Abstract:
A method for detecting photonuclear-induced delayed neutrons emitted from a material by exciting the material with high-energy photon stimulation to generate neutrons emissions, detecting said neutrons, generating electric pulse signals representative of detected neutrons, amplifying and shaping said electric pulse signals with an amplifier, low-pass filter and high-pass filter, and comparing said amplified and shaped electric pulse signal with a threshold voltage to generate output pulses to detect delayed neutrons.

Description:
[0001]     The United States Government has certain rights in this invention pursuant to Contract No. DE-AC07-94ID13223, DE-AC07-99ID13727, and Contract No. DE-AC07-05ID14517 between the United States Department of Energy and Battelle Energy Alliance, LLC. 
     
    
     TECHNICAL FIELD OF THE INVENTION  
       [0002]     The present invention relates to the detection of nuclear materials, such as highly enriched uranium (HEU).  
       BACKGROUND OF THE INVENTION  
       [0003]     With ever increasing demands for international trade and commerce, it is becoming more difficult to monitor the importation or smuggling of dangerous materials into a country. One very dangerous item that may be illegally imported into a country is nuclear material, such as highly enriched uranium (HEU). While any undeclared nuclear material is of concern and needs to be detected, HEU is of particular concern. HEU is a primary nuclear weapons component and can be used in limited quantities to produce an effective weapon of mass destruction. HEU is very difficult to detect in any shielded configuration due to its very low radioactive decay emissions. For example, HEU has a half life of 3.5×10 17  years with a neutron emission of 2×10 −4  n/s/g and a 186-keV gamma-ray emission that is easily shielded. Therefore, countries have to monitor their borders for nuclear materials in order to prevent adverse parties from building and detonating nuclear-type weapons within their borders.  
         [0004]     The problem of detecting the importation of nuclear materials is exacerbated by the high quantity of international trade in the world. For example, a port-of-entry typically has too many containers entering the country to individually inspect each container for nuclear materials and other contraband. The major problem with any manual container inspection is that the inspection process is extremely tedious and costly due to the large container sizes and voluminous quantities of goods involved.  
         [0005]     Nondestructive detection techniques of nuclear materials are characterized as passive or active depending on whether they measure radiation from the spontaneous decay of the nuclear material or from the radiation induced by an external interrogating source. Passive techniques can provide some capability in detecting nuclear materials; however, these techniques are limited due to the wide variety of possible nuclear material shielding configurations and the physical positioning within these large cargo containers. As indicated previously, the detection of HEU is even more difficult for passive detection techniques due to its very low radiation emissions.  
         [0006]     Low radiation emission levels from nuclear materials, whether emitted directly or as a result of attenuation from neutron/gamma shielding, require very sensitive detectors to enable detection. Even with the most sensitive detectors, the detection of nuclear material is limited by the detectors&#39; ability to distinguish natural background signals from the radiation generated by the nuclear material. Hence, in most cases involving containerized cargo, the passive detection techniques are challenged in the detection of most shielded nuclear materials, and especially shielded HEU.  
         [0007]     One active interrogation technique, using an external neutron source (isotope or neutron generator), can be used to detect nuclear materials via the neutron multiplication effect from the fissioning-events in nuclear materials. Unfortunately, discriminating between the external neutrons emitted by the interrogating source and the induced neutrons from the nuclear material is difficult within a large-volume environment, such as a cargo container. In addition, this technique is application limited because of the administrative restrictions on the required neutron source strength involved and the associated neutron energy, slowing-down considerations.  
         [0008]     Therefore, a need exists to overcome the above-described problems.  
       SUMMARY OF THE INVENTION  
       [0009]     A device for detecting photonuclear-induced neutrons is described herein. One embodiment of the device may comprise a neutron detector and a detection circuit. The neutron detector may comprise a detector output. The detection circuit may be operatively connected to the detector output and may comprise an amplifier, a low-pass filter, and a high pass filter. The amplifier may comprise an amplifier input and an amplifier output. The amplifier input may be being operatively connected to the detector output. The low-pass filter may comprise a low-pass filter input and a low-pass filter output. The low-pass filter input may be operatively connected to the amplifier output. The high-pass filter may comprise a high-pass filter input and a high-pass filter output. The high-pass filter input may be operatively connected to the amplifier output. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0010]      FIG. 1  is a top plan view of an embodiment of a nuclear detection system determining whether a container contains nuclear material.  
         [0011]      FIG. 2A  is a rear perspective view of a detector.  
         [0012]      FIG. 2B  is a side view of the detector of  FIG. 2A .  
         [0013]      FIG. 2C  is an end view of the detector of  FIG. 2A .  
         [0014]      FIG. 3  is a schematic diagram of an embodiment of a preamplifier of the nuclear detection system of  FIG. 1 .  
         [0015]      FIG. 4  is a detailed schematic diagram of an embodiment of the preamplifier of  FIG. 3 .  
         [0016]      FIG. 5  is a cutaway schematic view of an embodiment of a detector.  
         [0017]      FIG. 6  is an side, cross-sectional view of an embodiment of the detector of  FIG. 5 . 
     
    
     DETAILED DESCRIPTION  
       [0018]     A non-limiting embodiment of a interrogation system  100  used to detect the presence of nuclear materials, such as highly enriched uranium (HEU), is shown in  FIG. 1 . In the non-limiting example described herein, a container  106  is, or contains, the material to which a determination is being made regarding its nuclear material contents. It is to be noted that the example of the interrogation system  100  provided herein determines whether HEU, or any nuclear material, is present in the container  106 . The container  106  is illustrated as being a shipping container such as those used by cargo container ships. It is to be understood, however, that the container  106  may be virtually any container capable of transporting or smuggling nuclear materials. For example, the container  106  may be a truck or as small as a 55-gallon drum.  
         [0019]     In summary, the embodiment of the interrogation system  100  described herein has a photon generator  110  and a plurality of detectors  112  located in the proximity of the container  106 . The photon generator  110  generates photons that are directed into the container  106 . The photons induce photonuclear reactions, which causes elements within the container  106 , and possibly the container  106  itself, to emit neutrons. The photo-induced neutrons are detected by the detectors  112  and analyzed by electronics and other devices, such as computers, associated with the detectors  112 . The photo-induced neutrons are classified as either prompt or delayed. Prompt neutron emission results from the direct photon interaction with the nucleus of an element. Delayed neutrons occur from photofission-induced and neutron fission-induced, fission fragments. The emission of delayed neutrons from the container  106  is a positive indication that the container  106  contains a nuclear material.  
         [0020]     The detectors  112  enable the interrogation system  100  to discriminate between delayed neutrons and prompt neutrons. As set forth above, the detection of delayed neutrons indicates that the container  106  contains nuclear material. Prompt neutrons will help characterize any shield materials associated with the nuclear material. For example, if combined with conventional x-ray radiography, an anomalously high photoneutron signal from areas surrounding a very dense object may indicate significant low-density, neutron shielding materials.  
         [0021]     Having summarily described the interrogation system  100 , it will now be described in greater detail. In the non-limiting embodiment described herein, the photon generator  110  has an electron accelerator  116 , a converter  117 , and a collimator  118  among other components. The electron accelerator  116  may, as a non-limiting example, be an electron accelerator having selectable beam energies up to 12 MeV. The converter  117  may, as a non-limiting example, is a high atomic number converter. It should be noted that the photon generator  110  described herein is an example of a photon source and that other photon sources may be used in conjunction with the interrogation system  100 .  
         [0022]     In one embodiment, the accelerator  116  generates a beam of electrons and pulses up to a rate of several hundred hertz. The electrons are converted to high energy photons by way of the high atomic number converter  117 , wherein the high energy photons have energies up to the maximum electron beam energy. The photons are sometimes referred to as bremsstrahlung photons. The photons generated at the converter  117  are collimated to a preselected annular width by the collimator  118 . The process of generating a voluminous number of photons using a single pulse of electrons generated from a pulsed, electron accelerator is sometimes referred to as a photon flash event.  
         [0023]     The high-energy photons generated by the photon generator  110  are forward-directed toward the container  106 . The above-described high-energy photons have the ability to pass through many different shielding configurations. For example, the energy of the electron accelerator  116  may be selected in order to provide photons with energy spectra appropriate for optimal penetration of a given shield. Thus, the photons are able to pass through the walls of the container  106  as well as most shielding that may be used to conceal or smuggle nuclear materials in a given container. The photons react with the container  106  itself and materials within the container to induce photonuclear reactions with the container  106  and its contents causing neutrons to be emitted from the container  106 .  
         [0024]     Neutrons from a photoneutron reaction (γ,n) are emitted promptly after the reaction occurs. Thus, the photoneutrons are referred to as prompt photoneutrons. Photofission reactions (γ,fission) emit both prompt and delayed neutrons. The delayed neutrons occur from the decay of the unstable fission products. For a selected electron beam energy operation, the detected neutrons, measured between accelerator pulses, have a time-dependent response that allows the identification of delayed neutrons. These delayed neutrons, resulting from the fission process, are separable in time from the prompt neutron signature. As described above, the emission of delayed neutrons is a positive indication that the container  106  contains nuclear material. Furthermore, to identify or discriminate between nuclear material types, i.e., depleted uranium, HEU, and thorium, a ratio of delayed neutron counts at two different electron beam energies may be utilized. The use of two different electron energy beams is referred to as the dual-beam energy technique. In one embodiment, the dual-beam energy technique enables the interrogation system  100  to differentiate between the three common types of nuclear materials. Thus, most threats associated with the nuclear materials may be readily evaluated.  
         [0025]     The detectors  112  detect neutrons emitted from the container  106 . As described in greater detail below, the detectors  112  and their associated electronics are able to distinguish between prompt and delayed neutrons in the presence of background x-rays, i.e., photons, caused by the electron accelerator  116 , the collimator  118 , and other structural materials associated with the interrogation system  100 . A non-limiting example of a detector  130  is shown in  FIG. 2A . The detector  130  of  FIG. 2A  is an example of one of the detectors  112  of  FIG. 1 . The detector  130  illustrated herein is substantially parallel-piped, however, it should be noted that the detector  130  may be virtually any shape. For example, the detector  130  may be substantially cylindrical. The detector  130  described herein has a rear panel  132 , a top side,  134 , a right side  136 , a left side  138 , a bottom side  140 , and a front side  142 . As described in greater detail below, the rear panel  132  is removable and the front side  142  is adapted to face the container  106 ,  FIG. 1 .  
         [0026]     A side view of the detector assembly  130  with the rear panel  132  removed therefrom is shown in  FIG. 2B . The detector assembly  130  has a length L 1  extending between a rear side  144  to which the rear panel  132  fits and the front side  142 . The length L 1  may, as an example, be approximately 35.56 centimeters. The detector assembly  130  may have a height H 1  extending between the top side  134  and the bottom side  140 . The height H 1  may, as an example, be approximately 25.4 centimeters. As described above, the rear panel  132  shown in  FIG. 2B  is removed from the remaining portion of the detector  130 . As shown in  FIG. 2B , the rear panel  132  has a sealing portion  146  that fits into an interior portion of the detector  130 . The use of the rear panel  132  serves to prevent neutrons and electromagnetic interference from entering the interior of the detector  130 . More specifically, the detector  130 , based on the neutron absorptive materials selected (borated polyethylene), is more sensitive to neutrons entering the front side  142  than other areas. Thus, the majority of neutrons entering the detector  130  will have been emitted from the container  106 . The sealing portion  146  has a notch  147  formed therein that serves to pass wires between the interior and the exterior of the detector  130 .  
         [0027]     The interior of the detector assembly  130  is adapted to receive a detector block  150 . In one embodiment of the detector assembly  130 , the detector block  150  is inserted into and removed from the detector assembly  130  by way of the rear side  144  of the detector assembly  130 . As described in greater detail below, the detector block  150  has neutron detectors located therein and serves, in conjunction with the detector electronics  154 , to detect neutrons emitted by material within the container  106 ,  FIG. 1 . In addition to the detector block  150 , detector electronics  154  may be received within the interior of the detector  130  via the rear side  144 .  
         [0028]     A front view of the detector assembly  130  is shown in  FIG. 2C . The detector block  150  is formed from a virgin polyethylene block or a plurality of blocks. The detector block  150  has front surface  164  that has a plurality of holes  166  formed therein. The holes  166  described herein have diameters of approximately 2.64 centimeters and lengths of approximately eight centimeters. Each of the holes  166  are adapted to receive a neutron detector and a polyethylene plug having a diameter of approximately 1.27 centimeters. The polyethylene plug may be located between the neutron detector, not shown in  FIG. 2C , and the front surface  164  of the polyethylene block  150 . In one embodiment, each polyethylene plug is flush with the front surface  164  of the detector block  150 . One example of a neutron detector that may be located in one of the holes  166  is a 10-atmosphere helium-3 neutron detector. With additional reference to  FIG. 2B , the neutron detectors are operatively or otherwise electrically connected to the detector electronics  154 ,  FIG. 2B . The embodiment of the detector  130  described herein has four holes  166  to receive four neutron detectors. It should be noted that the detector assembly  130  may be adapted to have any number of neutron detectors located therein.  
         [0029]     The detector assembly  130  includes several layers of materials that serve to attenuate electromagnetic interferences in the detector electronics  154 ,  FIG. 2B  and to attenuate “room return” neutron noise signals. The detector  130  has an inner layer  158  and an outer layer  160 . The inner layer  158  is, as a non-limiting example, borated polyethylene and is approximately 5.08 centimeters thick. The polyethylene is borated at approximately five percent by weight. The neutron die-away time for this configuration is approximately sixty-eight seconds, wherein the die-away time is the time required for the neutron population within the detector block  150  to decrease by one exponential value. The outer layer  160  is approximately 0.05 centimeters thick and comprises cadmium. The outer layer  160  of cadmium serves to remove any external thermal neutrons below a defined energy value. In one embodiment of the detector assembly  130 , an electromagnetic shield surrounds the outer layer  160 . The shield may, as non-limiting examples, be a layer of copper or aluminum that is approximately 0.1 centimeters thick.  
         [0030]     Having described the detector block  150 , the detector electronics  154  will now be described. A block diagram of the detector electronics  154  is described followed by an embodiment of a more detailed circuit diagram.  
         [0031]     Referring additionally to  FIG. 3 , each of the neutron detectors described above has a preamplifier  200  operatively or otherwise electrically connected thereto. Accordingly, the detector electronics  154  of  FIG. 2B  has four preamplifiers  200  associated therewith; one for each of the helium-3 detectors. The non-limiting embodiment of the preamplifier  200  described herein is connected to a neutron detector  210  and has an input amplifier  214 , a low-pass filter (LPF)  216 , a high-pass filter (HPF)  218 , sometimes referred to as a differentiator, a comparator  220 , and a voltage adjustment  222 . The following description focuses on a single neutron detector  210  associated with a preamplifier  200  as shown in  FIGS. 3 and 4 . It should be noted that the detectors  112 ,  FIG. 1 , may have any number of neutron detectors associated therewith.  
         [0032]     In the embodiment of the preamplifier  200  described herein, the neutron detector  210  has two conductors; a case and a center conductor. The case of the neutron detector  210  is biased at approximately negative sixteen hundred to negative eighteen hundred volts relative to the center conductor, which operates at a nominal voltage of zero volts. This biasing arrangement allows the neutron detector  210  to be direct-coupled to the input amplifier  214 . The direct-coupling between the neutron detector  210  and the input amplifier  214  reduces the signal processing time between the neutron detector  210  and the subsequent processing by the other components of the preamplifier  200 . The preamplifier  200  enables the relatively weak signals generated by the neutron detector  210  to be processed.  
         [0033]     As described in greater detail below, the input amplifier  214  is a current-sensing amplifier. Current generated by the neutron detector  210 , upon detection of neutrons, is detected and amplified by the input amplifier  214 . The low-pass filter  216  serves as a pulse-shaping, low-pass filter. The low-pass filter  216  has a very short pulse rise time. For example, the pulse rise time may be approximately 0.5 microseconds. This short pulse rise time serves to reduce the probability of a detection pulse being detected and processed during the processing of an earlier detected pulse.  
         [0034]     The high-pass filter  218  serves to further shape the above-described pulse. This shaping of the pulse improves the ability of the preamplifier  200  to process individual neutron pulses even though the neutron detector  210  may have not completely recovered from a photon flash event. Because of the low-pass filter  216  being coupled to the high-pass filter  218 , the neutron detector assembly  130 ,  FIG. 2A , is capable of operating with minimal shielding in high radiation fields.  
         [0035]     The comparator  220  described herein is a high-speed differential comparator. The comparator  220  compares the pulse generated by the high-pass filter  218  to a threshold voltage that is established by the voltage adjustment  222 . When the input voltage from the high-pass filter  218  exceeds a threshold voltage established by the voltage adjustment  222 , the comparator  220  outputs a pulse. The pulse may be transistor-to-transistor logic (TTL) compatible so as to be registered by a conventional counter. The use of the comparator  220  reduces the probability that pulses generated by noise will be processed as detected neutrons. More specifically, only pulses that have a voltage high enough to be processed as detected neutrons are able to exceed the threshold voltage established by the voltage adjustment  222 . Pulses resulting from noise generally do not have voltages that are high enough to exceed the threshold established by the voltage adjustment  222 .  
         [0036]     Having generally described an embodiment of the preamplfier  200 , it will now be described in greater detail.  
         [0037]     A detailed schematic diagram of a non-limiting embodiment of the preamplifier of  FIG. 3  is shown in  FIG. 4 . The component values shown in  FIG. 4  and described herein are for illustration purposes only. One skilled in the art may change the values while achieving similar results within the scope of this description.  
         [0038]     The input amplifier  214  consists of an operational amplifier U 1  and its associated components. In one embodiment of the preamplifier  200 , the operational amplifier U 1  is an LM7171 amplifier that is commercially available from the National Semiconductor Corporation. The inverting input of the operational amplifier U 1  is connected to the center conductor of the neutron detector  210  by way of a resistor R 1 . The non-inverting input of the operational amplifier U 1  is connected to ground by way of a parallel combination of a capacitor C 1  and a resistor R 2 . The combination of the capacitor C 1  and the resistor R 2  serves as an input bias for the non-inverting input of the operational amplifier U 1  as well as noise reduction. Frequency compensation is provided by a capacitor C 2 .  
         [0039]     The feedback of the operational amplifier U 1  consists of a resistor R 3  connected in parallel to the combination of a transistor Q 1  and a transistor Q 2 . The resistor R 3  provides a high gain for the input amplifier  214 . The transistor Q 1  and the transistor Q 2  are wired as back-to-back diodes in order to prevent output saturation of the operational amplifier U 1 . The operational amplifier U 1  is powered by positive and negative twelve volt power supplies, which are used throughout the preamplifier  200 . The voltage inputs to the operational amplifier U 1  are connected to ground by a capacitor C 3  and a capacitor C 4  in order to attenuate noise on the power lines.  
         [0040]     The output of the input amplifier  214  is direct coupled to the input of the low-pass filter  216 . The low-pass filter  216  consists of a operational amplifier U 2  and its associated components. In one embodiment, the operational amplifier U 2  is an LM7171 and is commercially available from the National Semiconductor Corporation. The output of the operational amplifier U 1  is connected to the non-inverting input of the operational amplifier U 2  via a resistor R 4 . The feedback components of the capacitor C 5 , the capacitor C 6 , the resistor R 5 , the resistor R 6 , and the resistor R 7  provide low-pass filtering. The configuration of the low-pass filter  216  provides for a pulse rise time of approximately 0.5 microseconds. The response of the low-pass filter  216  described herein provides for approximately 3.0 dB attenuation at approximately 338 kilohertz.  
         [0041]     The output of the low-pass filter  216  is connected to the high-pass filter  218 . More specifically, the output of the operational amplifier U 2  is connected to the non-inverting input of an operational amplifier U 3  by way of a resistor R 8  and a capacitor C 7 . In one embodiment, the operational amplifier U 3  is an LM7171 operational amplifier and is commercially available from the National Semiconductor Corporation. The feedback associated with the operational amplifier U 3  consists of a resistor R 10  and a resistor R 11 , which provides for a gain of approximately five. The differentiation or high-pass filtering is achieved by way of the resistor R 8 , the resistor R 9 , and the capacitor C 7 . The response of the high-pass filter  218  described herein provides approximately 3.0 dB at approximately 319 kilohertz.  
         [0042]     The output of the operational amplifier U 3  and, thus the high-pass filter  218 , is an analog representation of neutrons detected by the neutron detector  210 . This analog output may be measured at the terminal referenced ANALOG OUT, which is connected to the output of the operational amplifier U 3  via a resistor R 12 . Voltage pulses on the ANALOG OUT terminal represent the detection of neutrons.  
         [0043]     The output of the operational amplifier U 3  may have some photon-related noise. In order to distinguish between noise and detected neutrons, the output of the differentiator  218  is connected to the comparator  220 . The comparator  220  includes an operational amplifier U 4  and its associated components. In one embodiment, the operational amplifier U 4  is an industry standard LM361 high-speed differential comparator. The inverting input of the operational amplifier U 4  is connected to the output of the operational amplifier U 3  by way of a resistor R 13 .  
         [0044]     Voltage comparison is achieved by creating a desired voltage at the voltage adjustment  222  and applying this voltage to the non-inverting input of the operational amplifier U 4 . A feedback resistor R 14  is also used in conjunction with the voltage comparison. The voltage adjustment  222  consists of a variable resistor VR 1 , a resistor R 15 , a resistor R 16 , a resistor R 17 , and a capacitor C 8 . The voltage applied to the non-inverting input of the operational amplifier U 4  is determined by adjusting the variable resistor VR 1 . High and low limits of the voltage are established by the resistor R 15  and the resistor R 16 . Noise and ripple are attenuated by the capacitor C 8  and buffering is provided by the resistor R 17 . The voltage established by the voltage adjustment  222  may be monitored at the terminal designated THRESHOLD MONITOR.  
         [0045]     The output voltage of the operational amplifier U 4  is limited by the voltage at the VCC. In order to provide a TTL output, the VCC is set at five volts. In order to limit the number of power supplies required to operate the preamplifier  200 , the five volt VCC may be established by dropping seven volts across a zener diode Z 1 , which is connected between the twelve volt power supply and VCC. A capacitor C 9  is connected between VCC and ground to attenuate noise and ripple on the VCC. Resistor R 18  provides bias current for Z 1 . The operational amplifier U 4  has an inverting and non-inverting output. In the non-limiting embodiment described herein, the non-inverting output is monitored via a resistor R 19  at a terminal referenced as PULSE OUT.  
         [0046]     The embodiment of the preamplifier  200  described in  FIG. 4  has a buffer  228  operatively or otherwise electrically connected to the comparator  220 . The buffer  228  comprises two NPN transistors, Q 3  and Q 4 , and their associated biasing components. The transistors Q 3  and Q 4  described herein are industry standard 2N3904 devices. The base of the transistor Q 3  is connected to the inverting output of the operational amplifier by way of a resistor R 20 . The emitter of the transistor Q 3  is connected to ground and the collector is biased by resistors R 22  and R 23 . Accordingly, the transistor Q 3  functions as an inverter. The collector of the transistor Q 3  is connected to the base of the transistor Q 4  by way of a resistor R 21 . The transistor Q 4  functions as a non-inverting switch and is biased by the resistors R 24  and R 25 . The buffered output of the preamplifier  200  is at the emitter of the transistor Q 4 . A resistor R 26  is located between the emitter and an output referenced as BUFFER OUT, which serves to reduce oscillations.  
         [0047]     It should be noted that other configurations may be used to create a buffer. For example, the buffer  228  may comprise a line driver or buffer consisting of an LM6221 line driver available from the National Semiconductor Corporation.  
         [0048]     The preamplifier  200  is able to detect neutrons emitted from the container  106 ,  FIG. 1 , within microseconds of an accelerator-produced, photon flash event. This rapid detection of neutrons enables the preamplifier  200  to be able to distinguish between prompt and delayed neutrons in the presence of background noise created from the production of photon flash events. The rapid detection also enables the container  106 ,  FIG. 1 , to undergo repeated photonuclear stimulation. For example, the generator  110  may stimulate the container  106  with repetition rates up to several hundred hertz thereby increasing the accuracy of the detection and reducing the time required for detection. Therefore, the container  106 ,  FIG. 1 , is able to undergo rapid photonuclear stimulation and the preamplifier  200  is able to accurately and rapidly detect the emission of both prompt and delayed neutrons. As described above, the detection of delayed neutrons is a positive indication that nuclear materials are present within the container  106 .  
         [0049]     Having described the components of the interrogation system  100 , the operation of the interrogation system  100 , including the preamplifier  200  will now be described.  
         [0050]     Referring to  FIG. 1 , the interrogation system  100  is located in close proximity to the container  106 . In one embodiment, the photon generator  110  and the detectors  112  are placed approximately one meter from the container  106 . In some embodiments, this placement of the detectors  112  is approximately two meters from the center of the container  106 . High energy photons are emitted from the photon generator  110  to induce photonuclear reactions with the contents of the container  106 . The photonuclear reactions cause neutrons to be emitted from the container  106 , which are detected by the detectors  112 . As described above, many materials will emit prompt neutrons, however, only nuclear materials emit delayed neutrons.  
         [0051]     Referring to  FIG. 4 , the neutron detector  210  generates a voltage pulse upon detecting a neutron. It should be noted that the preamplifier  200  is able to detect neutrons during the detector&#39;s flash recovery period which may last up to several hundred microseconds after each accelerator pulse. Therefore, the detectors  112 ,  FIG. 1 , are able to detect both prompt and delayed neutrons. The pulse is amplified by the input amplifier  214 . The configuration of the transistor Q 1  and the transistor Q 2  in the feedback of the operational amplifier U 1  serves to prevent the operational amplifier U 1  from saturating and further serves to lock out the remaining pre-amplifier circuitry. Without saturating the operational amplifier U 1 , the input amplifier  214  is able to discriminate between rapidly detected neutrons. The input amplifier  214  outputs pulses that are representative of detected neutrons. The signal generated by the input amplifier  214 , however, may have some significant noise due to the photon flash response.  
         [0052]     The signal generated by the low-pass filter  216  consists of a plurality of pulses, wherein each pulse is representative of a detection by the neutron detector  210 . The pulses are shaped by the low-pass filter  216  and the high-pass filter  218  to be well defined pulses. These pulses may be monitored at the terminal ANALOG OUT. In order to further distinguish pulses detected by the neutron detector  210  and neutron signal noise, the output of the high-pass filter  218  is input to the voltage comparator  220 . The signal is compared to a pre-selected voltage set via voltage adjustment  222 . The signal that is representative of the detected pulses is typically greater than the noise floor. Therefore, by setting the voltage of the voltage adjustment  222  slightly greater than the noise floor, the comparator  220  will only pass pulses and the noise will be significantly attenuated. The pulses generated by the comparator  220  may be monitored at the PULSE OUT terminal. In order to make the output of the preamplifier  200  TTL compatible, the buffer  228  converts the pulses generated by the comparator  220  to five-volt pulses and enables the preamplifier  200  to be coupled with other instruments.  
         [0053]     The pulses generated by the preamplifier  200  may be monitored by a conventional monitoring device, such as a counter or an oscilloscope. Thus, a determination as to whether the container  106 ,  FIG. 1 , produced delayed neutrons may be readily determined. In addition, the interrogating photons can be used to provide an x-ray radiograph of the container contents and help located very dense materials representative of nuclear materials. Specifically, the axially-dependent neutron emission signals can be correlated with the container radiograph to help verify nuclear material detection. This correlation may be performed during the axial development of a radiograph, e.g., real time inspection, or when the interrogation system  100  is repositioned, e.g., manual inspection, at the suspicious object location.  
         [0054]     Another embodiment of the interrogation system  100  uses a modified detector assembly  300 ,  FIG. 5 , to increase counting efficiencies. The detector  300  of  FIG. 5  also uses the preamplifier  200  described above. The detector  300  may be cylindrical and may have a length X of approximately 44.5 inches and a diameter D of approximately 3.9 inches. The detector  300  may weigh, as a non-limiting example, approximately 35.0 pounds. The detector  300  includes a tube  308 . In the non-limiting embodiment described herein, the tube  308  is a 10.0 atmosphere, helium-3 tube that is approximately 30.0 inches long. It should be noted that the tube  308  is electrically or otherwise operatively connected to the preamplifier  200 .  
         [0055]     A side, cross-sectional view of an embodiment of the detector  300  is shown in  FIG. 6 . As shown in  FIG. 6 , the tube  308  is surrounded by a plurality of concentric rings of differing materials that serve to suppress “room-return”-type thermal neutrons, which are considered to be noise. A first ring  310  consists of a one-inch thick polyethylene sleeve to effectively moderate the energetic neutrons emitted from the container  106 ,  FIG. 1 . A second sleeve  312  is located adjacent the first sleeve  310  and consists of cadmium. In one embodiment, the second sleeve is approximately 0.045 inches thick. A third sleeve  314 , a fourth sleeve  316 , and a fifth sleeve  318  consists of multiple borated polyethylene segments enriched with 26% boron-10. The third sleeve  314 , the fourth sleeve  316 , and the fifth sleeve  318  each may be approximately 0.125 inches thick.  
         [0056]     Caps, not shown, may be located at the ends of the detector  300 . In an axial manner, the caps may match the radial shielding composition described with reference to  FIG. 6 . The detector  300  may be covered by a 0.094 inch thick aluminum housing to provide structural protection and electromagnetic shielding.  
         [0057]     While an illustrative and presently preferred embodiment of the invention has been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.