Photoacoustic sensor for trace detection of post-blast explosive and hazardous molecules
Introduction
As a result of invention of quantum cascade lasers (QCL) [1], laser absorption spectroscopy (LAS) in the mid-infrared is rapidly becoming a viable alternative to other analytical methods such as FTIR for trace-gas spectroscopy. The QCL is a new type of mid-IR tunable semiconductor laser based on intersubband transitions in a multiple quantum well heterostructure, designed by means of band-structure engineering grown by molecular beam epitaxy [MBE]. Recently, the QCL has attracted much attention due to its room temperature operation and its superior output power and mode quality as compared to that of lead salt diode lasers [2]. Many vapor phase chemical direct photoacoustic sensors based on QCL have been demonstrated [3]. The transduction displacement has been studied using the photoacoustic spectroscopy [4]. Laser photoacoustic spectroscopy has been used for trace detection of vapor, gas and aerosols [5], [6], [7], [8]. Photoacoustic spectroscopy has also been utilized for detection of explosives and other remote sensing applications [9], [10].
In present work, a photoacoustic laser spectrometer system with QCL emission in the mid-infrared range has been developed. A high quality factor resonant photoacoustic cell was used in the system. For the first time (to the best of our knowledge), a resonant cell catering to solid powder, liquid, thin film and vapor on adsorbed surfaces has been developed. The system is also capable to detect the samples taken on paper by swiping through the contaminated surfaces (sample quantity in nanogram). The resonant frequency of the photoacoustic cell was 25 kHz. The detection limit of the system was estimated to be of the order of nanogram for molecules of TATP and plastic explosive. With the developed system, we have been able to detect explosives (including Plastic explosives) and gun powder which are very challenging to detect otherwise. In the present scenario, the system has potential applications in ensuring homeland security by way of forensic investigation of traces of explosives and onsite post blast examinations of explosive materials.
Section snippets
Acoustics of the photoacoustic cell
The photoacoustic (PA) cell performs three functions: keeping the sample confined, providing a structure to connect the various components of the sensor, and increasing the photoacoustic signal. To maximize the later function, the acoustic properties of the PA cell need to be considered carefully. There are several approaches of cell design for influencing the PA signal: acoustic resonance, acoustic filters, buffer volumes, differential cell designs and multi-pass etc. Acoustic resonance is the
Experimental
The schematic diagram of the experimental setup is shown in Fig. 5. The laser source and the acoustic detector are situated away from the target sample. The samples were kept at the bottom of cylinder 1 of the PA cell and the microphone was integrated to the far end of cylinder 2 of the PA cell, so that there was no direct contact between sample and microphone. The samples setting procedure was as follows: The top lid (Mid IR transmission window) of the cylinder 1 of the photoacoustic cell used
Results and discussion
Tunable quantum cascade laser radiations were made to fall on the sample. The sample absorbs laser radiations and after absorption, the non-radiative relaxation takes place accompanied by emission of heat. Since the laser is modulated this emission of heat generates a pressure wave which comes to the boundary layer of the samples. The boundary layers were also coupled with the atmospheric air present inside the cell. This pressure wave is converted in the electrical signal by the Piezo
Conclusion
A system based on Photoacoustic spectroscopy has been developed which has unique capability to detect traces of explosives materials, hazardous chemicals and drugs in various forms such as powder, liquid and samples adsorbed on surfaces. T-type Photoacoustic cell was design and developed for the detection of traces of explosives in the post-blast materials. All the experiments have been carried out using indirect photoacoustic spectroscopy method in T-type Photoacoustic cell at 25 kHz resonant
Acknowledgements
Authors are thankful to Dr. Chris R Webster and Dr. Stan Sander, Jet propulsion laboratory NASA, USA for fruitful discussion for this work. Authors are also thankful to Professor J. P. Singh, MSU, USA, Professor Thomas Thundat, Alberta University, Canada and Professor S. N. Thakur, BHU, Varanasi for their valuable suggestions.
Ramesh C. Sharma is currently a Senior Scientist with the Laser Science and Technology Centre, Defense Research and Development Organization, Delhi, India. He received the Ph.D. degree in experimental laser spectroscopy from BHU Varanasi, Varanasi, India, in 1995. He was a Post-Doctoral Fellow with IIT Kanpur, Kanpur, India, University of California, San Diego, USA, and Mississippi State University; Starkville, MS, USA. He has worked several years as a Research Fellow/Scientist with the
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Ramesh C. Sharma is currently a Senior Scientist with the Laser Science and Technology Centre, Defense Research and Development Organization, Delhi, India. He received the Ph.D. degree in experimental laser spectroscopy from BHU Varanasi, Varanasi, India, in 1995. He was a Post-Doctoral Fellow with IIT Kanpur, Kanpur, India, University of California, San Diego, USA, and Mississippi State University; Starkville, MS, USA. He has worked several years as a Research Fellow/Scientist with the University of Tokyo, Tokyo, Japan, the University of Leeds, Leeds, and the University of Manchester, Manchester, U.K. He has worked in the field of laser technology, laser spectroscopy, nonlinear optics, nanotechnology, chemical dynamics, laser plasma physics, and laser remote sensing technologies. Dr. Sharma is the Group Leader and working on remote sensing of bio and hazardous molecules using ultraviolet laser-induced fluorescence technique and quantum cascade laser photoacoustic spectroscopy. He is a Life Member of the Indian Laser Association and the Laser and Spectroscopy Society of India.
Subodh Kumar received the M.Sc. degree in physics from Patna University, Patna, India, in 1998. He is currently pursuing the Ph.D. degree in physics with the Indian Institute of Technology Delhi, Delhi. He joined the Laser Science and Technology Centre, Defense Research and Development Organization, Delhi, India, in 2001, as a Scientist. He is involved in, laser Photoacoustic spectroscopy of explosive and other hazardous materials after having brief experience in the synthesis and characterization of laser materials (glasses). His research interests also include remote sensing of bio-molecules using ultraviolet laser-induced fluorescence technique and terahertz generation by laser plasma interaction. Mr. Kumar is a Life Member of the Indian Laser Association.
Surya Kumar Gautam received the B.E degree in Electronics and Communication engineering from Delhi Technological University, India, in 2012, and M.Tech degree in Applied Optics from Indian Institute of Technological Delhi, India, in 2015. He is currently a Research Fellow in Laser Science and Technology Centre with the Defense Research and Development Organization (DRDO), Delhi, India. He is having research interest in several field of optics like laser spectroscopy, imaging and quantum optics. He is currently involved in, laser photoacoustic spectroscopy and laser induced fluorescence LIDAR techniques.
Saurabh Gupta received his B.Tech. degree in Electronics engineering from Aligarh Muslim University, Aligarh, India in year 2000 and his M.Tech. degree in VLSI Design Tools and Technology from Indian Institute of Technology, Delhi in Dec. 2001. Since then, he is working with Defense Research and Development Organization as a scientist. He is involved in development of data acquisition and system development for detection of explosives and other hazardous chemicals. His interests include development of rugged and compact systems for military use. He is a life member of IETE.
Hari B. Srivastava received the B.E. degree in electrical engineering from the University of Roorkee, Roorkee, India, in 1983, and the M.Tech. degree in electrical engineering from the Indian Institute of Technology Kanpur, Kanpur, India, in 1989. He currently the Director of the Laser Science and Technology Centre with the Defense Research and Development Organization (DRDO), Delhi, India. He is a Life Member of the Instrument Society of India and the Optical Society of India, and a member of the Computer Society of India. He has authored more than 25 research papers in various national/international technical research papers in proceedings and journals. Mr. Srivastava was a recipient of the DRDO Group Technology Award.