Scientists achieve sensitive detection of calcium-41 single atoms

Professor Lu Zhengtian and Dr. Xia Tian of the University of Science and Technology of China collaborated with colleagues to achieve single-atom sensitive detection of the extremely rare isotope calcium-41 by using the atomic well trace analysis method, lowering the detection limit of the isotope abundance to the order of 10-17 (parts per billion), and demonstrating the analysis of calcium-41 isotope on typical samples such as bone, rock, and seawater. This work solves the problem of detecting calcium-41 isotopes in geological and biological samples, making calcium-41 promising to be used as a tracer dating isotope in earth science and archaeology. On March 2, the results were published online in Nature Physics.


Calcium-41 isotopes are induced by cosmic rays and are expected to be used in the dating of natural samples such as glaciers and paleontology Courtesy of China University of Science and Technology

Natural rocks and biological bones are generally rich in calcium, and their isotopic composition is mainly stable isotope calcium-40, while containing a very small amount of the radioactive isotope calcium-41. Calcium-41 has a half-life of 100,000 years, which is 17 times the half-life of carbon-14, so calcium-41 can cover an older dating range than carbon-14. Calcium-41 on Earth is produced mainly by the capture of cosmic ray neutrons by calcium-40 in the shallow layers of the surface (several meters deep), and its isotope abundance is only on the order of 10-16 to 10-15, far below the detection limit that can be achieved by commonly used mass spectrometers.

In the past half century, more than a dozen units around the world have used accelerator mass spectrometry methods to continuously tackle the detection problem of calcium-41, but due to the interference from potassium-41 of similar mass, only samples with high abundance (10-15) in nature can be measured, which hinders its practical application.


Atomic trap device for detecting calcium-41 single atoms Courtesy of China University of Science and Technology

In this work, the researchers first chemically extracted about 80 milligrams of calcium metal from rock, bone and seawater samples, and loaded it into an atomic furnace to heat it to produce atomic beams. Then, various laser manipulation methods such as cooling, focusing, deceleration, and magneto-optical traps based on cold atomic physics were used to trap calcium-41 atoms one by one from the beam. Counting of individual calcium-41 atoms is achieved by measuring the fluorescence emitted by captured atoms.

Finally, the researchers used the ultra-high selectivity of the atomic trap to exclude the interference of other isotopes, elements and molecules, and achieved quantitative analysis at the 10-16 isotope abundance level, with a measurement accuracy of 12%, and the detection limit was pressed to the order of 10-17.

In the next step, the researchers plan to carry out extensive cooperation with scientists at home and abroad to jointly explore the application of calcium-41 dating in the field of earth science and archaeology. (Source: Wang Min, China Science News)

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