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Lithium isotope separation with tunable diode lasers

Lithium isotope separation with tunable diode lasers
Ignacio E. Olivares, Andre´s E. Duarte, Eduardo A. Saravia, and Francisco J. Duarte A laser-isotope-separation study of lithium has been performed with two-step excitation involving UVlaser radiation and a visible tunable-diode laser.
The method yields a high degree of selectivity by tuning the narrow-linewidth diode laser to the D or D levels of the lithium atom.
excitation is simplified by the use of the tunable diode laser and the overall approach benefits from theapplication of a compact mass selector that includes a precision magnetic sector and an ion beam that isdesigned specifically for light atoms such as lithium.
300.6170, 300.6260, 300.6410, 300.6540, 300.6550.
Lithium isotopes are important for fission and fusion The laser-isotope-separation ͑LIS͒ method tively complex and require fairly sophisticated engi- is considered, in the literature, to be very attractive owing to the high selectivity that can be achieved.1–6 advantage is that they can yield relatively high CW This approach applies a two-step selective photoion- powers in a single-longitudinal mode. Even higher ization method and can be used for nearly all of the average powers are available from narrow-linewidth elements of the periodic table with commercial tun- class of laser, although very desirable for this type of application, has been demonstrated and operated In this paper we present a study of LIS in lithium only in a handful of laboratories around the with simple, compact, and inexpensive tunable diode lasers, which offer excellent spectral characteristics.
The overall method allows the complete separation of These lasers are relatively inexpensive and compact, the different isotopes, even in the case where the and yield narrow-linewidth single-longitudinal-mode lines of the different isotopes overlap.
An integral component of the experimental method ploratory nature of the experiments, these compact is a mass selector that includes a magnetic sector.
coherent sources are very well suited.
fairly detailed description of this relatively simple, It should be made clear that the experimental ap- compact, and inexpensive apparatus for separating proach described here was designed specifically for lithium isotopes is given in this paper.
the selective excitation of light atoms for spectro- Tunable lasers that are useful to this type of ap- plication include the cw dye lasers9 and narrow- high-power tunable lasers was beyond the scope ofthis study.
I. E. Olivares ͑[email protected]͒ is with the Departa- mento de Fı´sica, Universidad de Santiago de Chile, Avenida Ec- The upper limit for the number of ions produced by uador 3493, Casilla 307, Correo 2, Santiago, Chile.
the lasers at the end of a laser-ionizing pulse can be is with the Departamento de Fı´sica, Facultad de Ciencias, Univer- obtained with the result of an absorption measure- USA, Incorporated, Miami, Florida 33131.
the Eastman Kodak Company, Rochester, New York 14650.
When this research was performed I. E. Olivares was with theComisio´n Chilena de Energı´a Nuclear.
Received 18 June 2001; revised manuscript received 20 Novem- ͑͞h2v v ͒ ϭ 17100 m2͞W2, P is the average UV laser power, P is the exciter power, T is the exciter transmittance at the coincidence area A of both lasers in the interaction region, ␴ ϭ 7 ϫ 10Ϫ22 20 May 2002 ͞ Vol. 41, No. 15 ͞ APPLIED OPTICS complete spectrum was about 15 minutes in order toget enough resolution at the ionization spectra.
emission wavelength was monitored with an opticalwavemeter ͑Burleigh Model WA4500, Burleigh In-struments, Fishers, New York 14453͒. As noted byprevious authors15–18 tunable semiconductor lasersare ideally suited for this type of spectroscopic task,given their remarkable stability and the absence ofthermal and other media-related instabilities thattend to introduce short-term wavelength drifts that Diagram of the mass separator. O, Molybdenum crucible require sophisticated control systems.13,14 with lithium; P. E., Pierce extractor; E. L., Einzel lenses; M. S., The tunable-diode laser was focused into the neu- magnetic sector; FC1, FC2, Faraday cups; pA1, pA2, picoamper- tral lithium beam with a f ϭ 0.25 m lens, while for ionization of the excited atoms we focused the fourth placed behind the Pierce extractor for nonselective ion production harmonic of a Nd:YAG laser1 ͑Lee Laser Model and calibration of the mass separator.
815TQ, Lee Laser, Inc., Orlando, Florida 32809͒ de-ployed in the counter-propagating direction with afused silica f ϭ 0.58-m lens. Typical average power m2 is the ionization cross section,7 T densities of the exciter and ionizer at the focus were the UV laser pulses of the ionizing laser, h is the 35 W͞cm2 at 671 nm and 125 W͞cm2 at 266 nm, Planck’s constant, and v , v are the excitation and ionization frequencies, respectively.
tition rate with a KTP* intracavity crystal to produce of the different isotopes can be obtained with the integral of the absorption coefficient over each line as KD*P crystal ͑Inrad Model 5-301, Inrad, Northvale, described elsewhere18 when the laser intensity is suf- New Jersey 07647͒ to produce the FHG at 266 nm and a dispersive quartz prism to separate the greenfrom the UV radiation.
Experimental Details
density we used a f ϭ 0.2 m lens to focus the green The schematics of the experimental set up is depicted into the KD*P crystal and a f ϭ 0.2 m fused silica lens A beam of lithium atoms enters an optical length with a photodiode ͑EGG Model FND100Q, cell an excitation volume is created by focusing two EG&G Optoelectronics, Canada, Vaudreuil, Quebec, transverse laser beams of different frequencies, J7V8P7, Canada͒ obtaining 80 ns ͑FWHM͒. We which provide the two-step excitation energy neces- measured the focusing area of the red and UV laser sary for selective photo ionization.
using an ICCD ͑Model 576EMG͞RB, Princeton In- lithium beam is illuminated by a focused UV laser struments, Trenton, New Jersey 08619͒ at different beam and a spatially coincident focused tunable- ͑11.8 Ϯ 1.3͒ ϫ 10Ϫ9 m2 for the red laser and A ϭ doubly-excited lithium atoms enter a Pierce extractor ͑16.8 Ϯ 0.7͒ ϫ 10Ϫ9 m2 for the UV, respectively. In and proceed to a set of Einzel lenses.
this case we are using only the 71% of the UV light for doubly-excited lithium beam then enters a magnetic beam is separated into two subbeams corresponding The beam of lithium atoms ͑Fig. 1͒ was produced beams continues to a separate Faraday cup, where through an evaporation of metallic lithium from a heat-pipe cell ͑Model HP-802, Comstock, Inc., OakRidge, Tennessee 37830͒. The heat pipe used here can reach a temperature of 800 °C with a stability The CW tunable-diode laser used in these experi- better than 1 °C͞min. A detailed description is ments was a commercial device ͑EOSI Model 2010, Environmental Optical Sensors, Inc., Boulder, Colo- closed, and the other end was opened and connected rado 80301͒ configured in a Littrow grating cavi- to a vacuum chamber containing a mass selector.
The aperture used to collimate the beam has a 0.5-cm The collimator and the Pierce extractor21 longitudinal mode at a linewidth of Ͻ100 kHz. The were held at the same positive potential.
beam divergence is diffraction limited at an output between them is used as the laser-excitation volume power of 9 mW. This laser was tuned with an elec- The Pierce extractor yields a divergent ion tronically controlled servomechanism that rotates beam that is focused with an Einzel lens system into electric transducer driven by a slow triangular wave was made in-house and is comprised of an ion gun generator ͑HP Model 3310B, Hewlett-Packard Corp., Englewood, Colorado 80155͒. The time to scan one To calibrate the mass selector we have used a lith- APPLIED OPTICS ͞ Vol. 41, No. 15 ͞ 20 May 2002 ium ion cell ͑STD 250x, HeatWave Labs, Inc., was measured at saturation with the same laser in- Watsonville, California 95076͒, which is a ceramic tensity that was used at the current measurement.
beta eucriptite source containing a 30%͞70% mixture This was done at resonance and slightly off- formance of these cells has been disclosed else- To determine the density of neutral lithium atoms we removed the focusing lens and reduced further the The Pierce voltage V determines the intensity by means of a neutral density filter.
spectrum was recorded with an optical power meter is proportional to the inverse of the applied voltage at ͑Model 1815C, Newport Corp., Irvine, California a given magnetic field strength and geometry, we can 92606͒ and a digital-storage oscilloscope ͑LeCroy Model 9314A͒. The background light was sub- value was obtained experimentally at 572 Ϯ 1 V with a stabilized high-voltage power supply ͑Model EH05P20, Glassman High Voltage, Inc., HighBridge, New Jersey 08829͒ after focusing the beam at The overall experimental setup described in the sub- beam we have used a beam profile monitor ͑Model sections 3.A and 3.B fits in the space provided by two BPM80, Natural Electrostatics Corporation, Middle- 1.21 ϫ 2.43 m commercial optical tables with a total ton, Wisconsin 53562͒ and adapted the length of the utilized surface area of approximately 4.5 m2.
vacuum chamber to the position of the optimum fo- main two items contributing to this reduced area are This focal point is 15 cm from the exit of the ion the tunable-diode laser and the in-house mass sepa- The best focusing voltage V was held at 464 Ϯ The tunable-diode laser is only a fraction of 1 V and was determined experimentally by the posi- the size of an alternative cw dye laser or a CVL- tion of the entrance slit of the magnetic sector.
To measure the mass spectrum we employed a magnet comprising the mass selector was designed 2.5-mm section copper wire that was moved by a gear specifically for applications involving light atoms system connected to a rotary-motion feedthrough with a stepper motor ͑Model BRM275-03, MDC, Hay- of the size of a conventional commercial mass spec- ward, California 94545͒. The ion current reaching Ease of operation is a further experimen- the wire was measured by a picoampermeter ͑Model 485, Keithley Instruments, Inc., Cleveland, Ohio44139͒ and recorded with a digital storage oscillo-scope ͑Model 9314, LeCroy Corp., Chestnut Ridge, New York 10977͒. By using a mixed cell we were In these experiments a beam of lithium atoms is pro- able to obtain a mass spectrum at one scan of the duced and illuminated by a two-step selective-laser The positions of the isotopes and the resolu- tion were obtained from the barrel graduation.
mass-selection apparatus two detectors, Faraday cup With this result we could replace the wire at the exit 1 and Faraday cup 2, are used to collect the spatially of the sector by two 9-mm-width copper plates sepa- separated isotopes 7Li and 6Li, respectively.
separation is shown, through a mass spectrum, in were determined from the mass spectrum.
For our experimental conditions we have a justed the position of the plates by measuring the resolution of ⌬M͞M ϭ 3, which is enough to separate obtained a collector suitable for mass 6 and mass 7 The isotopic beam detected in Faraday cup 1 gives origin to the resonance ionization mass spectrum of produced by the lasers, the ion cell was removed and the 7Li D and 7Li D resolved in doublets ͑Fig. 3͒.
replaced by the neutral beam and collimator de- Note that this spectrum is clear, well resolved, and tected in Faraday cup 2 gives origin to the resonance the 7Li collector was measured with a picoamperme- ionization mass spectrum of the 6Li D and 6Li D ter ͑Keithley Model 485͒ connected with a general- lines ͑Fig. 4͒. Note that this spectrum is character- purpose interface bus to a personal computer and ized by the lower intensity peaks, which correspond recorded with a Labview 5.0 application.
to this particular isotope exclusively.
rent of the 6Li isotope was recorded simultaneously son purposes the reader can observe the mixed, or with the same software and interface with a more combined, high-resolution spectrum of 7Li and 6Li sensitive picoampermeter ͑Keithley Model 595͒.
The time required by our system to take the data for Albeit rather insignificant, the average value for each pair of current values was 652 ms.
the background signal was subtracted in each case number of ions produced at the ionization area were obtained from an absorption measurement and com- ground level was measured when the lithium was pared with the ions collected at both plates behind cold ͑heat-pipe cell off ͒ obtaining 0.87 pA and 0.12 pA at the 7Li and at the 6Li collector plate, respectively.
20 May 2002 ͞ Vol. 41, No. 15 ͞ APPLIED OPTICS Resonance ionization mass hyperfine spectrum recorded Mass spectrum of mixed 7Li͞6Li beta-eucriptite source.
optical pumping ͑1 W͞cm2 is convenient͒. At 780 °Cthe density of the beam was n ϭ 2.5 ϫ 1016 mϪ3 with This background level is deemed to have a negligible installed behind the Pierce extractor has the impor- The hyperfine structure of the isotopes can be dis- tant function of efficiently repelling the thermal ions tinguished owing to a reduction in the Doppler width arriving from the heat-pipe cell, which could contrib- produced by the collimation and expansion of the ute to an increase of the background signal at the The transmittance T of each hyperfine line spectra, giving a loss of selectivity.
of the 7Li D line was 0.991 and 0.996, respectively, could be reduced by collision effects among atoms or and the ionization laser power was P ϭ ͑15 Ϯ 1͒ mW.
ions as excitation transfer, but these effects are quite In each case we considered the losses at the windows.
negligible owing to the low lithium density, which For these parameters Eq. ͑1͒ can be used to estimate gives a mean free path ␭ ϭ 1͞n␴ of the order of 10 m the upper limit for the number of ions.
or more, depending on the cross-section value of each those upper limits are estimated to be N ϭ 8.56 ϫ 105 and N ϭ 3.83 ϫ 105 at the peaks of the lines, also affect the selectivity, but this effect is absent because the Pierce extractor repels the electrons of The density of the neutral lithium beam was de- termined from the absorption spectra.
direct ionization of the lithium clusters produced by surement the intensity of the laser used for excitation the UV; this effect can be measured with the exper- was kept low enough to avoid saturation effects and imental arrangement of our previous work.17 corresponds to 104 clusters͞pulse or less. Theseclusters are filtered by the mass selector and do notcontribute to the picocurrent signal.
In these experiments we observe a negligible back- ground signal, and the spectral lines 7Li D , depicted in Fig. 3, and 6Li D depicted in Fig. 4, appear free of the simultaneous signal from the other isotope.
This indicates a high selectivity during the resonantionization process.
Discussion and Conclusion
In these experiments we have recorded a high-resolution spectrum corresponding to 7Li in oneFaraday cup detector, whereas the spectrum corre-sponding to 6Li was recorded in a second Faraday cupdetector.
tially from each other. This spatial mass separationresulted from the selective two-step laser excitation,using a UV laser beam and a visible tunable-diode Resonance ionization mass hyperfine spectrum recorded laser, of an atomic beam of lithium that propagated through a relatively simple mass selector.
APPLIED OPTICS ͞ Vol. 41, No. 15 ͞ 20 May 2002 tensity ratio and wavelength characteristics of the laser-pumped dye-laser oscillators,” Appl. Opt. 23, 1391–1394
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