Contraband detection apparatus and method

A contraband detection system (18) using a single, cone shaped neutron beam determines substances concealed in a sample object by developing total neutron cross section spectra for a plurality of elements, including carbon, nitrogen, oxygen, hydrogen and other potential contraband-indicating elements. A processor (26) performs a contraband determination classification based on the neutron total cross section spectra for the plurality of elements, including hydrogen and elements which do not have peaks in the energy range of interest. The contraband detection system (18) includes a neutron source (20) for producing a pulsed, cone shaped beam 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 neutron total cross section spectra of elements located in the sample object; and the processor (26). The neutron source (20) produces a pulsed beam (36) of fast white neutrons having sufficient energy range whereby removal of neutrons from the beam caused by the presence of a plurality of contraband-indicating elements can be determined. Various techniques of making contraband classification determinations are also disclosed.

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. 
PRIOR ART AND OTHER CONSIDERATIONS 
Small amounts of modern explosives are easy to hide in airport luggage, 
cannot be detected by current systems, and can destroy an airplane. A 
workable system for detecting explosives in airport luggage is urgently 
needed. The most accurate method would be to identify the number densities 
of elements throughout the luggage. The ratios obtained from these number 
densities could be used to identify explosives with great precision. For 
practical use in an airport, each scan would have to be completed in 
seconds. A system this advanced does not exist and is not possible under 
current technology. 
Current methods for detecting explosives in airport luggage use neutral 
particle probes, such as X-rays and neutrons, which can penetrate sealed 
luggage. However, existing systems cannot identify all of the elements 
which comprise explosives and have other shortcomings noted below. 
X-ray systems are sensitive to differences in X-ray absorption coefficients 
in luggage. Because explosives have absorption coefficients similar to 
items commonly found in luggage, X-ray systems, including X-ray computed 
tomography (CT) scanners, have high false alarm rates. 
Thermal neutron absorption (TNA) detects the n,.gamma. reaction on nitrogen 
and so searches only for nitrogen. Since many non-explosive items found in 
luggage are rich in nitrogen, TNA has an unacceptably high false alarm 
rate. Other problems with TNA include that the neutrons must be 
thermalized, the n,.gamma. cross section is in the millibarn range, it is 
difficult to obtain the spatial nitrogen concentration, and the background 
count rate is very high. "Explosive Detection System Based on Thermal 
Neutron Activation", IEEE AES Magazine, December 1989 and "Nuclear-Based 
Techniques for Explosive Detection", T. Gozani, R. Morgado, C. Seher, 
Journal of Energetic Materials, Vol. 4, pp. 377-414 (1986). 
Pulsed fast neutron absorption (PFNA) detects the inelastic scattered gamma 
rays from nitrogen, carbon, and oxygen. Problems with PFNA include that 
the cross sections are in the millibarn range, background count rates are 
very high, determination of concentration as a function of position has 
large uncertainties, and it is difficult to make a gamma ray detector with 
adequate energy resolution and still maintain high count rate capability. 
"PFNA Technique for the Detection of Explosives", Proc. of First-Int. Sym. 
on Explosives Det. Technology, FAA Tech. Ctr., Atlantic City Int. Airport, 
N.J., Feb. 1992. 
Grenier discloses a system based upon the n,.gamma. reaction with pulsed 14 
MeV neutrons. Grenier (U.S. Pat. No. 4,882,121). Grenier's system uses the 
inelastic scattering cross section or partial cross section. Since total 
cross sections are generally 100 to 1000 times larger than inelastic cross 
sections, a system based on total cross sections would be much more 
effective than Grenier's system. Grenier's system is based upon secondary 
interactions (detecting gamma rays resulting from first order 
interactions), would require a long counting time, and does not give 
hydrogen concentrations. 
As noted above, existing nuclear-based systems search for explosives in 
indirect ways, such as detecting gamma rays emitted from neutron 
interactions. A system is needed which can probe directly for explosives 
through first order interactions. The most accurate method would be to 
identify the number densities of the elements which make up explosives. 
Using a fast neutron probe in a neutron transmission/attenuation system 
would be ideal, because the neutrons can penetrate the sample and interact 
directly with the atoms. 
The Federal Aviation Administration Guidelines list numerous nuclear 
techniques for detecting explosives in luggage. "Guidelines for Preparing 
Responses to the Federal Aviation Administration's Broad Agency 
Announcement for Aviation Security Research Proposals, Revision 3, Nov. 1, 
1989" (the "Guidelines"). The Guidelines only briefly describe a fast 
neutron attenuation technique: 
A broad energy spectrum of pulsed neutrons is created. 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 beam position, yield a projected image of the elemental 
distribution in the bag. This method was published several years ago. It 
has not been applied to the airport security problem. 
Guidelines Section 1.1.3.6 at page 7 (emphasis supplied). The technique 
described in the Guidelines is not optimal, for reasons described below. 
The Guidelines state that "some of the methods measure only nitrogen; . . . 
the other methods seek to measure all other major elements in an 
explosive, carbon, oxygen, and nitrogen, by using fast neutrons for the 
interrogation." Guidelines at page 5 (emphasis added). The Guidelines 
technique searches only for elements which "absorb" neutrons corresponding 
to the characteristic neutron resonances of carbon (C), nitrogen (N) and 
oxygen (O). The Guidelines technique is based upon the absorption or 
partial cross section rather than the total cross section. However, the C, 
N and O would "absorb" only a small percentage of neutrons in the beam: 
only those neutrons with energies which are close to the resonance peaks 
of the elements in the beam. Hence the Guidelines technique could measure 
only a small percentage of neutron interactions, which would negatively 
affect both the statistics and the time required to complete a scan. 
A technique which uses the total neutron cross section would be more 
effective than the Guidelines technique. A system based on the total cross 
section would provide better statistics, would be more accurate, and would 
allow faster scanning. For example, consider the 1 MeV oxygen peak. The 
absorption cross section at 1 MeV is at most a few millibarns. In 
contrast, the total cross section is approximately 8.21 barns. Other 
resonance peaks give similar ratios between their resonance absorption 
cross sections and the total cross section. The resonance absorption cross 
sections are in the millibarn range while the total cross sections are in 
the barn range. 
Also, the Guidelines technique cannot detect hydrogen (H), since H does not 
have a resonance peak. Knowledge of the distribution of H in a sample 
would be useful in identifying explosives and other contraband. A system 
using the total cross section, and which can detect H as well as C, N and 
O, would be a significant improvement over the technique described in the 
Guidelines. 
In addition, the Guidelines technique is not optimal for use in an airport 
or for any use which requires a fast scan. The Guidelines describe a 
technique which scans a neutron beam across a suitcase and determines the 
location of the explosive "as a function of the beam position". In order 
to locate an explosive "as a function of the beam position", a system must 
scan the beam over numerous positions across the suitcase, identify the 
elements in the beam at each position, analyze for an explosive at each 
position, and identify the beam position relative to the luggage at the 
time an explosive is detected. Since small explosives can cause extensive 
damage, the beam must scan in small increments. This would require 
numerous scans over a single piece of luggage. A system which scanned an 
entire sample at the same time would be a significant improvement to the 
Guidelines technique. 
The Guidelines note that fast neutron attenuation has not been applied to 
airport security. Fast neutron attenuation has been applied to determine 
the composition of agricultural products, applications which do not 
require fast scanning or position sensitive detection. "Determination of 
H, C, N, O Content of Bulk Materials from Neutron-Attenuation 
Measurements," by J. C. Overley, Int. J. Radiat. Isot., Vol 36, No. 3, pp. 
185-191, 1985. "Element-Sensitive Computed Tomography with Fast Neutrons" 
by J. C. Overley, Nuclear Instruments and Methods in Physics Research, 
B24/25 (1987) pp. 1058-1062. Overley's work used small (2 cm) collimated 
neutron beams and required a considerable amount of time (10 minutes) to 
complete a scan at each location of the beam. In order to scan a suitcase 
60 cm by 75 cm, Overley's method would require hours. In contrast, an 
airport security system requires that an entire suitcase be scanned in 10 
seconds or less. Overley's technique is based upon numerous scans of a 
single sample by one or more neutron beams, could not operate in an 
airport or other environment requiring a fast scan, and is recognized by 
Overley as unworkable under current technology. 
Overley did not describe a workable technique for contraband detection. 
However, the technique described by Overley was a logical extension of his 
collimated beam method, or the use of multiple beams from a single 
accelerator to scan bulk material: 
Capital equipment requirements probably restrict practical application of 
the technique at the present time. Special purpose accelerators are 
beginning to evolve, however, and the possibility of producing several 
neutron beams simultaneously from one machine may reduce this impediment 
in the future. 
Overley (1985) at 191 (emphasis supplied). One of the limitations of 
current technology is that existing accelerators do not produce multiple 
beams. Even if a multiple beam accelerator is developed, it is doubtful 
that it could produce enough beams for practical use in a contraband 
detection system. For example, to cover an entire suitcase in a single 
scan would require hundreds of beams. Overley recognized these limitations 
under current technology when stating that the method is not of practical 
application at this time. Over a decade after publication of Overley's 
work, as noted in the Guidelines, a workable system has not been developed 
which can apply fast neutron attenuation to detect explosives and other 
illicit contraband in airport luggage. 
A system using only one neutron beam to scan an entire sample at one time 
would be an improvement over current proposals, which would use multiple 
beams or multiple scans. However, a single beam, single scan system would 
require solutions to several problems that are not obvious and are not 
anticipated by the Guidelines, Overley, or other references. 
One problem unsolved under current technology in creating a single beam, 
single scan system relates to the neutron probe. The methods outlined by 
Overley and the Guidelines use one or more collimated neutron beams. In 
contrast, a single beam, single scan system requires an uncollimated beam 
which expands in a cone shape, so that a sample object can be placed at 
the arc at the end of the cone for coverage by the single beam. A single 
beam, single scan system should use a neutron beam with an angular 
distribution of neutrons relatively flat around 0 degrees. This flat 
angular distribution would be required in order to obtain constant 
statistics across the sample. 
Another problem unsolved under current technology in creating a single 
beam, single scan system relates to the detection system. A system using a 
single cone shaped beam to scan luggage in a single pass requires a 
detection system which can detect small amounts of explosives and pinpoint 
their location in the luggage. A workable detection system would require 
numerous detectors with a relatively small spatial resolution. For 
example, a 4 cm by 4 cm spatial resolution is generally required in order 
to locate lethal amounts of explosives. To cover a 60 cm by 80 cm 
suitcase, a system would require approximately 300 detectors. In general, 
the detector array would be even larger to cover larger containers or to 
cover containers with a smaller spatial resolution (perhaps up to 625 
detectors). Each detector would require its own electronics. A system 
using 625 discrete detectors requires 625 electronics systems, processes 
hundreds of thousands of neutron interactions per second, and has only 10 
seconds (or less) to complete a scan of an entire suitcase, analyze the 
elemental distributions in the suitcase, make a classification regarding 
contraband, and sound an alarm. The large number of detectors would 
require optimizing the electronics and data analysis systems. A detection 
system meeting these requirements does not exist. 
Another unsolved problem in developing a single beam, single scan system 
under current technology is configuration for neutron time of flight 
measurements. As noted above, the detection system must be optimized in 
order to handle hundreds of thousands of neutron detection events over 
hundreds of detectors and include time of flight measurements. Simply 
stacking neutron detectors into a two-dimensional (x-y) array would allow 
detection of neutrons over increments of a sample placed between the beam 
and the detector array. However, an x-y array would cause neutrons of the 
same energy to register different times of flight for each detector, since 
the distance from the neutron source to each detector would vary. At a 
minimum this would require complex electronics and calculations which 
would correct every detector for every detection event. This problem is 
significant, since, as noted above, a single beam, single scan system 
would require many detectors and must be optimized. 
While x-y detectors have been constructed for thermal neutrons, such 
detectors could not be used in a fast neutron attenuation system. In 
general, thermal neutron detectors cannot be used to detect fast neutrons 
due to the lower detection efficiency. These detectors allow thermal 
neutrons to interact with an element that has a large fission cross 
section. A CCD camera placed outside the thermal neutron beam records the 
resulting scintillation and its position. E. W. McFarland, R. C. Lanza and 
G. W. Poulos, "Multi-dimensional Neutron-computed Tomography Using Cooled, 
Charge-Coupled Devices," IEEE Transactions on Nuclear Science, Vol. 38, 
No. 2, April 1991. A variation includes a neutron camera, which also must 
be used with thermal neutrons. Sulcoski and Brenizer "Neutron Radiography" 
by John P. Barton, 753-760, D. Reidell Publishing Company, Boston, 1986. 
Another variation uses an element that absorbs the thermal neutrons and 
emits x-rays or gamma rays, which are detected with film or scintillation 
sensors. Crispin, Roberty and Reis "Neutron Radiograph" by S. Fujinne, 
865-872, Kluwer Academic Publishers, London, 1989. 
The above types of x-y detectors will not satisfy the requirements for a 
fast neutron detector. A principal reason is that the cross section for 
fission is very small for fast neutrons and fission detectors have a very 
low efficiency. Also, such detectors are not configured for time of flight 
measurements. 
X-y detectors for fast neutrons do exist, but cannot be used for time of 
flight or neutron attenuation measurements. One type is the multi-wire 
proportional counter (MWPC) with a proton radiator at the entrance to the 
MWPC. "Neutron Radiography" by John P. Barton, 829-836, D. Reidell 
Publishing Company, Boston, 1986.; K. H. Valentine, S. Kaplan, V. 
Perez-Mendez and L. Kaufman, "A Multi-wire Proportional Chamber for 
Imaging Thermal, Epicadmium, and Fast Neutrons" IEEE Tr. on Nucl. Sc., 
Vol. NS 21, NO. 1, 1974, 178-183; B. Director, S. Kaplin and V. 
Perez-Mendez, "A Pressurized Multi-Wire Proportional Chamber for Neutron 
Imaging," IEEE Tr. on Nucl. Sc., Vol. NS-25, No. 1, Feb. 1978, 588-561. 
The MWPC consists of thin gas filled cells with small wires running 
parallel through the cells. The wires are s placed at high voltage and 
when a proton enters the cell close to a particular wire, a voltage pulse 
is created. By recording the position of the voltage pulse from a 
particular wire, the position of the event is known in the direction 
perpendicular to the wires. By placing a second ionization chamber with 
wires running perpendicular to the first set of wires, the position in the 
other direction is determined. 
A basic problem with this type of fast neutron detector is that the 
radiators must be very thin so that the recoil protons can escape from the 
radiator. In order to achieve reasonable efficiencies, many of these units 
must be placed in tandem. This problem is compounded when counting 
neutrons below 3 MeV. This is because the radiator would need nearly zero 
width for the lower energy protons to get through the first cell, making 
the efficiency near zero. 
De Volpi discloses a method for high-resolution radiography by using gamma 
rays or neutrons and a hodoscope. De Volpi (U.S. Pat. No. 4,092,542). De 
Volpi's system measures changes in the density of sample materials and is 
not workable in a neutron attenuation system using time of flight 
measurement. Also, De Volpi uses nuclear reactors as his source of 
neutrons and so the neutrons are in the KeV energy range or lower. 
Although De Volpi does not mention the type of neutron detector, detectors 
for KeV energy neutrons and lower energies generally are not useful for 
detecting neutrons in the MeV energy range. Neutron detectors for a 
workable fast neutron attenuation system must be capable of nanosecond 
timing resolution. There is no such timing requirement for De Volpi's 
patent. While De Volpi apparently stacks detectors vertically, the 
detection system is not configured for time of flight and no discussion is 
provided regarding the detection system. 
Another class of x-y fast neutron detectors uses a number of 
photomultiplier tubes placed behind a scintillator. Strauss (U.S. Pat. No. 
4,454,424). When neutrons are incident on the scintillator, some of the 
neutrons are absorbed and cause scintillations via fission. The recoil 
fission fragments create pulses of light which are detected by the 
photomultiplier tubes. The x-y position of the neutron interaction is 
determined by the particular photomultiplier tube which senses the light 
pulse. The Strauss detector uses a glass scintillator loaded with 
Lithium-6, which is not sensitive to fast neutrons. The Strauss detector 
does not measure neutron energy. The Strauss detector measures neutron 
interactions only on an x-y plane and so is not appropriate for use in a 
fast neutron attenuation system requiring time of flight measurements. 
Broadhurst (U.S. Pat. No. 5,278,418) discloses a technique to detect 
nitrates in a sample. Broadhurst's system detects only nitrogen and 
oxygen. Broadhurst's technique involves creating an energy variant neutron 
beam for measurement of neutron transmissions on and off the neutron 
resonances of nitrogen and oxygen. In this way, the Broadhurst technique 
seeks to infer the amount of nitrogen and oxygen present in a suitcase. 
Hence the Broadhurst technique measures the neutron attenuation over a 
very small energy interval using complicated equipment. A much better 
technique would be to measure the neutron attenuation over an energy range 
of several MeV. 
Gomberg discloses an explosive detection system based only on elastic 
scattering cross sections. Gomberg (U.S. Pat. No. 4,864,142). Gomberg 
describes a low count rate system because neutrons scatter at all angles, 
and his detectors are placed at back angles and so intercept only a small 
fraction of the scattered neutrons. Gomberg's neutron source must be 
varied from 0.1 to 4.2 MeV, which is a complicated procedure and 
cumbersome to implement. An airport system based on Gomberg's method could 
take hours to scan a single piece of luggage. 
In summary, no existing contraband detection system applies fast neutron 
attenuation over a broad energy range to identify explosives. Existing 
proposals are based upon multiple beams or multiple scans of a sample. No 
existing or proposed technique would allow detection of 
contraband-indicating elements which do not have a resonance peak, such as 
hydrogen. Current technology would allow only multiple scan or multiple 
beam systems, which, even if developed in the future, would be impractical 
for any use requiring a fast scan. Existing types of x-y detectors will 
not allow the accurate time of flight measurements required by a fast 
neutron attenuation system. Current technology and prior art do not teach 
how a fast neutron attenuation system could be built to solve these 
problems. 
OBJECTS 
Accordingly, it is an advantage of the present invention to provide an 
accurate and fast method and apparatus for detecting and identifying 
contraband substances. 
Another advantage of the present invention is to allow all portions of a 
sealed container to be analyzed simultaneously, by applying a single, cone 
shaped neutron beam. 
Still another advantage of the present invention is the use of the total 
neutron cross section to detect contraband. 
Yet another advantage of the present invention is the detection of 
contraband having small mass. 
Yet another advantage of the present invention is detection of hydrogen, 
which does not have a resonance peak. 
An additional advantage of the present invention is a method and apparatus 
for allowing time of flight measurement of neutrons over a 
multi-dimensional curved plane (R-.theta.-.phi. with constant R). 
SUMMARY 
A contraband detection system produces a single, cone shaped, pulsed white 
neutron beam with a relatively flat neutron angular distribution around 
zero degrees. A sample is placed in the beam at a point at which the beam 
has expanded sufficiently to cover the entire sample, allowing a 
simultaneous scan of the entire sample. The transmitted beam is then 
examined and compared to the original beam. The contraband detection 
system determines substances concealed in a sample object (such as 
luggage) by using the neutron total cross section spectrum for hydrogen 
and a plurality of other elements, including nitrogen, oxygen and carbon. 
A processor performs a contraband determination classification based on 
the spectra for the plurality of elements. The contraband detection system 
measures the neutron attenuation spectra for the sample and, using the 
total cross sections, determines the number densities of carbon, nitrogen 
and oxygen which possess resonance peaks, and provides the number density 
for hydrogen which does not have a resonance peak. 
The contraband detection system includes a neutron point source for 
producing a pulsed beam of fast white neutrons in the shape of a cone with 
a relatively flat neutron distribution around 0 degrees; a spatial neutron 
.theta.-.phi. detection array (R-.theta.-.phi. with constant R), which 
records fast neutrons at neutron energies from approximately 0.5 MeV to 
beyond 15 MeV; means for situating a sample object between the source and 
the detection array; a spectra analysis system for determining the neutron 
attenuation spectra of substances located in the sample object; and the 
classification processor. 
The neutron point source produces pulsed fast white neutrons having a 
sufficient energy range whereby removal of neutrons from the beam (by 
absorption or scattering) caused by a plurality of contraband-indicating 
elements is used to determine the neutron attenuation spectra of a sample 
object. 
The .theta.-.phi. detector array comprises an array of neutron detector 
elements arranged to form a curved surface. Each of the detector elements 
is 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. In one 
embodiment, the surface of the detector array is in the shape of a portion 
of a sphere, so that all detectors in the array are equidistant from the 
neutron point source. Various techniques of making a contraband 
classification determination are also disclosed.

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 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 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 2.0 MeV and 2.5 MeV. The accelerator utilizes 
a negative ion source at ground potential and accelerates the negative 
ions to the energy of 2.0 MeV to 2.5 MeV. The ions are then doubly 
stripped and accelerated back to ground at the opposite end of the 
accelerator, gaining another 2 to 2.5 MeV, giving them a total energy of 4 
MeV to 5 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 approximately 0.5 MeV to at least 5.0 MeV. The beam has a 
relatively flat neutron distribution and is configured to approximate the 
shape of a cone in order to scan an entire sample object at the same time. 
In the embodiment shown, the neutron detector array 40 is placed about 
three to six meters away from the target 34 along the flight path of the 
neutrons 36. The neutron detector array 40 is comprised of an 
.theta.-.phi. array 40 of neutron detector assemblies 22. The detector 
array 40 includes enough detectors to cover a large suitcase with a 
spatial resolution of 4 centimeters by 4 centimeters. Although not shown 
as such, shielding is provided around the detector array 40. 
The particular detector array 40 shown in FIGS. 2a and 2b includes 
twenty-five columns of detector elements 42, with each column consisting 
of twenty-five detector elements 42. Thus, six hundred twenty-five neutron 
detector elements 42 are provided in the array 40. 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. 
FIG. 3 shows a schematic diagram for the electronics. 
The neutron detector assembly 22 is comprised of a neutron detector element 
42, a photomultiplier tube 44, and a voltage divider 46. The 
photomultiplier tubes 44 have less than a nanosecond rise time and each 
voltage divider 46 is connected through an amplifier 48 to the spectral 
analysis system 24. 
The spectra analysis system 24 includes a deuteron beam pick-off 50; a time 
pick-off controller 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 array 60. 
The neutron detector assembly 22 can acquire configurations other than that 
described above. For example, the detector elements 42, photomultipliers 
44, and pulse shape discrimination circuit 60 can be replaced with 
scintillation and detection apparatus disclosed in my patent Miller (U.S. 
Pat. No. 5,155,366) Oct. 13, 1992, entitled Method and Apparatus for 
Detecting and Discriminating Between Particles and Rays, incorporated 
herein by reference. 
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 signal from the beam pick-off 50 causes the time 
pick-off 52 to generate a real time "stop" 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 the array 56 is 
associated with a corresponding one of the detector elements 42, and 
accordingly is associated with a corresponding one of the photomultiplier 
tubes 44. Each of the TAC units in array 56 is connected to receive a real 
time "start" pulse from the neutron detector assembly 22. Thus connected, 
each TAC in array 56 receives a real time stop pulse from the time 
pick-off 52 through a time delay 100 as the deuteron bunch travels through 
the beam pick-off 50. 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 
photomultiplier tube 44 and voltage divider 46, sends a real time "start" 
pulse to the associated TAC ("the activated TAC") in array 56. After the 
delayed stop pulse arrives at the activated TAC 56, the TAC 56 then 
generates a signal having an amplitude proportional to the time-of-flight 
from the beam pick-off 50 to the neutron detector element 42. 
In order to make the TAC units in array 56 more efficient, other 
embodiments can use signals from the detector elements 42 as a stop signal 
and signals from the time pick-off as the start signal as is well known in 
the prior art. 
The pulse shape discrimination circuit 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 discrimination circuits in 
network 60 discriminate gamma rays from neutrons for the multi-channel 
analyzer array 56, resulting in reduced background. 
The multi-channel analyzer array 58 includes a multi-channel analyzer (MCA) 
58 for each converter in TAC array 56. For the embodiment illustrated in 
FIG. 3, there are 625 MCAs in array 58. Each MCA 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 MCA 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 outputs which are 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 MCAs 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 noted, 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 neutron attenuation spectra for the plurality of 
contraband-indicating elements. In this regard, as noted, the output of 
each MCA 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 MCAs included in the MCA system 58 in order 
to produce a neutron attenuation 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 MCAs 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 neutron attenuation spectra for each neutron 
detector element 42 included in the neutron detector array 40. Data 
indicative of the neutron attenuation spectra 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 neutron attenuation 
spectra 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. 3. 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 
photomultiplier tubes 44 can be any suitable commercially available tubes, 
such as those manufactured by Burle as model 8575, or the HAMAMATSU R2083. 
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, most will have peaks in their 
neutron attenuation spectra at energies at which neutrons are removed from 
the beam. To this end, operation of the contraband detection system 18 of 
the present invention is optimum if several peaks or distinguishing 
features, which are not overlapping, for the contraband-indicative 
elements are present. Although hydrogen does not have a peak, the amount 
of hydrogen can be ascertained using particular classification 
determination techniques, known as the matrix or regression techniques. 
FIG. 4 is a graphic depiction of the superimposed total neutron cross 
section curves for hydrogen, carbon, nitrogen, and oxygen. The neutron 
cross section curves (Evaluated Nuclear Data Files) are available from 
Brookhaven National Laboratory and Oak Ridge National Laboratory. As shown 
in FIG. 4, there are several non-overlapping peaks for nitrogen, oxygen, 
and carbon. 
The peaks shown in FIG. 4 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 MeV that can also be used. There 
is a large oxygen peak at 1 MeV. 
Thus, if oxygen is present in a sample object, the presence of oxygen is 
signaled 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 attenuation 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. 
It is thus understood that the contraband detection system 18 of the 
present invention analyzes the neutron attenuation spectra for three 
elements (C, N, and O) which have neutron-removal peaks in the range of 
fast neutron energies, and a further element (H) which does not have a 
neutron-removal peak in the range of fast neutron energies. 
The processor 26 can utilize software including regression theory to 
determine not only the number of atoms per square centimeter for each of 
the contraband-indicating elements, but also a standard error associated 
with each element. An example of such software is Excel for Windows 
produced by Microsoft, which provides regression theory capability in 
connection with its advanced mathematical tools. 
To determine the number densities of the sample, known total neutron cross 
sections for each element for each energy in the energy range of interest 
are supplied to the processor 26 as independent variables. For each 
detector element 42, values of ln (N.sub.o /N), with the N values having 
been obtained from the associated MCA 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 atoms per square centimeter for 
each contraband-indicating element, as well as the standard error for each 
of the contraband indicating elements. 
The total cross sections used as the independent variable can be obtained 
from the ENDF cross sections and approximately "smeared" to fit the energy 
resolution of the spectrometer or they could be measured with the 
spectrometer. It has been found by the author of this patent that 
measuring the total cross sections with the neutron spectrometer gives the 
best results. T. G. Miller, "Application of Fast Neutron Scattering 
Spectroscopy (FNS/R) to Airport Security," SPIE Vol. 1737 Neutrons, X-rays 
and Gamma Rays (1992). FIG. 4 shows a graph of the total cross sections of 
H, C, N and O. FIG. 5 shows a graph of the measured neutron attenuation of 
an "average" suitcase, 4 cm of the explosive C-4, and 4 cm of the 
explosive C-4 imbedded in an "average" suitcase. The various peaks of C, 
N, and O are indicated. As can be seen from FIG. 4, adding the explosive 
to the suitcase dilutes the pure explosive spectrum to some extent, but 
most of the features of the explosives attenuation spectrum are 
maintained. FIG. 6 shows a regression theory fit to the C-4 attenuation 
curve of FIG. 5. As can be seen, the fit is good. FIG. 7 gives the 
regression theory statistics for the curve fit of FIG. 6. FIG. 7 gives an 
R-Squared of 0.997 and, as can be seen, the number densities of H, C, N 
and O are all determined with a standard error of less than 0.7%. 
For each detector, the resultant number of atoms per square centimeter for 
each of the four elements N, C, H, and O 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, the resultant 
numbers can be evaluated using atomic ratio expressions, (C/O, N/O and 
H/C), where the experimentally determined ratios are compared to the 
ratios of explosives, and a determination is made. It has been shown by 
the author of this patent that neural networks can be used to quickly 
optimize such data for the presence of explosives. "Decision Making Using 
Conventional Calculations Versus Neural Networks for Substance 
Identification," T. Gill Miller, SPIE Vol. 2093, pp. 182-193 (1993). 
Thus, by using the stored data which is available to the processor 26, 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 
classification mode described above, the processor 26 activates the alarm 
70 in the manner already described. 
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 N, C, H, and O 
can be detected. In this regard, 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.