CROSS REFERENCES TO RELATED APPLICATIONS
STATEMENT REGARDING FEDERAL SPONSORSHIP
This application is related to U.S. Pat. No. 5,760,331 entitled “Non-Lead, Environmentally Safe Projectiles and Method of Making Same”, dated Jun. 2, 1998, and U.S. Ser. No. ______, entitled “Non-Lead Hollow Point Bullet”, filed same day herewith, both hereby incorporated by reference in their entirety.
BACKGROUND OF THE INVENTION
 This invention was made with Government support under contract no. DE-AC05-000R22725 to UT-Battelle, LLC, awarded by the United States Department of Energy. The Government has certain rights in the invention.
There are thousands of materials that efficiently convert light incident at one color into light of different color(s). The process whereby absorption is followed by subsequent re-emission is known as luminescence. Depending on certain particulars, the terms fluorescence and phosphorescence are also used. Materials that are among the most effective at efficient luminescence are phosphors.
Phosphors are powdered materials that are used for such purposes as fluorescent lighting, television and computer-screen displays, x-ray detection, and xerography, to name a few applications. Decades of research by the lighting and display industries, as well as the laser research and development establishment, have produced a large database and knowledge base that has resulted in a wide variety of phosphor types for these various applications.
With the growing advances in forensic science, it is crucial to develop methods to aid in the process of crime investigation. One process that has yet to be mastered is the identification of bullets from a crime scene. Currently, rifling, caliber, and gunpowder residue are the predominant ways to identify the bullet's origin. In some cases the chemical makeup of the lead points to a specific origin. Through the combination of a new technology to make non-lead bullets and the old technology of luminescent materials, as seen in fluorescent lights and televisions, a technology has been developed to aid in the identification of bullets. By adding phosphors to the powdered metal bullets, the bullets can be “tagged” in order to identify the bullet manufacturer, person who last purchased the bullets, and other pertinent information. Essentially, a code having thousands of variations can be assigned to each batch of bullets as they are manufactured. Improvements in the identification of fired bullets would be of great value to law enforcement and other security agencies. Currently, rifling marks, caliber, and gun powder residue are the predominant ways to identify the bullet's origin. In some cases the chemical makeup of the lead can be used to identify a manufacturer or source of raw materials. Simple, field implemental techniques to identify possible origins for a bullet do not exist.
Methods to simplify identification of a bullet have been suggested. All examples utilize a tag inserted into the bullet as a means of identification. The suggested embodiments include tags with etched serial numbers or identification codes. The tags are inserted into cavities within the bullet, or at the base of a bullet, and remain intact after firing. These tags can be read upon recovery of the projectile. Other methods such as chemicals or colored materials are also suggested. Again, in most examples, an intact bullet must be recovered to access the identification means. Example U.S. patents teaching these methods include U.S. Pat. Nos. 5,485,789; 5,646,365; and 5,698,816.
Other methods for tagging sensitive materials such as explosives have been developed. These utilize “colored” or magnetic particles that can be recovered in the field and easily “read.” These methods have not been used for bullets in their current designs and materials.
Phosphor materials present several advantages for use as taggants. They are bright (i.e. only a small amount is needed in order to be detected), each material exhibits its own unique characteristics that can be easily detected and they are compatible with the host metal materials. Some of the best candidate materials are ceramics, and are thus capable of enduring harsh environments. There is no impact on functionality.
In U.S. Pat. No. 3,772,099, dated Nov. 13, 1973 a fluorescent tagging means for pre-detonation or post-detonation identification of explosives is described. A combination of spotting phosphor which is excitable by ultraviolet radiations to provide a band-type emission and coding phosphors which emit a line-type emission were added to explosives. All of the phosphors have very small particle sizes and are incorporated into phosphor grains held together by inorganic cement or a binder and the resulting conglomerate grains are dispersed in the explosive. After an explosion, the phosphor grains can be readily located by the ultraviolet response of the spotting phosphor, and information disclosed by the presence (or absence) of the coding phosphor can then be decoded. The spotting phosphor will also normally provide some additional indicia of information. Further refinements of this tagging system are disclosed in U.S. Pat. No. 3,967,990 dated Jul. 6, 1976.
To inhibit any tendency for the phosphor conglomerate particles to sensitize the explosives, the individual particles were encapsulated in an organic plastic material such as polyethylene and a method for so encapsulating these particles is disclosed in U.S. Pat. No. 3,961,106 dated Jun. 1, 1976. These encapsulated particles can incorporate an anti-static coating, such as amorphous carbon, in order to prevent electrostatic charging thereof and such a coating is described in U.S. Pat. No. 3,993,838 dated Nov. 23, 1976.
U.S. Pat. No. 3,897,284 dated Jul. 29, 1975 discloses micro particles for tagging of explosives, which particles incorporate a substantial proportion of magnetite that enables the particles to be located by means of magnetic pickup. Ferrite has also been used. More recently, modified tagging particles with strips of color coding material having a layer of magnetite affixed to one side and layers of fluorescent material affixed to both exterior sides has been developed In this system the taggant can be located by visual detection of the luminescent response, or magnetic pickup, or both. Both the ferrite and the magnetite materials are dark colored, however, and absorptive of the radiation which excites the luminescent material, thereby making the particles somewhat difficult to locate after an explosion. Further developments produced similar particles that take advantage of the magnetic properties without diminishing the luminescent response of the materials (U.S. Pat. No. 4,131,064).
An additional modification to this theme is the development of particles coded with ordered sequences of distinguishable colored segments (U.S. Pat. No. 4,053,433). Other patents utilize radioactive isotopes or other hazardous materials as taggants. These concepts and materials are not applicable to the current invention.
Although the aforementioned techniques have been applied to the identification and tracking of explosives, they have not been employed in the identification or tagging of bullets or other small caliber projectiles. Typically bullets are made from lead that can be swaged or cast into the appropriate shape or jacketed. To be effective and useful the aforementioned taggants would have to be chemically compatible with lead and be dispersible within the lead or lead alloy. The particles would need to be wet by molten lead or be mechanically alloyed into the metal. The taggants and taggant mixtures described in this disclosure cannot be easily added to lead or its alloys and thus may not be useful for tagging current bullets or projectiles.
Most of the aforementioned means for the identification of bullets rely on the insertion of plastic tabs or other forms into the base or core of a bullet. This adds complexity to processing and recovery of a significant portion of the bullet would be required for identification. The tags are likely plastic and thus the addition of these would decrease weight and alter weight distribution, diminishing performance. The methods are not practical and are limited in scope and flexibility.
Although the tagging of explosives with luminescent materials has been described, the use of luminescent taggants and taggant mixtures for bullets has not been addressed in patents or other literature. This is mainly due to the fact that the taggants would have to be added to the lead or lead alloys during melting and casting. The densities of the rare earths are much less than lead, and they are not wet by molten lead. The mixing of the luminescent materials into molten lead would be difficult if not impossible. The rare-earth oxides are easily blended with the powdered metals used in this invention. Of course, the lead could be alloyed with various metals, and identification accomplished through chemical analysis. Chemical analysis of lead would have to be conducted in a laboratory and would be costly and time consuming. The method described in this invention could be used to identify bullets or even small bullet fragments in the field. Very small quantities of the taggants and taggant mixtures are needed to be effective, thus the addition has no effect on performance.
- BRIEF SUMMARY OF THE INVENTION
However, bullets are being fabricated from mixtures of metal powders that are consolidated to form dense metallic materials, which mimic the properties of lead (U.S. Pat. No. 5,760,331). High-density metals such as tungsten are mixed with lighter, softer metals such as tin and zinc to produce components with controlled density and mechanical properties. Powder metal mixtures are simply cold pressed to produce a high-density material. No heat treating or sintering of the composite simulants is necessary to achieve the appropriate properties. This process as well as those that employ other powder metallurgy techniques (U.S. Pat. No. 4,428,295) and those using powder metals and polymer binders (U.S. Pat. Nos. 4,949,645 and 5,616,642) to make bullets and shot permit the addition of other materials such as taggants and taggant mixtures.
BRIEF DESCRIPTION OF THE DRAWINGS
This invention is an identifiable bullet having a slug made from an unsintered powdered metal composite core and a luminescent taggant mixture. The luminescent taggant mixture portion of the slug serves as an identifying agent that can be traced to the manufactured origin of the bullet thereby providing a means for matching the identity of a post-fired bullet with the identity of a pre-fired bullet manufacturer.
FIG. 1. Measurements for tin-tungsten bullet material with 0.5 wt % green emitter and 0.5 wt % red emitter.
FIG. 2. Measurements for bullet materials doped with 5.0 wt % blue emitter, and 0.5 wt % each of blue and red emitter.
FIG. 3. Measurements for bullet material with 0.01 wt % red emitting phosphor.
FIG. 4. Emission of some non-visible phosphors.
FIG. 5. Plot of the fluorescence showing the decrease in fluorescence intensity with time.
FIG. 6. A portable spectrometer system.
Non-lead bullets are being fabricated from powdered metals and a variety of binders. Although the tagging technique described herein is applicable to any bullet that contains a powder component, the examples used in the testing and evaluation of the technique utilized bullet fabricated from mixtures of powdered tin and tungsten. Powder mixtures containing 57 wt % tungsten powder (Osram M-70) and 43 wt % tin (Pyron Metal Powders TC-125) were blended and separated into small batches. This mixture was selected because it can be easily compacted to produce a composite simulant with properties that mimic those of the lead in a bullet.
Varying amounts of oxide phosphor powders were added to small batches of blended metal powder. The type and quantity of phosphor was varied, and mixtures of phosphors were also evaluated. The concentrations of phosphors examined were 5, 1, 0.5, 0.1, 0.05, and 0.01 wt %. Of the wide variety of phosphor types, rare earth and transition-metal doped phosphors are particularly pertinent to tagging applications. Some example dopants are europium (Eu), terbium (Tb), manganese (Mn), chromium (Cr), dysprosium (Dy), gadolinium (Gd), etc. The dopant determines the spectral-distribution, or color, of the fluorescence. The fluorescence spectra commonly consist of very narrow, monochromatic lines. Therefore emitted fluorescence from each of these dopants is easily distinguishable from the others on the basis of spectral distribution. The following materials were used for the instant invention:
1. Yttrium vanadate doped with europium (YVO4:Eu)—red
2. Magnesium fluorogermanate doped with manganese (MgFGeO6:MN)—red
3. Yttrium aluminum garnet doped with terbium (Y3Al5O12:Tb or YAG:Tb)—green
4. Barium orthophosphate doped with europium (Ba3(PO4)2:Eu)—blue
Cylindrical samples 0.356″ in diameter weighing 100 grains were fabricated from the “doped” powder blends. The samples were compressed in a steel die at a pressure of 50,000 psig and at room temperature. The samples were typical of the slugs used in the fabrication of non-lead bullets. The samples were then fractured and fragmented, and the pieces analyzed. A functional yet dated (circa mid-1970's) piece of equipment was used for the measurements described here. A Perkin-Elmer Model 650-10S spectrophotometer was employed to obtain the spectral data. It contains a xenon lamp light source that emits broadband light from 220 to 800 nm. A diffraction grating selects the wavelength to direct to the sample. It is not a perfect filter, however, and some broadband light will also strike the sample and will be scattered to the fluorescence detector. This detector is located at a right angle to the excitation source. Another diffraction grating intervenes which selects the wavelength to be detected.
For these tests, the bullet fragments were placed in the sample compartment and the operator would adjust the excitation wavelength until the desired color was observed. It was possible for the operator to visually detect the fluorescence with some, but not all, of the room lights turned off. The fluorescence intensity was measured as a function of fluorescence wavelength. All four phosphors selected for this study can be excited by ultraviolet light. They may also be excited by visible light but means to filter out visible excitation light and distinguishing it from the fluorescence were not available.
Representative fluorescence spectra are shown in FIGS. 1 through 3. In FIG. 1, spectra for a sample with mixtures of red (MgFGeO6:Mn) and green (YAG:Tb), each at 0.5 wt %, show the characteristic emission lines from those materials. In FIG. 2, the broad blue emission characteristic of Ba3(PO4)2:Eu is seen for a sample with 5 wt % of the given phosphor. Also given in that figure are the spectral features for a sample with a mixture of this blue phosphor at 0.5 wt % and a red phosphor also at 0.5 wt %. Another spectrum for a bullet doped with the red phosphor, YVO4:Eu is seen in FIG. 3. In that figure the large broad feature to the blue of the emission line is due to light from the illumination source that leaked through the filters.
For these tests, the red, YVO4:Eu, was easily detectable down to the lowest concentration, 0.01% by weight. The green-emitting phosphor, YAG:Tb, was detectable to this level also, though with more background noise. The blue was detectable down to 0.5%. The inability to detect the blue down to 0.1 wt % or lower was due to several factors, including the weakness of this emission and the interference from the excitation source in the blue region of the spectrum. This made it difficult to distinguish between fluorescence and this background light. With better spectral filtering, it will be possible to increase the detectability in the blue.
Phosphor materials present several advantages for use as taggants:
1. They are bright, ie. only small amount is needed in order to be detected.
2. Each material exhibits its own unique characteristics which can be easily detected.
3. They are compatible with the host metal material.
4. Some of the best candidate materials are ceramics, and are thus capable of enduring harsh environments.
5. There is no impact on functionality.
There are thousands of possible phosphor materials. We have concentrated mainly on rare-earth and transition-metal phosphors since the emission is usually narrow band. This means that all the output is concentrated at a specific color or a few specific colors, making it easier to discriminate against background light and other noise sources and, thus, easier to detect. FIG. 4 illustrates the emission of some non-visible phosphors that could be used. The following table is some of the main dopants that could be used:
|Some ||Wavelength (nm) || |
|Trivalent ||of major |
|Phosphors ||fluorescence lines ||Comments |
|Mn+3 ||650, 630 ||commercial red phosphor |
|Eu+3 ||619, 611, 538 ||main wavelength depends on europium |
| || ||concentration and host material. |
|Nd+3 ||1.06 ||common laser material |
|Dy+3 ||575, 480, 453 ||lamp phosphor |
|Tb+3 ||544, 484, 435 ||high concentration strengthens the blue |
| || ||end of the spectrum. |
|Sm+3 ||609 ||lifetime varied by samarium concentration. |
|Ce+3 ||420 ||common sensitizer |
|Pr+3 ||520 ||short lifetime. |
|Er+3 ||1500 ||many communications uses |
|Gd+3 ||315 ||emits ultraviolet |
|Tm+3 ||infrared |
|Ho+3 ||infrared |
|Cr+3 ||670 ||common laser dopant |
Each batch or production run of bullets is tagged with a unique combination of phosphors. In effect, then, the phosphor combination functions as a bar code. If one assumes that, say, nine of these materials are useful as tags and that, for each material, four relative brightness levels are possible, then the total combination of unique, characteristic spectral signatures is 49, or about 250,000. This number could easily be increased since a given dopant may emit at different wavelengths depending on the host material and the dopant concentration. For instance, a high concentration of Eu in yttrium oxide yields a phosphor with over 90% of the luminescence concentrated in the 611 nm line, while, if the host is yttrium vanadate, the main wavelength is at 619 nm. Fluorescence rise times and decay times also change as a function of dopant concentration. Furthermore, sometimes, a sensitizer or a desensitizer may be used. For instance, a deliberately added small amount of Pr is sometimes added to some europium phosphors to decrease the decay time at little or no expense of the overall brightness. FIG. 5 is a plot of the fluorescence from sample number 13 showing the decrease in fluorescence intensity with time. A short pulsed (˜3 ns) nitrogen laser illuminated the sample.
A variety of small, portable spectrometer systems are now on the markets that come close to meeting the needs of bullet tagging. A portable system is illustrated in FIG. 6. A laptop personal computer (PC) is connected to an analysis system that contains a light source, sample illumination compartment, and detector. A diffraction grating in front of the detector (not shown) would be scanned in order to acquire a spectrum. Software in the PC determines the composition of the tagging material by either:
1. Measuring the relative intensity of different emission lines;
2. Measuring the decay time of the various fluorescence wavelengths;
3. Measuring the variance of the excitation wavelength; or some combination of the above.
It is also possible to determine the time at which a bullet is fired. Thermoluminescence is a common dating technique that utilizes phosphor materials. In such applications, the time span is usually hundreds or even thousands of years. Nonetheless, with the right choice and/or design of phosphor material, it is possible to incorporate phosphors designed for determining minutes, hours, days, weeks, etc.