Doctoral Dissertations
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Browsing Doctoral Dissertations by Content Type "Thesis"
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- Direct and absolute measurements of average yield of neutrons in thermal fission of U²³⁵ and spontaneous fission of Cf²⁵²De Volpi, Alexander (Virginia Polytechnic Institute, 1966)Absolute measurements, using the manganese bath technique and prompt-neutron coincidence calibration of fission counters, have been carried out to obtain independent evaluations the total yield of neutrons in thermal fission of U²³⁵[ν̅(U²³⁵)] and spontaneous fission of Cf²⁵²[ν̅(Cf²⁵²)]. In this experiment, the neutron-fission coincidence phase was used to calibrate only the fission counter efficiency; this calibrated fission counter was then inserted in a large manganese bath where the fission rate and neutron rate were simultaneously monitored. The absolute efficiency of the manganese bath was determined with the aid of a series of supplementary experiments, including frequent comparison of absolute 4π beta-gamma coincidence counting with the liquid radio-assay system. The over-all neutron detection efficiency relies heavily on the published manganese/hydrogen cross-section ratio, now known to about 0.3%. Corroboration of the neutron efficiency is found by obtaining a source rate of (1.185 ± 0.007) x 10⁶ neutrons/ sec for the U.S. National Bureau of Standards secondary source (NBS-II), which has a most recent best value (1.180 ± 0.012) x 10⁶ neutrons/sec. It is believed that this method is free of certain possible systematic errors, namely (1) a priori with regard to the fission neutron spectrum; (2) calibration of a detector whose sensitivity is comparatively neutron-energy dependent; and (3) angular anisotropy of neutron emission in flat-foil fission chambers. Examination of the literature and of data obtained from specific experiments indicates that this latter problem may be responsible for the relatively large scatter compared to the reported precision of published values of the U²³⁵ fission cross section and v̅. The results of this experiment, ν̅(U²³⁵) = 2.407 ± 0.03 neutrons/fission and ν̅(Cf²⁵²) = 3.750 ± 0.028 neutrons/fission, are highly consistent with weighted averages determined by international data compilation centers.
- An investigation of methods for the determination of the colloidal particle size of viscoseSweet, Warren William (Virginia Agricultural and Mechanical College and Polytechnic Institute, 1932)
- An investigation of the effects of cold working upon copperDavis, G. Maslin (Virginia Agricultural and Mechanical College and Polytechnic Institute, 1929)
- Lower Extremity Biomechanical Response of Female and Male Post-Mortem Human Surrogates to High-Rate Vertical Loading During Simulated Under-Body Blast EventsCristino, Danielle M. (Virginia Tech, 2018-12)During an under-body blast (UBB) event, an improvised explosive device (IED) delivers a high-energy blast beneath a military vehicle, exposing mounted Warfighters to considerable risk of severe lower extremity injuries. Loftis and Gillich (2014) determined that the lower leg and ankle region is the most common body region to sustain skeletal injury in military mounted combat events, comprising twenty-one percent of cases reported in the Joint Trauma Analysis and Prevention of Injuries in Combat (JTAPIC) database between 2010 and 2012. Injuries of the lower extremity are not always life-threatening. However, from a survivability standpoint, these injuries may affect the ability of the Warfighter to self-extricate and ambulate in the immediate aftermath of an UBB event. In addition, lower extremity injuries can lead to long term health complications and reduced quality of life (Dischinger et al., 2004). While some comparisons can be drawn from the study of civilian automotive crashes; the impact level, rate, location, and directions in UBB are fundamentally different for the lower extremity. Therefore, substantial research efforts to characterize and assess injuries unique to UBB are essential. The Warrior Injury Assessment Manikin (WIAMan), the Tech Demonstrator version of which was introduced by Pietsch et al. (2016), is the only anthropomorphic test device (ATD) designed to evaluate injury patterns in UBB conditions. However, there are no known injury assessment tools for the female Warfighter at this time. The overarching goal of this research effort is to determine the origin of potential differences in the response of females and males in UBB conditions. The results of this work contribute to the body of research concerning high-rate axial loading of the lower extremity and form the first detailed biomechanical account of UBB effects on female PMHS. This work will inform future decisions regarding the requirements for a valid injury assessment capability for female Warfighters in the UBB environment and the subsequent research needed to support those requirements. Ultimately, advancements can be made in modeling and simulation capabilities, injury assessment criteria, test methodologies, and design approaches for safer military ground vehicles and personal protective equipment (PPE). Improvements in these technologies will reduce morbidity and mortality rates among the U.S. Warfighter population, both male and female.
- Novel Immune-Regulatory Mechanisms in a Mouse Model of Traumatic Brain InjuryHazy, Amanda Dawn (Virginia Tech, 2019-09-06)Traumatic brain injury (TBI) is a major health concern in the United States and worldwide and effective treatment options are limited. Differences in the magnitude and characteristics of the peripheral-derived immune cell response to TBI are key contributors to the secondary cascades of damage following brain trauma, and means of modifying this response to improve clinical outcome are a current area of active research. Our work elucidated the peripheral immune response to TBI by characterizing the transcriptomic profile of juvenile vs adult peripheral immune cells following TBI as well as discovering a novel role for the tyrosine kinase receptor EphA4 in the peripheral-derived immune response to brain trauma. Previous work has demonstrated significant differences in recovery from TBI in young vs adult animals, and some studies have indicated that the immune response contributes to these differences. We utilized next-generation sequencing to compare gene expression profiles of blood cell fraction samples in juvenile and adult mice. Our work demonstrated that juvenile peripheral immune cells show a more dynamic response to TBI than adult and that pattern recognition receptor signaling is a cornerstone of these differences. To assess the specific mechanisms involved in the peripheral response to TBI, we utilized a bone marrow chimeric mouse model lacking EphA4 in the hematopoietic compartment. These studies found decreased lesion infiltration of peripheral immune cells, specifically activated macrophages, in the absence of EphA4. We also showed that EphA4 interacts with the Tie2/Angiopoietin signaling axis to regulate macrophage phenotype on the M1/2 continuum. Overall, our work demonstrated a novel role for EphA4, mediated by Tie2, as a pro-inflammatory regulator of the peripheral-derived immune cell response to TBI.
- Predictive equations for milk yield and dry matter intake and development of a maximum-profit ration formulation modelBrown, Carl Alton (Virginia Polytechnic Institute and State University, 1976)
- A study of counseling services in selected small collegesClark, Martin E. (Virginia Polytechnic Institute and State University, 1974)The purposes of the study were: (1) to investigate the characteristics of counseling services in small colleges; (2) to determine if counseling services at church-related colleges differed from those at other small colleges; (3) to determine whether formal counseling services were deemed necessary for small colleges to meet the needs usually met by counseling services at larger universities, as perceived by faculty members and students. The Counseling Services Survey (CSS) instrument was adapted from standard instruments and sent to the chief student personnel administrator at each college holding membership in The Council for the Advancement of Small Colleges. Eighty-seven CSS instruments were returned, nineteen from Roman Catholic Colleges, forty-four from Protestant colleges, and twenty-four from independent colleges. CSS responses were compared to answer research questions regarding the existence, specific services, and operation of counseling services in these colleges. The Perceptions of Counseling Services (PCS) form was designed as an instrument for finding the perceptions by students, faculty, counselors, and administrator, regarding the availability, quality, and source of counseling for seven general problem areas. Thirty-seven CSS respondents administered the PCS in their colleges, twenty-nine of which had formal counseling services, and eight of which did not have formal counseling services. Results from the CSS study indicated that, with several minor variations on specific services, the existence of a formal counseling service along with its services and operation were not a function of institutional affiliation. Results from the PCS study showed that students and faculty members at colleges with formal counseling services generally perceived help for various problems to be available and to be of high quality more frequently than did respondents from colleges without formal counseling services. Also, counselors and administrators more frequently perceived most types of counseling help to be available and to be of high quality than did students and faculty members.
- A theoretical analysis of the temperature response in a bimetallic, composite geometry, nuclear reactor pressure vessel undergoing a loss-of-coolant accidentCoppari, Lawrence Americus (Virginia Polytechnic Institute and State University, 1977)