A stand-off Raman imaging system for the identification of explosive traces was modified for the analysis of substances in containers which are non-transparent to the human eye. This extends its application from trace detection of threat materials to the investigation of suspicious container content. Despite its limitation to containers that are opaque to the facilitated laser, the combination of Spatial Offset Raman Spectroscopy (SORS) with stand-off Raman imaging allows to collect spectral data from a broad range of different spatial offsets simultaneously. This is a significant advantage over SORS with predefined offset, since the ideal offset is unknown prior to the measurement and depends on the container material as well as the sample content. Here the detection of sodium chlorate in a white plastic bottle is shown. A 532nm-laser (pulse length 5ns, repetition 50kHz) was focused to a diameter of 10mm at 10m. A 1500mm Schmidt-Cassegrain telescope with a 152.4mm diameter collected the scattered light. An edge filter removed inelastically scattered laser light and a liquid crystal tunable filter was used to select 0.25nm broad wavelength ranges between 480 and 720nm. The sample area of 50×50mm was imaged on 1024×1024 pixels of an ICCD camera. For the conducted experiments an ICCD gate time of 5ns was selected and 70μJ-laser pulses were accumulated during 1s for each wavelength.
This paper describes how optical Kerr gating can be used for effective rejection of fluorescence from Raman spectra of explosives and explosives precursors. Several explosives are highly fluorescent, and this method enables Raman detection of explosives materials that would else be complicated or impossible to identify. Where electronic cameras (intensified charge-coupled devices, ICCDs) have showed not yet to be sufficiently fast to be used for rejection of this fluorescence, Kerr gating is here proved to be an efficient alternative, demonstrated by measurements on plastic explosives. Results were obtained using a gating time of ~30 ps. The Kerr gate was driven by the fundamental mode of an Nd:YAG laser, at 1064 nm, with pulses of ~8 mJ, 50 Hz and 30 ps. CS2 was used as a Kerr medium and Glan polarizing prisms were important features of the system. Raman spectra were obtained using a 532 nm probe wavelength, from the same Nd:YAG laser being frequency doubled, with a ~2 mJ pulse energy. Gating times of ~30 ps were thus achieved, with a fluorescence rejection factor of more than 1300, for the first time revealing detailed characteristics in Raman spectra from highly fluorescent PETN based plastic explosive.
A pulsed (4.4 ns pulse length) frequency doubled Nd:YAG laser, operating at 10 Hz, was used to generate Raman
scattering from samples at a distance of 12 m. The scattered light was collected by a 6 inch telescope and the Raman
spectrum recorded using an Acton SP-2750 spectrograph coupled to a gated ICCD detector. To extend the potential
applications further, employing a spatial offset between the point where the laser hit the sample and the focus of the
telescope on the sample, enabled collection of Raman photons that were predominantly generated inside the sample and
not from its surface. This is especially effective when the content of concealed objects should be analysed. Raman
spectra of H2O2 in a 1.5 mm thick, fluorescent HDPE plastic bottle were recorded at a distance of 12 m. From the
recorded spectra it was possible to determine the H2O2 concentration in the concentration range from 2-30%. Stand-off
Raman spectra of eleven potentially dangerous chemicals (commercial and improvised explosives) were recorded at a
distance of 100 m.
We present our work on stand-off Raman detection of explosives and related compounds. Our system employs 532 or
355 nm laser excitation wavelengths, operating at 10 Hz with a 4.4 ns pulse length and variable pulse energy (maximum
180 mJ/pulse at 532 nm and 120 mJ/pulse at 355 nm). The Raman scattered light is collected by a co-axially aligned 6"
telescope and then transferred via a fiber optic cable and spectrograph to a fast gating iCCD camera capable of gating at
500 ps. We present results including the effect of different excitation wavelengths, showing that 355 nm excitation gives
rise to significantly stronger stand-off Raman signals compared to that of 532 nm. We also show the effect of appropriate
detector gating widths for discrimination of ambient light and the reduction of high background signals in the obtained
Raman spectra. Our system can be used to identify explosives and their precursors in both bulk and trace forms such as
RDX and PETN in the low mg range and TNT in the 700 μg range at a distance of 20 m, as well as detection of a 1% or
greater H2O2 solution at a distance of 6.3 m.