Excitation of the metastable states of the noble gases (10 pages)

Tom J. Whitaker, Bret D. Cannon, Bruce A. Bushaw

Two new concepts for detecting noble gases are reported. Both techniques involve formation of the long-lived 1s5 metastable state of noble gases. The first technique utilizes the photon-burst method and should be capable of isotopically selective detection at extremely small relative abundances. The second concept incorporates a shelving technique that stores noble gas atoms in the metastable state and then pumps these atoms to a higher excited state that radiatively cascades to the ground state, emitting vacuum ultraviolet (VUV) photons. A significant advantage is that AlGaAs diode lasers can be used for the techniques rather than continuous wave cw dye lasers.

Гостев Валерий Анатольевич, D. V. Elakhovskii, A. D. Khakhaev

M. Abou Dakka

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.

Matthias Raunhardt

Kristen Matherson, R. D. Glover, D. E. Laban +1 more

We present progress towards measurements of total absolute collision cross sections of the 3P2 metastable state and the 3D3 excited state of neon with ground state noble gases utilizing a metastable neon magneto optical trap. The measurement is achieved by observing the decay rate of the trap as a function of added partial pressure of a noble gas. This technique will enable benchmark cross sections to be measured there by providing stringent tests of current quantum mechanical scattering calculations.

E. H. S. Burhop