Journal of NUCLEAR SCIENCE and TECHNOLOGY, Supplement 6, p. 101–104 (September 2008)
ARTICLE
Laser Isotope Separation of Zirconium for Nuclear Transmutation Process Hideaki NIKI1; , Takaaki KUROYANAGI1 , Yasunobu HORIUCHI1 and Shigeki TOKITA2 1
Graduate School of Engineering, University of Fukui, 3-9-1 Bunkyo, Fukui 910-8507, Japan 2 Institute for Chemical Research, Kyoto University, Gokasyo, Uji, Kyoto 611-0011, Japan (Received September 19, 2007 and accepted December 13, 2007)
Laser isotope separation process of zirconium based on polarization selection rules has been investigated using three dye lasers and a 1.06 mm Nd:YAG laser in relation to nuclear transmutation process in the fast reactor. A high selectivity larger than a hundred between odd and even isotopes has been obtained by keeping the laser fluence lower than 0.4 mJ/cm2 . A highly selective photoionization scheme employing only two dye lasers has been newly found. The process includes the excitation from the ground state by a 573.7 nm dye laser and the subsequent three-photon ionization by a 578.1 nm laser. KEYWORDS: laser isotope separation, zirconium, Zr, polarization slection rule, nuclear transmutation
I. Introduction In order to eliminate the long-lived fission products (LLFPs) by nuclear transmutation process in a power-generating fast reactor, isotope separation process is required especially for several elements such as Zr, Cs and Sn.1) As for Zr, seven isotopic components (8.1% 90 Zr, 9.7% 91 Zr, 11.9% 92 Zr, 14.9% 93 Zr, 16.8% 94 Zr, 19.5% 95 Zr, and 19.1% 96 Zr) are produced by fission reactions. Among these, 93 Zr and 95 Zr are radioactive. Though 95 Zr decays very fast, 93 Zr has a long half life of 1:53 106 years. This 93 Zr is desired to be separated and put back into a reactor for transmutation. Atomic vapor laser isotope separation (AVLIS) is a promising technique as a highly selective separation method. In a usual separation process the target isotope is selectively excited by a narrow-band laser and then ionized by another laser, while the other isotopes remain in their initial energy states. Absorption spectra of naturally occurring Zr atoms shows that the small absorption peaks of 91 Zr are widely spread over a few GHz due to its hyperfine structures while the peaks of the even isotopes lie in the spectral region of 91 2–4) Zr and the isotope shifts are small. Though no spectral data of 93 Zr are available, its spectral features would be similar to those of 91 Zr. Therefore it seems to be difficult to excite an specific isotope selectively especially when the Doppler width of the atomic beam is larger than the isotope shifts. Isotope separation method using polarization selection rules5,6) can be applicable to Zr. In this separation process the isotopes with nonzero nuclear spin are selectively excited by polarized lasers, while the isotopes with zero nuclear spin
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are prohibited from excitation by the selection rules. In the case of Zr the odd isotopes have nonzero nuclear spin (I ¼ 5=2) and the even isotopes have zero. Therefore the odd isotopes are selectively excited by this method. Since the isotopic selectivity is not affected by a spectral broadening, this method is useful especially for the elements having small isotope shifts or complex spectral features. In this paper we describe the experimental results of laser isotope separation of Zr based on polarization selection rules. Since a natural zirconium metal was used in the experiment, selectivity was measured only for 91 Zr. We also show some new results on separation experiment using only two dye lasers.
II. Principle of Zr Isotope Separation Basic principle of Zr isotope separation method based on polarization selection rules is shown in Fig. 1. Excitation pathways from the ground state of Zr atom are shown for (a) an even isotope and for (b) an odd isotope. In Fig. 1(a) J and mJ denote the angular momentum quantum number and the magnetic quantum number, respectively. Three-step photoexcitation through J ¼ 2 ! 1 ! 1 ! 0 pathway is considered. The three laser lights used for excitation are linearly polarized in the same direction. Each energy state of an even isotope consists of 2J þ 1 degenerate magnetic sublevels. According to the selection rules only the transitions between the magnetic sublevels with the same mJ are allowed for J ¼ 0, 1, and especially the second-step transition (J ¼ 1, mJ ¼ 0 ! J ¼ 1, mJ ¼ 0) is forbidden. Therefore an even isotope cannot be excited to the uppermost J ¼ 0 state. In the case of an odd isotope, the total angular momentum quantum number F is formed by the hyperfine interaction between J and the nonzero nuclear spin I (I ¼ 5=2). And F becomes a good quantum number in place of J in this
Atomic Energy Society of Japan 101
102 J =0
1064nm lonization Limit 53506cm-1
J =1
52604.47 cm-1 569.56nm
J =1 35046.95 cm-1
mJ −2 −1
0
1
2
567.63nm
J =2 17429.86 cm-1
(a) J =0
573.73nm -1
0 cm
J =1
J =1
m F−1/2
1/2
F =1/2
−3/2 −1/2 1/2 3/2 −5/2 −3/2−1/2 1/2 3/2 5/2 −7/2 −5/2−3/2 −1/2 1/2 3/2 5/2 7/2 −9/2−7/2 −5/2−3/2 −1/2 1/2 3/2 5/2 7/2 9/2
3/2
5/2
7/2
J =2
9/2
(b) Fig. 1 Allowed transitions between magnetic sublevels of Zr with J ¼ 2 ! 1 ! 1 ! 0 excitation steps for (a) an even isotope and (b) an odd isotope
experimental condition. An energy state of an odd isotope has the hyperfine structures. The ground state (J ¼ 2), for example, is split into five (F ¼ 1=2, F ¼ 3=2, F ¼ 5=2, F ¼ 7=2, and F ¼ 9=2) hyperfine energy states. Each hyperfine state has 2F magnetic sublevels mF as shown in Fig. 1(b). Transitions between the magnetic sublevels satisfying mF ¼ 0 for F ¼ 0, 1 are then allowed in this case. Thus, there are excitation pathways to the final J ¼ 0 state in the case of an odd isotope. The allowed and the forbidden transitions are indicated by solid lines and dotted lines, respectively in the figure. Of thirty magnetic sublevels in the ground state, six sublevels are not optically connected to the final J ¼ 0 state. Therefore 80% of the odd isotopes can be excited to the J ¼ 0 state. Thus, the isotopically selective excitation is possible when an appropriate J-value combination is used.
III. Experimental Procedure The atomic zirconium was photoionized with a four-step process as shown in Fig. 2. The atoms were sequentially excited by three dye laser pulses from the ground state to the 52604.5 cm1 high-lying state through J ¼ 2 ! 1 ! 1 ! 0 pathway and then ionized by a 1.06 mm pulse from the Nd:YAG laser. Among the known energy levels the first and the second excited states were chosen to have suitable energies, which could be excited by using the dye lasers employed in the experiment. The experimental arrangement is shown in Fig. 3. Zirconium atomic vapors were generated by an electron
J=0
J=1
J=2
Fig. 2 Real four-step photoionization pathway of Zr used in this experiment on highly selective ionization
beam heating in a vacuum chamber and collimated by a slit to form an atomic beam. The three dye lasers (Lambda Physik) were pumped by the second harmonic pulse of a Nd:YAG laser (Continuum). The repetition rate and the pulse width were 10 Hz and 6 ns, respectively. The dye lasers were operated at multimode and their bandwidths were 7 GHz. The fundamental frequency pulse from the Nd:YAG laser, which was not converted to the second harmonics, was used for ionization in the fourstep selective photoionization experiment. The temporal delay between the pulses from different lasers was adjusted by changing the optical path length so that four laser pulses could sequentially arrive at the interaction region in order to reduce ambiguity in the interpretation of excitation pathway. Typical delay between the adjacent pulses was set to 15 ns to avoid temporal overlaps of the laser pulses. Though the output pulses of the dye lasers were linearly polarized, they went through the polarizer in front of the entrance window of the chamber and these laser beams intersected the atomic beam perpendicularly. Wavelength tuning of the first excitation laser was done by monitoring the fluorescence from the atomic beam by a photomultiplier. In another experiment the polarization direction of one of the dye lasers was changed by using a wave plate and a polarizer. The ions generated at the interaction region were deflected and accelerated by a dc electric field (100 V/cm at the interaction region) to the direction that is orthogonal to the atomic beam and the laser beams. After passing through the 90 cm-long field-free region, the ions were detected by a microchannel plate (MCP). Thus the ion mass spectrum was analyzed by a time-of-flight technique.
IV. Results and Discussion Natural zirconium was used as an atomic vapor source in the experiment. It has four even isotopes (51.45% 90 Zr, 17.15% 92 Zr, 17.38% 94 Zr, and 2.8% 96 Zr) and one odd isotope (11.22% 91 Zr). Figure 4 shows the typical ion mass JOURNAL OF NUCLEAR SCIENCE AND TECHNOLOGY
103 Vacuum Chamber (~10−7 torr) PMT
10Hz 6ns Pulse +
Zr
Dye Laser 1 573.73nm
Zr Vapor Polarizing Filter
Dye Laser 2 567.63nm
0.9m Time-of-Flight Mass Spectrometer MCP
Dye Laser 3 569.56nm 2ω
Trigger
Nd:YAG
ω
Digital Oscilloscope
PC
Fig. 3 Schematic diagram of experimental arrangement
lon Signal (arb.unit)
0
90
92
94
96
Observed 90Zr/ 91Zr
3.0
sin 2 θ
2.0
1.0
0
90
Relative Polarization Angle θ 2 [deg.]
180
(a)
0.2
Fig. 4 Time-of-flight mass spectrum of Zr ions by selective ionization with three linearly polarized dye lasers and a 1.06 mm pulse from a Nd:YAG laser
Observed
91
90 Zr/ 91Zr
3.0 sin 2 θ
2.0 1.0
0
spectrum obtained in the experiment when zirconium atoms were photoionized via the scheme described previously with three lineally polarized dye lasers and a 1.06 mm pulse from a Nd:YAG laser as the selective excitation and the ionization sources, respectively. The signal was averaged over 64 laser shots and well reproducible. Selective ionization of 91 Zr is clearly seen in the figure. For the 90 Zr and 91 Zr isotopes, a selectivity, which is defined in this study as observed abundance ratio of 91 Zr to 90 Zr divided by natural abundance ratio of 91 Zr to 90 Zr, larger than a hundred was observed when the dye laser fluences at the interaction region were kept under 0.4 mJ/cm2 with the laser beam diameters of 2 mm. Selectivity was easily degraded when a fluence of a dye laser was increased typically to larger than 1 mJ/cm2 . SUPPLEMENT 6, SEPTEMBER 2008
90
Relative Polarization Angle θ 3 [deg.]
180
(b) Fig. 5 Dependence of the ratio of even isotope to odd isotope (90 Zr/91 Zr) on relative polarization angle (a) of the second-step excitation laser and (b) of the third-step excitation laser
Dependence of selectivity on relative polarization angle of the dye lasers was investigated. By changing the polarization direction of the second-step excitation dye laser 2 and that of the third-step 3 , the ion mass spectrum was measured. Figure 5(a) and 5(b) show the observed isotope ratio 90 Zr/ 91 Zr as a function of 2 and 3 , respectively. The 91 Zr ion signal showed no dependence on relative polarization angles. Therefore the signal intensity of the isotope ratio re-
104
Ion Signal (arb.unit)
0
575
575.5
576
576.5
577
577.5
578
1.0
B
2.0
3.0
A 4.0
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96 91 92
90
90
high energy region,8) few data are available for the thirdly excited states just below the ionization limit and the autoionizing states. Therefore we could not say whether these three-photon ionization process include a two-photon resonance excitation, three-photon resonance ionization or another process. Further investigation should be done to find the efficient isotope separation method.
94
A
91
B
6.0
Fig. 6 Ionization spectrum for the second second-step laser wavelength. Two mass spectra for the peak A and B are shown
flected the ion signal intensity of the even isotope 90 Zr. According to the theoretical consideration,7) the signal is expected to vary proportionally to sin2 2 (or sin2 3 ). Fitted curves are also shown in the figure. The selectivity is the lowest at about the angle of 90 in both cases. As described previously, highly selective ionization was achieved by keeping the laser power densities low because zirconium atoms were easily ionized by non-resonant multiphoton absorption or ionization process which would result in non-selective process. However, if there exist isotopically selective ionization pathways including multiphoton excitation, they might be practically useful, giving a simple process using a reduced number of lasers. A preliminary experiment on multiphoton ionization was performed. Only two of the dye lasers were used in this experiment. The time interval between the two laser pulses was 15 ns. The wavelength of the first dye laser was fixed to the resonance of the first excitation step (1 ¼ 573:73 nm), while the wavelength of the second one (1 ) was scanned. Thus the total ion signal was recorded. Laser fluence of 1 pulse was kept low as previously, while that of 2 set as intense as 6 mJ/cm2 . An example of the experimental result when the wavelength 2 was scanned from 575 nm to 578.2 nm is shown in Fig. 6. Reproducibility of the spectrum was confirmed by scanning 2 twice. More than ten peaks were observed in this wavelength region. Two small peaks were observed at 575.6 nm and 576.5 nm even without 1 pulse in this region. Therefore mostly the peaks seen in the figure are considered to result from three 2 photon ionization from the 17430 cm1 (J ¼ 1) state. Mass spectrum was also measured for each ion peak. Two results are inserted in Fig. 6. The isotope ratios are substantially the natural abundances for the peak A, while the mass spectrum for the peak B clearly shows a highly selective ionization. Actually, a high selectivity was observed only for the peak B in this wavelength region. Since the data for the energy levels are insufficient especially for
V. Summary Experimental study on laser isotope separation of zirconium has been performed using three dye lasers and a Nd:YAG laser, in relation to nuclear transmutation process in the fast reactor. A high selectivity larger than a hundred between odd and even zirconium isotopes has been obtained by using fourstep photoionization based on polarization selection rules. Selectivity was easily degraded when a fluence of a dye laser was increased typically to larger than 1 mJ/cm2 . A highly selective photoionization scheme employing only two dye lasers has been newly found. The process includes the excitation from the ground state by a 573.7 nm dye laser and the subsequent three-photon ionization by a 578.1 nm laser. References 1) H. Akatsuka, T. Ohsaki, T. Obara, M. Igashira, M. Suzuki, Y. Fujii-e, ‘‘Scientific Feasibility of Incineration in SCNES,’’ Prog. Nucl. Energy, 29, (Suppl.), 477–484 (1995). 2) G. Chevalier, J. M. Gagne, P. Panarosa, ‘‘Isotope shifts and 91 Zr from optogalvanic saturation spectroscopy,’’ Opt. Commun., 64, 127–130 (1987). 3) E. Langlois, J.-M. Gagne, ‘‘Zirconium isotope shift measurements using optogalvanic detection,’’ J. Opt. Soc. Am. B, 10, 774–783 (1993). 4) C. Lim, K. Nomaru, Y. Izawa, ‘‘Hyperfine structure constants and isotope shift determination in Zr I by laser-induced fluorescence spectroscopy,’’ Jpn. J. Appl. Phys., 37, 5049–5052 (1998). 5) L. C. Balling, J. J. Wright, ‘‘Use of angular-momentum selection rules for laser isotope separation,’’ Appl. Phys. Lett., 29, 411–413 (1976). 6) E. Le Guyadec, J. Ravoire, R. Botter, F. Lambert, A. Petit, ‘‘Effect of a magnetic field on the resonant multistep selective photoionization of gadolinium isotopes,’’ Opt. Commum., 76, 34–41 (1990). 7) L. W. Green, G. A. Mcrae, P. A. Rochefort, ‘‘Selective resonant ionization of zirconium isotopes using intermediate-state alignment,’’ Phys. Rev. A, 47, 4946–4954 (1993). 8) W. C. Martin, R. Zalubas, L. Hagan, Atomic energy levels, the rare earth elements, Natl. Stand. Ref. Data. Ser., circ. 60 Natl. Bur. Stand., Washington, DC, 1978.
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