Cranfield Defence and Security
Permanent URI for this community
Browse
Browsing Cranfield Defence and Security by Supervisor "Ahmad, S. R."
Now showing 1 - 4 of 4
Results Per Page
Sort Options
Item Open Access Interaction of laser radiation with urinary calculi(2009-11-25T17:07:23Z) Mayo, M. E.; Ahmad, S. R.; Bellerby, J.Urolithias, calculus formation in the urinary system, affects 5 – 10% of the population and is a painful and recurrent medical condition. A common approach in the treatment of calculi is the use of laser radiation, a procedure known as laser lithotripsy, however, the technique has not yet been fully optimised. This research examines the experimental parameters relevant to the interactions of the variable microsecond pulsed holmium laser (λ = 2.12 μm, τp = 120 – 800 μs, I ~ 3 MW cm-2) and the Q-switched neodymium laser (λ = 1064 nm, τp = 6 ns, I ~ 90 GW cm-2) with calculi. The laser-calculus interaction was investigated from two perspectives: actions that lead to calculus fragmentation through the formation of shockwave and plasma, and the prospect of material analysis of calculi by laser induced breakdown spectroscopy (LIBS) to reveal elemental composition. This work is expected to contribute to improved scientific understanding and development of laser lithotripsy. The results support the general model of thermal and plasma processes leading to vaporization and pressure pulses. Nd:YAG laser interaction processes were found to be plasma-mediated and shockwave pressure (~ 12 MPa) dependent on plasma and strongly influenced by metal ions. Ho:YAG laser-induced shockwaves (~ 50 MPa) were found to be due to direct vaporisation of water and dependent on laser pulse duration. The characteristics of the pressure pulse waveforms were found to be different, and the efficiency and repeatability of shockwave and the nature of the dependencies for the lasers suggest different bubble dynamics. For the Nd:YAG laser, LIBS has been demonstrated as a potential tool for in situ analysis of calculus composition and has been used for the identification of major and trace quantities of calcium, magnesium, sodium, potassium, strontium, chromium, iron, copper, lead and other elements.Item Open Access Laser induced breakdown spectrocscopy for elemental analysis in aqueous media(2012-02-02) Peel, C.; Ahmad, S. R.This thesis is based on extensive experimental work over a three year Ph.D. studentship program sponsored by the AWE on laser induced breakdown spectroscopy (LIBS) for elemental analysis in the water environment. An exhaustive up-to-date literature review has revealed widely different results on the dependence of some of the LIBS parameters on both laser and target parameters. In this research, parametric measurements of the laser-water interaction events were undertaken to validate some quoted values and to attempt to resolve discrepancies within the published results. Analysis of dielectric breakdown in water bulk by focussed laser beam has shown that the threshold laser pulse energy for this was approximately 60% lower in tap water than in distilled water due to impurity content of the former. However, the effect of analyte concentration was found to be much less drastic, giving only a 10% reduction for 2 orders of magnitude of concentration increase of dissolved sodium. Parameters of cavitation bubbles, resulting from the laser induced breakdown process in the water bulk, were measured using two different techniques i.e. probe beam deflection (PBD) and high speed imaging (HSI). Values of bubble diameter before collapse (maximum) and the frequency of the oscillation were found to be different, by 27% for the diameter and by 22% for the frequency, in the results obtained using the two different techniques. The values of the parameters obtained in this study and those found in the literature vary widely and do not show any trend on their dependence with laser pulse energy. It is concluded that large uncertainty in the estimation or control of sampled volume defined by the waist of the laser beam focus render large errors on such measurements. The present work includes parametric measurement of signal-to-noise ratio (S/N) for the detection of signals from analytes dissolved in water for different variable experimental parameters. Optimum values for achieving maximum S/N were obtained. In addition to the spectral and temporal filtering for improving S/N, mechanical filtering techniques were also investigated. This has resulted in the improvement of S/N by more than 25%. The thesis concludes with remarks, based on the analysis of the experimental data, on work for further improvement of sensitivity of the LIBS technique and its wider application as a portable device for in situ, real time point monitoring of elemental contaminants in water.Item Open Access Raman spectroscopy of biological tissue for application in optical diagnosis of malignancy(2009-11-25T17:41:59Z) Stone, N.; Ahmad, S. R.; Barr, Prof. H.The utilisation of near-infraredR aman spectroscopyfo r the discrimination of cancersa nd pre-cancers from normal tissue in the acro-digestive tract has been evaluated. A commercially available Raman microspectrometehr as been modified to provide optimum throughput, sensitivity and fluorescence suppression for epithelial tissue measurements. Laser excitation at 830nmw as demonstratedto be optimum. High quality (SN ratio 15-20) NIR-Raman spectrah ave been acquired from oesophageaal nd laryngeal tissues in time scales under 30 seconds. Pathological groupings covering the full range of normal and neoplastic tissues in the organs of interest have been studied. Both fresh (snap frozen) and formalin fixed tissue samples were investigated,f irstly to indicate whether tissue-typesc an be distinguishedi n vivo and secondlyt o demonstrateth e use of Raman spectroscopya s a tool for classificationi n the pathology lab. Results using multivariate statistical techniques to distinguish between spectra from specimens exhibiting different tissue pathologies have been extremely promising. Crossvalidation of the spectral predictive models has shown that three groups of larynx tissue can be separated with sensitivities and specificities of between 86 and 90% and 87 and 95% respectively. Oesophageal prediction models have demonstrated sensitivities and specificities of 84 to 97% and 93 to 98% respectively for a three-group consensus model and 73 to 100% and 92 to 100% for an eight-groupc onsensusm odel. Epithelial tissues including stomach, tonsil, endometrium, bladder and prostate have been studiedt o identify further tissuesw hereR amans pectroscopym ay be employedf or detection of disease.S pectraw ere similar to those obtainedf rom oesophagusa nd larynx, although sufficiently different for distinct discriminant models to be required. This work has demonstratedth e genericn atureo f Ramans pectroscopyfo r the detectiona nd classification of cancersa nd pre-cancerousle sionsi n many tissues.T he evidencep rovided by this study indicatest hat utilisation of Ramans pectroscopyfo r non-invasived etectiona nd classification of diseaseis a distinct possibility. Potentiald ifficulties in the transferabilityf rom in vitro to in vivo have been evaluated and no significant barriers have been observed. However, further in vivo probe development and optimisation will be required before 'optical biopsy' with Ramans pectroscopyc anb ecomea reality.Item Open Access Surface enhanced raman spectroscopy for ultra-sensitive detection of energetic materials(2010-11-03) Syed, A. A.; Ahmad, S. R.; Bellerby, J.The prospect of ultra-sensitive detection of molecular species, particularly those of energetic materials, has prompted the present research initiative. The combination of metal surface nano-technology and Raman spectroscopy has given rise to ‘Surface Enhanced Raman Spectroscopy’ (SERS). This is a very sensitive technique and has proved to be capable of detecting a single molecule. SERS was demonstrated by recording Raman spectra of the sample molecules adsorbed on various specially prepared SER-active surfaces both in the form of a colloidal suspension and on the solid roughened surfaces. Using a gold colloidal suspension, pyridine has been detected down to 10-11 molar (M) concentration. A silver slab was roughened to a dimension of a nano-scale by etching in nitric acid solution to make SER-active surface. Pentaerythritol Tetranitrate (PETN) explosive was detected using this surface after its 10-2 M solution was dropped, dried and washed (of any residue) from the surface. Lithographically engineered silver structures in the form of nanoarrays having a number of silver structures of approximately 106 in a region of 0.1 mm2 have been used for SERS. The major noise contribution to the scattering from impurities in an ordinary glass substrate has been eliminated by replacing glasses as substrates with pure quartz discs. The headspace vapours from peroxide explosives, Triacetone Triperoxide (TATP) and Hexamethylene Triperoxide Diamine (HMTD), were detected at approximately 70 parts per million (ppm) and 0.3 ppm concentrations respectively using a portable commercial Raman Spectrometer. PETN was also detected from its headspace vapour at about 18 parts per trillion (ppt) in spite of it having a much lower vapour pressure. The possibility of desorption of adsorbed molecules from a nano-structured surface by laser irradiation has been demonstrated experimentally with the aim of reusability of SER-active surfaces. Also demonstrated was the enhancement in Raman intensity through resonance Raman effect spectroscopy for the future use in surface enhanced resonance Raman spectroscopy (SERRS).