Method and apparatus for tagging explosives with a source of SF.sub.6 permitting the detection of their presence utilizing sensitive sniffing apparatus.
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.sub.6 to release over a period of time the SF.sub.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.sub.6, and in another embodiment a capsule is inserted containing liquid SF.sub.6 under pressure, the capsule being provided with a permeable window to permit a controlled release of the SF.sub.6 gas over a longer period of time.
Because of the penetrating nature of SF.sub.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.sub.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.sub.6 is adsorbed. The material selected for source 18 is one which has the characteristics of adsorbing large amounts of SF.sub.6 at elevated pressures and releasing the SF.sub.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.sub.6 by exposing a disc of each of the materials to SF.sub.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.sub.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.sub.6. It was found that if a moderately strong SF.sub.6 source is employed it is reasonably certain that barrier materials which allowed the SF.sub.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.sub.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.sub.6 in amounts which are detectable.
To test the effectiveness of this method over a period of time, several TFE pieces impregnated with SF.sub.6 were carefully measured for SF.sub.6 weight loss using the electrobalance and by measuring the SF.sub.6 concentration in dry air when passed over the pieces. Table VI lists the measured weight of remaining SF.sub.6 as a function of the number of days since initial loading for three of these pieces. The measured SF.sub.6 concentrations are also included.
The present invention depends for its effectiveness in part on the availability of apparatus to detect or "sniff" the presence of SF.sub.6. There are available commercially apparatus which have this capability in sensitivity required herein. For example, there is the Analog Technology Corporation's Model 140 wide range electron-capture detector system. In addition the Brookhaven National Laboratory has developed a SF.sub.6 Sniffer which is described completely in a paper "Tracing Atmospheric Pollutants by Gas Chromatographic Determination of Sulfur Hexafluoride" 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.sub.6 is released declines with time.
In order to provide for a more uniform rate of release of the SF.sub.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.sub.6 source 28 which consists of a sealed capsule of metal construction containing liquid SF.sub.6 under pressure. A window 32 of permeable material such as rubber permits SF.sub.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 material
Material 100.degree. C
25.degree. C
Material
wt., g.
Hours/21
64 18 117 166
__________________________________________________________________________
CTFE 0.045
2.2 4.6 0.3 0.2 0.1
E-CTFE
0.040
1.7 5.5 0.2 0.2 0.1
PFA 0.018
16.8 12.9 65.0 62.8 65.9
TFE-1
0.019
18.6 14.7 50.1 64.3 66.5
ETFE 0.010
10.1 9.6 1.2 4.7 5.3
TFE-2
0.016
10.9 9.3 33.8 38.2 37.3
FEP 0.014
18.4 14.9 60.0 71.6 74.3
__________________________________________________________________________
TABLE II
______________________________________
Weight of absorbed SF.sub.6, mg/g
TFE Predicted
Piece No.
Measured Second Order
Third Order
______________________________________
1 12.003 11.788 12.048
2 12.275 11.631 11.903
3 12.063 11.632 11.891
average deviation:
-0.430 -0.166
______________________________________
TABLE III
__________________________________________________________________________
Volume,
k Time to
Barrier
Barrier Material in..sup.3
%/hr 10%, hours
Capability
__________________________________________________________________________
Cardboard carton 1 -- <0.01 ineffective
112 -- 0.07 "
Polyethylene bottle
(6 dram)
1.4 0.9 11.1 moderate
(1 qt.)
58 0.85 11.8 "
Paint can (1/2 pint)
14.4 4 2.5 slight
(1 gal.)
231 0.70 14.3 moderate
Glass jar (1 ounce)
1.8 <0.0004
(>3 years)
very severe
(1 qt.)
58 0.70 14.3 moderate
Polyethylene zip-lock bag
1 31 0.32 negligible
50 0.52 19 mod. to severe
Brass 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.6
Teflon Time, Calculated Concentration .times. 10.sup.9
Piece 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
.+-.0.058
2 157 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
.+-.0.041
3 157 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
__________________________________________________________________________
