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Optics & Infrared Sensing

Optics & Infrared Sensing

Differential Remote Photoacoustic Spectroscopy (DIRPAS)

Figure 1  A possible configuration for DIRPAS to detect vehicle borne improvised explosive devices
Figure 1 A possible configuration for DIRPAS to detect vehicle borne improvised explosive devices

This project is focused on developing a new technology for standoff explosives/IED detection. A Differential Remote Photoacoustic Spectroscopy (DIRPAS) sensor combines eye-safe levels of mid wave infrared (MWIR) laser light excitation with an acoustic microphone/parabolic reflector detection scheme. The sensor will provide real time, < 0.5 sec "yes/no" detection and discrimination, and it is consumables-free.

The DIRPAS sensor uses the photoacoustic effect - if a pulsed laser beam is directed on a sample, an acoustic pulse will be generated whenever the laser wavelength is coincident with an optical absorption in the sample. Explosives that pose the greatest security risk (i.e. RDX, HMX, C4, Semtex, and TNT) have common broad absorptions, clustered in the MWIR around 6.35 mm. .9 None of these species absorb at 5 mm. Trace residues can be detected by comparing the PA signals that arise from two interleaved intensity-modulated 6.35 and 5.9 mm sources. It is important that the differential signal be monitored. Since bulk materials typically absorb continuously across the MWIR, the "background" signal must be removed.

It is anticipated that the first generation prototype DIRPAS sensor that PNNL is developing in collaboration with ARDEC (Picatinny Arsenal) and Maxion Technologies, will be capable of detecting fingerprint quantities of RDX on vehicle exterior surfaces at distances out to 5 m while operating at an outdoor vehicle check point. A second generation of the sensor should be capable of detecting fingerprint quantities of other types of explosives at distances of 50 m and potentially further.

Contact: Pamela Aker

Optics & Infrared Sensing

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