The quantitative detection of trace amounts of atomic isotopes in environmental analysis, geological dating and other fields, has a very wide range of applications. 85Kr (t1/2 = 11 y, abundance 2 × 10-11) and other long-lived isotopes of inert gas, is ideal dating tracer elements for ground water, ice cores and other samples. Due to the low isotopic abundance and limited usable sample volume, new sensitive detection method is needed.

Atom Trap Trace Analysis (ATTA) is an emerging method for the analysis of rare krypton isotopes at isotopic abundance levels as low as 10-14 using krypton gas samples of a few micro-liters. ATTA is a laser-based instrument, utilizing a magneto-optical trap to capture atoms of the desired isotope, which only occurs when the laser frequency precisely matches the resonance frequency of a particular atomic transition. Any small changes in the atomic transition frequency, such as the isotope shifts caused by changes in nuclear size and mass, are sufficient to perfectly distinguish between the isotopes. At present, multiple ATTA experimental apparatus are under construction in USA, China, Germany and India.

In Prof. Shui-Ming Hu’s work, 12 samples with 85Kr/Kr ratios in the range of 10-13 to 10-10, prepared at University of Bern, Switzerland, are measured using two separate ATTA instruments in Hefei, China, and Argonne, USA, respectively. They are also measured in a low-level-counting (LLC) laboratory in Bern, Switzerland. The three laboratories conducted the measurements independently. The comparison shows that the 85Kr/Kr ratios determined by both ATTA apparatuses agree well with the 85Kr activities determined by LLC at the precision level of 5%. The results demonstrate that the ATTA instruments can be used for dating environmental samples with a sample size as low as a few micro-liters of Kr gas at the standard temperature and pressure (STP). The results demonstrate that ATTA instrument is capable of quantitative analysis.

Current atmospheric 85Kr inventory is mainly attributable to nuclear fuel reprocessing activities. It has long been the most abundant man-made radioactive isotope in the troposphere. It can be used for dating samples in the 2–50 y age range. Two other radioactive noble gas nuclides, 81Kr and 39Ar, which have much longer half-life (229 ky) and (269 y), and lower natural abundance (6×10-13) and (8×10-16) respectively, together with 14C (t1/2 = 5730 y, abundance 1×10-12), can be used for dating older samples in the 1-100 y age range. Determination of the 39Ar/Ar ratio using ATTA will be feasible if the counting rate of 39Ar can be improved by an order of magnitude. High efficiency (thus less sample size is needed) and high selectivity (immune to the contamination from any other atoms or molecules) are the main advantages of the ATTA method compared to other trace analysis methods.

This work was supported by the USTC, Chinese Academy of Sciences, and National Natural Science Foundation of China (NSFC).