Researchers Realize All-optical Atom Trap Trace Analysis

The long half-life period of inert gases' radioactive isotope can be useful in radiometric dating. Since the discovery of 81Kr in the 1960s, the concentration of which is only 1×10-18 with a half-life time of 230,000 years, the researchers have been dreaming of tracing the water and ice circuit by dating ancient groundwater and glaciers.

In a study published in Physical Review Letters on July 6, Prof. LU Zhengtian and Doc. Florian Ritterbusch from University of Science and Technology of China realized optical excitation and trapping for single-atom detection of 81Kr.

Prof. LU has developed the method of atom trap trace analysis (ATTA), aiming at detecting isotope at an extremely low isotopic abundance. After being trapped in a magneto-optical trap, isotope atoms would transfer into a metastable state so that the atoms could be counted by their emitted fluorescence.

This approach hasn’t reached its full potential, as the main-stream method to get metastable atoms, via electron-impact excitation in a discharge, has problems such as low excitation efficiency, sample loss, and cross-sample contamination, and makes it difficult to reduce the water or ice sample size to extract enough 81Kr atoms.

To overcome the above limitations, Prof. LU's team tried optical excitation. The researchers chose photons with a wavelength of 124 nm to excite and trap krypton and saw the fluorescence of generated metastable krypton atoms at 819 nm.

Optical excitation schematic for 81Kr. (Image by WANG Jie et al.)

To deal with the self-absorption of 124 nm photons from the krypton discharge lamp (krypton in the lamp is strictly separated from the sample krypton in the measurement chamber), which caused unsatisfactory intensity of the vacuum ultraviolet (VUV), they conducted a Monte Carlo simulation of the photon transport in the lamp, and revealed a frequency change through scattering of the absorbed photons.

Subsequent experiments and measurements were consistent with the simulation results. The researchers optimized the lamp to develop a high intensity VUV lamp for ATTA. The all-optical ATTA offers a single atom loading rate of the rare 81Kr at 1800 atoms per hour.

Set up of the measuring chamber (left) and simulation results inside the lamb (right). (Image by WANG Jie et al.)

The design of the all-optical ATTA could reduce the sample size and extend the age range of radiometric dating with 81Kr, making the research of ancient groundwater and ice cores formed millions of years ago more efficient in earth and environmental sciences.

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(Written by JIANG Pengcen, edited by HOU Luyao, USTC News Center)

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