BACKGROUND OF THE INVENTION

Field of the Invention

The invention relates to the detection of explosives, and more specifically to methods and systems of detecting and locating explosives via thermal neutron analysis.

Description of Related Art

A great need exists for the detection of underground explosives, the scanning of luggage and passengers, and the detection of improvised explosive devices. An acceptable response to the explosive threat requires a highly sensitive and non intrusive technology to interrogate or scan a given target or zone at high velocity.

Explosive materials are well-separated from other benign materials because of the abundance of certain atomic elements in its atomic composition. In prior art, gamma rays stimulated by neutrons have been used for the qualitative detection and quantitative determination of many elements. Most nuclei emit characteristic gamma rays following the interaction of the neutrons with the nucleus. Because of the high binding neutron energy and the generally allowed transitions to the ground state of the excited nucleus, the emitted gamma rays have high penetrating energy with an energy spectra and yield that depends on the specific nucleus involved in the nuclear reaction, and then are very suitable for the interrogation of materials as a bulk.

Neutron capture and inelastic scattering were utilized in a wide variety of techniques to detect explosives and drugs. For example systems using thermal neutron analysis techniques (TNA) were described in U.S. Pat. No 3,832,545 to Bartko and No 5,076,993 to Gozani et al.; systems using fast neutron analysis (FNA) were described in U.S. Pat. No 5,098,640 to Gozani et al.; and pulsed fast neutron analysis techniques (PFNA) were described in U.S. Patent 5,076,993 to Gozani et al. There are variations in the methods by introducing different measuring techniques to improve the detection capabilities, for example using time gated thermal neutron analysis (GTNA), or the combination of several methods in a single equipment, for example like PFNA and GTNA techniques combined by Vourvopoulos et al. in "Pulsed fast/thermal neutron analysis: a technique for explosives detection" in Talanta 54 (2001). Other methods could be find in review articles, like the article written by Gozani : "The role of neutron based inspection techniques in the post 9/11/01 era", Nuclear Instruments and Methods in Physics Research B 213 (2004), S. Singh, et al. "Explosive detection systems (EDS) for aviation security", Signal Processing 83 (2003) or J. Yinon in "Field detection and monitoring of explosives ", Trends in Analytical

Chemistry, 21, (2002).

Using these neutron interrogations methods several commercial systems have been developed for different tasks and are available in the market. These systems are described in reviews of commercial equipments for example in "Commercial Systems for the Direct Detection of Explosives (for Explosive Ordnance Disposal Tasks)", ExploStudy, Final Report, Ecole Polytechnique Federale de Lausanne (EPFL) (2001) and "Guidebook on Detection Technologies and Systems for Humanitarian Demining" , Geneva International Centre for Humanitarian Demining, Geneva, March (2006).

Several thermal neutron interrogation technologies have concentrated their efforts on the detection of the gamma rays produced by the neutron capture with 10.83 MeV transition in nitrogen, because bulk nitrogen is a good indicative of the presence of explosive material. This approach has been considered in the Barko patent. Other fast neutron technologies have concentrated on the four cardinal constituents of explosives: hydrogen, carbon, nitrogen, and oxygen, as is considered in the Gozani and Sawla patents. Other equipments use thermal neutrons to measure the isotope concentration and isotope rate in order to identify the explosive content in unexploded munitions that may have been buried or exposed to the elements for years before they are recovered for example, as developed by Caffrey in the U.S. Pat. No 6,791,089 Bl.

Experience dictate that nature provides higher fidelity nuclear signatures at the cost of increasing the acquisition time, and these techniques are in general significantly time- consuming, as it was explained by Runkle et al. in "Photon and neutron interrogation techniques for chemical explosives detection in air cargo: A critical review ", in Nuclear Instruments and Methods in Physics Research A 603 (2009). Existing equipments require several seconds to minutes to detect, localize or identify an explosive hidden in a zone under interrogation.

This relative slow detection velocity is produced because even when the energy spectra of the gamma rays produced by the interactions between the neutrons and the interrogated nucleus are well defined, the signal produced after the gamma interaction with the detector and its electronic needs to give a very good signal to background ratio in order to produced net counts (signal minus background) well above the uncertainty limit. Runkle explained that the challenges for the signal to background discrimination are produced by the practically continuum nature of the background associated with the ubiquitous nature of hydrogen, carbon, nitrogen, and oxygen.

A more detailed explanation of the signals to background discrimination problems for fast neutrons is given by Gozani, who explains in "Understanding the physics limitations of PFNA- the nanosecond pulsed fast neutron analysis", in Nuclear Instruments and Methods in Physics Research B 99 (1995). In this paper, it is stated that a main source of background in the gamma detectors is the time correlated background resulting from Compton scattering in the inspected objects and the detector itself, the high energy continuum spectra in Fe and other higher Z elements, and the neutron induced gamma rays in the detector.

These background sources produced relative low signal to background ratio detection systems as it can be seen in the gamma ray spectra published for fast neutrons by Gozani et al. in "Advances in neutron based bulk explosive detection", in Nuclear Instruments and Methods in Physics Research B261 (2007), and Strellis et al. in "Classifying threats with a 14-MeV neutron interrogation system", in Applied Radiation and Isotopes 63 (2005). This low signal to background ratio conspires to create extended sensitivity methods or high velocity detection systems.

For thermal neutrons, similar low signal to background ratio has been found. For example,using an isotopic source of spontaneous fission neutrons ( 252 Cf) Cinausero et al. in "Development of a thermal neutron sensor for Humanitarian Demining" published in Applied Radiation and Isotopes 61 (2004) reported that the background strongly reduced the sensibility of the equipment. To improve the signal to background ratio Cinausero also used a much higher neutron yield - higher energy D-T neutron generator as a pulsed neutron source, and tried to reduce the background by acquiring the signals after the fast neutron pulsed vanished, but the resulting background was reduced only by a fraction of two, as consequence of the relative large life of the thermal neutrons in the moderator used in the source, compared with the time of flight (TOF) of the neutrons between the source and the target. Final results shows an underground explosive placed at few centimeters of the detector can be seen, but the process involves a few minutes of integration per measurements.

When a fast pulsed neutron source is used inside a moderator to obtain a pulsed thermal neutron source (as it is required for GTNA of TNA with neutron accelerators), after the original neutron pulse end only the fast neutron that were slowed down and thermalized in the detector and the surrounding environments exists, and their population decay with a time constant of the order to hundreds of microseconds to milliseconds. This distinctive behavior of pulsed thermal neutron sources compared fast neutron sources is well known and described in the text books as the "die away time" of a source of thermal neutrons (Chapter 9 of Beckurtz et al. "Neutron Physics" Springer Verlag, 1964. The thermal neutron "die away time" is very important in the combination of PFNA techniques with GTNA, and it was incorporated by Holslin et al. in the U.S. Pat. No 7,430,479 in the time analysis for elemental identification in a target irradiated with neutrons. Also it is considered in commercial PFNA/GTNA equipments like the equipment described by Vouropoulos et al.

It must be observed, however, that the use of simple moderator geometries to obtain an intense thermal neutron sources with a pulsed fast neutron sources does not necessarily indicate fast detection of suspected substances in GTNA or PFNA/GTNA equipments. The large die away time of the neutron population in the moderator produces a signal on the gamma detector that overlaps in time with the gamma signal produced by the target. This overlap is produced because the delay in the TOF of the neutrons between the source and the target position is smaller than the thermal neutrons life in the moderator. The time correlated signal produced by the source in the detector is interpreted as a background signal by the explosive detection process because it produces counts not related with the target

composition during the measuring time. Another clear disadvantage in present interrogation technologies with thermal neutrons is that the TOF have the information of the distance between the source and the target, but this information is lost, or strongly degraded, by the overlapping with the time-correlated background produced by the die away time of the source.

Norris recently described in an application of a U.S. Pat. No 2008/0017806 Al an interrogation system with thermal neutrons, with a rotary shielding in order to generate a pulsed neutron beam in the direction of objective to be interrogated with a detector placed far away from the source. In that system the challenges (apart of the mechanical challenge to rotate a thermal neutron shielding) of the thermal neutrons die away time remains because the die away time of the neutrons that exist the shielding copies the temporal behavior of the neutrons in the moderator. Moving the gamma detector away from the source reduces the background counting, but at the expense of increasing the effective die away time at the detector position, because of the TOF of the neutrons at lower energy. As it was explained above, the physics involved in the die away time of the thermal neutron is well known, and for homogeneous moderators and absorbers there are also well know relationship that can be used to estimate the time required for fast neutrons to reach the thermal energy range. Some typical figures will be given in order to understand the challenge of the synchronic background for the signal discrimination. The die away time depends mainly on the size and composition of the moderator. The average time for a 1 to 10 MeV neutron to achieve energies in the order of leV is approximately 2 μ$εο in water, and similar results could be expected in other hydrogenated moderators like the high dense polyethylene (HDP) usually used in TNA and GTNA equipments. If the moderator is very large, and the neutron leakage is very small, the die away time constant (usually called a) takes and asymptotic value of ₀ of about (200 Even for small water cubes or high dense polyethylene cubes with a size of 10 cm per side, the a time constant is about (70 μ8βΰ) ^_1 . Considering an exponential behavior a significant fraction of the original neutrons are in the moderator during 3 times the a time constant, then the total duration of the thermal neutron pulse will be around 210 μ8εα The velocities of neutrons with energies in the range from 0.025 eV to 0.1 eV corresponds to the range of 2200 to 4400 m/s. Then if the interrogation substance is placed at 40 cm from the neutron source, the neutrons with 0.025 eV arrives when there still neutrons from the source in the detector (the TOF is 180 μ8βΰ), or if is at 80 cm the neutrons with 0,1 eV arrives when there are also neutrons from the source in the detector (TOF is also 180 μ8βΰ). If the gamma detector is placed at a certain distance from the source, the respective TOF of the source spectra needs to be added to the total duration of the thermal neutrons in the source, and then need to be calculated the total duration of the synchronous background in the detector. Then is very clear that the all the gamma produced by substances at 50 cm or less will be measured at the same time of the synchronous background, and a large fraction of the gamma produced by substances between 50 to 100 cm will be measured at the same time of the synchronous background also. This ideal situation becomes much more complicated because usual moderators in TNA are much larger than a 10 cm HDP cube, the gamma detectors are usually at 10 to 20 cm of the source, and a usual penetration to look for demining is much less than 50 cm.

This example shows that the temporal behavior of the thermal neutrons in the source reduces the sensitivity of the equipment and hides the spatial information of the target position. These limitations are resulted as a consequence of the physics involved in the pulsed neutron source -moderator concept used at present. There is a well know neutron technique to measure the averaged thermal absorption cross section of a moderator which is done by diluting an absorber in a moderator and measuring the much shorter a constant (die away time) resulting from a fast neutron pulse injection, as is explained in chapter 18 of the Beckurtz text book. In Fluharty et

a\. "Moderator studies for a repetitively pulsed test facility (RPTF)", Nuclear Science and Engineering 35 (1969), a much shorter die away time was measured for a moderator of water with a given boron dilution, in the range between (10 to 40 Fluharty, who was interested in neutron diffraction, shows that a simple sandwich between two slabs of HDP moderator with a Cd sheet in the middle could have a time constant in the range between (10 to 25 μ8εΰ) ^_1 with higher neutron flux than the simple water cube with diluted boron. Ikeda et al. in "Wide energy range, high resolution measurements of neutrons pulse shapes of polyethylene moderators" , in Nuclear Instruments and Methods in Physics Research A 239 (1985) shows that slightly better time resolution could be produced by adding two Cd sheets in three slabs of high dense polyethylene moderator. In all these cases, the extractions of the neutron beams have been done in a perpendicular direction of the large face of the polyethylene slabs.

If the neutron yield of a given thermal neutron source need to be maximized, it does not seem useful to use an array of absorber strips standing normal to the direction of the beam extraction. But a solution for this limitation was proposed (for neutron diffraction

applications) by Mayer et al. in "New moderator for pulsed neutron diffraction" in 10 ^th Int. Collaboration on Advanced Neutron Sources (ICANS - X), Los Alamos, USA (1988), using a Cd square grid with moderating material in the form of a square base prism inserted in the spaces defined by the grid, without Cd strips normal to the direction of the beam extraction. The idea underlying this concept is to gain neutron intensity by the usual means of increasing moderator thickness, but restricting the lateral trajectories (normal to the beam extraction direction) of thermal neutrons by selectively absorbing those neutrons pruned to have higher residence time in the moderating media. Mayer et al. shows in "High efficiency moderator for pulsed neutron diffraction" in Nuclear Instruments and Methods in Physics Research A 288 (1990) that a time constant in the range of (10 μ8εΰ) ^_1 could be obtained with higher thermal neutron yield compared with the HPD - Cd sandwich proposed by Fluharty. Mayer et al. in "Neutron production and time resolution of a new class moderator for pulsed neutron diffraction" in Physica B 180 (1992), shows that the a time constant is independent of the moderator thickness and only depends on the Cd grid spacing, then the optimal moderator thickness can be attained, maximizing the thermal neutron flux of the beam independently on the a time constant.

An optimal solution for neutron diffraction using the moderator concept proposed by Mayer et al. but for a D-T pulsed neutron source, with neutrons with 14 MeV (Mayer used a source with to 2 MeV for neutrons), was obtained by Tracz et al. in "Pulsed thermal neutron source at the fast neutron generator" in Applied Radiation and Isotopes 67 (2009), obtaining similar results to the solutions proposed by Mayer.

Another solution to the disadvantages caused by putting an absorber in the direction of the beam extraction was carefully analyzed and optimized for neutron diffraction by Kiyanagi, in "Optimization of grooved thermal moderators for pulsed neutron sources, and its characteristics", in the Journal of Nuclear Science and Technology 21 (1984), by means of an optimal grooved emanating surface in a moderator to produce higher emanating neutrons with a time constant in the range of (20 to 50 The idea underlying this concept is to gain neutron intensity in the direction of the beam by means of the extraction of the thermal neutrons when are parallel to the beam direction. The resulting a time constant is not much smaller than the moderator block with the same dimension, but the thermal neutron beam is up to 4 times more intense.

The grooved and Cd grid moderator concepts have another different property compared with homogenous moderators: the neutrons beam do not follows the 1/r attenuation law. Because these are basically methods that use neutrons mainly when are in a given direction, then the neutrons have the behavior of a beam with a divergence produced by the specific geometry of the groove or the grid spacing and thickness.

It is clear that for neutron diffraction techniques some different moderator' s concepts have been explored, using much more elaborated designs than simple homogeneous geometries of highly hydrogenated materials. In these moderators, a strong reduction in the a time constant and strong improvements in the thermal neutron flux was achieved, by proper combination of mechanical heterogeneities and / or neutrons absorbers.

It is important to note that none of these methods has been applied to the neutron interrogation by gamma response of substances in the past.

A great diagnostic advantage in the sensibility and position sensitive response to pulsed thermal neutron sources may be obtained to interrogate a target or scan a zone in search for explosives or other dangerous substances, if other concept of thermal neutron source and gamma detector is used. A suitable low background measurement with all the TOF information can be obtained with thermal neutron analysis techniques, if the thermal interrogation pulsed has a die away time much shorter than the TOF between the neutron source and the target.

To produce a suitable thermal neutrons die away time, compared with the thermal neutrons TOF, a different source-moderator-shielding concept needs to be introduced, because prior art approaches basically accept the sensibility limits produced by the large die away time produced by the moderator compared with the thermal neutrons TOF.

SUMMARY OF THE INVENTION

In general and simple terms, the present invention provides a highly effective and direct manner to interrogate a target or to scan a zone with short pulsed thermal neutrons beam to measure the gamma response with TOF techniques, with high sensitiveness and velocity by using a new pulsed thermal source, with a thermal neutron die away time much shorter than the TOF of the thermal neutrons from the source to the target position.

The present invention provides a rapid and effective system for the reliable detection of explosives using a short pulsed thermal neutron analysis (SPTNA).

Unlike the prior art approaches, in which a pulsed source is immersed in a moderator built with a highly hydrogenated moderator block and surrounded by a moderator and absorber as a shielding, thereby causing a large thermal neutrons die away time, the present invention uses a pulsed fast neutron source next to a moderator in the forward direction corresponding to the direction of the beam used to interrogate the explosives, with material and mechanical heterogeneities conceived to obtain a forward collimated thermal neutron beam with high intensity and die away time shorter than the time of flight between the source and the target.

In another embodiment, the flux of the neutron beam resulting from the forward moderator could be increased with a backward zone designed with material and mechanical heterogeneities to be an efficient reflector without increasing the die away time of the forward moderator.

The resulting pulsed neutron beam can be directed to the object under investigation in order to cause thermal capture reactions in a limited small object volume that is defined by the intersection of the thermal neutron beam and the screened object. By choosing an adequate fast - thermal neutron and gamma shielding of the radiation from the source and moderator to the gamma detector, and an adequate gamma detector position and energy discrimination, the signal from the object arrives at the detector and it is discriminated by the electronics in time and energy with a temporal distribution produced by the convolution between the time of the flight of the different neutrons velocity and differential die away time at the different energies. As using this approach there is as minimal source-correlated background at the detector, a significant amount of net counts coming from the target, well above the uncertainty limit can be achieved in only few neutron pulses.

In one embodiment and just as an example, a high velocity explosive detection system can be conceived that triggers an alarm level when there is a simultaneous detection of a given combination of discriminated gamma (or relationship between count rates

corresponding to different atomic elements) that are considered to be representatives of an explosive. This equipment can be enhanced also using a temporal distribution figure of merit that corresponds with high probability that the specified explosive constituent elements are present in the object at the same position, and then minimizing the possibility of triggering a false alarm.

Another independent feature and advantage of a high velocity explosive detection system, is that the electronic processing system could trigger a warning alarm when certain elements are detected in positions that could be used to hide an explosive to the thermal neutrons used by the equipment to interrogate the zone.

In an alternative embodiment and by way of example only, a high velocity explosive detection system can be designed to scan a zone and to detect a target by the combination of the motion of the source and the detection system with a movable target zone. The object under investigation can be placed in a movable belt, or can fixed to the land and the detection system in a vehicle then producing a relative movement between the object and the interrogation system. If the explosive is large enough to produce a signal during several neutrons pulses, the electronic processing system needs to be able to compensate the higher intensity of the discriminated gamma produce by the larger target, with the different temporal distribution of the convolution of the different time responses.

Another independent feature and advantage of the high velocity explosive detection system, is that the system can be used to extend the interrogation time, or to reduce the relative velocity movement, by a decision of the operator of the equipment or because was triggered an alarm level. This reduction in velocity can be used to interrogate the target for more characteristics gamma rays that could produce a signal well above over background level or improve the time distribution count rate statistics. Then more interrogation time can be used for the suspicious target, because the other constituents have low concentration, low cross section or low gamma yield compared with the constituents included in the detection logic of the first alarm level.

Another independent feature and advantage of the high velocity explosive detection system is that the high sensitivity of the system with thermal neutrons can be used to design a system to scan persons in a very efficient gamma detection system embedded in a geometry that allows fast transient of the persons with extremely very neutron dose, only triggering few accelerator pulses when the person is at the middle of the detector geometry, walking at normal velocity. This system, with proper correction of the source and detectors positioning, die away time and attenuation introduced by the water contained in the human body could be used to detect explosive in small quantities and very low radiation doses, or to trigger a warning message.

It is still another feature of the invention to provide a explosive detection system to be used a relative large distances, using that the forward moderator can be designed with a beam behavior, not following the 1/r attenuation law. Also a focusing device can be added close to the forward moderator in order to reduce the divergence of the beam. The particular advantage of this concept is that could be used to detect explosive far away from the neutron source and the detector, and with sufficient large gamma detection panels, could be used to detect at thigh velocity or a vehicle in movement, improvised explosive devices in a distance in the range of several meters to a few tens of meters.

Generally speaking, the invention is a system and method for fast and effective detection and location of explosive substances, applicable for the detection of explosives in underground mines, general and carry-on luggage, passenger inspection, and improvised explosive devices (IED). The detection apparatus includes a pulsed thermal neutron beam, a gamma ray detection system, data collection modules and detection processing modules. The pulsed thermal neutron beam is produced by fast neutron moderation to thermal energies, with a fast extraction of the thermal neutrons in the direction of the explosives to be interrogated, with a die away time for the thermal neutrons shorter than the time of flight (TOF) between the source and the location of the interrogated substance. If the interrogation object or surface have explosive substances, characteristically gamma rays radiate isotropically from the interrogated substance when is irradiated with thermal neutrons. A portion of these gamma rays are detected by the gamma ray detection system, which is placed apart from the short pulsed thermal neutron source. The detectors electronics include a set of different energy discrimination regions of interest (ROI) in pulse which are related with the different energies of the characteristics gamma rays selected for the elemental constituents. The temporal distribution of the count rate obtained in the different ROI' s occurs when there is no synchronic background from the interaction between the neutron source and its shielding with the gamma ray detection system.

The detailed shape of the temporal distribution obtained in the different ROI' s is produced by the convolution between the velocity and time distribution of the thermal neutrons beam with the TOF due to the distance between the moderator and the position of interrogated elements. The detection processing modules determines if the different candidates of nuclear reactions coincide in space and quantity to trigger an alarm level and/or define the size and position of the explosive. A small gamma ray detection system could be used for underground mines detection and humanitarian demining. A very high efficient gamma ray system could be used for fast detection of explosives in luggage and to inspect passengers by virtue of the use of few pulsed thermal neutrons with correspondent low dose- conversion coefficient and proper source triggering when the passenger and luggage is in proper position. A fast and long distance IED interrogation system is produced by combining large gamma ray panels with intrinsic narrow beam characteristics of short thermal pulsed moderators and special focusing elements.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the pulsed fast neutron source injecting the neutrons in the forward moderator to produce a short pulsed thermal neutron beam (SPTNB).

FIG. 2 is a schematic representation that shows a schematic Cd grid with moderator between the Cd strips, used as a forward moderator for a fast pulsed fast neutron source to produce a SPTNB.

FIG. 3 is a schematic representation that shows a grooved moderator used as a forward moderator for a fast pulsed fast neutron source to produce a SPTNB.

FIG. 4 is a schematic representation that shows a given combination of a schematic Cd grid with a grooved moderator used as a forward moderator for a fast pulse fast neutron source, to produce a SPTNB.

FIG. 5 shows a schematic representation of a given forward moderator combined with the backward reflector in order to increase the flux in the neutron beam without increasing the neutron die away time. FIG. 6 shows a schematic representation of a forward moderator combined with the backward reflector with a shielding for fast and thermal neutrons that are not in the direction of the neutron beam.

FIG. 7 shows a schematic representation of an apparatus for detecting explosives substances positioned in front of the neutron beam, and detected with gamma ray detection panels close to the short pulse thermal neutron source, in accordance with this disclosure.

FIG. 8 shows a schematic representation of an apparatus for detecting explosives substances by interrogating objects located in a moving belt, with increased gamma efficiency but putting more combination of gamma detectors.

FIG. 9 shows a schematic representation of an apparatus for detecting underground explosives substances and/or mines for military or humanitarian demining.

FIG. 10 shows a schematic upper view of a moving apparatus for detecting explosives substances hidden in a person while the person is walking, with very high total gamma efficiency when the person is in the middle of the high efficiency and irradiation detection system.

FIG. 11 shows a schematic representation of an apparatus for detecting remote explosives substances with an additional focusing device and large panels of gamma detectors.

DETAILED DESCRIPTION OF THE INVENTION AND DRAWINGS

Description will now be given with reference to the attached Figs. 1-11. It should be understood that these figures are exemplary in nature and in no way serve to limit the scope of the invention, which is defined by the claims appearing hereinbelow.

FIG. 1 illustrates graphically an apparatus with a moderator to produced pulsed thermal neutron beam with the a temporal behavior of a short die away time compared with the die away time of a similar pure homogenous moderator with the same size, for detecting explosives and other substances (not shown) in a target (not shown) by detecting the gamma rays (not shown) with gamma detectors (not shown) in accordance with the present invention. The apparatus has three main components, a pulsed fast neutron source 50, a heterogeneous moderator 60 located in the forward direction, defining the forward direction 70 as the direction in which the system is designed to produce the thermal neutron beam 80 to interrogate the target (not shown). The duration of the neutron pulse of the pulsed fast neutron sources need to be shorter than the moderator thermal neutrons die away time, or the time of flight of the main fraction of the thermal neutrons to arrive to the position of the explosive (not shown) to be detected. The heterogeneities in the heterogeneous moderators 60 are spatial distribution of the moderator substance (usually a solid or liquid highly hydrogenated material compound by hydrogen with heavier atoms with low thermal neutron capture cross section), combined or not with different zones of thermal neutron absorbers, conceived to have short die away time for thermal neutrons compared with the die away time for thermal neutrons for the same pure moderator substance with the same external dimension. The applicability of the

heterogeneous forward moderator 60 to detect explosives (not shown) located in the trajectory of the thermal neutron beam 80 is established when the short die away time is also smaller than the time of flight of the main fraction of the thermal neutrons to arrive to the position of the explosives (not shown) to be detected.

There are several alternatives to build a short pulsed thermal neutron beam source (SPTNBS) for detecting explosives, the embodiment of FIG. 2 illustrates one of the more simple techniques to produce this type of heterogeneous moderator utilizing thin thermal neutron absorber strips 100 in a square arrange, with a face without thermal neutron absorber sheet perpendicular to the forward direction of the thermal neutron beam, with highly hydrogenated solid or liquid moderator filling the space between the absorber strips, and a thermal neutron absorber sheet cover in the opposite face of thermal neutron beam face to absorb the thermal neutrons that are reflected from the surrounding media to the moderator.

The embodiment of FIG. 3 illustrates another simple technique to produce this type of short pulsed thermal neutron beam for explosive detectors utilizing a simple highly hydrogenated solid moderator 110 with mechanical grooves 120 in the face perpendicular to the forward direction of the thermal neutron beam 80, with a neutron absorber covering all the other 5 faces of the heterogeneous moderator. A similar grooved moderator concept could be build with a highly hydrogenated liquid moderator if the grooved shape is produced by a solid made with a material with high atomic number and low thermal neutron capture cross section.

The embodiment of FIG. 4 illustrates another more complex technique to produce this type of short pulsed thermal neutron beam utilizing a combination of thin thermal neutron absorber strips 150 boxes surrounding with each box a small grooved 160 highly

hydrogenated moderator. The embodiment of FIG. 5 illustrate an improvement of the apparatus of FIG 1, adding a backward reflector 200 to those fast neutrons produced by the pulsed fast neutron source 50 of FIG. 1. This reflector is not in the solid angle of the forward short pulsed thermal neutron beam moderator 60. This backward moderator in conceived to increase the income flux of those neutrons with higher energy than the thermal neutrons to the forward short pulse thermal neutron beam moderator but without increasing at the same time the thermal neutron die away time in the neutron beam without reflector. This can be achieved by using a homogeneous mixture of a highly hydrogenated reflector and an thermal neutron absorber, or with a proper spatial distribution of the highly hydrogenated reflector substances combined or not with different zones of neutron absorbers. Alternatives for this backward reflector could be similar to the forward moderator alternatives of FIG. 2, FIG.3 and FIG. 4, but using grid spacing or grooved characteristics to ensure a die away of the thermal neutrons shorter than the one of the forward moderator, orienting the reflecting neutrons face in the same direction of the neutron beam.

The embodiment of FIG. 6 shows an improvement of the apparatus of FIG. 2 adding a fast and thermal neutron shielding 210 to reduce the income flux to the backward reflector and a forward moderator 60 to reduce the time dependant background in the gamma detectors (not shown) and also to reduce the neutron dose to the operator and public. This thermal neutron shielding is a combination of a highly hydrogenated moderator with size and concentration of a neutron absorber, high enough to ensure that its die away time is shorter than the SPTNBS die away time.

Referring to FIG. 1, FIG. 5 and FIG. 6, the FIG. 7 illustrates a schematic view of a complete explosive detection equipment designed using the SPTNBS of FIG. 6. The interrogated object 300 intercepts the thermal neutron beam 80 then producing characteristics gamma rays 310 which are radiated isotropically and detected with an array of gamma detectors 320 adjacent to the neutron beam 80. Each detector has their own pulse-height detector and time electronics 330, surrounded with shielding 340 for fast and thermal neutron and gamma rays. The output of the electronics of the detectors is then analyzed by the detection processing modules 350 to elaborate the energy and time information in order to produce the information about the composition and location of the interrogated object.

One particular type of pulsed fast neutron source 50 have the capabilities to generate fast neutrons pulses having a neutron yield of up to 3 x 10 ¹⁰ neutrons per second at present, with a neutron energy of 14 MeV per neutron using D-T fusion reactions. Typically these pulsed neutron generators are in the form of a tube of about 2 to 5 cm in diameter and 1 meter long.

One particular type of highly hydrogenated material for the heterogeneous moderator 60 is a high dense polyethylene or water, if proper casing is assured. A particular type of thin metal thermal neutron absorber neutron strips 100 and 150 sheets are made by Cd thin metal sheets, which have a very high neutron absorber cross section for neutrons with less than 0,7 eV. If the Cd sheet are about 1mm of thickness, the sheet is consider as black for neutrons with less than 0,7 eV. This energy limit has been taken as the upper limit of the thermal neutrons for simplicity, but this is only for a general description point of view because this limit is more corrected called the Cd cut off limit (according with Beckurtz et al.) because there is a small fraction of neutrons in the range between the 0,7 eV and 1 eV that still have upper scattering then correspond to the thermal neutron scattering zone. For the present invention this difference is not relevant because in this energy range the proposed short pulsed thermal neutron beam die away time is very small and the time of flight is also very small, then it does not have a relevant effect on the signal to background ratio in the gamma detectors 320.

One particular type of gamma detector 320 panels could be build by using arrays of large Nal(Tl) scintillations crystals, in the range between 3 x 3 x 3 to 5 x 5 x 10 inches. These scintillators have a very large detector and photo-peak efficiencies at high energy as could be expected to be used for the interrogation of Nitrogen, which produced gammas rays 340 with 10,83 MeV with relative high yield. The electronics 330 for pulse high discrimination and timing discriminate the energy and timing separately for each detector to reduce the effect of pulse high analysis distortion by pile up, and to reduce the dead time in the electronics. But if a particular embodiment does not have the problems related with high count rate (pile up, dead time), the detectors can be grouped in a few or even a single one electronic box for pulse-height discrimination and timing.

The detection processing modules 350 can be a simple alarm level if the signal produced by the detector corresponds to a suspicious count rate in the selected energy ROI with a suspicious time distribution, or could have an algorithm to invert the time of flight convolution produced by the target distance and size with the die away time at the thermal neutron source in order to calculate the spatial distribution of the interrogated nucleus. Then the technique can be used to look for the spatial and energy distributions of atomic elements matching the explosive composition. The embodiment of FIG. 8 illustrates an schematic representation of an apparatus for the detection of explosive substances 500 hidden in a close box or luggage 510 and being interrogated with a SPTNBS 520 apparatus, with several arrays of gamma rays detectors 530 located around the irradiation position in order to increase the gamma ray detection efficiency to have fast velocity to detect small quantities of explosives. The different position of the gamma ray detectors do not influence the time of flight because the only relevant time of flight corresponds to the neutron source to target distance, because average thermal neutrons flight at 2200 m/s, and gamma rays flight at 3 x 10 m/s. Then the high number of detectors will increase the equipment sensibility to have a higher detection velocity, with the belt running continuously at least at 0.5 m/s. This embodiment could be used to extend the interrogation time by stopping or reducing the velocity of the moving belt based on the decision of the operator of the equipment or because an alarm level was triggered. With lower velocity, it would be ease to interrogate the target for more characteristics gamma rays or improved the statistics of the time distribution count rate, and to give more information to the operator. The system could also trigger an alarm if a large hydrogenated volume has been put inside the inspected box or luggage, because is very easy to detect large hydrogen volume with the gamma detector 530. This can be useful because of the potential risk of a countermeasure placed by an attacker, based on the attenuation of neutron signal with a highly hydrogenated volume. In alternative embodiment of FIG. 8, a second neutron short pulsed thermal neutron beam 520 apparatus can be placed in the opposite side of the inspection volume in order to reduce the effect of the neutrons moderation.

The embodiment of FIG. 9 illustrates a schematic representation of an apparatus for the detection underground explosives substances and/or mines for military and humanitarian demining. A combination between a SPTNBS 520 apparatus and detector 530 is placed in a platform in front of the moving vehicle 610 to prevent the detonation of the vehicle if an explosive substance 620 is below the vehicle. With a proper neutron yield and gamma detection panels the beam 80 can scan for explosives 620 continuously in the zone between 10 to 20 cm below the earth level at a velocity of at least 1 to 2 m/s. In this embodiment, the interrogation time can also be extended by stopping or reducing the velocity of the vehicle based on a decision of the operator or because an alarm level was triggered. This reduction of the scanning velocity can be used to thoroughly interrogate the target for more characteristics gamma rays or to improve the statistics of the time distribution count rate, and then giving more information to the operator. The embodiment of FIG. 10 illustrates a schematic representation of an apparatus for detection of explosives substances hidden in a person 700 while the person is walking; with very high total gamma efficiency produced by the combination of moving gamma detection panels 710 and fixed gamma detection panels 720. When the person is in the middle of the high efficiency detection system the apparatus triggers just a few neutron pulses from the SPTNBS 520 to interrogate the person. The embodiment was shown as a rotary panel and this geometry could be used to scan a continuous walking person stream if in the different rotary panels 730 gamma detectors are placed looking for a gamma signal in both faces of each rotary panels. In another embodiment a different equipment for the same objective could be built with less gamma ray detectors if the system is built with a sequence of two sliding door open and closing continuously, triggering just a few neutron pulses during the small instant in which the two doors are closed. This type of equipment is possible because the dose quality factor for thermal neutrons is just a small fraction of that corresponding to the fast neutrons and because the sensibility of the detection has been strongly increased by the reduction of the time correlated background.

The embodiment of FIG. 11 illustrates a schematic representation of an apparatus for the interrogation and detection of remotely hidden explosive substances 800 corresponding to an improvised explosive device (IED). The apparatus is designed using a SPTNBS 520 combined with an additional thermal neutron beam focusing element 810. An example of the thermal neutron beam focusing element can be built with a structure of sheets of small angle scattering materials like Ni. The gamma response is measured by the gamma array detection panel 530. The scheme shows a very small gamma detection panel for illustrative purposes, but a large panel, in the range of 1 to a few square meters of area could be needed depending on the desired detection distance 820 between the explosive 800 and the SPTNBS 520, the efficiency of the short pulsed thermal neutron moderator 60, the divergence angle of the beam 80 and the neutron yield of the fast pulsed neutron source 50.

It should be emphasized that while most of the detection details where base on the rationale about to detect explosives by interrogating the target with thermal neutron and measuring the gamma response, similar identification objective could be achieve with other non explosives substances if the elemental composition could be distinguish by the gamma response to thermal neutrons without departing from the scope of the invention.

Having described certain embodiments of the invention, it should be understood that the invention is not limited to the above description or the attached exemplary drawings. Rather, the scope of the invention is defined by the claims appearing hereinbelow and includes any equivalents thereof as would be appreciated by one of ordinary skill in the art.