The reporter learned from the University of Science and Technology of China that Professor Hu Shuiming and Dr. Yang Guomin from the Laser Pointer Trace Detection and Precision Measurement Research Group of the National Research Center for Microscale Physical Science at the University of Hefei cooperate with the International Atomic Energy Organization (IAEA) and other units to deep groundwater in the North China Plain Important progress has been made in the research, and related results have recently been published online in the Earth and Planetary Science Express.
The project collected deep (300-700 meters) groundwater samples in several areas of the North China Plain, and separated trace amounts of radon dissolved in them. The laser pointers were used to measure the content of long-lived isotope 81 to confirm the age of deep groundwater in the area; And by comparing the content ratios of helium and neon in the groundwater with the isotopic composition, it is determined that 90% of the helium 4 in the groundwater in the area is generated in situ by the crust; using the absolute age of 81 to estimate the effective flux of helium 4 into the aquifer, Corrected the helium 4 dating model. Among them, the 81 dating data provided key time information, identified the source of helium 4 in the groundwater, and measured that the oldest groundwater in the area reached 1 million years, with an average flow velocity of less than 1 m / year.
The radioactive isotope plutonium 81 is mainly produced by the reaction of cosmic rays and stable plutonium isotope in the atmosphere, with a half-life of 230,000 years. As an inert gas, thorium has no chemical reaction in nature, and its transport mechanism is simple. After entering the groundwater, plutonium 81 is isolated from the atmosphere and its concentration decays and decreases over time. In the range of 50,000-1.3 million years, 81 is the most ideal dating isotope. However, due to the extremely low isotopic abundance, its detection is extremely difficult.
Scientific researchers first proposed a laser pointer cold atomic trap trace analysis method, which uses laser cooling to capture specific isotope atoms and performs single-atom counting to obtain the isotope content. The research team of the Chinese University of Science and Technology cooperated with the geological research unit to conduct groundwater dating studies in several large basins in China. Previously, the research team cooperated with Pang Zhonghe and the Research Institute of the Institute of Geology and Geophysics, Chinese Academy of Sciences to determine the age of geothermal water in Xianyang area of the Guanzhong Basin using the 81 method, and found that the geothermal water age was between 300,000 and 1.3 million years. So far, these are the only two confirmed "fossil" waters in China.