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Ultrasensitive trace-isotope analysis has been an important tool in modern science. Two well developed methods, Low-Level Counting (LLC) and Accelerator Mass Spectrometry (AMS), have been used for archaeological dating, medical diagnostic procedures, and for studying the transport processes in the ocean, atmosphere, and groundwater. They also have applications in physics, such as studying cosmic rays and detecting solar neutrinos. We have recently developed a widely applicable method, Atom Trap Trace Analysis (ATTA) , and demonstrated its feasibility by analyzing two rare isotopes in a natural krypton sample at the parts-per-trillion level. ATTA can enhance the capability and expand the applications of ultrasensitive trace-isotope analysis.
85Kr has a half-life of 10.8 years and an isotope abundance of ~10-11 in the atmosphere, while 81Kr has a half-life of 2.3 x 105 years and an abundance of 6 x 10-13. The present-day 85Kr in the environment has been released primarily by nuclear-fuel reprocessing plants. It has been used as a general-purpose tracer to study air and ocean currents, date shallow groundwater, and monitor nuclear-fuel reprocessing activities. In contrast, 81Kr is produced in the upper atmosphere by cosmic-ray induced reactions and is shielded from man-made fission products by stable 81Br. 81Kr is an ideal tracer for dating ancient groundwater and ice on the time scale of 105-106 years. A method of counting 81Kr atoms would also make possible a solar neutrino detector that is sensitive to both 7Be and 8B neutrinos from the sun.
Photo by George Joch
Figure 1. Dr. Chun-Yen Chen aligning the optics of the atom trap used to count 81Kr atoms and demonstrate the new Atom Trap Trace Analysis (ATTA) method.
LLC  has generally been used to measure the abundance of an isotope by counting its nuclear decays. It is currently used to count 85Kr. Although once used to count 81Kr, this is no longer possible because of the high present-day decay background of 85Kr. Taking a different approach, AMS  counts atoms instead of decays, thereby greatly enhancing the detection efficiency and avoiding the radioactive background problem. AMS is now routinely used as the standard method of 14C-dating. Recently, AMS has been used to count 81Kr and date groundwater . In this work, a large heavy-ion accelerator (K1200, MSU) was used to remove all of the electrons from the atoms so that 81Kr can be separated from its abundant isobar 81Br.
Figure 2. ATTA apparatus (about three meters long. (a) Discharge Source; (b) Skimmer; (c) Transverse Cooling; (d) Zeeman Slower; (e) Trap; (f) Photon Detector; (g) Balance Coil.). In this work, a krypton gas sample is injected into the system through a nozzle, around which a DC discharge is maintained. The discharge excite a fraction of the atoms into the 5s[3/2]2 metastable level so that the atoms can be manipulated by laser light at the convenient wavelength of 811nm. Two-dimensional transverse cooling is used to reduce the atomic beam divergence and amplify the atom flux in the forward di"RECT"ion. The thermal (300°C) atoms are then decelerated with the Zeeman slowing technique, and loaded into a magneto-optical trap. A photo-diode viewing the trap region detects the fluorescence of and count the trapped atoms.
Atom Trap Trace Analysis
A laser-based technique is attractive because it is simple, small and inexpensive. ATTA is based on techniques of laser manipulation of neutral atoms. In our system(see figure 2), individual krypton atoms are trapped by laser light inside a vacuum chamber and detected by viewing their fluorescence. An atom typically spends 0.1 second in the trap and scatters one million photons. The sensitivity of the fluorescence detector is such that a single trapped atom provides a signal-to-noise ratio of 50 (see figure 3).
Figure 3. Single atom counting. (a) Signal showing the arrival and departure of individual 83Kr atoms; (b) Signal of a single trapped 81Kr atom.
Figure 4. Number of trapped krypton atoms versus laser frequencies. (a) Dark peaks show the signals of abundant isotopes, while the resonant frequencies of the rare isotopes are marked. (b) Signal of the rare isotopes. The integration time of each data point of 81Kr(85Kr) signal is 3 hours(0.5 hours).
For isotope separation, ATTA relies on the fact that different types of atoms and molecules have different resonant frequencies. Even different isotopes of the same element differ due to the so-called isotope shifts. The atom trap is a wonderful isotope separator because it only traps atoms of a particular isotope when the laser frequency is tuned to its resonance position (see figure 4).
While the demonstration has been focused on krypton isotopes, ATTA can be applied to many other trace-isotopes for a wide range of potential applications including measuring solar neutrino flux, searching for exotic particles, tracing atmospheric and oceanic currents, archeological and geological dating, monitoring bone-loss rates in the diagnosis of osteoporosis, and monitoring fission products in the environment for safe-guarding nuclear wastes.
This work is currently pursued by a collaboration between the Physics Division (Kevin Bailey, Chun-Yen Chen, Xu Du, Yimin Li, Zheng-Tian Lu, Tom O'Connor) and the Chemistry Division (Linda Young) of Argonne National Laboratory. It is supported by the U.S. Department of Energy, Nuclear Physics Division, and LY by the Office of Basic Energy Sciences, Division of Chemical Sciences, under contract W-31-109-ENG-38.
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