Mid-infrared laser absorption spectroscopy utilizing a high-finesse optical cavity enables high precision trace analysis of gas molecules. In particular, optical detection of radiocarbon (14C) based on cavity ringdown spectroscopy using a quantum cascade laser (QCL) is gaining attention as an alternative to accelerator mass spectrometry. This paper reports a compact-packaged narrow-linewidth QCL system utilizing resonant optical feedback from an external V-shaped cavity. Based on frequency noise analysis, the derived laser linewidth is 44 kHz for 100 μs integration time with the capability to perform seamless frequency scanning around 10 GHz. We installed this laser system within a table-top cavity ringdown spectrometer for 14CO2. A single-shot detection limit of 1.2 × 10−9 cm−1 Hz−1/2 leading to a detectable abundance evaluated from a noise analysis of 0.2 in fraction modern 14C for a 10-s averaging time was achieved. This capability of rapid analysis for 14CO2 is suitable for various applications requiring trace 14C analysis.
A rapid and simple tritium analysis method is required for tracer application and quantitative evaluation of radioactive waste. In this study, we focused on cavity ring-down spectroscopy, which is an ultra-sensitive laser absorption spectroscopy, and developed a spectrometer for tritium analysis. A current modulation-assisted acoustic optical modulator switching method was developed in the prototype setup containing a 2.2 μm diode laser for accessing the 2ν1 absorption band of HTO water vapor. The benefit of this switching method was investigated using the Allan deviation and compared to conventional acoustic optical modulator-only and current-only switching methods. Using the prototype setup with the proposed switching method, cavity ring-down spectroscopy of stable H2O vapor was demonstrated. The detection limit for liquid tritium water analysis was estimated to be 2× 101 kBq/10 µL for ten minutes measurement.
2.2 µm cavity ring-down spectrometer was developed for tritium and deuterium isotope analysis of water samples. Spectra were obtained during flow-through measurement with H2O and D2O mixed liquid injection and carry-over was negligible.
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