5. NONTAGGED DETECTION DEVELOPMENT

The detection tagging efforts outlined in Section 4 require taggant addition at manufacture. It is anticipated that the addition of a detection taggant will greatly reduce the severity of the explosives threat. However, certain categories of explosives are not amenable to detection tagging, such as military, homemade, and smuggled foreign explosives. An increasing proportion of the threat that will remain after detection tagging is implemented may be expected to comprise these explosives. Under the terms of a contract with the Law Enforcement Assistance Administration, Aerospace has evaluated a broad spectrum of techniques for the detection of untagged explosives. The most promising of these techniques have been selected for further investigation and development under Bureau of Alcohol, Tobacco and Firearms sponsorship and with the concurrence of the Technical Subcommittee to the Advisory Committee on Explosives Tagging. A small subcontract has been awarded to assess the feasibility of dual-energy tomography, sources have been selected, and subcontracts are presently being negotiated to investigate plasma chromatography and letter- and book-bomb detection by dielectric measurement techniques.

5.1

5.1.1

LETTER- AND BOOK-BOMB DETECTION BY DIELECTRIC MEASUREMENT
TECHNIQUES

Description

Explosives have been empirically found to have somewhat higher dielectric constants than most of the materials commonly found in letters or report-size envelopes (flats). Explosives may also exhibit conductivities which have a characteristic dependence on frequency. These dielectric properties may be employed in conjuction with the weight, size, and metal content of an envelope to discriminate between letter- or book-bombs and innocent mail.

A subcontract is currently being negotiated between Aerospace and the Georgetown University Physics Department to investigate dielectric letter- and bookbomb detecton techniques. The purposes of this contract are the following:

Determine if the frequency dependence of the conductivity of explosives is indeed unique;

If it is unique, determine if it provides a greater capability of detecting explosives than a capacitance-measuring technique (e.g., does it allow for the detection of bombs in packages);

5.1.3

Determine the effectiveness of the capacitance-measuring technique against a broad spectrum of mail, using equipment previously developed using U.S. Postal Service funds; and

Obtain design parameters for an optimal dielectric letter- and book-bomb detector.

Plans

The subcontract with Georgetown University should run through FY 78. The results of this subcontract should be definitive as far as the potential capabiliites and limitations of this technique are concerned. No plans beyond this point have been made. DUAL-ENERGY TOMOGRAPHY

5.2 5.2.1

Description

Computerized tomography is a rapidly evolving technology wherein cross-sectional radiodensity mappings of an object ("slices") are analytically reconstructed from sets of one-dimensional x-ray projections. To date, computerized tomography has found application exclusively in the medical field, with some 13 companies producing and marketing computerized tomographic scanners capable of imaging any slice through the human body.

The techniques embodied in medical computerized tomograpic scanners can also be employed to scan suitcases or packages to automatically detect concealed explosives based on the relatively high density and low atomic number of these materials. (Density and atomic number may be obtained by performing radiodensity mapping at two energies.) The vast majority of packaged high explosives, including slurries/water gels, have densities of between 0.95 and 1.6 g/cm3, and typically have densities of approximately 1.2 g/cm3. Military explosives and commercial plastic explosives typically have even higher densities: 1.5 g/cm3 up to 1.85 g/cm3. The effective atomic numbers of explosives are unknown and will have to be determined empirically.

A dual-energy computerized tomographic device configured specifically for explosives detection would consit of a conveyor belt to transport the suitcases or packages, a stationary x-ray source, a stationary array of radiation detectors, associated electronics, and dedicated minicomputer. Detection would be automatic, based on composition (rather than shape), although an image could be readily provided to an operator.

A small, fixed-price contract has been awarded to the Georgetown University Medical School to perform dual-energy computerized tomographic scanning of a test box containing simulated explosives as well as typical suitcase contents (books, toiletries, etc.) using an existing research medical scanner. Data obtained in this manner will be of great use in assessing the capabilities of this approach, although the low energy of the medical unit (120 kVp maximum) makes it unsuited for taking measurements through densely packed suitcases. Preliminary scans taken of the test box at 75 and 120 kVp indicate that the simulated explosives (cast dinitrotoluene and an ammonium nitrate, sugar, and water slurry) can be readily identified. Computer simulation studies have also been initiated at Aerospace in El Segundo, California, to model the performance of a dedicated computerized tomographic explosives detector. These studies utilize a reconstruction algorithm previously developed at Aerospace and should provide the foundation for the software required for a practical system.

Preliminary studies should be completed during the second quarter of FY 78 and system requirements determined for breadboard detector. A competitive subcontract will be let for the construction of a breadboard detector capable of imaging a single plane through a large suitcase or package. Off-the-shelf hardware will be employed to the greatest extent possible and reconstruction will be performed by a nondedicated computer. Once constructed, this breadboard will be used to fully assess the capabilities of this technique, using a broad spectrum of actual explosives within ralistic packages and suitcases.

If the vapor detection of explosives, either tagged or untagged, is to be realized, it will be necessary to obtain a detector with a detection threshold and selectivity suitable for the explosive-specific vapor species sought. Plasma chromatography is a technique identified as having a detection threshold and selectivity sufficient to justify further evaluation as a vapor detection device based on the differential mobility constants of specific ions.

+

Several researchers have been active in advancing the state-of-the-art in plasma chromatography. Plasma chromatography has been demonstrated to be a technique with an outstanding detection threshold for the detection of trinitrotoluene vapors

(1 part in 1012). It has also been shown to have a potential for good selectivity. The plasma chromatograph has the advantages of operating at atmospheric pressures while being relatively simple and rugged. Therefore, it has the potential to be configured as a field instrument. Unfortunately, many questions remain to be answered concerning plasma chromatography before it may be deemed suitable for specific development as a detector of explosives or taggant vapors. These questions involve: (1) the response of the plasma chromatograph to explosives vapors other than trinitrotoluene; (2) the response of the plasma chromatograph to potential vapor taggants; (3) the response of the plasma chromatograph to identified background interferents at concentrations less than 1 part per billion (where the plasma chromatograph can still operate); and (4) the effect of background interferents on the detectability of explosives or taggant vapors. 5.3.3

Plans

Under direction from the Advisory Committee on Explosives Tagging, chaired by the Bureau of Alcohol, Tobacco and Firearms, Aerospace has drafted a Statement of Work and other necessary procurement materials to proceed with an evaluation study for the plasma chromatograph.

These techniques offer substantial promise for the detection of untagged explosive devices. The dual-energy tomography effort (experimental effort started 23 September 1977) has already produced distinct images with excellent resolution of a box containing explosives and other objects. The explosive dinitrotoluene and a simulated slurry were clearly discernible. These three techniques are not viewed as mutually exclusive, but rather as complementary depending on the operational scenario. The dielectric method, as presently viewed, is only suitable for letter or flat determination and is ideally suited for mailroom security. The dual-energy tomograph is excellent for package and baggage screening scenarios (i.e., transportation facilities). However, due to the requirement of the use of ionizing radiation (approximately 300 kVp rays), the screening of people is precluded. Plasma chromatography has the potential to perform controlled-access or large-area searches of packages or personnel via natural vapor or taggant vapor detection.

OTHER RECOMMENDED STUDIES

ACCES PORTRETION FITTION IDENTIFICATION OF EXPLOSIVES

Desert the preser ettors identification tagging of explosives, there are substanti categorie z explosive materials 10 which identification taggant addition is anticipates 1. be difficu“ sec sector. 251er impossible. Identification taggant addition I jak: smokers Dovets ir energ Doosters or primers, military explosives, Nomerat; EXDIOSes and those explosives in. Inventory prior to National implementation C. The teentificatio taggin, concer: represent broad categories of explosives for which convenziona Dom: debt, ana's wide require: to some time and, in some cases, ineelimiter:

Preser: anatic metho: results summarized by the Federal Bureau of Investigation Nations Bom: Cente: reves that I on 63 percent of the incidents is the 19 explosive matera use. I i candestine device known by laboratory analysis. Faure & reentor the materia responsibe: 10 the 2 December 1975 La Guardia Airport bies is a gering examber. The following section or enzyme, chromatographic, and nowe matching method development: as assigned to improve the forensic laboratory's Suessil anwysi Bercentages

Enzymes are complex organic molecules that are capable of catalyzing highly spectórt biechemica reactions. B counting ar enzymatic reaction to a second, lightPRVOMONĘ PRACLio r. is possiber to detect extremen small amounts of the initial reactant materias. The high specifics and tow threshold at detection of this technique thus make nana cake to the posteetonation identification of explosives residue. Follow ng standart procedures to extract explosives residue from debris and to separate The rescue Di Tu-aver chromatogram, ar enzymatic technique could be employed to detect extrasives or a thin-aver chromatographic plate that would otherwise be judged to

*Ar excellent example of the damage potentia of homemade explosive devices is the Sata bombing at the Los Angeres international Arront by the widely publicized "Rupnaber Bomber. The explosive charge consister of property mitiated nitrobenzene as confirmed on the Los Angeles Police Department forensic laboratory analysis.