Energetic nitrogen-based compounds utilized in solid rocket propellants break down under typical environmental conditions. The breakdown of these energetic propellant compounds requires stabilizing additives to absorb excess acids that form. These chemical changes result in a reduction of stabilizers and an increase of inert compounds over time which decrease propellant performance. Vibrational spectroscopic techniques such as Raman can detect changes in chemical concentrations due to the strong spectrum that these compounds demonstrate. In this study two wavelengths, 532 nm and 783 nm, are used to analyze the Raman spectra of propellant samples to characterize the changes to compounds over time. Computational techniques are demonstrated to mitigate fluorescence and single out the ratio of chemical peaks specific to stabilizer compounds. In addition, fluorescence in the 532 nm spectrum is examined as a method for characterizing propellant compounds, as 2NDPA traditionally has more fluorescence than MNA, and the 532 nm Raman system traditionally detects more fluorescence than the 785 nm Raman system. Detection of the stabilizer MNA in concentrations of greater than .70% and lower than .40% are demonstrated. Raman spectroscopy is shown to provide a rapid method for analyzing high and low concentrations of stabilizer compounds to determine the remaining viability of propellant.
Researchers from the U.S. Army, along with university scientists, are implementing efforts to develop a hyperspectral/broadband and/or ultraviolet (UV) sensing technology for target discrimination. The Army’s primary goal is to advance the development of fast reliable broadband optics techniques that can quickly identify and ascertain an indication of the geometric shape/compositional structure of various materials encountered on the battlefield. Samples of a variety of cement-based, metal, and composite materials are assembled and investigated to determine each sample’s spectra optical reflectance and absorption properties after being exposed to varying optical wavelengths. The optical wavelengths are generated from deuterium, halogen, and white light sources. The light intensity ratios are used to create data points which allowed for the identification of unique characteristics of each sample material. Broadband visible and near infrared (NIR) sources (deuterium and halogen) are reflected off the samples and the spectrometric reflections were captured. Several light intensity ratios are used and compared to distinguish the samples and error bars are created. While the initial results indicated that the halogen source may be used to distinguish most of the sample materials (and perhaps stand-alone versions of the other wavelength sources may not be sufficient), combinations of wavelengths/laser diodes with broadband light sources were used to determine if the identification/characteristics of each sample could be achieved. Results outlined in this paper include the current progress made toward the development of broadband optics sensing methodologies and instrumentation for identifying and discriminating the geometric configurations/formations of various battlefield materials.
A sensitive Raman spectroscopy technique is used for detection and possible quantification of the propellant stabilizer, nmethyl nitroaniline (MNA), in solid rocket propellants used in multiple domestic missile systems. Over time, the energetic ingredients of the propellant will degrade and react with the stabilizer, causing issues with the propellant useful safe life. Currently, there are no non-destructive analytical techniques for which MNA can be detected in solid rocket fuel inside a missile. Therefore, after a certain amount of time, missiles in inventory must be disassembled and tested for reliability and safety. This methodology is labor intensive, costly, and time consuming so a less intrusive approach is warranted to determine a missile useful safe life. Raman spectroscopy provides a possible solution to this problem, where a small fiber optic probe line may be inserted into the rocket motor of the missiles, which can be tested within seconds without the need for dismantling the missiles. A 785 nm portable Raman analyzer is used for all measurements reported in this paper with integration times ranging from 10 to 60 s. It is found that Raman sensing is a viable option for detection of MNA in solid rocket fuels.
Raman measurements, using a 785nm laser, are taken of Ammonium Nitrate and Sodium Nitrate buried in sand. Nitrate is kept in clear plastic containers and buried underneath sand at various depths. Raman measurements are then taken at distances of 5m and 20m, with the sand being completely dry as well as completely wet. A different set of experiments was conducted with Nitrate buried in sand in a glass container, where no Raman signal was seen in dry sand. Water was then added at the edge of the container and allowed to migrate to the bottom. Raman measurements are then taken at a distance of 7mm over time to detect Nitrates brought to the surface by water as it wicks to the surface.
Model human epidermal samples are used for transmission measurements at varying ambient humidity. Light is used
from four different light emitting diodes (LEDs), of UVA wavelength of 365nm, and three visible wavelengths of
460nm, 500nm, and 595nm. A humidity-controlled chamber was used to house the samples while transmission
measurements were taken. Many different types of measurements were taken, including raising ambient humidity from
20% to 75% then adding 0.5mL of water to the sample; lowering humidity from near 100% to 60%; and alternately
raising and lowering of the ambient humidity. The results show higher transmission of light through the samples at very
high ambient humidity, about 100%; whereas the transmission is much lower at lower ambient humidity. A simple
model of epidermis as a turbid medium and reduced light scattering by refractive index matching is used to explain the
results. Implications of these results are discussed.
The identification and real time detection of explosives and hazardous materials are of great interest to the Army and
environmental monitoring/protection agencies. The application and efficiency of the remote Raman spectroscopy system
for real time detection and identification of explosives and other hazardous chemicals of interest, air pollution
monitoring, planetary and geological mineral analysis at various standoff distances have been demonstrated. In this
paper, we report the adequacy of stand-off Raman system for remote detection and identification of chemicals in water
using dissolved sodium nitrate and ammonium nitrate for concentrations between 200ppm and 5000ppm. Nitrates are
used in explosives and are also necessary nutrients required for effective fertilizers. The nitrates in fertilizers are
considered as potential sources of atmospheric and water pollution. The standoff Raman system used in this work
consists of a 2-inch refracting telescope for collecting the scattered Raman light and a 785nm laser operating at 400mW
coupled with a small portable spectrometer.
Commercial substrates used for surface-enhanced Raman spectroscopy (SERS) are investigated for their reusability following cleaning with 254-nm UV light from a mercury lamp. SERS of Rhodamine 6G (Rh6G, a dye) and RDX (an explosive) is investigated. It is found that without UV irradiation, the substrate is usable only once, since it is not possible to dislodge the analyte either by prolonged immersion in distilled water or by ultrasonic cleaning. However, prolonged exposure to 254-nm UV followed by immersion in distilled water removes most of the analyte, making the substrate reusable for new SERS measurements. The technique of UV cleaning is demonstrated by recycling the same substrate several times and comparing SERS spectra taken after each cleaning cycle.