Patent Number: 050769936
Section: description

DETAILED DESCRIPTION OF THE INVENTION The following description is of the best mode presently contemplated for carrying out the invention. This description is not to be taken in a limiting sense, but is made merely for the purpose of describing the general principles of the invention. The scope of the invention should be determined with reference to the claims. Referring first to FIG. 1, a contraband detection system 10 made in accordance with the present invention is schematically depicted. The system includes a pulsed accelerator 12 that generates a collimated pulsed neutron beam 13. The beam 13 is directed towards an interrogation chamber 18 defining a path 15. An object 14 to be investigated is passed by means of a conveyor belt 16, or equivalent movement mechanism, through the interrogation chamber 18 in a direction 17 transverse to the beam path 15. Thus, at any instant of time, the beam 13 passes through a known volume of the object 14, where this volume is centered around the beam path 15. (As explained more fully below, the kinematics of the neutron producing reaction cause the beam to form a narrow cone about the desired beam path 15.) It is to be emphasized that while the object 14 shown in FIG. 1 is depicted as a suitcase, this is only exemplary as many different types of objects, of all different sizes, may advantageously be scanned for contraband in accordance with the teachings of this invention. A detector array(s) 20 is positioned adjacent the beam path 15, typically mounted inside the interrogation chamber 18, or equivalent casing, through which the object passes as it is probed with the penetrating pulsed neutron beam. This detector array 20 includes one or more gamma ray detectors that detect any gamma rays incident thereto that are emitted from the object 14 as a given pulse of the beam 13 passes therethrough. Each gamma ray thus detected causes a detection signal to be generated that is sent to process circuitry 22. The process circuitry 22 measures the time at which the neutrons are produced, the energy of the detected gamma ray, and the time at which the detection signal is received. Knowing when a given pulse of the neutron beam 13 starts, and measuring the time when a given gamma ray is detected, it is thus possible for the process circuitry 22 to determine the time of flight of the neutron in the pulsed beam 13 responsible for producing the detected gamma ray. This time-of-flight information, coupled with the known velocity of the neutron as determined by the kinematics of the neutron producing reaction, allow the process circuitry 22 to make a fairly accurate determination as to the location within the object 14 along the beam path 15 whereat the gamma ray originated. The determination of the particular location of the origin of the gamma ray within the object 14 is based on the time-of-flight measurement, and on the fact that the velocity of the neutron is appreciably slower than the velocity of the gamma ray. For example, assuming the fast neutron has an energy of 10 MeV, the velocity of the neutron will be on the order of 4.37.times.10.sup.7 m/sec. In contrast, an emitted gamma ray has a velocity that is very nearly the speed of light (3.times.10.sup.8 m/sec). Thus, e.g., the gamma ray that is produced as a result of a nuclear interaction between the neutron and an atomic nucleus will travel to the detector much faster than the scattered neutron. Accordingly, one may assume that the measured time-of-flight is the time of flight of the neutron that induced the emission of the detected gamma ray. Nonetheless, as explained below, corrections can be made as required in order to account for the gamma ray travel time portion of the time-of-flight measurement. Because the energy of the detected gamma ray allows the particular element from which the gamma ray originated to be identified, as is commonly done in nuclear-based detection systems, the system shown in FIG. 1 is thus able to determine directly the location of a particular element within the object 14. By scanning the object 14 with the beam 13 in a controlled fashion, so that substantially all relevant portions of its volume have a pulsed neutron beam pass therethrough during some portion of the scanning operation, it is thus possible to make a direct determination of the distribution of the elements within the object (or at least the elements of interest, i.e., those commonly found in the particular type of contraband being detected). This distribution information may be considered as an elemental map, and may be represented by appropriate data signals available as output data from the process circuitry 22, e.g., on data line 26. From this data, a direct finding may be made as to whether the object 14 likely contains any contraband, as the composition and distribution of elements within the object provides a valid indicator as to whether such elements are contraband, e.g., explosives or illicit drugs. If a finding is made that the object may contain contraband, appropriate control signals are generated, either by the process circuitry 22 or an auxiliary computer coupled thereto, to trigger an alarm (audible and/or visual) to automatically divert the object 14 to a location where further examination may be performed. In some embodiments, the elemental map may be displayed on a CRT screen. Such map, while being displayed on a two-dimensional surface, may nonetheless appear as a three-dimensional image. Advantageously, using conventional computer aided graphics capabilities, the volumes of interest of the object thus displayed may also be selectively turned, rotated, enlarged, etc. Such an image display of the object advantageously assists an operator in further analyzing whether the object contains contraband. Thus, as indicated from the above overview, the basic concept of the present invention is to use a pencil beam of pulsed energetic neutrons whose pulse length is much shorter than the travel time of the pulsed neutrons across the whole object to be interrogated. Through timing measurements, the position of the neutron pulse along the beam can be determined, thereby allowing the contents of the object at the particular volume element (voxel) where the neutron pulse encounters atomic nuclei to be determined. The neutrons interact in various ways (mostly by inelastic scattering) with the nuclei inside the interrogation volume. The intensities of the characteristic de-excitation gamma rays resulting from these reactions determine the relative abundance of the elements within that volume. It is contemplated that at least two methods may be used to produce the collimated beam of neutrons. The first method exploits the large center of mass velocity obtained when a heavy ion beam (HI) impinges on a hydrogen nucleus (H) with energies near the threshold for the H(HI,n)HI' [or (n,p) threshold in the c.m. system] reaction. Product neutrons produced from this method are confined to a forward cone with a small open angle, as shown schematically in FIG. 2. In FIG. 2, the c.m. velocity (v.sub.cm) of the system is large because it is supplied by the massive incoming heavy ion. Close to the threshold, the velocity component of the neutron in the c.m. system (v.sub.n,cm), shown in FIG. 2, is small. The outgoing neutrons are thus focused in the laboratory system into a cone whose maximum angle is given by sin.PSI..sub.L,MAX =v.sub.n,cm /.sub.cm. The second method of producing a collimated beam of neutrons is based on the forward peaking of neutrons produced in the D(d,n).sup.3 He reaction using energetic deuterons and an external collimator. Referring next to FIGS. 3A and 3B, a possible technique for scanning an object with a D(d,n) neutron beam is shown. FIG. 3A shows an end schematic view and FIG. 3B shows a top schematic view of an object 14 being investigated in accordance with the present invention. The point S represents a pulsed neutron source. A pulsed beam 13 originates at the source S, following a path 15. This pulsed beam 13 is bounded within a fan shaped cone 13a, for the reasons previously explained (see FIG. 2). A suitable collimator (not shown) may be used to narrow the fan shaped cone in one direction, e.g., as seen in the top view of FIG. 3B, thereby directing the beam to pass through a narrow slice defined by the dotted lines 14a and 14b of the object 14. Appropriate detectors 20a, 20b, 20c and 20d may be positioned on either side of the object 14. Each detector 20a, 20b, 20c or 20d may comprise a single detector, or an array of a plurality of detectors, as desired. Other detector configurations and placements may also be used. In operation, the narrow fan pulsed beam 13 passes in between the detectors 20a and 20b into the narrow volume slice of the object 14 bounded by the lines 14a, 14b. Any pulsed neutrons that strike the detectors 20a-d will create a large number of background gamma rays. Advantageously, however, such background gamma rays can be readily excluded using the time-of-flight measurement techniques herein described. (Using such techniques, gamma rays originating within detectors 20a and 20b would be detected too soon and gamma rays originating within detectors 20c and 20d would be detected too late, to have possibly originated within the object 14. Thus the measurement of such early and late gamma rays can be advantageously excluded, thereby eliminating a large source of background noise.). Only gamma rays originating within the narrow slice volume 14a, 14b would arrive at one of the detectors 20a-d within the appropriate time window. Thus, only these gamma rays provide a measure of the constituent elements within the volume slice bounded by the lines 14a, 14 b. As the object 14 moves relative to the beam 13, a different volume slice (voxel) of the object is investigated. As the entire object 14 moves in front of the beam 13 (or as equivalent relative motion is created between the object 14 and the beam 13, using appropriate scanning means, which relative motion is represented by the arrows 17a,), all of the volume slices within the object may thus be investigated. Moreover, as explained below in connection with FIG. 4, the measured time of arrival of a particular gamma ray within the acceptable time window further defines the location within the object 14 where the gamma ray originated. It will be appreciated, of course, that numerous variations of the basic scanning method shown in FIGS. 3A and 3B could be used. For example, multiple pulsed neutron sources could be used, each directing a pulsed neutron beam at a different side of the object 14. (In FIGS. 3A and 3B, for example, another pulsed beam source S' is shown on the side of the object 14 opposite the source S.). Additional collimators may be used, as desired, to limit the volume slice irradiated by the pulsed beam to a fraction of the volume slice bounded by lines 14a, 14b, e.g., to one-half, one-third, or one-fourth thereof. Appropriate means may then be used, either my moving the object 14 and/or moving or steering the beam 13, in order to ensure that the entire object 14 is sufficiently irradiated by the pulsed beam(s), voxel-by-voxel. FIG. 4A illustrates an enlarged section of the object 14 and depicts the concept of "voxels", or small volume elements, through which the pulsed fast neutrons travel. As explained above, when the pulsed beam of fast neutrons 13 is directed to a spot P on the surface of the object 14, it typically propagates through the object 14 within a region defined by a narrow cone (see FIG. 2). Thus, any particular neutron within the beam 13 may actually impinge anywhere within an area Al on the surface of the object 14, rather than on a spot P centered in the area A1. For the purpose of clarity in illustrating the concept of voxels, the area A1 in FIG. 3A is shown as a square, thereby allowing the voxels to be illustrated as cubes. In actuality, however, it is to be appreciated that the area A1 is normally rectangular or circular, and the voxels thus assume a shape that is more like solid sections of a cylinder than a cube. Behind the area A1 lies a small volume element, or voxel, V1 through which the beam 13 initially passes. The depth of the voxel is mainly determined by the length of the neutron burst. Similarly, behind the voxel V1 lies a second small volume element, or voxel V2, also centered about the beam path 15, through which the pulsed fast neutron beam 13 passes after it has traversed voxel V1. Other voxels, V3, V4, V5, V6, and so on, are also centered about the beam path 15 and define a string of small volume elements through which the beam 13 may pass after if has traversed through the first voxel. As the beam 13 passes through a given voxel V.sub.n, it may encounter an atomic nucleus, in which case a gamma ray may be emitted. For example, in FIG. 4A, a gamma ray 40, represented as a wavy arrow, may be produced as a result of a nuclear interaction between a neutron in the pulsed beam 13 and an atomic nucleus within voxel V1. This gamma ray 40 may be detected at spot 42 with a suitable gamma ray detector. Similarly, other gamma rays 44 and 46 may be produced within voxels V3 and V6, respectively, as a result of nuclear interactions occurring therewithin. Once a gamma ray is produced within a given voxel V.sub.n, it may travel out unscattered from the voxel and be absorbed in a gamma ray detector. If desired, of course, detectors may be positioned at several locations, around the voxels so as to increase the probability that a given gamma ray will be detected. As represented in FIG. 4A, the gamma rays 44 and 46 travel to the same detector location 48, whereas the gamma ray 40 travels to a different detector location 42. As explained more fully below in connection with FIG. 4B, it matters little which particular detector senses a gamma ray. All that matters is that the gamma ray is detected and that the time of its detection is noted. FIG. 4B illustrates the manner in which the approximate location, i.e., the particular voxel, of the origin of a given gamma ray is determined. This determination is made based upon the time of flight of the neutron. The time of flight of the neutron is measured by noting the time the neutron starts in the pulsed beam 13, represented in FIG. 4B as time t.sub.0, and by measuring the time of arrival of a gamma ray at any detector, which time of arrival for any given voxel n may be represented as t.sub.n. The kinematics of the neutron producing reaction are known, and hence the velocity of the neutron is known. Further, the time of flight of the neutron represents the predominant portion of the total time of flight of the neutron and gamma ray. In other words, for most purposes, the time of flight of the gamma ray represents only a small portion of the total time between the start time of a neutron, at time t.sub.0, and the arrival time of an associated gamma ray, at time t.sub.n. Hence, for most purposes, the gamma ray flight time can be ignored. Thus, the relative location of the atomic nucleus within the object 14 responsible for producing a given gamma ray is proportional to the arrival, time t.sub.n (and in fact, the approximate location of gamma ray origin can be calculated from the arrival time t.sub.n and the known velocity of the neutron, v). This concept is illustrated in FIG. 4B where the depth or location of the source of gamma ray 50 is at distance 1=vt.sub.1, where t.sub.1 represents the arrival time of the gamma ray. Similarly, the location of the source of gamma ray 52 is at distance 1.sub.2 =vt.sub.2 ; and the location of the source of gamma ray 54 is at distance 1.sub.n =vt.sub.n, where t.sub.2 and t.sub.n represent the arrival times of gamma rays 52 and 54, respectively. By noting that the distance 1.sub.1 is less than the distance 1.sub.2, and by knowing the location of the interrogated object, i.e., the spacing between the source of the neutron beam and the object being interrogated, it is a simple matter to show that the gamma ray 50 originated from within voxel V1 and the gamma ray 52 originated from within voxel V2. Advantageously, because the depth of the location of the origin of the gamma ray along the beam path 15 is proportional to the arrival time of the gamma ray, the particular voxel V.sub.n from which a gamma ray arriving at time t.sub.n originated can be simply determined by measuring by the value of t.sub.n. It is noted that the voxel interrogated by the pulsed fast beam need not be limited to a small voxel relative to the total volume of the object 14, as is suggested in FIG. 4A. Rather, for some applications, a particular voxel may be, e.g., a substantially large fraction of the object 14 being investigated. In fact, in some instances, a single voxel may comprise the entire volume of interest of the object being examined. Moreover, while several voxels may typically have to be examined in order to confirm the presence of contraband within the object 14, it will be sufficient in some instances for the invention to simply detect the appropriate elements indicative of contraband in a single voxel, in which case the object can be immediately flagged as one having contraband. That is, as the pulsed fast neutrons search the object for contraband voxel-by-voxel, there is no need to continue searching once contraband has been identified in one voxel. Rather, the searching can stop, the object can be flagged, and the next object to be investigated can be searched, thereby increasing the average throughput of the system. Referring next to FIG. 5A, a schematic diagram of a preferred pulsed fast neutron direct imaging system in accordance with the present invention is illustrated. An accelerator 12 produces a beam 13 of pulsed neutrons according to known. In general, these techniques involve accelerating a heavy ion, such as .sup.14 N or .sup.2 H towards a target 58, in which target the neutrons are produced. A cyclotron 60 is an exemplary device that may be used for accelerating the ions because it produces an intense ion beam having an inherently short duration. However, it is to be understood that other devices, such as properly instrumented electrostatic generators, could also be used to produce the pulsed neutron beam. As described above, the pulsed neutrons enter the object 14 following a path that carries the neutrons through respective regions or voxels of the object. Five such voxels are schematically illustrated in FIG. 5A, with benign materials present in three of the five voxels and explosive material in the other two voxels. A gamma ray 28 is produced as a result of the interaction between a given neutron and an atomic nucleus. FIG. 5A shows, as an example, the gamma ray 28 originating from explosive material 59 within the object 14. The gamma ray 28 is detected by the gamma ray detector 20'. For each gamma ray detected by the gamma ray detector 20' (or other detectors used in conjunction with the gamma ray detector 20'), the energy of the gamma ray and the time of flight of the neutron is determined. From this time of flight, a direct indication is advantageously made as to the location within the object 14 of the origin of the detected gamma ray 28, and hence, the location of particular elements of interest, e.g., explosive material, within the object 14. The time of flight is determined by noting the time between the start of the neutron pulse and the arrival of the gamma ray at the detector 20'. This time of flight is determined, in a preferred embodiment, as follows: a timing pulse associated with the accelerator 12 indicates the beginning of the neutron flight. Alternatively, a gamma ray "flash" from the target 58 may also indicate the beginning of the neutron flight. The time of the occurrence of the interaction is derived from the de-excitation gamma ray 28 signal's arrival at the gamma ray detector. This time may be suitably corrected for the gamma ray transit time, if desired. However, as noted above, the gamma ray transit time represents only a small portion of the overall time of flight. In a preferred embodiment, as shown in FIG. 5A, the time of flight (TOF) for instrumental reasons is determined by measuring the time that elapses between the arrival of the gamma ray at the detector and the time of production of the next reaction burst. The timing associated with such a measurement is illustrated in FIG. 6. Note that the repetition rate period, T, of the neutron burst is selected to be only slightly longer than the longest time of flight anticipated for a neutron to traverse the entire width of the object 14. As shown in FIG. 5A, a time to amplitude converter (TAC) 66 is used to generate an analog signal 70 having an amplitude that is proportional to the time between a start pulse and a stop pulse. For the embodiment shown in FIG. 5A, the start pulse, as stated above, is derived from the time at which the gamma ray 28 is detected at gamma ray detector 20'. A suitable timing discriminator 64 is used to ensure good definition of the start times presented to the TAC 66. The stop pulse is provided by timing signals derived from the pulsed accelerator 12. Thus, as represented in FIG. 5A, the output signal 70 of the TAC 66, gives rise to a time of flight spectrum, where the direction of the time axis is reversed. The output from the gamma ray detector 20' also gives rise to energy spectra similar in kind to spectra obtained using other detection systems as described, e.g., in prior patent applications, and the manner in which such output is detected and processed may likewise be the same. See, e.g., U.S. Patent Application Serial No. 07/367,534, filed 06/16/89; and U.S. patent application Ser. No. 07/053,950, filed 05/26/87 which applications are incorporated herein by reference. By associating the gamma ray energy of an event with each time of flight occurrence, a two-dimensional time-energy spectrum 72 is obtained. This time-energy spectrum advantageously depicts energy-time information from which the relative concentration of specified elements and their position within the object 14 is directly deduced. To illustrate, each vertical pattern within the spectrum 72 represents the energy distribution of the gamma rays for the five different volume elements or voxels of the object 14. For the representative spectrum 72 illustrated in FIG. 5A, for example, it is seen that each gamma ray spectrum (vertical patterns) consists of lines indicating the elements present within that region or voxel. For instance, the energy spectra 73 and 74 (which energy spectra are associated with gamma rays produced in the voxels of the object 14 wherein the explosive material is located) depict a high concentration of nitrogen and oxygen lines. This depiction thus indicates that nitrogen and oxygen are located within these voxels, thereby leading to the deduction that contraband is present within these voxels. To demonstrate the diagnostic power of the present invention, in FIG. 5B are shown the gamma ray spectra that were measured with simulants of dynamite, drugs (narcotics), and polyester cotton (cloth) using a pulsed beam of D(d,n) neutrons. As will be appreciated by those skilled in the art, there are several parameters that influence the accuracy and efficiency of the above-described detection system. Primary among these parameters are those associated with the kinematics of the neutron producing nuclear reactions. Preferably, the neutrons produced by either the HI+p or the D+d reactions should have a well defined and sharp energy, E.sub.n. Thus, the distance they travel into the interrogated object will be known from the multiplication of the time the neutron travels from the neutron producing target 58 to the point of interaction in the object with the neutron velocity, and can be expressed, with sufficient accuracy, as: ##EQU1## where M.sub.n is the neutron mass. As seen from the above expression, the intrinsic uncertainty in the spatial (longitudinal) position of the source neutrons may arise from uncertainty in both the neutron energy and the production time of the neutrons. Further uncertainties may arise if there is instrumental uncertainty in determining the various times associated with the time of flight measurement, mainly due to the timing circuitry. The energy spread in the kinematically focused neutrons produced in the H(HI,n) reaction depends on how far over the threshold is the incoming beam, and this spread leads to an uncertainty in the position of the product neutron packet. A .sup.14 N beam 0.5 MeV over threshold, for example, has an energy spread on the order of 0.83 MeV with a mean energy of 5.60 MeV. The other major contribution to the longitudinal uncertainty is the position of the neutron due to the length of the ion beam packet striking the neutron production target 58. This ion beam packet length is a characteristic of the cyclotron (or other accelerator/pulsing device). Most known cyclotrons have relatively short narrow beam pulses. For example, .sup.14 N beams around 90 MeV are available with -5 nsec pulse widths. For a .sup.14 N beam, 0.5 MeV over threshold, this leads to a spatial uncertainty of the produced neutrons of around 16 cm, which is relatively large. Other spatial uncertainties are shown in Table 1. Table 1 thus emphasizes the desirability of using a short beam pulse for this application. It is noted that the two uncertainties in the interaction position referenced above add in quadrature along with any uncertainties arising from the instrumentation. TABLE 1 ______________________________________ Spatial Uncertainty of Various Energy Neutrons Due to Time Spread of the Production Beams E (MeV) .DELTA.t (ns) .DELTA.d (cm) ______________________________________ 4 5 14 5 5 15 6 5 17 4 2 6 5 2 6 6 2 7 ______________________________________ Once a time-energy spectrum 72, or equivalent, has been obtained for each interrogation spot of the scanning beam, a final processing step is utilized to make a decision as to whether the detected data represents contraband or not. The gamma ray data (energy) and neutron data (time of flight) plus other system information are processed together to arrive at this decision. Such other system information may include, e.g., weight of the object, a neutron and/or X-ray image of the object and the like. The decision is made in a way that minimizes the probability of false alarm for a given probability of detection. In general, the decision analysis methods and techniques developed for TNA (thermal neutron activation) detection systems, see, e.g., U.S. Patent Application Ser. No. 07/367,534, filed 06/16/89, are applicable to the present invention. The input data obtained from the two systems are similar in kind, comprising gamma ray spectroscopic and spatial distribution information. The techniques of discriminant analysis and image analysis used in the TNA systems are therefore appropriate for the collimated fast, pulsed neutron system of the present invention. The main difference between these prior systems and the present system relative to processing the data is the direct multielemental mapping in the present invention while avoiding the complex image reconstruction. This is a significant advantage, as it allows the decision analysis to move forward at a rapid rate, thereby allowing a suitable feedthrough rate to be maintained for the objects being examined. Another significant advantage of the present invention over prior nuclear-based detection systems, such as those disclosed in the aforecited patent applications, is that the pulsed collimated neutron burst used by the present invention significantly reduces the amount of shielding required. This is because the employed pulsed collimated neutron burst is very directional in nature. Hence, shielding need only be positioned around those areas of the interrogation chamber 18 (FIG. 1) that will receive a neutron burst. Only minimal shielding, if any, need be placed around the areas of the chamber 18. This allows the overall detection system size to be smaller than prior art systems. A further advantage of the present invention is its ability to readily detect the specific light nuclei, e.g., oxygen, nitrogen, and carbon, commonly existing in contraband, while easily discriminating other nuclei not found in contraband. This advantageous result is achieved because the particular energy selected (.about.8MeV) for the incoming neutron burst generates preferentially gamma rays from the light nuclei of interest. In contrast, prior art systems using fast neutrons of higher energies (.about.14MeV) typically generate a host of gamma rays from nuclei not of interest. Thus, the prior art systems must sort the detected gamma rays to ascertain those of interest from those not of interest to a greater degree than is required with the detection system of the present invention. Moreover, because of the ability of the present invention to readily detect just the light nuclei of interest in contraband, the present invention further lends itself to the rapid detection of concealed drugs and the analysis of agricultural products in bulk. Table 2, below, identifies the elemental composition of, e.g., common drugs and narcotics, including the ratio of carbon to oxygen found in such substances. Data such as that shown in Table 2 thus aids in making the determination as to whether such substances are present in an interrogated object. TABLE 2 ______________________________________ Elemental Composition of Narcotics and Common Materials MATERIAL WEIGHT Narcotics: C H O N OTHER C/O ______________________________________ Heroin 68.2 6.28 21.66 3.79 3.1 Heroin 62.14 5.69 19.71 3.45 Cl 8.74 3.15 Hydrochloride Cocaine 67.3 6.92 21.1 4.61 3.2 Cocaine 60.03 6.53 18.83 4.12 Cl 10.43 3.19 Hydrochloride Morphine 71.56 6.71 16.82 4.91 4.2 PCP 85.87 10.35 0 5.76 .infin. LSD 74.27 7.79 4.95 12.99 15 Alcohol 52.2 13.0 34.8 0 1.5 Sugar (Sucrose) 42.1 6.43 51.5 0 0.82 Oil 77 12 11 0 7 Barley 43.2 6.85 4.90 1.0 .88 Soybeans 49.0 7.45 35.1 8.44 1.40 Pine Hartwood 54.38 6.31 39.16 1.39 ______________________________________ Table 3 below presents representative data of the constituent elements found in various types of contraband, as well as non-contraband. Data such as that shown in Table 3 further aids in making the determination as to whether a particular object being interrogated contains contraband or not. Advantageously, a contraband detection system as above described does not require a radioactive materials license because no "byproduct material" is in the system, and further because the levels of activation products are very low. However, because the system is a "radiation producing machine", it must be registered as such with appropriate governmental radiation safety authorities. Shielding design techniques similar to those commonly used in other radiation producing machines may be used to maintain radiation levels within prescribed limits. For example, composite shielding may be used to thermalize and absorb neutrons, and lead may be used as needed to absorb gamma rays. However, as previously indicated, the amount of shielding required when only short directed neutron bursts are used is significantly reduced from that required when continuous neutron irradiation is used, and in which the emission of the neutrons is isotropic. It should be again emphasized that while most of the schematic representations of the invention presented herein show a single source of the fast pulsed neutron beam, i.e., a single pulsed beam, alternative embodiments of the invention contemplate the use of multiple pulsed beams, each scanning the object from a different vantage point. In such an embodiment, appropriate timing measurements are used to keep track of which gamma ray originated from which beam (and hence, the location of the origin of each detected gamma ray within the multiple-beam scanned object). The use of multiple beams in this fashion advantageously allows the object to be more thoroughly and efficiently examined. While the invention herein disclosed has been described by means of specific embodiments and applications thereof, numerous modifications and variations could be made thereto by those skilled in the art without departing from the scope of the invention set forth in the claims. TABLE 3 __________________________________________________________________________ Various Physical Properties and Approximate Composition of Explosives and Other Materials PHYSICAL DENSITY % Weight Composition MATERIAL STATE (g/cm.sup.3) H C N O OTHER O + N __________________________________________________________________________ Nitroglycerine Liquid 1.6 2.2 15.9 18.5 63.4 0 81.9 (NG) EGDN Liquid 1.48 2.4 22.0 17.1 58.5 0 75.6 Amn. Nitrate Solid 1.7 5.0 0 35.0 58.0 0 93 Black Powder Solid 1.7-1.95 0 .about.22 10 36 S (3), K (29) .about.46 Nitrocellulose Solid 1.50-1,7 2.4 24.3 14.1 59.2 0 73.3 (9-14% N) PEIN (Pure) Solid 1.76 2.4 19.0 17.7 60.7 0 78.4 PEIN (Data Sheet) Solid 1.48 4.3 31.4 12.2 52.1 0 64.3 TNT (Pressed) Solid 1.63 2.2 37.0 18.5 42.3 0 60.5 Composition B Solid 1.71 2.7 24.4 30.5 42.7 0 73.2 Lead Styphnate Solid 3.02 0.7 15.4 9.0 30.8 Pb: 44.2 39.8 Tetryl Solid 1.57-1.71 1.8 29.3 24.4 44.6 0 69 Dynamite Solid 1.25 4.0 14.0 15.-20 59.0 Na: 10.0 74-79 Octogen (HMX) Solid 1.90 2.8 16.2 37.8 43.2 0 81 Composition 3 Putty-like 1.58-1.62 2.9 22.8 32.8 41.6 0 74.4 (C-3) Solid Composition 4 Putty-like 1.64-1.66 3.6 21.9 34.5 40.2 0 74.7 (C-4) Solid Picric Acid Solid 1.76 1.3 31.4 18.3 48.9 0 67.3 Lead Azide Solid 4.48 0 0 28.9 0 Pb 28.4 (Detonator) Triacetone Solid 1 (?) 9.7 38.7 0 51.6 0 59.7 Triperoxide Hexametylene Solid 1.57 5.77 34.6 13.5 46.2 0 59.7 Triperoxide Diamine NON EXPLOSIVE Packed Clothes Solid &lt;0.1 Polyester Solid (1.38) 3.7 66.7 0 29.6 0 29.6 Dacron Solid (1.38) 4.2 62.5 0 33.3 0 33.3 Cotton Solid (1.30) 6.0 48.0 0 46.0 0 46.0 Wool Solid (1.32) 4.7 37.5 21.9 5.1 0 27.0 Silk Solid (1.25) 5.3 39.5 28.8 26.3 0 55.1 Nylon Solid (1.14) 9.7 63.7 12.4 14.2 0 26.6 Orlon, Acrylan Solid (1.16) 5.7 67.9 26.4 0 0 26.4 Other Materials ABS (Acetonitrile Solid 1.20 8.92 84.5 76.5 0 0 76.5 Butadiene Styrene) Melamine- Solid 1.48 5.5 43.6 50.9 0 0 50.9 Formaldehyde Neoprene (Wet Solid 1.25 4.4 64.0 0 0 Cl: 31.6 0 Suites) Polyurethane Solid 1.50 7.9 52.2 12.2 27.8 0 40 Polyethylene Solid 0.92-0.96 14.3 85.7 0 0 0 0 Polypropylene Solid 0.89-0.91 14.3 85.7 0 0 0 0 Lucite, Acrylic Solid 1.16 9.1 54.6 0 36.4 0 36.4 Plexiglass PVC Solid 1.2-1.55 4.8 38.4 0 0 Cl: 56.8 0 Saran Solid 1-1.7 3.1 30.0 0 0 Cl: 66.9 0 Water Liquid 1 11.1 0 0 89.9 0 89.9 Ethyl Alcohol Liquid 0.79 13.1 52.1 0 34.0 0 34.0 Sugar Solid 1.59 6.5 42.0 0 51.4 0 51.4 __________________________________________________________________________