High Power Laser - Plasma Interactions for Homeland Security Applications

Advance in laser technology over the last few decades have allowed progress in intense laser-plasma interaction research. The relativistic plasma generated by intense laser pulses can generate many different forms of radiation. This radiation, including X-rays, has been studied intensively due to the numerous potential applications of these sources. For example, for Homeland Security, radiation sources are already utilized to detect dangerous materials and hidden items that threaten civil safety. Neutrons and THz radiation have been studied as candidates for next generation screening, which may complement typical X-ray techniques. This thesis contains three experimental studies of high-power laser-plasma interactions as sources of radiation for Homeland Security applications, especially at kilohertz repetition-rates using few- millijoule pulses. First, a neutron generation experiment was conducted using a high repetition-rate laser system (1⁄2 kHz) at the University of Michigan. A heavy water (D2O) stream was irradiated by 40 fs pulses, each containing a few millijoules of energy. Acceleration of deuterons (to E < 1 MeV) was achieved through plasma sheath acceleration. Ensuing DD nuclear fusion reactions, in turn, generated neutron fluxes of up to 10^5 s^−1 into 4π steradians. In order to understand the neutron source characteristics, deuteron spectra were measured with CR39 detectors and compared to particle-in-cell (PIC) relativistic plasma dynamics simulations. The neutron source characteristics were analyzed using various neutron detection techniques, including Time-of-Flight measurements, bubble detectors, and neutron-capture gamma-ray measurements. Second, THz generation from laser filamentation in air was investigated. For security applications, THz can complement X-ray scanning, because THz can detect non-metallic materials and dangerous chemicals while not ionizing the sample. Even though there have been extensive studies on THz generation from laser filamentation processes, the exact generation mechanisms are yet to be determined. In this thesis, optimization of THz radiation using an adaptive optic with active feedback was demonstrated. Using a genetic algorithm, the THz radiation was improved six-fold without the need for detailed knowledge of the mechanisms. In particular, the use of a high repetition-rate laser system accelerated the optimization of the THz signal. Another strength of this optimization system is that it can enhance certain THz generation mechanisms depending on the experimental circumstances. Lastly, using a nanosecond pulsed high-power laser system (10 Hz), a long-range detection technique was developed for detection of special nuclear materials. Although direct detection of radiation from nuclear materials can be defeated by radiation shielding, leakage of radiation-ionized gases can provide an alternative indicator of the existence of nuclear materials. For instance, in the presence of ionizing radiation, the ratio of ionized nitrogen to neutral nitrogen would be higher than in no-source air-plasma conditions. By inducing optical breakdown (plasma) near a sample’s position, the ionization levels of the surrounding air were analyzed. To enhance the detection efficiency, an adaptive-optic feedback system was introduced with this ratio as a figure-of-merit. This resulted in a 50 % enhancement in the spectral ratio of the nitrogen lines. In addition, aerosol-initiated plasma spectra were distinguished from the original air-breakdown plasma, as a step toward practical deployment.PHDNuclear Engineering & Radiological SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttps://deepblue.lib.umich.edu/bitstream/2027.42/145909/1/jmhah_1.pd

Spectral Enhancement of Organic Photodetectors

Solution-processed photodiodes with infrared sensitivities at wavelengths beyond the bandgap of silicon would be a significant advance towards cost-effective imaging. Colloidal quantum dots are highly suitable as infrared absorbers for photodetection. The concept of organic bulk heterojunctions sensitized with PbS nanocrystalline was proved with efficient near-infrared detection up to 1.8 mm for NIR imaging on active matrix TFT backplanes and demonstrated x-ray sensitivity

Amorphous lead oxide (a-PbO) for direct conversion X-ray imaging detectors

Modern medical imaging revolutionized the way that medical conditions are diagnosed. It became possible after significant advancements in science and technology related to the medical visualization field. X-ray imaging is one of the most commonly used modalities; it now relies on the digital Flat Panel X-ray Imagers (FPXIs) that can provide high-quality radiographs while benefiting from many advantages inherent to the digital detectors technology, including reduced radiation exposure and convenience of use. Although some FPXIs have already been commercialized, their application is very limited, and they do not exploit the full potential of this technology. Therefore, there is a niche that can only be filled through extensive research and development. The work carried out and presented in the dissertation contributes to this effort. In this work, an Amorphous Lead Oxide (a-PbO) photoconductor is investigated as a material of choice in the direct conversion digital FPXIs for various medical and industrial X-ray imaging applications, including general radiography, real-time fluoroscopy, digital breast tomosynthesis, and non-destructive testing. a-PbO is a recently developed material with high X-ray stopping power in a wide X-ray energy range and high theoretical X-ray–to–charge conversion efficiency. [...

Spectral Enhancement of Organic Photodetectors

Solution-processed photodiodes with infrared sensitivities at wavelengths beyond the bandgap of silicon would be a significant advance towards cost-effective imaging. Colloidal quantum dots are highly suitable as infrared absorbers for photodetection. The concept of organic bulk heterojunctions sensitized with PbS nanocrystalline was proved with efficient near-infrared detection up to 1.8 mm for NIR imaging on active matrix TFT backplanes and demonstrated x-ray sensitivity

Lead oxide (PbO) for direct conversion detectors

There is great interest in the utilization of non-crystalline photoconductors for direct conversion medical X-ray imaging detectors. Lead Oxide (PbO) is one of the most promising candidates for application in general radiography and fluoroscopy since it possesses high theoretical X-ray-to-charge conversion gain and high X-ray stopping power due the high Z of Pb (Z = 82). A further advantage of PbO compared with other photoconductors (like polycrystalline layers of HgI2, PbI2, CZT) is the absence of heavy absorption edges up to 88 keV, which inherently offers higher spatial resolution. Therefore, PbO exhibits many of the requirements for an effective detector material. However, although very promising, poly-PbO layers are known to exhibit a relatively high dark current, an incomplete charge collection and a residual signal after the end of X-ray exposure, called signal lag. The reported lag was the main obstacle of this poly-PbO-based detector prototype, since this restricts applications to static imaging only and obscures the full potential of PbO in medical imaging. Another disadvantage is the high porosity of poly-PbO and its structural instability in air, which makes this material challenging for practical utilization in X-ray detectors. To combat the above problems (i.e. lag, porosity and structural degradation in air) we have advanced PbO deposition technology and developed a novel type of amorphous lead oxide (a-PbO). The obtained a-PbO layers have near single-crystal bulk densities and are stable in air. In addition, the layers are capable of withstanding higher electric fields, while at the same time iii exhibiting lower dark currents than previously achieved. Also, the temporal response and charge yield were significantly improved and compare favorably with published results on conventional films of polycrystalline PbO and even a-Se – the only photoconductor currently utilized in direct conversion detectors. Our measured X-ray response show almost complete elimination of signal lag to a level sufficient for the high-speed operations. Our advances in PbO technology allow us to utilize the full potential of PbO for medical X-ray imaging applications

Dark Current modeling and characterization of amorphous lead oxide-based x-ray photoconductive devices for applications in medical imaging

High atomic number (Z) polycrystalline and amorphous photoconductors are currently being investigated to extend direct conversion X-ray detectors to real-time and high-energy lowdose applications. Amorphous lead oxide (a-PbO) is one of the most promising photoconductor candidates because of its negligible signal lag and high theoretical X-ray conversion efficiency. However, a-PbO layers are still experimental; PbO technology has been developed to the point where material science and engineering approaches must be applied to make a-PbO detector prototypes suitable for low-dose X-ray imaging. This includes determining the most appropriate a-PbO multilayer detector structures with specially designed blocking layers that will withstand the high electric fields needed for efficient (i.e., complete) collection of X-ray generated charge while maintaining an acceptable dark current (DC) level. DC is a source of noise in the detector structure that degrades the signal-to-noise ratio (SNR) of the detector system in low-exposure applications. Here we investigate the use of polyimide (PI) as a hole-blocking layer. PI blocking layers were proven successful in the only commercially used direct conversion detectors, which are based on layers of photoconductive amorphous selenium (a-Se). Experimentally, PI was shown to have the most suitable electrical and physical properties for our a-PbO technology. In addition, PI has a straightforward application process of spin coating. Therefore, PI was chosen as a hole blocking layer to decrease DC to tolerable levels in an a-PbO-based detector. [...

Nevada Test Site-Directed Research and Development, FY 2007 Report

The Nevada Test Site-Directed Research and Development (SDRD) program completed a very successful year of research and development activities in FY 2007. Twenty-nine new projects were selected for funding this year, and eight projects started in FY 2006 were brought to conclusion. The total funds expended by the SDRD program were $5.67 million, for an average per-project cost of$153 thousand. An external audit conducted in September 2007 verified that appropriate accounting practices were applied to the SDRD program. Highlights for the year included: programmatic adoption of 8 SDRD-developed technologies; the filing of 9 invention disclosures for innovation evolving from SDRD projects; participation in the tri-Lab Laboratory Directed Research and Development (LDRD) and SDRD Symposium that was broadly attended by Nevada Test Site (NTS), National Nuclear Security Administration (NNSA), LDRD, U.S. Department of Homeland Security (DHS), and U.S. Department of Defense (DoD) representatives; peer reviews of all FY 2007 projects; and the successful completion of 37 R&amp;D projects, as presented in this report. In response to a company-wide call, authors throughout the NTS complex submitted 182 proposals for FY 2007 SDRD projects. The SDRD program has seen a dramatic increase in the yearly total of submitted proposals--from 69 in FY 2002 to 182 this year--while the number of projects funded has actually decreased from a program high of 57 in FY 2004. The overall effect of this trend has helped ensure an increasingly competitive program that benefited from a broader set of innovative ideas, making project selection both challenging and rewarding. Proposals were evaluated for technical merit, including such factors as innovation, probability of success, potential benefit, and mission applicability. Authors and reviewers benefited from the use of a shortfalls list entitled the 'NTS Technology Needs Assessment' that was compiled from NTS, National Weapons Laboratory (NWL), and NNSA sources. This tool continues to be of considerable value in aligning the SDRD program with mission priorities, and was expanded in FY 2007 to include technology development needs from the DHS and other agencies with missions closely aligned to that of the NTS

Photodetectors

In this book some recent advances in development of photodetectors and photodetection systems for specific applications are included. In the first section of the book nine different types of photodetectors and their characteristics are presented. Next, some theoretical aspects and simulations are discussed. The last eight chapters are devoted to the development of photodetection systems for imaging, particle size analysis, transfers of time, measurement of vibrations, magnetic field, polarization of light, and particle energy. The book is addressed to students, engineers, and researchers working in the field of photonics and advanced technologies

An iterative method to deconvolve coded-mask images

Efficent astronomical imaging at energy greater than 20 keV is mainly achieved through modulation, either time (i.e. HXMT) or spatial (i.e. IBIS/INTEGRAL), techniques. Currently, the coded mask technique is widely used with the true spatial intensity distribution reconstructed from the data by the cross-correlation (CC) method. As the sensitivity of instruments increases, so must the angular resolution in order to avoid problems with source confusion. The IBIS 12’ angular resolution is clearly not sufficient to distinguish all the sources in the crowded field of the Galactic Centre. One possibility to overcome this problem is to change the deconvolution method. The objective of this thesis is to evaluate the real imaging capability of the Direct Demodulation (DD) method. It deconvolves incomplete and noisy data by iteratively solving the image formation equation under physical constraints. With the goal of exploiting the DD technique, in the early of the 1990s the HXMT mission was designed, where the imaging capability is obtained through the temporal modulation of the detected counts by a set of mechanical collimators. To achieve this goal, we developed the Lucy-Richardson (LR) code to reconstruct directly hard-X/soft- ray images. It assumes that the data and the noise follow a Poisson distribution and it guarantees the non-negativity of the restored images. For the moment, any kind of regularization or constraint was implemented in the underlying optimization problem, so this will be ill-posed yet. Due to the general nature of the DD and the fact that HXMT has still to fly, the IBIS/INTEGRAL data and its PSF were used to check our own code. The pure geometrical PSF considering only the effects due to the photon propagation from the mask to the detector was created. Our CC code implements the same balanced cross-correlation as the standard software for IBIS/INTEGRAL analysis. The CC deconvolved images are the reference for the image quality obtained with the LR. The great improvement in the theoretical angular resolution and location precision is evident. It is independent on the source position in the total FOV, the iteration number and the source flux. Within the parameters of the simulations used, the LR statistical uncertainty was found to be a factor of 10 smaller than that obtained with the CC. Furthermore, the LR deconvolved images have less fluctuating reconstructed background. The main LR drawback is the flux evaluation of the reconstructed source. It is mainly due to the choice of the correct iteration number. The use of a-priori information about the unknown object allows a complete regularization of the problem, so probably solving the problem with the flux estimation. Keywords: Coded-mask, Lucy, Richardson, INTEGRAL, IBI