Abstract:
Method and apparatus for tagging explosives with a source of SF 6  permitting the detection of their presence utilizing sensitive sniffing apparatus.

Description:
The invention described herein was made in the course of, or under a contract with the United States Atomic Energy Commission and/or the United States Energy Research and Development Administration. 
    
    
     BACKGROUND OF THE INVENTION 
     There has been increasing interest in the development of techniques for the detection of explosive materials. Recent terrorist activities including that of attempts to cause the destruction of civil aircraft in flight, as well as efforts to detonate explosives in places where large groups of people congregate, have heightened this interest. In addition, there is interest in preventing theft of such explosive materials from manufacturing plants. 
     Present approaches to the detection of explosives involve the use of comprehensive physical searches, X-ray and similar equipment, and dogs trained to sniff out the presence of certain types of explosive materials. 
     These approaches are either unwieldy or are of limited usefulness. 
     SUMMARY OF THE PRESENT INVENTION 
     The present invention overcomes many of the disadvantages of the techniques now in use by providing a simplified yet reliable approach to the problem of the detection of explosive materials. 
     In accordance with a preferred embodiment of this invention there is provided a method of tagging an explosive comprising the step of enclosing within the blasting cap a source of SF 6  to release over a period of time the SF 6  in sufficient amounts of the latter to permit detection. In one embodiment, the source is a solid member fully saturated initially with the SF 6 , and in another embodiment a capsule is inserted containing liquid SF 6  under pressure, the capsule being provided with a permeable window to permit a controlled release of the SF 6  gas over a longer period of time. 
     Because of the penetrating nature of SF 6 , this invention makes it possible to detect the presence of tagged explosives inside of closed packages merely by employing a so-called sniffer to monitor these packages. 
     Other advantages and objects of this invention will hereinafter become evident from the following description of preferred embodiments of this invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 is an elevation view in partial section of a preferred embodiment of this invention. 
     FIG. 2 is an elevation view in partial section of an alternative preferred embodiment of this invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     FIG. 1 shows a detonator or blasting cap 10 of conventional construction consisting of a shell 12 containing an explosive or detonating material 14. A rubber stopper 16 is crimped into place to seal the interior of shell 14. Within shell 12, but located on the other side of stopper 16 from the sealed material 14 is located a source 18 of the SF 6 . A pair of electrical leads 19a and 19b enter cap 10 to permit electric initiation. 
     Source 18 is a disc of suitable material in which SF 6  is adsorbed. The material selected for source 18 is one which has the characteristics of adsorbing large amounts of SF 6  at elevated pressures and releasing the SF 6  at a slow rate at ambient conditions over a long period of time. Such materials are available commercially and include the various fluoropolymers sold commercially under various trademarks including Teflon, a trademark of the DuPont Company for tetrafluoroethylene propylene. Other such fluoropolymers known in the art include chlorotrifluoroethylene copolymer (CTFE), ethylene-chlorotrifluoroethylene copolymer (E-CTFE), perfluoroalkoxy copolymer (PFA), ethylene-tetrafluoroethylene (ETFE), and fluorinated ethylene propylene copolymer (FEP). Extensive studies and tests were conducted to establish the effectiveness of the methods and apparatus described herein. 
     EXAMPLES 
     Table I shows the results of loading several different materials with SF 6  by exposing a disc of each of the materials to SF 6  at 300 psig at the temperature and for the time period indicated in the table. All of the materials listed in Table I are fluoropolymers available commercially. The ability of these materials to retain the adsorbed SF 6  330 days from loading is shown from measurements taken of three other samples of TFE appearing in Table II. Results are similar for all the other materials. 
     Studies were made to determine whether certain materials could be employed as effective barriers to the detection of the SF 6 . It was found that if a moderately strong SF 6  source is employed it is reasonably certain that barrier materials which allowed the SF 6  concentration to reach 10% of steady state within 10 hours or less should not present any significant problem to detection. By a moderately strong SF 6  source is meant herein a source with an elution rate of at least 1 nanoliter per minute or greater. Table III shows the results of tests taken employing selected barrier materials. The nature of diffusion phenomena is such that it appears that there are few effective ways of preventing the permeation of SF 6  in amounts which are detectable. 
     To test the effectiveness of this method over a period of time, several TFE pieces impregnated with SF 6  were carefully measured for SF 6  weight loss using the electrobalance and by measuring the SF 6  concentration in dry air when passed over the pieces. Table VI lists the measured weight of remaining SF 6  as a function of the number of days since initial loading for three of these pieces. The measured SF 6  concentrations are also included. 
     The present invention depends for its effectiveness in part on the availability of apparatus to detect or &#34;sniff&#34; the presence of SF 6 . There are available commercially apparatus which have this capability in sensitivity required herein. For example, there is the Analog Technology Corporation&#39;s Model 140 wide range electron-capture detector system. In addition the Brookhaven National Laboratory has developed a SF 6  Sniffer which is described completely in a paper &#34;Tracing Atmospheric Pollutants by Gas Chromatographic Determination of Sulfur Hexafluoride&#34; appearing in Environmental Science and Technology, Vol. 7, pp. 338-342, Apr. 1973. Other companies also have available apparatus which would be useful. 
     In the embodiment shown in FIG. 1, the rate at which the SF 6  is released declines with time. 
     In order to provide for a more uniform rate of release of the SF 6 , the embodiment shown in FIG. 2 may be utilized. There is shown a detonator or blasting cap 20 consisting of a shell 22 containing explosive or detonating material 24. A rubber stopper 26 is crimped into place to seal the interior of shell 24. A pair of electrical leads 27a and 27b are provided for initiation. 
     Embedded within stopper 26 is SF 6  source 28 which consists of a sealed capsule of metal construction containing liquid SF 6  under pressure. A window 32 of permeable material such as rubber permits SF 6  to be released at a uniform, controlled rate over a longer period of time as compared to the embodiment shown in FIG. 1. 
     
                                           TABLE I__________________________________________________________________________     SF.sub.6 Loading, mg per gram materialMaterial  100° C                25° CMaterialwt., g.     Hours/21           64   18   117  166__________________________________________________________________________CTFE 0.045     2.2   4.6  0.3  0.2  0.1E-CTFE0.040     1.7   5.5  0.2  0.2  0.1PFA  0.018     16.8  12.9 65.0 62.8 65.9TFE-10.019     18.6  14.7 50.1 64.3 66.5ETFE 0.010     10.1  9.6  1.2  4.7  5.3TFE-20.016     10.9  9.3  33.8 38.2 37.3FEP  0.014     18.4  14.9 60.0 71.6 74.3__________________________________________________________________________ 
    
     
                       TABLE II______________________________________Weight of absorbed SF.sub.6, mg/gTFE                  PredictedPiece No.  Measured      Second Order                            Third Order______________________________________1      12.003        11.788      12.0482      12.275        11.631      11.9033      12.063        11.632      11.891  average deviation:                -0.430      -0.166______________________________________ 
    
     
                                           TABLE III__________________________________________________________________________               Volume,                     k     Time to                                 BarrierBarrier Material     in..sup.3                    %/hr  10%, hours                                 Capability__________________________________________________________________________Cardboard carton    1     --   &lt;0.01  ineffective               112   --    0.07  &#34;Polyethylene bottle         (6 dram)               1.4  0.9   11.1   moderate         (1 qt.)               58   0.85  11.8   &#34;Paint can     (1/2 pint)               14.4 4     2.5    slight         (1 gal.)               231  0.70  14.3   moderateGlass jar     (1 ounce)               1.8  &lt;0.0004                          (&gt;3 years)                                 very severe         (1 qt.)               58   0.70  14.3   moderatePolyethylene zip-lock bag               1    31    0.32   negligible               50   0.52  19     mod. to severeBrass pipe    (3/4 inch)               1.5  0.0017                          5900   very severe         (2 inch)               23   0.37  27     severe__________________________________________________________________________ 
    
     
                                           TABLE IV__________________________________________________________________________      Weight (W) of Absorbed SF.sub.6, mg/g                        SF.sub.6Teflon Time,     Calculated  Concentration × 10.sup.9Piece No.  days      Meas. 2nd order                  3rd order                        Meas. Calc.__________________________________________________________________________1      157 17.085            16.964                  17.009  171 16.321            16.382                  16.364                        1.065 0.721  211 14.846            14.919                  14.861                        0.774 0.540  238 13.995            14.071                  14.053                        0.512 0.456  261 13.508            13.422                  13.460  269                   0.538 0.384      S.D.  ±0.096                  ±0.0582      157 17.108            16.967                  17.036  171 16.308            16.359                  16.356                        1.150 0.781  196 15.306            15.376                  15.322                        0.838 0.642  211 14.773            14.841                  14.788                        0.727 0.577  238 13.974            13.967                  13.953                        0.539 0.485  261 13.345            13.299                  13.343  269                   0.537 0.406      S.D.  ±0.083                  ±0.0413      157 16.859            16.826                  16.900  171 16.239            16.239                  16.243                        1.058 0.727  185 15.711            15.692                  15.657                        0.860 0.651  211 14.697            14.768                  14.719                        0.695 0.541  238 13.905            13.916                  13.903                        0.443 0.456  261 13.300            13.265                  13.306  269                   0.534 0.383      S.D.  ±0.040                  ±0.031__________________________________________________________________________