Snavely, Benjamin B.

Solarz, Richard W.

Tuccio, Sam A.

UNT Digital Library

This ib a preprint of a paper intended lor publication in « journal or proceedings. Since changea may be made before publication, thia preprint it made available with the underatandingthai it mil not be cited or reproduced without the permiaaion of the author. UCRL - 76923 PREPRINT &BAJ. 7$OCS3..( LAWRENCE LIVfcRMORE LABORATORY University ol Cattomia/Uvermore. California SEPARATION OF URANIUM ISOTOPES BY SELECTIVE PHOTOIONIZATION Benjamin B. Snavely, Richard W. Solarz and Sam A. Tuccio May 27, 1975 —NOTICE-the United SHIM Gawtnmcm Kralwt ilii Uniltd SUM M I in* Unitrd Sula t'flnfy Hneu'h «ad IXntopnem M-i^numm*, not iny of Dick «i»pto>m, M I i»Y ol ibrit tt*««>ioit, mbconiiKlott. ol lOrt tvpiotttt, «•»*• —r •UDM>, «ipiea ol unpUtd, ot Mmmei i n Irpl kabtiiy <w pttpm*«iy f« il» M W K J . mw*liu»™ a ••[ulDt* •>( mr mfiniatiK».irpi>»iwi,pi.«]"fioi THIS PAPER WAS PREPARED FOR SUBMISSION TO THE SECOND LASER SPECTROSCOPY CONFERENCE MEGEVE, FRANCE, JUNE 23-27, 1975 U) OISTRiBlS7i0.":0F7-|:;; .•;ins n SEPARATION Of URANIUM ISOTOPES BY SUZCTiU PHOTOIORIZATION* Benjamin B. Snavely, Richard W. Solarz and San A. Tuccio Considerable progress has been made during the past year in the development of selective photoionijation processes for the separation of iirantum Isotopes. Uranium enrichment by selective photoionization has been scaled from the microscopic level reported by Tuccio £ ! a! in June 197A to the milligram per hour rate^ '. This progress has been supported by developments in the understanding of the uranium spec­trum resulting from the application of Unable dye lasers as spectroscopic tools. In this paper, recent results of experiments on the laser photoseparation of uranium isotopes are reported. The high density of uranium encrqy levels in the 10,000 cm"' to 50,000 cm energy range provides the opportunity for selective photoionization of uranium by a number of different schemes. Some of these are represented in Figure 1. In Figure )A is shown a two-step process in which the Isotopically selective step, hv,, 1s followed by the absorption of photons with energy hv ? to produce ions of the de­sired isotope. In the diagram, hv, is shown terminating on an autoionization state or discrete state, above the ionization continuum of the atom to take advantage of the relatively large absorption cross section associated with these states. r T'-T ^ • i " " . Isotope ihitt i 1 T V I so tops shift i isoiops sni - f + FIG. 1 Alternat ive Excitation Schemes for the Isotopically Selective Photoionization of Uranium A potentially serious problem with the two-step photoionization process is that a small value of the Dhotoionization cross section may prec):--ic efficient ut":7ir.i-tion of the laser producing hv,. This could seriously impair the economics of yjc~\ a process. A variation of the two-step process is shown in Figure IB, In this case, photons of energy hv 2 excite the atoms to an energy level slightly below the *This wort: was performed under the auspices of the U. S. Energy Research and Development Administration 2 ionization continuum. lon-2ation is accomplished by the absorption of a third photon of energy hv^. This scheme w..s suggested by Nebonzahl and Levin* ' as a niout>b i.^  which small photoionizatian cross sections could be utilized effectively in a separa­tion system. By using an efficient infrared laser, such as a CO? laser, to provide the photons for the ionization step, process economics may not be affected adversely by a small photoionization cross section. Another type of three-step process is diagrammed in Figure 1C. If the photons, or energy levels, can be properly chosen, it would be possible to Ionize with photons approximately equal energy, that is: ftv, = hv, = hv-, . This places the required laser wavelengths in a spectre! region which is readily ac­cessible to presently available tunable lasers. A variation on this scheme, Figure ID, utilizes the infrared photon hvft to overcome a small photoionization cross section. In all of the diagrams, photons have been used to accomplish the ionization. If, however, the desired isotope is excited to a state which lies slightly M o w the ioni­zation energy, there are alternatives for the Ionization step. These include: field ionization by an electr'c field or by electron Impact. These techniques have also been proposed as means of jvercomlng effectively small photoionization cross sections. Continuum y//'1 " ' • ' • . '>'///. Krypton •U400A 3507 A, 3&MA . ion laser, 3781A * GiDund le*oi, P * 45% P • population @ zuiQ'K FIG. 2 Excitation Scheme Used in the Photoseparation of Small Macroscopic Quantities of Enriched Uranium The excitation scheme used in an experiment carried on at Liver-more to separat-macroscop^c quantities of enriched uranium is show in Figure 2. Tv.-o Users dre u''-.'. for the selective photoionization. A xenon ion laser operating 3781 A excites ura­nium atoms from a metastatic state at 620 cm" above the ground state, to an energy level at 27,OS8 cm" , approximately one-half of the ionization energy. Excited atoms 3 arc then ionized by the ultraviolet output of a krypton i'jo laser operating simul­taneously on two lines at 3507 A and 3504 A. These lines were chosen for conveni­ence and do not necessarily carry the atom to an autoionization state. The terminal state is considerably higher in energy than any of the autoionization states which have been found to date. Therefore, the cross section for this process is ex--17 2 pected to be of the order of 10 cm. The xenon laser i.as chosen for the iso-topicaUy selective excitation step because of the fortuitous coincidence between the xenon laser line at 3781 A and a uranium-atomic transition. In the experiment, the atomic vapor source was operated at a temperature of 2G00°K. At this temperature,, approximately 27X of the atoms are in the 620 cm"' state. The ground state contains approximately 45% of the atoms. If a selective photoionizatlon process is to utilize the greatest fraction of the uranium atoms in the vapor stream, then it is necessary to provide a second laser for excitation front the Ground level. Since the purpose of this experiment was proof of principle, and not large scale operation, a laser accessing the ground state was not used. FIG. 3 Geometry of the Photoseparation System The geometry of the separation experiment is shown in Figure 3. The uranium oven produces a stream of uranium atoms In a fan-shaped beam. The xenon laser beam, passing near the slit as show, produces the isotopically selective excitation. Ioni2ation of the excited atoms is then produced by the krypton laser beam traveling 235 in ihc- opposite direction. The II ir^ thus produced, are collected on « beryl­lium plate by an electric ficltl produced by tnaintainino the collector pi''1" *•. :* negative potential of 1500 volts with respect to the uranium oven. Neutral atoms are collected on a plate provided for the purpose as shown. The xenon laser beam power ir this experiment was approximately 70 -\-M, The 4 Increased by placing the uranium vapor within the cavity of the krypton la'^r. ]i, order to accomplish this, the output mirror of the xenon laser served as one of tt,' reflectors for the krypton laser. The circulating power 1n the Ionizing laser beam was between 30 and 50 watts. The deposition pattern of uranium ions on the collector plate is shown in Figure 4 as determined from densitometer tracings of an autoradiograph of the plate. Approximately 1 mg of material enriched to slightly over 1% 1n U was deposited on thii plate. In subsequent experiments, approximately 4 mg of uranium enriched tr between 2.5 and 3% has been obtained during a run lasting approximately 2 hours. Relative nnmlwr/«m FIG. 4 Uranium Ion Distribution on the Beryllium Collector Plate Calculations have been carried out to describe the ion trajectories in the collector system. The two'band pattern observed on the beryllium plate is pre­dicted by these calculations and results from distortion of the electric field by the neutral atom collector structure. At the right hand side of Figure 4 is shown the calculated trajectory distribution os a function of distance across the be-yl-lium collector plate. The quantity and the enrichment of material collected in these experiments was determined primarily by the power of the krypton laser. The number of thermal ions produced by the hot uranium vapor source corresponds to about 0.1% of the total num­ber of atoms in the vapor stream. With the laser powers used in this experiment, enrichment greater than approximately 32 cannot be expected. There was no provi­sion in these experiments for suppression of thernally generated ions to in:,)i*ove the enrichs'icnt. Knowledge of uranium atom spectroscopic parameters is essentia? in the assess­ment of the economic feasibility of enrichment processes based on uranium metal vapor. The value of t^e photoionizat'ion cross section is of particular importance 5 in this regard and a system has bet-n assembled to study Lhe [jhotoioni<?dtior. s|.ccLri.i , Jn this apparatus, a uranium beam is produced by an atomic vapor source bar.e<l on tho alloy URe2* K The uranium beam is excited to a selected energy level by the tun­able output of a CW dye laser. Ihe dye laser, with an output power of about 30 mW, produces sufficient intensity 1n the beam region to nearly saturate the chosen tran­sition. In a scheme which is very similar to that used to demonstrate selective tvo-step photoionization, a second, ultraviolet, optical beam intercepts the excited uranium beam. The ultraviolet beam is produced by a 2500 watt mercury arc filtered by a monochromator. As the ultraviolet wavelength is scanned, the rctc at which uranium ions are produced is measured. Uranium 238 ions are filtered out from spuri­ously produced ions of other types by a quadrupolo mass filter. By measuring tho 238 U ion current as a function of the ultraviolet photon wavelength, it is possible to determine the photoionization spectrin) related to the selected state of uranium. i - 5,6, or 7 (txlri paiity) 5 in I in b i 1 1 J = 6 (even parity) OcnT1 J< f i (oddparity) FIG. 5 High Lying Odd Parity Levels of Uranium A photoionization spectrum obtained at a resolution of 8 A is shown in Figure 5. This spectrum represents photoionization from a state at 16906 cm" of the MT level. This is an even-parity state reached from the odd parity L°6 ground state by the tunable CW dye laser. The M ? level is a level of the f^dsp electron configuration. From this state, it is expected that autoionization states 3 2 can he reached corresponding to the electron configuration f dp . Several distinct peaks are seen in the photoionization spectrum. In the ex­periments reported in Reference 1, an average cross section for tho reoicn f.v™1 3200 A to 2100 A of about 10" cm" was deduced for photoionization transitions from the same state. From the spectrum of Figure 5, it appears that the peak cross sec­tion for transitions to autoionization states from the M ? level is approximately 6 10 tin*"-„ larger than the l>adqround crc^s section ..it *.hort vhtvcletuiths. Tin- n.'-plieJ VJI'JC of -v 10" cm is large enough to be used in large scale ur<miui- ry.ira tion systems. Subsequent measurements at higher resolution have been obtained and it ha\ been found that the peaks shown in Figure 5 have a half viidth (FVJHM) of approxirilcly 3 A. Photoionization spectra such as that shown in Figure 5 yield ouito at*.urate values of ionization potential of the uranium atom. The value deduced frow these experiments is 6.15 + .02 eV. Second Photon Wavelcngtn Irw) FIG. C Autoionization Spectrum from the M ? Level at 16,904 cm"1 in Uraniu.i One possible excitation scheme for selective photoionization involves a two-quantum transition to a/) isotopically selected state. There are practical advan­tages for such a process in that the effective linewidth of the uranium transition could be narrowed to less than the doppler linewidth' . This, in turn, could re­sult in more efficient utilization of the excitation laser. To take advantage of such an excitation scheme, odd-parity levels of the uranium atom lying between 30,000 cm and 40,000 cm would be required. Very few odd-parity tei ns in this energy region have been observed and experiments have beon undertaken to identify such levels^ ', The experimental arrangement for these experiments was similar to that used in studying the photoionization spectrum. In this case, however, a tun­able nitrogen-laser-puiriped dye-laser was used for the excitation step. Ii; o>.>' -"i of experiments an even-parity level at 17,360 cm was e;:citM using the CW dye laser. Tne pulsed dye laser operating at over a wavelength range of approximately 5800 to 5G60 A was used in the second and subsequent ionization step excitations. Transitions to odd parity states are readily observed since the same photons which populate the odd parity levels can also produce ionizing transitions from the odd parity states. Assuming relatively little structure in the photoionization spectrum in the energy region accessed by the photons used, the ion current produced as a function of pulsed laser wavelength shf ild show peaks corresponding to the odd-parity terms. Six of the odd parity 1 >els found by transitions from the 17,360 a f level are shown in Figure 6. Utilizir, this information, experiments are now under­way to measure the two-photon cross sections for transitions from the ground state to these high lying odd-parity states. Several experiments of Importance in the assessment of the feasibility of metal vapor processes for laser photoseparation of uranium isotopes have been described. At this point, there are many additional experiments which need to be carried out before the problems of scaling and economic feasibility of metal vapor processes can be fully adJressed. References: 1. S. A. Tuccio, J. W. Dubrin, 0. G. Peterson, and B. B. Snavely, "Two-Step Selective Photoionization of " 5 U in Uranium Vapor", presented at VIII i.-.tomationa! Conference on Quantum Electronics, San Francisco, June 1974. 2. S. A. Tuccio, R. W. Foley, J. W. Dubrin, and 0. Krikorian, "Macroscopic Isotope Separation of Uranium by Selective Photoionization", presented at the Conference on Laser Engineering and Applications, Washington, D.C., May, 1975. 3. I. Nebenzahl and H. Levin, "A Process for Isotope Separation", Patent*DT2312194, Federal Republic of Germany. 4. T. w". Ha'nsch, K. C. Harvey, G. Meisel, and A. L. Schawlow, Optics Coirai. 11, 50 (1974). • ~ 5. Richard Solarz, Lee Carlson and Charles Hay, "Autoioni-tation Spectra and High Lying Odd Parity Levels in Uranium Vapor", presented at the Conference on Laser Engineering and Applications, Washington, B.C., Hay, 1975. 

Separation of uranium isotopes by selective photoionization

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Separation of uranium isotopes by selective photoionization

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