Note: Descriptions are shown in the official language in which they were submitted.
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
--4--
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
1153986
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~
--10--
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.
