LOW-LIMIT PHOTO-ACOUSTIC DETECTION OF SOLID RDX AND TNT EXPLOSIVES WITH CARBON DIOXIDE LASER A. K. Chaudhary, a* G. C. Bhar, b and S. Das b UDC 621.375:826:662.47 We report the absorption spectra of RDX and TNT explosive samples in solid form at room temperature using a carbon dioxide laser-based photo-acoustic technique. A continuous wave 12 C 16 O 2 laser, tunable between the 9.25–10.74 μm wavelength region, was used as an energy source. Interference-free limits of detection of solid RDX and TNT in simulated samples are estimated to be 10.0 and 16.5 ppbw, respectively. Keywords: RDX, TNT, 12 C 16 O2 laser, photo- acoustic, lock-in amplifier, chopper. Introduction. Recently there has been a great deal of interest in exploring the possibility of examination of secondary energetic materials in the field of forensic investigation. It is often necessary to establish the nature as well as the amount of ingredients present in the exhibit samples. The secondary energetic materials are categorized into three classes, viz. nitrates, nitro compounds, and nitramines. Explosive substances recovered from debris are highly contaminated and cannot be identified on the basis of their physical parameters such as color, melting point, micro- scopic analysis, etc. In addition, infrared spectroscopic techniques using a spectrophotometer, FTIR, are extremely useful in the de- tection, identification, and quantitative analysis of complex explosive. However, if the provided compounds are impure or of questionable purity, it is essential to subject them to one or more purification processes such as recrystallization, distillation under reduced pressure, etc., to adapt them for the spectrophotometer technique. The spectroscopic tech- niques also require additional arrangements for sample preparation. Several research groups have exploited the various types of laser-based properties such as photodissociation, multiphoton dissociation, photo-ablation, laser desorption, etc., in the investigation of RDX and TNT [1–4]. Most of the above-mentioned works which claimed detection of explosives at ppm level have used explosives in vapor form, which was obtained by sublimation of milligram- order samples. Xinsheng Zhao et al. [5] have studied the infrared multiphoton dissociation of RDX in a molecular beam by loading the RDX sample into the oven chamber. Gary M. Boudreaux et al. [6] have reported the detection of explosive by using a photo-fragmentation technique. They have used 400-mg sample of TNT and simulated soil mixture. Although this technique is mainly employed in the detection of atmospheric pollutants in gaseous form [7, 8], yet samples of explosives such as nitroglycerine (NG), and ethylene glycol dinitrate (EGDN), and dinitrotoluene (DNT) have also been detected in gaseous form using this technique [9, 10]. R. L. Prasad et al. [11] have reported the photo- acoustic spectra and modes of vibration of TNT and RDX at the CO 2 laser. In this paper we report an improvised form of a similar technique, which proved to be an effective, efficient, and nondestructive in nature and able to detect solid explosives at the ppb level on a quantitative basis for the first time. We have also quantitatively recorded the 12 C 16 O 2 laser absorption spectra of hexahydro-1,3,5-trinitro-1,3,5- triazene (RDX) and 2,4,6-trinitrotoluene (TNT) in the powder form by employing a photo-acoustic (PA) cell. The em- ployed 12 C 16 O 2 laser system provides not only stable emission frequency with fine tunability but also higher resolution as compared to a conventional spectrophotometer. As a result, more PA absorption bands have been observed as com- pared to previously reported work [11]. The spectral interval between the longitudinal modes of a cavity ∆ν m = C/2L, * To whom correspondence should be addressed. a Department of Physics, Addis Ababa University, P.O. Box No.1176, Addis Ababa, Ethiopia; e-mail: anil- phys@yahoo.com; b Laser Laboratory, Physics Department, Burdwan University, Burdwan-713104, India. Published in Zhurnal Prikladnoi Spektroskopii, Vol. 73, No. 1, pp. 113–118, January–February, 2006. Journal of Applied Spectroscopy, Vol. 73, No. 1, 2006 0021-9037/06/7301-0123 ©2006 Springer Science+Business Media, Inc. 123