Masaki Oba

Enhancement of intensity in microwave-assisted laser-induced breakdown spectroscopy for remote analysis of nuclear fuel recycling

An enhancement of emission intensity from a laser ablation plume, obtained by coupling a pulsed microwave using a simple wire antenna, is demonstrated to compensate the sensitivity reduction of a high resolution spectrometer that is required for nuclear fuel analysis. A gadolinium oxide sample was irradiated with 2.45 GHz, 250 W microwave pulse, and passed through a loop antenna. As a result, up to 50-fold enhancement of the emission signal was achieved for gadolinium ions. The enhancement enabled us to measure the mass concentration of europium per gadolinium, ranging from 5% to 100 ppm, and based on the extrapolation of the calibration curve the detection limit for microwave-assisted laser induced breakdown spectroscopy (MA-LIBS) was estimated to be 40 ppm. This offers a flexible and compact system of MA-LIBS for nuclear fuel analysis.

Development of RIMS apparatus for isotope analysis of calcium in nuclear waste materials

We have developed a frequency stabilized laser system for RIMS analysis of a long-lived radionuclide of calcium (41 Ca) in concrete waste materials. A stable reference laser has been developed using magnetically induced circular dichroism of atomic rubidium. The frequency stability was estimated to be 50 kHz over 8-h period from beat frequency measurement. Furthermore, a computer-controlled fringe offset lock system has been developed to transfer the high frequency stability to the other lasers for three-step excitation of atomic calcium. By using the system precise frequency tuning of 1 MHz error over 1 GHz scan was realized. The overall frequency tuning precision was also confirmed by a multistep fluorescence and resonance ionization spectroscopy of calcium.

Total angular momenta of even-parity autoionizing levels and odd-parity high-lying levels of atomic uranium

Using three-step resonance ionization spectroscopy, over 200 even-parity autoionizing (AI) levels of atomic uranium, including Rydberg series converging to the second lowest ionic level (6L 11/2o), were observed in the 49 930–51 200 cm−1 energy range. Total angular momenta (J values) of these levels were determined by a polarization combination method as well as a method based on the J-momentum selection rule. Using the AI levels of which J values were determined unambiguously, unique J values were also assigned for about 70 high-lying odd-parity levels. The observed J-dependence on autoionization linewidth is interpreted as being due to a centrifugal potential barrier.

Ablation‐initiated Isotope‐selective Atomic Absorption Spectroscopy of Lanthanide Elements

For remote isotope analysis of low‐decontaminated trans‐uranium (TRU) fuel, absorption spectroscopy has been applied to a laser‐ablated plume of lanthanide elements. To improve isotopic selectivity and detection sensitivity of the ablated species, various experimental conditions were optimized. Isotope‐selective absorption spectra were measured by observing the slow component of the plume produced under low‐pressure rare‐gas ambient. The measured minimum line width of about 0.9 GHz was close to the Doppler width of the Gd atomic transition at room temperature. The relaxation rate of high‐lying metastable state was found to be higher than that of the ground state, which suggests that higher analytical sensitivity can be obtained using low‐lying state transition. Under helium gas environment, Doppler splitting was caused from particle motion. This effect was considered for optimization for isotope selection and analysis. Some analytical performances of this method were determined under optimum conditions an...

Isotope Shift Measurement for High-Lying Energy Levels of Atomic Uranium by Resonance Ionization Mass Spectrometry

Employing three-step resonance ionization mass spectrometry, isotope shift measurements of atomic uranium have been performed in the 32000–36000 cm -1 and 49950–50500 cm -1 regions. On the basis of the measured isotope shifts of several autoionizing Rydberg levels, isotopic difference in the ionization potential has been determined to be +445 mK (mK=10 -3 cm -1 ).

Determination of Ionization Potential of Calcium by High-Resolution Resonance Ionization Spectroscopy

High-resolution resonance ionization spectroscopy has been utilized to determine a precise ionization potential of Ca. Three-step resonance excitation with single-mode extended-cavity diode lasers populates long and unperturbed Rydberg series of 4snp ( 1 P 1 ) and 4snf ( 1 F 3 ) states in the range of n =20–150. Using an extended Ritz formula for quantum defects, the series convergence limit has been determined to be 49305.9240(20) cm -1 with the accuracy improved one order of magnitude higher than previously reported ones.

Development of Frequency Stabilized Diode Laser Based on a Spectroscopic Study of Magnetically Induced Circular Dichroism of Atomic Rubidium

We developed a frequency-stabilized diode laser using magnetically induced circular dichroism. From a spectroscopic study of the Rb-D2 line, the most stable Doppler-broadened and Doppler-free lines were found to be the transitions from the Fg=2 ground state of 85Rb and the crossover resonance from the Fg=3 ground state to the Fe=3 and 4 excited states of 85Rb, respectively. Furthermore, we measured the fluctuation of a laser locked to the Doppler-free line to be ±50 kHz/8 h and thus revealed the applicability to a long-term frequency stabilization. The laser developed can be used as a reference laser in a frequency stabilization system for multiple tunable lasers for resonance ionization mass spectrometry (RIMS) analysis.

Effect of defocusing on laser ablation plume observed by laser-induced fluorescence imaging spectroscopy

We used laser-induced fluorescence imaging with a varying beam focal point to observe ablation plumes from metal and oxide samples of gadolinium. The plumes expand vertically when the focal point is far from the sample surface. In contrast, the plume becomes hemispherical when the focal point is on the sample surface. In addition, the internal plume structure and the composition of the ablated atomic and ionic particles also vary significantly. The fluorescence intensity of a plume from a metal sample is greater than that from an oxide sample, which suggests that the number of monatomic species produced in each plume differs. For both the metal and oxide samples, the most intense fluorescence from atomic (ionic) species is observed with the beam focal point at 3–4 mm (2 mm) from the sample surface.

Effect of Defocusing on Ablated Volume of Gadolinium Oxide

We used nanosecond (ns) and femtosecond (fs) laser pulses to ablate gadolinium oxide samples and measured the ablated volume as a function of the laser pulse focal position. The effect of defocusing on the ablated volume, which decreases as the ablation pulse focal position approaches the sample surface, was observed for both ns and fs pulses.

Optical Parametric Oscillator with Periodically Poled KTiOPO4 Pumped by 100 Hz Nd:YAG Green Laser

An optical parametric oscillator with a periodically poled KTiOPO4 (KTP) as a gain medium pumped by a pulsed Nd:YAG green laser with a maximum pulse repetition frequency of 100 Hz is demonstrated. Output energy of 5.8 mJ at the pulse repetition frequency of 100 Hz with the conversion efficiency of about 40% is obtained by a double-pass pumping scheme. Threshold pumping energy is 1.6 mJ with a peak power as low as 18 MW/cm2. At a pulse repetition frequency between 10 Hz and 100 Hz, the conversion efficiency is almost constant and undesirable thermal effects are not observed.