ATTA-3: a State-of-the-Art Instrument for Radio-Krypton Dating

The ATTA-3 instrument at Argonne has recently enabled routine Kr-81 dating. The instrument is based on Atom Trap Trace Analysis (ATTA), a novel laser based atom counting technique that allows detection of long lived noble gas radioisotopes (Kr-81, Kr-85 and Ar-39) with extremely low abundance (1E-16 to 1E-10). At the center of the instrument is a magneto-optical trap (MOT), which traps and counts only the atoms of the desired isotope. This unique feature makes ATTA free of interference from any other isotopes or molecular species. For Kr-81 dating in the age range of 150 - 1,500 kyr, the required sample size is 5 - 10 micro-L STP of krypton gas, which can be extracted from approximately 100 - 200 kg of water or 40 - 80 kg of ice. Several recent developments in our lab may lead to further improvements to the current ATTA-3 apparatus: 1) The isotopic abundance ratio between the unknown, rare isotope (either Kr-81 or Kr-85) and the stable, abundant isotope (Kr-83) is measured. Here the stable isotope serves as a control isotope. A new method has been developed that allows more accurate measurements of the control isotope Kr-83. Combined with the ability to measure the rare Kr-81 and Kr-85 isotopes, this scheme allows ATTA-3 to directly determine 81Kr/Kr and 85Kr/Kr ratios without other supplemental measurements, to reduce the overall uncertainties of the measured isotope ratios, and also to improve the long term stability of the system. 2) The current capacity of the ATTA-3 instrument is about 120 samples per year. The throughput is mainly limited by the so called 'memory effect', which is caused by the residual samples trapped in the system after each measurement. These residual samples are gradually released in subsequent measurements, causing cross-sample contaminations. In order to mitigate this problem, we wash the system with a xenon discharge for about 36 hours between measurements. This practice limits the overall sample processing speed. Preliminary investigations of the memory effect indicate that it can be reduced and the wash time between measurements shortened. This will increase the number of samples that the ATTA-3 instrument can handle annually in the future. This work is supported by DOE, Office of Nuclear Physics, under contract DE-AC02-06CH11357.