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Towards Purely Laser Based Generation and Trapping of Metastable Krypton

M. Abou Dakka

Published Jan 1, 2019• 0 citations
OpenAlex

Abstract

Radioisotopes that break down at a constant rate over time through radioactive decay allow scientists to understand the development of geological and anthropological history by dating samples that contain such isotopes. While many techniques exist to measure the age of a sample, in this thesis we explore the possibility of enhancing the atom trap trace analysis (ATTA) method, which has made significant strides in the area of ultra-sensitive radioisotope detection, especially with the use of metastable noble gas radionuclides. A metastable atom is an atom in a stable, long-lived (>1 ms) excited state. Noble gases have the longest metastable lifetimes, which combined with their geochemical inertness, make them ideal candidates for geological and environmental radiometric techniques. The production of metastable noble gases for this purpose has traditionally relied on discharge sources, but this method suffers from important drawbacks such as high levels of crosscontamination and poor excitation efficiencies. In an effort to overcome these drawbacks, the method described in this work presents a new platform for implementing, characterising and comparing purely laser-based production of metastable noble gases. In the first part of this thesis, we present both state-of-the-art experimental results in the purely laser-based generation of metastable krypton atoms, and a physically realistic simulation with good agreement with our experimental results. Our calculation of the twophoton absorption cross section for krypton (to a metastable-generating excited state) using 215nm deep ultraviolet (DUV) laser radiation, obtained by fitting with theory, confirms that high metastable production efficiencies are possible with laser-based techniques. In addition to this parameter, the branching ratio and photoionisation rate from the excited state are calculated and agree with the literature. In the second part, we present the experimental progress to date for a brand new method of manipulating metastable noble gas atoms using a two-stage approach of metastable production then trapping via a magneto-optical trap (MOT). While the results are limited due to experimental difficulties, they are complemented by a physically realistic 3D simulation based on the Monte Carlo method. This simulation suggests that the production and trapping of metastable noble gases is feasible without the use of a discharge source, a Zeeman slower, and even a secondary excitation chamber. However, the loading rate for such a MOT is not sufficient for ATTA measurements without drastic changes in the experimental configuration. In the final part, we propose a next-generation ATTA system based on stimulated rapid adiabatic passage (STIRAP) to transfer noble gas atoms to their metastable state with near 100% efficiency. This technique for the generation of metastable noble gases would be purely laser-based and would compete with the loading rates of conventional production techniques, while avoiding their deleterious effects. We conclude by identifying limits of laser-based techniques and explore approaches to overcome them.

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Radioisotopes that break down at a constant rate over time through radioactive decay allow scientists to understand the development of geological and anthropological history by dating samples that contain such isotopes. While many techniques exist to measure the age of a sample, in this thesis we explore the possibility of enhancing the atom trap trace analysis (ATTA) method, which has made significant strides in the area of ultra-sensitive radioisotope detection, especially with the use of metastable noble gas radionuclides. A metastable atom is an atom in a stable, long-lived (>1 ms) excited state. Noble gases have the longest metastable lifetimes, which combined with their geochemical inertness, make them ideal candidates for geological and environmental radiometric techniques. The production of metastable noble gases for this purpose has traditionally relied on discharge sources, but this method suffers from important drawbacks such as high levels of crosscontamination and poor excitation efficiencies. In an effort to overcome these drawbacks, the method described in this work presents a new platform for implementing, characterising and comparing purely laser-based production of metastable noble gases. In the first part of this thesis, we present both state-of-the-art experimental results in the purely laser-based generation of metastable krypton atoms, and a physically realistic simulation with good agreement with our experimental results. Our calculation of the twophoton absorption cross section for krypton (to a metastable-generating excited state) using 215nm deep ultraviolet (DUV) laser radiation, obtained by fitting with theory, confirms that high metastable production efficiencies are possible with laser-based techniques. In addition to this parameter, the branching ratio and photoionisation rate from the excited state are calculated and agree with the literature. In the second part, we present the experimental progress to date for a brand new method of manipulating metastable noble gas atoms using a two-stage approach of metastable production then trapping via a magneto-optical trap (MOT). While the results are limited due to experimental difficulties, they are complemented by a physically realistic 3D simulation based on the Monte Carlo method. This simulation suggests that the production and trapping of metastable noble gases is feasible without the use of a discharge source, a Zeeman slower, and even a secondary excitation chamber. However, the loading rate for such a MOT is not sufficient for ATTA measurements without drastic changes in the experimental configuration. In the final part, we propose a next-generation ATTA system based on stimulated rapid adiabatic passage (STIRAP) to transfer noble gas atoms to their metastable state with near 100% efficiency. This technique for the generation of metastable noble gases would be purely laser-based and would compete with the loading rates of conventional production techniques, while avoiding their deleterious effects. We conclude by identifying limits of laser-based techniques and explore approaches to overcome them.
The determination of isotope ratios of noble-gas atoms has many applications, e.g. , in physics, nuclear-arms control, and Earth sciences. For several applications, the concentration of specific noble-gas isotopes ( e.g. , Kr and Ar) is so low that single-atom detection is highly desirable for a precise determination of the concentration. As an important step in this direction, we demonstrate the operation of a krypton atom trap trace analysis (ATTA) setup based on a magneto-optical trap (MOT) for metastable Kr atoms excited by all-optical means. Compared to other state-of-the-art techniques for preparing metastable noble-gas atoms, all-optical production is capable of overcoming limitations regarding minimal probe volume and avoiding cross-contamination of the samples. In addition, it allows for a compact and reliable setup. We identify optimal parameters of our experimental setup by employing the most abundant isotope 84 Kr, and demonstrate single-atom detection within a 3D MOT.
We demonstrate the generation of metastable krypton in the long-lived 1s^{5} state using laser excitation. The atoms are excited through a two-photon absorption process into the 2p^{6} state using a pulsed optical parametric oscillator laser operating near 215 nm, after which the atoms decay quickly into the metastable state with a branching ratio of 75%. The interaction dynamics are modeled using density matrix formalism and, by combining this with experimental observations, we are able to calculate photoionization and two-photon absorption cross sections. When compared to traditional approaches to metastable production, this approach shows great potential for high-density metastable krypton production with minimal heating of the sample. Here, we show metastable production efficiencies of up to 2% per pulse. The new experimental results gained here, when combined with the density matrix model we have developed, suggest that fractional efficiencies up to 30% are possible under optimal conditions.
Atom-Trap Trace-Analysis (ATTA) is a technique aimed at a measuring the relative (in comparison to the common isotope) abundance of exceedingly rare noble gas radio-isotopes (Ar-39, Kr-81 and Kr-85), for the purposes of dating water, ice and other materials that contain trapped gases. These isotopes exist with an extremely low natural abundance and require measurement capability down to parts in 1017. The noble gas dating protocol is similar to that of radio-carbon dating but, the three isotopes together, allow dating of materials in complementary ranges to that possible with the radiocarbon technique. In a facility jointly created by CSIRO, Griffith and Adelaide Universities, we use laser guidance, cooling and trapping to perform isotopic separation using the isotopic dependence of atomic energy levels. The facility is currently undergoing construction but has been shown to be able to produce metastable Ne and Ar, which can then be laser cooled and trapped. When combined with the existing gas separation facility at the CSIRO Waite facility, which efficiently extracts pure noble gases from that are dissolved in water or ice samples, we should be in a position to start measuring the age of real-world samples in the early part of 2020. In addition, to describing the current state of construction, we will also discuss two more efficient approaches for creating metastable atoms that can reduce samples size and avoid unwanted cross-contamination between samples.
n this presentation, we discuss results from two separate bodies of work. In the first, we investigate all-optical excitation methods to produce metastable-state krypton. The high energies required to excite rare gases out of their ground state present a unique challenge in the context of laser experiments. Laser physics work with rare gases often relies on excitation within an RF discharge. All-optical excitation is a promising replacement for RF discharges, avoids problems caused by ion production and may eventually allow for higher efficiencies. We examine three separate methods of all-optical metastable-state production: using an ArF excimer laser, using the third harmonic of the Jefferson Lab free-electron laser (FEL), and using a commercially available UV lamp. In the second body of work, we perform photoassociation spectroscopy in an ultracold, heteronuclear Rb + Ar* system. Photoassociation is a method that may be used to both 1) produce ultracold molecules, and 2) perform high-resolution spectroscopy on excited molecular states. Most work within the field has been performed within all alkali-metal systems. We examine a unique noble gas–alkali system within a dual species magneto-optical trap. We have performed a comprehensive search for evidence of excitation to molecular levels of RbAr. We illuminate atom pairs with a photoassociation laser in a spectral region detuned a few GHz below an atomic asymptote of argon, analogous to previous work in the homo-nuclear argon or krypton systems. We discuss our methodology and findings. We conclude with a discussion of how our results may be applied to future spectroscopic searches
Nobel gases, with their closed electronic shells, require large energy to transfer the atoms from the electronic ground state to the first accessible excited state. For this reason, it is common to use various discharge sources to transfer atoms from the ground state into excited states, in particular into a long-lived metastable state for optical manipulation. There are specific applications, however, that would benefit from optical methods over discharge methods to produce metastable atoms. These include Atom Trap Trace Analysis and velocimetry/wind tunnel experiments, where a discharge source is impractical.
Metastable noble gas atomic beam is widely used in atomic and molecular physics studies.Using radio-frequency discharge and transverse laser cooling, we produced a well-collimated intense meta-stable Krypton beam.Numerical simulation is also used to analyze the trajectories of atoms in an optic field produced by transverse cooling laser beams.The charactersisticl of the atomic beam are determined by measuring the laser induced fluorescence.The atomic beam flux measured at 230 cm downstream is 1.61016 atoms/(s*sr), which is enhanced by two orders of magnitude.The Kr atoms are finally trapped in a magneto-optic trap.A total of 1.31010 meta-stable 84Kr atoms can be simultaneously trapped with a loading rate of 3.01011 atoms/s.The same setup is also successfully used to obtain a bright metastable atomic argon beam and trap.