WO2005083422A2

WO2005083422A2 - Methods and materials for tracking manmade airborne plumes

Info

Publication number: WO2005083422A2
Application number: PCT/US2004/038556
Authority: WO
Inventors: William S. Barney; Kevin Beltis; Daniel J. Ehntholt; Per Onnerud; Peter RALBOVSKY; Mehmet Rona
Original assignee: Tiax Llc
Priority date: 2003-11-17
Filing date: 2004-11-17
Publication date: 2005-09-09
Also published as: WO2005083422A3

Abstract

Tracers serve as a proxy for the presence of one or more target species, biological or chemical agents, in an airborne plume by mimicking the aerodynamic behavior of the target species. The tracers include rare-earth-doped luminescent particles. The target species are tracked by exciting the tracer luminescent particles and detecting a specific electromagnetic response therefrom. The tracers can be disposed in the airborne plume by deployment of munitions having such tracers.

Description

METHODS AND MATERIALS FOR TRACKING MANMADE AIRBORNE PLUMES

BACKGROUND OF THE INVENTION

1. Field of the Invention The invention relates to detecting a target species that may be present in an airborne plume and, in particular, to utilizing a tracer that mimics the aerodynamic behavior of a target species by detecting the tracer in an airborne plume.

2. Description of Related Art Techniques have been disclosed for tracing the source of explosive materials. For example, Ryan et al., in U.S. Patent No. 4,018635, disclose phosphor combinations particularly adapted for use with explosives for providing a distinctive information label. Schvoerer et al, in U.S. Patent No. 4,939,372, disclose a process for marking objects by the use of memory micro-crystals and markers for its implementation. Nelson et al., in U.S. Patent No. 6,432,715, disclose a method for marking items for identification. Further, O'Holleran, in U.S. Patent No. 4,744,919, discloses a method of dispersing particulate aerosol tracers.

SUMMARY OF THE INVENTION

In accordance with one or more embodiments, the invention is directed to a method of identifying a target species having a first aerodynamic equivalent diameter in an airborne plume comprising acts of directing an energy discharge at the plume to promote a response of a tracer having a second aerodynamic equivalent diameter and detecting the response of the tracer. The second aerodynamic equivalent diameter is typically substantially equal to the first aerodynamic equivalent diameter. In accordance with one or more embodiments, the invention is directed to a weapon system comprising an explosive and a tracer comprising a luminescent species coated with at least one compound selected from the group consisting of alumina, silica, and an organosilicone. In accordance with one or more embodiments, the invention is directed to a tracer comprising a luminescent particle coated with at least one compound selected from the group consisting of an organosilicone, alumina, and silica. Other advantages, novel features, and objects of the invention should become apparent from the following detailed description of the invention when considered in conjunction with any accompanying drawings, which are schematic and not intended to be drawn to scale.

BRIEF DESCRIPTION OF THE DRAWING

Preferred, non-limiting embodiments of the invention will be described by way of example with reference to the following, accompanying drawing. For clarity, not every component may be labeled in the drawing and not every component may be shown where illustration is not necessary to allow a person of ordinary skill in the art to understand the invention. FIG. 1 is a schematic illustration of tracers, in accordance with one or more embodiments of the invention, exemplarily showing sacrificial coatings that can burn or vaporize thereby exposing the tracer particles having sensitive or luminescently-active materials, and sacrificial matrices having sensitive materials, tracers, embedded therein;

DETAILED DESCRIPTION OF THE INVENTION The invention is not limited in its application to the details of construction and arrangement of components, systems or subsystems set forth in the description, including the example or as illustrated in the drawing. The invention is capable of other embodiments and of being practiced or of being carried out in various ways. The terms used herein for the purpose of description should not be regarded as limiting. The use of the terms "comprising," "including," "carrying," "having," "containing," "involving," and the like are to be understood to be open-ended, i.e., to mean including but not limited to. Only the transitional phrases "consisting of and "consisting essentially of shall be closed or semi- closed transitional phrases, respectively, as set forth, with respect to the claims. Aerosols are typically suspensions of solid particles and/or liquid droplets in a gaseous medium, e.g. air but can also refer to a single airborne solid particle or liquid droplet. Particle refers to a solid particle or a liquid droplet. Demolition of buildings, explosive or combustive disposal of hazardous material, or an explosive or combustive attack on a location containing hazardous materials can produce debris clouds or plumes that may include the hazardous materials or their associated compounds. Release of such plumes to the environment may be accidental, as in leakage from a chemical incinerator, or unavoidable, as in bombardment of a location including target materials or species such as chemical, biological, or nuclear species, including but not limited to, anthrax and asbestos. Components of airborne plumes typically begin to separate over time. Thus, for example and depending on several factors, gases would migrate at a rate that differs from the migration rate of particles. Further, larger particles would exhibit a migration behavior that differs from the migration behavior of smaller particles. The migration behavior of particles can be represented according to their respective fundamental aerodynamic characteristics. The invention is directed to remotely detectable tracer particles that can migrate with one or more target species or components that may be present in the plume. The invention can be further characterized as a method that can remotely detect one or more tracers that simulate the aerodynamic characteristics of one or more airborne target species. The invention can also be characterized as utilizing techniques that protect the simulating particles such that their intended tracer function without impairing response detection when exposed to conditions typically associated with munition storage, delivery, and detonation. The tracers of the invention are typically compounds or materials used to characterize the transport or migration of one or more materials through the environment. Tracers can serve as a proxy for the material of interest and preferably are detectable or at least easier to detect and/or do not have any changes or losses due to chemistry, adsorption, or other non- transport-related loss processes. Thus, the tracers of the invention can be environmentally stable or inert or, in some cases, can undergo physical changes that correspond to, or at least simulate, any physical changes that a target species can undergo during the lifetime of the one or more target species. The tracers of the invention can possesses transport properties similar to those of the material or species of interest; are typically easier to detect relative to the target species; are typically not environmentally present, or present in very small concentrations; and do not undergo chemical or physical loss processes or changes, other than those typically associated with dilution due to mixing or dispersion, or have chemical and physical loss processes are predictable. Tracers typically display aerodynamic behavior similar to those exhibited by plume particles so that they can travel or migrate with the plume particles for sufficient distance and time and provide an assessment of where contamination may occur. In some cases, tracers can have some one or more intrinsic or excitable signatures that can be detected remotely against the natural background. In accordance with one or more embodiments of the invention, one or more tracers can be disposed in an airborne plume, for example, resulting from detonation of a munition. The traces can also be disposed in the plume by introduction during incineration or demolition operations. In particular, a munition can include one or more tracers which, upon delivery of the munition, become incorporated in an airborne plume created by the detonation of the munition. Existing munitions can be modified or retrofitted with one or more tracers of the invention. Typically, the tracers of the invention, and/or components thereof, are stable with long term storage in or as a component of a munition. Further, the tracers typically do not degrade the performance of the munition having the tracers incorporated therein and, preferably, do not destabilize any component of the munition. Thus, for example, the tracer can be chemically inert relative to the explosive component of the munition. In some cases, the tracers of the invention can be rendered inert by being disposed in a chamber isolated from other component of the munition. Further, the tracers can be comprised of materials or compounds that have high melting points and shatter-resistant, relative to plume generating processes. Typically tracers, or at least a portion thereof, can survive an explosive event such that their characteristic signatures or response behavior can be detected. Thus, for example, tracers can be exposed to blast conditions associated with large military weapons such as temperatures that typically range from about 2000 °C to about 2400 °C and/or pressures that can range from about 225 to about 300 kbar. In some cases, the tracers are unchanged or have predictable behavior after exposure to frictional heating associated with high speed ballistic motion and/or forces. In accordance with further embodiments of the invention, tracers can have one or more protective or sacrificial coatings that preserve the luminescent and/or sensitizing nature of the species utilized in the luminescent component of the tracer and can also preserve aerodynamic characteristics prior to placing the tracers in service or in use. In some cases, the tracers, as coated and/or uncoated particles, can also be embedded in a protective matrix, which can be evaporated or otherwise releases the tracers upon deployment. For example, the one or more coatings can be detached, shattered, oxidized, reduced, vaporized, or otherwise removed, under conditions associated with munition deployment. The coating or matrix may provide resistance to shattering, thermal insulation, heat absorption, cooling, scavenging against oxygen, OH radicals, other oxidants, or other structure-altering molecules or radicals, that may be present in the ambient air or may be generated by the explosion/combustion conditions. Thus, the one or more coatings can be removed to provide a tracer having a specific stimulus response and a specific aerodynamic equivalent diameter after exposure to temperatures in a range of from about 2000 °C to about 2400 °C and/or pressures in a range of from about 225 to about 300 kbar. Examples of suitable coatings or matrices include, but are not limited to, silica, alumina, organosilicates and organosilicones. The protective material can absorb enough energy from an explosion or combustion process to limit the damage to the tracer. The protective material can be chemically compatible with the sensitive material, so that no degradation of either material takes place over the useful shelf life of the tracer particles. The protective material may slow or prevent diffusion of gases such as atomic or molecular oxygen, which may react with the sensitive material. For example, metal coatings or metals embedded within the coating matrix may have the ability to attract oxidants such as oxygen or OH. The protective material can be a material that easily can be coated on tracer particles, or into which the sensitive material easily can be incorporated. Thus, as exemplarily shown in FIG. 1, the tracer particle, such as neodymium- doped yttrium aluminum garnet (Nd:YAG) can have a sacrificial coating comprising alumina (Al₂O₃) or the luminescent tracer Nd:YAG particles can be embedded in an alumina protective matrix. The coatings can be applied at any suitable thickness that imparts the desired protective capacity. For example, the coating can be about 1 μm thick to about 10 μm, typically at least about 1000 Λ. Manufacturing of the coated or matrix-embedded particles may be performed by various techniques, including but not limited to, spray drying, condensation, chemical vapor deposition, electrochemical deposition, solution synthesis optionally followed by precipitation, and synthesis of bulk material, or millimeter-scale particles, followed by milling. Further, a releasing layer, which can be a gap or separation, disposed between the doped carrier and the protective coating that can prevent undesirable adhesion of the coating on the surface of the tracer particle. In other cases, the coating can have features that controllably direct fracture propagation and provide for a predictable removal behavior. The tracers of the invention typically have a distinguishable signature, and/or comprise one or more components that provide a distinguishable signature, which can be utilized to uniquely identify the presence of the tracer. Thus, a first tracer can have a signature associated with a specific element or compound, or in some cases, with a combination of particular compounds, that differs from a signature of other tracers. In some cases, the tracers of the invention can exhibit one or more signatures that can be triggered, in some cases selectively, in response to one or more specific and/or predetermined stimulating conditions. One or more tracers of the invention can have one or more luminescent components that can exhibit, typically by emission, one or more specific responses in reaction to one or more stimuli. The one or more responses emitted by the tracers can be a particular electromagnetic spectrum including particular wavelengths typically associated with the one or more luminescent components. Typically, the responses can be distinguishable from ambient or background spectra such that the detection of the particular responses is unambiguous. One or more tracers of the invention can further be characterized as a luminescent particle which can comprise a carrier having, or being doped with, one or more luminescent components. The one or more luminescent components can include one or more luminescent species and, optionally, one or more sensitizing species. Tracers can further include one or more protective or sacrificial coatings or layers surrounding the carrier having the luminescent component. The luminescent species can include any species that typically emit electromagnetic energy at a particular or desired wavelength and the sensitizing species can include any species that can absorb electromagnetic energy at a particular or desired wavelength and can transfer energy non-radiatively, typically through vibrational, phonon, modes of the carrier, to the one or more luminescent species. For example, the luminescent component can have a sensitizing species that absorbs a stimulating energy and effects emission of a response, typically as a particular electromagnetic energy, by the luminescent species in reaction to the stimulation. For example, the stimulating energy can comprise electromagnetic radiation transmitted at a first wavelength. The sensitizing species can absorb the stimulating energy and provoke emission of a response by the luminescent species, wherein the response as an electromagnetic emission having a wavelength that differs from the stimulating energy. Typically, one or more sensitizer species can be utilized if the luminescent species does not absorb, or has limited absorption, at a desirable stimulation wavelength and/or at a wavelength that can promote a detectable response. Examples of luminescent species and/or sensitizing species that can be utilized in the accordance with one or more embodiments of the invention include, but are not limited to rare earth elements typically listed in the lanthanide series including cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb). The luminescent species can also be any one or more transition metal such as, but not limited to, chromium (Cr), vanadium (V), manganese (Mn), and cobalt (Co). The luminescent species and the sensitizing species are typically present as a dopant on a carrier such that they are typically, though not necessarily, in the 3+ valence states. The carrier can be any suitable material that can be doped with the one or more luminescent species and/or one or more sensitizing species, preferably without attenuating or limiting a response of the luminescent species and/or the sensitizing species. Typically, the carrier, which can be transparent or substantially transparent in the pertinent electromagnetic spectrum, renders the tracer as having an aerodynamic characteristic that replicates an aerodynamic characteristic of the target species. Thus, in some cases, the tracer has the same or substantially the same aerodynamic equivalent diameter as the aerodynamic equivalent diameter of the target species such that, for a desired or predetermined period after initial plume formation, the tracer would be within a vicinity of the target species. For example, the tracer can have an aerodynamic equivalent diameter such that the tracer remains within about one-half kilometer of the target species even forty-eight hours or about 50 meters in about six hours after plume formation. Thus, in accordance with some embodiments of the invention, the tracer aerodynamic equivalent diameter can be within about 50 %, in some cases, within about 25 %, and in still further cases, within about 10 % of the aerodynamic equivalent diameter of the target species. Examples of suitable carrier materials include, but are not limited to, oxides such as, but not limited to, silica (SiO₂), alumina (Al₂O ), yttria (Y₂O₃), strontia (SrO), zirconia (ZrO ), and magnesium oxide (MgO); oxysulfides such as lanthanum oxysulfide (La O₂S), yttrium oxysulfide (Y₂O₂S), and gadolinium oxysulfide (Gd₂O₂S); glass such as phosphate glass; as well as other materials such as, but not limited to, yttrium aluminum garnet or YAG (Y Al₅O₁₂), gadolinium scandium garnet or GSGG

(Gd₃Sc Ga₃O₁₂), yttrium scandium gallium garnet or YSGG (Y₃Sc₂Ga₃Oι₂), yttrium orthovanadate (YVO₄), strontium aluminate (Sr₂Al₄O ), and yttrium phosphate (YPO₄). The doping amount of luminescent species and/or sensitizing species in the carrier can vary to provide a desired reaction intensity without promoting noise or undesirable emissions. For example, the tracer can comprise a rare earth element dopant such as erbium at a level in a range of about 0.1 % to about 3 % (by weight) in a YAG carrier matrix. Dopant response is typically not linear with respect to dopant concentration, so that increasing dopant concentrations may decrease the effective response, whereas dopant concentrations of less than about 1 % may not provide a response having sufficient intensity detectable under certain conditions, e.g. during daylight. However, the amount of dopant can vary based on other factors including the choice of carrier material. Thus, a glass-based carrier can be doped with one or more rare-earth luminescent species and/or one or more sensitizing species in a range of about 7 % to about 10 % (by weight). Particular examples of tracers include about 1 % by weight europium-doped yttria (Y O₃:Eu), about 1 % by weight ytterbium/ 1 % erbium-doped yttria (Y₂O₃:Yb/Er), about 1 % by weight praseodymium-doped yttrium aluminum garnet (YAG:Pr); and 1 % by weight dysprosium-doped yttrium aluminum garnet (YAG:Dy), commercially available from, for example, Phosphor Technology Ltd, United Kingdom, Stanford Materials Corporation, Aliso Viejo, California, and PhosphorTech Corporation, Lithia Springs, Georgia. In some cases, the tracers of the present invention can comprise materials that provide a distinguishable signature or response that can be detected by Raman scattering techniques or other techniques that provide unique identification thereof. In some cases, the tracer can have one or more distinct vibrational modes that promote a distinguishable response in reaction to an excitation energy. For example, the tracer can comprise one or more of a carbide such as, but not limited to, silicon carbide, vanadium carbide, tungsten carbide, and boron carbide; a boride such as, but not limited to, molybdenum boride, vanadium boride, tungsten boride, and zirconium boride; and a nitride such as, but not limited to, boron nitride and aluminum nitride. The aerodynamic behavior of particles is typically determined by their aerodynamic equivalent diameter, also referred to as aerodynamic diameter, which is defined as the diameter of a unit density sphere that has the same terminal velocity as the particle in question. The aerodynamic diameter can be related to the physical volume of the particle by the relationship

where d_a is the aerodynamic diameter, d_e is the "equivalent" diameter (diameter of a sphere with the same volume as the particle), p_p is the particle's density, po is "standard density" (1000 kg/m ) and χ is a shape factor that is empirically determined. Shape factors for common non-spherical particle types maybe found in the teaching of, for example, W.C. Hinds, in Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles, Second Edition, John Wiley & Sons, New York, 1999, p.52. One skilled in the art can appreciate that the concept of aerodynamic diameter can provide a starting point for the design problem, but that the relationship provided above may not apply to larger respirable particles transported by an appreciable wind, and that computational fluid dynamic modeling using computer software such as FLUENT computer program, available from Fluent Inc., Lebanon, New Hampshire, may further be utilized to evaluate the transport behavior of particles in a plume. Typically, a particle's aerodynamic diameter typically determines whether it is likely to deposit in the lung upon inhalation. Particles with d_a greater than about 10 microns are likely to be removed in the nasal passages and throat, and so are less likely to be responsible for lung damage or serious respiratory disease. Particles with d_a less than about 1 micron are likely to be exhaled without depositing in the lung, so that these are also less likely to cause health problems. Thus, when tracking hazardous particulates, the primary concern is particles with d_a between about 1 and about 10 microns. Further, the aerodynamic equivalent diameter of the tracers of the invention refers to tracer properties in the airborne plume, typically without the sacrificial or protective coating. However, in some cases, tracers can have the desired aerodynamic equivalent diameter even with at least a portion of the protective coating. The desired time and distance over which to track a hazardous particle can vary and be dependent on conditions and the nature of the hazard. Several considerations that may apply include, for example: • in the absence of stratospheric injection, which typically occurs only with nuclear explosions and volcanic eruptions, the particles may settle gravitationally at a rate that is typically dependent on their size and the height to which they are lifted. Particles larger than a few microns typically have gravitational settling velocities in the m/s range and may fall out within a few hours • particles that do not settle gravitationally may be captured in raindrops and rain out; thus, in very low rainfall areas, particles smaller than about 10 microns may persist in the atmosphere for weeks or longer • other fates of hazardous particles include oxidation and photolysis or photo- induced damage for chemical and biological hazards, and dehydration for some biological hazards • as the plume is transported it will likely be diluted, so that eventually the degree of hazard may be reduced below the level of a toxic or infectious dose Each type of threat will typically have an environmental lifetime beyond which the target species need not be tracked. The desired tracking time is a design criterion that can be used to determine the appropriate tracer particle size. In some cases it may be desirable to have the particle's physical diameter be different from its aerodynamic diameter. Particles with small surface area may be less susceptible to vapor condensation and shear forces, two potential conditions typically associated with an explosion that may damage the particles or reduce response detectability. Typically materials with density much greater than about 1000 kg/m could have smaller diameter but may still mimic the aerodynamic behavior of bioaerosols. Particles with large surface area can absorb and scatter light more efficiently. Particles that are porous or hollow may be much larger than bioaerosols but still mimic their aerodynamic behavior, while being detectable. One or more tracers of the invention can further include one or more hygroscopic components that can provide a simulation of any hygroscopic behavior of one or more of the target species. One or more of the hygroscopic components can exhibit behavior that is reversible such that if the one or more target species is exposed to appropriate conditions, the one or more hygroscopic components behaves correspondingly. Hygroscopic components can, for example, include one or more hygroscopic species or compounds that can capture water molecules to produce a surface that is energetically favorable and/or stable. Thus, the ceramic carrier materials, including, for example, silica and alumina, which are typically hygroscopic, can be utilized to have a hygroscopic behavior similar to the hygroscopic behavior of the target species. Some ceramics that may be utilized may exhibit hydrophobic characteristics such as those disclosed in, for example, U.S. Patent No. 5,518,780. Thus, the tracers of the invention can be tailored to incorporate hygroscopic and hydrophobic portions. Further, the tracers can have electrostatic properties that are similar to the electrostatic properties of the target species. The tracer electrostatic properties can be tailored by, for example, including one or more components, such as metal oxides, that can render the tracer with a desired or predetermined electrically conductivity exposed to the airborne plume conditions. Thus, for example, the one or more tracers of the invention can have about 1 part in 10000 of an electrically conducting metal oxide. Detection of the tracers can be performed utilizing using light scattering or spectroscopic techniques. Detection can utilize systems and techniques based on, for example, standoff methods, including fluorescence, laser mterferometry for detecting and ranging (LIDAR), Laser Induced Fluorescence (LIF), Raman scattering, microwave emissions, and even multivariate detection methods such as fluorescence combined with scattering characteristics patterning. Any suitable illuminating energy source or system can be utilized that delivers the desired excitation energy level to the sensitizing species and/or the illuminating species. Typically, a laser system is utilized because it can be controllably directed at the airborne plume and/or portions thereof. In some cases, the illuminating or exciting energy source preferably does not discharge in the visible spectrum. For example, a dysprosium-doped YAG particle can be illuminated with an excitation energy, such as 1.064 micron infrared light directed from a Nd:YAG laser, to effect a response detectable by, for example, multispectral detectors, point or array optical detectors, including but not limited to CMOS or CCD detectors, photomultiplier tubes, or photodiodes based on, for example, silicon, germanium, PbS/PbSe, or HgCdTe. The detector can further utilize one or more photodetector elements to selectively categorize the captured response following excitation of the tracer particle. For example, different response wavelengths may be separated spatially by a dispersion element and recorded on one or more or different photodetectors. The detection system may provide an output as a time-resolved signal, either to determine the range to the plume, or if the range is known, to allow spurious light signals to be rejected except within the expected arrival time of signals from the plume. In other cases, such as tracers without luminescent species, based on, for example, carbides, borides, and nitrides, Raman scattering techniques can be utilized to assess the presence of the tracers. Example of detectors or detector system components include those commercially available from Hamamatsu Photonics K.K., Japan; EG&G Technical Services, Inc., Gaithersburg, Maryland; Hitachi Ltd., Japan; Meade Instruments, Irvine, California. Further, techniques can be utilized to confirm the response and reduce the likelihood of noise and/or false positives. For example, illumination can be performed at periodic, non- random intervals thereby eliciting a corresponding periodic, non-random response. Moreover, one or more detectors can be utilized to detect, receive, amplify, and/or analyze the emitted response for a signature corresponding to one or more tracers. For example, one or more detectors can have one or more interference filters that selectively transmit emission characteristics of Eu, e.g. at about 610 nm and at about 720 nm; or Pr, at about 690 nm, about 650 nm, and about 480 nm. A ratio of the measured peak intensity at particular wavelengths can be also used, alone or in combination with other techniques, as an identifying signature corresponding to the one or more luminescent species of the tracers. For example, Eu can have a ratio of detected intensities at about 610 nm and at about 720 nm of about about 5:1.

EXAMPLE The function and advantages of these and other embodiments of the invention can be further understood from the example below. The following example illustrates the benefits and/or advantages of the one or more systems and techniques of the invention but do not exemplify the full scope of the invention. This prophetic example illustrates the operation of the tracer-detector system in accordance with one or more embodiments of the invention. A plume of smoke and dust generated by an explosion aerosolizes and disperses the one or more tracers. The plume is expected to contain hazardous materials in the form of about 5 micron diameter spherical particles of density 1000 kg/m³. The tracer material selected is Y₂O₃ (density 5000 kg/m³) doped with about 1 weight percent Dy^34". The tracer particles are typically spherical and have a physical diameter that results in d_a of about 5 microns, i.e., the tracer diameter is about 2.23 microns. In this example the hazard and tracer particles are assumed to have a monodispersed size distribution. About 1 kg of tracer material is aerosolized, which is equivalent to about 3 xlO¹³ particles. At the time of illumination and detection, the plume has dispersed to have a length of about 1.6 km and a horizontal dimension of about 25 x 25 m, giving a total volume of about 10⁶ m³. The number density of tracer particles is about 3 xlO⁷ per m³. The plume is illuminated from a distance of about 1000 m with a d:YAG laser characteristic of commercially available lasers, e.g. a DUALCHIP NANOLASER laser, available from JDS Uniphase Corporation, San Jose, California. This laser is expected to have a pulse energy at 1.064 microns of about 12.5 μj and pulse rate of about 50 KHz. The beam divergence full-angle is about 1500 microradians so that the beam area at 1 km distance is about 1.7 m . The particle response is detected by a photodetector coupled to about a telescope with effective aperture of about 14-inches, which can be co-located with the laser. The divergence of the telescope field of view (FOV) is about 1500 microradians, so that the effective aperture at 1 km distance is approximately 1.7 m . The laser beam excites tracers in the cloud along about a 25 meter cylindrical path with diameter of about 1.5 m, encompassing about 1.5 x 10⁹ particles. Each Dy ion typically has a fluorescence cross-section on the order of about 10^"20 cm² (at about 1.3 μm, as noted by, for example, Shin et al. , in J. Mater. Res. , 2001 , 5, 1318) and each particle contains about 10¹¹ ions. The total fluorescence cross-section illuminated then comes to about 1.6 cm². Each laser pulse typically has a photon density at the plume of about 3.8 x 10⁹ photons per cm². Ignoring extinction within the plume, the expected total Dy^34" emission at 1.3 μm is then a total of about 6 x 10⁹ photons. The plume is illuminated from a distance of about 1000 m with a Nd:YAG laser characteristic of commercially available lasers, e.g. a DUALCHIP nanolaser, available from JDS Uniphase. This laser is expected to have a pulse energy at 1.064 microns of about 12.5 μJ and pulse rate of about 50 KHz. The beam divergence full-angle is aboutl500 microradians so that the beam area at 1 km distance is about 1.7 m². The particle response is detected by a photodetector coupled to about a telescope with effective aperture of 14-inches, which can be co-located with the laser. The divergence of the telescope field of view (FOV) is about 1500 microradians, so that the effective aperture at 1 km distance is approximately 1.7 m . The laser beam would excite tracers in the plume cloud along about a 25 meter cylindrical path typically at diameter of about 1.5 m, encompassing about 1.5 10⁹ particles. Each Dy ion typically has a stimulated emission cross-section on the order of about 10^"20 cm² (at about 1.3 μm, as noted by, for example, Wei et al., in Optics Letters, 1994, 12, 904) and each particle typically contains about 10¹¹ ions. The total fluorescence cross-section illuminated then would be about 1.6 cm². Each laser pulse typically has a photon density at the plume of about 3.8 x 10 photons per cm . Ignoring extinction within the plume, the expected total Dy³⁺ emission at 1.3 μm is on the order of 10¹⁰ photons. Atmospheric attenuation is expected to be weak for 1.064 and 1.3 micron wavelengths, so it should not significantly affect the excitation or signal light strength. The fraction of emitted photons that are captured by the receiver can be represented as A_r/4πR² where A_r is the effective aperture of the telescope or receiving apparatus and R is the distance from the emitting particle to the receiver, which can be approximated in this example as the distance to the plume, about 1000 m. The fraction detected in the foregoing example would be about 10 or a total of about 10 photons, well within the detection capability of typical photodetectors. This is on the same order of brightness as the sky at 1.3 microns, so that with phase-sensitive detection it should be possible to distinguish the tracer response from the background. The sensitivity can also be increased by varying some of the parameters given here, e.g. amount of taggant or tracer, laser power, telescope diameter, etc. Having now described some illustrative embodiments of the invention, it should be apparent to those skilled in the art that the foregoing is merely illustrative and not limiting, having been presented by way of example only. Numerous modifications and other illustrative embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the invention. In particular, although many of the examples presented herein involve specific combinations of method, acts, system, or elements, it should be understood that such components, elements, and/or acts may be combined in other ways to embody the invention. Acts, elements and features discussed only in connection with one embodiment are not intended to be excluded from a similar role in other embodiments. It is to be appreciated that various alterations, modifications, and improvements can readily occur to those skilled in the art and that such alterations, modifications, and improvements are intended to be part of the disclosure and within the spirit and scope of the invention. Moreover, it should also be appreciated that the invention is directed to each feature, system, subsystem, or technique described herein and any combination of two or more features, systems, subsystems, or techniques described herein and any combination of two or more features, systems, subsystems, and/or methods, if such features, systems, subsystems, and techniques are not mutually inconsistent, is considered to be within the scope of the invention as embodied in the claims. Further, any of means-plus- function limitations are recited in the following claims, the means are not intended to be limited to the means disclosed herein for performing the recited function, but are intended to cover in scope any means, known now or later developed, for performing the recited function. Use of ordinal terms such as "first" and "second" and the like in the claims to modify a claim element does not by itself connote any priority, precedence, or order of one claim element over another or the temporal order in which acts of a method are performed, but are used merely as labels to distinguish one claim element having a certain name from another element having a same name (but for use of the ordinal term) to distinguish the claim elements. Further, as used herein, "plurality" means two or more. As used herein, a "set" of items may include one or more of such items. Those skilled in the art should appreciate that the parameters and configurations described herein are exemplary and that actual parameters and/or configurations will depend on the specific application in which the systems and techniques of the invention are used. Those skilled in the art should recognize or be able to ascertain, using no more than routine experimentation, equivalents to the specific embodiments of the invention. It is therefore to be understood that the embodiments described herein are presented by way of example only and that, within the scope of the appended claims and equivalents thereto; the invention may be practiced otherwise than as specifically described. What is claimed is:

Claims

1. A method of identifying a target species having a first aerodynamic equivalent diameter in an airborne plume comprising: directing an energy discharge at the plume to promote a response of a tracer having a second aerodynamic equivalent diameter; and detecting the response of the tracer; wherein the second aerodynamic equivalent diameter is substantially equal to the first aerodynamic equivalent diameter.

2. The method of claim 1 wherein the target species comprises a biologically active agent.

3. The method of claim 1 wherein the tracer comprises at least one of a ceramic compound and a glass.

4. The method of claim 3 wherein the tracer comprises a material selected from the group consisting of carbides, borides, nitrides, oxides, vanadates, garnets, aluminates, phosphates, and oxysulfides.

5. The method of claim 4 wherein the tracer comprises at least one compound selected from the group consisting of boron carbide, silicon carbide, vanadium carbide, and tungsten carbide.

6. The method of claim 4 wherein the tracer comprises at least one compound selected from the group consisting of molybdenum boride, vanadium boride, tungsten boride, and zirconium boride.

7. The method of claim 4 wherein the tracer comprises at least one of aluminum nitride or boron nitride.

8. The method of claim 4 wherein the tracer comprises at least one compound selected from the group consisting of SiO₂, Al₂O₃, Y₂O₃, SrO, ZrO₂, MgO, or TiO₂.

9. The method of claim 4 wherein the tracer comprises at least one compound selected from the group consisting of La₂O₂S, Y₂O₂S, Gd₂O₂S.

10. The method of claim 5 wherein the tracer comprises at least one compound selected from the group consisting of Sr₂Al₄O₇, Y₃Al₅Oι₂, Gd Sc₂Ga₃Oι , Y₃Sc₂Ga₃O₁₂, YVO₄, and YPO₄.

11. The method of claim 1 wherein the tracer comprises at least one luminescent species.

12. The method of claim 11 wherein the at least one luminescent species comprises at least one lanthanide series rare earth element.

13. The method of claim 11 wherein the at least one luminescent species comprises a transition metal selected from the group consisting of Cr, V, Mn, and Co.

14. The method of claim 11 where the tracer comprises a carrier doped with at least one sensitizing species.

15. The method of claim 14 wherein the at least one sensitizing component comprises at least one lanthanide series rare earth element.

16. The method of claim 15 wherein the at least one sensitizing component is selected from the group consisting of Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, andYb.

17. The method of claim 15 wherein the at least one sensitizing component comprises a transition metal selected from the group consisting of Cr, V, Mn, and Co.

18. The method of claim 1 wherein the tracer comprises yttria doped with erbium.

19. The method of claim 1 wherein the tracer comprises yttria doped with ytterbium and erbium.

20. The method of claim 1 wherein the tracer comprises yttria doped with chromium and thulium.

21. The method of claim 1 wherein the tracer comprises yttrium aluminum garnet doped with praseodymium.

22. The method of claim 1 wherein the tracer comprises yttrium aluminum garnet doped with samarium and praseodymium.

23. The method of claim 1 wherein the tracer comprises yttrium aluminum garnet doped with dysprosium.

24. The method of claim 1 wherein the tracer comprises yttrium aluminum garnet doped with samarium and dysprosium.

25. The method of claim 1 wherein the tracer comprises a phosphate glass doped with erbium.

26. The method of claim 1 wherein the tracer comprises a phosphate glass doped with ytterbium and erbium.

27. The method of claim 1 wherein the tracer comprises a phosphate glass doped with chromium.

28. The method of claim 1 wherein the tracer comprises a phosphate glass doped with chromium, ytterbium, and erbium.

29. A weapon system comprising an explosive and a tracer comprising a luminescent species coated with at least one compound selected from the group consisting of alumina, silica, and an organosilicone.

30. The weapon system of claim 29 wherein the tracer has an aerodynamic equivalent diameter in a range of about 1 μm to about 10 μm.

31. A tracer comprising a luminescent particle coated with at least one compound selected from the group consisting of an organosilicone, alumina, and silica.

32. The tracer of claim 31 wherein the luminescent particle comprises a carrier selected from the group consisting of glass, yttrium aluminum garnet (Y₃Al₅O₁₂), yttrium orthovanadate (YNO₄), yttrium phosphate (YPO₄), lanthanum oxysulfide (La₂O₂S), yttrium oxysulfide (Y₂O₂S), and gadolinium oxysulfide (Gd₂O₂S), gadolinium scandium garnet (Gd₃Sc₂Ga₃O₁₂), yttrium scandium gallium garnet (Y Sc₂Ga₃Oι₂), and strontium aluminate (Sr₂Al₄O ), doped with at least one rare earth element.

33. The tracer of claim 31 wherein the luminescent particle comprises yttria doped with erbium.

34. The tracer of claim 31 wherein the luminescent particle comprises yttria doped with ytterbium and erbium.

35. The tracer of claim 31 wherein the luminescent particle comprises yttrium aluminum garnet doped with praseodymium.

36. The tracer of claim 31 wherein the luminescent particle comprises yttrium aluminum garnet doped with dysprosium.

37. The tracer of claim 31 wherein the luminescent particle has an aerodynamic equivalent diameter in a range of about 1 μm to about 10 μm.

38. The tracer of claim 32 wherein the luminescent particle comprises phosphate glass doped with ytterbium and erbium.

39. The tracer of claim 32 wherein the luminescent particle comprises phosphate glass doped with chromium and thulium.