Since its invention in 1947, carbon dating has revolutionised many fields of science by allowing scientists to estimate the age of an organic material based on how much carbon-14 it contains. However, carbon-14 has a half-life of 5,700 years, so the technique can’t determine the age of objects older than around 50,000 years.
In 1979, scientists suggested using calcium-41, with a half-life of 99,400 years, instead. It’s produced when cosmic rays from space smash into calcium atoms in the soil, and is found in the earth’s crust, opening the door to dating fossilised bones and rock. But several problems need to be overcome before it can be used to reliably date objects.
One important advancement was reported in Nature Physics in March 2023.
What is radiometric dating?
When an organic entity is alive, its body keeps absorbing and losing carbon-14 atoms. When it dies, this process stops and the extant carbon-14 starts to decay away. Using the difference between the relative abundance of these atoms in the body and the number that should’ve been there, researchers can estimate when the entity died.
A significant early issue with carbon dating was to detect carbon-14 atoms, which occur once in around 10 12 carbon atoms. Calcium-41 is rarer, occurring once in around 10 15 calcium atoms.
In the new study, researchers at the University of Science and Technology of China (USTC), Hefei, pitched a technique called atom-trap trace analysis (ATTA) as a solution. ATTA is sensitive enough to spot these atoms; specific enough to not confuse them for other similar atoms; and fits on a tabletop.
How does ATTA work?
A sample is vaporised in an oven. The atoms in the vapour are laser-cooled and loaded into a cage made of light and magnetic fields.
We know that in an atom, an electron in one orbital can transition to the next if it’s given a specific amount of energy; then it jumps back by releasing that energy. In ATTA, a laser’s frequency is tuned such that it imparts the same energy as required for an electron transition in calcium-41. The electrons absorb and release this energy, revealing the presence of their atoms.
The researchers reported being able to spot one calcium-41 atom in every 10 16 calcium atoms with 12% precision in seawater. “However, there was only one sample analysed,” Tian Xia, an associate scientist at USTC and a coauthor of the paper, told The Hindu by email.
Can ATTA be improved?
In future, “we hope that from the effusive atomic beam, the loading efficiency of the Ca-41 atoms into the trap can be improved,” so the measurement time for each sample is lower and the sensitivity is higher, Dr. Xia added.
USTC group leader Zheng-Tian Lu told Physics Today that ATTA’s success is due to innovations with lasers: “laser power is a lot higher, and laser frequency control is better – everything got better.”
ATTA also avoids potassium-41 atoms, which are similar to calcium-41 atoms but lack the same electron transition. It can also be modified to study isotopes of some noble gases that have defied techniques developed for carbon-14, such as argon-39, krypton-81, and krypton-85. They can be used to stay groundwater and frozen water.
What are the applications of ATTA + calcium-41?
“The successful application of [ATTA] to a calcium isotope now opens the possibility of extension to other metal isotopes,” University of Adelaide physicist Rohan Glover wrote in Nature in April.
The researchers are currently exploring an earth-science application. In warmer climate, glaciers retreat and allow rock below to accumulate calcium-41. In colder climate, glaciers advance and block the calcium-41 from reaching the rock. This way, scientists hope to use ATTA to study how long some rock has been covered by ice.
“We are collaborating with geo-scientists … by measuring the Ca-41 abundance in some rock samples,” Dr. Xia said.
- When an organic entity is alive, its body keeps absorbing and losing carbon-14 atoms. When it dies, this process stops and the extant carbon-14 starts to decay away. Using the difference between the relative abundance of these atoms in the body and the number that should’ve been there, researchers can estimate when the entity died.
- In ATTA, a laser’s frequency is tuned such that it imparts the same energy as required for an electron transition in calcium-41. The electrons absorb and release this energy, revealing the presence of their atoms.
- “The successful application of [ATTA] to a calcium isotope now opens the possibility of extension to other metal isotopes,” University of Adelaide physicist Rohan Glover wrote in Nature in April.
