Stability of nuclear decay rates

A few interesting publications

Here are two excellent articles related to Zach Etienne's colloquium




Selected Publications

Thumbnail of figure from publication
A neutral strongly coupled laser-produced plasma by strong-field ionization in a gas jet
S. D. Bergeson, M. Lyon, J. B. Peatross, N. Harrison, D. Crunkleton, J. Wilson, and S. Rupper (et al.)
We report measurements of a neutral strongly coupled plasma generated by focusing a femtosecond-duration laser pulse into a room-temperature gas jet. The ion temperature in this plasma is determined by the plasma density through the disorder-induced heating effect. We present measurements of the mass, radius, and energy dependence of the time-varying ion density as the plasma expands. Molecular dynamics model indicate that higher values of the strong coupling parameter could be achieved if the plasma is ionized again by a second laser pulse that follows the first one. However, the final value of the coupling parameter appears to be only weakly dependent on the final ionization state.
Thumbnail of figure from publication
Towards Stronger Coulomb Coupling in an Ultracold Neutral Plasma
M. Lyon and S. D. Bergeson
Ultracold neutral plasmas are strongly coupled Coulomb systems that are generated by photoionizing lasercooled atoms close to the ionization threshold. The strong coupling parameter Γ is limited at times later than ∼100 ns by disorder-induced heating. A recent simulation predicted that higher values of Γ can be realized in ultracold neutral plasmas if the plasma ions are excited to higher ionization states. In this paper we present recent results from an experiment that increases the strong coupling of an ultracold neutral plasma by promoting the plasma ions to the second ionization state. Using laser-induced fluorescence we map out the ion velocity distribution of the Ca+ ions in a partially doubly ionized plasma and show that the heating due to the second ionization depends on the timing of the second ionization laser pulses. We compare our results to MD simulations, which estimate that Γ increases from approximately 2.5 to 3.6.
Thumbnail of figure from publication
Using higher ionization states to increase Coulomb coupling in an ultracold neutral plasma
M. Lyon and S. D. Bergeson (et al.)
We report measurements and simulations of the time-evolving rms velocity distribution in an ultracold neutral plasma. A strongly coupled ultracold neutral Ca +   plasma is generated by photoionizing laser-cooled atoms close to threshold. A fraction of these ions is then promoted to the second ionization state to form a mixed Ca + −Ca 2+   plasma. By varying the time delay between the first and the second ionization events, a minimum in ion heating is achieved. We show that the Coulomb strong-coupling parameter Γ  increases by a factor of 1.4 to a maximum value of 3.6. A pure Ca 2+   plasma would have Γ=6.8 , moving these strongly coupled systems closer to the regime of liquid-like correlations.

Theses, Captstones, and Dissertations

Figure from thesis
Ion Friction at Small Values of the Coulomb Logarithm (by Robert Tucker Sprenkle)
We create a dual-species ultracold neutral plasma (UNP) by photo-ionizing Yb and Ca atoms in a dual-species magneto-optical trap. Unlike single-species UNP expansion, these plasmas are well outside of the collisionless (Vlasov) approximation. We observe the mutual interaction of the Yb and Ca ions by measuring the velocity distribution for each ion species separately. We model the expansion using a fluid code including ion-ion friction and compare with experimental results to obtain a value of the Coulomb logarithm of Λ= 0.04.
Figure from thesis
Quantum Interference in the Hyperfine Transitions of Ytterbium (by Adam Dodson)
Emission spectra from atoms with hyperfine structure typically show closely-spaced atomic transitions. This happens because the hyperfine interaction splits and shifts the fine-structure energy levels in both the ground and excited state by a small amount. In laser-induced fluorescence measurements, the atoms are driven into a superposition of excited hyperfine states which then decay into a range of ground hyperfine states. Interference in different quantum pathways for this process influences the probability of excitation. Unless this is properly accounted for, this interference effect systematically shifts the apparent center of the fluorescence lineshape. We report measurements of this quantum interference (QI) effect in Yb-171 and Yb-173 and show that QI shifts the line centers by up to 5 MHz. We extend and verify a published QI model for Yb-171. We show that optical pumping complicates a straightfoward application of the model to the experiment for Yb-173. We then demonstrate that optical pumping-induced variations in the distribution of magnetic sub-levels in the hyperfine structure are insufficient to explain observed shifts in Yb-173.
Figure from thesis
Construction of a Compact Two-Dimensional Magneto-Optical Trap for a Cold Calcium Beam (by Daniel Woodbury)
We present the construction and characterization of a two-dimensional magneto-optical trap (MOT) as a source of cold calcium atoms. Atoms are cooled transversely from a hot effusive source and captured in the 2-D MOT without axial confinement. We present calculations for and creation of the calcium source, as well as calculations for a paired ytterbium 2-D MOT. The results demonstrate that the 2-D MOT is feasible for calcium, with atomic density reaching a peak of $1.3 \times 10^9$ atoms/cm$^3$ in the trap. While we predict these densities can provide a loading rate 25 times higher than loading directly from an effusive beam, we were not able to successfully create a 3-D MOT. We present possible reasons for these results, as well as our future plans for experimental development.