A compact cavity ring-down spectrometer aimed at the detection of trace amounts of radiocarbon (¹⁴C) in biomedical samples is demonstrated. Rapid sampling, as required for large scale studies, is made possible by modifications to a CHNS elemental analyzer. After conversion of the carbon content of a sample into CO₂, spectral analysis using a high-finesse cavity in conjunction with a quantum cascade laser system reveals the ratio of ¹⁴CO₂ in relation to the stable isotopologues of ¹²C and ¹³C. Cooling and temperature stabilization of the cavity is achieved by a combination of liquid cooling and thermo-electric elements. The system is studied in terms of reproducibility, linearity, and sensitivity as well as contamination and memory effects of the sampling process. While the performance of the system is still limited mainly due to etaloning and other systematic effects, first spectra of biomedical samples, such as urine and feces, have been analyzed at ¹⁴C concentration levels above ten times natural abundance. Results are compared with those from a traditional liquid scintillation counter system. Possibilities for improvement of the sensitivity are discussed.

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 linewidth reduction of a distributed feedback quantum cascade laser (DFB-QCL) based on optical feedback for a mid-infrared (MIR) cavity ring down spectroscopy (CRDS) ¹⁴C spectrometer is presented. A cat-eye reflector as well as a path-length enhancement by a Herriott cell were employed for a compact optical setup. The laser linewidth was evaluated by monitoring the beat frequency between the DFB-QCL and a MIR optical frequency comb (OFC). The linewidth reduction by optical feedback was clearly observed although slow frequency drifts caused by environmental changes were still visible. A low-bandwidth beat-note lock to the OFC was conducted for long-term stabilization as well as for precise frequency scanning and thereby ¹⁴C measurement by MIR CRDS with optical feedback was demonstrated successfully.

Identification method for a hidden or shielded gamma-ray source using a vehicle-mounted 4$\pi$ gamma-ray imager was proposed. Since 4$\pi$ gamma-ray imaging provides direction of incident gamma-rays into the imager and its intensity, a position and an activity of a gamma-ray source can be estimated by several 4$\pi$ gamma images obtained by the imager moving around the source in combination with locations and orientation data of the imager. Using a 3D pixel array CdTe detector as a prototype of 4$\pi$ Compton gamma-ray imager, both the position and activity of a single ¹³⁷ Cs point source was estimated quantitatively in 3D voxel space by 4$\pi$ Compton gamma-ray images measured at three positions. In addition, 4$\pi$ gamma images were obtained by off-line processing of data acquired during the movement of the vehicle-mounted imager with processing time less than measurement time.