Realization of radio-krypton dating with an atom trap

An atom trap system for practical 81Kr-dating has been developed. Calibration of the system has been accomplished. With this system, 81Kr-dating of ancient groundwater has been realized.; 81Kr (t_1/2 = 230 kyr, isotopic abundance ∼5 × 10−13) is a long-lived cosmogenic isotope, ideal for dating old groundwater and ice in the range of 50,000 years to a million years. Due to the extremely low isotopic abundance of 81Kr (∼1000 81Kr atoms are contained in 1 liter of modern ice or groundwater), analysis of 81Kr at or below the atmospheric isotopic abundance has been an experimental challenge.; This work presents the development of the Atom Trap Trace Analysis method for practical 81Kr-dating. This method is based on laser manipulation of neutral atoms. The atom trap system is capable of detecting individual 81Kr and 85Kr atoms with an isotopic abundance as low as 0.1 parts per trillion. With the counting efficiency of 2 × 10 −4 and the required sample size of below 100 μl STP of krypton gas, the system has met the requirement of implementing 81Kr-dating of ancient groundwater. A demonstration of the system capability on quantitative measurements has been performed by analyzing the ratios of 85Kr/ 81Kr in a group of Kr samples prepared by an Environmental Science group at the University of Bern. Among these samples derived from young (age less than 70 years) air samples, the 81Kr/Kr ratios are expected to be identical while the 85Kr/Kr ratios (measured with Low Level Counting at Bern group) are expected to vary. A good agreement between our results and LLC results has demonstrated the validity of our method.; The system has been used to date the ancient groundwater of the Nubian Aquifer. Two groundwater samples have been analyzed and their residence times are 596 ± 195 kyr and 474 ± 89 kyr, respectively.