Laser Isotope Separation Research Articles

IR multiphoton absorption and isotopically selective dissociation of CHCIF2 in a herriott multipass cell

The small absorption is a major problem in isotopically selective IR multiphoton dissociation. Usually the radiation diverges before it is fully absorbed. To solve this problem, we have demonstrated the application of a refocusing (Herriott) multipass cell. It can generally help in laser isotope separation to use the photons more efficiently. Employing such a cell and a Q-switched CO2 laser at high repetition rate, the dissociation yield of CHCIF2 was 23 times higher than in a single pass. The number of passes used (up to 60) was more than is conventionally possible in such small cells. The increased number was permitted by making use of spherical aberration. With 18 passes, we also measured the multiphoton absorption for various wavelengths and pressures, in part separately for12CHCIF2 and13CHCIF2, and also for two-wavelength irradiation. Appropriate change of pressure or wavelength increased the absorption. But the corresponding increase of the dissociation was larger in every case. To explain this and other observations, we invoke the molecular distribution over the energy levels.

High precision and spatial resolution sulfur isotope analysis using MILES laser microprobe

The MILES ( Micro In situ Laser Extraction System) laser microprobe permits high spatial resolution (< 10 −3mm 3, or < 0.2 μmol S) in situ sampling of geological material for sulfur isotope analysis. Sulfides are combusted in F 2 by absorption of CO 2 laser radiation and converted to sulfur hexafluoride (SF 6). The product SF 6 is purified by cryogenic distillation. In combination with a high-sensitivity dualinlet isotope ratio mass spectrometer, sulfur isotope analyses of powders of pyrite, galena, and sphalerite yield δ 34S cdt values with a high precision, ranging from 0.03 to 0.09%. The sulfur isotope ratios measured are accurate and exhibit no matrix-dependent sulfur isotope effects over the range of 62%. A minimum F 2 pressure of 20 kPa (for MILES) is required to mediate against small isotopic fractionations between multiple sulfur species apparently caused by laser isotope separation and/or reaction with oxygen during analysis. The precision and accuracy of δ 34S cdt values from in situ analyses are good ( $ ̌ 0.2‰), but isotopically homogeneous working standards or intercomparison materials are not available thus far. Sulfur isotope ratios derived by conventional-SO 2 and laser-SF 6 are well correlated ( r 2 = 0.99999), but a slope different from unity ( m = 1.035) arises, probably due to inadequate corrections to SO 2 data for oxygen isobaric interferences. Sulfur isotope isopleths in a large, cubic metamorphic pyrite porphyroblast, determined from 79 in situ analyses, are discordant to crystallographic zonation. Concordance between crystallographic and isotopic zonation needs be tested using high precision and spatial resolution analyses such as those described here. Sampling crystallographic zones in minerals can result in erroneous conclusions if isotopic and crystallographic zoning are not coincident.

Short-cavity high-repetition-rate CO&lt;inline-formula&gt;&lt;sup&gt;&lt;roman&gt;2&lt;/roman&gt;&lt;/sup&gt;&lt;/inline-formula&gt; laser

We report on the construction and optimization of a TEA CO 2 laser with a discharge volume of 15 cm 3 and cavity length of 20 cm. Such a short cavity facilitates single longitudinal mode operation. A roots blower is employed to achieve the necessary gas flow rate for highrepetition-frequency operation in a compact design. Output has been obtained at 1 kHz and a stable discharge to a repetition rate of 2 kHz has been demonstrated. The laser is part of a program aimed at the development of an efficient laser system for molecular laser isotope separation. Additional applications in materials processing are envisioned.

Near-Resonant Effects on Propagation of Frequency-Swept Laser Pulse in Atomic Vapor Laser Isotope Separation

Near-Resonant Effects on Propagation of Frequency-Swept Laser Pulse in Atomic Vapor Laser Isotope Separation

Numerical study of propagation of laser pulses in a multilevel photoionization system

Propagation of laser pulses in a multilevel photoionization system is studied for atomic laser isotope separation. For this purpose a simulation code called cealis-p has been developed. Through numerical solutions of coupled Maxwell–Bloch equations we illustrate the propagation characteristics and the influence on the photoionization efficiency. It is shown that near-resonant interactions with the nontarget isotope will significantly alter the laser-pulse shape, carrier frequency, and propagation velocity, resulting in degradation of the photon-utilization efficiency and ionization efficiency.

Continuously tunable CO&lt;inline-formula&gt;&lt;sup&gt;&lt;roman&gt;2&lt;/roman&gt;&lt;/sup&gt;&lt;/inline-formula&gt; MOPA chain for a molecular laser isotope separation pilot plant

A continuously tunable 3-atm mixed isotope CO 2 oscillator power amplifier (MOPA) chain is developed for a molecular laser isotope separation pilot plant. The characteristics of this laser such as tunability, bandwidth, and output energy are reported. A closed loop gas flow system with a catalyst is employed and its performance is reported.

Limitation of CO&lt;inline-formula&gt;&lt;sup&gt;&lt;roman&gt;2&lt;/roman&gt;&lt;/sup&gt;&lt;/inline-formula&gt; laser-induced gas breakdown

To obtain economical extraction of ²³⁵U in the molecular laser isotope separation (MLIS) process, 16-μm laser beams must be generated in a parahydrogen Raman cell with high repetition rates and sufficient intensity. Because the intensities of the 16-μm laser beams are dependent on the intensity of the incoming pump laser beams, the intensity of the CO₂ lasers must be kept as high as possible. The maximum intensity has, however, been found to be controlled by the onset of gas breakdown in the Raman cell at too low a level for efficient Raman conversion. Through tests, the origin of gas breakdown in a 2-kHz-repetition-rate Raman cell is identified as particle contamination. The effect of the degree of contamination is determined and compared with experimental results. Conditions are set and modifications implemented on the Raman cell to ensure efficient Raman conversion.

Small cell experiments for electrolytic reduction of uranium oxides to uranium metal using fluoride salts

Electrolytic reduction of uranium oxide was proposed for the preparation of uranium metal feed for the atomic vapor laser isotope separation (AVLIS) process. A laboratory cell of 25-cm ID was operated to obtain additional information in areas important to design and operation of a pilot plant cell. Reproducible test results and useful operating and control procedures were demonstrated. About 20 kg of uranium metal of acceptable purity were prepared. A good supply of dissolved UO2 feed at the anode is the most important controlling requirement for efficient cell operation. A large fraction of the cell current is “nonproductive” in that it does not produce a metal product nor consume carbon anodes. All useful test conditions gave some reduction of UF4 to produce CF4 in addition to the reduction of UO2, but the fraction of metal from the reduction of UF4 can be decreased by increasing the concentration of dissolved UO2. Operation of large continuous cells would probably be limited to current efficiencies of less than 60 Pct, and more than 20 Pct of the metal would result from the reduction of UF4.

The internal and translational energy dependence of molecular condensation coefficients: SF6 and CCl4

In this paper we describe a series of beam–surface scattering experiments which examine the internal and translational energy dependence of the molecular condensation probabilities for CCl4 or SF6 colliding with their respective condensed phases. It has been conclusively shown that thermal excitation of a polyatomic molecule’s rotational and vibrational degrees of freedom can inhibit its probability of sticking upon impact with a cryogenically cooled surface. This effect is most pronounced in the limit of low incident kinetic energy, and essentially vanishes at higher velocities. As part of these experiments, we have also obtained the angular and velocity distributions for reflected SF6 and Kr which have been used to examine the energy and momentum exchange of these gases with their respective condensed phases. Our findings suggest that heterogeneous laser isotope separation schemes based on precollision molecular excitation may warrant further investigation.

Industrial Exposure to Uranium Aerosols at Laser Enrichment Processing Facilities

Comprehensive studies of the radiotoxicological risk at a new uranium enrichment processing facility using laser isotopic separation were motivated by the formation of industrial uranium oxide aerosols identified as UO 2 +U metal and U 3 O 8 . Taking the new ICRP recommendations into account, this study was aimed at determining for each of those uranium oxides the physicochemical and biokinetics parameters which are required in order to calculate committed dose per unit of intake (DPUI)

Industrial Exposure to Uranium Aerosols at Laser Enrichment Processing Facilities

Comprehensive studies of the radiotoxicological risk at a new uranium enrichment processing facility using laser isotopic separation were motivated by the formation of industrial uranium oxide aerosols identified at UO2+Umetal and U3O8. Taking the new ICRP recommendations into account, this study was aimed at determining for each of those uranium oxides the physicochemical and biokinetics parameters which are required in order to calculate committed dose per unit of intake (DPUI). Activity median aerodynamic diameter was measured between 5.2 and 10 µm with the presence of fine particles under 1µm reaching, in some cases, up to 20%, concentration at the workstation varied from 1.8 to 125 Bq.m-3 and biological half-times were calculated from in vitro and in vivo experiments, leading to a main biological half-time of 77 days for UO2+Umetal particles and 187 days for U3O8 particles. DPUI values using these experimental results were assessed between 1.7x10-6 and 2.9 x 10-6 Sv.Bq-1 for UO2+Umetal and between 1.8x10-6 and 3.2x1-6 Sv.Bq-1 for U3O8.

Selective Excitation of Branched Vibrational Ladder in Uranium Hexafluoride Laser Isotope Separation

Theoretical investigation was made on the dynamics of initial excitation process in molecular laser isotope separation for uranium hexafluoride (UF6) based on the generalized N-level density-matrix equation derived by Goodman et al. Branched vibrational model due to anharmonic-splitting components for the v 3 mode of UF6 molecule were formalized and phase interferences of density-matrix elements were considered to analyze the selective excitation. Because of a power-broadening effect, no more than 0.05 J/cm2 fluence of a laser pulse completely masks high enrichment peaks in broadband (δ=2GHz) pumping case. Even when matching the laser frequency with v 3 band (n=0→1) of 238UF6, the enrichment factor (α ≡R product/R feed) R≡[235UF6]/[238UF6]; R is the abundance ratio and [ ] means the mole fraction.) does not decrease. A narrow spectral linewidth (δ=400 MHz) is shown to be essential to achieve a high concentration ratio in the system with a branched N-level ladder as expected from a general two-level system. The exciting frequency, which does not necessarily accord with the v 3 band of 235UF6, gives the maximum enrichment peak. Both the detuning of the optimum frequency and the sharp enrichment peak suggest a direct excitation due to multiphoton resonance. Spectral stability of the laser pulse is also required to excite only the desired isotope for the system with complicated anharmonic-splittings.

Sodium-layer laser-guide-star experimental results

We describe a series of experiments to characterize the sodium-layer guide star that was formed with the high-power laser developed for the Lawrence Livermore National Laboratory Atomic Vapor Laser Isotope Separation program. An emission spot size of 3.0 m was measured, with an implied laser irradiance spot diameter of 2.0 m. The rms spot motion at the higher laser powers, with active beam-pointing control, was less than 0.5 arcsec and had little effect on the observed spot size under these conditions. We measured the resonant backscatter from the sodium layer as a function of laser power to obtain a saturation curve. With a transmitted power of 1100 W and an atmospheric transmission of 0.6, the irradiance from the guide star at the ground was 10 (photons/cm2)/ms, corresponding to a visual magnitude of 5.1. The implications for the performance of wave-front sensors with a laser guide star of this magnitude and resulting closed-loop adaptive-optics performance are discussed.

Design, layout, and early results of a feasibility experiment for sodium-layer laser-guide-star adaptive optics

We describe the design and the early results of a feasibility experiment for sodium-layer laser-guide-star adaptive optics. Copper-vapor-laser-pumped dye lasers from Lawrence Livermore National Laboratory’s Atomic Vapor Laser Isotope Separation program are used to create the guide star. The laser beam is projected upward from a beam director that is located ~5 m from a 0.5-m telescope and forms an irradiance spot ~2 m in diameter at the atmospheric-sodium layer (at an altitude of 95 km). The laser guide star is approximately fifth magnitude and is visible to the naked eye at the top of the Rayleigh-scattered laser beam. To date, we have made photometric measurements and open-loop wave-front-sensor measurements of the laser guide star. We give an overview of the experiment’s design and the laser systems, describe the experimental setup, show preliminary photometric and open-loop wave-front-sensor data on the guide star, and present predictions of closed-loop adaptive-optics performance based on these experimental data. The long-term goal of this effort is to develop laser guide stars and adaptive optics for use with large astronomical telescopes.

Selective Exciation of Branched Vibrational Ladder in Uranium Hexafluoride Laser Isotope Separation.

Theoretical investigation was made on the dynamics of initial excitation process in molecular laser isotope separation for uranium hexafluoride (UF6) based on the generalized N-level density-matrix equation derived by Goodman et al. Branched vibrational model due to anharmonic-splitting components for the ν3 mode of UF6 molecule were formalized and phase interferences of density-matrix elements were considered to analyze the selective excitation. Because of a power-broadening effect, no more than 0.05 J/cm2 fluence of a laser pulse completely masks high enrichment peaks in broadband (Δν=2 GHz) pumping case. Even when matching the laser frequency with ν3 band (n=0→4) of 238UFG6 the enrichment factor (α≡Rproduct/Rfeed, R≡[238UF6]/ [238UF6] ; R is the abundance ratio and [ ] means the mole fraction.) does not decrease. A narrow spectral linewidth (Δν=-400 MHz) is shown to be essential to achieve a high concentra-tion ratio in the system with a branched N-level ladder as expected from a general two-level system. The exciting frequency, which does not necessarily accord with the ν3 band of 235UF6, gives the maximum enrichment peak. Both the detuning of the optimum frequency and the sharp enrichment peak suggest a direct excitation due to multiphoton resonance. Spectral stability of the laser pulse is also required to excite only the desired isotope for the system with complicated anharmonic-splittings.

Determination Method of Dissociation Yield from HF Fluorescence in Infrared Multiphoton Dissociation of Supercooled UF6.

A method is presented for the determination of the dissociation yield of UF6 in infrared laser isotope separation of uranium. The fluorescence intensity from vibrationally excited HF molecules in the dissociation with 16 μm infrared radiation is related to that in the dissociation with 266 nm ultraviolet radiation for which the absorption cross section and the quantum yield are known. One can calibrate the dissociation yield of UF6 from the fluorescence intensity in 16 μm irradiation, using the experimental result that the fluorescence signal linearly increases with the pulse energy of the 266 nm beam, and the relation derived theoretically between its energy and the number of dissociated molecules. The authors discuss how the fast thermalization of F atoms in both irradiation cases guarantees such a calibration method in spite of the difference in the translational energy of dissociation fragment F atoms.

レーザ法ウラン濃縮技術開発の現状

レーザ法ウラン濃縮技術開発の現状

Modeling of Multifrequency Infrared Multiphoton Dissociation for Laser Isotope Separation.

A mathematical model is proposed for multifrequency infrared multiphoton dissociation of polyatomic molecules with implications for laser isotope separation. In this model, the dissociation probability is expressed by the functional form based on the product of power-law terms for individual fluences. The data (on the fluence dependence of reaction volume) in the experiments performed on multiphoton dissociation of UF6 cooled at 90 K with multifrequency 16-μm laser beams are utilized to verify our proposed model. This model can show the fluence conditions for high contrast ratios where the dissociation probability for isotopically selective dissociation overwhelms that for nonselective dissociation.

Near-Resonant Effects on Propagation of Frequency-Swept Laser Pulse in Atomic Vapor Laser Isotope Separation.

The propagation characteristics of frequency-swept laser pulse in near-resonant medium were studied by the numerical calculation and the experiment. The characteristics of the frequency-swept laser pulse has been largely modified in comparison with the frequency-fixed laser pulse. These modifications were demonstrated also experimentally for Sm vapor. The results agreed well with the numerically calculated.

Selective photoionization of pare-earth element isotopes

We have shown that the problem of laser isotope separation with an isotopic shift not exceeding 1.5 GHz, a typical value for rare-earth elements, can be solved at a productivity level 1–3µg/sec. Many questions connected with compensation for Doppler broadening, development of methods of lowering the broadening of laser pulses both in the course of their formations as well in the course of their propagation in a strongly absorbing medium of resonant atoms have not yet been solved.