Isotope Separation of Gadolinium

Transcription

1 Proceedings of ibe 6ih International Symposium on Advanced Nuclear Energy Research -INNOVATIVE LASER TECHNOLOGIES IN NUCLEAR ENERGY - Laser Isotope Separation of Gadolinium H.Niki/N.Aly/K.Koh/K.Nomaru/'Y.-W.Chen^Y.Izawa/S.Nakai and "C.Yamanaka Faculty of Engineering, Fukui University, Bunkyo 3-9-1, Fukui 910, Japan 'Institute of Laser Engineering, Osaka University, Yamada-oka 2-6, Suita, Osaka 565, Japan 'Institute for Laser Technology, Yamada-oka 2-6, Suita, Osaka 565, Japan Basic studies on laser isotope separation of gadolinium were performed. Spectroscopic data were obtained such as isotope shifts and hyperfine structures using an atomic beam. Enrichment of 157 Gd up to 80% was observed by three-step photoionization experiment using linearly polarized dye lasers. Design of an separation system was discussed by the help of computer calculation of excitation dynamics. Keywords: laser isotope separation, gadolinium, isotope shift, hyperfine structure 1. INTRODUCTION Laser Isotope Separation (LIS) technique has several potential advantages over the traditional methods in its high isotopic selectivity and low energy consumption. Technology has been developed so far intensively for uranium enrichment. The applications of LIS, however, are expected to be put into practice in many other fields. Gadolinium enrichment is one of the interests in the field of nuclear engineering. Natural abundance of gadolinium is currently used as a burnable poison in the light water reactors to control the fuel reactivity. If enriched gadolinium including 60-80% of 157 Gd was available, many improvements could be achieved in economics and safety. In order to discuss the feasibility of laser gadolinium enrichment, basic studies were performed. Spectroscopic data were obtained such as isotope shifts and hyperfine structures using an atomic beam and cw single-frequency dye lasers 970

2 by means of laser-induced fluorescence technique. Enrichiment of 157 Gd up to 80% was observed by three-step photoionization experiment using three linealy polarized pulsed dye lasers. Efficient selective photoionization scheme was also discussed by the help of computer calculation of atomic excitation dynamics. 2. ISOTOPE SHIFTS AND HYPERFINE STRUCTURES Among basic spectroscopic data for laser isotope separation isotope shifts and hyperfine structures are especially important to discuss the selectivity and the efficiency. Natural gadolinium has seven isotopes ('""Gd, 15a Gd 157 J Gd, '"Gd, 154 Gd and 152 Gd) among which two odd isotopes have hyperfine structures due to their nonzero nuclear spin (1=3/2). Measurements were performed by means of laserinduced fluorescence spectroscopy using cw dye lasers and an atomic beam for the first and the second excitation steps assuming a three-step photoionization scheme. And the isotope shifts and the hyperfine constants were determined. 160Qd 158Gd cm Gd Type A I J I 157rvi ', 0 I 1t;q Cd LAJLi V (GHz) 154 Gd AJLA 160 Gd 158Gd "17618 cm Gd Type B 160Qd 158Gd 156 Gd cm-1 Type C r~> >-157Gd d- A -l55gd- JLU UAJAJLJU! V (GHz) 157Qd _J 1 2 v (GHz) 15SGd >54 Gd Fig.1 Classification of the absorption spectra for the first-step excitation 971

3 For the first-excitation steps absorption spectra from the four low-lying levels were measured". The results show that these spectra are classified into three types as shown in Fig.1. In type A the spectral regions of 157 6d and 155 Gd are overlapping with each other due to their widely spread hyperfine structures and the absorption line of 156 Gd lies in the spectral region of 157 Gd. In type B the spectral regions of two odd isotopes are separated but the peak of 156 Gd still lies in the region of 157 Gd. In type C the absorption regions of each isotope are completely separated. The isotope shifts per unit mass difference are about MHz through all the obtained data. For the second-excitation steps absorption spactra were measured by using two dye lasers. The first dye laser was used to excite one of the even isotopes or one component of the hyperfine lines of the odd isotopes from the low-lying levels. The wavelength of the second dye laser was scanned through the transitions of the second-excitation step. Selective excitation by the first laser made it easy to analyze the obtained s spectra". 問 2l Figure 2 shows the energy levels of atomic gadolinium and the transition used in this experiment. Two cm-l cm cm" 1. 9D cm-l (J=2)' 533 cm' cm- 1 J=4 J=5 Fig.2 Energy levels of Gd and the transitions used for the measurement of the second-step spectra 972

4 >5«Gd l58 Gd l««gd cm' i-l i s «Gd cm' 1 i^gd l5«gd H*??. s Gd 157Gd 355 Gd H^ * ^ l^gd c-b d' T d S4 Gd: i l L life Frequency (GHz) Frequency (GHz) Fig.3 Typical absorption spectra of Gd for the second-step excitation typical second-step spectra are shown in Fig.3. The isotope shifts per unit mass difference are about MHz which are smaller than those of the first-step excitation and also smaller than the hyperfine-structure broadenings. Balling and Wright 3 ' proposed a method for laser isotope separation of certain 3. GADOLINIUM ENRICHMENT EXPERIMENT nonzero nuclear spin isotopes based on the polarization selection rules in a multistep photoionization. This method requires to choose an excitation scheme with a proper combination of J quantum numbers of the levels and the laser polarizations. We applied this method for the 157 Gd enrichment experiment. One possible three-step excitation scheme is shown in Fig.4 where the initial state is the ground state (J- 2) and the final state is an autoionizing state (J= 0). If three exciting laser lights are all linearly polarized in a same direction, the first-step transition U-2, tn- 0 -> J- 2, m- 0) is forbidden where m represents the m~ magnetic 隠れ c quantum number. Thus the final level can not be reached The excitation scheme we used is presented in Fig.5. Three linearly polarized in the case of an even isotope. On the other hand there exist several pathways to the final level in the case of an odd isotope due to its hyperfine structures. pulsed dye lasers pumped by a Q-switched Nd:YAG laser were used as the excitation 973

5 -J = 0 J = cm' cm- 1 J=I J = cm" A J = 2 J = 2 J = 2 ^i J= cm' 0 cnr 1 Sc~ 隠 of Fig.4 Excitation scheme of a even isotope with linearly polarized lasers Fig.5 Photoionization schemes used in the 157 Gd enrichment experiment Gd laser spectrum A., c X3 c <x> 3 o c o Mass Fig.6 First-step excitation spectrum used in the 157 Gd enrichment experiment Fig.7 Mass spectrum of the Gd ions produced by collinearly polarized lasers 974

6 sources. Laser-produced gadolinium ions were extracted from the atomic beam by an electric field and detected through a qudrupole mass filter. The absorption spectrum of the first-step excitation we used is classified into type B as shown in Fig.6 The spectral width of the first excitation laser was MHz, the wavelength of which was tuned to the absorption region of 157 Gd. Thus the selectivity was spectrally achieved between 157 Gd and 155 Gd. The spectral widths of the second and the third lasers were about 10GHz. Typically obtained ion mass spectrum is shown in Fig.7. Polarization selectivity is clearly seen in the figure because the natural abundances of 157 Gd and 156 Gd are 15.6ao~ 15.68% and 釦 d 20.47%, respectively. Enrichment of,57 Gd up to 80% was observed in this experiment. 4. SELECTIVE PH0T0I0NIZATI0N SCHEME FOR GADOLINIUM ENRICHMENT As described in the previous section high isotopic selectivity can be obtained by the separation method based on the polarization selection rules. High ionization efficiency, however, can not be expected in this scheme because only the small number of the ground sublevels of an odd isotope can reach the final states with acceptable efficiencies. Therefore we consider here selective photoionization schemes based on isotope shifts. In the analyses the four-level ladder and the lambda excitation schemes were considered 4 ' as shown in Fig.8. The atoms are excited to the autoionization levels by three or four laser beams through the first and second excited levels. Q, and y represent the Rabi frequency for each excitation step and the ionization loss rate, respectively. Coherent excitation dynamics were calculated by using the computer simulation code CEALIS 5) which is based on the density matrix equations. The input parameters of this code are J values, hyperfine constants, loss rates for the levels, Rabi frequencies, Doppler widths for the transitions and isotope shifts. First we show the results of two-step photoionization scheme, where the atom is excited by two laser beams with Rabi frequncies of i and Q 2 and ionized by the ionization loss rate of y. Considering the thermal population in the low energy levels, the ground level (Ocnr 1, J- 2) and the two metastable levels (

7 1 Z^^utoionizationlever^^^ 49834atit U///A (222Z ZZZZSESZZioni^tinnli limit a A second excited level -33S3S cm" 1 -lowest level level 1 level cm -1 ~yste Ladder system 町 1 (a) Lambda system (b) Fig.8 Three-step photoionization schemes used in the calculation analyses - - «- 0 (J=2)-17618(3)-33535(4)cm-1» - 215(JE3) 21S(Jc3).17G18(3)-33S35(4)em 自 (3) 33535(.c )cm'~._ 215(J=3)-17931(4)-33535(4)cm- 1 fw I i i i ' i i i i [ i ) i i i i i i j i i i i [ i i i i j l i j j : (a) m S33(J«4)-17931(4)-33535(4)CITr' S33(J«4)-1761B(3)-33535(4)cm (J«4)-180B4(5)-33535(4)cm' 1 i i i l i i i i i i i l i i i i i i i l i i i l i (a) ~30 0 I' ' i " i I r " ' " " ' " " ' " " ' r ' fl, ~ (GHz) fi, ~ (GHz) ~ ~ ~ 90 ~ L (b) -^ 80E- g70t I 60 ' ' ' ' ' ' ' * ' fit ~ (GHz) Fig.9 Ionizations and enrichments in two-step photoionizations from the ground and the 215cm -1 levels 100,T 1 1 I IT I I I I 1 I ITITpT I I III I I I I 1, c 50 UJ iiiiiiiniti.Miitinjmilnii' o.o fit (GHz) Fig.10 Ionizations and enrichments in two-step photoionizations from the 533cm~ 1 level 976

8 cnr\ J- 3 and 533cnf\ J- 4) were chosen as the initial levels in the excitation ladder, all of which are almost equally populated at about 2000K. The ionization probability and the concentration of 157 6d ions were calculated for different twenty transitions. Results are shown in Fig.9 and 10 for six transitions as a function of Q,, where rectangular laser pulse shapes with durations of 40ns and Qi - r = 0.2GHz are assumed. The laser wavelength is tuned to the strongest line of the hyperfine components of 157 Gd in each excitation step. In the figures the ionization is defined as the ratio of produced 157 Gd ions to the initial number of 157 Gd atoms of the initial state, while the enrichment is defined as the ratio of the ionized 157 Gd to the total ionized atoms. In each excitation pathway, the ionaization probability reaches a maximum at Q, GHz and then decreases slightly for further increasing i. This is because the balance of the Rabi frequencies and the ionization loss rate is important for efficient ionization. Maximum efficiency is achieved when all these values are approximately equal as mentioned by Shore and Ackerhalt 6). In the strongest transition of 157 Gd The transition dipole moment between different magnetic sublevels changes depending on nk. Figure 11 shows the relative transition dipolem oment as a function of irk of the lower level for J- 2->3 (AJ = 1), F = 7/2->9/2a nd J- 3->3 {AJ- changes for different m? values in the case of AJ- case of AJ- 0), F= 9/2->9/2. The transition dipole moment dramatically 0, while it does not in the 1. Thus the excitation pathways having AJ- 1 are desirable for efficient ionization because of the reason mentioned above. Enrichment decreases with increasing? t because the excitation probabilities of other isotopes increase. For the pathway cm~ 1, the enrichment drastically decreases with increase of O^. Here,60 Gd transition frequencies are different by 1.88GHz and -1.90GHz from the laser frequencies for the first- and second excitation step, respectively. So the 160 Gd is possibly excited to the second excited level by twophoton resonance. For the pathway cm _1, the enrichment is low. This is due to the complete overlapping of the strongest hyperfine line of 157 Gd with the absorption line of ' 58 Gd in the first-step excitation. From the above considerations the pathway cnr 1 gives the highest ionization 977-

9 ~ -AJ=1 (J=2-3) -AJ=0 (J=3-3) 6.S cm" I Ionization loni7~tìon decay rat~ rate y "f (GHz) Fig.11 Transition dipole moment as a function of irk for AJ = 1 and 0 transitions Fig.12 Ionization as a function of r in the three-step photoionization and a relatively high enrichment. In practical three-step photoionization scheme the ionization loss rate is thought to be pretty large. From our autoionization spectrum measurement we found a high peak at the transition from the level 33535cm" 1 to the autoionization level 49834cm" 1, the ionization loss rate r of which is 1-10GHz. Figure 12 shows the ionization in the three-step photoionization as a function of y under the conditions Q, GHz and D 2 = 0 3 =0.2GHz. It was found that the ionization 0 V > cm-l / 100 : -- ^_ 30 < - ^ ^ " ^ ^ ~ 25 c 2 «o ~ I 'J.. i i-l i i_i 1, i i i L-L.. i... 1 i i J Pulse width (ns) _ -60 s u Fig.13 Ionization and enrichment as a function of a laser pulse width for the four-color three-step photoionization 978-

10 does not change appreciably. Finally we consider the four-color three-step photoionization scheme for efficient use of an atomic vapor, where the ground level and the lowest 鵬 metastable ~tast 油 1e level are simultaneously excited to the common level (17618cm -1 ). In order to avoid the effect of population trapping in the two lowest levels one of the two lasers for the first-excitation step was detuned by Ay = -100MHz and the Rabi frequencies were imbalanced 75. The ionization and enrichment are shown in Fig. 13 as a function of the laser pulse width under the conditions Q\ GHz and Qi- r - 0.2GHz. The maximum ionization of 30% with the enrichment of >80% was obtained for the laser pulse width of 100ns. 5.SUMMARY Basic studies on laser isotope separation of gadolinium were performed. The isotope shifts and the hyperfine stuctures were measured to discuss the isotopic selectivity and the excitation efficiency. Enrichment of 157 Gd up to 80% was obtained by using three linearly polarized dye lasers based on polarization selection rules. Excitation dynamics were analyzed by the calculations based on the density matrix equations. An ionization efficiency of 30% with 157 Gd concentration of >80% was obtained in the four-color three-step photoionization scheme, where the ground and the 215cm -1 metastable levels are simultaneously excited to the common 17618cm -1 level. References 1) H.Niki, T.Miyamoto, Y.Izawa, S.Nakai and C.Yamanaka, 0pt.Carm.7Q,16(1989). 2) N.E.S.Aly, K.Koh, K.Nomaru, H.Niki, Y.Izawa and S.Nakai, to be published in Rev.Laser Eng. 3) L.C.Balling and J.J.Wright, Appl.Phys.Lett.29,411(1976). 4) N.E.S.Aly, K.Nomaru, Y.-W.Chen, H.Niki, Y.Izawaand S.Nakai, Rev.Laser Eng.22,108(1994). 5) A.Adachi, Ph.D.dissertation, Osaka University(1991). 6) B.W.Shore and J.Ackerhalt, Phys.Rev.A]5,1640(1977). 7) S.Adachi, H.Niki, Y.Izawa, S.Nakai and C.Yamanaka, 0pt.Cami.8l,364(1991). 979