Patent Publication Number: US-2009230315-A1

Title: Neutron Imaging Camera, Process and Apparatus for Detection of Special Materials

Description:
ORIGIN OF THE INVENTION 
     The invention described herein was made by one or more employees of the United States Government and may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     FIELD OF THE DISCLOSURE 
     This disclosure relates generally to measurement apparatus for detection of special nuclear materials, in particular, to a neutron imaging camera, and more particularly, to techniques and apparatus capable, for example, of functioning as an inspecting and/or monitoring device in many applications on land, sea, and air platforms, both manned and unmanned to determine presence and/or location of special nuclear materials using both passive and active interrogation methods. 
     BACKGROUND 
     The increasingly large volume of international trade is typically effectuated by shipping goods in containers, also known as ISO containers or isotainers. “ISO” refers to The International Organization for Standardization, an international standard-setting body composed of representatives from various national standards bodies which produces world-wide industrial and commercial standards. 
     Such containers are constructed so that they may be manipulated via cranes or other heavy equipment, and thus loaded and sealed intact onto and/or readily transferred between container ships, railroad cars, planes, and trucks, for example, to effectuate intermodal transport capability. In the context of ship-borne cargo, containers are stored in container storage yards prior to and after transfer from ship to shore and vice versa, and are typically stacked about four containers high within the storage yard. 
     The sheer number of containers transported by a single container ship or box ship, or one train or other vehicle, from one port or country to another, together with the rather large internal volume of each container, render comprehensive inspection of incoming goods shipped in this manner impractical and ineffectual. Among other things, the delay involved in opening each individual container, removing the contents, inspecting the materials by hand, replacing the contents etc. represent excessive costs. This also would result in delay crippling to international trade, and inspection via this means additionally and necessarily results in damage to some fraction of the items being shipped. Over one million containers enter the United States daily, via a combination of sea, air and land transportation. 
     Alternative methods for attempting to detect illegal importation of fissile materials, that is, materials which could be employed in forming a “dirty bomb” or other nuclear explosive device, rely on scanning procedures that introduce onerous delays in trans-shipment of materials, incur unreasonably high costs in practice, and do not pinpoint location of potentially devastatingly deadly materials with sufficient accuracy. 
     At the same time, increasing concern regarding illegal importation of even relatively small amounts of special nuclear materials, including weapons-grade fissile materials, such as Plutonium-239 ( 239 Pu), has resulted in desire to promote more thorough inspection of goods being imported into a country, with a goal of interception and interdiction, prior to reaching or passing through US ports. However, it is not practical, for many reasons, including inability to effectively search for some types of nuclear materials contraband via hand inspection, to attempt comprehensive hand inspection of the contents of each container. 
     Nuclear materials may be tracked via indicia such as detection of a number of different particle types, including alpha particles, beta particles (energetic electrons, e − ), neutrons, and gamma rays emitted from these types of matter. However, of these indicia, alpha particles, beta particles (energetic electrons, e − ), and gamma rays are also readily masked via suitable shielding. 
     These various problems and developments indicate increasing need for new tools and/or processes facilitating rapid location and identification of any particular container or other repository containing special nuclear materials, such as weapons-grade plutonium, without requiring excessive labor, and without inducing delay in trans-shipment of goods. For the reasons stated above, and for other reasons discussed below, which will become apparent to those skilled in the art upon reading and understanding the present disclosure, there are needs in the art to provide improved detectors in support of increasingly stringent and exacting performance and measurement standards in settings such as “hands-off” or “stand-off” inspection of relatively large volumes of goods or materials via passive or active interrogation. 
     SUMMARY 
     The above-mentioned shortcomings, disadvantages, and problems are addressed herein, which will be understood by reading and studying the following disclosure. 
     In one aspect, the present disclosure contemplates a multiple-cell neutron-sensitive camera. Each cell of the camera includes a combination of a time expansion chamber and a micro-well detector array coupled to the time expansion chamber. 
     In another aspect, a neutron momentum measurement apparatus includes a plurality of neutron defection cells. Each neutron detection cell of the plurality includes a time expansion chamber and a micro-well detector array coupled to the time expansion chamber. Individual micro-wells in the array are arranged in an addressable mosaic and provide electrical connections to at least two conductors. The conductors form at least two buses. The neutron momentum measurement apparatus also includes front end electronics coupled to at least one of the at least two buses. The front end electronics includes an array of charge amplifiers, shaping amplifiers, and analog-to-digital conversion circuitry coupled to at least one of the at least two buses. 
     In a further aspect, the present disclosure describes a process for determination of a location of a source of fast neutrons. The process includes detecting presence of ionizing radiation in a first cell of a neutron detection apparatus when a first threshold condition is exceeded. The process also includes determining, responsive to detecting, when a fast neutron has been detected, via presence of characteristic signature associated with a second threshold condition. The process further includes calculating momentum of the detected fast neutron when determining indicates that a fast neutron has been detected. The process additionally includes combining the calculated momentum with other calculated momentum data from at least a second cell of the neutron detection apparatus to derive a location of the source relative to the neutron detection apparatus. 
     Systems, apparatus, and processes of varying scope are described herein. In addition to the aspects and advantages described in this summary, further aspects and advantages will become apparent by reference to the drawing, and by reading the detailed description that follows. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS  
         FIG. 1  is a simplified block diagram of an overview of a shipping container storage area, illustrating one of many applications of the subject matter of the present disclosure. 
         FIG. 2  is a simplified block diagram of an array of the detector portions of a neutron imaging camera useful in the context of  FIG. 1 . 
         FIG. 3  is a simplified block diagram illustrating a plan view of a micro-well detector and associated components useful in the context of the neutron imaging camera of  FIG. 2 . 
         FIG. 4  is a simplified composite of a side view taken along section lines IV(i)-IV(i) of  FIG. 2  in combination with a side view taken along section lines IV(ii)-IV(ii) of  FIG. 3 , illustrating operating principles of a time expansion ionization chamber and a micro-well. 
         FIG. 5  depicts experimental gas gain versus voltage for three different micro-well depths. 
         FIG. 6  is a simplified exemplary representation of elastic neutron scattering in a hydrocarbon medium. 
         FIG. 7  is a simplified exemplary representation of boron-ten ( 10 B) neutron capture. 
         FIG. 8  is a simplified representation of an n-p interaction on He-three ( 3 He). 
         FIG. 9  graphically depicts triton particle range, and 
         FIG. 10  illustrates proton range, for two different conditions applicable to the n-p reaction of  FIG. 8 . 
         FIG. 11  is a graph descriptive of representative sensitivity for the n-p reaction depicted in  FIG. 8 . 
         FIGS. 12 and 13  compare Examples 1 and 2 ( FIG. 12 ) and Examples 2 and 3 ( FIG. 13 ). 
         FIG. 14  is a flowchart providing a blueprint of a process for characterization of momentum for a fast neutron using the apparatus disclosed herein. 
         FIG. 15  compares neutron spectra for shielded vs. unshielded weapons-grade plutonium. 
         FIG. 16  shows, for a one cubic meter device, simulated neutron imaging camera integral neutron detection rate from one kilogram of weapons-grade plutonium, scaled by the integration time divided by the square of the distance. 
         FIG. 17  is a simplified diagram illustrating a deployment scenario for a neutron imaging camera or cell, in accordance with an embodiment of the subject matter of the disclosure. 
         FIG. 18  is a simplified diagram illustrating a deployment scenario for a neutron imaging camera or cell, in accordance with an embodiment of the subject matter of the disclosure. 
     
    
    
     DETAILED DESCRIPTION  
     In the following detailed description, reference is made to the accompanying drawings that form a part hereof, and in which are shown, by way of illustration, specific embodiments which may be practiced. These embodiments are described in sufficient detail to enable those skilled in the art to practice the embodiments, and it is to be understood that other embodiments may be utilized, and that logical, mechanical, electrical, and other changes may be made, without departing from the scope of the embodiments. As used herein, the term “drift” as applied to ions and charged particles implies motion of the individual ions or charged particles, responsive to an applied electrical field (in contradistinction to motion of particles via other physical processes, such as diffusion, etc.). 
     Ranges of parameter values described herein are understood to include all subranges falling therewithin. The following detailed description is, therefore, not to be taken in a limiting sense, 
     As used herein, the term “neutron imaging camera” is defined to describe a device capable of triangulation of a source of neutrons via multiple “cells” of such a neutron imaging camera. Each cell, in turn, is defined to include a single active gas volume bounded by a micro-well detector array on one side, a drift electrode on an opposed side, and a field shaping grid surrounding a volume between the one and the opposed side. 
     The detailed description is divided into eight sections, in the first section ( FIG. 1  and associated text), a system level overview is provided. In the second section ( FIGS. 2 through 5  and associated text), a physical example of a neutron detection system capable of identifying small quantities of special nuclear materials (including highly enriched uranium, weapons-grade plutonium, etc.), and their location, is presented, in the third section ( FIGS. 6 through 11  and associated text), several examples of neutron interactions are described. 
     In the fourth section ( FIGS. 12 and 13  and associated text), general characteristics applicable to the examples of the preceding sections are described, in the fifth section ( FIG. 14  and associated text), a process for characterization of ionization events is described. In the sixth section ( FIGS. 15 and 16  and associated text), general characteristics of the principles of the disclosed detection apparatus are described. In the seventh section, a variety of alternative sensing and deployment scenarios are presented, in the eighth section, a conclusion of the detailed description is provided. A technical effect of the systems and processes disclosed herein includes at least one of facilitating capability for measurement of direction of fast neutrons emitted from special nuclear materials and rapid determination of location of even relatively small amounts of such special nuclear materials. 
     §I. System Overview 
       FIG. 1  is a simplified block diagram of an overview of a shipping container storage area  100 , illustrating one of many applications of the subject matter of the present disclosure. The container storage area  100  includes a large number of shipping containers  102 , typically stacked four high. In the example of  FIG. 1 , only one container  104  (shaded) contains weapons-grade plutonium. A neutron imaging camera  106 , which includes a processor (not illustrated in  FIG. 1  for simplicity of illustration and ease of understanding) and is able to employ multiple neutron detection events in order to define a range  108  of angles within which the container  104  is located. When the neutron imaging camera  106  comprises multiple cells  210 , both angle and distance may be found using well-known principles of back projection and triangulation. 
     Neutrons having energies E n  of less than 200 keV (200 kilo electron volts) have severely limited angular and energy information vis-a-vis the source from which they originated, due to scattering events. “Fast neutrons” (those having energies of greater than ˜0.2 MeV, or mega electron volt, and particularly, those having energies above one-half MeV) are difficult to shield, and display minimal scattering in atmosphere. As a result, directional information is retained for fast neutrons within a radius consistent with inspection (for example, on the 
     When multiple cells  210  or neutron imaging cameras  106  are utilized, the range can be defined via stereoscopic principles. As a result, the neutron imaging camera  108  is capable of uniquely, non-invasive and rapidly identifying the particular container  104  containing the weapons-grade plutonium or other special nuclear materials by detection of neutrons having an energy above 0.2 MeV or in a range extending at least from 200 keV MeV to several MeV or more. 
     Detection of neutrons, rather than other fission products, by the presently-disclosed neutron imaging camera  108  presents advantages in that neutrons are not as readily shielded as many other types of radiation emergent from such radioactive decay, i.e., beta particles, gamma rays and alpha particles. Fast neutrons are also not as readily scattered by the atmosphere and other materials as are other types of radiation. As a result, determination of the three-dimensional direction and energy of reaction products allows determination of the angle of the source from which the neutrons originated relative to the neutron imaging camera  106 . Determination of the angles from multiple cells in the neutron imaging camera  106  allows determination of the location of the source via stereoscopic comparison of data from at least two cells or via triangulation from a neutron camera  106  that includes two or more cells. As a result, a stand-off or remote sensing capability for rapidly determining presence of special nuclear materials is realized. The neutron imaging camera  106  is described in more detail in §II, infra. 
     §II. Simple Example Of A Neutron Imaging Camera 
       FIG. 2  is a simplified block diagram of neutron detection apparatus or neutron imaging camera portions  206  including an array of neutron detector portions or neutron detection cells  210  useful in the context of the neutron imaging camera  106  of  FIG. 1 . The neutron detector portions or neutron detection cells  210  in the group of such cells  210  forming the neutron imaging camera  206  each include a time projection tower or time expansion ionization chamber  212  having a drift electrode  214  at one end. In one embodiment, an array of field shaping electrodes or wires  216 ,  216 ′, . . . ,  216 ″ extend around a body of each of the time expansion chambers  212 . In one embodiment, a single array of field shaping electrodes or wires  216 ,  216 ′, . . . ,  216 ″ surround the ensemble of time expansion chambers  212 . 
     A detector array  218  is located at an end of each time expansion chamber  212  distal from the respective drift electrode  214 . Typical dimensions for the cells  210  are on the order of  50  cm by  50  cm (corresponding to the area of the drift electrode  214  and thus to the area of the detector array  218 ). As a result, in this example, four cells occupy a area of about one square meter, however, larger or smaller cells may be employed, and may be chosen specifically for the task at hand. 
     The detector array  218  is biased positively with respect to the associated drift electrode  214 . As a result, and as is described in more detail below with respect to  FIG. 4 , electrons e −  arising from ionizing events in each of the time expansion chambers  212  drift from a respective point of origin towards the detector array  218  associated with that time expansion chamber  212 . 
     It has been found that the time required for an electron e −  to drift (vertically downward) to the detector array  218  is reduced by some three orders of magnitude, with relatively little diffusion (lateral motion), via introduction of an electronegative gas in appropriate proportions. As an example, an electron e −  combines with a carbon disulfide gas molecule CS 2  to form a drift ion, CS 2   − . Alternatively, any of many other gases might in principle be usefully employed. These may include methane and other hydrocarbons, other electronegative gases, such as sulfur hexafluoride (SF 6 ), nitro-methane (CH 3 NO 2 ), carbon tetrachloride (CCl 4 ), and other known gases. 
       FIG. 3  is a simplified block diagram illustrating a plan view of a micro-well detector array  318  and associated components useful in the context of the neutron imaging camera  200  of  FIG. 2 . The view of  FIG. 3  corresponds to looking downward towards the detector array  218  in the representation of  FIG. 2 . 
     The micro-well detector array  318  includes a mosaic of micro-wells  319  having a vertical pitch  321   V  and a horizontal pitch  321   H . Individual micro-wells  319  within the array  318  are Illustrated as being arranged in rows  322  and columns  323 . Output buses  324  and  325  are illustrated as forming a Cartesian array, allowing signals from each micro-well  319  to be independently identified, processed and characterized. Front end electronics  328  are only shown as being associated with the rows  322  for ease of illustration and simplicity of description. It will be appreciated that similar or other signal processing and conditioning circuitry is associated with the columns  323 . 
     Typical values for the vertical pitch  321   V  and the horizontal pitch  321   H  are on the order of four hundred micrometers, although larger or smaller pitches  321  may foe employed. Also, while the example shown in  FIG. 3  represents the vertical pitch  321   V  and the horizontal pitch  321   H  as being approximately equal, for simplicity of illustration and ease of understanding, it will be understood that the vertical pitch  321   V  and the horizontal pitch  321   H  need not be equal. It will also be appreciated that while the individual micro-wells  319  are shown as being arranged in an array  318  in conformance with a right-angled Cartesian coordinate system, any coordinate system may be employed in arranging the micro-wells  319 , provided that the addressing scheme associated with the buses  324  and  325  is appropriately adjusted. 
     The front end electronics include charge amplifiers  327  individually coupled to each row  322  and having outputs coupled to pulse-shaping amplifiers  328 , The pulse-shaping amplifiers  328  have outputs coupled to respective inputs of analog-to-digital converters A/D  329 , which include sample-and-hold circuits as an integral portion thereof. Digital signals representations of the analog signals on the row lines  322  thus are output on the bus  325 . 
     The charge amplifiers  327  associated with the examples disclosed herein typically have noise characteristics of ˜1,000 e −  RMS and sensitivities of ˜2 milliVolts per femto-Coulomb. In part due to the time expansion properties of the time expansion ionization chamber  212  of  FIG. 2 , the front-end electronics  326  may have combined characteristics supporting, for example, one to two and a half mega samples per second with, for example, twelve bits of resolution and a buffer capability of 20,000 samples per channel. 
     In practice, some 10,000 front end electronic channels may be needed. An ASIC (application specific integrated circuit) may be an attractive way to realize these functions. 
       FIG. 4  is a simplified composite of a side view taken along section lines IV(i)-IV(i) of  FIG. 2 , in combination with a side view taken along section lines IV(ii)-IV(ii) of  FIG. 3 , of a portion  406  of the neutron imaging camera  206  of  FIG. 2  and cells  306  and  406  of  FIGS. 3 and 4 .  FIG. 4  illustrates operating principles of a time expansion ionization chamber  412  (corresponding to section lines IV(i)-IV(i) through the time expansions chamber  212  of  FIG. 2 ) and a micro-well  419  (corresponding to section lines IV(ii)-IV(ii) through a micro-well  319  in  FIG. 3 ).  FIG. 4  is not drawn to scale. 
     The active tracking volume in the time expansion chamber  212  ( FIG. 2 ) or  412  ( FIG. 4 ) is bounded by a drift electrode  214  or  414  at one end, and a detector array  218  or micro-well detector array  318  ( FIG. 3 ) or a micro-well  419  ( FIG. 4 ) forming a portion of a detector array such as  318  at an opposed end. The drift electrode  414  is negatively biased to a drift voltage V D  by a power supply  420  with respect to the micro-well  419 . The drift voltage is set relative to the cathode voltage to provide an electric field of about one thousand Volts/centimeter in the drift volume. Ionization electrons e −  are formed along the trajectory of ionizing particles, and those electrons combine with electronegative gas molecules to provide negative ions  436  to drift towards the micro-well detector element  419 . The ionized gas molecules  436  drift toward the anode (formed by the micro-well  419 ) of the time expansion chamber  412 . The electrons are stripped from the negative ion in the much higher electrical fields within the micro-well detector array  318  and micro-wells  319 ,  419  and an avalanche of secondary gas ionization results in the strong electric field (circa 40 kiloVolts/centimeter) set up by a high voltage V M  applied between the anodes and cathodes of the micro-wells  319 ,  419 . The avalanche charge is collected on the anode electrode  440  and an equal but opposite image charge is collected on the cathode electrode  444 . Signals from the anode electrode  440  and the cathode electrode  444  are produced essentially simultaneously, thereby allowing the x, y position of the micro-well  419  with the avalanche to be determined from a time correlation of the charge pulses in the transient digitizer outputs. 
     Negative ions  436  resulting from ionization of the electronegative gas molecules drift much more slowly than an electron e −  would. This results in substantial effective time expansion with 
     respect to the arrival of these ions  436  at the detector array  218  or micro-well detector array  318  or the micro-well  419 . Consequently, speed requirements for the electronic detection apparatus (e.g., front end electronics  329  of  FIG. 3 ) are greatly reduced and yet allow them to be able to determine relative times of arrival of the tons in a series of micro-wells  319  within the micro-well array  318  or a series of micro-wells  419  along a two-dimensional projection of the path of the ionizing particle. 
     A dielectric substrate  439  supports the micro-well  419 . A bottom conductor forms an anode  440  of the micro-well  419 . A dielectric material  442  separates the anode  440  from a cathode  444 . 
     The micro-well  419  has a depth  446  that is typically on the order of seventy-five to several hundred micrometers, and a width  447  that may be one hundred to several hundred micrometers. The width  447  may be defined as a fraction of the pitch  321 , with values of about one-half providing useful results, although larger or smaller ratios may be employed. 
     A power supply  448  provides a multiplication voltage V M . As a result, a high field gain region  449  is realized deep in the micro-well  419 , and can give rise to a gain of at least 30,000 via avalanche multiplication of the primary electron e −   438  without suffering instability. 
     One benefit to this geometry is that ultraviolet radiation from the avalanche process giving rise to the gas gain of the micro-well  419  is shielded. The electronegative gas, if poly-atomic as is CS2, is strongly absorbing of UV photons. As a result, most of that radiation is absorbed within each micro-well  419 , avoiding breakdown from photon feedback and thus obviating need for a quench gas, 
     The dielectric material  442  is typically about 400 micrometers thick, i.e. has a thickness similar to the diameter of the micro-well  419 . When the cathode  444  is 800 volts more negative than the anode  440 , a field of about 20,000 volts per centimeter is realized within the micro-well  419 . The micro-well detector  419  is a type of proportional counter detector and gas gain can be realized with a wide range of gases and mixtures. For example, use of argon is able to provide gas gains of in excess of 10,000, under such conditions (see  FIG. 5  and associated text, infra). Use of noble gases, also known as the helium family or the neon family, i.e., one or more of helium (He), neon (Ne), argon (Ar), krypton (Kr), or xenon (Xe), for example, as a proportional gas or ionization gas, within the micro-well  419 , provides significant gas gains. Gas gain can be realized with tend to provide relatively favorable ionization energies vis-a-vis at least some other gases, however, other gases may be usefully employed. 
     Micro-wells  419  having a pitch of circa four hundred micrometers and a diameter of about two hundred micrometers provide sufficient spatial resolution to track a two-dimensional plot of the trajectory of the ionizing particles  432 . Measurement of differences in time of arrival provides the third dimension, allowing the trajectories of the ionizing particles  432  to be determined in three dimensions. In turn, the three-dimensional trajectory of the incoming fast neutron may be inferred from the trajectories of the ionizing particles  432 . 
     Within the time expansion chamber  412 , an ionizing particle  432  causes an ionization event  434 , resulting in an electron e −  which then forms a negative ion  438  and a spalation ion or particle  438 . For example, when an incoming particle (not illustrated in  FIG. 4 ) is a fast neutron, and that fast neutron is captured by  10 B in the form of  10 BF 3 , the resultant ionizing particles are  7 Li and an alpha particle, with energies of &gt;0.95 MeV and &gt;1.7 MeV, respectively, and respective ranges of circa 2 to 3 and 4 to 8 millimeters, when the time expansion chamber  412  contains one atmosphere of enriched (90%)  10 BF 3 . In turn, these reaction products  432  each give rise to a series of ionization events  434  as they travel, and the resulting pattern of electrons e −  and thus of negative ions  438  resulting from electrons released by those ionization events  434  track the trajectories of the ionizing particles  432  and thus allow reconstruction of the trajectory of the incoming fast neutron. Similar analysis using known parameters applies to use of other gases, such as the  3 He(n, p)T reaction, described below with reference to §III(C). 
       FIG. 5  depicts experimental gas gain (up to the limit of stability) versus voltage for three different depths of micro-well  419  using P-10 (90% argon, 10% methane) at a pressure of slightly less than one atmosphere. The graph  550  in  FIG. 5  has an abscissa  552  calibrated in voltage difference across and an ordinate  554  calibrated in gas gain (e.g., in electrons per electron) on a logarithmic scale. A first curve  556  corresponds to depth  446  of 3 mils or about 75 micrometers, a second curve  557  corresponds to a depth  446  of five mils of about 125 micrometers and a third curve  558  corresponds to a depth  446  of eight mils or about 200 micrometers. The curves demonstrate gas gains in excess of 10,000 for depths  446  of five mils (125 micrometers) or more, at voltages of more than 600 volts. 
     A variety of different nuclear processes may be employed in the neutron imaging camera  106  of  FIG. 1 . In §III of the present disclosure, infra, several examples illustrative of the principles of operation of such neutron imaging cameras  106  and  206  are described. 
     §III. EXAMPLES  
     Different types of neutron interactions may be harnessed to determine directional data from fast neutrons. These include inelastic scattering, one form of which is described below in §III(A), in Example 1, with respect to a hydrocarbon scattering medium. Another type interaction is discussed in §III(B) and involves capture of a neutron by the nucleus of an atom which is then rendered unstable and undergoes radioactive decay. Example 2 describes this type of event with boron 10 ( 10 B) as the target. Yet another type of reaction, represented in §III(C), involves conversion of  3 He to triton (heavy hydrogen). Example 3 describes this type of event. Other types of known nuclear interactions, e.g., recoil from helium four, may also be employed in conjunction with the teachings of the present disclosure. 
     §III(A). Example 1 
       FIG. 6  is a simplified representation  660  of inelastic neutron scattering in a hydrogen-rich (e.g., methane, CH 4 ; or a mix of methane with ethylene, aka ethene, C 2 H 4 ) medium. An incoming neutron  662  traveling along a first trajectory  663  is incident on a scattering site  664 , such as a hydrogen atom which is part of a gaseous molecule. As a result, a recoil proton  665  is ejected from the molecule  664  and travels along a trajectory  666 . Protons, in general, are highly ionizing particles. This event, and the trajectory  666 , are marked by a trail of ionizing events, such as  434  of  FIG. 4 . 
     The neutron  662  is scattered, and, consequently the neutron trajectory  663  is modified to a new trajectory  667 . The neutron  662  continues to travel, albeit with less energy as a result of the scattering, and then undergoes a second collision at a second scattering site  668 . A second proton  671  is ejected along a trajectory  672 , and gives rise to further ionizing events. The neutron  662  continues to travel along a new trajectory  669 , striking a third molecule  870 . 
     A third proton  675  is ejected along a trajectory  676 , which also is measurable via the ionization trail created by the third proton  675 . The neutron  662  continues along a third trajectory  674 . 
     In Example 1, only the recoil protons  865 ,  671 ,  675  are ionizing particles. As a result, the trajectories  663 ,  667 ,  670  do not give rise to ionization frails. Consequently, multiple collisions are required in order to determine the angle and energy of the incoming neutron  662 . Three (as depicted in  FIG. 6 ) or more (not shown) interactions, when property sequenced, provide an estimate of the incoming neutron  682  energy and scatter angle. This approach does not provide true imaging, rather a collection of overlapping circles, one for each neutron  662  detected. The density of the overlapping circles provides a measure of the probability of the direction of the detected neutrons  662 . Multiple neutron sources, or moving sources, add substantial confusion to this approach. 
     §III(B). Example 2 
       FIG. 7  is a simplified representation  760  of boron-ten ( 10 B) neutron capture. In Example 2, the time expansion chambers  212 ,  412  ( FIGS. 2 and 4 , respectively) include a boron-ten containing gas such as boron triflouride ( 10 BF 3 ). Typically, the pressure of this gas is on the order of one atmosphere, however, other pressures may be employed. 
     An incoming neutron  762  traveling on a trajectory  782  strikes a  10 B nucleus  783 . The  10 B nucleus  783  then is transformed to excited  11 B which then promptly disintegrates to  7 Li and an a particle. Both the α particle and the lithium ion produce are ionizing particles and produce ionization trails. 
     In Example 2, fast neutrons  762  captured on  10 B give rise to  7 Li and a breakup ions having respective energies of &gt;0.95 MeV and &gt;1.07 MeV, corresponding to respective ranges of about two millimeters and six millimeters under the conditions described herein. Thus, the RMS angular uncertainty of the  7 Li and a breakup ions is ˜4.5° and ˜1.9°, respectively. The resulting angular uncertainty for the neutron  762  is estimated from the quadrature sum to be &lt;5°. 
     The reaction described above (boron neutron capture) creates a characteristic “V”-shaped pair of trajectories. In general, nuclear reactions resulting in relatively low mass of the reaction products or breakup fragments provide relatively longer resultant trajectories, and thus facilitate accuracy in directional assessments. Consequently, analysis of the data from the micro-well detectors  419  allows discrimination between neutron reaction products and other forms of incident radiation. 
     §III(C). Example 3 
       FIG. 8  is a simplified representation  860  of an n-p interaction on helium-three ( 3 He). In Example 3, the time expansion chambers  212 ,  412  ( FIGS. 2 and 4 , respectively) include  3 He and CS 2  gasses. Typically, the pressure of this gas mixture is on the order of one to several atmospheres, however, other pressures may be employed. 
     An incoming fast neutron  880  traveling on a trajectory  881  strikes an atom of  3 He  882 . This, in turn, causes a proton  883  to be ejected and to travel on a trajectory  884 . The  3 He  882  is converted to triton  885  (an atom of  3 He) traveling along a trajectory  886 . 
       FIG. 9  illustrates a graph  900  depicting range for triton  885 , The graph  900  has an abscissa  992  and an ordinate  994 . The abscissa  992  represents neutron energy on a log scale, while the ordinate  994  represents triton  885  range, also on a log scale. A curve  996  (solid trace) corresponds to triton  885  range at a pressure of one atmosphere, while a curve  998  (dashed trace) corresponds to triton  885  range at a pressure of three atmospheres. Vertical bar  999  denotes a neutron energy of one-half MeV. 
       FIG. 10  shows a graph  1000  illustrating proton range for the conditions described with reference to  FIG. 9 . The graph  1000  has an abscissa  1002  and an ordinate  1004 . The abscissa  1002  represents neutron energy on a log scale, while the ordinate  1004  represents proton  884  range, also on a log scale. A curve  1006  (solid trace) corresponds to proton  884  range at a pressure of one atmosphere, while curve  1008  (dashed trace) corresponds to proton  884  range at a pressure of three atmospheres. Vertical bar  1010  denotes a neutron energy of one-half MeV. 
       FIG. 11  is a graph  1100  descriptive of representative sensitivity for the n-p reaction depicted in  FIG. 8 . The graph  1100  has an abscissa  1102  and an ordinate  1104 . The abscissa  1102  abscissa  1102  represents neutron energy on a log scale, while the ordinate  1104  represents scaled sensitivity, also on a log scale. A curve  1106  (upper trace) corresponds to a pressure of three atmospheres, while a curve  1108  (lower trace) corresponds to a pressure of one atmosphere. Vertical line  1110  corresponds to a neutron energy of one-half MeV. 
     Three examples of nuclear interactions, relevant at least to fast neutrons, and giving rise to ionizing breakup ions, have been provided. These are discussed in comparative terms below in §IV. 
     §IV. Comparison of Examples 1, 2 and 3.  
     Some comparisons of salient characteristics of §§III(A), (B) and (C) (i.e., Examples 1, 2, and 3, supra) are provided below, in general, the n-p reaction on helium-three ( 3 He) of §III(C) requires  3 He, which presently is much more costly than other detection gases, but which is also capable of providing relatively high sensitivity. The boron-ten ( 10 B) neutron capture reaction of §III(B) (Example 2) provides less sensitivity than the n-p reaction on helium-three ( 3 He) of §III(C) (Example 3), but more sensitivity than the proton scattering process of §III(A) (Example 1). 
     Table I below summarizes examples of gases usable in various roles in neutron detection apparatus, such as are described herein. Table I includes a list of examples of gases which find utility in one or more of a variety of roles. 
     
       
         
           
               
             
               
                 TABLE I 
               
             
            
               
                   
               
               
                 Exemplary lists of gases having utility in the context of the present 
               
               
                 disclosure, including examples of gases having multiple utility. 
               
            
           
           
               
               
               
               
               
            
               
                   
                   
                   
                 e −  negative 
                   
               
               
                   
                   
                 Neutron 
                 diffusion 
               
               
                 GAS 
                 Ionization 
                 detection 
                 suppression 
                 UV quench 
               
               
                   
               
               
                   3 He 
                 Yes 
                 Yes 
                 Possible 
                 No 
               
               
                   4 He 
                 Yes 
                 Yes 
                 Possible 
                 No 
               
               
                   6 Li 
                 Possible 
                 Yes 
                 Possible 
                 Possible 
               
               
                   10 BF 3   
                 Yes 
                 Yes 
                 Possible 
                 Possible 
               
               
                 Ne 
                 Yes 
                 Unknown 
                 Possible 
                 No 
               
               
                 Ar 
                 Yes 
                 Unknown 
                 Possible 
                 No 
               
               
                 Kr* 
                 Yes 
                 Unknown 
                 Possible 
                 No 
               
               
                 Xe 
                 Yes 
                 Unknown 
                 Possible 
                 No 
               
               
                 CH 4   
                 Yes 
                 Yes‡ 
                 Unknown 
                 Yes 
               
               
                 C 2 H 4   
                 Yes 
                 Yes‡ 
                 Unknown 
                 Yes 
               
               
                 C 2 H 6   
                 Yes 
                 Yes‡ 
                 Unknown 
                 Yes 
               
               
                 C 2 H 5 OH† 
                 Yes 
                 Yes‡ 
                 Unknown 
                 Yes 
               
               
                 C 3 H 6   
                 Yes 
                 Yes‡ 
                 Unknown 
                 Yes 
               
               
                 C 4 H 8   
                 Yes 
                 Yes‡ 
                 Unknown 
                 Yes 
               
               
                 CO 2   
                 Yes 
                 Unknown 
                 Possible 
                 Yes 
               
               
                 CS 2 † 
                 Yes 
                 Unknown 
                 Yes 
                 Yes 
               
               
                 CCl 4 † 
                 Yes 
                 Unknown 
                 Yes 
                 Yes 
               
               
                 CH 3 NO 2 † 
                 Yes 
                 Possible 
                 Yes 
                 Yes 
               
               
                 SF 6   
                 Yes 
                 Unknown 
                 Yes 
                 Yes 
               
               
                   
               
               
                 *non-radioactive forms only 
               
               
                 †liquids at STP; maintained in gaseous form by keeping partial pressure below vapor pressure 
               
               
                 ‡via inelastic scattering; generally applicable to hydrocarbons 
               
            
           
         
       
     
       FIG. 12  depicts a graph  1200  illustrating relative cross-sections for triple neutron proton scattering (Example 1, §III(A)) and the boron neutron capture reaction (Example 2, §III(B)). The graph  1200  includes an abscissa  1202  and an ordinate  1204 . The abscissa  1202  corresponds to neutron energy expressed on a logarithmic scale. The ordinate  1204  corresponds to probability of detection, also expressed on a logarithmic scale. 
     A curve  1208  (solid trace) represents cross-section for the boron neutron capture reaction (Example 2, §III(B)) using 90% enriched  10 BF 3 , at one atmosphere, while a relative cross-section for triple neutron proton scattering is represented by a curve  1208  (dashed trace) in CH 4  at three atmospheres (Example 1, §III(A)). Vertical bar  1210  denotes a neutron energy of one-half MeV. Comparison of the curves  1206  and  1208  shows that the  10 BF 3  reaction provides at least one order of magnitude greater sensitivity. 
       FIG. 13  provides a graph  1300  representing a comparison of sensitivities for the triton reaction (Example 3, §III(C)) and the boron triflouride reaction (Example 2, §III(B)). The graph  1300  has an abscissa  1302  and an ordinate  1304 . The abscissa  1302  represents neutron energy on a logarithmic scale. The ordinate  1304  corresponds to probability of detection, also expressed on a logarithmic scale. 
     A curve  1306  corresponds to the triton reaction at a pressure of three atmospheres. A curve  1306  represents the triton reaction at a pressure of one atmosphere. A curve  1308  (analogous to the curve  1206  of  FIG. 12 ) illustrates probability for neutron capture on  10 B. Vertical bar  1310  denotes a neutron energy of one-half MeV. Comparison of the curves  1305  and  1306  to the curve  1308  shows that the  3 He(n, p)T reaction provides overall higher interaction probability than the  10 B reaction for neutrons with energies greater than about 1 MeV at a pressure of 1 atmosphere and at all neutron energies at a pressure of 3 atm. 
     Increasing the pressure of the  10 BF 3  gas increases the probability for neutron capture, but the angular resolution, derived from the measured of the  7 Li and α tracks, is decreased, because the track lengths are decreased. Increasing the  3 He pressure likewise decreases the  3 H and p track lengths. In this case, however, the angular resolution is improved because the  3 H and p tracks are more fully contained within the drift volume, providing a more accurate measurement of their energies. 
     The examples of §III(A) through §III(C) involve common acts when implemented as described above, using the apparatus exemplified by the discussion in §III. The acts collectively form a process and are summarized below in §III with reference to  FIG. 14  and associated text. 
     §V. Process 
       FIG. 14  is a flowchart  1400  providing a blueprint of a process for characterization of momentum for a fast neutron, and thus for determining location of a source of the fast neutrons, using the apparatus disclosed herein. The process  1400  begins with an act  1405 , 
     In an act  1410 , the process  1400  responds to one or more signals indicative of ionization events from at least one cell  210 . In other words, the act  1410  detects an ionization event that has occurred by assessing two degrees of freedom of motion of ionization electrons via two-dimensional data from a micro-well detector coupled to a first cell in a time expansion chamber  212 , via signals corresponding to that ionization event manifested on signal lines  325 ,  328 . When an ionization event has been detected via the acts of block  1410 , control passes to a query task  1415 . 
     In the query task  1415 , the process  1400  determines when the ionization event detected in the block  1410  provides indicia in excess of a first programmable threshold level. The one or more signals may arise from one or more of the signal lines  322 ,  323  of  FIG. 3 , and be manifested via appropriate elements of either or both of buses  324 ,  325 . Setting a programmable number representing a suitable duration composed of sequential time slots in conformance with clocking/timing signals provided to the front end electronics  326  comprises specification of a portion of the criteria for determination that the first programmable event criteria have been observed. The block  1410  and the query task  1415  thus collectively detect presence of ionizing radiation in a first cell  210  of a neutron detection apparatus  106  when a first threshold condition is exceeded. 
     When the query task  1415  determines that the ionization event detected via the block  1410  does not achieve the first threshold, control returns to the block  1410 . In other words, the process  1400  resets to wait for another ionization event to be detected. 
     When the query task  1415  determines that the first threshold has been achieved, control passes to a query task  1420 . The query task  1420  determines when a second threshold has been achieved. In one embodiment, the query task  1420  includes determining, from data regarding the ionization electrons, at least two paths each corresponding to a respective ionized entity via relative timing data from multiple wells of the micro-well detector array  318 , When the query task  1420  determines that the second threshold has not been achieved, control returns to the block  1410 , as described above. When the query task  1420  determines that the second threshold has been achieved, control passes to a block  1425 . 
     In the block  1425 , momentum of the incoming neutron is estimated by first characterizing the momenta of the tons which have been detected. A first portion of the momenta information may come from the query task  1420 , e.g., from assessing two degrees of freedom of motion of ionization electrons via two-dimensional data from a micro-well detector coupled to a first cell. The third component of motion is calculated by determining time differences between collisions (thus allowing the energy loss neutron velocity to be determined) and timing differences between data from the series of micro-wells along the two-dimensional projection of the ton path (allowing the three-dimensional motion of the ion to be calculated). 
     In one embodiment, the second threshold corresponds to detection of characteristics of a fast neutron being scattered in a hydrocarbon medium. In one embodiment, the second threshold corresponds to detection of characteristics of a fast neutron colliding with helium three, i.e., conversion of  3 He to triton (heavy hydrogen,  3 H). When either the query task  1415  indicates that the first threshold was not achieved, or the query task  1420  indicates that the second threshold was not achieved, the data from the most recent iteration of the block  1410  are discarded. 
     In the block  1425 , a momentum of a particle giving rise to the ionization event detected in the block  1410  and confirmed via the query tasks  1415 ,  1420  is calculated. The way in which this is done depends on the detection mechanism being employed. Trajectories and intervals between detection events are employed to calculate the momentum of the fast neutron. Control then passes to a block  1430 . 
     In the block  1430 , the momentum data calculated in the block  1425  are stored in a memory. Control then passes to a query task  1435 . 
     In the query task  1435 , the stored data from multiple detected ionization events are analyzed to determine when the cumulative amount of data is sufficient to provide an accurate estimate of a position of a source of the radiation being detected. When the query task  1435  determines that insufficient data exists for forming an accurate estimate of the location of a source of the radiation, control passes to a block  1440 , and the process  1400  iterates. 
     When the query task  1435  determines that the stored data permit an accurate estimate of the position of a source to be identified, control passes to a block  1445 . In the block  1445 , calculated angular and velocity data (momentum data) are combined with stored data from other fast neutron detection events. Control then passes to a block  1450 . 
     The types of information assessed in the query task  1435  in determining when the stored data are sufficient to estimate a source include the number of “hits” associated with each cell  210  of the neutron imaging camera  106  and the number of cells  210  which provide data that could be associated with a single source  104 . When the data indicate that multiple sources  104  are likely, the stored data elements are grouped according to the apparent direction of the source  104 , and are analyzed in the block  1445  within the context of the resulting separate groups. 
     In the block  1450 , a source  104  location is estimated from the combined data from the block  1445 . The process  1400  then ends in a block  1455 , and can iterate to refine the source  104  location estimate or trigger an annunciator to indicate presence of a source  104  comprising special nuclear materials. 
     The process  1400  incorporates characteristics common to the examples shown above with reference to §III. The characteristics common to the neutron imaging camera  106  ( FIG. 1 ) and described in more detail in §II of this disclosure are summarized below in §VI with reference to  FIGS. 15 and 16 . 
     §VI. Characteristics Relevant to Neutron Imaging Cameras 
     In this section, characteristics common to the examples of neutron imaging disclosed in §III are described. Neutron imaging is based on measuring neutron momenta, the direction and velocity or energy of the neutrons. The direction and energy of the incoming neutrons may be measured by the ionization tracks and energy deposited by recoil protons (§III(A), Example 1) or breakup particles (§III(B) and (C), Examples 2 and 3, respectively). 
     A variety of particles are emitted by special nuclear materials, including α, β particles, and γ rays. These types of particles are readily absorbed by shielding materials. Slow neutrons (E n &lt;one keV) can, in principle, provide large count rates, however, the angular and energy data they provide is severely limited. Fast neutrons (E n &gt;0.5 MeV) provide substantially fewer counts, are difficult to shield and exhibit minimal scattering along their path. Therefore, they present a long mean free path, several hundred meters, thereby preserving directional and energy information. These characteristics are employed by the neutron imaging camera  106 ,  206  of  FIGS. 1 and 2  to detect fast neutrons via processes such as are described above in §III with reference to Examples 1, 2, and 3, using the detection apparatus described in §II with reference to  FIGS. 2 through 5 . 
     A first level triggering signal is generated by comparing the signals from each of the channels (e.g., on the bus  326  of  FIG. 3 ) to a programmable threshold value. When the signal on any one channel of the bus  324  is above the threshold value for several (e.g., ˜three to five) contiguous samples, the first level triggering signal is generated. 
     The second-level trigger signal results in analysis of timing information from the micro-well detectors. This timing information, in turn, allows an estimation of the particles relative emission angles. That data can be used to construct an estimate of angular information which defines the incoming neutron. By a suitable comparison of signals from multiple detection cells, the location of one or more special nuclear material targets may be identified with substantial accuracy. 
       FIG. 15  is a graph  1500  illustrating relative neutron flux versus neutron energy for shielded and unshielded targets. The graph  1500  has an abscissa  1502  and an ordinate  1504 . The abscissa  1502  corresponds to the neutron energy expressed on a logarithmic scale. The ordinate  1504  represents neutron flux, also represented on a logarithmic scale. A curve  1506  (dashed trace) corresponds to neutron flux from a target comprising one kilogram of weapons-grade plutonium (e.g., six percent  240 Pu). A curve  1508  (solid line) describes neutron flux from the same target, but shielded by a ten centimeter thick shell of water surrounding a 5 centimeter thick special shell of iron surrounding the target. Vertical bar  1510  denotes a neutron energy of 0.5 MeV. 
       FIG. 16  shows a graph  1600  of the simulated integral neutron detection rate, generated by the neutron imaging camera produced by one kilogram of weapons-grade plutonium scaled by the integration time divided by the square of the distance. The graph  1600  has an abscissa  1602  and an ordinate  1604 . The abscissa  1602  corresponds to the neutron energy expressed on a logarithmic scale. The ordinate  1604  represents the integral of the number of neutrons multiplied by the measurement time interval (Δt), all divided by the distance squared, on a linear scale, with dimensions of neutrons per second. 
     A curve  1606  corresponds to simulating the expected detection rate from one kilogram of weapons-grade plutonium scaled by the integration time divided by the square of the distance in meters. Vertical bar  1610  denotes a neutron energy of 0.5 MeV. 
     §VII. Alternative Examples and General Discussion 
     In the preceding six sections, a number of operational principles were described, and some discussion of known phenomena as applied to new situations was presented, in this section, a variety of different implementation considerations are presented with reference to  FIGS. 17 and 18 . 
     The example described above in §I, with reference to inspection of incoming shipping containers in the context of a seaport, fails to address a number of current problems. For example, should a critical amount of special nuclear materials arrive and be off-loaded in a seaport, irreparable damage may well have already been done. Detonation of a nuclear device or dispersal of special nuclear materials in a major shipping area in the receiving country may present significant disruption of shipping, as well as major loss of life, and/or significant nuclear contamination. Consequently, what is needed is what is called “very forward deployment” of detection technologies. 
     In other words, what is needed is to detect special nuclear materials well outside of the local area. For example, it would be highly desirable to ensure that special nuclear materials are not included in shipments to a designated port by inspection of the contents (not merely the manifest) of a shipping vehicle, which may be an airborne, seaborne or land transportation vehicle, at and/or en route to the port of departure. Additional inspection, well away from the port of destination, may also be desirable, because release or dispersal of some types of special nuclear materials at a port can present disastrous consequences as well as irreparable harm. 
     A fixed location for the nuclear imaging camera  106  of  FIG. 1  or one or more of the cells  210  of  FIG. 2  may fail to provide the desired information relative to a moving vehicle containing a suspicious or undesirable payload, such as special nuclear materials. Some highly dangerous materials, such as highly enriched uranium or even moderately enriched uranium, may not be detectable based solely on passive neutron signature alone. Such materials, however, may provide recognizable neutron signatures when fission is induced via active interrogation, for example by irradiation with a suitable flux of particles, for example, by an appropriate flux of neutrons or gamma rays. 
       FIG. 17  is a simplified diagram illustrating a deployment scenario including a dispersed detection station or apparatus  1700  including a plurality of N-many neutron defectors  1706 (N), in accordance with an embodiment of the subject matter of the disclosure. The neutron detector array  1706 , in this example, includes one or multiple neutron detectors  1706 ( 1 ),  1706 ( 2 ) . . .  1706 (N), each having a respective data communications path  1750 ( 1 ),  1750 ( 2 ) . . .  1750 (N) to one or more processors  1752 . The neutron detectors  1706  may be relatively stationary, and may each comprise one or more neutron detection cells, such as the neutron detection cells  210  of  FIG. 2 , which may be organized in various ways. 
     The data communications paths  1750  may be unidirectional, that is, only supplying information from the neutron detectors  1706  to the processor  1752 , or may be bidirectional, that is, also capable of conveying instructions or other information from the processor to specific ones of the neutron defectors  1750 . In general, data from each neutron detector  1706  includes timing information as well as the type of path data provided by ionization events within each of the neutron defectors  1706 . 
     An object  1760  to be inspected is moving along a predetermined path  1764 , as indicated by the dashed line and arrow. It should be recognized that while only one row of neutron detectors  1706  are illustrated along one side of a path  1764  for simplicity of illustration and ease of understanding, both sides of the path  1764  may include linear or other forms of arrays of neutron defectors  1706 . It should also be noted that the neutron detectors  1706  need not be deployed in linear arrays and need not all be in any one plane; the neutron detectors  1706  may be at different altitudes (or depths) and may be arranged in any fashion suited to the type of inspection being performed. 
     Neutrons may be emitted from the object, and neutrons traveling along any of the paths  1768 ( 1 ),  1768 ( 2 ) . . .  1768 (N) may be detected by the respective neutron detector  1706 ( 1 ),  1706 ( 2 ) . . .  1788 (W) intersected by the corresponding path. Back projection coupled with timing data is communicated to the processor  1752  and allows the processor  1752  to use data from the discrete neutron detectors  1706  to determine which particular object  1760  (for example, a railroad car which is part of a train moving along the path  1764 , or a portion of a ship passing over or through a detection station  1700 ) includes special nuclear materials within it. 
     Some types of special nuclear materials are of concern, but provide relatively sparse amounts of neutrons in comparison to  240 Pu. This is true, even though the physical amounts of these other special nuclear materials needed to cause a nuclear event are larger than the amount of  240 Pu that could be employed for such purposes. For example, under ordinary conditions, highly enriched uranium (HEU) or even modestly enriched uranium (that is, uranium which has been processed to segregate  235 U from the dominant natural isotope,  238 U) emits neutrons at a rate that is orders of magnitude lower than the rate at which weapons-grade plutonium or  240 Pu emit neutrons. 
     Consequently, providing one or more optional sources  1780 , each capable of producing an appropriate flux  1782  of particles, such as neutrons or gamma rays, can enhance the rate of neutron emission from some types of special nuclear materials, such as enriched uranium, which may be contained within the object  1760 , to levels consistent with practical detection, that is, to produce sufficient neutrons per unit time while irradiated via the optional source (or sources)  1780  to make detection practical. When only one neutron detector  1706  is employed, and the special nuclear materials contained in the object  1760  are moving, it may not be possible to determine the location of the special nuclear materials. Use of multiple, but physically separated, neutron detectors  1706 ( 1 ),  1706 ( 2 ), . . .  1706 (N) at known locations can allow accurate determination of the presence of fissile materials and can be used to detect even fissile materials having long half-lives. In other words, irradiation of special nuclear materials that produce relatively few neutrons per unit time under ordinary conditions, may enhance a neutron emission rate to promote efficient and timely detection capabilities. 
     This can be accomplished, for example, when one or more excitation or particle sources  1780  are combined with, or dispersed near or along the path  1764 , or are configured to be able to operate in conjunction or cooperation with the neutron detectors  1706 , to provide a flux of particles along a path  1782  (indicated by a dotted line and arrow), such as a flux of neutrons or gamma rays. 
     It will be appreciated that other types of conventional monitoring equipment (e.g., infrared and/or visible light cameras, etc.) may be co-integrated into the apparatus  1700 , or with any of the other embodiments described above with reference to  FIGS. 1 through 16 . As a result, a suite of different types of sensors (e.g., visible and/or infrared cameras together with neutron detectors  1706 ) may be combined in order to achieve detection capability for a wide range of different types of materials, some of which may not be neutron sources, via a single detection station  1700 . 
     The neutron detectors  1706  may comprise individual detectors  1706 , or discrete, separated neutron detection cells (each analogous to one of the cells  210  of  FIG. 2 ) or may comprise individual (multi-cell) neutron imaging cameras (analogous to the camera  106  described above with reference to  FIG. 1 , et seq.). The links  1752  at least allow data to go from the individual respective detectors  1706  to the processor  1752 , or may be bi-directional links allowing commands to go from the processor  1752  back to the detectors  1706 , and the links  1750  may include hard-wired links, which may be encrypted and which may include RF, microwave, acoustic, or optical links. 
     The embodiment depicted in  FIG. 17  includes at least some neutron detectors  1706  having relatively fixed positions vis-a-vis a path of travel  1764  for goods  1760 . The path of travel  1764  may constitute a road, a railway, a shipping lane, a flight path, a path taken by goods being transferred by crane, etc. Relative positions of the neutron detectors  1706  and the path of travel  1765  are known, for example, via conventional GPS receivers, and the processor  1752  may include present relative positional data for the neutron detectors  1706  and the path of travel  1764 , even when one or more of the neutron detectors  1706  may be in motion. 
       FIG. 18  is a simplified diagram illustrating a deployment scenario for a neutron detector  1800 , in accordance with an embodiment of the subject matter of the disclosure. A neutron detector  1806  is depicted as traveling along a path  1808 , from a path including at least a first position  1810  to at least a second position  1812 . An object  1860  is illustrated at a relatively fixed position, and the object  1860  may contain special nuclear materials. 
     Neutrons traveling along the path  1868 ( 1 ) from the object  1860  at a first time, when the neutron detector  1806  is intersected by the path  1868 ( 1 ), will be detected by the neutron detector  1806  at the position  1810 . Similarly, neutrons traveling along the path  1868 ( 2 ) will be detected by the neutron detector  1806  when the neutron detector  1806  is intersected by the path  1868 ( 2 ), i.e., at a later point in time, when the neutron detector is at the position  1812 . 
     In contrast to the scenario shown in  FIG. 17 , where the object  1760  is moving and the neutron detector or detectors  1706  are relatively stationary, a single neutron detector  1806  which is moving can be used to determine the position of special nuclear materials which may be contained in the object  1860 , or multiple such neutron detectors  1806  may be employed, using well-known principles of triangulation to determine a unique locus associated with such special nuclear materials. As noted above, a suite of detectors of varying types may be incorporated or associated with the neutron detector  1806  to enable defection of a broad variety of signatures indicative of different types of materials of interest. Also, as noted above with reference to  FIG. 17 , one or more particle sources, such as the particle source  1880 , providing a stimulus comprising suitable flux  1882  as noted above, may be included in the deployment scenario  1800 . 
     §VIII. CONCLUSION  
     Apparatus, systems, and processes implementing a novel imaging camera based on neutron detection are described. The disclosed neutron imaging arrangements provide capability for stand-off detection of special nuclear materials via passive and/or active interrogation and find application in a wide range of terrestrial, airborne and/or marine scenarios. 
     Earth-based situations where the disclosed neutron imaging technology finds utility include facility/installation protection, border crossing monitoring (aerial or ground based), portal and high seas monitoring via active or passive detection techniques. The camera and the techniques employed by the camera are unusually rugged, respond to radiation that is difficult to obscure, provide high sensitivity, and achieve large field-of-view and accurate point-source imaging and location identification capabilities in modest form factor. 
     Although specific embodiments have been illustrated and described herein, it will be appreciated by those of ordinary skill in the art that any arrangement which is calculated to achieve the same purpose may be substituted for the specific embodiments shown. This disclosure is intended to cover any adaptations or variations. For example, although described in procedural terms, one of ordinary skill in the art will appreciate that implementations can be made in a procedural design environment or any other design environment that provides the required relationships. 
     In particular, one of skill in the art will readily appreciate that the names or labels of the processes and apparatus are not intended to limit embodiments. Furthermore, additional processes and apparatus can be added to the components, functions can be rearranged among the components, and new components to correspond to future enhancements and physical devices used in embodiments can be introduced without departing from the scope of embodiments. 
     One of skill in the art will readily recognize that embodiments are applicable to future elements capable of the functionality described herein. The terminology used in this disclosure is meant to include all alternate technologies which provide the same functionality as described herein.