Development of a Compact, High-Specificity, Single-Use Sensor/Sampler

Project # 23-041 | Year 1 of 1

David Baldwin, Manuel Manard, Steven Koppenjan, Eric Larson, Eliseo Pizano, Ian Bortins

Special Technologies Lab (STL)
_{This work was done by Mission Support and Test Services, LLC, under Contract No. DE-NA0003624 with the U.S. Department of Energy, the NNSA Office of Defense Programs, and supported by the Site-Directed Research and Development Program. DOE/NV/03624–1914.}

Abstract

Detection levels for some extremely high value fugitive gases from chemical and nuclear weapons proliferation activities are significantly limited by the thermal partitioning of the population among several low energy states of the material. These characteristics make very sensitive optical (visible ultraviolet (UV‐VIS), shortwave infrared (SWIR), longwave infrared (LWIR), or terahertz (THz)) detection impossible at significant standoff distances due to the limited population available for absorption for a given transition without very long pathlengths and loss of location information. We propose a solution that provides a single high sensitivity and specificity spectral measurement in a self‐contained sampling and analysis device. It will consist of a pre‐evacuated combined gas sampler, pressurized pulsed expansion to vibrationally and rotationally cool the sample, and a pulsed source timed to coincide with the expansion within a high Q cavity. Ring down signal from a single measurement will provide high sensitivity localized identification and concentrations for a specific gas.

Background

Some of the most significant signatures of chemical weapons and nuclear fuel cycle effluents are fugitive chemical vapors. The exploitation of some of these signatures is stymied by relatively poor spectral sensitivity or significant spectral interferences for current near infrared (NIR) and LWIR remote sensing methods. We are proposing to develop a compact, self-contained sensing device with high sensitivity and specificity that could be deployed to identify and measure a concentration of specific gases that are difficult to detect using existing methods. We proposed to develop a handheld device that samples ambient gas and analyzes for a particular target material using a single-pulse Fourier transform microwave cavity device with a pulsed expansion to maximize sensitivity and eliminate spectral interferences. Fugitive molecular gases are currently detected using NIR or LWIR remote detectors that sense the rotational-vibrational transitions of these gases in an ambient mixture with other spectrally active gases. Most notably, water vapor obscures large portions of the electromagnetic spectrum due to its high and variable atmospheric concentrations and strong transitions. Some target gases also have spectral sensitivity that is limited by the spread of the available population across a large number of low-energy rotational-vibrational states at ambient temperatures. With significantly limited sensitivity and high probability for spectral interferences, the value of some of these signatures for locating the source term is limited. Samples of these gases could be collected in a sealed container or through adsorption on a sorbent material and returned to a laboratory for mass spectral analysis. However, the delay would prohibit the intelligent collection of samples based on immediate feedback.

Technical Approach

We proposed to develop a single shot, battery operated rechargeable system where a small sample of ambient air containing the target gas is sequentially sampled, pressurized with a buffer noble gas from a prefilled pressurized ampule, pulsed through a nozzle into a high-Q evacuated cavity, and the signal from a single pulse ring down absorption is used to measure the concentration of the target gas. As envisioned, the ambient air would be sampled into a small, moderately evacuated chamber by manually opening and closing a valve. The sample gas would be pressurized with a small amount of a buffer gas using a second manual valve. Finally, a manually triggered event chain will pulse this material into an evacuated high-reflectivity optical cavity and fire a single pulse of a microwave or THz source. The pulsed expansion will cool the gas (target and interferents), typically to about 10 K, and concentrate the population into a small number of rotational and vibrational states. This will increase the sensitivity and reduce the interferences in the spectral measurement. The signal will be extracted by detecting the source signal as it decays, leaking from the cavity. A Fourier transform of this signal will result in a pair of Doppler-shifted peaks for the selected transition. The integrated intensity of these peaks would be proportional to concentration. Following its use, the system’s evacuated chamber could then be used to collect and return a gas sample for laboratory spectral analysis.

Sensitivity enhancement in such a system occurs both from the concentration of the species into a single or small number of low energy states, but also through the long absorption pathlength associated with a cavity absorption where the average number of roundtrip passes through the cell before detection may be hundreds or thousands of times the physical size of the cavity. The reduction of interferences is a result of both the rotational and vibrational cooling of the interfering species, as well as the choice of rotational spectroscopy that has inherently high resolution and sparse spectral features in a cooled jet source.

Although previously applied using a repeatedly pulsed version, the application of this type of detection to transient chemical species at low concentrations of more complex molecules from hot flame gases makes our approach with stable ambient gases and less demanding cooling requirements appear to be analytically feasible¹. Early design stages will model the sensitivity limitations based on available sources, detectors, cooling factors, cavity quality, and high-resolution transmission (HITRANS) spectral information.

Ultimately, sensitivity will depend upon the nature of the target chemical, with optimum sensitivity being obtained for monoisotopic polar materials with large rotational constants. This same technique would be applicable to THz/microwave (rotational), LWIR (rovibrational), and UV-VIS (electronic) transitions of a variety of species, using different solid-state sources, detectors, and cavity designs. However, we would target a rotational feature for our first application. We anticipate the eventual development of a handheld “soda can” form factor that would allow a user to activate and determine a concentration and collect a sample with each unit. The device would be recyclable for re-use after recharging the battery, evacuating the system, and replacing the buffer gas. A series of measurements could be made with a “six-pack” of sensor/collectors for plume tracing and localization.

In the first phase of this work, we intended to target a material of interest to chemical weapons sensing. We intended to develop and test the individual components of the device as described above (evacuated sampling chamber, pressurized buffer gas, single pulse valve, single pulse microwave source, evacuated ringdown cavity, detector, signal processing and display) to determine whether any of these components are unable to function in an ultra-compact self-contained form. The components would have been evaluated for single shot sensitivity for the selected material. Follow-on years would entail assembling a small device based in these evaluations and best choice compromises for sensitivity and size, weight, and power minimization.

• [1] N. Hansen, J. Wullenkord, D.A. Obenchain, I. Graf, K. Kohse-Höinghaus, and J.-U. Grabow. RSC Adv. 7, 37867 (2017).

Results and Technical Accomplishments

Failed to recruit necessary PhD staff, therefore stopped project.

Conclusions and Path Forward

Although the spectroscopist was not procured, several achievements were made. Spectral models of microwave and IR spectra of some high value toxic industrial and proliferation gases were calculated, indicating significant (>100x) increases in sensitivity versus ambient monitoring. The initial target list has difficult spectral limitations.

Publications

• Title: Hyperfine structure and isotope shifts of xenon measured for near-infrared transitions with Doppler-free saturated absorption spectroscopy
  Journal / Conference: Spectrochimica Acta Part B: Atomic Spectroscopy 202 (2023) 106635
  Year: 2023
  Author(s): James Bounds, Alexandre Kolomenskii, Rusty Trainham, Manuel Manard, Hans Schuessler

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