Photoacoustic detection by means of a differential double resonator cell applied to security and defence

A.S. VALLESPI; V SLEZAK; A PEURIOT; F. GONZÁLEZ; A PEREYRA; G SANTIAGO

Dresden

Congreso; Conference 8901A Optics and Photonics for Counterterrorism, Crime Fighting and Defence IX; 2013

SPIE

The purpose of this article is to present a sensitive optical system for immediate detection of trace substances associated with security in public spaces (TNT, RDX, nytroglicerine, ammonium nitrate, NH3, etc.) and storage or explosive detection. Since these substances present a strong absorption band between 9 and 11 𝜇𝜇𝑚𝑚, we propose its detection by means of photoacoustic spectroscopy (PAS) and study some properties with both a pulsed CO2 laser (TEA) and a CW CO2 laser. The laser beam is aimed to an innovative dual resonator differential cell (Fig. 1) designed by means of finite elements method, which lowest resonant frequency is the first longitudinal mode at 1204 Hz. The beam is modulated at the cell?s resonance by means of a chopper with a special blade which allows both reflection and transmission of the laser beam. A differential microphone (Knowles NR 23160) is coupled to both resonators [1] [2] at the midpoint of the cell. The characteristics of the differential microphones would minimize noise from distant sources, such as ambient noise and local heating of the cell windows. With the aim of characterizing the system we started the studies with ammonia. The chosen cell?s material is polypropylene, suitable for reducing the effects of adsorption due to the polarity of the ammonia molecule [3]. In order to take into account error sources during the measurements of low concentrations, physical adsorption-desorption at the cell?s walls is studied by means of the record of the PA signal decay using the TEA CO2 laser. A theoretical model, based on Langmuir?s isotherms, fits well the experimental results. As a result, a 5 % PA signal decay from an enclosed sample of 248 ppmV of NH3 in N2 was registered within 1 hour. Based on this result, minimum errors of the concentrations values are expected from measurements carried out on flowing gas mixtures passing through both resonators. The setup for CW CO2 laser excitation takes advantage of the differential microphone by picking up out of phase signals. For this purpose, a polished chopper wheel was prepared to allow generating the direct and the reflected beam, alternatively aimed to one resonator and the other (Fig. 2).The measurements show that for the double resonator configuration a signal increase is achieved as expected from the study of the sensitivity of both resonators separately, which had been previously characterized. The first measurements with this system indicate a limit of detection of around 90 ppbV at 1W, deduced from one standard deviation of the PA signal from pure N2. Figure 1: schematics of the double resonator cell. Figure 2: simplified sketch of the excitation of the PA Cell. A) The beam is transmitted through the chopper blade. B) The beam is reflected by the polished surface and redirected to the PA Cell with a system of spherical mirrors. References: [1] R. Bernhardt, G. Santiago, V. Slezak, A. Peuriot, M. González.Sensors and Actuators B 150; (2010) 513?516. [2] A. Schmohl, A. Miklós, and P. Hess. Appl Opt. 41; (2002) 1815-1823. [3] N. Melander and J. Henningsen. AIP Conf. Proc. 463; Proceedings of the 10th international conference on photoacoustic and photothermal phenomena, (23-27 Aug 1998) 78-80.