CA1153986A - Isotope separation by solar photoionization - Google Patents

Abstract

ABSTRACT
ISOTOPE SEPARATION BY SOLAR PHOTOIONIZATION
Isotope separation, particularly separation of isotopes of lithium, is achieved by exposing a beam of atoms to radiation that selectively excites atoms of a particular isotope without exciting atoms of other isotopes of that element. The excited atoms are ionized by solar radiation and the ions attracted to an ion collector plate maintained at a negative potential. The atoms not ionized are condensed on a grounded atom col-lector plate. Optionally, the solar radiation not absorbed by the system may be used to generate electricity. Lithium isotopes are useful in nuclear reactors and nuclear weapons.

Description

1153~3136 DESCRIPTION
ISOTOPE S~PARATION BY SOLAR PHOTOIONI ZATION
BACKGROUND OF THE INVENTION
1. Field of the Invention This invention relates to separation of isotopes by solar photoionization and particularly to separation of 6Li and 7Li.

2. Description of the Prior Art There are two naturally occurring isotopes of lithium _ 6Li and 7Li. The natural abundance of the iso-topes is 7% 6Li and 93% 7Li. Both isotopes find several uses in purified form. 6Li undergoes fission on exposure to thermal neutrons, producing tritium. Thus, it has applications in nuclear weapons and, potentially, in fusion reactors. 7Li, in the form of LiOH or Li2CO3, is used as a pH controller in nuclear reactors but has a much larger potential market as a heat exchanger fluid in nuclear reactors. High purity is necessary, because 7Li does not readily undergo fission upon exposure to thermal neutrons.
Laser methods for isotope separation are well known and have been described in both patents and the scientific literature. In recent years, several reviews of laser separation of isotopes have appeared (See e.g.
Sov. J. Quant. Electron. 6, 129 (1976); 6, 259 (1976) and Scientific American 236, 2, 86 (1977)). In partic-ular, laser-induced fractionation and separation of lithium isotopes have been described by Rothe et al.
Chem. Phys. Lett. 53, 74 (1978); 56, 336 (1978). Their , , 1153~6 process involves sequential two-photon ionization of Li2.
Initial excitation and ionization are both produced by laser irradiation (from one or two argon ion lasers).
U.S. Patent 4,149,077, issued April 10, 1979 to Yamashita et al., discloses substantially the same method for laser separation of lithium isotopes.
If large-scale lithium isotope production were based on this process, a great deal of expensive electrical energy would be consumed.
There are several other known processes for separating lithium isotopes - diffusion, mass spec-trometry and electroylsis with an amalgam. These methods are also energy intensive and the amalgam pro-cess has pollution problems as well. Thus, a lithium isotope separation process which requires less energy and/or uses a renewable energy source would be attrac-tive, particularly if it posed minimal pollution prob-lems.
SUMMARY OF THE INVENTION
In accordance with the present invention, a process is provided for separating a particular isotope of an element from a beam of atoms of the particular isotope and at least one other isotope of the element.
The process comprises exposing the beam to electro-magnetic radiation of a predetermined wavelength to selectively excite atoms of the particular isotope of the element to an excited electronic state without substantially exciting atoms of other isotopes of the element, exposing the beam to solar radiation to selec-tively ionize the excited atoms of the particularisotope without substantially ionizing atoms containing other isotopes of the element and separating ions of the particular isotope from the remainder of the beam.
The ions of the particular isotope may be drawn toward a negatively biased ion collector plate, while the other isotopes, depleted or entirely free of the particular isotope, continue undiverted and are con-densed on an atom collector plate. The solar radiation 1153~36 --3~
that is not absorbed by the isotopes being separated may be converted to electrical energy by a conventional solar energy converter. Thus, the process requires minimal input of energy in nonrenewable form and may, in fact, generate more electricity than it uses. Moreover, the process is substantially pollution-free and gen-erates no troublesome by-products.
The process of this invention is particularly suitable for separating isotopes of lithium, although it may be used for separating isotopes of other elements such as uranium.
BRIEF DESCRIPTION OF THE DRAWINGS
Fig. 1 shows certain energy levels of lithium that are significant in one embodiment of the present invention.
Fig. 2 is a top plan view in partial cutaway of an apparatus suitable for practicing this invention.
Fig. 3 is a side elevation of the apparatus of Fig. 2 with sor,le parts removed for clarity.
DETAILED DESCRIPTION OF THE INVENTION
_ This invention concerns a process for separating isotopes of an element using photo-excita-tion. The initial excitation, which may be called "bound state excitation," preferentially excites atoms of a particular isotope without substantially exciting atoms of other isotopes of that element. A laser, such as a tunable dye laser, may provide this bound state excitation. Solar radiation then provides the energy for ionization of the excited atoms. The excited state is preferably within about 3 ev or less of the ionization level to permit efficient ionization with solar radiation. Likewise, the ionization potential of the atom is preferably greater than about 3 ev to prevent appreciable ionization of atoms from the ground state.
The separation of isotopes of lithium, the preferred aspect of this invention, is discussed herein-after in detail.

53~6 Fig. l shows the energy levels of 6Li and 7Li and transitions that are most important in the selective ionization of 6Li by the process of this in-vention. The separations of the levels of the 22P
state are exaggerated for clarity. All the states with n _ 3 are seen to lie within 3 ev of the ionization level.
Ionization of a lithium atom by the process of this invention is accomplished by two or more photo-excitation steps, including one or more bound stateexcitations followed by solar photoionization. The first bound state excitation must be isotope selective, discriminating between corresponding levels of 6Li and 7Li, which are typically about 5 x 10 5 ev apart. If one or more additional bound state excitations are required, the excitation energy must correspond to the gap between levels to be effective and must not excite unwanted atoms from the ground state to avoid contamination of the desired isotope.
The most suitable two-step ionization method of this invention involves selectively exciting Li from the ground state to the 3 P state by 323.268 nm radia-tion, then photoioniziny the excited atoms with solar radiation.
A particularly suitable three-step ionization route to separating 6Li isotopes is carried out as follows:
a) A beam of lithium atoms is exposed to mono-chromatic light of 670.81 nm to excite 6Li to the 22Pl~2 30 state, while avoiding 670.78 nm and 670.79 nm light, which excites 7Li to the 22P1~2 and 22P3/2 levels, res-pectively, b) 6Li atoms are excited from 22P to 32D with 610.36 nm light and c) Li atoms are ionized from the 32D state with solar radiation.
Among the characteristics of lithium that make this three-step route particularly suitable are the 1153~6 large absorption cross-section for 670.81 nm light (~10 12 cm2), high degeneracy of the excited states and large photoionization cross-section (7 x 10 18 cm2) of the 32D state.
Bound state excitation can be provided in at least three ways: lasers, the sun or a lithium vapor lamp. Laser bound state excitation is summarized in the Table, which lists suitable dyes and preferred pump lasers. When the first excitation is to 22P, second excitation to 32S or 32D is preferred, because these transitions have higher cross-sections than the others.
TABLE
Wavelength Pre$erred Pump Transition (nm) ~Y~ Laser 152 S-2 P 670.81 Rhodamine 640 Krypton ion 22p_32D 610.36 Rhodamine 6G Argon ion 22p_32s 812.65 Oxazine Krypton ion 22p_42D 460.3 Coumarin or Argon ion stilbene 22p_42s 497.2 Coumarin 480 Krypton ion At midday, the sun, under typical clear sky conditions, provides to a surface facing the sun about 6.5 mW/m irradiance in a 0.0045 nm band centered at 670.81 nm. If this light is collected with 80%
efficiency by a 4000 m2 collector ~e.g. by an array of heliostats), then the hourly yield of excited (to 2~P) 6Li is 0.4 mole x Q.Y., where Q.Y. is the quantum yield. Before photoionization is energetically possible, excitation from the 22P to a higher level is required. The most suitable wavelengths are listed in the Table and can likewise be provided by the sun.
A preferred source of bound state excitation is a Li vapor lamp, which emits precisely those wave-lengths which are needed to excite 6Li atoms. Of course, it also emits the wavelengths which excite Li atoms. If the lamp contains the natural abundance of Li, 93% of the atoms are the heavier isotope, and consequently the emitted radiation will be richer in the llS3~6 (unwanted) wavelengths which excite 7Li. Even if the lamp contained pure 6Li, however, an unwanted wavelength would be emitted, since the energies of transition for 6Li 22S~ > 22p3/2 and 7Li 22Sl/2 > 22P1~2 nearly coincide.
Except when the bound state excitation is monochromatic (i.e. from a laser), it is necessary to filter out radiation which excites 7Li atoms. A
preferred method for filtering uses a heat pipe containing 7Li vapor and a quenching gas, preferably hydrogen, deuterium or a saturated hydrocarbon having vapor pressure of at least about 10 kPa at room temper-ature, such as methane, ethane, propane or butane.
15 The Li7 vapor strongly absorbs 670.78 nm and 670.79 nm radiation but passes 670.81 nm radiation, which excites 6Li to the 22P state. In the heat pipe, before an excited 7Li atom (in the 22P state) can absorb another photon, it yields its energy to the quenching gas and decays to the ground state. The wavelenyths that excite 7Li from the 22P state to higher states are nearly the same as those that excite 6Li from the 22P
state; thus, it is important that these wavelengths not be absorbed by 7Li atoms in the filter. D2 or CH4 in the range of about 10 kPa accomplishes the necessary quenching of excited 7Li. Reactivity of these gases with lithium is low, and quenching cross-sections are large.
As indicated above, various sources are suit-able for providing the bound state excitation in thepresent process. EIowever, efficient photoionization of the excited atoms is accomplished only by solar radiation. In order to ionize substantially all the 6Li atoms in the atomic beam and leave pure 7Li, high solar radiation intensity (~1 kW/cm2 or, preferably, even higher) is required. With typical bound state excitation intensities ~3 W/cm2), about 60% of 6Li atoms are in the 32D or 32S states. These states are most readily excited and solar photoionization is most efficient from them. At a temperature of about 400C-600C, the Li atoms ~ove at a velocity of about 1.7 + 0.2 x 105 cm/sec. If the solar radiation is concentrated in an area of about 100 cm linear dimen-sion, the incident ionizing photon flux must be at least about 500 W/cm2 in order to ensure that at least about 75~ of the 6Li atoms are ionized. The cross-section for ionizing atoms in the 32D or 32S states is about 7 x lO 18 cm2; thus, most of the incident solar photons are transmitted. The transmitted photons can be reflected back through the atomic beam to nearly double the effective photon flux. Alternatively, the trans-mitted photons can be made to fall on a solar energy converter and generate electricity.
The 6Li atoms that are not ionized by the solar radiation are collected with and thereby contam-inate the 7Li. But low photoionization intensity also causes 7Li contamination of 6Li. The reason involves the near coincidence of the 6Li 22P3/2 and 7Li 22Pl~2 levels. The heat pipe filtering of the bound state excitation prevents direct excitation of the Li 2 Pl/2 level. However, this level can be indirectly excited by the following route, which may be called "radiation trapping":
1) bound state excitation of 6Li to 22Pl~2, 2) further bound state excitation of 6Li from 22Pl/2 to 32D3/2, 32Sl/2 or another higher-lying state,

3) emission from a higher-lying state to the 6Li 22P3/2 level and 7 2 4) resonant transfer of energy from 6Li 2 P3/2 Radiation trapping can be reduced by decreasing lithium vapor density. Alternatively, if the photo-ionization rate is higher than the emission rate, radiation trapping is nearly eliminated, because the higher-lying 6Li states are ionized before they emit.
In addition to contamination of 6Li with 7Li by 1153$~

radiation trapping, another source of this contamination is charge exchange. After a 6Li positive ion is formed by the photoionization process, it is attracted to a negatively biased ion collector plate. To reach the plate it must move some distance through the rest of the atomic beam, most of which is 7Li. For lithium ions with about 50 to 100 V of translational energy, the cross-section for charge exchange is about 2 x 10 14 cm2.
The cross-section decreases with increasing voltage;
thus, higher voltage minimizes this effect, but it increases the energy expended per ion collected. At a beam density corresponding to about 1011/cm3, charge exchange adds about 10% to 20% 7Li contamination. Beam densities below about 1012/cm3 are preferred, because higher beam densities yield higher contamination from both radiation trapping and charge exchange.
The above description has dealt only with the absorption of individual photons, but in the vicinity of strong transitions such as the 670.81 nm 22S 22P and 20 610.36 nm 22p ~ 32D, 2-photon "near-resonant" absorption can occur, if the combined energy of the 2 photons equals the total gap energy (22S ~ 32D). By this effect, highly excited states (such as 32D) are generated.
The effect falls off as l/(v-vr)2, where vr is the 25 frequency of the (670.31 nm) resonant transition. It slightly increases the rate of 6Li excitation and also excites some 7Li, adding another 10% to 20% 7Li contam-ination to the 6Li. Increasing the path length and pressure of 7Li vapor in the heat pipe filter reduces the near-resonant light absorption by increasing pres-sure-broadened single-photon absorption. This slightly decreases the 6Li excitation rate but, depending on the application, the increased 6Li purity might offset this.
Similar apparatus and procedure can be used to selectively excite, ionize and collect 7~i. In that case, of course, the bound state excitation must provide the wavelength appropriate for exciting 7Li and the filter must selectively remove wavelengths which excite ~ , 1~5~3~6 6Li. Thus, an appropriate heat pipe filter could contain 6Li vapor and hydrogen, deuterium or a saturated hydrocarbon having vapor pressure of at least about lO
kPa at room temperature.
Details of the present process can be under-stood by referring to Figs. 2 and 3, which depict a typical apparatus suitable for separatiny lithium isotopes. Vacuum chamber 10, in which the pressure is maintained below about lO 2 Pa, includes oven 11 in its lower portion. Lithium metal 12 is vaporized in oven 11. The vapor emerges from opening 13 in the top and rises through channels 14 formed by corrugated metal foil 15. The foil serves to define and collimate the beam of lithiurn atoms. Ions formed in oven 11 are electrostatically trapped on the foil, but condensation of atoms from the beam is minimized by maintaining the foil at an elevated temperature with heaters (not shown). Lithium vapor lamp 16 or other suitable light source provides a beam of bound state excitation 16a, which passes through window 17 and is filtered by filter 18, unless source 16 is a laser, in which case filter 18 is not needed. After passing through window l9, beam 16a selectively excites atoms of one of the lithium isotopes. Filter 18 is preferably a heat pipe containing a quenching yas and a vapor of the Li isotope that is not to be ionized.
For clarity, lamp 16, window 17 and filter 18 are removed from Fig. 3. Solar radiation l9a from a concentrator (not shown) passes through window 20 and is incident on the atomic beam, ionizing excited atoms.
Solar radiation not absorbed by the atomic beam passes through window 21, is reflected by mirror 22 and passes through the atomic beam a second time. Alternatively, mirror 22 could be oriented to direct the unabsorbed solar radiation to a solar energy converter (not shown).
Windows 20 and 21 are heated by the solar radiation, thus minimizing condensation of lithium atoms on their surfaces. Window 19 is kept warm for the same reason.

1153~

Solar radiation may provide both the bound state excitation and photoionization, in which case but a single (filtered) beam is needed. Ions are collected on ion collector plate 23, which is maintained at a negative potential. Neutral atoms are condensed on grounded atom collector plate 24, which is clamped to the top of chamber 10 for cooling purposes. Oven walls, corrugated foil and collector plates must all be of materials which don't react with lithium. Tungsten, tantalwn, rhenium and molybdenum are examples of suitable materials. Molybdenum is preferred because of machinability and cost considerations.
The apparatus and procedure for separating uranium isotopes by the method of this invention are basically the same as that described above for separating lithium isotopes. Since uranium is a highly refractory metal, heating to the melting temperature is not convenient. Instead, an atomic beam of uranium atoms may be obtained by electron bombardment, as was described by Janes et al., IEEE J. ~uant. ~lectron. QE-12, 111 (1976). Alternatively, the alloy URe2, which melts at a much lower temperature, may be heated in an oven, as described by Carlson et al., J. Opt. Soc. Am. 66, 846 (1976).
Although the spectrum of atomic uranium is far more complex than that of lithium, appropriate levels for laser isotope separation have been determined (L. J.
Radziemski, Jr., et al., Opt. Comm. 15, 273 (1975)). As with the lithium isotope separation, 235U and 238U may be separated by selectively exciting one of the isotopes to a state lying less than about 3 ev below the ioniza-tion level and then ionizing the excited atoms with solar radiation. For example, two-step ionization may be accomplished using bound state excitation of 343.55 nm or 348.94 nm followed by solar photoionization.
Alternatively, three-step ionization can use bound state excitations of 591.54 nm and 545.61 nm or 682.69 and 548.89 nm followed by solar photoionization. If ~",..~

11~3~6 necessary, filtering may be provided by a heat pipe containing a ~uenching gas, such as xenon, and the uranium isotopes that are not to be ionized. Ions of the desired isotope are attracted to an ion collector plate maintained at a negative potential, while atoms of other isotopes are condensed on a grounded atom collector plate.
EXAMPLE
Natural isotopic abundance lithium metal is heated to about 400C in an oven within a vacuum chamber maintained at a pressure of 10 3 Pa. The lithium vapor pressure in the oven is about 10 Pa. Lithium atoms emerge from the top of the oven, rise through channels formed by corrugated molybdenum foil and are exposed to bound state excitation from two dye lasers pumped by an argon ion laser. Radiation of 670.81 nm (from rhodamine 640) and 610.36 nm (from rhodamine 6G) selectively excite 6Li atoms to the 32D state. Solar radiation concentrated from a series of heliostats is focused on and ionizes the excited 6Li atoms. The solar radiation not absorbed is directed to a thermal solar energy converter to generate electricity. For a lithium density of 1011 atoms/cm3, about 1.5 x 1017/sec ions of 6Li are attracted to and condensed on an ion collector plate maintained at a negative potential of about 100 V.
About 2 x 1018/sec atoms of 7Li are condensed on a grounded atom collector plate. Both collector plates are held at a temperature below about 180C.

Claims (19)

I claim:

1. A process for separating a particular iso-tope of an element from a beam of atoms of the particular isotope and at least one other isotope of the element which comprises:
a) exposing the beam to electromagnetic radiation of a predetermined wavelength to selectively excite atoms of the particular isotope of the element to an excited electronic state without substantially exciting atoms of other isotopes of the element, b) exposing the beam to solar radiation to selectively ionize the excited atoms of the particular isotope without substantially ionizing atoms containing other isotopes of the element and c) separating ions of the particular isotope from the remainder of the beam.

2. The process of claim 1 wherein the element comprises lithium.

3. The process of claim 1 wherein the element comprises uranium.

4. The process of claim 1 wherein the source of the predetermined wavelength is a laser.

5. The process of claim 1 further comprising between steps (a) and (b) exposing the beam to a second predetermined wavelength to excite to a higher electronic state excited atoms of the particular isotope without substantially exciting atoms of other isotopes of the element.

6. The process of claim 5 wherein the element comprises lithium.

7. The process of claim 5 wherein the element comprises uranium.

8. The process of claim 5 wherein the first and second predetermined wavelengths are produced by at least one laser.

9. The process of claim 5 wherein the first and second predetermined wavelengths are produced by filtering radiation from a source of polychromatic radiation.

10. The process of claim 9 wherein the source of polychromatic radiation is solar radiation.

11. The process of claim 9 wherein the par-ticular isotope is 6Li and the polychromatic radiation is filtered by passing through a medium comprising 7Li vapor and a gas selected from the group consisting of hydrogen, deuterium and saturated hydrocarbons having vapor pressure of at least about 10 kPa at room temperature.

12. The process of claim 11 wherein the gas comprises deuterium.

13. The process of claim 11 wherein the gas comprises methane.

14. The process of claim 9 wherein the particular isotope is 7Li and the polychromatic radiation is filtered by passing through a medium comprising 6Li vapor and a gas selected from the group consisting of hydrogen, deuterium and saturated hydro-carbons having vapor pressure of at least about 10 kPa at room temperature.

15. The process of claim 14 wherein the gas comprises deuterium.

16. The process of claim 14 wherein the gas comprises methane.

17. The process of claim 11 or claim 14 wherein the source of polychromatic radiation is a lithium vapor lamp.

18. The process of claim 1 or claim 5 wherein the solar radiation incident on the beam has intensity of at least 500W/cm2.

19. The process of claim 1 or claim 5 further comprising the step of converting to electrical energy solar radiation not absorbed in the isotope separation process.