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IMPACT of LASER and NUCLEAR TECHNOLOGIES V.S.LETOKHOV

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IMPACT of LASER and NUCLEAR TECHNOLOGIES V.S.LETOKHOV Proceedings of the 6th International Symposium on Advanced Nuclear Energy Research -INNOVATIVE LASER TECHNOLOGIES IN NUCLEAR ENERGY- IMPACT of LASER and NUCLEAR TECHNOLOGIES V.S.LETOKHOV Institute of Spectroscopy Russian Academy of Sciences Triotzk, Moscow Region, 142092, RUSSIA A brief review is given of the applications of laser spectroscopy techniques and laser separation processes at an atomic-molecular level in nuclear physics and technology. Keywords: laser spectroscopy, laser separation, isotopes, nuclear isomers. 1. INTRODUCTION A series of principally new technologies have been developed over the 20th century that have formed the basis for a revolution in modem science and engineering. While lagging in its evolution some 30-40 years behind the nuclear technology, the laser technology has already reached a level allowing it to exert a noticeable effect on other fields of science and technology, nuclear physics and technology in particular. It should be emphasized in this respect that the laser technology is now at its exponential development stage still far from saturation. And so its application trends observed to exist today will be enhanced many times over in the years to come. Table 1 generally lists the main trends in the application of lasers in the nuclear technology, the types of lasers needed for particular applications being indicated. The lecture briefly considers only the first two trends based on the resonance laser- matter interaction, which enable one to implement laser detection (and diagnostics) and laser separation at an atomic-molecular level. The applications based on nonresonance laser- matter interaction are equally important but they fall outside the scope of my interests. The laser methods for detection and separation at an atomic- molecular level are potentially capable of being successfully used at all the stages of the nuclear fuel cycle. First, the laser detection of the traces of radioactive and dangerous chemical species contaminating the environment can be employed at the nuclear power station itself, at the fuel- reprocessing and chemical conversion plants, -11- and at the radioactive waste storage sites. Secondly, laser diagnostic techniques can be used to monitor the state of the critical reactor elements and to prospect for uranium deposits. Thirdly, laser separation techniques can be employed to achieve a more efficient separation of uranium isotopes, to produce monoisotopic materials for some construction reactor components, and probably to extract radioactive isotopes for other applications in the chemical reprocessing of radioactive wastes. Table 1. LASER SCIENCE and TECHNOLOGY for NUCLEAR TECHNOLOGY Laser Spectroscopy _>. Laser Tunable LasersLaser Detection Fiber Optics and Diagnostics Resonant Interaction of Laser Separation Pulsed High Average Laser Light with Atoms (Isotopes,....) Power Tunable Lasers and Molecules Laser High Power Pulsed Processing of and CW Lasers Materials Nonresonant Interaction of Laser Light with Substance Laser High Energy and Thermonuclear """" Peak Power Pulsed Fusion Lasers 2. LASER SPECTROSCOPY Unique properties of laser radiation (monochromaticity, temporal and spatial coherence, high power, controllable pulse duration) has made it possible to revolutionize all the characteristics of optical spectroscopy. It has now become possible to attain the ultimate of these characteristics (Table 2). All these characteristics are potentially important for nuclear physics and technology applications. I will restrict myself to the discussion of the possibilities of implementing the laser spectroscopy for study of atoms with short- lived nuclei in ground and excited metastable state (nuclear isomers), detection of atoms with very rare long- lived isotopes and traces of radioactive atoms. - 12- Table 2. LASER SPECTROSCOPY CHRACTER1ST1CS UMJTATJONS 1. Spectral Resolution - Homogeneous Width, - Interaction Time with Laser Wave -103- 1 Hz 2. Temporal Resolution - Period of Light Oscillations - 10-14-10'15s 3. Sensitivity - Atomic- Molecular - Single Atom Structure Molecule 4. Selectivity - Overlapping of Wings - Very Rare of Spectral Lines Isotopes 10"10- 10-20 5. Spatial Resolution - Light Wavelength, -0.1- 1.0 mem - de-Broglie Wavelength - 1-10 A 6. Remoteness - Length of Propagation - Cross- section of Scattering -0.1- 100 km Laser spectroscopic techniques are greatly diversified (see monograph [1]), but a unique position among them is held by the laser resonance ionization spectroscopy (RIS) [2, 3] which allows one to achieve ultimate characteristics, specifically very high sensitivities and selectivities in detecting and separating of atoms and molecules. Figure 1 presents a schematic diagram of a laser resonance ionization spectrometer used to study the proportion of short- lived nuclei ( mean- square charge radius variation A <r^>, magnetic moment M, and quadrupole moment Q obtained by bombarding a target widi high- energy protons. The above nuclear characteristics are derived from the hyperfine structures and isotope shifts of optical resonance transitions being measured [4]. This method has been used at the Konstantinov Institute of Nuclear Physics and the Institute of Spectroscopy to investigate the isotopic shifts and hyperfine structures of the isotopes of Nd (A= 132, 134- 142), Sm (A= 138- 145, 147, 149, 150, 152, 154), Eu (A= 138-115), Ho (A= 152- 165), and Tm (A= 156- 172) and other rare- earth elements [5]. These measurements have yielded the mean square charge radii of the nuclei of these isotopes, their electromagnetic moments (with the exception of the even-even isotopes and the Eu isotopes), improved values of the spins of certain nuclei, and the isotopic behavior of the charge radius A <A> of rare earth elements forN< 82 and 88< N < 94, i.e., on either side of the magic number N= 82. Fig. 2 shows the values of A <r^> for the isotopes of the these elements. Forthe isotope chains ofNd, Sm, Eu, there is a clear shell effect, i.e., the rate of change of the radius changes at N= 82. Figure 2a shows the isotopic variation in A <fi> for N< 82 for Eu, Nd and Sm, and the previously investigated [6] isotopes of Ba, Cs, Xe (using a different system). As can be - 13 — seen, A <r^> has a clear Z- dependence that is probability due to the different rate of growth of deformation along each of the isotopic chains as N is reduced. From the standpoint of nuclear physics, the success of laser optical spectroscopy in the study, detection, etc. of nuclei is based on two properties: (1) the large cross section for the resonance excitation of optical transitions in the electron shell (a0pt= X^/ 2n= 10**0 Cm2= 10^ barn) and (2) the high intensity of even relatively modest laser beams (1 W/cm^ corresponds to an intensity 1= 10^ photon/ cm^s). This ensures the high rate of resonance excitation Wext= o"0ptI= 10" s"* of atoms in a beam for given nuclear charge Z and given neutron number N. Fig. 1 General view on laser resonance ionization spectrometer of Leningrad Institute of Nuclear Physics and Institute of Spectroscopy for study of short- lived isotopes generated by lGeV proton beam from accelerator. Moreover, the scope of laser methods is actually much more extensive: it is possible to achieve not only isotopically, but also isomerically selective excitation of atoms, which means that isotopic and isomeric nuclei can be detected and separated by laser radiation [7], Selective laser photoionization of nuclear isomers was first observed in on- line experiments on the hyperfine structure and isotopic shift of the atomic lines of the radioactive isotopes of europium [5a]. — 14 — A<r*>,im2 ' A<r*>, fm* Number of neutrons Number of neuirons a b Fig. 2 Variations of mean- square charge radius A <r2> as a function of neutron number N: a) in the range of N< 82 for Sm, Eu, Nd (from [5]) and Ba, Cs, Xe (from [6]); b) in the range N> 82 for Nd, Sm, Ho and Tm (from [6]). The nuclear isomers of samarium- 141g, 141m, and thulium- 164g, 164m have been separated for the first [8] time by selective laser photoionization of atoms in an on- line experiment, using a proton accelerator and a mass separator of radioactive isotopes. The principle employed was similar to that illustrated in Fig. 3. Photoions with an excited nucleus of 141mSm or 164Tm, produced in isomer- selective three- step photoionization, were extracted by the electric field from the region of interaction between the laser radiation and the atomic beam, and were deposited on the cathode of a secondary- electron channel multiplier. Fig. 3 shows the photoionization spectrum of a mixture of the Sm isomers (laser wavelength tuned to the first step) together with an interpretation of the results. It is clear that the photoionization spectrum contains well- resolved ions belonging to 141mSm. This means that, when the laser frequency corresponding to the first step is tuned in the range 1-4 GHz, a beam of photoions containing isomeric nuclei is produced in the direction perpendicular to the atomic beam. This system can be used to investigate isomers with half- lives of the order of the time necessary to liberate them from the target (down to 1 s). - 15 — Ground slate Excited state "Sm,. of the nucleus ol the nucleus 1-1/2 /- tt/2 3/2 6161,SA J226s,6c»f •J/2 6751,5% ir'esspfy -2 -! 0 1 2 J * 5 6 GHZ 6004,2% m+g || | g III 111 I I I I , 7 S 11 13 15 A III II V*6*%f 2Fj.j ' Z 2'T 2' 2 2 ml 1 III III II a b Fig. 3 Resonance ionization spectrum (a) for the first step (X]) of excitation of '41sm with the ground (g) and excited (m) nuclear states. The diagram (b) shows the position of the lines of HFS due to ^'Sm atoms with ground- state (g) and isomeric (m) nuclei (from [8]). There is relatively large number of very rare long-lived radioactive isotopes of cosmogenic and technogenic origin.
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