Browsing by Author "Lundeen, Stephen, committee member"

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Open Access
Barium tagging in solid xenon for the EXO experiment
(Colorado State University. Libraries, 2011) Mong, Brian, author; Fairbank, William, Jr., advisor; Lundeen, Stephen, committee member; Berger, Bruce, committee member; Van Orden, Alan, committee member
Neutrinoless double beta decay experiments are searching for rare decay modes never before observed to uncover the absolute mass of the neutrino, as well as to discover if it is a Majorana fermion. Detection of the daughter nucleus can help provide positive identification of this event over most radioactive backgrounds. The goal of the Enriched Xenon Observatory (EXO) is to measure the rate of 0νββ decay in 136Xe, incorporating 136Ba daughter identification by laser induced fluorescence spectroscopy. Here, we investigate a technique in which the 136Ba daughter is grabbed with a cryogenic probe by freezing it in solid xenon ice, and detected directly in the solid xenon. The absorption and fluorescence spectra of barium in solid xenon were observed for the first time in this work. Identification of the 6s2 1S0 → 6s6p 1P1 transition in both absorption (558 nm) and emission spectra (594 nm) were made. Additional blue absorption and emission lines were observed, but their transitions were not identified. Saturation of the 6s2 1S0 → 6s6p 1P1 transition was not observed with increased excitation rates using resonance excitation at 558 nm. From this a limit on the metastable decay rate was deduced to be greater than 104 s-1. Finally a fluorescence spectrum was obtained from a sample with only 20,000 atoms in the laser beam. With potential improvements of 107 in detection efficiency, single barium atom detection seems possible in solid xenon. A fiber probe detector based on a bare single mode fiber was also constructed and tested with fluorescing dye molecules. Successful detection of a few dye molecules in solution at the probe tip was demonstrated.
Open Access
Dynamics of low-density ultracold plasmas in externally applied electric and magnetic fields
(Colorado State University. Libraries, 2013) Wilson, Truman M., author; Roberts, Jacob, advisor; Krueger, David, committee member; Lundeen, Stephen, committee member; Yalin, Azer, committee member
The experiments described in this thesis were focused on the influence of external electric and magnetic fields and electron evaporation on the evolution of ultracold plasmas (UCPs). The UCPs were created from the photoionization of 85Rb which was first captured in a magneto-optical trap (MOT) and then magnetically trapped and transferred by a set of magnetic coils attached to a motorized translation stage to a region of the vacuum chamber with a set of electrodes. The first experiment studied the response of the UCP to sharp electric field pulses, which included 2 cycles of a sine wave pulse. These experiments showed a resonant response to the 2 cycles of rf that was density dependent, but was not a collision based mechanism. Instead, the response was caused by a rapid energy transfer to individual electrons through the collective motion of the electron cloud in the UCP. This density-dependent response allowed us to develop a technique for measuring the expansion rate of the UCPs in our system. It was also observed in second set of experiments that electron evaporation from the UCP had a significant effect on the amount of energy that was transferred to the ions to drive the UCP expansion. Model calculations show that we should expect electron evaporation to have a more significant influence on the UCP expansion rate at the relatively low densities of the UCPs that we create compared to other experiments. By modeling electron evaporation during expansion, our data are consistent with evaporation reducing the electron temperature significantly, which lowers the overall UCP expansion rate. In addition to these studies, we also performed an experiment in which it was observed that in the presence of a magnetic field there was a significant increase in the initial UCP expansion rate coupled with a deceleration of the ion expansion at later times in the UCP evolution. Our observations to date are consistent with the magnetic field influencing electron screening and UCP formation. By restricting the electrons motion in the direction transverse to the magnetic field lines to circular orbits around the magnetic field lines, the electrons cannot move appropriately to screen the internal radial electric fields produced by the excess of ions. Studies of this effect are currently under way. Future studies include direct measurements of the electron temperature and collision rates between the components of the UCP as we move towards trapping the UCP in a Penning trap.
Open Access
Experimental realization of two-isotope collision-assisted Zeeman cooling
(Colorado State University. Libraries, 2013) Hamilton, Mathew, author; Roberts, Jacob, advisor; Lundeen, Stephen, committee member; Gelfand, Martin, committee member; Bartels, Randy, committee member
The work presented in this thesis focuses on the demonstration and initial evaluation of a novel non-evaporative cooling method called collision-assisted Zeeman cooling. For this realization, an ultracold gas consisting of a mixture of 87Rb and 8Rb was used. Cooling was accomplished through interisotope inelastic spin-exchange collisions that converted kinetic energy into magnetic energy. Continual optical pumping spin polarized the 85Rb which ensured that only kinetic energy reducing collisions occurred and the scattered pump photons carried entropy out of the system. Thus, cooling of the ultracold gas can be achieved without requiring the loss of any atoms in order to do so. This represents a theoretical advantage over forced evaporative cooling, which is the current state-of-the-art cooling technique in most experiments. This thesis discusses the details of collision-assisted Zeeman cooling, as well as how the theory of the technique has been extended from cooling a single species to cooling with two species. There are many predicted advantages from using two rather than one species of atom in this type of cooling: greater flexibility in finding favorable spin-exchange collision rates, easier requirements on the magnetic fields that must be used, and an additional means to mitigate reabsorption (the primary limitation in many if not most non-evaporative cooling techniques). The experimental considerations needed to prepare a system that simultaneously trapped two isotopes to be able to perform collision-assisted Zeeman cooling are discussed. Because this cooling scheme is highly reliant on the initial conditions of the system, a focused experiment examining the loading of the optical trap with both isotopes of Rb was conducted and the results of that experiment are described here. The first experimental observations of spin-exchange collisions in an ultracold gas mixture of Rb are described as a part of this work. The experiments where collision-assisted Zeeman cooling were demonstrated are then described and evaluated. In this first implementation of the cooling technique the initial densities were too low and optical-pump-induced heating and loss too high for achieving the full predicted performance of the cooling technique. Through additional modeling, these limitations were understood and the necessary improvements for the next iteration of CAZ cooling experiments are laid out at the end of this work.
Open Access
Mobility and fluorescence of barium ions in xenon gas for the EXO experiment
(Colorado State University. Libraries, 2014) Benitez Medina, Julio Cesar, author; Fairbank, William, advisor; Berger, Bruce, committee member; Lundeen, Stephen, committee member; Menoni, Carmen, committee member
The Enriched Xenon Observatory (EXO) is an experiment which aims to observe the neutrinoless double beta decay of 136Xe. The measurement of this decay would give information about the absolute neutrino mass and whether or not the neutrino is its own antiparticle. Since this is a very rare decay, the ability to reject background events by detecting the barium ion daughter from the double beta decay would be a major advantage. EXO is currently operating a detector with 200 kg of enriched liquid xenon, and there are plans to build a ton scale xenon detector. Measurements of the purity of liquid xenon in our liquid xenon test cell are reported. These results are relevant to the research on detection of single barium ions by our research group at Colorado State University. Details of the operation of the purity monitor are described. The effects of using a purifier, recirculation and laser ablation on the purity of liquid xenon are discussed. Mobility measurements of barium in xenon gas are reported for the first time. The variation of mobility with xenon gas pressure suggests that a significant fraction of molecular ions are formed when barium ions interact with xenon gas at high pressures. The measured mobility of Ba+ in Xe gas at different pressures is compared with the predicted theoretical value, and deviations are explained by a model that describes the fraction of molecular ions in Xe gas as a function of pressure. The results are useful for the analysis of experiments of fluorescence of Ba+ in xenon gas. It is also important to know the mobility of the ions in order to calculate the time they interact with an excitation laser in fluorescence experiments and in proposed 136Ba+ daughter detection schemes. This thesis presents results of detection of laser induced fluorescence of Ba+ ions in Xe gas. Measurements of the pressure broadening of the excitation spectra of Ba+ in xenon gas are presented. Nonradiative decays due to gas collisions and optical pumping affect the number of fluorescence counts detected. A model that treats the barium ion as a three level system is used to predict the total number of fluorescence counts and correct for optical pumping. A pressure broadening coefficient for Ba+ in xenon gas is extracted and limits for p-d and d-s nonradiative decay rates are extracted. Although fluorescence is reduced significantly at 5-10 atm xenon pressure, the measurements in this thesis indicate that it is still feasible to detect 136Ba+ ions directly in high pressure xenon gas, e.g. in a double beta decay detector.
Open Access
Spatially-selective optical pumping cooling and two-isotope collision-assisted Zeeman cooling
(Colorado State University. Libraries, 2014) Wilson, Rebekah Ferrier, author; Roberts, Jacob, advisor; Krueger, David, committee member; Lundeen, Stephen, committee member; Marconi, Mario, committee member
In this thesis I describe two non-evaporative cooling schemes for cooling Rb atoms. The first is a Sisyphus-like ultracold gas cooling scheme called Spatially-selecTive Optical Pumping (STOP) cooling. In principle, STOP cooling has wide applicability to both atoms and molecules. STOP cooling works by exploiting the fact that atoms or molecules in a confining potential can be optically pumped out of an otherwise dark state in a spatially-selective way. Selecting atoms or molecules for optical pumping out of a dark state in a region of high potential energy and then waiting a fixed time after the optical pumping allows for the creation of a group of high kinetic energy atoms or molecules moving in a known direction. These can then be slowed using external fields (such as the scattering force from a resonant laser beam) and optically pumped back into the dark state, cooling the gas and closing the cooling cycle. I present theoretical modeling of the STOP cooling technique, including predictions of achievable cooling rates. I have conducted an experimental study of the cooling technique for a single cooling cycle, observing one dimensional cooling rates in excess of 100 micro-K per second in an ultracold gas of 87Rb atoms. I will also comment on the prospects for improving the cooling performance beyond that presented in this work. The second cooling scheme I investigated is called Two-Isotope Collision Assisted Zeeman (2-CAZ) cooling. Through a combination of spin-exchange collisions in a magnetic field and optical pumping, it is possible to cool a gas of atoms without requiring the loss of atoms from the gas. I investigated 2-CAZ cooling using 85Rb and 87Rb. I was able to experimentally confirm that the measured 2-CAZ cooling rate agreed with a cooling rate predicted though a simple analytic model. As part of the measured cooling rate, I quantitatively characterized the heating rates associated with our actual implementation of this cooling technique and found hyperfine-changing collisions to be a significant limitation for the 85/87Rb gas mixture. Possible improvements to this experiment will be discussed as well as the prospects for improved cooling performance using an atom without hyperfine structure as the optically pumped atom.
Open Access
Table-top, full-field, actinic microscope for extreme ultraviolet lithography mask characterization
(Colorado State University. Libraries, 2010) Brizuela, Fernando, author; Rocca, Jorge J., advisor; Marconi, Mario C., committee member; Lundeen, Stephen, committee member; Attwood, David T., committee member
The development of increasingly smaller, faster, and more complex electronic devices that significantly impact everyday life is driven by the ability of printing smaller and smaller components onto semiconductor chips. The number of transistors printed onto an integrated circuit has increased from about one thousand in the 1970 to over a billon in recent years. This exponential growth has been possible thanks to great advances in microlithography processing, and is expected to continue with the implementation of Extreme Ultraviolet Lithography (EUVL) for the printing of the next generation of semiconductor chips. Although EUVL is conceptually similar to conventional lithography in that a mask is projected onto the wafer with a set demagnification, the unique characteristics of extreme ultraviolet light have generated a myriad of technological challenges in the development of this new lithographic technique, including the availability of bright sources, photoresists, reflective optics, and metrology tools at these wavelengths. Of these challenges, the need for microscopes capable of characterizing the printability of absorber patterns on the reflective Mo/Si coated lithographic masks, has risen be to one of the highest priorities for chip manufacturers as they prepare to implement EUVL at high-volume manufacturing. Currently, only a very limited number of EUV microscopes for mask characterization are available. And although these few synchrotron-based microscopes have significantly contributed to the development of EUVL masks, their building-size illumination source make them unsuited for mask characterization in an industrial setting. This dissertation describes the development of the first compact, full-field microscope for at-wavelength characterization of EUVL masks. This microscope combines the output of a table-top 13.2 nm wavelength laser with state-of-the-art diffractive optics to render high quality images of the patterns on EUVL masks with 55 nm spatial resolution and acquisition times of less than 90 seconds. From these images we have demonstrated for the first time measurements of line-edge roughness and normalized intensity line slope of an EUVL mask using a compact microscope. This is significant because with this microscope that emulates the imaging conditions of a 4xdemagnification stepper it is possible to evaluate the mask quality and printability independently of photoresist response. It is foreseeable that these microscopes will not only contribute to the development of EUVL mask technology, but will also play a significant role in the path for the realization of convenient stand-alone metrology systems for on-site evaluation of EUVL masks.