WO1992012415A1

WO1992012415A1 - Contraband detection apparatus and method

Info

Publication number: WO1992012415A1
Application number: PCT/US1991/009762
Authority: WO
Inventors: Thomas G. Miller
Original assignee: General Research Corporation
Priority date: 1990-12-31
Filing date: 1991-12-30
Publication date: 1992-07-23
Also published as: AU1150992A

Abstract

A contraband detection system (18) determines substances concealed in a sample object by developing total neutron cross section spectra for a plurality of elements, including nitrogen, oxygen, hydrogen, and carbon. A processor (26) performs a contraband determination classification based on the total neutron cross section spectra for the plurality of elements. Included are a neutron source (20) for producing a pulsed beam (36) of fast white neutrons; a spatial neutron detection array (40); a conveyor system (28) for situating a sample object (29) between the source (20) and the detection array (40); a spectra analysis system (24) for determining the total neutron cross section spectra of elements located in the sample object (29); and, the processor (26). The neutron source (20) produces a beam (36) of fast white neutrons having a sufficient energy range whereby removal of neutrons from the beam caused by the presence of a plurality of contraband-indicating elements can be determined.

Description

CONTRABAND DETECTION APPARATUS AND METHOD

BACKGROUND

1. Field of Invention

This invention pertains to the detection of contraband, and particularly to the detection and identification of explosives and illicit drugs

concealed in luggage and the like.

2. Prior Art and Other Considerations

Concealed contraband poses an obvious hazard to the general welfare. Therefore, the rapid and accurate detection and identification of concealed contraband is of critical importance to the public sector. This is true whether the contraband be an explosive or an illicit drug.

The potential concealment and subsequent activation of explosives in luggage and air cargo has gravely concerned aviation authorities and the

travelling public. Numerous explosive detection techniques have been proposed, including

electromagnetic (e.g., "X-ray") or

photon probes, vapor detectors, and nuclear-based techniques.

One widely documented prior art nuclear-based technique is Thermal Neutron Activation ( "TNA" ).

Gozani et al., "Explosive Detection System Based On Thermal Neutron Activation", IEEE AES Magazine,

December 1989. Morgado et al., "The Detection of Bulk Explosives Using Nuclear-Based Techniques". The TNA technique operates upon the premise that an explosive contains a large amount of nitrogen. According to the TNA technique, luggage and cargo are conveyed through a chamber having a cloud of thermal neutrons. As

nitrogen absorbs the thermal neutrons, a number of nuclear reactions occur. In one of these nuclear reactions, a gamma ray having an energy of 10.8 MeV is emitted. The detection of gamma rays having an energy of 10.8 MeV thus becomes an indication or "signature" for nitrogen.

In implementation, the TNA technique typically involves the placement of an array of gamma ray

detectors around the sample luggage. The luggage is carried by a conveyor through a chamber housing the detectors. A neutron source is also housed in the chamber for producing the cloud of thermal neutrons.

Thermal neutrons employed in the TNA technique are relatively slow, low energy neutrons. In general, neutrons can be created from radioisotopes, such as Californium (²⁵²Cf) or by particle accelerators.

Neutrons created from Californium have a spectrum of energies ranging from near zero to 10 MeV. The high energy neutrons, also called "fast neutrons", primarily scatter when interacting with matter until the neutrons diffuse into matter having essentially the same kinetic temperature as the surrounding matter. Therefore, in order to produce thermal neutrons, the neutron source must be surrounded by a moderating material in order to slow down the fast neutrons for the production of thermal neutrons.

One problem with the TNA technique is that diffused neutrons cannot be focused into a ray or beam. Consequently, many of the neutrons diffuse in a

direction away from the sample luggage and the

detection apparatus, for example into the shielding or moderating material. Absorption of these neutrons by materials other than the luggage (such as the

shielding) causes the generation of gamma rays at characteristic energies other than 10.8 MeV. The detection of these other non-10.8 MeV gamma rays occasions background noise, which makes discernment of the nitrogen-indicative 10.8 MeV gamma rays more difficult.

Moreover, it was noted above that the generation of 10.8 MeV gamma rays is just one of several possible nuclear reactions that occur when nitrogen absorbs thermal neutrons. Accordingly, the TNA technique sees only a partial "cross section" of the total resultant thermal neutron/nitrogen nuclear reaction.

Consequently, the probability of detection of gamma rays generated by the thermal neutron/nitrogen nuclear reaction is proportionately less than would be a detection probability based on total cross section.

Further, the gamma rays created by the thermal neutron/nitrogen reaction are isotropic, meaning that the gamma rays also scatter in all directions. The probability of detection of the gamma rays is thus further reduced in view of the fact that only a

fraction of the 10.8 MeV nitrogen-indicative energy gamma rays travel toward the detectors.

A further shortcoming is that the TNA technique detects only the presence of one element ╌ nitrogen. It has been recognized that explosives generally include other elements to significant degree, including oxygen and carbon. Gozani, "Nuclear-Based Techniques For Explosive Detection", Journal of Energetic

Materials. Vol. 4, p. 377-414 (1986). Explosive detection techniques involving fast neutrons have also been proposed by Morgado et al.

According to this fast neutron proposal, a beam of deuterons is accelerated at low energy onto a tritium target, thereby liberating alpha particles and

neutrons. Subsequent neutron collisions would produce characteristic gamma rays from inelastic reactions with the main constituents of explosives. Thus, this fast neutron proposal also involves the generation and detection of gamma rays. This technique also makes use of a partial neutron cross section and hence suffers many of the same deficiencies as the TNA system. The gamma ray detectors are a special problem as conventional gamma ray detectors can not be used and one would probably have to employ liquid nitrogen cooled detectors.

In November 1989 the Federal Aviation

Administration released its "Guidelines For Preparing Responses To The Federal Aviation Administration's Broad Agency Announcement For Aviation Security

Research Proposals", Revision 3, November 1, 1989,.

The "Guidelines" mentions, on page 7, a nuclear method which "has not yet been applied to the airport security problem." That method involves the creation and detection of a broad energy spectrum of a pulsed neutron beam. According to the "Guidelines", the elements in the path of the beam absorb those neutrons whose energies correspond to the characteristic neutron resonances of the elements. The dips in intensity spectrum of the neutrons that pass through the luggage, measured as a function of the neutron energy, yields a projected spectrum of the elemental distribution in the luggage. Despite numerous proposals, to date an explosive detection system that overcomes the disadvantages associated with the TNA technique has not been

developed.

Accordingly, it is an object of the present invention to provide an accurate method and apparatus for detecting and identifying contraband substances.

An advantage of the present invention is the provision of method and apparatus for the detection of contraband wherein the total cross section of a nuclear reaction is utilized.

A further advantage of the present invention is the provision of method and apparatus for the detection of contraband which facilitates three dimensional location of contraband in a piece of luggage or the like.

Yet another advantage of the present invention is the provision of method and apparatus for the detection of contraband which detects a plurality of possible constituent elements of an explosive.

Another advantage of the present invention is the provision of method and apparatus for the detection of contraband, which detects illicit drugs as well as explosives.

Another advantage of the present invention is the provision of method and apparatus for the

generation of white neutrons for use in the detection of contraband.

Still another advantage of the present invention is the provision of method and apparatus for analyzing spectra for a plurality of elements having neutron-removing peak energies in a range of fast neutron energies, and for using that analysis to determine the presence of yet a further element, such as hydrogen, which does not exhibit a resonance peak.

Yet another advantage of the present invention is the provision of method and apparatus for the detection of contraband which are extremely sensitive, particularly in view of the total neutron cross section utilization, thereby reducing the probability of false detection. SUMMARY

A contraband detection system determines substances concealed in a sample object (such as luggage and the like) by developing a total neutron cross section spectrum for a plurality of elements, including nitrogen, oxygen, hydrogen, and carbon. A processor performs a contraband determination

classification based on the spectra for the plurality of elements. The contraband detection system of the present invention analyzes the neutron removal spectra for three elements (Carbon, Nitrogen, and Oxygen) which have neutron-removal peaks in the range of fast neutron energies, and advantageously uses that analysis to determine the presence of yet a further element (e.g., Hydrogen) which does not have a resonance peak in the range of fast neutron energies.

The contraband detection system includes a neutron source for producing a pulsed beam of fast white neutrons; a spatial neutron detection array;

means for situating a sample object between the source and the detection array; a spectra analysis system for determining the total neutron cross section spectra of substances located in the sample object; and, the classification processor. The neutron source produces a beam of fast white neutrons having a sufficient energy range whereby removal of neutrons from the beam (by absorption and/or scattering) caused by a plurality of contrabandindicating elements can be determined. As used herein, neutron "removal" refers to removal of neutrons from a beam, whether the removal is by neutron absorption, neutron scattering, or both.

The detector array comprises a two-dimensional array of neutron detector elements. Each of the detector elements is essentially aligned along a neutron path with a corresponding three-dimensional sector of the sample object, whereby a two-dimensional coordinate of the location of contraband in the sample object can be specified. The processor makes a

classification determination with respect to a two-dimensional coordinate location of the sample object after the sample object is situated in a first

orientation between the neutron source and the

detection array. Thereafter the processor makes a classification determination with respect to a three dimensional coordinate location of the sample object after the sample object is situated in a second

orientation between the neutron source and the

detection array.

In some embodiments, the neutron source

comprises a composite target toward which deuterons are directed for the production of white neutrons. In one embodiment, a composite target includes a first layer of Boron-10 and a second layer of Carbon-13. In another embodiment, a composite target includes a first layer of Oxygen-16 and a second layer of Carbon-13. In yet another embodiment, a composite target includes a first layer of Beryllium and a second layer of Carbon- 13. The composite targets assure the prompt production of white neutrons across the entire energy spectrum necessary for obtaining total neutron cross section spectra for the contraband-indicative elements of interest. Various techniques of making a contraband classification determination are also disclosed, including a neural network (ALN) technique; the

evaluation of empirical expressions; and, a matrix solution technique.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing and other objects, features, and advantages of the invention will be apparent from the following more particular description of preferred embodiments as illustrated in the accompanying drawings in which reference characters refer to the same parts throughout the various views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of the invention.

Fig. 1 is a schematic view of a detection system according to an embodiment of the invention.

Fig. 2 is an isometric view of a suitcase being conveyed past a detector array of the system of the embodiment of Fig. 1.

Fig. 3 is a graph showing total neutron cross section curves for hydrogen, carbon, nitrogen, and oxygen.

Fig. 4a - 4b are graphs showing total neutron cross section spectra for the following respective items: weapon; cocaine; explosive; hashish; and a normal suitcase. Fig. 5A is an isometric view showing a sample object in a first orientation before a detection array of the embodiment of Fig. 1.

Fig. 5B is an isometric view showing a sample object in a second orientation before a detection array of the embodiment of Fig. 1.

Fig. 6 is a side view of a fast white neutronproducing composite target utilized in an embodiment of the invention.

DETAILED DESCRIPTION OF THE DRAWINGS

Fig. 1 shows a contraband detection system 18 including a neutron source 20; a neutron detector assembly 22; a spectra analysis system 24; and, a classification processor 26. Fig. 1 also shows a conveying system 28 for introducing a sample object 29, such as a piece of luggage or a suitcase, between the neutron source 20 and the neutron detector assembly 22.

The neutron source 20 includes an accelerator 30 for generating a pulsed deuteron beam 32 and for directing the pulsed deuteron beam to a target 34. The beam 32 is on the order of 3.0 MeV to 8.0 MeV. The pulses of the deuteron beam 32 have a pulse length of about 1 nanosecond or less . The neutron source 20 is enclosed in shielding 38 which, in the illustrated embodiment, is in the shape of a sphere or the like with an aperture oriented so that only those neutrons that are heading in the direction of the sample object 29 are released from the shielding 38.

In one embodiment, the accelerator 30 is a small tandem accelerator with a terminal voltage of between 1.5 MeV and 4.0 MeV. The accelerator utilizes a negative ion source at ground potential and accelerates the negative ions to the 2.0 MeV to 4.0 MeV center of the accelerator. The ions are then doubly stripped and accelerated back to ground, gaining another 1.5 to 4.0 MeV, giving them a total energy of 3.0 MeV to 8.0 MeV.

The target 34 has a composition such that impingement of the pulsed deuteron beam 32 produces a pulsed white neutron beam 36. As used herein, the term "white neutron beam" means a beam of neutrons having energies in a range from about 0.5 MeV to about 3.0 MeV. In one embodi-ment of the invention, the target 34 is a thick carbon (Carbon 12) target. As used herein, the term "thick", when used to describe a target, includes a target sufficient to prevent

substantially any deuterons of a specified energy from passing through the target. Thus, the deuterons slow down and stop as they travel through the thick carbon target. Some of the deuterons produce a nuclear reaction at each deuteron energy from the accelerator energy down to the threshold of the reaction.

In another embodiment of the invention, the target 34 is a thick beryllium target. The

deuteron/beryllium reaction produces an order of magnitude more neutrons than does a carbon 12 target, but many of the neutrons are above the energy range of interest.

The neutron detector assembly 22 is placed about three to five meters away from the deuteron detector 50 along the flight path of the neutrons 36. The neutron detector assembly 22 comprises a two-dimensional array 40 of neutron detector elements 42. The detector array 40 include enough detectors to cover a large suitcase with a spatial resolution of 5 centimeters by 5

centimeters. Although not shown as such, shielding is provided around the detector assembly 22. The particular detector array 40 shown in Fig. 2 (as well as Figs. 5A and 5B) includes twenty five columns of detector elements 42, with each column consisting of fifteen detector elements 42. Thus, 375 neutron detector elements 42 are provided in the array 40. A two dimensional subscript notation is employed for identifying the detector elements 42 in the array 40, with element 42_1,1 being the detector element in row one of column one; with element 42_2,1 being the detector element in row one of column two; with element 42_1,2 being the detector element in row two of column one; and so forth. It should be understood that the array 40 may take on other sizes in accordance with the type of objects for which the contraband detection system is designed to operate.

The neutron detector assembly 22 also comprises an array of photo multiplier tubes 44, with each of the photo multiplier tubes being connected to receive signals from a corresponding one of the neutron

detector elements 42. The photo multiplier tubes have less than a nanosecond rise time. Each photo

multiplier tube in array 44 has an output terminal connected to a corresponding voltage divider 46. Each voltage divider 46 is connected, in turn, through an amplifier 48 to the spectra analysis system 24.

The spectra analysis system 24 includes a deuteron beam pick-off 50; a time pick-off 52; an amplifier 54; an array 56 of time-to-amplitude

converters (TACs); a multi-channel analyzer array 58; and, a pulse shape discrimination circuit network 60. The neutron detector assembly 22 can acquire

configurations other than that described above. The deuteron beam pick-off 50 is a cylinder which senses when a charged deuteron pulse travels through the cylinder. The electric current sensed by the beam pick-off 50 is amplified by the amplifier 54 and is sensed by the time pick-off 52. The conduction of the electric current between the pick-off 52 and the beam pick-off 50 causes the time pick-off 52 to

generate a real time "start" pulse which is applied to each of a plurality of converters in the array 56 of time-to-amplitude converters.

Each of the time-to-amplitude converters included in array 56 is associated with a corresponding one of the detector elements 42, and accordingly is associated with a corresponding one of the photo multipliers in array 44. Each of the TAC converters in array 56 is connected to receive a real time "stop" pulse from the neutron detector assembly 22, and particularly from the amplifiers 48 connected

downstream from the array 44 of photo multiplier tubes. Thus connected, each TAC in array 56 receives a real time start pulse from the time pick-off 52 as a

deuteron travels through the deuteron detector 52.

When a neutron impinges on one of the detector elements 42 and creates a measurable pulse in the neutron detector assembly 22, the impinged-upon detector 42, via its associated photo multiplier tube 44 and voltage divider 46, sends a real time "stop" pulse to the associated TAC ("the activated TAC") in array 56. The activated TAC 56 then generates a signal having an amplitude corresponding to the time-of-flight from the beam pick-off 50 to the neutron detector assembly 22.

In order to make the converters in TAC array 56 more efficient, it should be understood that in other embodiments one can use the signal from the detector elements 42 as a start signal and a delayed signal from the time pick-off as the stop signal, as is well known in the prior art.

The pulse shape discrimination circuit network 60 includes a number of pulse shape discrimination circuits corresponding to the number of detector elements 42 included in the array 40. The pulse shape circuits in network 60 discriminate gamma rays from the neutrons for the multi-channel analyzer array 56, resulting in a reduced background.

The multi-channel analyzer array 58 includes a multi-channel analyzer for each converter in TAC array 56. For the embodiment illustrated in Fig. 1, there are 375 analyzers in array 58. Each multi-channel analyzer in array 58 is connected to receive the output amplitude signals from a corresponding converter in TAC array 56.

In view of the fact that the amplitude of the output signal from an activated TAC in array 56

reflects time-of-flight, the associated multi-channel analyzer in array 58 sorts the amplitude pulses from the activated TAC to give a time of flight spectrum for the activated TAC. The amplitude pulses are then categorized into channels, with each channel

corresponding to a small range of neutron energies.

Each multi-channel analyzer in array 58 generates output which is indicative of the number of counts for each channel.

The processor 26 is a conventional data

processing system having a central processing unit, memory, an arithmetic logic unit, and an input/output interface/controller 62. The processor 26 has its input/output interface/controller 62 connected by bus 64 to the multi-channel analyzers included in array 58 to receive the data utilized to generate the total neutron cross section spectra curve for each detector element 42 with respect to the sample object 29. As indicated previously, the term "total neutron cross section" is the sum of the neutron absorption cross section and the neutron scattering cross section. The input/output interface/controller 62 of the processor 26 is also connected to a printer 66; to a CRT display screen 68; and, to an alarm 70.

The central processing unit of the processor 26 executes instructions for evaluating the total neutron cross section spectra information for the plurality of contraband-indicating elements. In this regard, as indicated above, the output of each multi-channel analyzer in array 58 is connected to the input/output controller 62 of the processor 26 by a corresponding line in bus 64. The processor 26 performs calculations for each of the analyzers included in the multi-channel analyzer system 58 in order to produce a total neutron cross section spectra corresponding to each of the detector elements 42 included in the array 40. The types of calculations performed by the processor 26 with respect to the data obtained from each of the analyzers included in array 58 for generating the spectra is in accordance with standard techniques such as those understood with reference to Marion and

Fowler, Fast Neutron Physics, 1960.

Thus, the processor 26 creates a total neutron cross section spectrum for each neutron detector element 42 included in the neutron detector array 40. Data indicative of the spectrum for each detector element 42 is stored in memory and also ported to the printer 66. Still further, the processor 26 produces a graphic depiction of the total neutron cross section spectrum for each neutron detector element 42. The graphic depiction is selectively displayable both on the CRT display screen 68 and on hardcopy output generated by the printer 66. Numerous commercially available devices may be employed for the elements of the analysis system 24 of Fig. 1. For example, the time pick-off 52, amplifier 54 (as well as amplifiers 48), the TACs included in array 56, and the pulse shape discrimination circuits included in network 60 are available from Canberra as model numbers 2126, 2111, 2143, and 2160A,

respectively. A suitable scintillator is a liquid scintillator manufactured by Nuclear Enterprises, Inc. as model NE-213. The photo multiplier tubes 44 can be any suitable commercially available tubes, such as those manufactured by RCA as model 8575, or the AMPERE XP L702. A suitable voltage divider 46 is manufactured by ORTEC as model 261.

The contraband detection system 18 of the present invention detects the presence of a plurality of contraband-indicative elements, including nitrogen, hydrogen, oxygen, and carbon. Of these contraband-indicative elements, in an energy range of interest (e.g., 0.5 MeV to 3.0 MeV) most will have peaks in their total neutron cross section spectrum at energies whereat neutrons are removed from the beam. To this end, operation of the contraband detection system 18 of the present invention requires detection of at least one peak from contraband-indicative elements having peaks in the energy range of interest, with the

detected peak not overlapping with peaks from other elements in a sample object. Although hydrogen does not have a peak, the amount of hydrogen can be

ascertained using one of a particular classification determination technique, known as the matrix or

regression technique, described hereinafter as the first classification determination mode.

Fig. 3 is a graphic depiction of the

superimposed total neutron cross section curves for four elements ╌ htyrdarogen, carbon, nitrogen, and oxygen. The neutron cross section curves for these three elements are well known, such as the Evaluated Nuclear Data Files which are available from Brookhaven. National Laboratory and Oak Ridge National Laboratory. Fig. 3 shows the total cross sections of hydrogen, carbon, nitrogen, and oxygen plotted on the same graph and to the same scale from 0.5 MeV to 3.0 MeV, the range of interest for the present invention. As shown in Fig. 3, there are several non-overlapping peaks for nitrogen, oxygen, and carbon in the energy range of interest.

The peaks shown in Fig. 3 correspond to neutron energies at which neutrons are absorbed and/or

scattered (i.e., "removed" from a beam) by the

respective elements. For example, carbon has one large neutron removal peak at 2.07 MeV and a smaller neutron removal peak at 2.9 MeV. Oxygen has a large doublet at 1.69 MeV and 1.65 MeV. Nitrogen has two prominent peaks, one on each side of the large oxygen doublet:

1.78 MeV and 1.6 MeV. There is another large oxygen peak located at 1.32 MeV with two nitrogen peaks too close to clearly resolve. There are three more

nitrogen peaks located at 1.21 MeV, 1.18 MeV, and 1.12 that can also be used.

Thus, if oxygen is present in a sample object, the presence of oxygen is signalled by the absorption and/or scattering of neutrons at the illustrated oxygen peaks. Similarly, the presence of carbon and nitrogen are indicated by the absorption and/or scattering of neutrons at the respective peaks.

In addition to generating the neutron total cross section spectra for each of the detector elements 42, the central processing unit of the processor 26 includes instructions which, when executed, make a classification determination regarding a potential contraband substance located by each detector 42 in the sample object 29. When a detector element 42 locates elements in sample object 29 for which the processor 26 makes a contraband classification determination, the processor outputs a signal to the alarm device 70.

There are several possible modes for making a

classification determination. Three modes are

discussed below.

Classification Determination Mode #1

As yet another example of the classification determination, the processor 26 evaluates the

expression (eqn 1) In (N_o/N) = n_Hxs_H + n_cxs_c + n_Nxs_N + n_oXS_o which represents the total spectrum of a sample object having the elements hydrogen, carbon, nitrogen, and oxygen. In the expression of equation 1 (eqn 1),

N_o is the number of incident neutrons without sample in path.

N is the number of detected neutrons with

sample in path.

n_s is the number of atoms per cubic centimeter of an element (H = hydrogen, C = carbon, N

= nitrogen, O = oxygen).

x is the length of the sample object. s_E is the cross section of an element. The quantities n_Hx, n_cx, n_Nx, n_ox are four unknowns, which are the number of atoms per square centimeter of the respective element in the beam path. These equations are valid if multiple scattering of neutrons is small enough to be neglected, which is the case for the present application. Since each point on the total cross section spectrum curve must satisfy equation 1, and since all the cross sections are known at all energies, four equations with four unknowns can be written, along with the corresponding value of In (N_o/N). Thus, equation 1 can be evaluated at four points in matrix form. For convenience of evaluation, four points are picked at which each of the elements having a peak at one of the four points. In particular the points .66 MeV, 1.0 MeV, 1.116 MeV, and 2.077 MeV are chosen. When written as four equations matrix at these points, equation 1 becomes

(eqn 2) In (N_o/N) = n_Ex S where In (N_o/N) is a 1 × 4 matrix consisting of four elements In (N_o/N) for the four evaluation points;

where n_Ex is a 1 × 4 matrix consisting of the unknowns at the four evaluation points; and, were S is a 4 × 4 matrix consisting of cross sections for each of the four elements H, C, N, and O at each of the four evaluation points.

As a case study illustration of the matrix type classification determination, 4 centimeters of an explosive known as Compound B and 16 centimeters of cotton were concealed in a piece of luggage. For the case study illustration, the following matrices

contained the following values (the S matrix being in units of 10^-24):

2.4 5.367 3.099 2.451 2.774

2.98 4.252 2.577 2.385 8.268 In (N_o/N)= 2.23 S = 3.994 2.431 4.007 3.364

1.96 2.773 6.044 1.505 1.457

Solving equation 2 for n_Ex gives (eqn 3) n_Ex = S^-1 In (N_o/N) where the inverse matrix S^-1 of S is (in units of 10⁺²⁴)

.425 -.045 -.186 -.123

-.101 .005 -.026 .226

S^-1 = -.270 -.104 .480 -.011

-.110 .174 -.035 -.004

Evaluating equation 3 for n_Ex gives (in units of 10⁺²⁴ atoms/cm²)

.23004

.157

(n_Ex)_calc = .090

.169

These values of .23004 for hydrogen, .157 for carbon, .090 for nitrogen, and .169 for oxygen are then

evaluated, such as by the expressions of mode 2, for example, to determine whether the alarm 70 should be activated with respect to any particular detector element 42.

The choice of the four particular points utilized to solve the four matrix equations is either pre-determined or is selectable by the processor 26. In this regard, the processor 26 has stored in its memory a table of four known element cross sections for each point in the spectrum, with the result that the processor 26 can judiciously select the four points for solving the equations. Moreover, the processor 26 can select more than four points, if desired.

It is thus understood that the contraband detection system 18 of the present invention analyzes the neutron removal spectra for three elements (Carbon, Nitrogen, and Oxygen) which have neutron-removal peaks in the range of fast neutron energies, and

advantageously uses that analysis to determine the presence of yet a further element (e.g., Hydrogen) which does not have a neutron-removal peak in the range of fast neutron energies. As a variation of the first Classification Determination Mode, the processor 26 can utilize software including regression theory to determine not only the number of neutrons per square centimeter for each of the contraband-indicating elements, but also a probability or error value associated with each element. An example of such software is QuattroPro Version 2 produced by Borland International, which provides regression theory capability in connection with its advanced mathematical tools.

In connection with the first mode variation, known total neutron cross sections for each element for each energy in the energy range of interest is supplied to the processor 26 as independent variables. For each detector element 42, values of In (N_o/N), with the N values having been obtained from the associated

multichannel analyzer in array 58, are supplied to the processor 26 as dependent variables. The processor 26 then outputs, for each detector element 42, the number of neutrons per square centimeter for each contrabandindicating element, as well as an error or probability value for the output neutron number.

The known total neutron cross section data used as the independent variables is based on the

aforementioned Evaluated Nuclear Data Files which are available from Brookhaven National Laboratory and Oak Ridge National Laboratory. In order to account for the finite resolution of the system, these data files are preferably recalculated in terms of channel averaged cross sections. Channel averaged cross section

(represented by sigma bar) is calculated as follows:

where E_L is the lower energy of the channel and E_U is the upper energy of the channel.

For each detector, the resultant number of neutrons per square centimeter for each of the four elements Nitrogen, Carbon, Oxygen, and Hydrogen, can be further examined to determine whether the degree of presence of these elements indicates that contraband is concealed in a suitcase. In this respect, for example, the resultant numbers can be evaluated using atomic ratio expressions in the manner described below in Classification Determination Mode #2.

Classification Determination Mode #2 As another example of the classification determination, the processor 26 evaluates one or more empirically derived atomic ratio expressions related to the probability of the presence of contraband. One set of example expressions for the detection of explosives are the following (the symbol "*" being used to signify a multiplication operation):

N * (O/C)

N/O

N/C

where C, N, and O represent the sums of the counts in the carbon, nitrogen, and oxygen peaks, respectively, as obtained from each multichannel analyzer in array 58. Advantageously, the values for the atomic ratio expressions listed above do not depend upon the

thickness of the contraband material.

The processor 26 then evaluates one or more of the atomic ratio expressions, and compares the results to a table of data stored in read only memory (ROM) available to the processor 26. If the evaluation results of the atomic ratio expressions compare

favorably with data stored for an average (i.e., devoid of contraband) suitcase, the processor 26 does not send a signal to activate alarm 70. If the evaluation results resembled data stored for any one of a

plurality of contraband-containing suitcases, the processor 26 sends a signal to activate alarm 70.

As indicated above in connection with

Classification Determination Mode #1, the atomic ratio expressions can also be used after values of N, C, O, and H are determined in ways other than peak counts. In fact, when the matrix technique of Classification Determination Mode #1 is employed, additional atomic ratio expressions can be utilized, including atomic ratio expressions involving Hydrogen. A set of atomic ratio expressions appropriate in such circumstances are as follows:

N * (O/C)

(N * O)/(C * H)

N/O

N/C

N/H

O/C

O/H

C/H

For each detector element 42, the evaluated expressions are compared by the processor 26 to stored data available to the processor 26. In this respect, the values for one or more of the expressions are compared to stored sets of values for comparable expressions, including a stored set of values for comparable expressions for an average or normal (i.e., devoid of contraband) suitcase; a stored set of values for comparable expressions for a suitcase containing a first type of contraband substance; a stored set of values for comparable expressions for a suitcase containing a second type of contraband; and so forth for as many types of contraband substances as are of concern.

Table I is an example of stored values for example atomic ratio expressions. In Table I (as well as Table II discussed below), an average suitcase is defined as having a width of 30 centimeters, being constructed of polyurethane; having a wall thickness of .15 centimeters (totaling .30 centimeters for two walls); and containing 4.7 centimeters nylon and 25 centimeters cotton. Table I also shows values for atomic ratio expressions for a 30 centimeter-wide suitcase (wall thickness totaling .30 cm) having (1) only 5 centimeters of polyurethane inside [second row across Table I]; (2) only 5 centimeters of the

explosive Composition B inside [third row]; (3) only 5 centimeters of black powder inside [fourth row]; (4) only 5 centimeters of cocaine inside [fifth row].

Provide in parentheses in Table I are values showing how the atomic ratio expressions for non-average suitcases differ from the value of the expression for the average suitcase, the differences being the

quotient of the non-average suitcase value divided by the average suitcase value.

Table II is yet another example of stored values for example atomic ratio expressions. Table II shows values for atomic ratio expressions for an average suitcase in which five centimeters of cotton have been replaced by (1) five centimeters of polyurethane [row 2 of Table II]; (2) five centimeters of Composition B [row 3]; (3) five centimeters of Black Powder [row 4]; and, (4) five centimeters of cocaine [row 5].

It should be understood that expression

evaluations for additional scenarios for other types of non-contraband containing suitcases can also be

developed and stored, for example in another table in a ROM accessed by processor 26. In connection with examining the specific atomic ratio expression values it calculates, the processor 26 can examine trends established by the values. For example, with reference to Table I, the processor 26 can distinguish between a predominately polyurethanecontaining suitcase and a predominately Composition Bcontaining suitcase by noting the fact that the

expression differences (provided in parentheses) for the atomic ratio expression O/C, O/H, and C/H are greater than unity (1.0) for Composition B, but are less than unity for polyurethane.

Thus, using the stored data available to the processor 26 (such as the data in Table I), the

processor 26 can determine whether, the suitcase

contains polyurethane and other similar plastics; and can also determine the type of explosive or plastic in the suitcase.

When the processor 26 determines that any detector element 42 has detected contraband in

accordance with the aforedescribed classification mode, the processor" 26 activates the alarm 70 in the manner already described.

Classification Determination Mode #3 In another mode of the invention the processor

26 can operate in conjunction with an adaptive learing system, such as a neural network system. As one example of this mode, the total neutron cross section spectra information obtained from each multi-channel analyzer in the array 58 is input into an a pattern classifier. The pattern classifier falls in the group method of data handling category of neural networks, such as the automatic learning network ("ALN")

disclosed in commonly-assigned U.S. Patent 4,213,183 to Barron et al., which is incorporated herein by reference.

The ALN methodology is an empirical approach used to design detection and classification models.

The ALN model synthesis process includes the extractingof a number of candidate features from the total

neutron cross section spectra information obtained from the multi-channel analyzer 58 input. This set of features is then submitted to a model synthesizing program that generates a polynomial network model ("the ALN model") that relates the input feature values to an output variable which, for a classifier, determines the likely identity of substances located by a detector 42 in the sample object 29.

Each primitive element of the ALN network is a polynomial combination of the feature inputs selected for that element or of the output of a preceding

element. For example, for a two-input element whose inputs are X1 and X2, the output is

A₁ + A₂X₁ + A₃X₂ + A₄X₁X₂ + A₅X₁ ² + A₆X₂ ² + A₇X₁ ³ +A₈X₂ 3

2

The classification decision is made based upon the value of the output. That is, if the ALN output is above a pre-determined threshold value, the alarm 70 is activated.

As one example of the operation of the ALN, total neutron cross section spectra were generated for ten suitcases containing explosives and eleven

suitcases without explosives. The ALN of the processor 26 was trained on these two sets of data. Ten

additional spectra were generated for a blind test of the ALN to determine whether the ALN would be able to determine the presence of explosives in previously unseen spectra.

The candidate features chosen for the analysis were generated by dividing the spectra of the suitcases into twenty bands and computing the area under the cross section curve in each band. This area was then normalized to the area under the entire curve. These twenty features were extracted from each of the

training spectra and their values were input to the ALN training program. The training data sets were

classified correctly when the ALN output threshold value was set to 0.0. Sets whose ALN output values were above 0.0 were classified as explosives and sets whose ALN output values were below 0.0 were classified as non-explosives. The ten unknown data sets were then analyzed by the ALN. The ALN program correctly

identified the suitcases containing explosives with no false alarms. Moreover, the ALN was also able to recognize an explosive not used in any of the training data sets.

High Neutron Yield Target Embodiments As mentioned above, an important requirement of the present invention is that the neutron source 20 produce a sufficient number of white neutrons across a sufficiently wide neutron energy range in order to detect the peaks of all elements of concern. The number of neutrons depends on the nature of the target 34 and the number of deuterons produced by the

accelerator 30. An adequate number of white neutrons can ultimately be produced across the entire energy range using a carbon 12 target when a sample object 29 remains situated between the neutron source 20 and the neutron detector assembly 28 for a sufficient length of time. However, in some environments it may be

desirable to detect the contraband more quickly, thereby necessitating a more intense neutron source.

In the above regard, Fig. 6 is a side view of a fast white neutron-producing target 34' utilized in an embodiment of the invention in order to assure the production of a sufficient number of white neutrons across a sufficient neutron energy range, e.g. from 0.5 MeV to 3.0 MeV. The target 34' is a composite target comprising two elements, in particular a first element layer 82 and a second element layer 84. As used herein, the first element layer 82 has a high neutron yield for deuteron energies in a first subrange; the second element layer 84 has a high neutron yield for deuteron energies in a second subrange. The first and second subranges can, and most likely do, overlap within the overall range of neutron energies.

In one embodiment, the first element layer 82 comprises Boron-10 and the second element layer 84 comprises Carbon-13. Boron-10 is chosen as a first element layer 82 in view of the fact that Boron-10 has a high neutron yield for deuteron energies in a first subrange from 2.5 to 3.0 MeV. Carbon-13 is chosen as a second element layer 84 since Carbon-13 has a high yield for deuteron energies in a second subrange from 2.5 MeV down to lower energies, providing ten to twenty times as many white neutrons at the lower energies than does Carbon 12.

In yet another embodiment, the first element layer 82 comprises Oxygen-16 and the second element layer 84 comprises Carbon-13. Since Oxygen-16 is a gas in the pure state, the Oxygen-16 layer 84 is a gas target or is included in a layer of an oxygen compound such as silicon dioxide.

In still another embodiment, the first element layer 82 comprises Beryllium and the second element layer 84 comprises Carbon-13. In this embodiment, deuterons in a deuteron beam (of about 7 MeV) first strike the first layer 82 (Beryllium), lose

approximately 5 MeV, and then interact with the second layer 84 (Carbon-13), losing the remaining energy. The neutron spectra produced by each of the two target layers thus add, producing many more neutrons in the energy range of 0.5 MeV to 3.0 MeV than from a single layer target alone.

It should be understood that other elements may be employed for layers 82 and 84, and that more than two layers can be utilized. Each layer 82, 84 is formed from an element or compound having a large cross section for producing neutrons in a corresponding subrange of the 0.5 MeV to 3.0 MeV range, so that white neutrons are effectively uniformly produced across the entire 0.5 MeV to 3.0 MeV range.

As indicated above, neutrons for the contraband detection apparatus 18 are created by allowing a charged particle beam to impinge on a specially

prepared target. When the charged particle beam strikes the target, some of the individual particles of the beam interact with the target material to produce neutrons. The energy of the resulting neutron depends on the energy of the charged particle, the angle at which the neutron is emitted with respect to the incoming charged particle beam and the nature of the nuclear interaction.

The energy of the resulting neutron for a particular charged particle energy can be solved exactly by solving the conservation equations

(conservation of energy and momentum). If a thick target is used, some of the charged particles will penetrate into the target material, losing energy and hence some of the deuterons will react with the target at lower energy and will produce neutrons at a lower energy. Hence, a beam of charged particles incident at a given energy incident on a thick target can produce a neutron beam with a so-called neutron energy spectrum, i.e., a neutron beam with various neutron energies from a threshold up to some maximum value, determined by the nuclear reaction. By judiciously selecting the charged particle beam energy and the target material, the resulting neutron energy spectrum can be tailored and maximized (to produce the maximum number of neutrons per incident deuteron).

Fig. 6 shows a sandwich type or composite target 34 wherein a charged particle beam passes through at least two target materials 82, 84 as it passes through the target starting at some maximum energy and going to zero energy. Hence, the neutron spectrum is tailored and maximized. The number of neutrons that are

produced per second by a charged particle at a

particular energy is proportional to the number of target nuclei per cubic centimeter present in the target, the current of the charged particle beam, and a proportionality constant known as the cross section for producing the neutrons. The cross section is a

function of the incoming charged particle energy as well as the angle of emission (differential cross section). By integrating the differential cross section over all possible angles, one obtains the total cross section. In general, the apparatus 18 of the present invention makes use of only the differential cross section as it utilizes the neutrons emitted in a small angle around zero degrees with respect to the incoming charged particle beam.

For the contraband detection apparatus 18 of the present invention, the neutron spectrum should extend from about 0.5 MeV up to about 3.0 MeV, and the number of neutrons emitted per second per incoming charged particle should be maximized. Suitable target

materials have the following properties:

(1) The emitted neutrons have a neutron spectrum that extends from about 0.5 MeV to about 3.0 MeV so as to include some major peaks of carbon, nitrogen, and oxygen.

(2) The number of neutrons emitted per second around zero degrees per incident deuteron is as large as possible, so that the measuring time is as short as possible. Hence, suitable target materials have a large differential cross section around zero degrees for producing neutrons in the 0.5 MeV to 3.0 MeV range for incoming deuteron energies of from 3.0 MeV to 8.0 MeV.

(3) Suitable materials are stable under charged particle bombardment and are relatively easy for fabrication.

Figs. 4A - 4E show total cross section neutron spectra produced by the contraband detection system 18 for sample objects containing various substances. Fig. 4a shows a spectrum for a weapon; Fig. 4b shows a spectrum for cocaine; Fig. 4c shows a spectrum for an explosive; Fig. 4d shows a spectrum for hashish; and, Fig. 4e shows a spectrum for a suitcase not having explosives or drugs concealed therein.

The weapon spectrum of Fig. 4a is characterized by a significant oxygen peak (OP) and significant nitrogen peaks (NP). The cocaine spectrum of Fig. 4b is characterized by a significant carbon peak (CP) and a significant nitrogen peak (NP). The explosive spectrum of Fig. 4c is characterized by two significant oxygen peaks (OP) and significant nitrogen peaks (NP). The hashish spectrum of Fig. 4d is characterized by a significant carbon peak (CP).

Unlike the prior art TNA technique, the present invention utilizes the total neutron cross section of nitrogen. The detection system 18 of the present invention is thus orders of magnitude more sensitive than detectors which see only elastic scattering of neutrons. Accordingly, the detection system 18 of the present invention increases the probability of

detection and decreases false alarm probability.

Moreover, the present invention detects not only the presence of nitrogen, but of additional elements such as the explosive-indicative element oxygen and the drug-indicative element carbon. The invention not only determines the presence of elements having neutron-removal energy peaks in the range of white neutron energies, but also the presence of elements which do not have peaks in the range of white neutron energies.

Furthermore, the present invention elegantly detects substantially all significant neutron

interactions and is not constrained to detecting only a fraction of the neutron interactions as in elastic scattering where counters are placed at back angles. In this regard, the present invention has enhanced sensitivity since each neutron directed to the sample object, and which undergoes an interaction in the sample object, is detectable by its absence.

Spatial Location of Contraband Substances

The provision of the detector array 40

facilitates a three-dimensional, spatial location of contraband concealed within a sample object. In this regard, Fig. 5A shows a sample object 29 such as a suitcase positioned in the manner of Fig. 2 between the neutron source 20 and the detector array 40. The sample object 29 is situated on a conveyor 90 of the conveying system 28 for travel in a direction indicated by arrow 92. Direction 92 is parallel with an X axis shown in Figs. 5A and 5B.

During a first pass of the sample object 29 before the detector array 40 , the sample object 29 has the orientation shown in Fig. 5A. The conveyor 90 is stopped when the sample object 29 has a lower leading edge thereof aligned with an imaginary origin labelled as "ORIGIN" in Fig. 5A. When stopped at the ORIGIN, the lower leading edge of the sample object 29 is aligned with detector element 42_1,1 of the detector array 40. At the ORIGIN, in Fig. 5A a front leading vertical edge 29a of the sample object is then aligned with a Y axis as shown in Fig. 5A, and the front leading horizontal edge 29b is then aligned with a Z axis as shown in Fig. 5A.

With the sample object 29 oriented in the manner shown in Fig. 5A, white neutrons from the neutron source 20 are directed toward the sample object 29 and the detector array 40. A total neutron cross section spectrum is generated for each of the detector elements 42 included in the array 40. Each detector element 42 corresponds to a two-dimensional coordinate of a plane of the sample object 29. In the orientation of Fig. 5A, the sampled plane is of the face of the suitcase lying along the XY axes. The sectors of the sample object 29 are labeled with subscripts in similar manner as are the detector elements with which each sample object sector is aligned, but the sectors are further identified by a third subscript indicating position along the Z axis. That is, sectors S_1,1,1through S_1,1,3 are aligned (in the sense of the Z axis) with detector element 42_1,1, sectors S_1,2,1 are aligned with detector element 42_1,2; and so forth.

For illustrative purposes, suppose than an explosive EXP is concealed in the sample object at sector S_12,6,2. During the first pass of the sample object 29 before the detector array 40, with the object 29 having the orientation of Fig. 5, the processor 26, using either of the classification determination modes described above, will determine that an explosive exists in the sample object based on the total neutron cross section spectrum for the detector element 42_12,6. The processor 26 accordingly activates the alarm 70; and stores in its memory data including the identity of the particular detector element(s) which located an explosive.

As a result of the activation of the alarm 70, the sample object 29 is subject to a second pass through the contraband detection system 18 of the invention. During the second pass, the sample object 29 acquires the orientation shown in Fig. 5B. The object 29 is stopped at the ORIGIN with edge 29b on the Y axis and the edge 29a on the Z axis. When the neutron source 20 is turned on, the spectra are again generated for each of the detector elements 42 in the array 40. From the total neutron cross section

spectra, the processor 26 determines that an explosive was located by the detector element 42_12,2. Correlating the first pass data with the second pass data for the sample object, the processor 26 then calculates the sector of the sample object 29 that contains the explosive, i.e., sector S_12,6,2. Thus, the processor 26 is able to pinpoint the three-dimensional spatial location in the object whereat the contraband is concealed.

While the invention has been particularly shown and described with reference to the preferred

embodiments thereof, it will be understood by those skilled in the art that various alterations in form and detail may be made therein without departing from the spirit and scope of the invention. For example, the presence of elements other than Nitrogen, Hydrogen,

Carbon, and Oxygen can be detected. In this regard, in the Classification Determination Mode #1 the known total neutron cross sections of other elements can be included in the calculations to obtain an indication of the presence of those elements in the sample object.

Claims

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. Apparatus for detecting a contraband substance internally located in a sample object, the apparatus comprising:

means for producing a pulsed beam of fast white neutrons having energies in a range from about 0.5 MeV to about 3.0 MeV;

means for detecting neutrons;

means for situating a sample object between the neutron producing means and the detecting means;

means for determining the total neutron cross section spectra of a plurality of elements located in the sample object, the producing means producing a beam of fast white neutrons having a sufficient energy range whereby the total neutron cross section of a plurality of contraband-indicating elements can be determined.

2. A method for detecting a contraband substance internally located in a sample object, the method comprising:

producing a pulsed beam of fast white neutrons having energies in a range from about 0.5 MeV to about 3.0 MeV with a neutron producing means;

situating a sample object between a neutron producing means and a neutron detecting means;

detecting neutrons;

determining the total neutron cross section spectra of a plurality of elements located in the sample object, the producing means producing a beam of fast white neutrons having a sufficient energy range whereby the total cross section spectra of a plurality of contraband-indicating elements can be determined.

3. An apparatus for producing a pulsed beam of fast neutrons comprising means for producing a pulsed beam of deuterons and for directing the pulsed beam of deuterons toward a two-layered target, a first layer of the target having a large cross section for producing neutrons in a first energy subrange and a second layer of the target having a large cross section for

producing neutrons in a second energy subrange.

4. The apparatus of claim 3, wherein the first layer comprises Boron-10 and the second layer comprises Carbon-13.

5. The apparatus of claim 3, wherein the first layer comprises Oxygen-16 and the second layer

comprises Carbon-13.

6. The apparatus of claim 3, wherein the first' layer comprises Beryllium and the second layer comprises Carbon-13.

7. Apparatus for detecting a contraband substance internally located in a sample object, the apparatus comprising:

means for producing a pulsed beam of fast white neutrons;

means for detecting neutrons which are not removed from the beam;

means for determining neutron removal spectra for a plurality of contraband-indicative elements potentially located in the sample object, the producing means producing a beam of fast white neutrons having a sufficient energy range to include energies at which neutrons are removed from the beam due to the presence of the plurality of neutron-removing contraband- indicating elements in the sample object; and,

processing means for evaluating the neutron removal spectra for the plurality of neutron-removing contraband-indicating elements having peak neutron- removal energies in the energy range of the fast white neutrons, and for using the evaluation to determine the potential presence of a further element in the sample object, the further element not having a peak neutronremoval energy in the energy range of the fast white neutrons.

8. The apparatus of claim 7, wherein the processing means further uses the neutron removal spectra

evaluation and the determination of the potential presence of the further element to make a

classification determination regarding a potential presence of a contraband substance located in the sample object.

9. The apparatus of claim 8, wherein the processing means evaluates, at four energies in the energy range of the fast neutrons, the expression

In (N_o/N) = n_Hxs_H + n_cXs_c + n_Nxs_H + n_oXs_o wherein

N_o is the number of incident neutrons without a

sample object in a path between the producing means and the detecting means;

N is the number of detected neutrons with a sample object in a path between the producing means and the detecting means;

n_E is the number of atoms per cubic centimeter of an element (H = hydrogen, C = carbon, N = nitrogen, 0 = oxygen); x is the length of the sample object;

s_E is the cross section of an element; and, wherein the quantities n_Hx, n_cx, n_Hx, n_oX are the number of atoms per square centimeter of the respective elements Hydrogen, Carbon, Nitrogen, and Oxygen in the beam path.

10. A method for detecting a contraband substance internally located in a sample object, the method comprising:

(1) producing a pulsed beam of fast white neutrons with a neutron producing means;

(2) situating a sample object between a neutron producing means and a neutron detecting means;

(3) detecting neutrons which are not removed from the pulsed beam;

(4) determining, for a plurality of contrabandindicating elements potentially located in the sample object, spectra indicating removal of neutrons from the beam, the producing means producing a beam of fast white neutrons having a sufficient energy range whereby peak neutron-removing energies of the plurality of neutron-removing contraband-indicating elements can be determined; and,

(5) using the determination of step (4) to determine the potential presence of a further contraband-indicating element which does not have a peak neutron-removing energy in the energy range of the fast white neutrons.

11. The method of claim 11, further comprising:

using the spectra determination of step (4) and the determination of step (5) to make a classification determination regarding a potential presence of a contraband substance located in the sample object.

12. The method of claim 11, wherein, at four energies in the energy range of the fast neutrons, the following expression is evaluated:

In (N_o/N) = n_Hxs_H + n_cXs_c + n_Nxs_N + n_oXs_o wherein:

N_o is the number of incident neutrons without a

sample object in a path between the producing means and the detecting means;

n_E is the number of atoms per cubic centimeter of an element (H = hydrogen, C = carbon, N = nitrogen, 0 = oxygen);

x is the length of the sample object;

S_E is the cross section of an element; and,

wherein the quantities n_Hx, n_cx, n_Nx, n_ox are the number of atoms per square centimeter of the respective elements Hydrogen, Carbon, Nitrogen, and Oxygen in the beam path.

13. Apparatus for detecting a contraband substance internally located in a sample object, the apparatus comprising:

means for producing a pulsed beam of fast white neutrons;

means for detecting neutrons which are not removed from the beam;

means for determining a neutron removal spectra of a plurality of elements located in the sample

object, the producing means producing a beam of fast white neutrons having a sufficient energy range whereby peak neutron-removal energies of a plurality of contraband-indicating elements can be determined;

processing means for using data from the neutron removal spectra to evaluate a plurality of atomic ratio expressions and using the results of the evaluations to determine the potential presence of a contraband substance.

14. The apparatus of claim 14, wherein said plurality of atomic ratio expressions include (N * O)/C; (N * O)/(C * H); N/O; N/C; N/H; O/C; O/H; C/H; C/O; and H/O, wherein N is

the number of Nitrogen atoms per square centimeter in the neutron beam path, wherein O is the number of

Oxygen atoms per square centimeter in the neutron beam path, and wherein C is the number of Carbon atoms per square centimeter in the neutron beam path; and wherein H is the number of Hydrogen atoms per square centimeter in the neutron beam path.

15. A method for detecting a contraband substance internally located in a sample object, the method comprising:

(2) detecting neutrons which are not removed from the beam with a neutron detecting means;

(3) situating a sample object between the neutron producing means and the detecting means;

(4) determining a neutron removal spectra of a plurality of elements located in the sample object, the producing means producing a beam of fast white neutrons having a sufficient energy range whereby peak neutron-removing energies of a plurality of contraband-indicating elements can be determined;

(5) using data from the neutron removal spectra to evaluate a plurality of atomic ratio expressions;

(6) using the results of the evaluations of step (5) to determine the potential presence of a contraband substance.

16. The method of claim 16, wherein said plurality of atomic ratio expressions include (N * O)/C; (N * O)/(C * H); N/O; N/C; N/H; O/C; O/H; C/H; C/O; and H/O, wherein N is the number of Nitrogen atoms per square centimeter in the neutron beam path, wherein O is the number of Oxygen atoms per square centimeter in the neutron beam path, and wherein C is the number of

Carbon atoms per square centimeter in the neutron beam path; and wherein H is the number of Hydrogen atoms per square centimeter in the neutron beam path.