Atom Trap Trace Analysis For ⁴¹Ca At Ultralow Isotopic Abundance

Calcium is an abundant metal element on Earth,widely distributed in reservoirs at the Earth’s surface:the hydrosphere,the biosphere,and the crust.The potential methodology of calcium dating is expected to promote the knowledge of Earth evolution,paleoanthropology and paleontology.The isotope of calcium 41 Ca is radioactive with a half-life of≈100 ka,produced predominantly by capturing cosmicray-induced thermal neutrons via the 40Ca(n,γ)41Ca reaction.It has long been proposed as a tracer for dating samples in the range of 5 0 ka～1Ma.However,41 Ca dating method has not been built up so far,because accelerator mass spectrometry(AMS)is unable to meet the needs of 41Ca dating,whose instrumental background of 41Ca/Ca ratio is at the 10-15 level,comparable to the saturated abundance of 41Ca on the earth surface.In order to fully explore the applications of 41 Ca dating,the detection background of AMS needs to be further suppressed by at least one order of magnitude.The build-up of 41Ca dating method meets the bottleneck-insufficient 41Ca measurement capability at ultra-low abundance.In 2004,there reported a demonstration of 41Ca sample analysis with 41Ca/Ca ≈10-8 with atom trap trace analysis(ATTA)method.In an ATTA system,laser is employed to realize high isotopic selectivity of atomic cooling and trapping,without isobaric or molecular interferences.In this project,we design and construct a highperformance Ca-ATTA system,and explore the procedure of preparing Ca-ATTArequired metallic Ca from Ca compounds in reference and environmental samples,including granite,animal bones,and seawater.We perform measurement at the 41Ca/Ca level of 10-16,with a detection limit at 10-17 level and precision of 12%.The precision and accuracy of the 41Ca/Ca ratios are verified by measuring a series of reference samples in the range of 5.3 × 10-16-3.6 × 10-13,one order of magnitude below the saturated abundance.In the system,Ca atoms pass through transverse cooling zone,transverse focusing zone,and Zeeman slower,and finally get captured and detected in the magneto-optical trap(MOT).We introduce a 453-nm laser beam irradiating the trapped atoms as the repumper to extend the capture lifetime,and program a time sequence to suppress isotopic interference.The high-performance Ca-ATTA system sheds light on the 41Ca analysis of environmental samples.The 41Ca/Ca results of the aforementioned environmental samples are at 10-15 and 10-16 level.This table-top Ca-ATTA system has exhibited more powerful strength than AMS in 41 Ca/Ca analysis,and met the requirements to establish 41Ca dating method,and is poised for 41Ca dating studies in geoscience and archeology.