Torgny E. Carlsson

Stand-off detection of explosives particles by multispectral imaging Raman spectroscopy.

A Raman multispectral imaging technique is presented, which can be used for stand-off detection of single explosives particles. A frequency-doubled Nd:YAG laser operating at 10 Hz illuminates the surface under investigation. The backscattered Raman signal is collected by a receiver subsystem consisting of a 150 mm Schmidt-Cassegrain telescope, a laser line edge filter, a liquid-crystal tunable filter, and a gated intensified charge-coupled device (ICCD) detector. A sequence of images is recorded by the ICCD, where, for each recording, a different wavelength is selected by the tunable filter. By this, a Raman spectrum is recorded for each pixel, which makes it possible to detect even single particles when compared to known spectra for possible explosives. The comparison is made using correlation and least-square fitting. The system is relatively insensitive to environment and light variations. Multispectral Raman images of sulfur, ammonium nitrate, 2,4-dinitrotoluene, and 2,4,6-trinitrotoluene were acquired at a stand-off distance of 10 m. Detection of sulfur particles was done at a distance of 10 m.

Spatially offset hyperspectral stand-off Raman imaging for explosive detection inside containers

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.

Stand-off detection of explosive particles by imaging Raman spectroscopy

A multispectral imaging technique has been developed to detect and identify explosive particles, e.g. from a fingerprint, at stand-off distances using Raman spectroscopy. When handling IEDs as well as other explosive devices, residues can easily be transferred via fingerprints onto other surfaces e.g. car handles, gear sticks and suite cases. By imaging the surface using multispectral imaging Raman technique the explosive particles can be identified and displayed using color-coding. The technique has been demonstrated by detecting fingerprints containing significant amounts of 2,4-dinitrotoulene (DNT), 2,4,6-trinitrotoulene (TNT) and ammonium nitrate at a distance of 12 m in less than 90 seconds (22 images × 4 seconds)1. For each measurement, a sequence of images, one image for each wave number, is recorded. The spectral data from each pixel is compared with reference spectra of the substances to be detected. The pixels are marked with different colors corresponding to the detected substances in the fingerprint. The system has now been further developed to become less complex and thereby less sensitive to the environment such as temperature fluctuations. The optical resolution has been improved to less than 70 μm measured at 546 nm wavelength. The total detection time is ranging from less then one minute to around five minutes depending on the size of the particles and how confident the identification should be. The results indicate a great potential for multi-spectral imaging Raman spectroscopy as a stand-off technique for detection of single explosive particles.

Laser Triggering of Spark Gap Switches

Switching time jitter is an important property to consider when selecting a closing switch for a pulsed-power system, and time-precise triggering may be achieved through the use of lasers. For a midgap laser-triggered spark gap, three different physical mechanisms can be used: nonresonant multiphoton ionization, resonant-enhanced multiphoton ionization, and electron tunneling. The first one is traditionally used, whereas the latter two are more exploratory. In this paper, the traditional method is employed to study the delay time and time jitter of a laser-triggered spark gap using an Nd:YAG laser at 1064 and 532 nm, where the laser pulse is guided via an optical fiber to the spark gap; the laser pulse energy and the applied voltage have been varied with nitrogen as the working gas. One draw back of the current laser triggering technology compared with other triggering techniques is that laser systems are more complex and prone to electromagnetic interference. Another downside is that the pulse-repetition rate is poor. A discussion about the development of lasers to overcome these issues is included, together with a deliberation about the pros and cons of the two exploratory methods of laser triggering.

Interaction Between Solid Copper Jets and Powerful Electrical Current Pulses

The interaction between a solid copper jet and an electric current pulse is studied. Copper jets that were created by a shaped-charge device were passed through an electrode configuration consisting of two aluminum plates with a separation distance of 150 mm. The electrodes were connected to a pulsed-power supply delivering a current pulse with amplitudes up to 250 kA. The current and voltages were measured, providing data on energy deposition in the jet and electrode contact region, and flash X-ray diagnostics were used to depict the jet during and after electrification. The shape of, and the velocity distributions along, the jet has been used to estimate the correlation between the jet mass flow through the electrodes and the electrical energy deposition. On average, 2.8 kJ/g was deposited in the jet and electrode region, which is sufficient to bring the jet up to the boiling point. A model based on the assumption of a homogenous current flow through the jet between the electrodes underestimates the energy deposition and the jet resistance by a factor 5 compared with the experiments, indicating a more complex current flow through the jet. The experimental results indicate the following mechanism for the enhancement of jet breakup. When electrified, the natural-formed necks in the jet are subjected to a higher current density compared with other parts of the jet. The higher current density results in a stronger heating and a stronger magnetic pinch force. Eventually, the jet material in the neck is evaporated and explodes electrically, resulting in a radial ejection of vaporized jet material.

Speckle interferometry for measurement of continuous deformations

An improved method for measurement of continuous displacement and deformations with digital speckle pattern interferometry is presented. The initial random phase of the speckle pattern is evaluated by having many phase-shifting steps before the deformation. By this way the accuracy of the initial phase estimation can be increased and the handling of the image noise is improved. This makes it also possible to use the phase stepped speckle patterns as references for comparison with the speckle patterns of the deformed object, thereby increasing the reliability and accuracy of the phase estimations of the deformed patterns. The technique can be used for measuring deformations such as transients and other dynamic events, heat expansion as well as other phenomena where it is difficult to accomplish phase shifting during deformation.

Laser-triggering of spark gap switches

Switching time jitter is an important property to consider when selecting a closing switch for a pulsed-power system, and time-precise triggering may be achieved through the use of lasers. For a midgap laser-triggered spark gap, three different physical mechanisms can be used: nonresonant multiphoton ionization, resonant-enhanced multiphoton ionization, and electron tunneling. The first one is traditionally used, whereas the latter two are more exploratory. In this paper, the traditional method is employed to study the delay time and time jitter of a laser-triggered spark gap using an Nd:YAG laser at 1064 and 532 nm, where the laser pulse is guided via an optical fiber to the spark gap; the laser pulse energy and the applied voltage have been varied with nitrogen as the working gas. One draw back of the current laser triggering technology compared with other triggering techniques is that laser systems are more complex and prone to electromagnetic interference. Another downside is that the pulse-repetition rate is poor. A discussion about the development of lasers to overcome these issues is included, together with a deliberation about the pros and cons of the two exploratory methods of laser triggering.

Combining finite element analysis with digital light-in-flight recording by holography

Point clusters of measured shapes achieved with the digital light-in-flight recording by holography method, is combined with CAD - Computer Aided Design software to obtain convertible formats for design and simulation software. The point cloud is constructed to a b-spline surface by least square fitting. The b-spline surface is converted into IGES- format in order to be readable for CAD and other software. A simple deformation experiment is made which demonstrates the advantages of combining holographic measurements with FEA- analyzes in order to adjust the constraints, the boundary conditions and loads in the simulation model to better fit experimental results.

Ultrashort-pulse interferometric sensors for optical shape and deformation measurement

Basic principles for interferometry with ultrashort-pulse lasers are described. Different interferometer configurations for use as sensors for optical shape and deformation measurement are discussed. The sensor configurations that are mainly treated, are: the non-scanning white-light interferometer as distance and pulse length sensor. Speckle interferometry with ultrashort pulses for three-dimensional shape measurement. Light-in-flight recording by holography for visualization of light propagation and for three-dimensional shape measurement. Digital light-in-flight recording by holography. Holographic interferometry with ultrashort pulses.

Evaluation of digital flash x-ray images produced using computed radiography

Methodologies to extract information from flash X-ray images produced using Computed Radiography (CR) are described. In this process, the exposed image plate is directly and accurately scanned by a laser beam into digital format. Compared with photographic X-ray recordings, better geometrical stability and higher contrast are achieved. This enables more elaborate image and signal processing techniques to be applied to the images, resulting in a higher capacity to extract information from the recordings. Here, the density and velocity profiles along non-particulated shaped charge jets from flash radiography imaging have been evaluated. The mass distribution was estimated using the Abel transform, where the contrast to density calibration was done directly from information extracted from the images. The velocity evaluation was performed in two steps. The sums of intensities along cross sections were evaluated using image processing. By assuming constant jet density and identifying corresponding parts of the curves, the velocity profile along the jet is evaluated.