Single-atom Counting Research Articles

Single-atom counting in a two-color magneto-optical trap

Recording the fluorescence of a magneto-optical trap (MOT) is a standard tool for measuring atom numbers in experiments with ultracold atoms. When trapping few atoms in a small MOT, the emitted fluorescence increases with the atom number in discrete steps, which allows to measure the atom number with single-particle resolution. Achieving such single particle resolution requires stringent minimization of stray light from the MOT beams, which is very difficult to achieve in experimental setups that require in-vacuum components close to the atoms. Here, we present a modified scheme that addresses this issue: Instead of collecting the fluorescence on the MOT (D2) transition, we scatter light on an additional probing (D1) transition and collect this fluorescence with a high-resolution microscope while filtering out the intense MOT light. Using this scheme, we are able to reliably distinguish up to 17 $^{40}$K atoms with an average classification fidelity of 95 \%.

60Fe deposition during the late Pleistocene and the Holocene echoes past supernova activity

Nuclides synthesized in massive stars are ejected into space via stellar winds and supernova explosions. The solar system (SS) moves through the interstellar medium and collects these nucleosynthesis products. One such product is 60Fe, a radionuclide with a half-life of 2.6 My that is predominantly produced in massive stars and ejected in supernova explosions. Extraterrestrial 60Fe has been found on Earth, suggesting close-by supernova explosions ∼2 to 3 and ∼6 Ma. Here, we report on the detection of a continuous interstellar 60Fe influx on Earth over the past ∼33,000 y. This time period coincides with passage of our SS through such interstellar clouds, which have a significantly larger particle density compared to the local average interstellar medium embedding our SS for the past few million years. The interstellar 60Fe was extracted from five deep-sea sediment samples and accelerator mass spectrometry was used for single-atom counting. The low number of 19 detected atoms indicates a continued but low influx of interstellar 60Fe. The measured 60Fe time profile over the 33 ky, obtained with a time resolution of about ±9 ky, does not seem to reflect any large changes in the interstellar particle density during Earth's passage through local interstellar clouds, which could be expected if the local cloud represented an isolated remnant of the most recent supernova ejecta that traversed the Earth ∼2 to 3 Ma. The identified 60Fe influx may signal a late echo of some million-year-old supernovae with the 60Fe-bearing dust particles still permeating the interstellar medium.

Methods for Gibbs triple junction excess determination: Ti segregation in $$\hbox {CoSi}_2$$ thin film

Methods are presented determining the Gibbs triple junction excess ( $${\varGamma }^{\text{TJ}}$$ ) of solute segregation in polycrystalline materials from single atom counting in 3D volumes. One method bases on cumulative profile analysis, while two further methods use radial integration of solute atoms. The methods are demonstrated and compared on simulated model volumes which include three grain boundaries joining together at a triple junction with set values for Gibbs grain boundary and triple junction excess. An experimental technique that provides 3D volumes with single atom detection and spatial resolution close to atomic scale is atom probe tomography. An atom probe tomography volume of a $$\hbox {CoSi}_2$$ thin film that contains three grain boundaries and a triple junction has been acquired. Ti segregation is found qualitatively at the grain boundaries and triple junction. The quantification of the Ti excess at the investigated $$\hbox {CoSi}_2$$ triple junction reveals for the three introduced methods positive Gibbs triple junction excess values. It demonstrates that there is an excess of Ti at $$\hbox {CoSi}_2$$ triple junctions and provides opportunities for its quantification.

Ultra-sensitive radionuclide analyses: new frontiers in radioanalytics

The most sensitive technologies for ultra-low-level analyses of long-lived radionuclides have been accelerator mass spectrometry and inductively coupled plasma mass spectrometry, reaching detection limits about 1 nBq/g. Together with underground operation of large volume HPGe detectors they have had great impact on underground physics experiments, approaching new frontiers in radioanalytics—a single atom counting.

Laser-based radiocarbon detection in the laboratory: How soon?

Research over the past 25 years and the use of accelerator mass spectrometry (AMS) have demonstrated benefits of single-atom counting of 14 C compared with scintillation monitoring of 14 C radioactive decay for a multitude of applications in drug development studies. These include pharmacokinetics and metabolism studies, microdosing studies, and quantification of DNA adducts. In the last decade, the possibility of single-atom counting using lasers has been demonstrated, providing the possibility of simplified laboratory-based systems, which can equal or excel AMS sensitivity and provide scintillation system convenience without high levels of radioactivity. To achieve the required sensitivity, optical storage cavities have been used to enhance the laser interaction of the low densities of radiocarbon present. Two types of laser technologies have been used-cavity ring-down spectroscopy (CRDS) and intracavity opto-galvanic spectroscopy (ICOGS). Problems to be overcome to achieve routine use have included separation of the 14 C signal from backgrounds, achievement of acceptable precision and accuracy, reduction of measurement times for small samples, and improvement in the ease of use for the operator. Both technologies have achieved impressive results to date using samples of order 1 mg with CRDS and 10 μg with ICOGS. Commercial development is the next step.

Developments in radioanalytics: from Geiger counters to single atom counting

Progress in radioanalytical science, mainly in radiometrics and mass spectrometry technologies for ultra-sensitive analyses of radionuclides applied in natural sciences is shortly reviewed. While in the radiometrics sector the greatest developments have been made in underground Ge gamma-spectrometry, in the mass spectrometry sector the accelerator mass spectrometry had dominant position reaching the status of single atom counting and compound specific analysis for some long-lived radionuclides.

Precise measurement of the thermal and stellar Fe54(n,γ)Fe55 cross sections via accelerator mass spectrometry

The detection of long-lived radionuclides through ultra-sensitive single atom counting via accelerator mass spectrometry (AMS) offers opportunities for precise measurements of neutron capture cross sections, e.g. for nuclear astrophysics. The technique represents a truly complementary approach, completely independent of previous experimental methods. The potential of this technique is highlighted at the example of the $^{54}$Fe($n, \gamma$)$^{55}$Fe reaction. Following a series of irradiations with neutrons from cold and thermal to keV energies, the produced long-lived $^{55}$Fe nuclei ($t_{1/2}=2.744(9)$ yr) were analyzed at the Vienna Environmental Research Accelerator (VERA). A reproducibility of about 1% could be achieved for the detection of $^{55}$Fe, yielding cross section uncertainties of less than 3%. Thus, the new data can serve as anchor points to time-of-flight experiments. We report significantly improved neutron capture cross sections at thermal energy ($\sigma_{th}=2.30\pm0.07$ b) as well as for a quasi-Maxwellian spectrum of $kT=25$ keV ($\sigma=30.3\pm1.2$ mb) and for $E_n=481\pm53$ keV ($\sigma= 6.01\pm0.23$ mb). The new experimental cross sections have been used to deduce improved Maxwellian average cross sections in the temperature regime of the common $s$-process scenarios. The astrophysical impact is discussed using stellar models for low-mass AGB stars.

Analysis of Krypton-85 and Krypton-81 in a Few Liters of Air

Long-lived radioactive krypton isotopes, (81)Kr (t1/2 = 229,000 year) and (85)Kr (t1/2 = 10.76 year), are ideal tracers. (81)Kr is cosmogenic and can be used for dating groundwater beyond the (14)C age. (85)Kr is a fission product and can be applied in atmospheric studies, nuclear safety inspections, and dating young groundwater. It has long been a challenge to analyze radio-krypton in small samples, in which the total number of such isotopes can be as low as 1 × 10(5). This work presents a system developed to analyze (81)Kr and (85)Kr from a few liters of air samples. A separation system based on cryogenic distillation and gas chromatographic separation is used to extract krypton gas with an efficiency of over 90% from air samples of 1-50 L. (85)Kr/Kr and (81)Kr/Kr ratios in krypton gases are determined from single-atom counting using a laser-based atom trap. In order to test the performance of the system, we have analyzed various samples collected from ambient air and extracted from groundwater, with a minimum size of 1 L. The system can be applied to analyze (81)Kr and (85)Kr in environmental samples including air, groundwater, and ices.

Ion current as a precise measure of the loading rate of a magneto-optical trap

We have demonstrated that the ion current resulting from collisions between metastable krypton atoms in a magneto-optical trap can be used to precisely measure the trap loading rate. We measured both the ion current of the abundant isotope 83Kr (isotopic abundance=11%) and the single-atom counting rate of the rare isotope 85Kr (isotopic abundance∼1×10(-11)), and found the two quantities to be proportional at a precision level of 0.9%. This work results in a significant improvement in using the magneto-optical trap as an analytical tool for noble-gas isotope ratio measurements, and will benefit both atomic physics studies and applications in the earth sciences.

Normalization of the single atom counting rate in an atom trap

The single atom counting rate of a rare isotope and the loading rate of another stable isotope with an abundance over 10 orders of magnitude larger are measured in one atom trap. The linear correlation between the measured counting/loading rates is examined to determine the (84)Kr/(82)Kr and (85)Kr/(83)Kr ratios of a Kr gas sample. Experiments show that the relative uncertainty is reduced to 1.3% when the single atom counting rate of (85)Kr is normalized by the measured (83)Kr loading rate. The measurement of the normalized single atom counting rate can be used to determine extremely low (10(-16)-10(-11)) isotope abundance. This normalization method is robust and can also be applied in other atomic systems.

Towards ultrahigh sensitivity analysis of 41Ca

An atom trap trace analysis system based on the technique of laser manipulation of neutral atoms is being developed to count individual 41Ca atoms present in natural samples with an isotopic abundance of 10 −15. Trapping of all stable calcium isotopes has been demonstrated and single-atom counting has been realized. For the most abundant isotope, 40Ca (97% isotopic abundance), a magneto-optical trap loading rate of 2×10 10 atoms/s has been reached at the overall capture efficiency of 1×10 −4. System improvements could increase the efficiency by at least an order of magnitude.

Resonance ionization spectroscopy and the detection of 81Kr

Development of resonance ionization spectroscopy (RIS) has made the concept of single-atom counting, whether radioactive or stable, practical for most elements of the periodic table. Tunable narrow-band lasers are used to efficiently and selectively excite and ionize a chosen element without interference from the much more abundant background, thereby achieving sensitivity at the level of a few atoms. Applications of the selectivity and sensitivity of RIS are numerous, including the direct analysis of solid, chemically processed liquid and noble gas samples. For example, 81Kr, a 2.1 × 10 5 year half-life cosmogenic radioisotope was detected with less than 400 81Kr atoms in the RIS analysis system.