Efficient laser isotope separation of 13C using novel linear multi-pass cavity

¹³C isotope has been separated in the form of enriched product C₂F₄ by selective multi-photon dissociation (MPD) of Freon-22 (CHClF₂) using the 9P(26) laser line of a transversely excited atmospheric CO₂ laser. The non-linearity factor, γ, that determines the dependence of the yield of ¹³C isotope on the fluence (J/cm²) has been determined for various laser rotational lines (9P(20)–9P(26)) and the advantage of a lower γ in the case of 9P(26) is highlighted for macroscopic production of ¹³C isotope. It is also shown that a higher value of the optimum fluence at 9P(26) not only results in a higher enrichment efficiency but in a relatively lower value of γ also. The laser pulse energy is efficiently utilized for selective MPD of Freon-22 by focusing the pulse energy repeatedly with the help of a novel linear multi-pass cavity (LMPC). The novelty of this optical arrangement lies in its ability to maintain the laser fluence around an optimum value for a desired enrichment of ¹³C in the product. This also ensures a higher quantity of enriched product because of the higher reaction volume. The advantage of the LMPC over the conventionally used Herriott multi-pass cell has also been presented. The gain in reaction volume in the present optical cavity having 20 passes with a constant fluence in each pass is as high as 12. Isotope-selective MPD of Freon in a LMPC with constant fluence in each pass showed a distinct advantage in energy utilization to separate ¹³C isotope over the gradually reducing fluence case.This revised version was published online in August 2005 with a corrected cover date.     

Multifrequency radiation pressure laser,isotope separation

Isotope separation by laser deflection of an atomic beam is limited in its efficiency by the accumulation of atoms in metastable states. This restriction can be removed with the use of lasers which excite metastable atoms to states from which spontaneous decay to the ground state is allowed. This is demonstrated in the separation of barium isotopes, where efficiency was improved from a lower limit of 70% to at least 83% using a second laser. Efficiency approaching 100% can be achieved in barium with a second laser but the required wavelength is not available.     

A study of isotopically selective photoionization of ytterbium atoms for laser isotope separation

Atomic-vapor laser isotope separation (AVLIS) is studied experimentally and theoretically for ytterbium vapors. The optimum ionization scheme and the process dynamics are determined. The photoionization scheme uses the transitions $$6^1 S_0 \mathop \to \limits^{555.648nm} 6^3 P_1 \mathop \to \limits^{581.067nm} (7/2,3/2)_2 \mathop \to \limits^{582.79nm} (52353cm^{ - 1} ).$$ For a numerical study of photoionization dynamics, the mathematical model of the AVLIS process is used, which is based on the density matrix formalism and Maxwell's equations. Selective photoionization of the¹⁶⁸Yb isotope is simulated numerically. The yield and selectivity of the process are determined. It is shown that the length of the photoionization region is limited because the laser radiation is absorbed by atoms of¹⁷¹Yb and¹⁷³Yb isotopes. The advantage of the laser method over the method using an electromagnetic separator is demonstrated.     

Atomic vapor laser isotope separation

Atomic Vapor Laser Isotope Separation (AVLIS) is a general and powerful technique applicable to many elements. A major present application to the enrichement of uranium for lightwater power reactor fuel has been under development at the Lawrence Livermore National Laboratory since 1973. In June 1985, the Department of Energy announced the selection of AVLIS as the technology to meet future U.S. needs for the internationally competitive production of uranium separative work. Major features of the AVLIS process will be discussed with consideration of the process figures of merit.This paper is a synopsis of a presentation made at the Royal Swedish Academy of Engineering Sciences IVA Symposium Laser Technology in Chemistry, Stockholm, Sweden, November 10, 1987     

Lithium isotope separation by laser

A lithium isotope separation was performed using a laser isotope separation method. It was found that the lithium atoms with a natural isotopic abundance enhanced its⁶Li concentration up to over 90% by tuning the laser wavelength to the² P _1/2 of⁶Li. Too high power, however, leads to a loss of enrichment due to the power broadening effect which was analysed by the equation of motion of density matrices.     

Rotational-coherence molecular laser isotope separation

We have proposed a laser isotope separation method, utilizing rotational coherence of a simple molecule. In the scheme, photoexcited molecules are isotopically separated by difference of rotational period between them. To illustrate this method, two-pulse photodissociation of mixed ⁷⁹Br₂/⁸¹Br₂ isotopes has been investigated theoretically. The photodissociation probabilities of ⁷⁹Br₂ and ⁸¹Br₂ have been calculated as functions of time delay between the photoexcitation and dissociation laser pulses. We have demonstrated that isotope enrichment factor of ⁷⁹Br relative to ⁸¹Br can be changed from 0.34 to 1.8, by simply changing the time delay only by 0.2 ns. Additionally, we have shown that this method is effective for heavy isotopes, based on mass dependence of the isotope enrichment factor.     

Enrichment of carbon-14 by selective laser photolysis of formaldehyde

Using a frequency-doubled dye laser, simultaneous high-resolution spectra of¹⁴CH₂O and¹²CH₂O between 290 and 345 nm were measured. About 30 lines with spectral selectivities for¹⁴C≳50 were found. Photolyses on one such line at 326.94 nm of a dilute mixture of¹⁴CH₂O in natural CH₂O gave one-step enrichment factors of up to 150, with μg yields of enriched product. Since a factor of 150 in¹⁴C concentration corresponds to ∼7.2 half lives or 41,000 years, laser enrichment of archaeological samples, especially when combined with direct detection methods of¹⁴C abundance measurement, can greatly improve the range of radiocarbon dating.     

Sources of atomic vapor for laser isotope separation

Conclusions Thus, our experimental investigations of the methods of obtaining a vapor of neutral atoms of the refractory and chemically active rare element (Gd) have shown that both investigated methods — electron-beam evaporation and cathode sputtering — are promising. The fact that the levela ⁹D₅ is equally singled out in the electron-beam evaporation and in the cathode sputtering is evidence of the decisive importance of the processes of establishment of the distribution of the populations over the sublevels of the ground state of the evaporated atoms. It is obvious here that the determination of the properties of this distribution for other elements calls for investigations similar to those described above.The question of the choice of source of neutral atoms should be solved separately for each chosen laser isotope separation scheme.Translated from Trudy Ordena Lenina Fizicheskogo Instituta im. P. N. Lebedeva Akademii Nauk SSSR, Vol. 114, pp. 24–37 (1979)     

A waveguide reactor for IR multiphoton dissociation and its application to13C laser isotope separation

A waveguide reactor for infrared multiphoton dissociation reactions has been presented and applied to laser isotope separation of¹³C. The reactor is a sealed vessel containing a hollow waveguide for a CO₂ laser, and the laser beam is condensed in the waveguide. The waveguide is a pipe of total-reflection glass or metal-coated glass with a funnelled inlet; it is 50 cm long with a 3 mm inner diameter. The isotope separation of¹³C has been made by multiphoton dissociation of CHF₂Cl using a TEA CO₂ laser. The result has shown that the waveguide reactor increases the dissociation yield by four to ten times as much as that without the waveguide at the pulse rate of up to 150 Hz, while the separation factor remains on the same level. It is also found that an optical breakdown which is induced at the wall surface plays a significant role to reduce the isotope selectivity.This work is for a partial fulfilment of B.S. degree of Osaka Electric Communication University     

On-line computer controlled cw dye laser spectrometer for laser isotope separation

An on-line cw dye laser multichannel spectrometer is described, which can be controlled by FORTRAN. Automatic tuning of a dye laser to the wavelength of maximum excitation selectivity for photochemical isotope separation is achieved. Applying the system to the selective photoaddition of iodine chloride ICl to acetylene, an enrichment factor of³⁷Cl in the photoproduct C₂H₂ICl of β=100±5 was obtained. Both stable chlorine isotopes can be enriched to more than 97% purity in a single step, starting from natural samples in both cases.     

Laser isotope separation of 13C: A comparative study

IR laser chemistry of (CF₃Br/Cl₂) mixture and neat CF₂HCl are examined in the context of ¹³C enrichment. Decomposition extent, enrichment factor and energy absorbed are measured for both systems at their respective optimum conditions. A direct comparison is obtained by keeping extraneous factors such as laser, its pulse duration, cell, irradiation geometry etc. the same. The halogen scavenged CF₃Br MPD requires lower fluence compared to neat CF₂HCl irradiation. Overall throughput for a product with 60–65% ¹³C content in a single stage is the same for both systems requiring a similar amount of energy. However, at lower enrichment levels, CF₂HCl MPD is better than (CF₃Br/Cl₂) photolysis in terms of both product yield and energy absorption.     

Ion extraction and charge exchange in laser isotope separation

In the atomic vapor laser isotope separation (AVLIS) process, a vapor is ionized by pulsed laser beams, and the ions are extracted by negatively biased collectors. We compute the unsteady dynamics of the photoplasma using a two-dimensional (2-D) particle-in-cell (PIC) code. Collisions between ions and neutral species are simulated by a Monte Carlo technique. The plasma dynamics is visualized by snapshots of particle positions showing the directions of their velocities. The three kinds of particles (electrons, photo-ions, and ions created by charge exchange) are marked by different colors. The graphic outputs illustrate the motion of the electrons toward the anodes, the vertical drift of the plasma, its erosion by the transient ion sheath, and nonselective ionization by charge exchange     

飞秒激光分离同位素的模拟实验研究

利用飞秒激光与铜镍材料作用,在不同条件下得到了铜镍材料的分离,分离比可达30%.分析认为,激光等离子体中的自生轴向磁场和环形磁场在同位素的分离过程中可能起着相反的作用,二者相互竞争.不同的磁场构形将对同位素分离产生不同的结果,因此,通过控制不同的激光等离子体相互作用过程可以实现对激光分离同位素的控制 关键词： 飞秒激光等离子体 同位素 铜镍 自生磁场     

Spectral considerations in the laser isotope separation of Uranium Hexafluoride

Key spectral features important in the laser isotope separation of Uranium Hexafluoride are reviewed. Specifically the (v ₃+v ₄+v ₆) band lying in the frequency range of the CO₂ laser, the 3v ₃ band which is covered by the CO laser, and the fundamentalv ₃ band of UF₆ are considered. Computer-calculated spectra show that the ternary combination bands (v ₃+v ₄+v ₆) and 3v ₃ are completely dominated by the solid Q-branches of some 300 to 1000 hot bands, but that the fundamentalv ₃ band has structured P-and R-branches with peaks and holes, provided the pressure is below a few torr. Laser isotope separation can be achieved either on the isotope-shifted slopes of the Q-branches envelopes (“Q-slope method”) or on the coincidence of a peak and a hole in the P-or R-branches regions (“peak-hole method”). Room temperature experiments were carried out in 1972/1973, which used a CO₂ laser with an internal U²³⁸F₆ filter tube that forced lasing on what is believed to have been the P-16 line falling on the right-hand Q-slope of the (v ₃+v ₄+v ₆) band of UF₆. With a chemical reaction for final isotope separation, the measurements yielded a U-235 enrichment factor of about 1.1, in agreement with the maximum possible value of 1.5 for Q-slope operation and the fact that a high cut was used.     

原子法激光同位素分离光功率优化理论研究

利用速率方程方法研究了光学厚介质中原子多步电离过程的激光功率的配置问题.计算结果表明,在总激光功率一定的情况下,存在使原子电离几率为最大的激光功率分配方式.更重要的一个结果是,在保持激光照射原子数量不变的条件下,当总功率一定时,缩小激光束截面积,增加激光束在原子蒸汽中的传播距离可以大大提高原子的平均电离几率.     

Deuterium isotope effects on carbon-13 chemical shifts in selected amino acids as function of pH

The carbon-13 chemical shifts of glycine, alanine, and lysine have been studied as a function of pH, when dissolved in water as well as in deuterium oxide. No correction of the apparent pH measured in the deuterium oxide samples was made. A comparison of the two sets of data thus obtained shows that isotope shifts of up to 0.9 ppm are present. A more detailed analysis of the data reveals that the isotope shifts are caused not only by an isotope effect on the titration shifts but are, in the high pH region, also due to isotope effects on the pK values that correspond to the dissociation of the ammonium groups. Thus relatively small isotope effects on these pK values cause significant changes of the ¹³C chemical shifts at pH values equal or close to the pK values. While the isotope effect on the titration shifts in the acid and neutral pH regions causes upfield shifts, as normally observed, downfield shifts have been found in the basic pH region. The titration shifts and pK values necessary for the analysis were obtained by a nonlinear least squares fit of the appropriate theoretical model to the shift data lot all carbons in a given amino acid, simultaneously. This procedure allows a precise determination of macroscopic pK values and titration shifts even in the case of strongly overlapping deprotonation reactions, as for lysine.