Senior Projects of the Class of 2001
(updated October 3, 2000)
Brian Blackmore with Cyrus Taylor
Period and Periodicity of Classes of Cellular Automata
Cellular automata (CA), or lattice systems coupled with local rules, can be used to model a variety of dynamical systems including neural networks, chaotic systems, biological systems, and gas lattice systems. CA have a variety of physically-realizable properties such as dissipation, reversibility, and periodicity, all of which are poorly explained by current theories.
Periodicity of CA will be examined, starting with W-Rule systems, in an attempt to quantify periodicity and a period-to-parameter relationship for the W-Rule. Generalization will proceed to arbitrary states, neighborhood sizes, and lattice sizes. Generalized periodic CA will then be examined in an attempt to find reduced rule sets encapsulating large numbers of periodic rules for a given cellular space.
David Hanneke with Arnold Dahm
A New Technique for Measuring Isotopic Impurity Diffusion in Solid Helium
At low temperature and high pressure, 4He forms a solid with an hcp lattice. At temperatures near the melting point, 3He atoms diffuse by tunneling into delocalized vacancies about which little is known experimentally. Reported diffusion coefficients and their temperature dependencies are at variance with each other and may result from an anisotropic diffusion tensor. We propose to design and build a system that uses NMR techniques to observe this behavior. Using a gradient coil, we will observe NMR in a thin slice. We will saturate the spins in this slice with a large rf pulse, causing the NMR signal to vanish, and watch the signal recover as thermalized 3He atoms diffuse into the slice. The diffusion coefficient along the field gradient will thus be measured. Three orthogonal gradient coils will be used to measure the diffusion tensor in a single hcp helium crystal.
Moshe Katz-Hyman with Dan Akerib
Detection and Veto of High Energy Neutrons in the CDMS II Experiment
The cryogenic detectors used in the CDMS (Cryogenic Dark Matter Search) experiment are a novel type of device that can distinguish between nuclear and electron recoils. The detector accomplishes this by independently measuring both the ionization and phonon energies deposited by the recoils. This method of detection allows photons, electrons, and other charged particles to be distinguished from WIMPs, or Weakly Interacting Massive Particles, the type of dark matter we are searching for. However, neutrons are less distinguishable from WIMPs on an event by event basis because both are massive, uncharged particles which have a large phonon energy to ionization energy ratio. Great steps have been taken to shield the experiment from extraneous backgrounds. The CDMS II Experiment is located in the Soudan mine in Minnesota, which provides shielding from direct cosmic rays and other forms of radiation. Low energy neutrons can be shielded effectively from the detectors by hydrogen rich materials, such as polyethylene. High energy neutrons cannot be effectively shielded because the cross-section and energy deposition is too low in these materials. It is likely that the chief source of high energy neutrons is from hadron showers produced in the surrounding rock by cosmic ray muons. A possible method for detecting these showers and thereby allowing a veto of these high energy neutron events is the instrumentation of the surrounding rock. By placing an array of streamer tubes in the walls and ceiling of the the cave, it should be possible to detect the charged particles created in the hadron showers. In this experiment, Monte Carlo simulations of the CDMS II site at the Soudan site will be set up and cross-checked against data taken from the Stanford Underground Facility to confirm their reliability. Simulations will be performed to determine the feasibility of instrumenting the rock using different lattice spacings and pattern-detection algorithms. If these studies result in a practical design, then prototype tubes will be built and a field test, possibly using Lake Erie as the test site, will be carried out.
Oleg Kogan with Dr. Chih-Jen Sung, (Mech./Aero. Engr.)
Quantitative Temperature Measurement in Reacting and Non-Reacting Flows by Vibrational and Rotational Raman Spectroscopy
Non-intrusive techniques for measurement of scalar and hydrodynamic structures of a flow field have obvious advantages over the intrusive ones, as they do not modify the flow field itself. Using laser-based techniques to study the structure of a flow field are especially helpful because of the associated high temporal and spatial resolutions.
In the present study, we plan to utilize Raman spectroscopy to measure the temperature distributions in reacting and non-reacting flows. Particular interests include (1) the determination of the temperature based on the vibrational and rotational Raman spectra and the comparison of these vibrational- and rotational- Raman determined temperatures; and (2) the theoretical calculations of the corresponding Raman spectra.
Marta Lewandowska with Corbin Covault
Development of a CCD-Based Photometry System for Atmospheric Monitoring
High-energy astrophysical gamma rays produce extensive air showers when they strike the upper atmosphere of the Earth. The Solar Tower Atmospheric Cherenkov Effect Experiment (STACEE) collects the Cherenkov radiation produced in these air showers via an array of solar heliostat mirrors. Because STACEE uses the atmosphere to detect gamma rays, careful monitoring of atmospheric conditions is necessary in order to accurately calibrate the detector.
The STACEE Atmospheric Monitor (SAM) is a system for automated real-time photometry and cloud detection. It consists of a photomultiplier tube placed in the eyepiece of a computer-controlled Meade 8" LX200 Schmidt-Cassegrain telescope, controlled and read out by a serial link to a PC. Due to the photometer's narrow (8 arcminutes half-angle) field of view, the absolute pointing and tracking of the telescope must be accurately calibrated and maintained. The goal of the project is to design and construct a secondary optic outfitted with a CCD for the purpose of verifying the pointing of the telescope, and relaying that image to the control room. This addition will provide an alternative to the current photometric system. Once this optic is constructed, it will be mounted onto the telescope at the STACEE site in New Mexico and tested. If time allows, STACEE data will be taken, and work will be done to integrate the new optic into the photometry system.
Genevieve Mathieson with Phil Taylor
Models of Energy Flow in a Complex Society
In recent years, there have appeared a number of alternatives to traditional energy sources. As natural resources are depleted, it becomes more important to find reasonable replacements from these new energy sources. Some of these energy alternatives include photovoltaic, wind, hydroelectric, and nuclear power - both fission and fusion. As a student of history and physics, I sought a project that would combine historical and societal issues with scientific and technological ones. An examination of the physical processes of the various available energy sources for their efficiency, availability, and viability seems to be ideal; demonstrating the societal impact of physics. A major component of this project will be the presentation of this information in the form of an educational online game for use by the students of Physics 196 - Energy and Society in the Spring semester 2001.
Cameron McBride with Glenn Starkman
The Propagation of Light and Gravity through Matter-filled Spacetime with Stabilized Compactified Extra Dimensions
The idea of extra dimensions beyond the usual four is far from new. In the 1920s, Kaluza and Klein postulated an extra dimension in an attempt to unify the theory of gravity with electromagnetism. While not completely successful, the idea continues. Modern string theory requires 10 spatial dimensions to maintain mathematical consistency. There are also attempts to use large extra dimensions to solve the hierarchy problem -- the large ratio between the electroweak and Planck energy scales, which is manifested by the weakness of gravity in relation to the other forces.
In the standard approach to string theory, the extra dimensions exist as finite curled up ("compactified") spatial dimensions with sizes on the order of the Planck length (10^-35 meters). In 1998, Arkani-Hamed, Dimopoulos and Dvali proposed that these could be as large as 100 microns to a millimeter, if ordinary matter (standard model particles) is confined to a three-dimensional membrane (or "brane") within the multi-dimensional space (or "bulk"). The spreading out of gravitational field lines in the bulk would then explain the apparent weakness of gravity.
Whether large or small, one can ask why the extra dimensions are the size that they are -- why don't they expand or contract like the ordinary dimensions? The usual answer is that there is some dynamical mechanism (vaguely specified) which stabilizes them. Whatever that mechanism, we should expect the size of the extra dimensions to respond, even if only slightly, to the presence of mass. Gravity and light (unless it is confined to a brane) propagating through space should 'sense' the changing size of the extra dimensions. This should cause phenomena such as bending or retardation of light or gravity waves.
The behavior of waves traveling through inhomogeneous extra dimensions will be investigated. An attempt will be made to use the results to constrain theories of extra dimensions.
Brandon Miller with Rolfe Petschek
Liquid Crystal Alignment Properties
Alignment of liquid crystals is essential to a variety of devices and materials properties. First order phase transitions to highly ordered phases (such as the first order phase transition from the nematic to smectic C phase) often result in poor alignment. This project will examine theoretically and experimentally the possibility that regular, ordered nucleation sites for the more ordered phase will allow for formation of well-aligned liquid crystal phases. This work is expected to consist first of understanding nucleation theory and then hypothesizing theoretically the nature of the nucleation droplet for the smectic C phase in a nematic liquid crystal. The possibility of using photolithography or locally varying rubbing to promote such nucleation sites and so to promote uniform alignment of the liquid crystal will then be studied theoretically and experimentally. The experimental work will be done in Chuck Rosenblatt's lab under the supervision of Rolfe Petschek.
Tim Peshek with Daniel S. Akerib
Characterization of a Particle Detector for Use in the Cryogenic Dark Matter Search
The best evidence for dark matter comes from the observation that the luminous mass in galaxies is too small to account for the measured galactic rotational velocity. Big Bang Nucleosynthesis suggests that the fraction of critical density contributed by baryons, WB, is equivalent to 0.05 ± 0.005. Comparing measurements of the potential energy of galactic clusters to predictions obtained using the virial theorem and other methods provide values of WM = 0.35 ± 0.1, where WM is the ratio of the total matter density to the critical density. Non-baryonic dark matter must make up the difference. One candidate for dark matter provided by particle physics is the lightest supersymmetric partner, a particle belonging to a class known as Weakly Interacting Massive Particles or WIMPs.
The Cryogenic Dark Matter Search (CDMS) is one experiment which is designed to look for WIMPs. The detectors employ a thin film tungsten layer sputtered onto germanium or silicon crystals. This allows the simultaneous measurement of ionization and phonon energy deposited in an event, which aids in distinguishing between nuclear and electron recoils. WIMPs preferentially scatter directly off of an atomic nucleus, whereas background electromagnetic radiation typically scatters off electrons. To improve shielding, CDMS will begin to use these detectors in a deep underground facility; therefore it is imperative that each detector’s performance be fully tested before being incorporated in the WIMP search. CWRU has been made a test-site for the CDMS collaboration and will begin to fully characterize CDMS detectors.
One aspect to consider in running these detectors is their response to electrons. Past experiments have shown that electrons interacting with the detector may contaminate the surface and make the ionization measurement difficult. New detectors should have had this problem resolved and this thesis will focus on exposing the detectors to an electron source and a neutron source as a means to characterize surface versus bulk events for discrimination.
Christopher T. Sica with Robert Brown
A Nuclear Magnetic Resonance-based Desktop Imaging System
According to the principles of Nuclear Magnetic Resonance (NMR), a sample placed in a constant magnetic field and then subsequently exposed to an oscillating magnetic field will respond with a measurable magnetic signal. The goal of this project is to make the first step in designing a desktop imaging system based on NMR principles. Previous attempts have not been entirely successful, due in part to a lack of a sufficiently strong magnetic field. Through the use of a stronger magnet and fast computers, we hope to achieve a stronger signal, and then add a gradient field in order to examine whether we can measure spatial density variations in at least a one-dimensional body. An experiment such as this could be extended ultimately as a portable imaging system, or for use as a research tool.
Megan Smith with Ken Singer
Trap Spectroscopy in Photo-refractive Polymers
The mechanisms of photoconduction are of great interest due to the role of photoconduction in xerography and photorefractive holography. Photoconduction involves the excitation of electrons in response to light, leading, in the case of the materials of interest here, to the transport of electron holes. Some of the holes move to locations classified as traps because of the location of their energy levels. Hence a space-charge field develops in response to the electromagnetic field, which can act as a record of the light intensity pattern incident on the material.
Thus, hole traps are essential to the photorefractive process. An understanding of the nature of these sites is therefore useful in the study of photorefractive materials. This project will study the energy spectrum of trapping sites in photorefractive polymers using temperature dependent measurements of the time-of-flight mobility and xerographic discharge. The material investigated consists of a polymer called polyvinylcarbazole (PVK) which acts as the charge transporting agent, 4,4'-n-pentylcyanobiphenyl (5BC) as the nonlinear optical element and C60 as a sensitizer. Other materials may also be investigated in order to examine structure-property relationships.
Andrew Stickrath with Ken Singer
Orientational Mobility of Chromophores in Photorefractive Polymer Composites
Polymer composites doped with nonlinear optical species have been shown to have large nonlinear optical susceptibilities, and are of prime interest in the study of photorefraction. These composites consist in part of a moiety responsible for a linear electrooptic effect, essential for photorefraction and directly related to its strength. This role is filled by chromophores (the nonlinear optical species), each acting as a dipole, in the polymer composites to be studied. In order to observe photorefraction in polymers an external electric field must be applied to align these chromophores, thereby creating a noncentrosymmetric media and making second order effects such as the electrooptic effect possible. The dynamics of how the chromophores align with the electric field and with the photogenerated space charge field associated with the photorefractive effect are of great interest in the understanding of the photorefractive response and dynamics.
Previous work focused on electric field-induced second harmonic generation (EFISH) and ultra-fast laser pulses for the purpose of understanding this orientational dynamic has also resulted in the observation of several surprising phenomena. Among these are an unexpected initial rise and fall in second harmonic generation (SHG) in several composites and an increase in SHG in polymers doped with C60 (buckyballs) when exposed to He-Ne light. These processes and their relation to chromophore orientation will be studied and characterized both experimentally and theoretically.
Nick Wagner with Kathy Kash
Laser Trapping and Cooling
The use of lasers to trap and cool neutral atoms has attained temperatures in the millikelvin range. These cold atoms have allowed for high-resolution spectroscopy, increased the accuracy of atomic clocks, and are used to make Bose-Einstein condensates. We intend to build a laser trapping and cooling apparatus, that has been described in literature, for use in Senior Lab. This involves building external cavity diode lasers and associated electronics for locking the frequency on to atomic absorption lines. In the process, we will perform detailed laser spectroscopy on rubidium around 780nm to determine the energy levels used for trapping. Rubidium atom will then be trapping in a UHV chamber using two diode lasers: one to excite the trapping transition and one for hyperfine pumping. Once the trap is working, the fill rate and capacity of the trap can be measured as a function of laser power, laser polarization, and rubidium pressure. The cooled and trapped atom can be bounced off a magnetic field to demonstrate the quantization of angular momentum, or further cooled in a magnetic trap.
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