Hyperfine Interact (2013) 216:59–64 DOI 10.1007/s10751-012-0736-y
High Resolution-Resonance Ionization Spectroscopy on uranium Amin Hakimi · Thomas Fischbach · Sebastian Raeder · Norbert Trautmann · Klaus Wendt
Published online: 19 December 2012 © Springer Science+Business Media Dordrecht 2012
Abstract High Resolution-Resonance Ionization Spectroscopy (HR-RIS) allows for sensitive probing of atomic structures and energy level schemes even for highly complex systems. This work explores the applicability of commercial diode lasers for isotope selective spectroscopy of uranium. Using narrow bandwidth continuouswave (cw) diode lasers, multi step excitation processes were investigated involving levels which could be populated with the radiation of 405 nm BluRay© laser diodes as a first step for ultra trace analysis of uranium. Keywords Resonance Ionization Spectroscopy · Uranium
1 Introduction Uranium is one of the few actinide elements occurring in nature with significant abundances. With a ground state configuration 5 f 3 6d7s2 , its electronic shell allows for manifold couplings producing a broad variety of configurations. These are distributed along the entire energy range from the atomic ground state at 0 cm−1 up to the ionization potential at 49958.4 cm−1 . As the six valence electrons populate up to four open shells, most of the energy levels are not pure but have a mixed character, which significantly complicates the conclusive assignment of configurations. By
A. Hakimi (B) · T. Fischbach · K. Wendt Institut für Physik, Johannes Gutenberg-Universität Mainz, Staudinger Weg 7, 55128 Mainz, Germany e-mail:
[email protected] S. Raeder TRIUMF, 4004 Wesbrook Mall, Vancouver V6T 2A3, BC, Canada N. Trautmann Institut für Kernchemie, Johannes Gutenberg-Universität Mainz, Fritz-Strassmann-Weg 2, 55128 Mainz, Germany
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employing radiation of several lasers multi step excitation into intermediate and high-lying levels along strong optical resonance lines becomes possible. From there, the atoms may be ionized either by non-resonant photoionization or by populating an autoionizing state. Subsequently efficient detection of laser-generated ions completes the sensitive and selective technique of resonance ionization spectroscopy. By using narrow bandwidth laser radiation of about 3 MHz spectral linewidth, HR-RIS investigations even enable the resolution of hyperfine structures and isotope shifts in the different transitions. The analytical application of this dedicated laser-based technique concerns the determination of uranium isotope ratios of 236 U/238 U in various samples. The ratio varies from 10−12 up to 10−9 for natural sources and may reach up to 2.4 · 10−3 for samples stemming from the nuclear fuel cycle [1, 2]. This turns 236 U into a very sensitive tracer for the determination of any anthropogenic contamination [3]. The extraordinary selectivity of the multi step resonant excitation process with respect to element as well as isotope characterizes HR-RIS compared to other ionization techniques presently used in commercial mass spectrometers. A second major benefit of this technique is the possibility for an in-situ check of the signal background during a measurement by simply detuning the laser wavelengths off resonance, which greatly helps for the validation of measurements. For the analytical case of 236 U, the capability of HR-RIS has been first demonstrated in [4], where the determination of isotope ratios for 236 U/238 U as low as 10−7 was reported. Earlier HR-RIS investigations in uranium focused on excitation schemes which involved first step diode laser excitation at 415 nm, starting from the atomic ground state. Nowadays the well advanced development process for blue laser diodes results in a fixed lasing wavelength of 405 ± 1 nm for the BluRay© data storage technology. Correspondingly, spin-off products useful for 415 nm generation in laser spectroscopy are presently rapidly disappearing from the market and a laser based analytical technique has to search for alternative solutions. For HR-RIS on uranium this fact either implies the use of extensive and delicate cw frequency doubling systems for existing red diode laser systems or, as second option, the search and identification of alternative excitation schemes using available laser wavelengths in the blue spectral range. The latter approach will be discussed here.
2 Experimental setup A detailed description of the general layout of the laser system and the spectrometer, as used for HR-RIS, was already given in [5]. The laser system and in particular the wavelength control has meanwhile undergone substantial changes, which are subject to a further publication presently in preparation. The ionization schemes which were investigated in this work are summarized in Fig. 1 and have been studied earlier with pulsed lasers of about 4 GHz spectral band width [6]. All of them start from the thermally populated level 5 f 3 6d7s2 5 K5O at 620.323 cm−1 , which is expected to comprise about 30 % of the ground state population density at the typical evaporation temperature of 2000 K. First step excitations around 405 nm were driven with about 5 mW power from a home-built Littrow-type diode laser employing a BluRay© laser diode as gain medium. The intermediate transitions of schemes (a) and (b) were excited using a commercial Littrow-type diode laser (DL-PRO 780,
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Fig. 1 Ionization schemes for uranium based on first excitation steps driven by BluRay laser diode light with 405 ± 1 nm
TOPTICA, 40 mW), whereas for scheme (c) and (d) a home-built Littrow-type diode laser operating at 830 nm with about 20 mW was in use. In all four cases shown the ionization step was induced by powerful radiation of ∼ 1 W from a tapered amplifier laser setup (TA-PRO 780, TOPTICA). The frequency stabilization and control system for all three lasers, as described in [5], was extended substantially by upgrading the formerly used fringe-offset-locking technique with a second interferometric measuring device (iScan, TEM Mess-technik). The combination allows for precise and fast frequency jumps (within a few ms) over large intervals of up to several GHz with an absolute precision of about 1 MHz, which is an important prerequisite for the intended analytical application of isotope ratio determination. The atomic beam–mass spectrometer arrangement used for HR-RIS is shown schematically in Fig. 2. A hot cavity graphite tube furnace is used for sample atomization. The sample is introduced in form of an aqueous uranium sample dropped and dried upon a piece of Zr foil which acts as reducing agent. By resistive heating the sample is slowly evaporated from the cavity exit hole, forming a rather well collimated atomic beam. A threefold electrostatic repeller setup serves for suppression of surface ions which are formed at the high temperatures required for efficient evaporation of uranium. As shown in Fig. 2, the blue laser beam counter propagates the two overlapped red laser beams in the crossed atomic-beam - laserbeam ionization arrangement, resulting in a substantial suppression of the residual Doppler width arising from the velocity distribution of the atomic beam. The laser ions are extracted, guided by electrostatic ion optics and afterwards bent by a 90◦
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Fig. 2 Spectrometer setup for HR-RIS measurements
electrostatic quadrupole deflector, which separates ionic from neutral species to reduce unspecific background. A subsequent quadrupole mass filter serves for further suppression of ionic background of incorrect mass and to investigate elemental and isotopic sample composition by mass spectrometry. Finally, low background ion detection is accomplished in a single channel secondary electron multiplier (channeltron). Aside of several minor modifications of the ion optics of the system, a major change in respect to the apparatus layout as described previously in [5] is the installation of a new atomic beam source. Higher absolute temperatures up to 2300 K together with a smoother temperature profile could be realized, preventing the occurrence of cold spots near the exit hole and corresponding efficiency losses due to adsorption. The new oven type was tested in the geometry of a laser ion source on other actinide elements using pulsed Ti:sapphire lasers, showing good performance during laser spectroscopic investigations while no characterization for analytical uranium measurements was attempted yet [6, 7]. However, in the case of Pu an overall efficiency of 1 · 10−3 was demonstrated [8]. For uranium a similar determination of the absolute detection efficiency with this source type in crossed beam geometry with cw laser light is subject of future studies.
3 Results and discussion In the comparison of the four schemes shown in Fig. 1, scheme (a) exhibits the highest ion yield and therefore is the most promising candidate for analytical investigations. Scheme (b) involves an autoionizing state which is broader than 30 GHz, resulting in a very low cross-section of this quasi-non resonant step in particular for
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Fig. 3 Scan of the high power laser across the 778 nm transition into the intermediate level at 37180 cm−1 with a second laser in resonance. As seen in the inlay, signal enhancement is observed only for both lasers in resonance, indicating unexpected strong non-resonant ionization
narrow-bandwidth radiation. Schemes (c) and (d) both populate sufficiently narrow autoionizing states; nevertheless, in both cases only a relatively low total ion yield was achieved. Presumably, the 828.967 nm transition into the intermediate level at 37412 cm−1 has a particularly weak transition strength, preventing competitive excitation and subsequent ionization for analytical purposes. A very interesting feature of scheme (a) is the fact that it requires only two lasers, significantly simplifying the laser system. The intermediate level at 38170 cm−1 shows a rather strong coupling to the continuum without a specific autoionizing level in resonance or even in the vicinity. This fact has been verified in the measurement shown in Fig. 3. The intermediate level was populated with the DL-PRO with laser power of 40 mW while the TA-PRO was scanned across a range of 30 GHz with a power of 900 mW. Observed is a weak non-resonant photoionization background when the TA-PRO frequency is off resonance together with a resonance enhancement of about a factor of 10, when both laser frequencies are coinciding, quantitatively somewhat below expectation. The value indicates that in resonance the high laser power totally saturates both the second excitation step as well as the (non resonant) ionization. This fact slightly reduces optical selectivity as compared to a resonant three-step excitation, while this simple excitation scheme already provides a high ion yield and simplifies the measurement procedure enormously. Correspondingly, further investigations are in progress for full characterization of this scheme, regarding. i.e., isotope shifts for computing the achievable optical selectivity and absolute ionization efficiency to extract a level of detection for the technique.
4 Conclusion and outlook The possibility to perform HR-RIS by employing BluRay© laser diode radiation for the first excitation step in uranium has been demonstrated. Different ionization schemes based on first step excitation around 405 nm were identified. One scheme seems particularly promising for analytical applications on the 236 U/238 U isotope
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ratio using a two-color three-photon ionization. Next step will be the determination of the absolute ionization efficiency and the LOD for uranium with the setup described herein, as well as identification of its benchmarks for the 236 U/238 U isotope ratio determination by measuring an isotope dilution series with a set of calibrated samples. Acknowledgements Funding from the “Deutsche Forschungsgemeinschaft” (DFG) under grant WE as well as the framework within the interdisciplinary Research Training Group GRK826 “Trace Analysis of Elemental Species” are gratefully acknowledged.
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