Patent Application
20220355248
Proposals for separating and enriching isotopes came about almost immediately after isotopes were discovered. In 1919, Lindemann and Aston examined a vast array of possible methods including fractional distillation, chemical separation, gaseous diffusion, and gravitational and centrifugal separation, along with separation of positive ions with electric and magnetic fields. (See, F. A. Lindemann, et al. XLVIII. The possibility of separating isotopes. Philos. Mag. Ser. 6. 37, 523-534 (1919).) Their early analysis concluded that isotopes “must be separable in principle though possibly not in practice.” Today, isotope separation and enrichment underpin advanced technologies in a wide variety of fields, including isotopic labeling in medicine, energy, and quantum computing.
Gaseous diffusion, distillation, and gas centrifuges exhibit small isotopic separation effects which are overcome through large scale installations where many separation steps are performed in sequence. Alternatively, laser-based techniques such as atomic vapor laser isotope separation (AVLIS) and magnetically activated and guided isotope separation (MAGIS) can separate isotopes to a much higher degree, but require ionization or excitation of the target isotope. (See, T. R. Mazur, B. Klappauf, M. G. Raizen, Demonstration of magnetically activated and guided isotope separation. Nat. Phys. 10, 601-605 (2014); P. A. Bokhan et al., Laser Isotope Separation in Atomic Vapor (2006).)
Illustrative embodiments of the invention hereafter will be described with reference to the accompanying drawings.
Other principal features and advantages of the invention will become apparent to those skilled in the art upon review of the following drawings, the detailed description, and the appended claims.
Provided are apparatus and methods for enriching and separating isotopologues and/or isotopes from a supersonic beam comprising two or more isotopologues and/or two or more isotopes. The apparatus and methods utilize the narrow velocity distribution of a high Mach number supersonic beam where, essentially, all components entrained in the supersonic flow travel with the same velocity. Different isotopologues and/or different isotopes collide with a condensed film of the isotopologue mixture or the isotope mixture with differing impact energies due to their differing atomic masses. The apparatus and methods may be used to enrich or separate isotopologues or isotopes based on the differential condensation or reflection of the isotopologues and/or isotopes.
Because the probability of gaseous condensation depends on the incident gas-surface collision energy, the supersonic conditions result in differential condensation (and reflectivity) of the different isotopologues or isotopes due to differences in the energy exchanged between the surface and the different isotopologues or isotopes entrained within the monovelocity, or nearly monovelocity, incident supersonic beam. The methods described herein take advantage of these differences in energy exchange. Key to the enrichment and separation methods described herein is the inventors' discovery that the ratio of the isotopologues and isotopes in the condensed films can be tailored to provide the condensed film with a phonon density of states and, in the case of isotopologue condensation, a rovibrational density of states, that provides preferential condensation of one isotopologue with respect to other isotopologues or preferential condensation of one isotope with respect to other isotopes. Because the condensed film is formed from the incident supersonic beam, the extent of the enrichment and separation for a given isotopologue and/or isotope can be tailored by adjusting the ratio of the different isotopologues or isotopes in the incident supersonic beam, as well as their incident velocity.
In the methods for enriching or separating isotopologues, a substantially monovelocity supersonic beam containing a first isotopologue of a first molecule and a second, heavier, isotopologue of the first molecule is directed onto a surface containing the condensed forms of the first and second isotopologues. The methods are carried out with supersonic beam isotopologue ratios, incident supersonic beam velocities, incident supersonic beam angles, and surface temperatures at which the heavier isotopologue is preferentially condensed onto the surface relative to the lighter isotopologue and, conversely, the lighter isotopologue is preferentially reflected from the surface of the film formed from the condensed mixed gas, relative to the heavier isotopologue. The methods further include the step of collecting and isolating the heavier isotopologue from the surface condensate, the lighter isotopologue from the gas scattered from the surface, or both.
Analogous methods can be used to enrich or separate isotopes. Thus, in the methods for enriching or separating isotopes, a substantially monovelocity supersonic beam containing a first isotope of a first element and a second, heavier, isotope of the first element is directed onto a surface containing the condensed forms of the first and second isotopes. The methods are carried out with supersonic beam isotope ratios, incident supersonic beam velocities, incident supersonic beam angles, and surface temperatures at which the heavier isotope is preferentially condensed onto the surface relative to the lighter isotope and, conversely, the lighter isotope is preferentially reflected from the surface of the film formed from the condensed mixed gas, relative to the heavier isotope. The methods further include the step of collecting and isolating the heavier isotope from the surface condensate, the lighter isotope from the gas scattered from the surface, or both.
The surface, which comprises a steady-state mixture of the incident isotopologues (or isotopes) entrained within the supersonic beam, can be formed rapidly on an underlying support substrate via the initial condensation of the entrained isotopologues (or isotopes) in the supersonic beam. After a short time, the composition of the growing surface film will reach a steady state and its composition will reflect the preferential condensation of the heavier isotopologue or isotope. In order to form a condensed film that provides preferential condensation of the heavier isotope or isotopologue, it may be necessary to alter the atomic ratio of the isotopes or the molecular ratio of the isotopologues in the supersonic beam relative to their naturally occurring ratios (i.e., on Earth). This is particularly true in situations where a heavier atomic isotope is present in only a trace amount in nature relative to its lighter counterpart. Thus, in some embodiments of the methods that use a mixture of isotopologues in the supersonic beam, the ratio of isotopologues comprising a heavier isotope of an atom to isotopologues comprising a lighter isotope of that atom is higher than the naturally occurring ratio. Similarly, in some embodiments of the methods that use a mixture of isotopes in the supersonic beam, the ratio of heavier isotope to lighter isotope is higher than the naturally occurring ratio. The extent to which the isotopologue or isotope ratio in the supersonic beam and the condensed film will differ from the naturally occurring ratio will depend on the particular isotopologues and isotopes being used, their respective natural abundances, and the incident beam velocity. By way of illustration, in various embodiments the supersonic beams and/or the steady-state condensed films, molar ratio of isotopologues or the atomic ratio of isotopes may differ from their naturally occurring ratios by at least 10%, at least 100%, at least 1000%, at least 10,000%, or more (e.g., 10% to 10,000%). The ratios in the supersonic beams and the steady-state condensed films may be higher or lower than the naturally occurring ratios.
The material of the support substrate on which the condensed film is formed is not particularly limited, but should be selected to allow for cooling to a temperature at or below the condensation temperature of the material (isotope or isotopologue) to be enriched or separated. In order to maintain the support substrate and the overlying condensed film at a temperature sufficiently low that the isotopologues or isotopes continue to condense, the support substrate may be placed in thermal contact with a cooler, such as a cryogenic cooler.
The supersonic beam may be continuous or pulsed, or modulated, for example, via mechanical chopping. The supersonic expansion may be free or may be passed through an aperture, such as a skimmer or collimator, before it strikes the surface. This skimming effect will cause a slight enrichment of the heavier isotopologue or isotope present in the beam before it strikes the surface, as the heavier isotopologue or isotope is preferentially concentrated toward the centerline of the supersonic beam during the expansion. This skimming effect, which may influence the composition of the incident beam under favorable expansion conditions, can therefore be used advantageously to further increase the desired enrichment.
One example of an ultra-high vacuum (UHV) scattering apparatus that can be used to carry out the methods is shown schematically in
Experimentally, the sticking probability for an isotopologue (or isotope) can be measured using the King and Wells technique, where
P1 is the background signal for the isotopologue (or isotope), P2 is the full flux of the isotopologue (or isotope) with a flag blocking the substrate, and P3 is the surface adsorption of the isotopologue (or isotope) with the flag removed. Such measurements are demonstrated in the Example.
As discussed above, the absolute sticking coefficient can be controlled by adjusting the ratio of the isotopologues (or isotopes) in the incident supersonic beam, the incident kinetic energy of the supersonic beam, the incident angle of the supersonic beam, and/or the temperature of the surface. By way of illustration, heating the nozzle and/or seeding the isotopologues (or isotopes) in a low molecular weight carrier gas (such as helium or hydrogen) will increase the kinetic energy of the supersonic beam and lower the absolute sticking coefficient. Conversely, cooling the nozzle and/or seeding the isotopes in a high molecular weight carrier gas (such as xenon) will increase the absolute sticking coefficient. Additionally, the sticking coefficient can be modified by the angle of incidence; a more glancing angle of incidence will produce a lower sticking coefficient than an angle of incidence more perpendicular to the plane of the surface. As sticking coefficients are highly system dependent, a precise sticking coefficient as a function of isotopologue (or isotope) ratios, gas temperature, angle of incidence, and surface temperature may be determined experimentally. The velocity of the seeded beam is not highly dependent on the concentration of the carrier gas, (e.g., helium). If one wants to tune the velocity precisely, then a variable temperature nozzle is preferably used to heat or cool a given seed gas/carrier gas combination. The expansion is isenthalpic, so the overall kinetic energy is predictable in the gas mixture. For example, pure helium has a kinetic energy that is predicted by thermodynamics. Mixing in a low percentage of the substance to be enriched with the helium results in a beam with a velocity close to that of pure helium. More precisely, one can calculate using a weighted average of the heat capacity of the carrier gas plus that of the seed gas. Therefore, it is preferable to tune the energy of the beam by nozzle temperature for a given composition rather than by composition. This allows one to broadly tune the incident kinetic energy. For different regions, one might use Hz, He, or Ne, for example, as the seed gas with temperature tuning used for overall energy control. See equation 8 of “Development of a Supersonic P(3PJ), O(1D2) Nozzle Beam Source,” S. J. Sibener, R. J. Buss, C. Y. Ng, and Y. T. Lee, Rev. Sci. Instrum. 51, 167-182 (1980).
For the enrichment or separation of any given isotope or isotopologue, once a vacuum condensation temperature is obtained, one can optimize the conditions by, for example, changing the ratio of the isotopologues or isotope in the incident beam or decreasing the beam velocity from a high velocity (i.e., higher nozzle temperature), while keeping the substrate at a fixed temperature below the vacuum condensation temperature and observing the beam velocity at which the material first condenses. One can then adjust isotopologue or isotope ratio in the beam and/or the beam velocity to find a point of maximum enrichment of the condensate and/or scattered material, depending on the desired product. The substrate temperature may also be varied, but the effect on the amount of enrichment is expected to be smaller.
The supersonic beam may be generated by a variety of atomic beam sources configured to supersonically expand a gas through a variable temperature nozzle. The gas comprises the first and second isotopologues (or isotopes) in a selected ratio. By “first and second isotopologue” it is meant two forms of the same molecule, wherein one or more atoms of the first isotopologue have equal numbers of protons but different numbers of neutrons relative to the corresponding atoms of the second isotopologue. Similarly, by “first and second isotope” it is meant two forms of the same element that contain equal numbers of protons but different numbers of neutrons. The gas may comprise additional (i.e., more than two) isotopologues and/or isotopes, each of which may be enriched or separated using the method.
Straightforward collection techniques used for molecular beam apparatus can be employed to collect condensed and reflected fractions of the isotopologues (or isotopes). By way of illustration, the scattered gas can pass through an aperture which leads to a separate vacuum chamber that is pumped by a high vacuum pump, where the exhaust of the high vacuum pump contains the reflected portion enriched in the lighter isotope. Additionally, the condensed portion can be collected by periodically warming the condensate above its condensation temperature. As the condensed portion desorbs and enters the gas phase, the exhaust of the chamber's high vacuum pump will be enriched in the heavier isotope (or isotopologue). The isotopologues (or isotopes) collected from both the condensed portion and the reflected portion can separately be recompressed with a mechanical compressor. Once recompressed, each portion can be further enriched through subsequent passes (e.g., 2, 10, 100, 1000, or more passes) through the enrichment method until a desired level of enrichment is obtained.
The types of isotopologues and/or isotopes that can be enriched and separated by the methods described herein are not particularly limited. However, because the enrichment and separation are higher for isotopologues (or isotopes) having molecular weights that differ by a greater percentage, the methods are more effective for lower molecular weight isotopologues and isotopes. In addition, because molecules have more degrees of freedom than atoms and are, therefore, more efficient at energy transfer, the methods are particularly useful for the enrichment and separation of isotopologues.
Illustrative examples of isotopologues that can be enriched or separated by these methods include but are not limited to: 6LiH and 7LiH; 12CH4 and 13CH4; 28SiH4 and 29SiH4; 28SiF4 and 29SiF4; two or more of H2, HD, HT, D2, DT, and T2; two or more of 1H216O, 1H2H16O, 2H216O, 1H216O, and 1H218O; 16O2 and 18O2; 14N2 and 15N2; HBr and DBr; HCl and DCl; HI and DI; H2S and D2S; two or more of CH3F, CD3F, 13CH3F, and 13CD3F; two or more of CH3Cl, CD3Cl, 13CH3Cl, and 13CD3Cl; two or more of 13C16O2, 1C16O2, 13C18O2, and 12C18O2; two or more of 12C18O, 13C18O, 13C16O, and 13C18O; two or more of NH3, 15NH3, ND3, and 15ND3; NO and 15NO; two or more of 14N2O, 15N14NO, and 15N2O; two or more of PH3 and PD3; COS and 13COS; CHCl2F and CDCl2F; two or more of 12CH2F2, 13CH2F2, 12CD2F2, and 13CD2F2; two or more of CH3Br, 13CH3Br, and CD3Br; two or more of 12C3H6, 12C3D6, 13C3H6, and 12C3H4D2; two or more of 12CH3N, 13CH3N, 13CH315N, 12CH315N, 12CD3N, and 12CH2DN; 12C2H4, 12C2H2D2, two or more of 12CH4, 12CD4, 13CH4, 13CD4, and 12C2H3D, two or more of 13C2H4, 13C2D4, and 12C2H3D; 12C2H6, 13C2H6, and 12C2D6; and two or more of 32SF4, 33SF4, 34SF4, and 36SF4.
Illustrative examples of isotopes that can be enriched or separated by these methods include but are not limited to: 12C, 13C, and/or 14C; 28Si and 29Si, 6Li and 7Li; 10B and 11B, 36Ar and 40Ar; 22Ne and 20Ne; 24Mg and 26Mg; 124Xe, 129Xe, 131Xe, 132Xe, and 134Xe; and 78Kr, 82Kr, and 86Kr.
In order to illustrate the methods, the sticking probabilities of CH4, CD4, and a mixture of CH4 and CD4 on condensed CH4 and CD4 surfaces. The sticking probabilities were measured using the King and Wells technique and the apparatus shown in
This example illustrates how the isotopic sticking probability of CH4 and CD4 can be adjusted as a function of translational beam energy on CH4 and CD4 thin films under ultrahigh vacuum (UHV) conditions at low temperatures using the King and Wells method complemented by in situ infrared spectroscopic studies of gaseous condensation. (King, D. A. et al., Surf. Sci. 1972, 29, 454-482.) VENUS code molecular dynamics (MD) simulations show the same sticking probability differences between isotopologues as were observed in the gas-surface scattering experiments. Taken together, these results accurately and independently determine the sticking probability, allowing exploration of how differences in isotopic composition of the surface and incident molecular mass can impact the overall energy accommodation, and thus adsorption of the gaseous species onto the film.
Key to these studies was the use of essentially monoenergetic seeded supersonic beams so that the CH4 and CD4 had the same incident velocity but different kinetic energies and momenta. It was shown that as the incident velocity of CH4 and CD4 increased, the sticking probabilities for both molecules on a CH4 condensed film decreased systematically, but that preferential sticking and condensation occurred for CD4. These observations differed when condensed CD4 was used as the target interface, demonstrating that the film's phonon and rovibrational densities of states and collisional energy transfer cross sections played a role in differential energy accommodation between isotopically substituted incident species. In addition, a mixture of gaseous CH4 and CD4 was grown on a methane thin film. While both species adsorbed creating a mixed isotopologue condensate, an increased abundance of CD4 vs. CH4 was also seen within the film as opposed to initial beam concentration. An isotopic enrichment for CD4 was demonstrated in the mixed surface based on the difference in sticking probabilities between CH4 and CD4.
The studies reported here illustrate how differences in mass, energy, and surface composition can be adjusted to form a condensed surface film that absorbs and dissipates energy from impinging molecules or atoms to allow preferential adsorption of a selected isotopologue or isotope onto the film structure.
This example presents differential condensation between methane isotopologues under specifically selected substrate, gas-mixture, and incident velocity conditions. However, the demonstrated methods and outcomes can be used for the development of isotopic enrichment and separation techniques for a variety of isotopologues and/or isotopes.
Experimental Methods
All experiments were conducted in a molecular beam scattering instrument previously discussed in full detail. (Gibson, K. D. et al., J. Chem. Phys. 2011, 134, 034703\1-7.) Briefly, this instrument includes a UHV chamber with base pressures of 10−10 Torr connected to a triply differentially pumped molecular beamline. In the main chamber, a state-of-the-art closed-cycle helium-cooled sample manipulator (Advanced Research Systems) enabled precise and accurate temperature control of the Au(111) sample substrate between 16 and 800 K. The crystal was exposed to the impinging beam at normal incident angle and monitored in real time with optics for in situ reflection absorption infrared spectroscopy (RAIRS). Gas scattering and incident flux monitoring occurred with a residual gas analyzer (RGA).
All RAIR spectra were analyzed with Gaussian peak fitting atop cubic baselines. Spectra were acquired with a Nicolet 6700 infrared spectrophotometer (Thermo Fischer) using incident p-polarized IR radiation at an angle of 75° to the Au(111) crystal and a liquid nitrogen-cooled mercury cadmium telluride (MCT/A) detector. Each RAIR spectrum was an average of 25-200 scans taken using 4 cm−1 resolution with a clean Au(111) sample used for the background correction.
CH4 or CD4 was dosed on the Au(111) substrate via beam deposition at 18 K prior to measurements at 20 K. Dosing conditions resulted in a deposition rate of 0.5 layers per second.
CH4 and CD4 beams were produced by expanding 1% CH4 in H2 or 1% CD4 in H2 at stagnation pressures of 150-400 psi through a 15 μm platinum pinhole. Resistively heating the beam nozzle from room temperature up to 1100 K resulted in beam velocities of up to 4600 m/s and rotationally cold molecules resulting from the seeded expansion. The translational energy distribution widths (Δv/v) ranged from 5 to 24%. It was noted that the velocity slip between the two isotopologues varied between 0 and at most 100 m/s with velocities spanning 2400 to 4600 m/s; therefore, the incident velocities of the two isotopologues were essentially identical for the purposes of a given experiment. Incident velocities were measured by time-of-flight methods using a mechanical chopper to modulate the beam prior to detection with an in-line quadrupole mass spectrometer. To confirm all the results and further understand phonon interactions at cold temperatures, a mixed beam was produced by expanding 1% CD4, 3% CH4 in H2.
Sticking probability was determined using the King and Wells technique. (King, D. A. et al., Surf. Sci. 1972, 29, 454-482.) This was previously described in more detail for this system, and a typical King and Wells experiment conducted on a CH4 surface temperature at 20 K is shown in
King and Wells measurements were performed at 20 K for all results presented in this example. This temperature was carefully chosen due to the methane surface interaction and the King and Wells method itself. UHV conditions at 20 K accurately model astrophysical chemistry rich environments such as dense molecular clouds. Additionally, at 20 K, multilayer CH4 is stable on a gold substrate and frozen condensate films which enables measuring the condensate via RAIRS. (Collings, M. P. et al., Astrophys. Space Sci. 2003, 285, 633-659; and Ayotte, P. et al., J. Geophys. Res. Planets 2001, 106, 33387-33392.) The liquid helium cooling of the sample manipulator could impact the pumping speed and thus the reflected portion of the beam; therefore, all measurements were taken at a single sample temperature. This ensured that the unknown pumping speed remained consistent across measurements. Sticking probability was also calculated by using the initial CH4 indirect flux instead of the value at saturation.
Results and Discussion
To fully demonstrate the role that mass matching and pre-adsorbed hydrocarbons play in trapping dynamics for CH4 and CD4, sticking probability data for CH4 and CD4 were obtained on top of amorphous CH4 and CD4. Films were grown for ˜80 layers to achieve a film with a steady-state composition and self-similarity in film structure. The measured sticking probabilities for CH4 and CD4 on a CD4 substrate are shown in
However, a higher sticking coefficient was noted for CD4 on CH4 ice than for CH4 on CH4 ice particularly at high incident translational energies,
This isotopic effect increased with increasing translational energy. To understand this, the Baule model was examined, which predicts that a more efficient collision occurs when the gas and surface masses match due the singularity in the momentum case. (Kao, C. L. et al., Surf. Sci. 2004, 565, 70-80.) An incident molecule containing mass m and energy E encounters a square well potential of depth D and a surface species of mass M, resulting in an energy transfer (A) in the collision: (Carlsson, A. F. et al., Surf. Sci. 2000, 470, 62-80.)
For this, it was assumed that the energy of the incoming molecule is much greater than the well depth of the potential. Thus, for the CH4 film, the energy transfer for the CD4 projectile was greater than that of the CH4, which would generally indicate a higher sticking probability. This model does not account for the density of states of the film or the internal modes of the molecule, as discussed later. These contributions can influence sticking probabilities. Due to the role of these molecular degrees of freedom, complex and multi-phonon interactions between the surface and the incident projectiles CH4 film and CD4 clearly need to be taken into consideration.
To further explore and confirm the experimental results, a beam comprised of both CH4 and CD4 in a 3:1 ratio was considered; this ratio was not selected to optimize condensation differences, but rather to demonstrate the robust nature of differential sticking. This allowed for the quantification of the sticking probability as well as condensate composition. While dosing a multilayer film of both CH4 and CD4 at 20 K, the integrated area of the degenerate ν4 mode was tracked over time using RAIRS. Once the condensate reached a self-similar structural steady state of at least 100 layers, at least 10 spectra per experiment on at least three different days were averaged to determine the film composition. As depicted in
The differential sticking probability of CH4 and CD4 on CH4 and CD4 condensates were examined using RAIRS for measuring on-surface gaseous condensation and complementary King and Wells mass spectrometry techniques for monitoring sticking probabilities. It was found that as the incident velocity of CH4 and CD4 increased (up to 4600 m/s), the sticking probability decreased for both films. Interestingly, it was concluded that preferential sticking and condensation occurred for CD4 when striking the surface in comparison to the outcome for CH4. This observation was confirmed both experimentally from infrared spectroscopy of the condensation and via mass spectrometric detection of the reflected molecules. Next, a mixed incident supersonic beam comprised of both CH4 and CD4 in a 3:1 ratio was employed to measure the condensate as well as the sticking probability. When doing so, the same effect was seen in the condensed mixed film, supporting an isotopic enrichment of the heavier isotope. These results are consistent with enhanced multi-phonon interactions attributable to the film's phonon and rovibrational densities of states and inelastic cross sections including intermolecular energy exchange between the incident CD4 projectile and the CH4 film, allowing for more efficacious gas-surface energy transfer.
This example demonstrates the importance of film structure and surface lattice coupling to allow for efficient energy transfer and an isotopic enrichment of the heavier isotope (CD4). These results also offer a new route for isotope enrichment via the preferential condensation of heavier isotopes and isotopologues during gas-surface collisions under carefully selected substrate, gas-mixture, and incident velocity conditions.
The word “illustrative” is used herein to mean serving as an example, instance, or illustration. Any aspect or design described herein as “illustrative” is not necessarily to be construed as preferred or advantageous over other aspects or designs. Further, for the purposes of this disclosure and unless otherwise specified, “a” or “an” means “one or more”.
The foregoing description of illustrative embodiments of the invention has been presented for purposes of illustration and of description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and as practical applications of the invention to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto and their equivalents.
The present application claims priority to U.S. provisional patent application No. 63/182,204 that was filed Apr. 30, 2021, the entire contents of which are incorporated herein by reference.
This invention was made with government support under FA9550-19-1-0324 awarded by the Air Force Office of Scientific Research and under U.S. National Science Foundation awarded by CHE-1900188. The government has certain rights in the invention.
| Number | Date | Country | |
|---|---|---|---|
| 63182204 | Apr 2021 | US |
