using a system of the kind described herein to selectively ionize a first target isotope, then separately collecting the ionized atoms and the neutral atoms downstream of the optical resonator. Where the system is highly selective for the first target isotope, comparing the weight (or numbers) of collected atoms of each type can yield information about the abundance of the first target isotope in the original sample. In some embodiments, the neutral atoms can be returned to an atom source to perform further isotopic separation with a different target isotope. Similar principles can be applied in assays to determine elemental composition of a sample, with the selection being made for particular atomic species (or elements).

[0090] In another example use-case, systems of the kind described herein can be used in isotope tracing, e.g., to determine concentrations of the target isotope in biological or other samples.

[0091] In yet another example use-case, resonant cavities of the kind described herein can be applied in frequency-modulated (FM) spectroscopy. In FM spectroscopy, a wavelength ofa tunable laser is scanned across an atomic transition wavelength of a target species or isotope in a sample; when the laser wavelength matches the transition wavelength, a modulation in the optical absorption can be detected. In some embodiments of the invention, a tunable optical resonator of the kind described herein can be used to boost sensitivity of FM spectroscopy.

[0092] In various embodiments, any device or structure that creates an electric field transverse to the beam path can be used to steer ionized atoms of the target type (e.g., target isotope or target atomic species) out of the main beam, thereby physically separating the atoms of the target type from the rest of the atoms. As noted above, atoms can be collected from the neutral beam and / or from the ionized beam, depending on intended use.

[0093] All processes described herein are also illustrative and can be modified. Operations can be performed in a different order from that described, to the extent that logic permits; operations described above may be omitted or combined; and operations not expressly described above may be added.

[0094] All numerical values and ranges provided herein are illustrative and may be modified. Unless otherwise indicated, drawings should be understood as schematic and not to scale.

[0095] Accordingly, although the invention has been described with respect to specific embodiments, it will be appreciated that the invention is intended to cover all modifications and equivalents within the scope of the claims.

Claims

WHAT IS CLAIMED IS:

1. A method comprising: preparing a sample of atoms, the sample including atoms of a target type and atoms of one or more other types, wherein the target type has an initial abundance in the sample of atoms; producing an atom beam from the sample of atoms; irradiating the atom beam with a pumping laser operating at a first wavelength, the first wavelength being capable of selectively exciting atoms of the target type to an excited state; passing the atom beam through an optical resonator driven by an ionization laser operating at a second wavelength, the second wavelength being capable of selectively ionizing atoms of the target type from the excited state; applying an electric field to the atomic beam downstream of the optical resonator to divert ionized atoms from a beam path of the atomic beam; and collecting the ionized atoms, wherein an abundance of the target type among the collected ionized atoms is higher than the initial abundance.

2. The method of claim 1 wherein irradiating the atom beam with the pumping laser is performed prior to passing the atom beam through the optical resonator.

3. The method of claim 1 wherein irradiating the atom beam with the pumping laser is performed while the atom beam is passing through the optical resonator.

4. The method of claim 1 wherein producing the atom beam includes heating the sample of atoms in an oven having an aperture, wherein the atom beam is formed by vapor pressure in the oven.

5. The method of claim 1 wherein the target type and the one or more other types are different isotopes of an element.

6. The method of claim 5 wherein the element is an alkali, lanthanide or alkaline earth element.

7. The method of claim 5 wherein the element is one of strontium, ytterbium, mercury, lutetium, or lithium.

8. The method of claim 1 wherein the target type is a target atomic species and the one or more other types are one or more other atomic species.

9. The method of claim 8 wherein the target atomic species is a precious metal.

10. The method of claim 9 wherein the precious metal is gold.

11. The method of claim 9 wherein the one or more other atomic species include one or more elements in the platinum group of metals.

12. The method of claim 1 further comprising: collecting neutral atoms from the atom beam downstream of the electric field, wherein the neutral atoms exhibit depletion of the target type.

13. The method of claim 1 wherein the excited state is a metastable excited state having a lifetime of at least 10 microseconds.

14. The method of claim 1 wherein the excited state is a metastable excited state having a lifetime of at least 1 millisecond.

15. The method of claim 1 wherein the abundance of the target type among the collected ionized atoms is at least 99%.

16. A system comprising: an atom beam source to produce an atom beam that includes atoms of a target type and atoms of one or more other types, wherein the target type has an initial abundance in the atom beam; a pumping laser having a beam path that intersects a path of the atom beam, the pumping laser having a first wavelength that is capable of selectively exciting atoms of the target type within the atom beam to a metastable excited state; an ionization laser having a second wavelength that is capable of selectively ionizing atoms of the target type from the excited state; an optical resonator positioned to intersect the path of the atom beam and having a resonant wavelength equal to the second wavelength, wherein the ionization laser is locked in transmission to the optical resonator; andan ion collector positioned along the path of the atom beam downstream of the optical resonator.

17. The system of claim 16 wherein the atom beam source comprises an oven having an aperture to emit vaporized atoms.

18. The system of claim 16 wherein the optical resonator comprises opposing reflective surfaces.

19. The system of claim 18 wherein each of the opposing reflective surfaces has a radius of curvature of at least 30 km.

20. The system of claim 18 wherein each of the opposing reflective surfaces has a radius of curvature of at least 100 km.

21. The system of claim 18 wherein each of the opposing reflective surfaces is a flat surface.

22. The system of claim 18 wherein each of the opposing reflective surfaces has a reflectivity of at least 99.9%.

23. The system of claim 16 wherein the pumping laser and the optical resonator are arranged such that the beam path of the pumping laser intersects the atom beam upstream of the optical resonator.

24. The system of claim 16 wherein the pumping laser and the optical resonator are arranged such that the beam path of the pumping laser intersects the atom beam within the optical resonator.

25. The system of claim 16 wherein the ion collector includes a charged plate to attract positive ions.

26. The system of claim 16 wherein the target type and the one or more other types are different isotopes of an element.

27. The system of claim 16 wherein the target type is a target atomic species and the one or more other types are one or more other atomic species.

Images