DETECTION OF EXPLOSIVES VAPOR AT THE PPT LEVEL WITH A LASER INTERFEROMETRIC CALORIMETER

SCIENCE RESEARCH LABORATORY (SRL) HAS DEMONSTRATED, UNDER FUNDING FROM SDIO, AN ULTRASENSITIVE TECHNIQUE FOR THE REAL-TIME MEASUREMENT OF ABSORPTION DUE TO TRACE GASES IN THE ATMOSPHERE. THE SRL TECHNIQUE, CALLER LASER INTERFEROMETRIC CALORIMETRY (LIC) IS SIMILAR IN PRINCIPLE TO LASER OPTO-ACOUSTIC (LOA) SPECTROSCOPY WHICH HAS BEEN UTILIZED FOR OVER 15 YEARS TO DETECT TRACE CONCENTRATIONS OF VAPORS FROM THE MORE VOLATILE EXPLOSIVES SUCH AS EGDN, NG AND DNT AT THE 1-10 PPB LEVEL. THE NEW LIC TECHNIQUE, HOWEVER, HAS DEMONSTRATED THE ABILITY TO DETECT ABSORPTION AT THE 2 X 10-10 CM -1 LEVEL WITH A SINGLE PULSE FROM A LASER TUNED TO A CHARACTERISTIC ABSORPTION LINE. THIS IS ABOUT 100 TIMES THE SENSITIVITY THAT OPTOACOUSTIC TECHNIQUES HAVE BEEN ABLE TO DEMONSTRATE IN PRACTICE. BY SUMMING SIGNALS FROM A LASER OPERATING AT 10 HZ RATE FOR 10 SECONDS, AN ADDITIONAL FACTOR OF 10 IN SENSITIVITY CAN BE OBTAINED, DROPPING THE MINIMUM DETECTABLE VAPOR CONCENTRATION TO THE 1-10 PPT LEVEL. A PORTABLE LIC SYSTEM COULD THUS EXTEND REAL-TIME DETECTION CAPABILITIES TO LESS VOLATILE EXPLOSIVES SUCH AS TNT. BY USING PRECONCENTRATION TECHNIQUES, OR LASER DESORPTION FROM SURFACES, EVEN LOW VAPOR PRESSURE EXPLOSIVES SUCH AS RDX COULD BE DETECTED.