WO2019032223A2

WO2019032223A2 - Isotope enrichment via differential condensation and reflection of isotopes during supersonic beam gas-surface scattering

Info

Publication number: WO2019032223A2
Application number: PCT/US2018/040609
Authority: WO
Inventors: Steven J. SIBENER; Kevin J. NIHILL; Jacob D. GRAHAM; Alison A. MCMILLAN
Original assignee: University of Chicago
Current assignee: University of Chicago
Priority date: 2017-07-07
Filing date: 2018-07-02
Publication date: 2019-02-14
Anticipated expiration: 2020-01-07
Also published as: WO2019032223A3

Abstract

Provided are apparatus and methods for enriching and separating isotopes or isotopologues. The apparatus and methods enrich and/or separate isotopes or isotopologues present in a supersonic beam incident upon a surface via the differential condensation or reflection of different isotopes or isotopologues on the surface.

Description

ISOTOPE ENRICHMENT VIA DIFFERENTIAL CONDENSATION AND REFLECTION OF ISOTOPES DURING SUPERSONIC BEAM GAS-SURFACE

SCATTERING

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority from U.S. provisional patent application number 62/529,660, which was filed on July 7, 2017, the entire contents of which are hereby incorporated herein by reference.

REFERENCE TO GOVERNMENT RIGHTS

[0002] This invention was made with government support under Grant No. FA9550-15- 1-0428 awarded by the Air Force Office of Scientific Research. The government has certain rights in the invention.

BACKGROUND

[0003] 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 underpins advanced technologies in a wide variety of fields, including isotopic labeling in the life sciences and radioisotopes in medicine.

Microelectronics may also begin to utilize isotopic enrichment, as highly enriched ²⁸Si wafers have markedly increased thermal conductivity and electron transport characteristics. (See, T. Ruf et al. , Thermal conductivity of isotopically enriched silicon. Solid State Commun. State Commun. 115, 243-247 (2000); J.-Y. Li et al , Extremely high electron mobility in isotopically-enriched ²⁸Si two-dimensional electron gases grown by chemical vapor deposition. Appl. Phys. Lett . 103, 162105/1-4 (2013).) 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).)

SUMMARY

[0004] Provided are methods and apparatus for enriching and/or separating isotopes or isotopologues. In the methods, a supersonic beam of a gas that includes a first isotope of a first element (or a first isotopologue of a first compound) and a second isotope of the first element (or a second isotopologue of the first compound) is directed onto a surface, wherein the first isotope (or isotopologue) has a higher incident kinetic energy than the second isotope (or isotopologue). Depending on the energy of the supersonic beam, the angle of incidence, and the temperature of the surface, the separation of the first and second isotopes (or isotopologues) can take place in one of two separation regimes. In the first separation regime, the isotope (or isotopologue) having a higher incident kinetic energy is preferentially adsorbed onto the surface and the isotope (or isotopologue) having a lower incident kinetic energy is preferentially reflected from the surface. In the second separation regime, the isotope (isotopologue) having a lower incident kinetic energy is preferentially adsorbed onto the surface and the isotope (or isotopologue) having a higher incident kinetic energy is preferentially reflected from the surface.

[0005] Thus, one embodiment of a method for enriching and/or separating isotopes includes the step of directing a supersonic beam comprising a gas comprising a first isotope of a first element and a second isotope of the first element onto a surface, the first isotope having a higher incident kinetic energy than the second isotope, wherein the first isotope is preferentially adsorbed onto the surface, relative to the second isotope. The method further includes collecting either or both of the gas scattered from the surface and the adsorbate on the surface, wherein the gas scattered from the surface is enriched in the second isotope and the adsorbate is enriched in the first isotope, where enrichment in a given isotope is determined relative to the concentration of that isotope in the incident supersonic beam.

[0006] Another embodiment of a method for enriching and/or separating isotopes includes the step of directing a supersonic beam comprising a gas comprising a first isotope of a first element and a second isotope of the first element onto a surface, the first isotope having a higher incident kinetic energy than the second isotope, wherein the second isotope is preferentially adsorbed onto the surface, relative to the first isotope. The method further includes collecting either or both of the gas scattered from the surface and the adsorbate on the surface, wherein the gas scattered from the surface is enriched in the first isotope and the adsorbate is enriched in the second isotope.

[0007] One embodiment of a method for enriching and/or separating isotopologues includes the step of directing a supersonic beam comprising a gas comprising a first isotopologue of a compound and a second isotopologue of the compound onto a surface, the first isotopologue having a higher incident kinetic energy than the second isotopologue, wherein the first isotopologue is preferentially adsorbed onto the surface, relative to the second isotopologue. The method further includes collecting either or both of the gas scattered from the surface and the adsorbate on the surface, wherein the gas scattered from the surface is enriched in the second isotopologue and the adsorbate is enriched in the first isotopologue, where enrichment in a given isotopologue is determined relative to the concentration of that isotopologue in the incident supersonic beam.

[0008] Another embodiment of a method for enriching and/or separating isotopologues includes the step of directing a supersonic beam comprising a gas comprising a first isotopologue of a compound and a second isotopologue of the compound onto a surface, the first isotopologue having a higher incident kinetic energy than the second isotopologue, wherein the second isotopologue is preferentially adsorbed onto the surface, relative to the first isotopologue The method further includes collecting either or both of the gas scattered from the surface and the adsorbate on the surface, wherein the gas scattered from the surface is enriched in the first isotopologue and the adsorbate is enriched in the second isotopologue, where enrichment in a given isotopologue is determined relative to the concentration of that isotopologue in the incident supersonic beam.

[0009] One embodiment of an apparatus for enriching and/or separating isotopes includes: a pump configured to maintain a low pressure within a chamber; a first beam source configured to provide a supersonic beam comprising a gas comprising a first isotope of a first element and a second isotope of the first element, wherein the apparatus is configured to direct the supersonic beam onto a surface in the chamber, such that at least a portion of at least one of the first isotope and the second isotope adsorbs onto the surface and at least a portion of at least one of the first isotope and the second isotope scatters from the surface; a temperature control device in thermal communication with the surface; and a collector configured to collect the gas scattered from the surface.

[0010] Another embodiment of an apparatus for enriching and/or separating isotopes includes: a pump configured to maintain a low pressure within a chamber; a beam source configured to provide a supersonic beam comprising a gas comprising a first isotope of a first element and a second isotope of the first element, wherein the apparatus is configured to direct the supersonic beam onto a surface in the chamber, such that at least a portion one of the first isotope and the second isotope preferentially adsorbs onto the surface and at least a portion of the other of the first isotope and the second isotope preferentially scatters from the surface; a temperature control device in thermal communication with the surface and configured to (a) maintain the surface within a first temperature range in which the first isotope or the second isotope is preferentially adsorbed onto the surface relative to the other isotope, and (b) maintain the surface within a second temperature range at which the adsorbate is desorbed from the surface; a controller configured to turn the supersonic beam on and off and to direct the temperature control device to change between the first and second temperature ranges; and a collector configured to collect the adsorbate desorbed from the surface.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] Illustrative embodiments of the invention hereafter will be described with reference to the accompanying drawings, wherein like numerals denote like elements.

[0012] FIG. 1 shows a schematic of the ultra-high vacuum surface scattering instrument employed in an example of the methods described herein.

[0013] FIG. 2 displays the ⁶Ar/⁴⁰Ar isotopic ratios measured to determine the enrichment factors of the methods described herein. Depicted are the ratios of the incident beam, the portion reflected from the argon condensate at 26 K, and the argon condensate itself at three incident velocities (1650 m/s, 1760 m/s, and 1027 m/s); error bars represent 1σ as determined from Poisson statistics. The ⁶Ar/⁴⁰Ar isotopic ratio of the incident beam was found to be 0.00264 +/- 0.00001, 0.00272 +/- 0.00001, and 0.00261 +/- 0.00001 for beam velocities 1650 m/s, 1760 m/s, and 1880 m/s, respectively. The ⁶Ar/⁴⁰Ar isotopic ratio of the reflected beam was found to be 0.0058 +/- 0.0003, 0.0040 +/- 0.0002, and 0.00305 +/- 0.00007 for beam velocities 1650 m/s, 1760 m/s, and 1880 m/s, respectively. The ⁶Ar/⁴⁰Ar isotopic ratio of the condensate was found to be 0.00242 +/- 0.00007, 0.00235 +/- 0.00006, and 0.00219 +/- 0.0003 for beam velocities 1650 m/s, 1760 m/s, and 1880 m/s, respectively.

[0014] FIG. 3 depicts a single thermal desorption spectrum showing desorbed ⁴⁰ Ar and 6Ar as a function of increasing surface temperature (T_s) after the condensation of a helium seeded Ar beam with velocity of 1650 m/s onto a 26 K amorphous argon condensate. The background-subtracted and integrated waveforms for ⁶Ar and ⁴⁰ Ar provided an isotopic ratio of ⁶Ar/⁴⁰Ar of 0.00242 +/- 0.00007 in the condensate.

[0015] FIG. 4A shows the scattering enrichment factors for ⁶Ar derived from measured isotopic ratios. FIG. 4B shows the condensation enrichment factors for ⁴⁰Ar derived from measured isotopic ratios. The scattered fraction is enriched in ⁶Ar while the condensed fraction is enriched in ⁴⁰ Ar.

[0016] FIG. 5 depicts the isotopic ratio of ⁶Ar/⁴⁰Ar, incident and reflected, from an argon condensate at 29 K at two incident velocities (563 m/s and 1027 m/s); error bars represent 1σ as determined from Poisson statistics. The ⁶Ar/⁴⁰Ar isotopic ratio of the incident beam at both velocities was 0.00308 +/- 0.00002. Enrichment of ⁴⁰ Ar and depletion of ⁶Ar in the reflected beam were observed at both incident beam velocities. The isotopic ratio of ⁶Ar/⁴⁰Ar after reflection was found to be 0.00245 +/- 0.00033 for the lower velocity beam and 0.00261 +/- 0.00033 for the higher velocity beam. The ⁴⁰ Ar enrichment factors for the lower and higher velocity incident beam were found to be A = 1.25 and AR2 = 1.18, respectively

[0017] FIG. 6 depicts a single thermal desorption spectrum showing desorbed ⁴⁰ Ar and 6Ar as a function of increasing surface temperature (T_s) after the condensation of a helium seeded Ar beam with velocity of 1027 m/s onto a 29 K amorphous argon condensate. The background-subtracted and integrated waveforms for ⁶Ar and ⁴⁰ Ar provided an isotopic ratio of ⁶Ar/⁴⁰Ar of 0.00327 in the condensate.

[0018] FIG. 7 depicts the ⁶Ar/⁴⁰Ar isotopic ratio incident on, and as condensed on, a 29 K amorphous argon surface at two incident velocities (563 m/s and 1027 m/s); error bars represent 1σ as determined from the standard deviation of eight experimental runs. The 6Ar/⁴⁰Ar isotopic ratio of the incident beam at both velocities was 0.00308 +/- 0.00002. Enrichment of ⁶Ar and depletion of ⁴⁰ Ar in the condensate were observed at both incident beam velocities. The isotopic ratio of ⁶Ar/⁴⁰Ar after condensation was found to be 0.00313 +/- 0.00004 for the lower velocity beam, and 0.00328 +/- 0.00001 for the higher velocity beam. The ⁶Ar enrichment factor for the lower and higher velocity incident beam was found to be Aci = 1.02 and Ac2 = 1.06, respectively.

DETAILED DESCRIPTION

[0019] Provided are apparatus and methods for enriching and separating isotopes (or isotopologues). The apparatus and methods are based on using 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 isotopes (or isotopologues) therefore collide with the surface with differing impact energies due to their differing atomic masses. Given that the probability of gaseous condensation depends on the incident gas-surface collision energy, these conditions result in differential condensation (reflectivity) of varying isotopes (or isotopologues) due to changes in the energy exchanged between the surface and the components entrained within the incident and nearly

monovelocity supersonic beam. The apparatus and methods may be used to enrich or separate isotopes (or isotopologues) via differential condensation or reflection of various isotopes (or isotopologues) involving such a supersonic beam.

[0020] In an embodiment, a method for enriching or separating isotopes comprises directing a monovelocity supersonic beam containing a first isotope of an element and a second isotope of the element onto a surface, wherein the heavier of the two isotopes has a higher incident kinetic energy and momentum than the lighter of the two isotopes. The method can be carried out under conditions (e.g., incident supersonic beam velocities, incident supersonic beam angles, and surface temperatures) that allow for either the heavier isotope or the lighter isotope to be preferentially condensed onto the surface, relative to the other isotope. Guidance for identifying supersonic beam velocities, incident angles, and surface temperatures that allow for the preferential condensation of either the heavier isotope or the lighter isotope are provided in the Examples. The methods further include the step of collecting and isolating the heavier isotope, the lighter isotope, or both, from the gas scattered from the surface or the condensate on the surface.

[0021] For high energy supersonic beams, the larger incident energy of the heavier isotope relative to the lighter isotope results in the portion of the supersonic beam that is not condensed on the surface (the scattered portion) being enriched in the lighter isotope, as illustrated in Example 1. Conversely, the condensed portion (which may be amorphous or crystalline) that grows on the surface will be enriched in the heavier isotope and then can be recovered from the surface by heating the surface to a temperature that facilitates its desorption. Opposite enrichment trends can be achieved using a lower energy supersonic beam, as illustrated in Example 2.

[0022] As a further detail of this embodiment, the relative enrichment effect observed between the scattered portion and condensed portion is influenced by the absolute sticking coefficients of the isotopes. By way of illustration, if 100% of the supersonic beam condensed on the surface, no enrichment effect would be observed in the condensed portion. Likewise, if 0% of the supersonic beam condensed on the surface, no enrichment effect would be observed in the scattered portion. When sticking coefficients are near, but not at, these limits, the maximum enrichment is observed for each condition. By way of illustration, if 99% of the supersonic beam condensed on the surface, the largest change in isotopic abundances would be observed in the scattered portion, and a relatively smaller change in isotope abundances would be observed in the condensed portion. Likewise, if 1 % of the supersonic beam is condensed on the surface, the largest change in isotope abundances would be observed in the condensed portion, and a relatively smaller change in isotope abundances would be observed in the scattered portion. This effect can be described by the relationship Ri = RF(1-S2) + RcS2, where Ri is the ratio of the first isotope to the second isotope that is present in the incident supersonic beam before it strikes the surface, RF is the ratio of the first isotope to the second isotope that is present in the scattered portion of the supersonic beam, Rc is the ratio of the first isotope to the second isotope that is present in the condensed portion of the supersonic beam, and S2 is the absolute sticking coefficient of the second isotope. Furthermore, the following relationship relates the enrichment factor of the first isotope in the scattered portion (EFi,s) to the enrichment factor of the second isotope in the condensate (EF2,c) through the sticking coefficient of the second isotope (<¾): EFi,s (1- S2) +

[0023] Notably, the absolute sticking coefficient can be manipulated by adjusting the incident kinetic energy of the supersonic beam, the incident angle of the supersonic beam, and/or the temperature of the surface. Heating the nozzle and/or seeding the 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, thus increasing the observed change in isotope abundances in the condensed portion. 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, thus increasing the observed change in isotopic abundances in the scattered portion. 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 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 H2, 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 0( Py),

Nozzle Beam Source," S.J. Sibener, R.J. Buss, C.Y. Ng, and Y.T. Lee, Rev. Sci. Instrum. 51, 167-182 (1980).

[0024] 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, 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 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.

[0025] 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 isotope. 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 isotopes, each of which may be enriched or separated using the method. In certain embodiments, the gas may comprise molecules that are isotopologues, e.g., hydrogen, which may be comprised of two or more of H2, HD, HT, D2, DT and T₂. It will be apparent to one of skill in the art that if the supersonic beam contains two, three or more isotopes (or isotopologues), the invention can be used to enrich or deplete a sample of one or more of the isotopes (or isotopologues). The composition of the gas and the type of isotopes are not particularly limited. 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 isotope (or isotopologue) present in the beam before it strikes the surface, as the heavier isotope (or isotopologue) 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. The wide range of incident beam conditions can be used to dial in a desired sticking coefficient for both weakly interacting, physisorption systems and strongly interacting, chemisorption systems.

[0026] Because these methods are able to enrich or separate isotopes of a given element based on minor differences in their incident momenta due to mass differences, effectively any isotopes in the periodic table of the elements that can be co-expanded in a supersonic molecular beam can be enriched or separated by the methods. It will be apparent to one of skill in the art how to vaporize these isotopes to enable co-expansion in a supersonic beam. For example, depending on the mass and boiling point of the element, use of a pulsed laser may be preferable for prevolatilization.

[0027] Illustrative examples of isotopes that can be enriched or separated by these methods include but are not limited to ⁶Ar and ⁴⁰ Ar; ⁶Li and ⁷Li; ²²Ne and ²⁰Ne; ²⁴Mg and ²⁶Mg; ¹²⁴Xe, ¹²⁹Xe, ¹³¹Xe, ^{1 2}Xe, and ^{1 4}Xe; and ⁷⁸Kr, ⁸²Kr, and ⁸⁶Kr. This method also is applicable to elements extending across the periodic table.

[0028] Any isotopologues that can be co-expanded in a supersonic molecular beam can be enriched or separated by the methods. Isotopologues with fewer normal modes will be particularly amenable to these methods. It will be apparent to one of skill in the art how to vaporize these isotopologues to enable co-expansion in a supersonic beam.

[0029] Illustrative examples of isotopologues that can be enriched or separated by these methods include but are not limited to ¹²CH₄ and ¹ CH₄; ²⁸SiH₄ and ²⁹SiH₄; ²⁸SiF and ²⁹SiF ; Hi, HD, HT, D₂, DT, and T₂; ¹H₂ ¹⁶0, ¾²Η¹⁶0, ²H₂ ¹⁶0, ¾₂ ¹⁶0, and ¾₂ ¹⁸0; ¹⁶0₂ and ¹⁸0₂; ¹⁴N₂ and ¹⁵N₂; HBr and DBr; HC1 and DC1; HI and DI; H₂S and D₂S; CH₃F, CD₃F, ¹ CH₃F, and ¹ CD₃F; CH₃C1, CD₃C1, ¹ CH₃C1, and ¹ CD₃C1; ¹ C¹⁶0₂, ¹²C¹⁶0₂, ¹ C¹⁸0₂, and ¹²C¹⁸0₂; ¹²C¹⁸0, ¹ C¹⁸0, ¹ C¹⁶0, and ¹ C¹⁸0; NH₃, ¹⁵NH₃, ND₃, and ¹⁵ND₃; NO and ¹⁵N0; ¹⁴N₂0, ¹⁵N¹⁴NO, and ¹⁵N₂0; PH₃ and PD₃; COS and ¹ COS; CHChF and CDChF; ¹²CH₂F₂, ¹ CH₂F₂, ¹²CD₂F₂, and ¹ CD₂F₂; CH₃Br, ¹ CH₃Br, and CD₃Br; ¹²CH₃N, ¹ CH₃N, ¹ CH₃ ¹⁵N, ¹²CH₃ ¹⁵N, ¹²CD₃N, and ¹²CH₂DN; ¹²C₂H₄, ¹²C₂H₂D₂, ¹ C₂H₄, ¹ C₂D₄, and ¹²C₂H₃D; ¹²C₂H₆, ¹ C ₂H₆, and ¹²C₂D6; ²SF₄, SF₄, ⁴SF₄, and ⁶SF₄; and isopropane isotopologues, including ¹²C₃H6, ¹²C₃D6, ¹ C₃H6, and ¹²C₃H₄D₂. This method also is applicable to elements extending across the periodic table.

[0030] Effectively, any surface can be used, provided the temperature of the surface can be maintained at a temperature that is below the condensation temperature of the material (isotope or isotopologue) to be enriched or separated; as noted above, incident beam velocity and incident beam angle can modify these values and differ substantially from equilibrium conditions where the incident gas and surface are at the same thermal temperature. In order to cool the surface to a temperature sufficiently low that an isotope (or isotopologue) of interest adsorbs to it, the surface may be placed in thermal contact with a cooler, such as a cryogenic cooler. By way of illustration, in order to enrich or separate ⁶Ar and ⁴⁰ Ar, the surface should be cooled to a temperature below 31 K. By way of illustration, in order to separate ¾₂ ¹⁶0 and ²H₂ ¹⁶0, the surface should be cooled to a temperature below 140 K, the temperature at which water condenses in a vacuum. The vacuum condensation temperatures of other common materials may be found in the published literature (see, M. P. Collings, M. A.

Anderson, R. Chen, J. W. Dever, S. Viti, D. A. Williams, M. R. McCoustra, A laboratory survey of the thermal desorption of astrophysically relevant molecules. Mori. Not. R. Astron. Soc. 354, 1 133-1 140 (2004)) or may be readily determined experimentally.

[0031] This enrichment or separation method results from the preferential adsorption of a given heavier (or lighter) isotope (or isotopologue) on a substrate and the preferential scattering (reflection) of a lighter (or heavier) isotope (or isotopologue). When both isotopes (or isotopologues) interact with the condensate of the isotopes (or isotopologues), the scattered angle distribution is typically lobular, leading to negligible angular separation of the isotopes (or isotopologues). It will be appreciated by one of skill in the art that the condensate may be crystalline or amorphous, depending on the condensation conditions. The enrichment or separation process occurs at the surface, and straightforward collection techniques used for molecular beam apparatus can be employed. 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).

[0032] The isotopes (or isotopologue) 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 this isotopic (or isotopologue) enrichment technique until a desired level of enrichment is obtained. The same surface or different surfaces can be used in the subsequent passes.

EXAMPLES

Example 1

Methods

[0033] Isotopic ratios of scattered and desorbed argon were measured using an ultra-high vacuum (UHV) scattering apparatus shown schematically in FIG. 1. The instrument has been thoroughly described elsewhere. (See, B. Gans, P. A. Knipp, D. D. Koleske, S. J. Sibener, Surface dynamics of ordered Cu3Au(001) studied by elastic and inelastic helium atom scattering. Surf Sci. 4, 81-94 (1992).) Generally, it is composed of three sections: a differentially pumped supersonic beam source, a chamber that houses the surface, and a rotatable differentially pumped quadrupole mass spectrometer (QMS). The argon beam (with isotopic abundances ⁴⁰Ar = 99.6035%, ⁶ Ar = 0.3336%) was produced by supersonically expanding argon seeded in helium or hydrogen through a 30 μηι diameter nozzle held a temperature of 580 K or 650 K; this expansion then passed through a skimmer and was collimated by two additional apertures. Prior to striking the surface, the beam could be modulated with a 50% duty cycle via a square-wave chopping sequence or a pseudorandom chopping sequence for cross-correlation time-of-flight analysis. The beam-surface interaction parameters could be adjusted by changing the incidence angle (6>j), azimuth (φ), and tilt (χ) of the surface manipulator, as well as the temperature of the surface, which is controlled by a button heater and closed-cycle helium refrigerator to maintain temperatures between 24 and 500 K as measured by a silicon diode.

[0034] The ⁶Ar/⁴⁰Ar isotopic ratio of the scattered beam was determined by square-wave chopping the incident beam while the mass filter setting of the QMS was switched

(modulated) between mass 40 and mass 36 at 0.1 Hz. In order to allow for reproducibility of the argon overlay er on the substrate between runs, -30 minutes of beam dosing (with 50% duty cycle) were allowed before collecting data. The length of dose time necessary depended on beam velocity: more time was needed for experiments with higher beam velocities, since higher velocities correspond to lower sticking coefficients.

[0035] The complementary measurement of the ⁶Ar/⁴⁰Ar isotopic ratio condensed on the substrate was determined by temperature programmed desorption (TPD) experiments.

Desorption measurements were carried out by dosing the argon beam on the surface below the condensation temperature for 5-10 minutes (with 100% duty cycle), blocking the incident beam, and then raising the surface temperature at a rate of 1 K/min while the mass filter setting of the QMS was switched (modulated) between mass 40 and mass 36 at 10 Hz.

Results & Discussion

[0036] Adsorption is governed by the energy transfer between an impinging gas particle and a surface, with physisorption or chemisorption occurring when the kinetic energy of the particle is lower than its binding energy to the surface. This inelastic process is mediated via energy loss to multiphonon excitations. Particles that transfer proportionally more energy to the surface by inelastic processes are more likely to be condensed.

[0037] A mixture of gases in a supersonic molecular beam exhibit seeding, whereby species with different masses will exit the source with the same velocity; this phenomenon does not occur in effusive sources. (See, G. Scoles, Atomic and Molecular Beam Methods Volume I (Oxford University Press, New York, 1988).) In a supersonic beam of argon seeded in helium (10% Ar/90% He), the predominant isotopes (⁴⁰Ar = 99.6035%, ⁶Ar = 0.3336%) will therefore be moving at the same velocity (as confirmed by time-of-fiight measurements) but will have different energies due to their mass mismatch; for the 650 K argon seeded in helium beam used in this experiment, the incident energies of the argon isotopes were ⁴⁰ Ar = 562 meV and ⁶Ar = 506 meV. In a supersonic beam of argon seeded in hydrogen with a nozzle temperature of 580 K, the incident energies of the argon isotopes are ⁴⁰ Ar = 641 meV and ⁶ Ar = 577 meV. In a supersonic beam of argon seeded in hydrogen with a nozzle temperature of 650 K, the incident energies of the argon isotopes are ⁴⁰ Ar = 733 meV and 6Ar = 659 meV. The velocity of the helium seeded beam was 1650 m/s, while the velocities of the 580 K and 650 K hydrogen seeded beams were 1760 m/s and 1880 m/s, respectively. A direct outcome of this energy difference between the ⁶ Ar and ⁴⁰ Ar isotopes is that a supersonic beam of argon impinging upon an amorphous argon condensate grown on a surface below the sticking (i.e., condensation) temperature (measured to be -31 K) will exhibit a preferential sticking of the heavier ⁴⁰ Ar isotope. This is observed in FIG. 2, which shows the background-subtracted reflected (scattered) ratio of ⁶Ar/⁴⁰Ar at three incident beam velocities. The ⁶Ar/⁴⁰Ar ratios of the incident beams for the three velocities (measured by reflecting the beam off the surface while above the condensation temperature) were 0.00264 +/- 0.00001, 0.00272 +/- 0.00001, and 0.00261 +/- 0.00001 for beam velocities 1650 m/s, 1760 m/s, and 1880 m/s, respectively. For the lowest velocity incident beam, the reflected beam was enriched in ⁶ Ar and has an ⁶Ar/⁴⁰Ar isotopic ratio of 0.0058 +/- 0.0003. For the intermediate velocity incident beam, the reflected beam was less enriched in ⁶Ar and has an ⁶Ar/⁴⁰Ar isotopic ratio of 0.0040 +/- 0.0002. For the highest velocity incident beam, the reflected beam was even less enriched in ⁶Ar and has an ⁶Ar/⁴⁰Ar isotopic ratio of 0.00305 +/- 0.00007. The ⁶Ar enrichment factors, shown in FIG. 4A, for the lowest, intermediate, and highest velocity incident beams were found to be 2.2 +/- 0.1, 1.46 +/- 0.06, and 1.17 +/- 0.03, respectively. Here, the ⁶Ar enrichment factor is defined as the scattered 6Ar/⁴⁰Ar isotopic ratio divided by the incident ⁶Ar/⁴⁰Ar isotopic ratio.

[0038] Note that the isotopic ratio above the sticking temperature was below the

6Ar/⁴⁰Ar ratio of natural abundance Ar (0.00335); this is due to an effect of seeding whereby the heavier species in a beam (here, ⁴⁰Ar) is focused into the center of the beam, whereas the lighter mass is pushed to the edge of the beam, leading to a slight enrichment in the higher mass as the incident beam passes through the skimmer and collimating apertures.

[0039] Conversely, the condensate should exhibit enrichment of the heavier isotope: ⁴⁰ Ar. Experimental determination of the condensate composition is possible through temperature- programmed desorption (TPD), in which the surface is dosed with the supersonic Ar beam below the sticking temperature and subsequently heated at a rate of 1 K/min while switching between the two masses of interest on the mass spectrometer. FIG. 3 illustrates a single experimental TPD spectrum. Again, the incident beams had ⁶Ar/⁴⁰Ar isotopic ratios of 0.00264 +/- 0.00001, 0.00272 +/- 0.00001, and 0.00261 +/- 0.00001 for beam velocities 1650 m/s, 1760 m/s, and 1880 m/s, respectively. For the lowest velocity beam, the condensate was enriched in ⁴⁰ Ar and had an ⁶Ar/⁴⁰Ar isotopic ratio of 0.00242 +/- 0.00007. For the intermediate velocity beam, the condensate was further enriched in ⁴⁰Ar and has an ⁶Ar/⁴⁰Ar isotopic ratio of 0.00235 +/- 0.00006. For the highest velocity beam, the condensate was even further enriched in ⁴⁰ Ar and has an ⁶Ar/⁴⁰Ar isotopic ratio of 0.00219 +/- 0.0003. The ⁴⁰ Ar enrichment factors, shown in FIG. 4B, for the lowest, intermediate, and highest velocity incident beams were found to bel.09 +/- 0.03, 1.16 +/- 0.03, and 1.19 +/- 0.02, respectively. Here, the ⁴⁰ Ar enrichment factor is defined as the incident ⁶Ar/⁴⁰Ar isotopic ratio divided by the condensed ⁶Ar/⁴⁰Ar isotopic ratio.

Example 2

Methods

[0040] Isotopic ratios of scattered and desorbed argon were measured using an ultra-high vacuum (UHV) scattering apparatus. The instrument has been thoroughly described elsewhere. (See, B. Gans, P. A. Knipp, D. D. Koleske, S. J. Sibener, Surface dynamics of ordered Cu3Au(001) studied by elastic and inelastic helium atom scattering. Surf. Sci. 4, 81- 94 (1992).) Generally, it is composed of three sections: a differentially pumped supersonic beam source, a chamber that houses the surface, and a rotatable differentially pumped quadrupole mass spectrometer (QMS). The argon beam (with isotopic abundances ⁴⁰Ar = 99.6035%, ⁶ Ar = 0.3336%) was produced by supersonically expanding neat argon or argon seeded in helium through a 15 μιτι diameter nozzle held at 305 K; this expansion then passed through a skimmer and was collimated by two additional apertures. Prior to striking the surface, the beam could be modulated with a 50% duty cycle via a square-wave chopping sequence or a pseudorandom chopping sequence for cross-correlation time-of-flight analysis. The beam-surface interaction parameters could be adjusted by changing the incidence angle (6>j), azimuth (φ), and tilt (χ) of the surface manipulator, as well as the temperature of the surface, which is controlled by a button heater and closed-cycle helium refrigerator to maintain temperatures between 26 and 500 K as measured by a silicon diode.

[0041] The ⁶Ar/⁴⁰Ar isotopic ratio of the scattered beam was determined by square-wave chopping the incident beam while the mass filter setting of the QMS was switched

(modulated) between mass 40 and mass 36 at 0.1 Hz. In order to allow for reproducibility of the argon overlay er on the substrate between runs, 30 minutes of beam dosing (with 50% duty cycle) were allowed before collecting data. [0042] The complementary measurement of the ⁶Ar/⁴⁰Ar isotopic ratio condensed on the substrate was determined by temperature programmed desorption (TPD) experiments.

Desorption measurements were carried out by dosing the argon beam on the surface below the condensation temperature for 20 minutes (with 100% duty cycle), blocking the incident beam, and then raising the surface temperature at a rate of 1 K/min while the mass filter setting of the QMS was switched (modulated) between mass 40 and mass 36 at 10 Hz.

Results & Discussion

[0043] Adsorption is governed by the energy transfer between an impinging gas particle and a surface, with physisorption occurring when the kinetic energy of the particle is lower than its binding energy to the surface. This inelastic process is mediated by multiphonon excitations; a straightforward consequence of this is that a surface with more phonon modes available to excite (i.e., colder) has a greater ability to condense an impinging gas particle. Likewise, particles with less incident energy to be removed by inelastic processes are more likely to be condensed.

[0044] A mixture of gases in a supersonic molecular beam exhibit seeding, whereby species with different masses will exit the source with the same velocity; this phenomenon does not occur in effusive sources (See, G. Scoles, Atomic and Molecular Beam Methods Volume I (Oxford University Press, New York, 1988).) In a supersonic beam of neat argon, the predominant isotopes (⁴⁰Ar = 99.6035%, ⁶Ar = 0.3336%) will therefore be moving at the same velocity (as confirmed by time-of-flight measurements), but will have different energies due to their mass mismatch; for the 305 K pure argon beam used in this experiment, the incident energies of the argon isotopes were ⁴⁰Ar = 65.7 meV and ⁶ Ar = 59.1 meV. In a supersonic beam of argon seeded in helium with a nozzle temperature of 305 K beam, the incident energies of the argon isotopes are ⁴⁰Ar = 218.6 meV and ⁶ Ar = 196.8 meV. The velocity of the pure and helium seeded beams are 563 m/s and 1027 m/s, respectively.

A direct outcome of this energy difference between the ⁶ Ar and ⁴⁰ Ar isotope is that a supersonic beam of argon impinging upon an amorphous argon condensate grown on a surface below the sticking (i.e., condensation) temperature (measured to be -32 K) will exhibit a preferential sticking of the lighter ⁶Ar isotope. This is observed in FIG. 5, which shows the background-subtracted reflected ratio of ⁶Ar/⁴⁰Ar at two incident beam velocities. The ⁶Ar/⁴⁰Ar ratio of the incident beam for both velocities (measured by reflecting the beam off the surface while above the condensation temperature) was constant at 0.00308 +/- 0.00002. For the lower velocity incident beam, the reflected beam was enriched in ⁴⁰ Ar and has a ⁶Ar/⁴⁰Ar isotopic ratio of 0.00245 +/- 0.00033. For the higher velocity incident beam, the reflected beam was less enriched in ⁴⁰ Ar and has a ⁶Ar/⁴⁰Ar isotopic ratio of 0.00261 +/- 0.00033. The ⁶Ar enrichment factors for the lower and higher velocity incident beams were found to be A = 1.25 and AR2 = 1.18, respectively. This effect was not present when the surface was dosed with an effusive source, indicating the usefulness of supersonic beams in isotopic enrichment.

[0045] The two beam velocities were used to manipulate the absolute sticking coefficient of the beam and illustrate the relationship Ri = RF(1-S2) + RcS2, where for this example Ri is the ⁶Ar/⁴⁰Ar ratio of the incident supersonic beam, RF is the ⁶Ar/⁴⁰Ar ratio of the reflected portion of the supersonic beam, Rc is the ⁶Ar/⁴⁰Ar ratio of the condensed portion of the supersonic beam, and S2 is the absolute sticking coefficient of ⁴⁰ Ar. Therefore, increasing the incident velocity lowers the absolute sticking coefficient and one would expect a change in the ⁶Ar/⁴⁰Ar ratio. Indeed, from FIG. 5, one can observe that A > ARL

[0046] Note that the isotopic ratio above the sticking temperature was below the 6Ar/⁴⁰Ar ratio of natural abundance Ar (0.00335); this is due to an effect of seeding whereby the heavier species in a beam (here, ⁴⁰Ar) is focused into the center of the beam, whereas the lighter mass is pushed to the edge of the beam, leading to a slight enrichment in the higher mass as the incident beam passes through the skimmer and collimating apertures.

Conversely, the condensate should exhibit enrichment of the lighter isotope: ⁶ Ar.

Experimental determination of the condensate composition is possible through temperature- programmed desorption (TPD), in which the surface is dosed with the supersonic Ar beam below the sticking temperature and subsequently heated at a rate of 1 K/min while switching between the two masses of interest on the mass spectrometer. FIG. 6 illustrates a single experimental TPD spectrum. FIG. 7 illustrates this ⁶Ar enrichment in the condensate at the two incident beam velocities. Again, the incident beam had a ⁶Ar/⁴⁰Ar isotopic ratio of 0.00308 +/- 0.00002. For the lower velocity beam, the condensate was enriched in ⁶ Ar and had a ⁶Ar/⁴⁰Ar isotopic ratio of 0.00313 +/- 0.00004. For the higher velocity beam, the condensate was further enriched in ⁶Ar and has a ⁶Ar/⁴⁰Ar isotopic ratio of 0.00328 +/- 0.00001. The ⁶Ar enrichment factor for the lower and higher velocity incident beam was found to be Aci = 1.02 and Ac2 = 1.06, respectively.

[0047] 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".

[0048] 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.

Claims

WHAT IS CLAIMED IS:

1. A method for enriching isotopes, separating isotopes, or both, the method comprising:

directing a supersonic beam comprising a gas comprising a first isotope of a first element and a second isotope of the first element onto a surface, the first isotope having a higher incident kinetic energy than the second isotope, wherein either the first isotope or the second isotope is preferentially adsorbed onto the surface, relative to the other isotope; and collecting the gas scattered from the surface, the adsorbate on the surface, or both, wherein the gas scattered from the surface is enriched in either the first isotope or the second isotope.

2. The method of claim 1, wherein the first isotope is preferentially adsorbed onto the surface, relative to the second isotope, and the gas scattered from the surface is enriched in the first isotope.

3. The method of claim 1, wherein the second isotope is preferentially adsorbed onto the surface, relative to the second isotope, and the gas scattered from the surface is enriched in the second isotope.

4. The method of claim 1, wherein the adsorbate on the surface is collected.

5. The method of claim 4, wherein collecting the adsorbate on the surface comprises: heating the surface to a temperature that facilitates the desorption of the isotope that is preferentially adsorbed from the surface; and collecting the desorbed adsorbate isotope.

6. The method of claim 1, wherein the gas scattered from the surface is collected.

7. The method of claim 1, wherein the supersonic beam comprises monomers of the first and second isotopes.

8. The method of claim 1, further comprising passing the supersonic beam through an aperture prior to directing it onto the surface, wherein passage through the aperture enriches the supersonic beam in the first isotope.

9. The method of claim 6, further comprising compressing the gas scattered from the surface, forming a second supersonic beam from the compressed gas, and directing the second supersonic beam onto a second surface, wherein either the first isotope or the second isotope is preferentially adsorbed onto the second surface, relative to the other isotope.

10. The method of claim 9, wherein the first and second surfaces are the same surface.

11. The method of claim 9, wherein the first and second surfaces are different surfaces.

12. The method of claim 5, further comprising compressing the adsorbate desorbed from the surface, forming a third supersonic beam from the compressed adsorbate desorbed from the surface, and directing the third supersonic beam onto a third surface, wherein either the first isotope or the second isotope is preferentially adsorbed onto the third surface, relative to the other isotope.

13. The method of claim 12, wherein the first and third surfaces are the same surface.

14. The method of claim 12, wherein the first and third surfaces are different surfaces.

15. The method of claim 1, wherein the supersonic beam further comprises a second element that is lighter than the first element.

16. The method of claim 15, wherein the first element comprises about 0.1 mole percent to about 20 mole percent of the supersonic beam relative to the second element.

17. The method of claim 1, wherein the supersonic beam further comprises a second element that is heavier than the first element.

18. The method of claim 17, wherein the first element comprises about 0.1 mole percent to about 20 mole percent of the supersonic beam relative to the second element.

19. The method of claim 1, wherein the gas further comprises a third isotope of the first element.

20. An apparatus for enriching or separating isotopes, the apparatus comprising: a pump configured to maintain a low pressure within a chamber; a first beam source configured to provide a supersonic beam comprising a gas comprising a first isotope of a first element and a second isotope of the first element, wherein the apparatus is configured to direct the supersonic beam onto a surface in the chamber;

a temperature control device in thermal communication with the surface and configured to maintain the surface at a temperature at which the first isotope of the second isotope is preferentially adsorbed onto the surface relative to the other isotope; and

a collector configured to collect the gas scattered from the surface.

21. An apparatus for enriching or separating isotopes, the apparatus comprising: a pump configured to maintain a low pressure within a chamber; a beam source configured to provide a supersonic beam comprising a gas comprising a first isotope of a first element and a second isotope of the first element, wherein the apparatus is configured to direct the supersonic beam onto a surface in the chamber;

a temperature control device in thermal communication with the surface and configured to (a) maintain the surface within a first temperature range in which the first isotope or the second isotope is preferentially adsorbed onto the surface, and (b) maintain the surface within a second temperature range at which the adsorbate is desorbed from the surface;

a controller configured to turn the supersonic beam on and off and to direct the temperature control device to change between the first and second temperature ranges; and a collector configured to collect the adsorbate desorbed from the surface.

22. The apparatus of claim 21 further comprising an aperture between the beam source and the surface, wherein the aperture is positioned such that the beam source directs the supersonic beam through the aperture on its way to the surface.

23. A method for enriching isotopologues, separating isotopologues, or both, the method comprising:

directing a supersonic beam comprising a gas comprising a first isotopologue of a compound and a second isotopologue of the compound onto a surface, the first isotopologue having a higher incident kinetic energy than the second isotopologue, wherein either the first isotopologue or the second isotopologue is preferentially adsorbed onto the surface, relative to the other isotopologue; and collecting either or both of the gas scattered from the surface and the adsorbate on the surface, wherein the gas scattered from the surface is enriched in either the first isotopologue or the second isotopologue relative to the adsorbate on the surface.

24. The method of claim 23, wherein the first isotopologue is preferentially adsorbed onto the surface, relative to the second isotopologue and the gas scattered from the surface is enriched in the first isotopologue relative to the adsorbate on the surface.

25. The method of claim 23, wherein the second isotopologue is preferentially adsorbed onto the surface, relative to the second isotopologue and the gas scattered from the surface is enriched in the second isotopologue relative to the adsorbate on the surface.