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Current Areas of Research

Absolute Spectral Measurements and Calculations

Absolute spectral measurements of x-ray tubes were performed and used to develop methods to analyze chemical compositions using only fundamental parameters, such as absorption, fluorescence yield, incident spectrum, etc. This work served as the basis of most x-ray fluorescence analysis codes available today. These codes are currently being updated to include more recently measured data over a wider spectral range. In addition, experimentally verified theoretical models of tube spectral output were developed that permit analysis and prediction for many x-ray source geometries. Commercial x-ray films and detectors also were calibrated to provide absolute measurement capabilities for use with continuous and pulsed x-ray sources.

Radiation Effects in Electronics

The section developed the techniques for using low energy (10 keV) x-ray sources during chip fabrication. Previous work was limited to Co-60 sources that required the electronic devices be packaged for radiation testing. Exploiting the differences in radiation response of Si-based devices to low (x-rays) and high (Co-60) energy radiations, the group was able to develop considerable knowledge of processes involved in the production, trapping, and annealing of holes and interface states in these devices. In addition, the ability to follow microscopic energy flow within layered materials, allowed prediction of the subsequently measured effects of enhanced dose in sensitive regions of devices then under development.

Spectral Measurements of Pulsed X-Ray Sources

Methods for plastically and elastically curving x-ray diffracting crystals and multi-layered structures were explored and developed. Such diffracting elements disperse incident rays so that the complete spectral distribution can be measured simultaneously, a necessity for measuring single, high intensity x-ray pulses. Curved diffractor spectrographs are now routinely used to measure plasmas emissions from pulsed-lasers, dense-plasma-focus, and other short duration pulsed sources.

Capillary and Mirrored X-Ray Optics

Development of a speedy, sophisticated and flexible ray tracing code led to the ability to follow radiation transport through capillary, microchannel plate, and conical x-ray optics. Of particular interest were optics that provided collimation and divergence control for x-ray sources used in lithography for the production of microelectronic circuits. Optics with greater than 10^6 elements and calculations using more than 10^8 rays are easily modeled.

X-ray Absorption Fine Structure (XAFS)

The proximity of atoms within a solid material can be determined by x-ray absorption fine structure measurements performed at a synchrotron x-ray source. The spectrum of x-rays transmitted through the sample near an absorption edge is recorded as the incident photon energy is incrementally changed. The proximity of atoms in the sample produces fine structure in the spectrum, the measurement of which allows the average spacing between atoms to be determined, even though the sample may be noncrystalline. For crystalline samples, similar measurements can be carried out on diffracted beams, thus allowing the distance between specific lattice sites to be determined.

XRF Sensor for the Cone Penetrometer (SCAPS)

The evaluation of subsurface soils for metal environmental contaminants, such as Co and Pb, has in the past been complicated by undesired intercommunicaton of aquifers that proceeds after the typically performed, large diameter (~4 inch) core drilling operations. The small diameter Cone Penetrometer (Site Characteristization and Analysis Penetrometer System, SCAPS) incorporates an NRL designed miniature x-ray fluorescence measurement system within a 1.25" ID steel pipe that is rammed into the ground to depths of over 100 feet. The x-ray window is boron carbide, an exceptionally hard material that is nonetheless transparent to x-rays. A miniature x-ray tube, designed for the penetrometer by Dr. W.T. Elam of NRL, is smaller in diameter than a pencil and short enough to fit in a pen cap. The SCAPS probe has characterized sites around the country.

UAV Deployment of Miniature Sensors

The Section, in collaboration with Tactical Electronic Warfare Division and the Naval Center for Space Technology and the University of Michigan, has fitted a small NRL unmanned aerial vehicle (UAV) with a MEMS (microelectromechanical system) sensor suite (temperature, pressure, relative humidity and acceleration). The in-flight sensor data were input to a miniature software radio with error correction code, radiated to an NRL satellite, and relayed from an airfield in Virginia back to the ground station at NRL-DC. Further work continues on networking sensors, vehicles, and ground stations.

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