Patent Publication Number: US-7594447-B2

Title: Device for testing for traces of explosives and/or drugs

Description:
This application is a divisional of U.S. patent application Ser. No. 10/929,915, filed Aug. 30, 2004. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The invention relates to an apparatus for testing for the presence of trace amounts of a contraband material on a person. 
     2. Description of the Related Art 
     Terrorism risks continue to increase at transportation facilities, government buildings, banks, restaurants, hotels and other locations where there is a significant flow of pedestrian or vehicular traffic. 
     Airlines now routinely screen passengers and employees for explosives. Screening typically is carried out in several stages. For example, all passages are required to pass through a metal detector and all baggage is required to pass through an X-ray apparatus. However, a plastic explosive device could be concealed on a person or in a piece of luggage in a manner that might not be detected by a conventional metal detector or an X-ray apparatus. Even a small amount of a plastic explosive can cause sufficient damage to bring down an aircraft. 
     Most airports now include apparatus for detecting trace amounts of explosives. These devices operate on the principle that small amounts of the explosive materials will be transferred to the body, clothing and luggage of people who had handled the explosive. Some detectors employ small flexible fabric-like traps that can be wiped across a package or piece of luggage. The trap removes residue from the surface of the package or luggage. The trap then is placed in an apparatus, such as an ion trap mobility spectrometer, that tests the residue on the trap for trace amounts of explosive materials. A device of this type is disclosed in U.S. Pat. No. 5,491,337 and is marketed by the GE Ion Track. These devices typically are employed in proximity to the metal detectors, and security personnel will perform screening on some of the passengers based on a random sampling or based on a determination that the passenger has met certain criteria for enhanced screening. 
     The ion trap mobility spectrometer disclosed in U.S. Pat. No. 5,491,337 also can operate in a mode for detecting trace amounts of narcotics. Narcotics are illegal and insidious. Furthermore, it is known that many terrorists organizations fund their terrorism through the lucrative sale of narcotics. 
     The above-described ion trap mobility spectrometer and similar devices have been accepted at airports in view of the notorious efforts of terrorist groups to attack commercial airliners. The above-described detectors have not been accepted widely at other potential targets of terrorism, including train stations, bus terminals, government buildings and the like. The screening of personnel entering train stations, bus depots, government buildings and such by the above-described detection devices would significantly slow the flow of people into and through such buildings and would impose a significant cost penalty on the operators of such facilities. 
     Only a fraction of airline passengers have their baggage checked for trace amounts of explosives or narcotics using the available ion trap mobility spectrometers and similar devices. Efforts to use such devices to check all bags for trace amounts of explosives or narcotics would impose greater time and cost penalties on the airline industry. Additionally, explosive detectors typically are used only on luggage and other parcels. An apparatus of this type would not identify plastic explosives worn by a passenger who had no carry-on luggage. 
     U.S. Pat. No. 6,073,499 discloses a walk-through detector. The detector shown in U.S. Pat. No. 6,073,499 operates under the principle that a boundary layer of air adjacent to a person is heated by the person. This heated air adjacent a person is less dense than air further from the person. Less dense air rises. Accordingly, a thermal plume of air flows up adjacent to a person. Minute particles, including particles of explosives or narcotics, will be entrained in this thermal plume of air and will flow upwardly from a person. The walk-through detector disclosed in U.S. Pat. No. 6,073,499 employs an ion mobility spectrometer or ion trap mobility spectrometer to detect microscopic particles of interest that are likely to be entrained in the thermal plume of air flowing upwardly adjacent to a person who walks through and pauses briefly in the detector. The walk-through detector disclosed in U.S. Pat. No. 6,073,499 is very effective for detecting whether a person is carrying explosives or narcotics and whether the person has recently handled explosives or narcotics. 
     A person who had handled explosives or narcotics is likely to have microscopic residue of the explosive or narcotic materials on his or her fingers, and trace amounts of the explosive or narcotic will be transferred to objects that are handled by the person. U.S. Pat. No. 5,741,984 discloses an apparatus for dispensing tokens that preferably are made of PTFE or cotton. The dispensing apparatus is constructed so that the individual is required to grip the token tightly to separate the token from the dispensing apparatus. The token then is fed into a token handler that delivers the token into an ion mobility spectrometer for analyzing residue that may have been transferred to the token from the hand of the person who retrieved the token from the dispenser. The apparatus shown in U.S. Pat. No. 5,741,984 creates inventory control problems associated with the need for having a sufficient supply of tokens and then periodically loading tokens into the dispenser. Additionally, the system disclosed in U.S. Pat. No. 5,741,984 requires a separate complex dispensing apparatus for dispensing tokens with sufficient resistance for reliably transferring residue from the hand of the person retrieving the token. Additionally, a complex apparatus is required for handling the token, feeding the token into the ion mobility spectrometer and then removing the token after analysis. The inventor of the subject application has determined that residue of such contraband will be transferred from the fingers of the person to an airline ticket, a boarding pass or an identification card. Pending U.S. patent application Ser. No. 10/774,003 discloses a detector that identifies particles of interest on such a card-like object. Accordingly, the device disclosed in pending U.S. patent application Ser. No. 10/774,003 avoids problems associated with maintaining an inventory of tokens, dispensing tokens properly from a dispenser, handling tokens in a token handler and then removing the tokens from the token handler. 
     The above-described products that check for the presence of trace amounts of substance of interest on luggage, tickets, boarding passes and the like generally work very well. However, there continues to be a demand for a small, rapid, reliable and low cost detector for detecting trace amounts of substances of interest directly on a person. A device of this type would be useful at security checkpoints where a person is not likely to be carrying luggage (e.g., many commuter train stations or bus terminals) and at locations where a person is not likely to have a boarding pass (e.g., government buildings). The above-described walk-through portal provides an unobtrusive checking of passengers for explosives or narcotics without the need to check luggage or boarding passes. However, these devices are relatively large and relatively costly. Hence, devices of this type may be inappropriate for some security checkpoints. 
     Existing security checkpoints also are very labor intensive, and devices that could reduce the number of highly trained technicians would be received favorably. In this regard, a typical airport security checkpoint requires at least four and typically five or six trained technicians. A first employee reviews personal identification cards and boarding passes at the entry to the checkpoint. A second person coordinates the loading of carry-on luggage and personal effects onto a conveyor for movement through an X-ray scanning device. A third person continuously watches the monitor of the X-ray scanning device. A fourth person controls the movement of passengers through the walk-through metal detector while a fifth person remains available for conducting more detailed screening with a handheld metal detector. The above-described explosive/narcotics detection device that employs fabric-like wipes typically is positioned near the outlet end of the conveyor through the X-ray scanning device. As noted above, the small fabric-like trap is wiped across the surface of the luggage to pick up trace amounts of substances of interest that may have been transferred from the passenger to the luggage. The wipes then are placed in the detector and analyzed. A sixth technician generally is available to perform this screening and analysis. Alternatively one of the five technicians mentioned above must be redeployed for this screening and analysis. A more direct approach would be to detect the substances of interest directly on the passenger. Most preferably, such detection would be carried out without direct human intervention by the technicians who operate the checkpoint. 
     It is assumed that terrorists and other criminals frequently travel without carrying explosives, weapons or other contraband. Existing security checks at airports compare the name on a boarding pass to the name on a photo identification card and then compare the passenger to the photograph. However, there is virtually no checking of the physical characteristics of the passenger to physical characteristics of suspected terrorists and criminals. Additionally, there is virtually no checking of physical characteristics of the passenger to documented physical characteristics of the person whose name appears on the boarding pass or photographic identification. There is also no checking of whether a person with the physical characteristics of the passenger has traveled previously under a different name. 
     Devices are available for taking fingerprints of a person and for comparing the fingerprints to information in a database of fingerprint information. Such an apparatus can compare a scanned fingerprint to fingerprints of certain known suspects. Such an apparatus also can store fingerprint data for future reference or analysis. Other apparatus can identify people by scans of facial features or other characteristics. 
     Accordingly, an object of the subject invention is to provide a device for detecting the presence of substance of interest on a person at a security checkpoint. 
     Another object of the invention is to provide a device that can check for the presence of a substance of interest without intrusion on the passenger by security personnel. 
     A further object of the subject invention is to provide an apparatus that can substantially simultaneously check for the presence of a substance of interest and check the identity of the person at the security checkpoint. 
     An additional object of the invention is to provide a lightweight relatively, low cost, small apparatus for checking for the presence of substances of interest on a person. 
     Still a further object of the invention is to provide a sampling apparatus where the sampling medium is reusable and non-removably part of the sampling apparatus. 
     SUMMARY OF THE INVENTION 
     The subject invention is directed to a detector that can be used at a security checkpoint to check for the presence of explosives, narcotics or other substances of interest on a person. The detector includes a housing and a detecting apparatus within the housing or communicating with the housing. The detecting apparatus preferably is an ion trap mobility spectrometer, such as the detector disclosed in U.S. Pat. No. 5,491,337, the disclosure of which is incorporated herein by reference. A product of this type is marketed by GE Ion Track under the trademark ITEMIZER 3®. The detector also could be an ion mobility spectrometer, such as the type disclosed in U.S. Pat. No. 5,200,614. Other means for detecting trace amounts of explosives, narcotics or other volatile substances can be employed as the detecting apparatus in the detector of the subject invention. 
     The detector further includes a sample collection assembly for collecting samples directly from a person at the checkpoint and delivering the collected samples to the detecting apparatus so that the samples can be analyzed for substances of interest. The sample collection assembly can be removed and replaced with another type of sample collection assembly, such as the sampling apparatus disclosed in copending application Ser. No. 10/774,003. 
     The sample collection assembly includes a sampling sheet with a target section dimensioned and configured to accommodate a thumb, a palm or at least one forefinger of a person being screened by the detector. An instruction panel and/or a speaker may be provided in proximity to the sampling sheet to provide visual and/or audible signals instructing the person to place a palm, a thumb or at least one forefinger on the target section of the sampling sheet and/or to wipe a palm, a thumb or at least one forefinger across the target section. 
     The sample receiver may include a housing with a window disposed so that the target section of the sampling sheet is exposed at the window. The sample collection assembly further includes apparatus for moving the target section of the sampling sheet from the window to an inlet to the detecting apparatus. For example, the sampling sheet may be a generally cylindrical drum, a disc, a plate or a belt. The apparatus that moves the target section of the sampling sheet from the window to the inlet of the detecting apparatus may be an electric motor. The sampling sheet may be deflectable in response to pressure generated by the palm, thumb or forefinger of the person that is being scanned. Removal of the pressure, therefore, may trigger a pressure sensitive switch that activates the motor for moving the target section of the sampling sheet from the window to the inlet of the detecting apparatus. Slots may be provided in the sampling sheet to facilitate deflection. 
     The sample collection assembly may include a desorber and a transfer box in proximity to the inlet to the detecting apparatus. The target area of the sampling sheet may be advanced into the desorber. The desorber heats the sampling sheet sufficiently to vaporize material transferred from the hand to the target area of the sampling sheet. A vacuum pump then draws the vaporized material into the inlet of the detecting apparatus. The detecting apparatus functions to identify substances of interest and generates a signal when a substance of interest has been detected. 
     The sample collection assembly may further include a fingerprint scanning device for scanning and reading the fingerprint as finger is wiped across the target area of the sampling sheet. The fingerprints can be stored for future reference. Alternatively, the fingerprint can be compared to known fingerprint data for comparing the scanned fingerprint to other identification information pertaining to the person at the detector. Alternatively, the scanned fingerprint data can be compared to an existing database with fingerprint data for potential terrorists or other criminals. Information obtained by the fingerprint scan can generate an audible or inaudible alarm and can trigger an increased level of scrutiny at the checkpoint. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of a detector that incorporates the apparatus of the subject invention. 
         FIG. 2  is a schematic view of an ion trap mobility spectrometer of the detector shown in  FIG. 1 . 
         FIG. 3  is a front perspective view, partly in section, of a modular sample collection assembly of the detector for detecting substances of interest on a finger of a person. 
         FIG. 4  is a rear perspective view, partly in section of the modular sample collection assembly shown in  FIG. 3 . 
         FIG. 5  is a perspective view of a drum for use in the modular sample collection assembly of  FIGS. 3 and 4 . 
         FIG. 6  is a front perspective view of the inlet disposed between the drum of  FIG. 5  and the detector of  FIG. 2 . 
         FIG. 7  is a rear perspective view of the inlet shown in  FIG. 6 . 
         FIG. 8  is a perspective view of an alternate sample collection assembly. 
         FIG. 9  is a perspective view of another alternate sample collection. 
         FIG. 10  is a perspective view of the detector with an alternate sample collection assembly mounted therein. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A detector according to the invention is identified generally by the numeral  10  in  FIG. 1 . The detector  10  includes an outer housing  11  and a flat panel display monitor  12  such as an LCD monitor. An ion trap mobility spectrometer (ITMS) is disposed within the housing  11  and is illustrated schematically in  FIG. 2 . 
     The ITMS of  FIG. 2  comprises a cylindrical detector  20  having an inlet  22  at one end for receiving sample air of interest borne by a carrier gas which that has been doped with a low concentration vapor (typically a few parts per million) employed as a charge transfer mediator. More particularly, the inlet  22  communicates with a source of sample air of interest  14  and a supply of carrier gas and dopant  16  with flows of gases to the inlet  22  being enabled by a flow generator such as a pump illustrated schematically and identified by the numeral  18  in  FIG. 2 . A heated membrane  19  formed from a microporous refractory material or from dimethyl silicone is disposed near the inlet  22  and in communication with the source of the sample of air  14  for blocking passage of at least selected constituents of the air and for enabling passage of other constituents of the air, including the constituents of interest. The sample air, carrier gas, and dopant molecules pass through the inlet  22  and are spread by a diffuser  24  into an ionization chamber  26 . The ionization chamber  26  is in the form of a shallow cylinder with a diameter D, length L, and cylindrical wall  28  of a radioactive material, e.g., nickel 63  or tritium, which emits beta particles. Inlet  22  communicates with one end of the ionization chamber  26 . A grid electrode E 1  is provided at the end opposite the inlet  22 , and is normally maintained at the same potential as the inlet end and the walls of the ionization chamber  26 . Thus a largely field-free space is provided in which electrons and ion charges build up and interact with the sample molecules under bombardment by the beta-particles from the radioactive walls. Beyond the ionization chamber  26 , the ionized sample gases pass through open electrode E 1  and into an ion drift region  30  having several field-defining electrodes E 2 -E n . A collector electrode or plate  32  is disposed at the end of the drift region  30  for receiving the ion samples reaching that end. 
     Periodically a field is established across the ionization region  26 , by creating a potential difference between the grid electrode E 1  and the inlet diffuser  24  and radioactive source  28 , for about 0.1-0.2 mS, to sweep the ions through the open grid E 1  into the drift region  30  with the assistance of the switching of the field between electrodes E 1  and E 2 . The ions in the drift region  30  experience a constant electric field, maintained by the annular electrodes E 2 -E n , impelling them along the region and down toward the collector electrode  32 . The electrode  32  detects the arriving charge, and produces signals that are amplified and analyzed through their spectra in the spectrometer. The gases exit through an outlet in the wall next to the electrode  32 . After about 0.2 mS the field across the ionization region  26  is again reduced to zero and the ion population is again allowed to build up in the chamber  26  preparatory to the imposition of the next field. The polarity of the fields is chosen on the basis of whether the detector is operated in a negative or positive ion mode. When detecting explosives, a negative ion mode is usually appropriate, but when detecting narcotic samples positive ion mode is preferred. 
     The detector  10  includes a sample collection apparatus that is identified generally by the numeral  40  in FIGS.  1  and  3 - 7 . The sample collection apparatus  40  include a housing  42  that can be mounted removably into a receptacle  44  formed in the housing  11  of the detector  10 . More particularly, the sample collection apparatus  40  is a modular unit that can be releasably mounted to the housing  11  of the detector  10  so that the detector  10  can be adapted for a particular use. In this regard, the sample collection apparatus  40  can be removed and replaced with another sample collection apparatus, such as the card sampling apparatus as identified generally by the numeral  70  in  FIG. 10 , and as disclosed in copending application Ser. No. 10/774,003. Alternatively, the sample collection apparatus  10  can be replaced with the known apparatus for receiving a fabric-like trap that may be wiped across a surface of a parcel or piece of luggage to detect substances of interest. 
     The housing  42  of the sample collection apparatus  40  includes a window  46  at a position on the housing  40  that will face the person that is to be scanned for trace amounts of substances of interest. The window  46  is configured and dimensioned to receive substantially all of the gripping surface of the distal digit on a thumb. 
     The sample collection apparatus  40  further includes a generally cylindrical drum  48  mounted in the housing  42  for rotation about an axis that is parallel to the front face of the detector  10 . More particularly, the cylindrical drum  48  is disposed to be substantially internally tangent with portions of the housing  42  adjacent the window  46 . Hence, a target area on the exterior of the drum  48  will be exposed at the window  46 . The drum  48  is formed from a material that will retain residue from the hand of a person being screened. The material of the drum  48  also must be able to be heated quickly and repeatedly to sufficiently high temperatures for vaporizing residue received from the hand. Additionally, the material of the drum  48  should be capable of being cooled quickly to prevent discomfort when a finger is placed on the drum  48  and to maintain a desirably low cycle time for scanning. The material of the drum can be a non-metallic material or a thin metallic material, such as aluminum or stainless steel. Aluminum exhibits desirable heating and cooling characteristics and exhibits a long life. The thickness of the material of the drum  48  also is selected to facilitate rapid heating and cooling. A thickness in the range of 0.002-0.020 inch is preferred. The relatively thin material of the drum  48  also permits slight inward deflection of the drum  48  in response to digital pressure created by a thumb or forefinger placed on or wiped across the target area of the drum  48  exposed at the window  46 . This deflection can trigger a pressure sensitive switch to activate a scanning cycle. 
     The drum  48  further includes a plurality of slots  50  that extend entirely through the material of the drum  48 . The slots  50  perform several functions. The slots  50  remove mass from the drum  48  and break the conductive heat transfer path to facilitate rapid heating and cooling of the drum  48 . The slots  50  also accommodate air flow to facilitate cooling. Additionally, the slots  50  facilitate deflection of the drum  48  that may trigger the pressure sensitive switch. Slots  50  that are aligned parallel to the axis of the drum  48  permit the apparatus  40  to accommodate the optional fingerprint scanning. However, embodiments without the fingerprint scanning option can have slots oriented differently or can have perforations other than slots. 
     The sample collection apparatus  40  further includes a pressure sensitive switch  52  fixedly mounted to the housing  42  at a location radially aligned with the window  46  and inwardly from the drum  48 . The switch  52  senses small deflections of the drum  48  as the thumb or forefinger is wiped across portions of the drum  48  in the window  46 . Thus, the switch  52  can generate a signal to activate a scanning cycle. 
     A fingerprint reader  54  optionally is disposed inwardly of the drum  48  for reading fingerprint characteristics of a thumb as the thumb is moved relative to the slot  50 . Alternatively, the switch can include an optical switch that will sense the presence of light that occurs when the thumb is slid past the slot  50  and into a position where ambient light can again enter the slot  50 . 
     The sample collection apparatus  40  further includes a motor  56  mounted to the housing and operative to rotate the drum  48 . The motor  56  is connected to the switch  52  and functions to rotate the drum  48  a selected amount in response to the sensed completion of a wipe of a thumb or forefinger across portions of the drum  48  disposed in the window  46 . 
     The sample collection apparatus  40  further includes a desorber  58  mounted to the housing and disposed interiorly of and adjacent to the drum  48 . The desorber  58  functions to rapidly heat portions of the drum  48  aligned with the desorber  58  for vaporizing trace amounts of material transferred from the thumb or forefinger to the target area of the drum  48  that was exposed at the window  46 . The desorber  58  preferably is always on when the detector  10  is on to avoid a need for preheating during each scanning cycle. A sample transfer box  60  is mounted to the housing  42  at a location radially aligned with the desorber  58  but disposed exteriorly of and substantially adjacent the drum  48 . The desorber  58 , and the sample transfer box  60  have opposed facing surfaces that are curved with radii of curvature substantially corresponding to the inner and outer circumferential shapes of the drum  48 . The sample transfer box  60  further includes a sample tube  62  that communicates with the inlet  22  of the ion trap mobility spectrometer of  FIG. 2 . 
     The apparatus  10  is employed by providing audible or optically readable instructions to a person who desires access to a restricted or secured area. The optically readable instructions may be provided on the display monitor  12 . In particular, the person will be instructed to wipe a thumb across the target area of the drum  48  exposed at the window  46 . The fingerprint reader  54 , if provided, will optically scan the fingerprint as the thumb is wiped across the slot  50  in the drum  48 . The fingerprint data can be stored for future reference. Alternatively, the fingerprint data can be used to compare personal identification information associated with the fingerprint to other personal identification information presented by the user. For example, the identity of the person determined by the fingerprint scan can be compared with identity on an identification card or boarding pass that also is presented to the apparatus  10 . An incorrect match may generate a signal to instruct a human security technician to conduct enhanced checking to determine the cause of an inconsistency. Alternatively, the scanned fingerprint data can be compared to known databases that have fingerprints for suspects of terrorism or other crimes. A match with these known databases can generate an audible or inaudible alarm that will trigger additional screening by security personnel or by other available equipment at a security checkpoint. 
     The movement of a thumb across the target area of the drum  48  exposed at the window  46  will deflect the thin aluminum of the drum  48  and will actuate the pressure sensitive switch  52  aligned with the window  46 . Alternatively, a switch can detect changes in light level as the thumb moves clear of the slot  50 . The signal generated to indicate a completion of a wipe of a thumb across the target area of the drum  48  in the window  46  will cause the motor  56  to rotate the drum  48  an amount other than 180°, and preferably about 135°. Thus, the target area of the drum  48  that had been aligned with the window  46  will advance into the narrow space between the desorber  58  and the sample transfer box  60 . The motor  56  then stops. The heated desorber  58  raises the temperature of the drum  48  between the desorber  58  and the sample transfer box  60  sufficiently to vaporize residue transferred from the thumb to the drum  48 . A vacuum pump  18  in the detector of  FIG. 2  then will draw the vaporized material through the sample collection tube  62  and into the detector for analysis. The ITMS will detect the presence of substances of interest and will generate an appropriate signal for additional or enhanced testing by security personnel at the checkpoint. The ITMS also can check for “people peaks” indicative of a human being. The absence of a “people peak” may suggest that a person is wearing gloves or has placed a pen or other inanimate object on the drum  48  to avoid an accurate scan. The absence of a people peak may generate a signal for additional screening. The preferred 135° rotation of the drum ensures that the area of the drum  48  that was most recently heated by the desorber  58  will not be rotated directly back into alignment with the window  46 . Thus, target areas on the drum  48  are more certain to be sufficiently cool when aligned with the window  46 . 
     The sample collection apparatus can take other configurations. For example,  FIG. 8  shows a sample collection apparatus  40 A with a drum  48 A mounted for rotation about an axis aligned at an angle, and preferably a right angle, to the front face of the detector  10 . The window  46 A is sufficiently wide to place all forefingers of one hand on a portion of the drum  48 A exposed at the window.  FIG. 9  shows a detector  40 B with an aluminum disc  48 B in place of the drum. The disc  48 B rotates about a substantially vertical axis. Other options can include a thin plate that translates without rotation or a flexible belt that is driven about rollers. 
     While the invention has been described with respect to a preferred embodiment, variations can be made without departing from the scope of the invention as defined by the appended claims. 
     The illustrated apparatus shows the sample collection apparatus  40  as being a modular unit in the detector  10 . The sample collection apparatus can be removed and replaced by a card scanner  70 , as shown in  FIG. 10  and as disclosed in copending application Ser. No. 10/774,003. Alternatively, an apparatus for receiving fabric-like sample traps can be inserted in the receptacle  44 . However, the sample collection apparatus  40  can be part of a dedicated detector apparatus without an ability to convert to other testing formats. 
     The illustrated embodiment of the detector shows a monitor  12  facing generally in the same direction as the window  46 . However, the monitor  12  can face in the opposite direction so that a security screener can observe the monitor from one side of the apparatus while a person desiring security clearance will be on the opposed side of the apparatus. Alternatively, a monitor can be disposed at a remote location or can be replaced by or supplemented with other signal generators for indicating clearance or a need for further checking. One monitor can be used to assess scanning performed by a system of detectors  10 . 
     The sampling sheet can be formed entirely from a metallic material, a non-metallic material or combinations of metallic and non-metallic materials. For example, the sampling sheet can include a metallic support at peripheral locations on the sheet and a non-metallic material at intermediate positions.