CA1111800A - Isotopic separation - Google Patents

Abstract

ABSTRACT OF THE DISCLOSURE
Method and apparatus for separating isotopes of an element in an atomic or molecular mixture. The isotropic recoil momenta resulting from selective excitation and ioni-zation of an atomic beam is used to mechanically separate a desired isotope from the beam. Similarly, the isotropic recoil momenta resulting from selective photon excitation and promotion of dissociative electron attachment of a molecular beam of uranium hexafluoride is used preparatory to mechanical separation. The isotropic recoil momenta resulting from multi-photon dissociation recoil in a molecu-lar beam of UF6 or SF6 is also used for separation. And, matrix formation of UF6 in HBr so as to collapse the .gamma.3 vibrational mode of the UF6 molecule is used in conjunction with selective isotopic excitation to promote reduction of UF6 molecules containing U235 and facilitate simplified separation.

Description

BACRGROUND OF THE INVENTION.. .
Field of the Invention~
This invention relates to molecular and atomici~otope separation, particularly applicable to separation of --1-- , 1, i"

X ' .
.

- . . , , - ,, . ,, " .. , - .

s~, . ~

. :, ' - ' ` ' ' ^
- ' : , .

Uranium-235 from other uranium isotopes includ~ng Uranium-238.
De~crlption of the Prior Art:
Specific isotopes of a given element are used for many purposes including medical treatment, tracer studies of chemical and biological processes, and preparation of target materials and fuel for nuclear reactors. One of the most common and desirable processes is the separation or enrich-ment of Uranium-235 from other uranium isotopes, particular-ly Uranlum-238. The basic process presently in use for such separation is gaseous diffu~ion, requiring a complex cascad-ing network and large energy inputs.
Alternative methods being considered include centrifugal separation and, more recently, processes taking advantage of the isotope shiits in atomic or molecular mixtures so as to ~orm ions o~ a desired isotope, and then separating the ions Examples of the latter processes include those described in the above-referenced patent and also U.S. Patent No. 3,443,087 in the name of J. Robieux et al. Another U.S. Patent No. 3,558,877 in the name of Jerome Pressman teaches the use of a light beam to deflect selected isotopes to promote separation. Still another U.S.
Patent, No. 3,944,825 in the name of Richard H. Levy et al utilizes selectiYe ionization ~nd an expand$ng plasma to achieve separation. The references, while representing desirable improvements in the art, are not without defi-ciencies. Among the de~iciencies is the necessity for separation of ions from a neutral background requiring electric or magnetic fields formed from complex energy consuming separating devices. Another deficiency is the

-2-8~) power required in some proposed processes to ionize to a sufficiently high energy state. Further, the occurrence of charge exchange reactions can complicate the proposed methods. It is desirable to alleviate the complexities and requirements associated with such separating devices and procedures and to provide further alternatives in the field of isotope separation.
SUMMARY OF THE INVENTION
This invention provides method and apparatus for isotope separation from atomic and molecular isotopic mix-tures, particularly applicable to separation of isotopes of uranium. The invention eliminates reliance upon electric and magnetic field separation means and is generally unaf-fected by charge exchange possibilities. A basis relied upon for several of the preferred embodiments is that recoil momenta are imparted to atoms or molecules undergoing selec-tive irradiation resulting in ionization, dissociation or dissociative electron attachment reactions. Accordingly, a molecular or atomic isotopic gaseous beam having velocity in a given direction can be selectively irradiated such that a desired isotopic ~pecies, because of isotropically distributed recoil momenta, acquires a velocity in a direction different than the direction of the beam. me desired products of these reactions can therefore be collected down~tream of the irradiation area by simple mechan~cal means such as a slit plate upon which the products condense or from which they are continuously remo~ed. me balance of the beam, contain-ing a higher abundance of undesired species, passes through the slit.
In one embodiment a gaseous atomic isotopic beam lil~8~

containing, for example, a low abundance of a first isotope and a high abundance of a second isotope is irradiated so as to ~electively excite the first isotope. The excited first lsotope is further irradiated at a wavelength which takes advantage of the isotope shift and selectively ionizes the isotope and imparts isotropically distributed recoil momenta to the ions. The ions therefore move in a direction differ-ent than the direction of the beam and can be collected down~tream with comparati~ely simple mechanical separation means. Once the ions acquire momenta and a new direction f~om the recoil reaction, the occurrence o~ neutrallzation or charge exchange reactions ls of little consequence since it is the direction o~ the flow, and not the charge, which provides the basis for separation.
In another preferred embodiment the recoll momenta from a dissociative electron attachment reaction provides the ba~is for ~eparation of ~ 35F6 from an isotop~c mixture of UF6 including U238F6. me UF6 molecules are formed as a gaseous beam and the U235F6 is selectively Pxcited by narrow band radiation of a predetermined wavelength. me molecular beam i8 then exposed to a beam of electrons promoting disso-ciative electron attachment and production of U235F5- and other products including a free F(fluorine3 fragment and an energy release. A substantial portion of the energ~Y is distributed as kinetic recoil momenta, thereby changing the direction of flow of the U235F5-. Mechanical separation of molecules containing U235, as ions or neutrals, is then performed downstream of the electron attachment reaction region~
In another similar embodiment, an isotopic mixture 8$~

of UF6 is again formed into a gaseous beam and exposed to multi-photon, preferably 2-photon, irradiation. The first photon selectively excites U235F6 molecules and the second photon promotes dissociation to products including U235F5.
me energy released upon breaking the bond freeing the fluorine is distributed as vibrational motion and recoil momenta, changing the direction of the molecules containing U235 and thereby facilitating mechanical separation.
In yet another preferred embodiment, an abundance of ~ 35 is recovered from an isotopic mixture of UF6 by an isotopically selecti~e irradiation initiated reaction of Br (hydrogen bromide) and U235F6. UF6 and HBr molecules are deposited onto a non-reactive surface in a concentration providing a substantially greater abundance of Br than UF6. me UF6 and HBr are formed ~nto a solid matrlx so that the UF6 molecules are distributed throughout the Br and so that the rotational structure of the V3 mode of v~bration of the UF6 is collapsed. In the condensed state, the U235F6 molecules in the matrix are then selecti~ely excited. By proper ad~ustment o~ the molecular temperature by irradia-tion of the ~3 mode, a reduction of U235F6 molecules to products such as ~ 35F5 and ~ 35F4 is enhanced and, because of the differences in volatility, the other reaction pro-ducts can be driven off as gases while the ~ 35F5 and U235F4 remain as solids on the deposit~on surface.
It will be apparent to those skilled in the art that the various steps summarized above each have inherent ine~ficiencies and present technologioal limitations, It therefore is to be understood that the term "separation" and the like refer to increasing the concentration of a desired 8~

molecule or isotope, such as U235, as compared to the feed concentration. It will also be understood that each of the di~closed embodiments is compatible with multiple repetitive stages, as desired for a chosen end-point concentration.
And, it is also to be understood that while the following description refers to the separation of a desired species, such as Uranium-235, in the presence of an undesired species, such as Uranium-238, the actions and reactions directed toward the desired species can similarly be directed toward the undesired species, also accomplishing separation.
BRIEF DESCRIPTION OF THE DRAWINGS
me advantages, nature and additional features of the invention will become more apparent from the following description, taken in connection with the accompanying drawings, in which:
Figure 1 is a schematic representation of the i50-tropic velocity vector distribution of an ion and electron formed from an atomic beam;
Figure 2 is a side view, in cross-section~ of apparatu~ in accordance with the invention;
Figure 2A is a view taken at IIA-IIA of Figure 2;
Figure 3 is a schematic representation of a photo-dissociation reaction;
Figur~ 4 is a schematic representation of the isotropic velocity vector distribution of the reaction of Figure 3;
Figure 5 is a schematic representation of UF6 molecules distributed in an HBr matrix;
Figure 6 is a top view, in cross-section, of apparatus in accordance with another embodiment of the _~, 8~

invention; and Figure 7 is a schematic representation of photon $nteraction with a matrix surface on a substrate.
DESCRIPTION OF THE PREFERRED EM~ODIMENTS
Isotopic separation of an atomic or molecular isotopic mixture can be achieved by selectively imparting added momentum to the desired atomic or molecular species.
One method o~ obtaining added momentum, taking advantage of the isotopic shift, is through recoil, typically isotropic, upon fragmentation of the desired atom or molecule resulting from selective irradiatlon. Fragmentation, as used herein, re~ers to loss of an electron from an atom or molecule, or breaking apart of a molecule into plural parts. The frag-mentation can be of various forms, forming such products as ions, neutrals and electrons. The recoil phenomenon is common to those reactions typically termed photo-ionization, dissociative electron attachment, and photo-dissociation.
In any of these processes, conservation of momen-tum and energy requires, upon fragmentation, that any excess energy be distributed among the resulting products, in forms such as vibrational, rotational, and translational motion.
The translational motion imparted ~s referred to as reco~l.
PHOTO-IONIZATION
In the photo-ionization process, a selected iso-tope in an atomic mixture is ionized through multi-photon absorption. Two-photon absorpt~on is typically utilized in, for example, the selecti~e ionization of Uranium-235 in an atomic mixture including other uranium isotopes, such as Uranium-238. Uranium is used herein as an exemplary ele-ment, although the procedure is applicable to other elements.

In accordance with this invention uranium vaporor gas is formed into an atomic beam 10 illustrated in Figure 1, The beam i5 preferably collimated and ribbon-shaped. The Uranium-235 in the beam is then s~lectively irradiated, in accordance with the isotope shift, by radia-tion of a predetermined wavelength which raises the elec tronic state but is insufficient to ionize the Uranium-235.
m e initial photon can have an energy of, for example, 3.5 eV. mis initial selective irradiation of Uranium-235 in preference to Uranium-238, represented as a photon hVl Flgure 1, results in a beam mixture in which the population of excited atoms is enriched in Uranium-235. me second photon, represented as h~2, selectively ionizes the excited Uranium-235. This second photon can have any energy suffi-cient to ionize the excited atoms, up to about 6 eV, the ~onization limit for both Uranium-235 and Uranium-238.
When the sum of the two photon energies i8 in excess of the ionization energy of Uranium-235, an electron, represented as e in Figure 1, leaves the Uraniu~-235 atom with a kinetlc energy about equal to the difference between the total absorbed energy and the ionization potential.
The excess momentum vector has a random direction, as de-picted by the circular broken lines of Figure 1. L~ne 12 represents th~ distribution of the velocity vectors of the ionized U235 , and line 14 respresents a similar distribution of the ~reed electrons. The electron line 14 is of larger diameter than the ion line 12 to illustrate that the elec-tron velocity (ve) is substantially higher than the Uranium-235 ion velocity (~235) in accordance with Equations (1) and (Z) wherein m refers to mass and I.P. is 'he ionization potential.
h~ I h 2 ~ I.P. = 1/2 mU ¦v 2351 + 1/2 me IVe I (1) As a result of conservation of momentum:
mU235VU235 meVe (2) It is apparent that the mass of the uranium frag-ment, or any other atomic or ionization fragment, is sub-stantially greater than the mass of the electron. Accord-lngly, a substantial portion of the excess energy is given to the electron. I~ it ls assumed, for example, that the excess energy i~ 1 eV, the electron velocity is:
1 eV = 1.602 x 10~12ergs = 1/2 me ¦ve j2 Ive I = 5.9~1 x 107 cm/sec And, from the conservation Equations (1) and (2):

U2351 m 235 ¦ve ¦ = 137.~ cm/sec-eV

If it ~s further assumed that the atomic beam i8 formed at about 20~0K and a pressure o~ approximately 0.01 Torr, the isotopes in the beam have an average velocity given by:
- (8RT) 1/2 = 4.1g~ x 104 cm/sec where R is the universal gas constant; T is the absolute temperature; and M is the atomic mass.
The change in momentum, the recoil momentum, imparted to the isotopes by the ionization process is iso-tropic; hence the momentum vector has a random direction, as illustrated in Figure 1.

111~8~

As the recoil U235 ions acquire a differential veloc~ty in random directions of about 137.5 cm/sec-eV, most of the ionized U235 will move out of the beam volume, ln a direction oi motion difierent than that of the beam.
Because the added momentum is small compared to the iso-tope's initial momentum in the beam, the flow path of the ~ ~-V235 lons will fan out, maintaining a substantial forward directional component in the direction of the beam, as depicted in Figure 2. With a drlit region downstream of the ~ -ionizatlon of~ ior ex Q le, 300 cm, the U235 recoil ions w~ll diverge to a maximum Or 0.987 cm per eV of excess photon energy. -~
One iorm oi apparatus consistent with carrying out the inventive method is shown in Figure 2. It includes a means for forming the atomlc isotopic gaseous beam 10 such as an oven 16. me oYen ~hown includeq a crucible 18 for holding a BUpply of the ieed~toc~ 20~ such a~ natural uranlum, It further includes mean~ for vaporizing the ~eedstock 20, such as induction heating coils 22 and heat shield 240 To facilitate vaporization the oven is main-tained at vacuum conditions through conduit 26, connected to vacuum apparatus not shown. For uranium ieedstock the vacuum ~5 preferably maintained at about 10 8 Torr and the feedstock heated to approximately 2000K. me atom nux obtained can be in the region oi 1022atoms cm 2sec 1.
In order to form the high temperature uranium gas into a beam of desired configuration, collimat~ng means such as the elongated collimator 28 are used. Although the beam 10 can be of various geometric cross-sections, including circular, the preferred geometry is ribbon-shaped, as shown X

~ 8~ ~S,~22 in Ngure 2A. The beam lO then enters an elongated photo-ionization region 30, where it is irradiated with photons h~l and h~2 of preselected wavelength from one or more lasers 32. The irradiating photons preferably are oriented ~ ;~
to intersect the atomic beam lO at an angle to the beam direction. The efficiency of the irradiation process can be increased by use of mirrors which reflect the photons back ;~
and forth through the atomic beam 10. me efficiency and `
throughput of the system can be similarly increased by utilizlng mirrors and a wide ribbon beam 10 or a plurality of beams arranged side by ~ide.
The photo-ionization region 30 is also maintained at vacuum conditions, about 10 g Torr~ through conduit 34 connected to Yacuum maintaining mean~ not shown. Upon obtaining recoil momentum, the selectively ionized Uranium-235 diverges from the beam 10 in a h~gh vacuum field free area. The ma~nstream of the beam 10 therefore continues through a sllt 36 ~n mechanical separating means such as the conden~ing collecting surface 3~. The region 40 about the collecting surface i~ also maintained at vacuum conditions through conduit ~2. To increase collection efficiency~ the beam 10 mainstream can continue through another axially al~gned photo-ionlzat~on and collection region, It w~
also be noted that since the basis for collection is the differing d~rection Or the Uranium-235 ions, neutralization of the ions prior to impact upon the collecting sur~ace does not affect the separa~ion process. ~he collecting surface can be easily removed when a desired buildup of uranium, increased in the Uranium-235 concentration, is ach~e~ed.
me uranium can then be remo~ed from the collecting surface 8~i~
~5,~22 by chemical, scraping or other well known processes. De-pendent upon the geometry of the apparatus, a buildup of uranium may also occur on the walls ~ of the photo-ioniæation region 30, particularly do~mstream of the point of ionization. The ~ralls 44 are therefore preferably seg-mented to facilitate removal and subsequent processing to recover the uranium product.
DISSOCIATIVE ELECTRON ATTACHMENT
ln addition to photo-ionization, the recoil momen-tum upon fragmentation can also be utilized in dissociative electron attachment reactions. The process and apparatus utilized can be similar to that described above, addition-ally requiring means for exposing the molecular beam to an electron beam.
In a dissociative electron attachment reaction an iso~opic mixture of m~lecu es of, for ex~mple5 UF6 is formed into a gas~ous collimated beam flowing in a ~reset direc~
tion. The beam is then exposed to irradiation at a prede-termined wavelength so as to selectively excite the vibra-tional~J3 or other suitable combination mode of the U235F6 molecules in preference to other molecules such as U23~F6.
The energy of the irradiation should be less than the ioni-zation potential of the molecule and the excitation can be performed through single or multi~le photon absorbing steps.
The UF6 beam is then exposed to a beam of electrons of sufficient energy to selectively promote dissociative elec-tron attachment of the excited UF6 molecules in nreference to other UF6 molecules. ~Jith an asterick denoting an excited state, alternati~ely stated as a state of population in-version, and a double plus denoting excess ener~y, the 45,~22 reackion can be wr~tten as:

I
U235F6 ~ U235F6* e ~ U235F6 ---3 UF5 ~ + 1/2(F2) I E
While the reaction identifies U235F6-, free fluorine, and energy (E) as the products, it will be understood that other product~, such as U235F4- are ~lso possible. The dicsocia- ~
ti~e electron attachment reaction offers some advantages ~-with respect to the abo~e-described photo-ionization pro- -cess. First, it will be recogn~zed that since the fragments include free fluorine atoms as opposed to electrons, the percentage distr~but~on of recoil momentum to the UF5 lon i8 greater than the percentage distribution to a Uranium-23S
atom. Accordingly, the UF5- will move out of the mole¢ular UF6 be~m with a greater separation. Also, the temperatures required for w~rklng with UF6 are much lower and w~uld generally ~e below 1000C *or other ¢ompounds.

In addltion to photo-ionization and disso¢iati~e eleotron attachment reaction~, recoil momentum can al80 be advantageously utilized, similar to the proce~ses discussed `
above, in a photo-dissociati~e procedure in con~unction with a molecular isotopic beam. Here, moleculeR of, for example, -~
SF6 or UF6 are formed into a gaseous collimated molecular beam. The molecular beam i8 selectively exclted with a first photon irradiation o~ the vibrational1/3 or ~u~table combination mode so as to increa~e the excited population in U235F6 concentration. Neither the first or second photon irradiation, again, should be of sufficient energy to, alone, cause dissociation of the U23~F6 or U235F6 molecules.
The molecular beam ~s ~hen further ~rradiated at a predeter-s~a mlned wavelength at an energy sufficient to dissociate the ;
excited U235F6 molecules. It has been established by Rock-wood, S.D. and Rabideau, S.W., in the IEEE J. Quantum Electronics, QE-10, 789 (1974), that two-photon irradiation of SF6 ls achievable, and can be written as:
~ ' hV ,`:
SF6 ~ SF6* ~ SF5 + 1/2 F2 ~
Since the molecular makeup oi SF6 is similar to that Or ~ ;
UF6, lt can be pre~umed that two-photon dissociation is also achelvable with UF6. me process is illustrated in Figure 4. me bond energies of UF6 and SF6 are ~5.9 kcal/mole and 45.6 kcal/mole, respectively.
Upon breaking Or the bond, a substantial portion ~;
of the energy released goes into recoil motion of the frag-ments and, as in the above di~cussions, ~acilitates separa-tion Or the recoll product~ from the mainstream Or the molecular beam by relati~ely slmple mechanical ~eparation structure. Since the recoil products are neutrals, electric or magnetic ~ields need not be used for separation, MATRIX ISOLATION
Figure 6 illu~trates apparatus useful in ~epara-tion of a molecular mixture Or uranium ~sotope~ by matr~x i~olation also utilizlng selecti~e excitation principle~, The basis for the separation is the isolation of UF6 mole-cules in a precisely defined en~ronment characteristically reacti~e abo~e a preselected threshold temperature. The matrix can be formed by conden~ing a molecular species, ~uch as UF6, onto a cold surface at the same time that a ma~rix molecule or atom, inert or reacti~e, such as HBr, is con-den~ed. Whlle varying ratios o~ the matrix constituents can ,8~
45,~22 be used, the procedure as applied in lnfrared spectroscopy usually involves the matrix molecules or atoms at a number den~ity in excess of 50 to 100 times the primary molecule or atom.
In accordance with one embodiment of the inven-tion, UF6 and HBr, preferably in a gaseous state, are codeposited on a cold unreactive surface, thereby creating a thin solid layer of ind~vidual UF6 molecules trapped in an HBr matrix. As illustrated in Figure 5, the matrix forma-tion makes each UF6 molecule generally unable to interactwlth other UF6 molecules. Additionally, the freezing of the UF6 into the matrix substantially lessens the molecular rotational transitions, and specifically collapses the rotational ~tructure of the V3 fundamental vibration at 623.5 cm~l into a single ~harp line at a slightly different frequency (see Bar-Ziv, E., Frieberg, M., and Weis~S., Spectrochemicha Acta, 1972, 2~A, 2025-202g).
The sub~tantial elimination of rotational contri-bution~ greatly increases the efficient utilization of the vibrational frequency difference Or about 0.05 cm between the ~3 vibrational mode5 of U235 and U23g- An efficient selective excitation of the U235F6 V3 fundamental frequency can therefore be reali2ed.
Although other isolating species may be used, HBr is known, through ~York developed at the Union Carbide Corp-oration, Oak Ridge Gaseous Diffusion Plant reported by ~olf, A.S. 9 Hobby, W~E., and Rapp, K.E., in Inor~anic Chemistry 1965~ 4, #6, 755-757, to undergo the follo~ng reactions:
2UF6 + 2HBr--~ 2U~5 + 2HF + Br2 8~i~
45,~22 2UF5 ~ 2H8r~ 2U~4 1 2HF I Br2 It was found that ~rhile liquid UF6 which was allowed to stand overnight in contact wlth liquid HBr at room temperature produced only a small amount of reaction products, the reactions proceed vigorously at 65C. The reactions are therefore strongly temperature dependent.
Although the mechanism of the reaction has not been eluci-dated, in all probability it proceeds along the s~me axis as the ~3 fundamental mode:

F F F
10F U - F + H Br~ U - F + H-F I 1/2Br2 ~ - F
F F F

Accordingly, with U235F6 isolated and surrounded by HBr molecules, selecti~ely exc~ting the U235F6 ~3 mode with sufficient energy to produce an effective vibrational temperature of 65C wlll result in the molecules' reduction to UF5, UF4, and so forth. Once the U235F6 molecules have been selectively reduced in the presence of U23gF6 mole-cule~, separation of the isotopic species is accomplished wlth relative ease. For example, because of di~ferences in volatility, a mtld heating of the matrix under reduced pressure to about 300 Torr will volatize the U23gF6, HBr, HF and ~r2, while the solid U235F5 and some U235F~ remain on the deposition surface. Chemical reactions carr~ed out under matrix i~olat$on conditions are also discussed in l'Direct Synthesis And Characterization Of Debenzenechro-mium(0~ In An Argon M~trix At 1~Kn, John T~r. Boyd, John M.
Iavoie and Dieter M. Gruen, Journal of Chemical Physics, Vol. 60, No. 10, May, 197~; nMatrix Isolation Infrared Study /~-.8~
~5,~22 Of The Reacticn Between Ger~nanium ~apor And Molecular Oxy-gen. The Characterization And Mechanism Of Formation Of Molecular Germanium Dioxide And Ozone", A~ Bos, J. S.
Ogden, and L. Orgee, Journal of Physical Chemistry, Vol.
7~, No. 17, 1974; and "Infrared Spectra Of ~atrix-Isolated Uranium Oxide Species. I. The Stretching Region~, by S. D.
Gabelnick, G. T. Reedy, and M. G. Chasanov, Journal of Chemical Physics, Vol. 5~, No. 10, 1973.
It will be apparent that the disclosed invention offers substantial ad~antages as compared to other laser excitation methods. The exciting radiation is in the in-frared region, which, under present technology, is believed to be the most economical and efficient region for photon production~ Further enhancing the efficiency of the proce-dure i8 that the reactions are carried out in a solid state, thereby enabling a high density of reacting sites. Also, since no io~ization processes are involved, a relatively high density of excited molecules can be achieved ~thout concern for space charge effects. Further, selective i80-topic occurrences are le~s likely to be scrambled by inter-molecular collisions as a result of the dilution factor and general immobility in the solid matrix~ ~nd, since the final separation can merely be the process of removing volatiles from a solid product, complex field separation devices are unnecessary.
The apparatus shown in Figure 6 can be used to carry out the inventive method, and includes four basic regions denoted I throu~h IV. Region I is the matrix f~rma-tion area and includes a series of nozzles 60 and 62 lJhich respecti~ely deposit UF6 an~ HBr from the inlets 6~ and 66 111~8~
i at a molecular number ratio o~ UF6 to Br in the range of 1:50 to l:100. me UF6 and Br are deposited (as liquids or ~-possibly gases) onto a moving substrate 68 which iæ cooled to a temperature less than 100K. me spraying region is ~; preierably maintained at a pressure of about 10 4 Torr or less, through vacuum maintaining means (not shown) connected to conduit~ 70 and 71. Cooling means such as a refrigera~
tion coil 69 ln contact with or near the moving substrate can be utilized to cool the substrate. ~ ;
The substrate is non-reactive with the matrix materials and can include materials such as gold or platinum depo~ited on a copper or other band. Copper is preferred because o~ its good thermal heat transier characterlstics.
me thickneqs o~ the matrix mixture can be ad~usted by j varylng the amount deposlted and/or the speed of the sub-strate 68. The actual mass throughout the entlre system is al~o controlled by the number of spray nozzles, the mass flow through the nozzles, the irradiation absorptlon efii-ciency, and the laser power. me typlcal advantageous denslty diiferentlal between this embodiment and the molecu-lar beam teachings will be apparent from a comparison o~ the number density in the beams at 1 Torr and the number density ln the diluted matrix, For example, ~or a uranium feed having approx~mately 0.7% U235, at 20C and a dilution of 100:1, the number density of U235 as UF6 in the solid matrix is approximately 9.3 x 1016/CC. A molecular beam maintained at 1 Torr, which represents the maximum pressure reasonably applicable to molecular beams, will have a U235 number density of about 2.6 x 10l4/CC. me d~f~erential ccn~tl-tutes approximately a 350 times greater number density in a /
.

~ 5,~22 matrix than in a molecular beam, which ad~antageously allows higher throughput and greater efficiency.
Selecti~e excitation and chemical reaction of the deposited solid matrix occurs in region II. The matrix in region II is preferably maintained at t~mperature and pres-sure conditions similar to region I to allevlate the poten-tial for additional side reactions. A conduit 72 connected to appropriate apparatus can be used for this purpose.
Selective irradiation of the matrix is performed by irradia-tion means such as a laser head 74. In this region, thesystem ef~iciency is effected by the irradiation absorption efficiency and the power density of the laser 74. m e substrate speed can be compatibly ad~usted with the laser power available, being correspondingly faster for a high power and lower for a low power, Since the output of many lasers is polarized~ the laser head i8 preferably aligned, with respect to the sur-face o~ the ~ubstrate, at Brewster's Angle. As illustrated ln Figure 7, this arrangement lessens reflective losses in the laser beam transmtssion. At Brewster's Angle substan-tially f~ll transmission of the radiation i5 into the matrix.
With a platinized or gold coated copper substrate, the beam of photons will be reflected back into ~he matrix as at "A"
with only small losses as at "B". ~eviations from the Brel~ter's Angle compliment at the matrix-substrate inter-face 76 ~111 cause additional re~lections back into the matrix as shown at "A", ~ransmission of ~hotons at Brew~
ster's Angle thus creates a high efficiency configuration for photons subsequent to entering the matrix, as ~ell as assuring essentially 100% penetration of the photon beam 8~i~

into the matri~. Further assisting the photon reaction efficiency, illustrated at "C", is the result that photons re-emitted by an unreacting relaxation are subjected to the same internal reflections, enhancing the reactive absorption probability ~or U235F6 selective excitation.
Upon excitation causing a ~ 35F6 molecular temper-ature in excess of 650C, the reduction process discussed above rapidly occurs, producing U235F5 and U235F4.
m e matrix and substrate then move to region III
where separation of the reduction products occurs. Heating means such as a heating coil 78 warm the substrate and matrix to a temperature in the range of 600C. Because of the differing volatilities of the constituents, this mild heating drives off the unreacted U238F6, excess HBr, HF, and Br2, leaving behind the U235F5 and some U235F4 as a solid residue. m e volatilized compounds are pumped from the warm substrate through apparatus connected to conduit 80.
The substrate then continues to region IV, where the products having an increased U235 concentration are removed and the substrate is further cleaned and dried prior to returning to region I. m e removal o~ de~ired products can merely involve washing, dissolving, or scraping of the products from the ~ubstrate.
The ~ 35F6 molecules left on the substrate in region III should coalesce into stranded particles during vaporizat1on of the other materials. However, ~t can also be carried away as small particulates with the U238F6 and HBr gases. In this event, a particulate filtration system, including a filter 82, can be utilized to trap the solid U235F5 and U235F4.

8'~ ~5,~22 '.
The apparatus also preferably includes baffles ~4 between the respective regions which assist in maintaining the de~ired pressures and separation of the four regions.
The baffles ~2 can be maintained at low temperature thr~ugh co~ling by liquid nitrogen or liquid carbon dioxide.
EXAMPLE
An exemplary throughput utilizing matrix isolation can be shown analytically assumi~g, for example, that the ;~
UF6 deposition nozzles 60 have an effective orifice area of one square centimeter with a UF6 pressure of 1 Torr at t 273K. The nozzles can be distributed over approxlmately 20 cm. Further assuming a substrate velocity of 0.5 cm/sec., a deposition of approximately 2.09 grams per hour of trapped U235F6 is available for photo-excitation, corresponding to ~-approximately 1.41 grams per hour of pure U235~ as sh3wn by the follow~ngs To determine the molecular density (n) of U235F6 where ~ molecules/cm3, at, for example, 273K and 1 Torr (1/760 atmosphere), it i8 known that Pv = n RT
6.023 x 1023 Where v = 1 cm3, T = 273K, P = 1 Torr and R = ~2.057 cm3 atm mole~l K 1, and accordingly n =( 1/760 x 1 ) 6.023 x 1023 = 3.65 x 1016 molecule~/cm3, 2.~57 x 273 corresponding to n = 2.59 x 1014 U235F ~ cm3.
The average velocity (v) of the U235F6 can be ~ ( ~Rr )1/2 wher8 R = ~.317 x 107, T = 273 and ~rM
M = 3~9, corresponding to v = 1.55 x 10~ cm/sec.

8 ~
45,g22 The flux (F) at these exemplary conditions is defined by F = n~ v, or 145S x lO~ x _~ 9 x 101~ = 1.006 x 101~ molecules/cm2 sec. Accordingly, to determine the number of moles and grams deposited during one hour, assum- -ing one cm2 of orifice at l Torr and 273K, the deposition 1 oo6 x 10l~ 3 235 This corresponds to 6 x 10-3 x 3~9 = 2.09 g U235F~hr. In terms of pure U235 th~ deposition rate is 6 x 10-3 x 235 =
1-41 g U235 hr.
The exemplary process can be carried out under reasonable energy requirements. To exemplify the power output required for the laser to perform the selective excitation in a single photon excitation process assuming 1 cm2 of orifice and the other exemplary conditions aboYe, ~t was shown that the deposition rate of U235~6 is 1.006 x 101~ molecules/sec. Accordingly, the minimum number of ~elective photons (h~ ) w~uld be 1.006 x lOlg/sec. The energy (Joule/h~3) of each photon is 1.9g6 x 10-23 x 625 =
1.2~ x 10-2 J/h~3. Accordingly, the minimum desirable energy on a time basis is 1.006 x lO-l~ x 1~2~ x 10-2 =
1.25 x 10-2 J/sec, or a laser power of only l.Z5 x 10-Z
watts, or 12.5 milliwatts. Assum~ng a one ~ercent quantum yield and one percent wall plug efficiency for the laser, the po~ler re~uired is 125 ~atts, or 319 kw/g of separated U235. The electrical power expended ~y the laser for separ-ation of lg U235F6 is approximately 216 k~ and) for a yield of 1 kilo~ram of three percent enriched UF6, the po~rer required is 3.32 ~/kg.

45,~22 There have therefore been described a number of systems and methods useful in isotopic separation of atomic and molecular mixtures. It will be apparent that many ;
modifications and additions are possible in view of the above teachings. It therefore is to be understood that within the scope of the appended claims, the invention may be practiced other than as specifically described.
:

X

Claims (9)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. A method of separating isotopes of an element in an atomic mixture containing a first isotope and a second isotope, said method comprising:
a) producing a gaseous atomic beam of said iso-tope;
b) selectively exciting by radiation of a prede-termined wavelength said first isotope in preference to said second isotope in said beam;
c) selectively ionizing said excited first isotope in preference to said second isotope so as to impart iso-tropically distributed recoil momenta to said ions of said first isotope, thereby changing the direction of motion of said ions from the direction of motion of said beam; and d) separating said ions of said first isotope from said beam by mechanical separation structure.

2. The method of claim 1 additionally comprising the step of neutralizing said ions prior to said separation.

3. The method of claim 1 wherein said separation step comprises passing said beam through a collimating slit and collecting said ions on the surfaces forming said slit.

4. The method of claim 1 wherein one of said isotopes is Uranium-235 and the other is Uranium-238.

5. Apparatus for separating isotopes of an elec-ment in an atomic mixture containing a first isotope and a second isotope, said apparatus comprising:
a) means for producing a gaseous beam of said isotopes;
b) means for selectively exciting by radiation of a predetermined wavelength said first isotope in preference to said second isotope in said beam;
c) means for selectively ionizing said excited first isotope in preference to said second isotope so as to impart isotropically distributed recoil momenta to said ions of said first isotope, thereby changing the direction of motion of said ions from the direction of motion of said beam; and d) means for separating said directionally changed ions of said first isotope from the main stream of said beam.

6. The apparatus of claim 5 wherein one of said first and second isotopes is Uranium-235 and the other is Uranium-238.

7. The apparatus of claim 5 wherein said separ-ating means comprise a collimating slit aligned with said mainstream whereby said directionally changed ions are collected on the surface surrounding said slit.

8. A method of separating U235F6 from a mixture of UF6 molecules including U235 and a second uranium isotope comprising:
a) producing an isotopic gaseous beam of UF6 molecules;
b) selectively exciting by irradiation at a pre-determined wavelength said U235F6 molecules in said beam in preference to said UF6 molecules containing said second uranium isotope;
c) exposing said isotopic beam to a beam of elec-trons of sufficient energy so as to promote dissociative electron attachment of said excited U235F6 molecules in preference to other UF6 molecules thereby producing U235F5-and other products having isotropically distributed recoil momentum in a direction different than the direction of said beam; and d) separating said products from said beam by mechanical separation structure.

9. A method of separating isotopes of uranium in a mixture of UF6 containing U235F6 and U238F6, said method comprising:
a) producing an isotopic gaseous beam of UF6 molecules;
b) selectively exciting by radiation of a prede-termined wavelength said U235F6 molecules in said beam in preference to other UF6 molecules in said beam;
c) selectively adding to said excited U235F6 mole-cules by radiation of a predetermined wavelength sufficient energy to dissociate said excited U235F6 molecules such that the products produced by said dissociation, including U235F5, are imparted isotropically distributed recoil momenta there-by changing the direction of motion of said products from the direction of motion of said beam; and d) separating said products from said beam by mechanical separation structure.