Current Search: scintillation (x)
View All Items
 Title
 Stressinduced phase change sintering: A novel approach to the fabrication of barium chloride transparent ceramic scintillators.
 Creator

Shoulders, William, Gaume, Romain, Coffey, Kevin, Sohn, Yongho, University of Central Florida
 Abstract / Description

For the precise infield identification of dangerous radioisotopes, the desire for higher resolution, cheaper, and more rugged gammaray scintillator detectors has pushed the materials science community to investigate new compounds and processing techniques.(&)nbsp; One particular solution is Eu:BaCl2 transparent ceramics processed by the novel phasechange sintering technique. Typically, optical transparency in BaCl2 would be limited by birefringence scattering at grain boundaries due to...
Show moreFor the precise infield identification of dangerous radioisotopes, the desire for higher resolution, cheaper, and more rugged gammaray scintillator detectors has pushed the materials science community to investigate new compounds and processing techniques.(&)nbsp; One particular solution is Eu:BaCl2 transparent ceramics processed by the novel phasechange sintering technique. Typically, optical transparency in BaCl2 would be limited by birefringence scattering at grain boundaries due to mismatch in refractive index.(&)nbsp; Traditional ceramic processing routes would also predispose this volatile and hygroscopic material to the formation of defects, which interrupt the energy transfer in the scintillation process. Literature shows that these challenges have prevented halide gammaray scintillator ceramics, including Ce:LaBr3, Eu:SrI2, and Ce:Cs2LiYCl6, from reaching the performance of their singlecrystalline counterparts. The sintering method explored in this thesis utilizes a polymorphic orthorhombic to cubic phase transition followed by plastic deformation.(&)nbsp; The experimental implementation of this method necessitated the design of a unique airtight hotpressing device, capable of developing conditions for this phase conversion, and the synthesis of high purity powders. Systematic experiments on powder synthesis and on densification were carried out to demonstrate the feasibility of this approach and understand the conditions for phasechange sintering. These experiments, supported by characterizations including xray diffraction, electron microscopy, and thermal analysis, lead to the production of optically isotropic cubic barium chloride ceramic samples. Finally, the optical and scintillation properties of Eu:BaCl2 ceramic samples were investigated, revealing an energy resolution of 6% at 662 keV, an unprecedented value for a halide ceramic scintillator.
Show less  Date Issued
 2016
 Identifier
 CFE0006844, ucf:51779
 Format
 Document (PDF)
 PURL
 http://purl.flvc.org/ucf/fd/CFE0006844
 Title
 Barium Based Halide Scintillator Ceramics for Gamma Ray Detection.
 Creator

Shoulders, William, Gaume, Romain, Coffey, Kevin, Sohn, Yongho, University of Central Florida
 Abstract / Description

As our understanding of ceramic processing methods for the purpose of fabricating polycrystalline optical materials has increased over the past few decades, the race is on to bring ceramic technology to markets where single crystalline materials have traditionally been used. One such market is scintillators. This Master's thesis focuses specifically on a class of materials attractive for use as gammaray scintillators. These barium based halides can potentially be utilized in fields...
Show moreAs our understanding of ceramic processing methods for the purpose of fabricating polycrystalline optical materials has increased over the past few decades, the race is on to bring ceramic technology to markets where single crystalline materials have traditionally been used. One such market is scintillators. This Master's thesis focuses specifically on a class of materials attractive for use as gammaray scintillators. These barium based halides can potentially be utilized in fields applications ranging from ionizing radiation detection in the field to highenergy physics experimentation. Barium bromide iodide and barium chloride single crystals have already showed high light yield, fast scintillation decay, and high energy resolution, all desirable properties for a scintillator. This work attempts to show the likelihood of moving towards polycrystalline scintillators to take advantage of the lower processing temperature, higher manufacturing output, and overall reduced cost. The experiments begin with identifying appropriate sintering conditions for hot pressed ceramics of BaBrI and BaCl2. Possible sources of optical loss in the first phase of hot pressed samples are investigated using a wide range of characterization tools. Preliminary luminescence and scintillation measurements are reported for a translucent sample of BaBrI. Recommendations are made to move toward highly transparent ceramics with scintillation properties approaching those measured in single crystal samples.
Show less  Date Issued
 2013
 Identifier
 CFE0005258, ucf:50576
 Format
 Document (PDF)
 PURL
 http://purl.flvc.org/ucf/fd/CFE0005258
 Title
 COMPARISON OF THEORETICAL MODELS OF POWER SPECTRAL DENSITY TO THE EXPERIMENTAL VALUE FOR SPECTRUM OF IRRADIANCE FLUCTUATIONS.
 Creator

Hershberger, Craig, Phillips, Ronald, University of Central Florida
 Abstract / Description

A propagation experiment was designed, assembled, and conducted on an extended range to verify theoretical temporal models for weak to strong fluctuation theory. Laser light intensity was propagated over terrain at the Kennedy Space Center (Florida), and detected using optical receivers at a distance of 13.3 Km from the optical source. The intensity data from the experiment was used to generate an experimental Power Spectral Density (PSD) function. The theoretical Mutual Coherence Function ...
Show moreA propagation experiment was designed, assembled, and conducted on an extended range to verify theoretical temporal models for weak to strong fluctuation theory. Laser light intensity was propagated over terrain at the Kennedy Space Center (Florida), and detected using optical receivers at a distance of 13.3 Km from the optical source. The intensity data from the experiment was used to generate an experimental Power Spectral Density (PSD) function. The theoretical Mutual Coherence Function (MCF) and Wave Structure Function (WSF) as set forth by Andrews/Phillips , were evaluated to determine the effective relationship between the statistical moments of the random optical field and the laser light intensity. Two scales of interest were identified (refractive largescale and diffractive smallscale) and plotted revealing the characteristic shape of each component. In addition, statistical principles applied to the correlation/covariance function relationship and a graphical convolution process were used to generate a theoretical PSD function. Further, utilizing Taylor's "frozen turbulence" hypothesis an analysis of the theoretical temporal covariance function was performed. Functional forms for refractive and diffractive logirradiance components were developed and used to generate a second theoretical PSD function. Finally, the experimental and theoretical Power Spectral Density functions are plotted on the same graph and a comparison is performed.
Show less  Date Issued
 2008
 Identifier
 CFE0002403, ucf:47734
 Format
 Document (PDF)
 PURL
 http://purl.flvc.org/ucf/fd/CFE0002403
 Title
 THE SCINTILLATION INDEX IN MODERATE TO STRONG TURBULENCE FOR THE GAUSSIAN BEAM WAVE ALONG A SLANT PATH.
 Creator

Thomas, Fredrick, Young, Cynthia, University of Central Florida
 Abstract / Description

Scintillation is one of the most common statistics in the literature of mathematical modeling of laser propagation through random media. One approach to estimating scintillation is through the Rytov approximation, which is limited to weak atmospheric turbulence. Recently, an improvement of the Rytov approximation was developed employing a linear filter function technique. This modifies the Rytov approximation and extends the validity into the moderate to strong regime. In this work, an...
Show moreScintillation is one of the most common statistics in the literature of mathematical modeling of laser propagation through random media. One approach to estimating scintillation is through the Rytov approximation, which is limited to weak atmospheric turbulence. Recently, an improvement of the Rytov approximation was developed employing a linear filter function technique. This modifies the Rytov approximation and extends the validity into the moderate to strong regime. In this work, an expression governing scintillation of a Gaussian beam along an uplink slant path valid in all regimes of turbulence is presented, as well as results for the limiting cases of a plane wave and a spherical wave.
Show less  Date Issued
 2005
 Identifier
 CFE0000670, ucf:46509
 Format
 Document (PDF)
 PURL
 http://purl.flvc.org/ucf/fd/CFE0000670
 Title
 EFFECT OF INNER SCALE ATMOSPHERIC SPECTRUM MODELS ON SCINTILLATION IN ALL OPTICAL TURBULENCE REGIMES.
 Creator

Mayer, Kenneth, Young, Cynthia, University of Central Florida
 Abstract / Description

Experimental studies have shown that a "bump" occurs in the atmospheric spectrum just prior to turbulence cell dissipation.1,3,4 In weak optical turbulence, this bump affects calculated scintillation. The purpose of this thesis was to determine if a "nonbump" atmospheric power spectrum can be used to model scintillation for plane waves and spherical waves in moderate to strong optical turbulence regimes. Scintillation expressions were developed from an "effective" von Karman spectrum using...
Show moreExperimental studies have shown that a "bump" occurs in the atmospheric spectrum just prior to turbulence cell dissipation.1,3,4 In weak optical turbulence, this bump affects calculated scintillation. The purpose of this thesis was to determine if a "nonbump" atmospheric power spectrum can be used to model scintillation for plane waves and spherical waves in moderate to strong optical turbulence regimes. Scintillation expressions were developed from an "effective" von Karman spectrum using an approach similar to that used by Andrews et al.8,14,15 in developing expressions from an "effective" modified (bump) spectrum. The effective spectrum extends the Rytov approximation into all optical turbulence regimes using filter functions to eliminate midrange turbulent cell size effects to the scintillation index. Filter cutoffs were established by matching to known weak and saturated scintillation results. The resulting new expressions track those derived from the effective bump spectrum fairly closely. In extremely strong turbulence, differences are minimal.
Show less  Date Issued
 2007
 Identifier
 CFE0001559, ucf:47141
 Format
 Document (PDF)
 PURL
 http://purl.flvc.org/ucf/fd/CFE0001559
 Title
 FADE STATISTICS FOR A LASERCOM SYSTEM AND THE JOINT PDF OF A GAMMAGAMMA DISTRIBUTED IRRADIANCE AND ITS TIME DERIVATIVE.
 Creator

Stromqvist Vetelino, Frida, Young, Cynthia, University of Central Florida
 Abstract / Description

The performance of lasercom systems operating in the atmosphere is reduced by optical turbulence, which causes irradiance fluctuations in the received signal. The result is a randomly fading signal. Fade statistics for lasercom systems are determined from the probability density function (PDF) of the irradiance fluctuations. The expected number of fades per second and their mean fade time require the joint PDF of the fluctuating irradiance and its time derivative. Theoretical integral...
Show moreThe performance of lasercom systems operating in the atmosphere is reduced by optical turbulence, which causes irradiance fluctuations in the received signal. The result is a randomly fading signal. Fade statistics for lasercom systems are determined from the probability density function (PDF) of the irradiance fluctuations. The expected number of fades per second and their mean fade time require the joint PDF of the fluctuating irradiance and its time derivative. Theoretical integral expressions, as well as closed form, analytical approximations, were developed for the joint PDF of a gammagamma distributed irradiance and its time derivative, and the corresponding expression for the expected number of fades per second. The new approximation for the conditional PDF of the time derivative of a gammagamma irradiance is a zero mean Gaussian distribution, with a complicated irradiance depending variance. Fade statistics obtained from experimental data were compared to theoretical predictions based on the lognormal and gammagamma distributions. A Gaussian beam wave was propagated through the atmosphere along a horizontal path, near ground, in the moderatetostrong optical turbulence. To characterize the propagation path, a new method that infers atmospheric propagation parameters was developed. Scintillation theory combined with a numerical scheme was used to infer the structure constant, Cn2, the inner scale and the outer scale from the optical measurements. The inferred parameters were used in calculations for the theoretical PDFs. It was found that fade predictions made by the gammagamma and lognormal distributions provide an upper and lower bound, respectively, for the probability of fade and the number of fades per second for irradiance data collected in the moderatetostrong fluctuation regime. Aperture averaging effects on the PDF of the irradiance fluctuations were investigated by comparing the irradiance distributions for the three receiver apertures at two different values of the structure parameter and, hence, different values of the coherence radius. For the moderatetostrong fluctuation regime, the gammagamma distribution provides a good fit to the irradiance fluctuations collected by finitesized apertures that are significantly smaller than the coherence radius. For apertures larger than or equal to the coherence radius, the irradiance fluctuations appear to be lognormally distributed.
Show less  Date Issued
 2006
 Identifier
 CFE0001440, ucf:47069
 Format
 Document (PDF)
 PURL
 http://purl.flvc.org/ucf/fd/CFE0001440
 Title
 EXTREME ULTRAVIOLET SPECTRAL STREAK CAMERA.
 Creator

Szilagyi, John, Richardson, Martin, University of Central Florida
 Abstract / Description

The recent development of extreme ultraviolet (EUV) sources has increased the need for diagnostic tools, and has opened up a previously limited portion of the spectrum. With ultrafast laser systems and spectroscopy moving into shorter timescales and wavelengths, the need for nanosecond scale imaging of EUV is increasing. EUVÃƒÂƒÃ‚Â‚ÃƒÂ‚Ã‚Â's high absorption has limited the number of imaging options due...
Show moreThe recent development of extreme ultraviolet (EUV) sources has increased the need for diagnostic tools, and has opened up a previously limited portion of the spectrum. With ultrafast laser systems and spectroscopy moving into shorter timescales and wavelengths, the need for nanosecond scale imaging of EUV is increasing. EUVÃƒÂƒÃ‚Â‚ÃƒÂ‚Ã‚Â's high absorption has limited the number of imaging options due to the many atomic resonances in this spectrum. Currently EUV is imaged with photodiodes and Xray CCDs. However photodiodes are limited in that they can only resolve intensity with respect to time and Xray CCDs are limited to temporal resolution in the microsecond range. This work shows a novel approach to imaging EUV light over a nanosecond time scale, by using an EUV scintillator to convert EUV to visible light imaged by a conventional streak camera. A laser produced plasma, using a masslimited tin based target, provided EUV light which was imaged by a grazing incidence flat field spectrometer onto a Ce:YAG scintillator. The EUV spectrum (5 nm20 nm) provided by the spectrometer is filter by a zirconium filter and then converted by the scintillator to visible light (550 nm) which can then be imaged with conventional optics. Visible light was imaged by an electron image tube based streak camera. The streak camera converts the visible light image to an electron image using a photocathode, and sweeps the image across a recording medium. The streak camera also provides amplification and gating of the image by the means of a micro channel plate, within the image tube, to compensate for low EUV intensities. The system provides 42 ns streaked images of light with a temporal resolution of 440 ps at a repetition rate of 1 Hz. Upon calibration the EUV streak camera developed in this work will be used in future EUV development.
Show less  Date Issued
 2010
 Identifier
 CFE0003558, ucf:48905
 Format
 Document (PDF)
 PURL
 http://purl.flvc.org/ucf/fd/CFE0003558
 Title
 THE EFFECTIVENESS OF DATA CODES AND HARDWARE SELECTION TO MITIGATE SCINTILLATION EFFECTS ON FREE SPACE OPTICAL DATA TRANSMISSION.
 Creator

Stein, Keith, Phillips, Ronald, University of Central Florida
 Abstract / Description

The design of an optical communication link must plan for the random effects of atmospheric turbulence. This study analyses data from an experiment which transmitted from a laser located 8 meters above ground over a 13 Km range to coherent detection devices approximately 162 meters above ground. The effects of a fading and surging beam wave were considered in regards to code techniques for error correction, amplitude modulation and hardware architecture schemes. This study simulated the use...
Show moreThe design of an optical communication link must plan for the random effects of atmospheric turbulence. This study analyses data from an experiment which transmitted from a laser located 8 meters above ground over a 13 Km range to coherent detection devices approximately 162 meters above ground. The effects of a fading and surging beam wave were considered in regards to code techniques for error correction, amplitude modulation and hardware architecture schemes. This study simulated the use of arrays and large apertures for the receiving devices, and compared the resultant scintillation index with the theoretical calculations.
Show less  Date Issued
 2006
 Identifier
 CFE0001204, ucf:46945
 Format
 Document (PDF)
 PURL
 http://purl.flvc.org/ucf/fd/CFE0001204
 Title
 THE PDF OF IRRADIANCE FOR A FREESPACE OPTICAL COMMUNICATIONS CHANNEL: A PHYSICS BASED MODEL.
 Creator

Wayne, David, Phillips, Ronald, University of Central Florida
 Abstract / Description

An accurate PDF of irradiance for a FSO channel is important when designing a laser radar, active laser imaging, or a communications system to operate over the channel. Parameters such as detector threshold level, probability of detection, mean fade time, number of fades, BER, and SNR are derived from the PDF and determine the design constraints of the receiver, transmitter, and corresponding electronics. Current PDF models of irradiance, such as the GammaGamma, do not fully capture the...
Show moreAn accurate PDF of irradiance for a FSO channel is important when designing a laser radar, active laser imaging, or a communications system to operate over the channel. Parameters such as detector threshold level, probability of detection, mean fade time, number of fades, BER, and SNR are derived from the PDF and determine the design constraints of the receiver, transmitter, and corresponding electronics. Current PDF models of irradiance, such as the GammaGamma, do not fully capture the effect of aperture averaging; a reduction in scintillation as the diameter of the collecting optic is increased. The GammaGamma PDF of irradiance is an attractive solution because the parameters of the distribution are derived strictly from atmospheric turbulence parameters; propagation path length, Cn2, l0, and L0. This dissertation describes a heuristic physicsbased modeling technique to develop a new PDF of irradiance based upon the optical field. The goal of the new PDF is threefold: capture the physics of the turbulent atmosphere, better describe aperture averaging effects, and relate parameters of the new model to measurable atmospheric parameters. The modeling decomposes the propagating electromagnetic field into a sum of independent randomamplitude spatial plane waves using an approximation to the KarhunenLoeve expansion. The scattering effects of the turbulence along the propagation path define the randomamplitude of each component of the expansion. The resulting PDF of irradiance is a double finite sum containing a Bessel function. The newly developed PDF is a generalization of the GammaGamma PDF, and reduces to such in the limit. An experiment was setup and performed to measure the PDF of irradiance for several receiver aperture sizes under moderate to strong turbulence conditions. The propagation path was instrumented with scintillometers and anemometers to characterize the turbulence conditions. The newly developed PDF model and the GG model were compared to histograms of the experimental data. The new PDF model was typically able to match the data as well or better than the GG model under conditions of moderate aperture averaging. The GG model fit the data better than the new PDF under conditions of significant aperture averaging. Due to a limiting scintillation index value of 3, the new PDF was not compared to the GG for point apertures under strong turbulence; a regime where the GG is known to fit data well.
Show less  Date Issued
 2010
 Identifier
 CFE0003209, ucf:48576
 Format
 Document (PDF)
 PURL
 http://purl.flvc.org/ucf/fd/CFE0003209