NCJ Number
44884
Date Published
1977
Length
53 pages
Annotation
RESULTS OF A PROGRAM TO EVALUATE THE FEASIBILITY OF USING A LASER OPTOACOUSTIC SYSTEM TO DETECT HIDDEN EXPLOSIVES ARE REPORTED.
Abstract
THE EXPERIMENTATION WAS CONDUCTED IN 1977. NUMEROUS MEASUREMENTS WERE MADE ON ONE EXPLOSIVE VAPOR IN PARTICULAR -- ETHYLENE GLYCOL DINITRATE (EGDN) -- A COMPOUND SIMILAR TO NITROGLYCERINE WHICH IS ONE OF THE PRINCIPAL COMPONENTS OF MOST MODERN DYNAMITES. IN THE LASER OPTOACOUSTIC DETECTION TECHNIQUE, A LASER BEAM IS PASSED THROUGH AN ABSORPTION CELL WHICH CONTAINS A TRACE GAS OF INTEREST. IF THE LASER LINE IS ABSORBED BY THE TRACE GAS, THE RESULTING SOUND WAVES ARE DETECTED BY A SENSITIVE MICROPHONE. IN THESE EXPERIMENTS, CARBON 12 DIOXIDE AND CARBON 14 DIOXIDE LASERS WERE USED AND MEASUREMENTS MADE AT A NUMBER OF THE CO2 WAVELENGTHS AVAILABLE IN ORDER TO OBTAIN THE SPECTRAL ABSORPTION CHARACTERISTICS OF THE SUBSTANCE. THE OBJECTIVE OF THE PROGRAM WAS TO CONSTRUCT A LASER OPTOACOUSTIC SYSTEM HAVING A LIMIT OF DETECTION ABOUT TWO ORDERS OF MAGNITUDE BETTER THAN THAT REPORTED TO DATE, AND TO PERFORM A SERIES OF CRITICAL EXPERIMENTS TO ASCERTAIN WHETHER EXPLOSIVE VAPOR CONCENTRATIONS OF 100 PARTS PER TRILLION OR LESS COULD BE DETECTED AND DISTINGUISHED FROM A WIDE VARIETY OF OTHER VAPORS AND ORGANIC COMPOUNDS LIKELY TO BE PRESENT SIMULTANEOUSLY. AS A RESULT OF THE EXPERIMENTATION, RESEARCHERS DEMONSTRATED PERFORMANCE SUFFICIENT TO DETECT EGDN VAPOR AT CONCENTRATIONS OF ABOUT 10 PARTS PER BILLION IN THE PRESENCE OF 50 PERCENT RELATIVE HUMIDITY WATER VAPOR. EXTENDING THE SENSITIVITY BELOW 100 PARTS PER TRILLION WOULD REQUIRE A SOLUTION TO WATER VAPOR AND ANOMALOUS ABSORBANCE PROBLEMS. RESEARCHERS KNOW OF NO EXISTING DESICCANT, MEMBRANE, OR OTHER GAS SEPARATION DEVICE WITH THE CHARACTERISTICS NEEDED TO ALLOW THE LASER OPTOACOUSTIC PRINCIPLE TO BE EMPLOYED AT ITS THEORETICAL LIMITS, WHICH APPEAR TO BE IN THE ONE PART PER TRILLION RANGE. IT IS CONCLUDED THAT SINCE THE PRINCIPLE STUDIED HAS A SENSITIVITY SUBSTANTIALLY BETTER THAN MOST OTHER ANALYTIC TECHNIQUES, AND IT ALSO PROVIDES SPECTRA WHICH CAN BE USED FOR DISCRIMINATION AND IDENTIFICATION, IT IS AN ATTRACTIVE CANDIDATE FOR EXPLOSIVE DETECTION. CONTINUING DEVELOPMENT IS RECOMMENDED. EXPERIMENTAL DATA ARE PRESENTED IN TABLES, CHARTS, AND GRAPHS, AND A BIBLIOGRAPHY IS PROVIDED. (VDA)