An atom trap system for practical 81Kr dating

Abstract

81 Kr (t1/2=2.3×105 yr, 81Kr/Kr∼6×10−13) is a long-lived cosmogenic isotope, which is ideal for dating old groundwater and ice in the age range of 50,000 years to 1 million years. Here, we describe the apparatus and performance of an atom-counting system for practical 81Kr dating. This system is based upon the atom trap trace analysis method that was first demonstrated in 1999. Since then, significant improvements have been made to increase the system efficiency and to reduce the required krypton sample size. For a modern krypton gas sample of 100 μl STP, which contains 1.2×106 81Kr atoms, the system can accumulate approximately 240 81Kr counts in 20 h, thereby reaching a counting efficiency of 2×10−4. Detailed studies have been conducted to characterize the performance of this system. This system has been calibrated with a low-level counting method and has been used for 81Kr dating of ancient groundwater from the Nubian Aquifer (Egypt). It can also be used to measure the isotopic abundance of a fission-produced isotope 85Kr (t1/2=10.76 year,85Kr/Kr∼2×10−11).