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The investigators report the analysis of coupons of mineral flint and chert of archaeological interest and various provenances in comparison to uranium-doped glass, mineral single crystal quartz and thermally grown silicon dioxide. The measurements are motivated by a desire to quantify the concentration of uranium in lithic sources of paleoindian artifacts; uranyl oxide is suspected to be the origin of the observed UV fluorescence. Rutherford backscattering spectrometry (RBS) confirmed the concentration of uranium in these minerals is on the order of tens of ppm as γ-ray spectroscopy analysis suggested previously. The level of uranium present does not correlate, however, with the observed fluorescence behaviors. Optical effects such as absorption and quenching due to other impurities is suspected to be the cause of the differences.

Through statistical analysis of atomic force micrographs of sputtered-silicon films, we examine the determination of surface roughness parameters, and how this determination is affected by probe characteristics, such as sharpness and aspect ratio. We compare values for the roughness exponent, a, and the lateral correlation length, n, calculated from micrographs obtained with multiple standard atomic force microscopy (AFM) probes in contact and noncontact modes, as well as from micrographs obtained with a multiwalled-nanotube-enhanced AFM probe. The sharper, loweraspect-ratio nanotube probe is expected to provide a truer picture of the roughness parameters for these films. Using the nanotube probe, we obtained a ¼ 0:61 � 0:02, as compared to a � 0:83 obtained with conventional probes. We have also found the nanotube probe is able to detect a smaller lateral correlation length compared to that of the conventional AFM probes. 2002 Elsevier Science B.V. All rights reserved.

Individual carbon nanotube (CNT) field emission characteristics present a number of advantages for potential applications in electron microscopy and electron beam lithography. Mechanical and electrical reliability of individual CNT cathodes, however, remains a challenge and thus device integration of these cathodes has been limited. In this work, we present an investigation into the reliability issues concerning individual CNT field emission cathodes. We also introduce and analyze the reliability of a novel individual CNT cathode. The cathode structure is composed of a multi-walled carbon nanotube (MWNT) attached by Joule heating to a nickel-coated Si microstructure. The junction of the CNT and the Si microstructure is mechanically and electrically robust to withstand the strong electric field conditions that are typical for field emission devices. An optimal Ni film coating of 25 nm on the Si microstructure is required for mechanical and electrical stability. Experimental current–voltage data for the new cathode structure definitively demonstrates carbon nanotube field emission. Additionally, we demonstrate that our new nanofabrication method is capable of producing sophisticated cathode structures that were previously not realizable, such as one consisting of two parallel MWNTs, with highly controlled CNT lengths with 40 nm accuracy and nanotube-to-nanotube separations of less than 10 µm.

Low-cost, rapid and reliable detection technologies for soil nutrients can improve the effectiveness of data collection for site-specific management of soil fertility. In this study, an automated fluidic control system was developed for rapid detection of soil nutrients. Micro pump and valves, controlled by a lap-top computer, delivered a sequence of test solutions to a test chamber where ion selective electrodes are mounted. A 90 seconds test time with acceptable accuracy was demonstrated. This automated soil nutrient fluidic system can further be miniaturized and may be useful for in-field rapid detection of soil nutrients.

The lightning protection of various industrial installations has been investigated by injecting current of smaller amplitudes. Large deviations of the intended current distribution and high values of induced voltages have been observed. Local measures can remedy the deficiencies.

X- or gamma-rays have been produced in the developing discharge of a 2 MV lightning surge generator. The generator provides a voltage waveform with 1.2/50 mus rise/fall time. The waveform is truncated by a spark gap: two sharply pointed electrodes placed horizontally at 0.7 m distance with one electrode grounded. Gap breakdown occurs at about 850 kV. High energies has been recorded with BaF2 and LaBr3(Ce) gamma detectors facing the surge. Aluminum windows limit the detection to gamma's with energy above 17 keV. The scintillator- photomultiplier combination has been calibrated by the 662 keV line of a 137Cs source; the pulse output rise/fall time is 11/23 ns. With positive surges applied to the floating spark gap electrode, hard radiation is observed in 100% to 60% of the surges depending on the electrode's distance to the measurement equipments. The radiation occurred in the leading edge, during the streamer development before a conducting channel was formed between the electrodes. The gamma signal consists of several pulses, well resolved in time. Gamma pulses equivalent to 2-3 MeV were detected. From absorption measurements we conclude that these high energy pulses are pile-up of multiple gamma-quanta, each over 100 keV. The pile-up occurs within the rise time of the detector response, 11 ns. The consistent timing with the HV-surge excluded background radiation as a source for these high intensity pulses. For negative discharges three things change: more surges resulted in gamma-pulses, more gamma-pulses per surge, and more gamma energy. Gamma-pulses up to the equivalent of 30 MeV have been recorded. The high equivalent energies are caused by streamers from the measurement cabinet, right in front of the detector. X-ray was also measured from streamer-corona plasma in a wire-plate reactor. No breakdown occurred during the experiment. A high repetition rate 15/25 ns high HV-pulse with 100 ns width was applied to the wire. The voltage pulse's amplitude was 65 kV with an additional DC offset of +20 kV. Energies equivalent to 45 keV have been recorded.

In this paper, we present an empirical study of dynamic behavior of an electron source system which incorporates an individual CNT. We propose a representative circuit model that is simple yet particularly valuable for emission current control for each CNT emitter in an array to facilitate high throughput maskless lithography.

The effects of shape and the surface area of cathode support structure on the field emission properties of carbon nanotube cathodes are investigated and quantified in this paper. This study demonstrates that the overall structural geometry, specifically the area of the cathode support structure, plays a significant role and should be considered in the design rules for optimization of electron source for advanced electron microscopy.

This paper reports experimental data for individual multi-walled CNT (MWNT) emitters. For comparison, a thermodynamics-based variational method was used to obtain a closed-form expression for the radial electric field of a spherical cathode. Theoretical data generated from Technology Computer-Aided Design (TCAD) device simulation software is also reported. A fundamental understanding of the individual CNT emitter has important ramifications for electron microscope applications. Developing theoretical models for nanotube field emission properties using a thermodynamics-based variational approach (Gunther et al., 2004) as well as TCAD (Smith et al., 2005) would provide design rules for optimizing carbon nanotube film emitters for various applications

Carbon nanotubes (CNTs) used as a probe for scanning probe microscopy has become one of the many potential usages of CNTs that is finding real applications in scientific research and industrial communities. It has been proposed that the unique mechanical buckling properties of the CNT would lessen the imaging force exerted on the sample and, thus, make CNT scanning probes ideal for imaging soft materials, including biological samples in liquid environments. The hydrophobic nature of the CNT graphitic sidewall is clearly chemically incompatible with the aqueous solution requirements in some biological imaging applications. In this paper, we present electron micrograph results demonstrating the instability of CNT scanning probes when submerged in aqueous solution. Moreover, we also introduce a novel approach to resolve this chemical incompatibility problem. By coating the CNT probe with ethylenediamine, thus rendering the CNT probe less hydrophobic, we demonstrate the liquid imaging capability of treated CNT probes. Experimental data for imaging in aqueous solutions are presented, which include an ultrathin Ir film and DNA molecules on a mica surface.

We investigate the development of meter long negative discharges and focus on their X-ray emissions. We describe appearance, timing and spatial distribution of the X-rays. They appear in bursts of nanosecond duration mostly in the cathode area. The spectrum can be characterized by an exponential function with 200 keV characteristic photon energy. With nanosecond-fast photography we took detailed images of the pre-breakdown phenomena during the time when X-rays were registered. We found bipolar discharge structures, also called "pilot systems", in the vicinity of the cathode. As in our previous study of X-rays from positive discharges, we correlate the X-ray emission with encounters between positive and negative streamers. We suggest that a similar process is responsible for X-rays generated by lightning leaders., Comment: 31 pages, 13 figures

were recorded including rates of endoleaks, neurological complication, repeat procedures, and left arm claudication. Results: Median follow up was 552.5 days. LSCA was embolized in 41/104 (39%) of procedures. Indication for TEVAR in the embolized and non-embolized groups was dissection in 23/41 (56%) and 28/63 (44%), aneurysm 11/41 (26%) and 19/63 (30%), pseudoaneurysm 6/41 (14%) and 4/ 64 (6%), transection 1/41 (2%) and 9/64 (14%), and penetrating ulcers 0/41 (0%) and 3/64 (4%), respectively. The cerebrovascular accident (CVA) rate was 7/41 (17%) and 7/63(11%) and paralysis rate was 0/41 (0%) and 1/63 (2%) for the embolized and non-embolized groups respectively. The endoleak rate was 24/41 (58%) for the embolized group; 7/41 (17%) were persistent and resulted in aneurysm sac enlargement (6 type Ia and 1 type II endoleak) treated with sac embolization (n1⁄45), stent graft extension (n1⁄41), or surgical repair (n1⁄41). Endoleak rate was 22/63 (35%) for the non-embolized group; 5/63 (8%) were persistent and resulted in aneurysm sac enlargement (3 type Ia and 2 type II endoleaks) treated with LSCA embolization (n1⁄44) and both LSCA and sac embolization (n1⁄41). LSCA was covered without a carotid-subclavian bypass in 58/104 (56%). CVA rates for bypass and no bypass groups were 5/46 (11%) and 6/58 (10%) and paralysis rates were 1/46 (2%) and 0%, respectively. 10/58 (17%) patients required a subsequent carotid-subclavian bypass for left arm claudication. Conclusion: Coverage of the left subclavian artery during endovascular repair of the thoracic aorta is feasible with low complication rates. Embolization of the LSCA may be required to prevent or treat endoleaks.

We investigate structure and evolution of long positive spark breakdown; and we study at which stage pulses of hard X-rays are emitted. Positive high-voltage pulses of standardized lightning impulse wave form of about 1 MV were applied to about 1 meter of ambient air. The discharge evolution was imaged with a resolution of tens of nanoseconds with an intensified CCD camera. LaBr3(Ce+) scintillation detectors recorded the X-rays emitted during the process. The voltage and the currents on both electrodes were measured synchronously. All measurements indicate that first a large and dense corona of positive streamers emerges from the high voltage electrode. When they approach the grounded electrode, negative counter-streamers emerge there, and the emission of hard X-rays coincides with the connection of the positive streamers with the negative counter-streamers., 19 pages, 11 figures in Journal of Physics D: Applied Physics 2012

X-ray emission has been detected occasionally during the streamer-corona propagation in a wire-plate corona reactor open to ambient air. A 65 kV pulse with 15 ns rise time is applied to the wire anode superimposed on a 20 kV dc bias. The duration of the driving voltage pulse (110 ns) is less than 2.5 times the primary streamer transit time. Under this condition no arc discharge occurs between the wire and the cathode plates separated by 6 cm air. The onset of x-ray emission coincides with the initiation of the primary streamers near the wire anode. No x-rays were detected later, during or after the primary or secondary streamer development. X-ray energies ranged between 10 and 42 keV, as detected by a LaBr3 (Ce) scintillator–photomultiplier combination. Time resolved imaging of the streamer propagation highlights the different stages in the streamer discharge process. The energetic electrons originate near the anode, at the moment of streamer initialization.

A lightning surge generator generates a high voltage surge with 1.2 microsec. rise time. The generator fed a spark gap of two pointed electrodes at 0.7 to 1.2 m distances. Gap breakdown occurred between 0.1 and 3 microsec. after the maximum generator voltage of approximately 850 kV. Various scintillator detectors with different response time recorded bursts of hard radiation in nearly all surges. The bursts were detected over the time span between approximately half of the maximum surge voltage and full gap breakdown. The consistent timing of the bursts with the high-voltage surge excluded background radiation as source for the high intensity pulses. In spite of the symmetry of the gap, negative surges produced more intense radiation than positive. This has been attributed to additional positive discharges from the measurement cabinet which occurred for negative surges. Some hard radiation signals were equivalent to several MeV. Pile-up occurs of lesser energy X-ray quanta, but still with a large fraction of these with an energy of the order of 100 keV. The bursts occurred within the 4 nanosec. time resolution of the fastest detector. The relation between the energy of the X-ray quanta and the signal from the scintillation detector is quite complicated, as shown by the measurements., 7 pages, 7 figures

We report the effect of cathode structure on the field emission properties of individual carbon nanotubes. Experimental field emission data are obtained for two well-defined cathode structures: a multi-walled carbon nanotube (MWNT) attached to an etched Ni metal wire and a MWNT attached to a flat Ni-coated Si microstructure. We observed different macroscopic turn-on fields of 1.6 and 2.5 V µm−1, respectively, for the aforementioned experimental structures. This effect is investigated by detailed finite element analysis. We demonstrate that the geometry of the cathode structures significantly affects the microscopic tip field, leading to different turn-on voltages and field distributions for such individual MWNT emitters. Simulations show that changing the support geometry from a hemispherically capped shank to a cylindrical shank produces an increase in the macroscopic threshold field of 0.91 V µm−1. This effect is further investigated by varying the support radius from 0.5 to 30 µm for a cylindrically shaped support structure. The results show that such a variation in the radius of the support structure produces an increase in the macroscopic turn on field from 0.72 to 5.89 V µm−1. We also report quantitative evidence for the nonlinear relationship between the field enhancement factor as a function of support structure radius for nanostructures of three different aspect ratios.