Patent Publication Number: US-10775514-B2

Title: System for detecting and locating radioactive sources

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. patent application Ser. No. 16/389,980 entitled “System for Detecting and Locating Radioactive Sources” and filed on Apr. 21, 2019, which is a continuation of U.S. patent application Ser. No. 16/191,335 entitled “System for Detecting and Locating Radioactive Sources” and filed on Nov. 14, 2018, which claims the benefit of U.S. Provisional Patent Application No. 62/588,377 entitled “Directional Gamma Ray Detector” and filed on Nov. 19, 2017, and U.S. Provisional Patent Application No. 62/616,177 entitled “Directional Radiation Detector with Dual Shields” and filed on Jan. 11, 2018, and U.S. Provisional Patent Application No. 62/713,245 entitled “Directional Radiation Monitor with Middle Detector” and filed on Aug. 1, 2018, the entire disclosures of which are incorporated by reference as part of the specification of this application. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to nuclear weapon detection. More particularly, the present invention is directed in one exemplary aspect to a directional radiation detection system that determines an angle of the radiation source, relative to the system, including the sign and magnitude of the source angle. 
     BACKGROUND 
     Smuggled nuclear weapons are an urgent threat to the US and to all nations. Although radioactive weapons and their components emit gamma rays and neutrons, they are difficult to detect when shielded or obfuscated by clutter. An advanced particle detector is needed to reveal and localize threat materials rapidly and reliably. Localization data is also needed to increase the statistical power of each radiation scan in the presence of backgrounds, since even a few gamma rays or neutrons coming from a particular region of the cargo would reveal a hidden source, whereas non-directional detectors require hundreds or thousands of additional detections above background to raise a suspicion that some kind of source might be somewhere nearby. With source localization, the entire inspection process could be speeded up, reducing inspection times and entry waits at shipping ports. Clean loads could be cleared more quickly. Secondary inspections, when necessary, could use the location information as a starting point. 
     Some attempted solutions at directional detection include “gamma cameras” involving collimators such as pinhole or multi-channel collimators, or coded-aperture masks. Such detectors are notoriously inefficient since most of the gamma rays are absorbed in the collimator. Other attempted solutions use paired or elongate detector elements, which generally provide low efficiency and poor angular resolution, and require time-consuming iterative rotations to find the source. Even lower efficiency is characteristic of double-scattering type detectors that rely on measuring two Compton scatterings for directional gamma ray detection, or two proton-recoil scatterings for neutrons. 
     What is needed is a compact, rugged, efficient detector that indicates the specific direction of the source of gamma rays or neutrons without extensive searching or iteration. Preferably the new detector would have sufficient sensitivity to detect even well-shielded nuclear weapons, and sufficient angular precision to localize the source among clutter and obfuscation, rapidly, with high efficiency, and at low cost. 
     SUMMARY 
     Disclosed herein are systems for the detection and localization of a radioactive source. In an exemplary aspect, a system is described. In one embodiment, the system includes two slab-shaped shields, two side detectors, a middle detector, and a processor. The shields may be separated by a predetermined distance, and may be oriented parallel to a centrally positioned aiming plane, and may be configured to block most particles incident orthogonally thereon from the source. The side detectors may be positioned parallel to the shields, each side detector being positioned proximate to an exterior face of a respective one of the two shields. The middle detector may be positioned between the shields, closer to the front than the back of the system, and oriented perpendicular to the aiming plane. The side detectors and the middle detector may be configured to detect particles from the source and to emit signals upon detecting the particles. The processor may be configured to calculate the angle of the source relative to the aiming plane, based at least in part on the signals from the side detectors and the middle detector. 
     These and other embodiments are described in further detail with reference to the figures and accompanying detailed description as provided below. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1  is a sketch in cross-section of an exemplary system according to the disclosure comprising two side detectors, two shields, and a middle detector according to some embodiments. 
         FIG. 2  is a cross-section sketch of an exemplary system with shield protrusion and with light sensors conveying data to a processor according to some embodiments. 
         FIG. 3  is a sketch in perspective of an exemplary system facing out of the page according to some embodiments. 
         FIG. 4  is a cross-section sketch of an exemplary system with detectors comprising three scintillators of different types and a shared light sensor according to some embodiments. 
         FIG. 5  is a sketch in cross-section of an exemplary system with the addition of detector back-flanges according to some embodiments. 
         FIG. 6  is a sketch in cross-section of an exemplary system with outriggers to augment the detection efficiency according to some embodiments. 
         FIG. 7  is a sketch in cross-section of an exemplary system with trapezoidal side detectors and beveled shields according to some embodiments. 
         FIG. 8  is a sketch in cross-section of an exemplary system with an additional energy-resolving detector behind the middle detector according to some embodiments. 
         FIG. 9  is a sketch in cross-section of an exemplary system with three middle detectors according to some embodiments. 
         FIG. 10  is a sketch in cross-section of an exemplary system with three middle detectors comprising scintillators of different types, optically coupled together and viewed by a common light sensor according to some embodiments. 
         FIG. 11  is a cross-section sketch of an exemplary system in which the shields are truncated in the back region to reduce weight according to some embodiments. 
         FIG. 12  is a sketch in cross-section of an exemplary system with tapered shields to reduce weight according to some embodiments. 
         FIG. 13  is a sketch in cross-section of an exemplary system with scintillating shields and an optional scintillating light guide optically coupled to the middle detector according to some embodiments. 
         FIG. 14  is a sketch in cross-section of an exemplary system with a rear-facing detector positioned between the shields according to some embodiments. 
         FIG. 15  is a sketch in perspective of an exemplary system with split detectors abutting at the midplane, thereby enabling the system to determine whether a source is above or below the midplane according to some embodiments. 
         FIG. 16  is a perspective sketch of an exemplary vehicle inspection station incorporating multiple copies of the present system arrayed around an inspection zone according to some embodiments. 
         FIG. 17A  is a perspective sketch of an exemplary portable directional survey meter according to some embodiments. 
         FIG. 17B  is a perspective sketch of an exemplary wearable health and safety monitor according to some embodiments. 
         FIG. 17C  is a perspective sketch of an exemplary display showing two analysis results according to some embodiments. 
         FIG. 18  is a sketch in perspective, partly cut-away, of an exemplary walk-through portal with multiple copies of the system arranged in the walls of the portal according to some embodiments. 
         FIG. 19  is a sketch in perspective, partly cut-away, of an exemplary mobile radiation scanner containing multiple copies of the system according to some embodiments. 
         FIG. 20  is a graph showing the results of an MCNP6 simulation, showing the counting rates of the two side detectors versus the source angle. 
         FIG. 21  is a graph from the same simulation as  FIG. 20 , showing the calculated differential versus source angle. 
         FIG. 22  is a graph from the same simulation as  FIG. 20 , showing the counting rate of the middle detector versus the source angle. 
         FIG. 23  is a graph based on the simulation of  FIG. 20 , showing the angular correlation function that relates the source angle to the counting rate ratio. 
         FIG. 24  is a flowchart showing steps of an exemplary method for calculating the source angle according to some embodiments. 
         FIG. 25  is a flowchart showing steps of an exemplary method to rotate the system into alignment with the source according to some embodiments. 
         FIG. 26  is a flowchart showing steps of an exemplary staged analysis method to optimize the use of low detection rates according to some embodiments. 
         FIG. 27  is a flowchart showing steps of an exemplary method to update the calculated source angle. 
     
    
    
     DETAILED DESCRIPTION OF INVENTION 
     In the following description, reference is made to the accompanying drawings in which it is shown by way of illustration specific embodiments in which the invention can be practiced. Not all of the described components are necessarily drawn to scale in order to emphasize certain features and to better facilitate the reader&#39;s conception of the disclosed embodiments. It is to be understood that other embodiments can be used and structural changes can be made without departing from the scope of the embodiments of disclosed herein. 
     Disclosed herein are systems and methods for detection and localization of nuclear and radiological weapons, their radioactive components and materials, and other radiation sources that are to be located. In some embodiments, a directional radiation detection system (“the system”) can be configured to detect gamma rays or neutrons (the “particles”) from a radioactive source, and to determine the direction of the source, relative to the system, in one dimension. Embodiments of the system can determine both the sign and magnitude of the source angle, based deterministically on particle data acquired at a single orientation of the system, with high detection efficiency and superior angular resolution. Examples are presented for detecting gamma rays and/or neutrons, but the principles disclosed herein are readily applicable to any particle type. In any application involving detection of clandestine radioactive threats, the ability to rapidly determine the source direction is an enabling improvement. 
     In some embodiments, the system may comprise two spaced-apart slab-shaped shielding barriers (the “shields”) positioned facing each other and parallel to an “aiming plane” that runs centrally between the shields from the back to the front of the system, two slab-shaped particle detectors (the “side detectors”) positioned exteriorly and parallel to the shields, a third slab-shaped detector (the “middle detector”) positioned between the shields, and an electronic processor. The shields may be spaced apart by a predetermined “shield separation distance.” In some embodiments the shields may be configured to block most (at least 50%) of the particles orthogonally incident on each shield. The side detectors may be positioned on opposite sides of the system and closely proximate to the exterior surfaces or faces of the shields respectively, wherein the exterior surface of each shield is the shield surface which is farthest from the center of the system. The middle detector may be perpendicular to the aiming plane, parallel to the front of the system, and positioned closer to the front than the back of the system. The side detectors and the middle detector (collectively the “detectors”) may be configured to detect the particles and responsively emit distinct signals. The “signals” may comprise light pulses or electronic pulses or accumulated charge or counting rates or electrical voltages or currents or other detection data related to interactions of the particles, or their secondaries, in the detectors. As used herein, signals are “distinct” if they can be uniquely associated with a particular detector, thereby indicating which of the detectors detected each particle. For example, two electrical pulses on separate cables or conductors are distinct, two electronic pulses on the same conductor but with detectably different pulse shapes are distinct, two light pulses on separate light guides are distinct, and two light pulses having detectably different wavelengths are distinct, since in each case the signals can be analyzed and tallied separately. The processor may be configured to analyze the signals and to calculate the source angle based at least in part on the signals, wherein the “source angle” is the angle between the aiming plane and a line from the center of the system to the source location. For each detector, the processor may determine a “counting rate”, equal to the number of particle detections in each detector within a particular time interval. In addition, the system may include a “midplane” comprising a centrally positioned plane, orthogonal to the side detectors, orthogonal to the shields, orthogonal to the aiming plane, orthogonal to the middle detector, and parallel to the back-to-front direction of the system. The “thickness” of a slab is its smallest dimension; the “length” of a slab is its longest dimension; the “width” of a slab is its intermediate dimension. 
     In some embodiments, the angular sensitivity of the middle detector may be determined by its shape, its orientation perpendicular to the side detectors, and the partial collimating effect of the shields. The angular sensitivity of the side detectors may be determined by their shape, their orientation parallel to the shields, and the blocking effect of the shields which admit particles from only one side according to some embodiments. Consequently, the side detectors may have oppositely directed, antisymmetric angular sensitivities, while the middle detector may have a symmetrical angular sensitivity, relative to the aiming plane. The angular correlation function may exploit these different angular sensitivities, thereby relating the source angle to a formula or analysis of the detection data of each detector. By this means, the system can determine the sign and magnitude of the source angle from data acquired at a single orientation. 
       FIG. 1  is a cross-section sketch of an embodiment of the system including two side detectors  101  (shown in light stipple), two shields  102  (in diagonal hatch), a middle detector  103  (dark stipple), and a processor  105 . The system is facing to the right in this view and in all the figures unless otherwise noted. Also shown is the aiming plane  120  as a dashed line, since it is viewed edge-on in this cross-section view. The front is indicated as  106 , and the back is  107 . The midplane is not shown because it lies in the plane of the paper. A source  110  is indicated by a star, and a particle trajectory  111  as an arrow. The source angle  112  is indicated as an arc. The middle detector  103  is closer to the front  106  than the back  107 . Signals  113 , depicted as dotted arrows, flow from each detector  101  and  103  to the processor  105  for analysis. The processor  105  may comprise digital electronics, and optionally analog electronics, configured to calculate the source angle  112  from the signals  113  using a predetermined angular correlation function. 
     The particle  111  may be a neutral particle such as a gamma ray or a neutron, or it may be a charged particle such as an electron or a proton. The side detectors  101  and the middle detector  103  may be of any type suitable for detecting the particle  111 , such as scintillators, semiconductor detectors, and gaseous ionization tubes. Scintillators include organic types such as stilbene or polyvinyltoluene (PVT) based scintillators of which many varieties are available, or inorganic types such as NaI, CsI, BGO, LYSO and many others, typically viewed by light sensors such as photomultiplier tubes, photodiodes, or microchannel plate sensors. Semiconductor detectors include n-type or p-type reverse-biased junctions, typically amplified by pulse amplifiers and other electronics. Gaseous ionization tubes include Geiger, proportional, and other types of tubes and chambers in which ionization charges are electrostatically collected in a gas and amplified by further electronics. In some embodiments, the sensor may be included in the associated detector, such as a semiconductor detector with an amplifier built-in. In other embodiments, the sensor may be included in the processor or other electronics connected to the detector. The shields  102  may comprise any material suitable for blocking the particle  111  from passing therethrough. If the particle  111  is a gamma ray, the shields  102  preferably comprise a high-Z, high-density material such as lead, tungsten, bismuth and the like, Z being the atomic number. If the particle  111  is a neutron, the shields  102  preferably comprise a hydrogenous material such as HDPE or PMMA containing a neutron-absorbing material such as lithium or boron. 
     In some embodiments, each shield  102  may comprise a substantially planar, slab-like shape of material configured to block most of the particles  111  incident orthogonally on the shield  102 . Specifically, each shield  102  may be thick enough that over 50% of the orthogonally incident particles  111  are absorbed or stopped or attenuated or degraded, such that a downstream detector  101  is unable to detect the particles  111  or their secondary particles. Secondary particles are particles generated by interactions of the incident particle  111 , such as Compton electrons or photoelectrons from gamma ray interactions, or alpha particles or triton particles from neutron capture events, or recoil protons from neutron scattering events. Since most of the particles  111  strike the shield  102  at some angle other than orthogonal, the particles  111  on average encounter a tangentially longer distance traversing the shield  102 , such as typically 2 to 3 times the thickness of each shield  102 , thereby resulting in an average attenuation of 80%-90% for each shield  102  and 96%-99% for the pair of shields  102  together. 
     Thicker shields  102  provide greater attenuation and thus greater isolation between the side detectors  101 , but preferably the shields  102  are not so heavy that precise handling is difficult. Therefore, the shield  102  thickness may be a compromise between the isolation desired versus the weight of the system. In some embodiments, the shields  102  may be configured to block only 10% or 20% or 30% or 40% of the orthogonally incident particles  111 , thereby saving substantial weight while still providing sufficient isolation between the side detectors  101  to enable the source angle  112  determination. 
     The detectors  101  and  103  may be configured to detect gamma rays. For example, the side detectors  101  may comprise low-cost plastic scintillator material, while the middle detector  103 , being much smaller and thinner, may be made from a high-density scintillator such as CdWO 4  or BGO. Although inorganic scintillators often cost more than plastic, the higher detection efficiency of the denser material partially compensates for the small volume of the middle detector  103 , resulting in faster localization of sources. Alternatively, the detectors  101  and  103  may comprise semiconductors such as reverse-biased planar diodes, or they may comprise gaseous ionization tubes or chambers configured to detect gamma-generated electrons by ionization of the gas, such as Geiger or proportional tubes. The shielding for gamma rays may comprise a high-Z, high-density material such as tungsten, lead, bismuth, or uranium for example, or it may comprise other materials such as leaded glass or steel if sufficiently thick to block a sufficient fraction of the gamma rays. 
     The detectors  101  and  103  may be configured to detect neutrons. For example, the detectors  101  and  103  may comprise neutron-specific gamma-blind scintillators such as ZnS coupled with a lithium or boron converter material, or microbeads of self-transparent neutron-specific scintillator such as Li-glass or borosilicate glass scintillator, all embedded in a transparent hydrogenous matrix such as PMMA (polymethylmethacrylate). Alternatively, the detectors  101  and  103  may comprise a thin semiconductor type detector coated with a neutron-capture nuclide such as boron or lithium, such that the ions emitted by the capture nuclide may pass into the semiconductor. As a further alternative, the detectors  101  and  103  may comprise gaseous ionization detectors such as proportional chambers or Geiger tubes filled with a gas such as  3 He or BF 3 , or coated on the interior surface with boron or lithium. Shields  102  for neutrons may comprise a hydrogenous material such as HDPE (high-density polyethylene), preferably loaded with a capture nuclide such as Li or B. 
     In some embodiments, the dimensions of the middle detector  103  may determine its detection efficiency and may also determine the calculation of the source angle  112 . In a preferred embodiment, the width of the middle detector may be at least six times the thickness, and the length may be at least three times the width. For example, the size of the middle detector  103  in the direction perpendicular to the aiming plane may be at least six times the size of the middle detector  103  in the back-to-front direction, and the size of the middle detector  103  in the direction perpendicular to the midplane may be at least three times the size of the middle detector  103  in the direction perpendicular to the aiming plane. Such a configured middle detector  103  may provide higher detection sensitivity for particles from the front than the side, and may thereby result in a monotonic angular correlation function that can be used in the source angle  112  determination. 
     In some embodiments, the position of the middle detector  103  relative to the shields  102  may affect the angular sensitivity of the middle detector  103 , due to the partial blocking action by the shields  102  for particles  111  arriving from higher angles. In one embodiment, the middle detector  103  may be positioned flush with the front  106 , such that the front surface of the middle detector  103  is substantially coplanar or coterminous with the front surfaces of the shields  102 , wherein “substantially coplanar” means coplanar within a small distance such as within 1/10 or ¼ or ½ times the thickness of the middle detector  103 . In another embodiment, the middle detector  103  may be recessed from the front  106 , such that the front surface of the middle detector  103  is placed rearward relative to the front surfaces of the shields  102  by a “recess distance.” In some embodiments, the recess distance may be 0.5 to 1.5 times the shield separation distance. In some embodiments, the sensitivity distribution of the middle detector  103 , as modified by the shields  102 , may result in an angular correlation function that extends approximately linearly for source angles  112  ranging from zero to ±90 degrees, thereby enabling localization of the source  110  throughout the front half-space. 
     Some room-temperature semiconductor detectors are disk-shaped. In some embodiments, the middle detector  103  may comprise a plurality of separate disk-shaped semiconductor detectors, arranged in a vertical array that approximates the shape of a slab facing the front  106 . The data from the various semiconductor detectors may be added or otherwise combined to produce an overall signal  113  that indicates when a particle  111  is detected anywhere in the array. An advantage of such an array is that semiconductor detectors are typically very thin, and therefore satisfy a preferred feature that the middle detector  103  be thin. Also, semiconductor detectors require no bulky phototubes or high voltage, a further advantage since space is generally limited in the vicinity of the middle detector  103 . 
     Conventional detectors typically have a longitudinal ambiguity, in which they cannot determine whether the source  110  is in front or behind due to symmetry. Embodiments of the presently disclosed system have no such defect. For example, the embodiment of  FIG. 1  breaks the longitudinal ambiguity since the middle detector  103  is closer to the front  106  than the back  107 , and therefore has a higher counting rate, relative to the side detectors  101 , when the source  110  is in front of the system, and a lower counting rate when the source  110  is behind the system. The processor  105  may be configured to determine whether the source  110  is in front or behind the system by calculating the sum of the two side detector  101  rates, dividing the middle detector  103  rate by that sum, and then comparing that result (or its inverse) to an expected range of values for a source  110  being in front or behind the system. 
     In some embodiments, the side detectors  101  may substantially cover or enclose the exterior surfaces of the shields  102 , except for the shield edges and a small optional protrusion distance as discussed below. Enclosing the shields  102  in active detector material can result in high efficiency since nearly all of the arriving particles  111  encounter the side detectors  101  first, before reaching any shielding material. In contrast, conventional detectors generally include collimators or baffles with large exposed “dead” areas, resulting in many particles  111  being blocked without detection, and hence low overall efficiency. 
     In some embodiments, the side detectors  101  may comprise ionization-density-dependent detectors such as PSD (Pulse-Shape Discriminating) scintillators, thereby providing separate simultaneous determinations of gamma ray and neutron interactions according to their different pulse shapes. By separately analyzing the gamma and neutron interactions, the system can determine the locations of a gamma ray source and a neutron source simultaneously. This may be a crucial advantage in a situation where an adversary attempts to confuse a radiation scan by placing a benign gamma ray emitter near a clandestine weapon. 
     The processor  105  may comprise a digital calculating device such as a microcontroller or CPU or GPU or logic array or the like, configured to process the detector output signals  113  and determine the location of the source  110 . The processor  105  may be embedded in the system, or it may be mounted externally, or there may be multiple separate processors such as an embedded microcontroller internal to the system communicating with a facility computer elsewhere. In some embodiments, the processor  105  may be programmed to perform one or more of the angular analysis methods detailed herein, and further configured to indicate the calculated source angle  112  using a human-readable display or indicator and/or store the results in a non-transitory computer-readable record and/or transmit the results to an external computer. 
     To consider a specific example adapted to detect 1 MeV gamma rays, the side detectors  101  may be PVT plastic scintillator with a thickness of 15 mm and a width (in the back-to-front direction as depicted) of 60 mm. The length (going into the page as depicted) of the side detectors  101  may be 150 mm for a portable unit. For a large installation such as a cargo scanner, the length of the side detectors  101  may be much larger, up to four meters (assuming the scintillation light is collected efficiently). Such a tall detector can scan an entire vehicle or inspection zone at once. 
     The middle detector  103  may be, for example, a CdWO 4  scintillator with dimensions 3 mm thick by 30 mm wide, and a height equal to that of the side detectors  101 . The middle detector  103  may be oriented perpendicular to the side detectors  101 , and perpendicular to the shields  102 , and parallel to the front  106  of the system. In the depicted embodiment, the recess distance is zero, which means that the front surface of the middle detector  103  is flush or coplanar with the front surfaces of the shields  102  on the front  106  of the system. 
     The shields  102  may be spaced apart by 30 mm to accommodate the middle detector  103 . For shielding gamma rays, the shields  102  may be lead, with a thickness of 15 mm and a width of 60 mm. Such shields can provide 55% attenuation of 1 MeV gammas at orthogonal incidence, according to an MCNP6 simulation. The detection threshold in the simulation was set at 100 keV, meaning that a scattered gamma ray exiting the shield would not be detected if it had less than 100 keV of energy. 
     As a second exemplary embodiment, the particle  111  may be a neutron such as a low-energy (thermal or epithermal, E&lt;1 eV) neutron. The side detectors  101  may be gaseous ionization tubes such as proportional chambers containing a neutron-capture gas such as  3 He or BF 3  or other arrangement of neutron-capture nuclei. The middle detector  103  may be an ion-implanted or surface-barrier type semiconductor detector including LiF or B 4 C internally or as a thin layer, to promote neutron capture reactions generating energetic ions. The shields  102  for the neutron application may be HDPE with 5% addition of LiF, which may further moderate and absorb the neutrons. 
     As a third exemplary embodiment, the particle  111  may be a 1 MeV neutron, the side detectors  101  may be HDPE containing ZnS-coated optical fibers to detect recoil protons, or PMMA containing microbeads of a transparent scintillator for the same purpose. 
     The processor  105  may be configured to calculate the source angle  112  from the various detector rates. For example, the processor may first accumulate detection data such as counting rates from the detectors  101  and  103  for a period of time, and then calculate a differential equal to the detection rate of one of the side detectors  101  minus the detection rate of the other side detector  101 . Then the processor  105  may divide the differential by the detection rate of the middle detector  103 , thereby obtaining a ratio. Next, the processor  105  may use a predetermined angular correlation function that takes the ratio value as input and estimates the source angle  112  as output. In this way, the processor  105  may determine the source angle  112  from a single set of detection data, without iterations or rotations. 
     Cosmic rays are energetic particles, mostly muons at sea level, traveling generally vertically through the atmosphere, resulting in a background counting rate in each detector. In the usual orientation of the system, for measuring the horizontal angle of the source  110 , the side and middle detectors  101  and  103  are vertical and therefore are hit by fewer cosmic ray particles than if they were horizontal. In addition, each cosmic ray particle is likely to pass through an extended region of the detector  101  or  103 , and therefore to deposit a lot of energy, resulting in large pulses  113  that can be rejected on the basis of pulse height alone. For example, a scintillator may have a vertical dimension of between 5 and 15 cm which would be typical for a small device, and with a thickness of typically 1-2 cm. Depending on the cosmic ray angle, the path of the cosmic ray through the scintillator is substantially more than the thickness, such as at least 3 cm typically. Most cosmic rays drop energy at a rate of about 2 MeV per gram/cm 2  of material traversed, or at least 6 MeV in 3 cm of plastic scintillator of density about 1 gram/cm 3 . Most of the gammas from nuclear weapon materials are much lower in energy, typically 1-2 MeV. Therefore, even with the relatively poor energy resolution of plastic scintillators, most of the cosmic rays can be rejected by a threshold cut at 3 MeV. 
     As a further background reduction, any events that trigger more than one detector  101  or  103  may be vetoed or rejected. Cosmic rays usually travel all the way through the system and thus are likely to hit more than one detector  101  or  103 . Such a coincidence veto can eliminate many cosmic rays. As an additional advantage, the coincidence veto can also eliminate events in which neutron or gamma ray scatters in one side detector  101 , penetrates the shields  102 , and then scatters in the other side detector  101 . 
       FIG. 2  shows a cross-section view of an embodiment of the present system with improvements. Two side detectors  201  are shown flanking two shields  202 , which surround the middle detector  203 . The side detectors  201  and the middle detector  203  are scintillators in this example. Light sensors  204  such as phototubes can view the side detectors  201 , and a photodiode light sensor  214  can view the middle detector  203 . Signals depicted as dotted arrows  213  are passed from the light sensors  204  and  214  to a processor  205  for analysis. In the embodiment depicted, the shields  202  protrude frontward beyond the side detectors  201  by a protrusion distance  208  (which in this case is 25 mm). The middle detector  203  is shown recessed from the front of the shields  202  by a recess distance  209 , which in this example is 15 mm. In some embodiments, the angular correlation function depends on the shape and recess distance  209  of the middle detector  203 . The light sensors  204  and  214  may be mounted on the rear surface of each detector  201  and  203 , thereby avoiding material in the way of incident particles that may arrive from the front half-space. 
     In an embodiment wherein the shields  202  protrude frontward beyond the side detectors  201 , the shield protrusion distance  208  may be related to the thickness of the side detectors  201 , such as 0.5 to 1.5 times the thickness of the side detectors  201 , and often the protrusion distance is substantially equal to the side detector thickness (such as 0.9-1.1 times the side detector thickness). Such a shield protrusion  208  may enhance the isolation of the side detectors  201 , and may also provide improved angular resolution by blocking particles that arrive from various angles. For example, the shield protrusion  208  may be configured to prevent particles that arrive at an angle of 45 degrees from passing in front of the shields  202  and striking the downstream side detector  201 . 
     The processor  205  may include analog signal processing electronics such as amplifiers, electronic filters, pulse-height discriminators, pulse-shape analyzers and the like, as well as digital computing electronics capable of performing calculations according to a method such as the various analysis methods detailed herein. In some embodiments, the method may include counting the pulses from each detector for a particular time interval, performing arithmetic operations to derive the source angle from the data, and indicating or transmitting or otherwise reporting the results. Microcontrollers of many different types, CPUs and GPUs, gate arrays and ASICs of many types may be used for this purpose according to various embodiments. 
       FIG. 3  is a perspective sketch of the embodiment of  FIG. 2  but with the middle detector  303  now flush with the front  306  of the shields  302 . The system is pointing out of the page, generally toward the viewer&#39;s right side. The depicted embodiment comprises two side detectors  301  mounted outside and proximate to two shields  302 , and a middle detector  303  mounted between the shields  302 . The aiming plane  320 , shown in dash, is the centrally positioned plane of symmetry parallel to the shields  302 . The midplane  330  is a plane passing centrally through the system, perpendicular to the aiming plane  320 , perpendicular to the side detectors  301 , perpendicular to the shields  302 , and perpendicular to the middle detector  303 . The shields  302  and the side detectors  301  are parallel to the aiming plane  320  which passes centrally between the shields  302 . The middle detector  303  is oriented perpendicular to the aiming plane  320 , perpendicular to the shields  302 , perpendicular to the side detectors  301 , perpendicular to the midplane  330 , and parallel to the front  306  of the system. The back of the system is indicated as  307 . The width of the middle detector  303  is indicated by a curly brace  340 , and the height or length of the middle detector  303  is indicated by an arrow  350 . 
     The middle detector  303  can be positioned to have a symmetrical angular sensitivity distribution peaked in the forward direction. In contrast, the side detectors  301  can have antisymmetric angular sensitivities relative to the aiming plane  320 . As a result of this difference in angular sensitivities, there exists an angular correlation function that quantifies the correlation between the source angle and the detection rates. 
       FIG. 4  is a cross-section sketch of an alternative embodiment of the system in which the two side detectors  401  and  411  comprise two different scintillator materials emitting different pulse shapes. For example, the first side detector  401  (light stipple) may be PVT and the other side detector  411  (dark stipple) may be BGO, which have pulse decay times of 5 ns and 300 ns respectively. Likewise, the middle detector  403  may comprise yet a third scintillator material with a distinct pulse shape, such as CdWO 4  with a 14-microsecond decay time. A single light sensor  404 , such as a large-diameter phototube, may view both side detectors  401  and  411 , and also the middle detector  403 , through light guides  410  (light grid hatch). Thus, the signals from the various detectors are distinct in that they can be separated according to pulse shape, thereby determining which detector detected each particle. In addition, a precalibrated detection efficiency and/or a predetermined background rate may be applied as corrections for each detector. The configuration may be economical since only a single light sensor  404  is needed. 
     The configuration shown has zero shield protrusion, since the side detectors  401  and  411  are coterminous with the shields  402 . The lack of a shield protrusion can provide more detection area and therefore higher detection efficiency over a wide range of source angles, but also can result in reduced contrast or particle isolation between the two side detectors  401  and  411  when the source angle is intermediate (such as 45 degrees) due to particles passing by the shields  402  and striking the downstream side detector  401  or  411 . Designers can choose the amount of shield protrusion according to the relative importance of wide-angle efficiency versus narrow-angle resolution. 
       FIG. 5  is an embodiment of the system including detector back-flanges  511  and  512 . Here the side detectors  501  are rather thin, comprising a high-density scintillator material such as LYSO for example. LYSO has a high detection efficiency for gamma rays, and therefore a thin layer may be sufficient to detect incoming gamma rays from most angles. However, when the system is brought into alignment with a gamma ray source, very few gammas are observed in the thin side detectors  501  due to the small area presented edge-wise to the gamma ray source. That could make it difficult to determine when the source is aligned with the aiming plane by comparing the counting rates in the two side detectors  501 . To provide additional detection area when the system is aimed at the source, the back-flanges  511  and  512  can be mounted on the rear portions of the two side detectors  501 . The back-flanges  511  and  512  may extend laterally (away from the shields  502 ) beyond the adjacent side detectors  501  by a distance, which is preferably at least equal to the thickness of the adjacent side detectors  501 , and may be larger. Thus, the back-flanges  511  and  512  may provide extra detection area when the system is aligned with the source. 
     The figure shows one back-flange  511  comprising a segment mounted onto the exterior surface of a side detector  501 , and a second back-flange  512  comprising a larger segment mounted behind the adjacent side detector  501  according to some embodiments. As another option, the side detector  501  and its back-flange  511  or  512  may be made as a single L-shaped part. All of these options perform similarly. Each light sensor  504  can view one side detector  501  and its back-flange  511  or  512  together. Also shown are shields  502  and a middle detector  503 . 
     While the detector materials listed in this example are mainly scintillators applicable to gamma rays, the same principles may be used for improving the low-angle detection efficiency of high energy or low energy neutrons using suitable neutron-sensitive detector materials and/or other detector types. For example, a low-energy neutron detector based on neutron capture in a thin layer of B or Li, which is proximate to an ionization chamber or thin scintillator, may have even thinner dimensions than indicated in the figure, and thus would benefit from back-flanges to detect low-energy neutrons from sources near zero degrees. 
       FIG. 6  shows another embodiment of the system, now with outriggers  611 , comprising detector panels mounted adjacent to the exterior surfaces of the side detectors  601 . The outriggers  611  may comprise planar slabs of detector material, which may be the same material as the side detectors  601  or a different detector material, depending on the application. A purpose of the outriggers  611  may be to increase the detection efficiency by providing more detection volume. In a preferred embodiment, each outrigger  611  may be cut shorter than the adjacent side detector  601 , so as to remain in the “shadow” of the shields  602  for obliquely-arriving particles. Thus, the front end of the outrigger  611  may be substantially shortened relative to the front end of the adjacent side detector  601 . The outrigger shortening distance may be related to the thickness of the outrigger  611 , for example being equal to the thickness of the outrigger  611 . The shortening distance of the outrigger  611  may be configured so that any particles that are shielded from hitting the side detector  601  are also shielded from hitting the outrigger  611 . When so configured, the outriggers  611  may provide enhanced detection efficiency while causing little or no loss in angular resolution. 
     The side detectors  601 , middle detector  603 , and the outriggers  611 , may comprise scintillators configured to detect gamma rays or neutrons. Each side detector  601  and its adjacent outrigger  611  may be viewed together by a single light sensor  604 . The outriggers  611  can nearly double the detection volume. The outriggers  611  may comprise the same material as the side detectors  601  as in this example, while in other embodiments they may be different types of scintillators, or other types of detectors. The outriggers  611  are shown cut shorter than the side detectors  601 , by an amount that ensures that the outrigger  611  remains shadowed by the shield  602  for particles arriving at a particular angle, such as 45 degrees. Preferably the outrigger  611  is shadowed by the shield  602  in the same way that the side detector  601  is shadowed. For example, if the outrigger  611  has the same thickness as the side detector  601 , then the outrigger  611  is preferably shortened relative to the side detector  601  by the same amount that the side detector  601  is shorter than the shield  602 . 
       FIG. 7  shows another embodiment, in this case with the side detectors  701  comprising gaseous ionization chambers configured for neutron detection. Each side detector  701  is in the shape of a trapezoid as viewed from the top. The trapezoidal shape can maximize the detection volume while remaining in the shaded zone of the shields  702 . Also, the shields  702  may be beveled at the same angle as the side detectors  701 . The middle detector  703  may be a lithium-coated semiconductor detector in this example, mounted between the shields  702  which may comprise borated HDPE. Neutrons arriving at larger angles than the bevel angle are blocked by the shields  702  from reaching the trapezoidal side detectors  701 . In some embodiments, the shields  702  may be configured to substantially match, or have the same size, as the side detectors  701  on the surface where they meet, thereby providing optimal shielding. With such shaping, the trapezoidal side detectors  701  can be made much thicker, and thereby achieve greater detection efficiency, without extending beyond the shielded zone and without sacrificing contrast between the two side detectors  701 . 
       FIG. 8  is a cross-section sketch of an embodiment of the system including a fourth detector  827  positioned between the shields  802 , behind the middle detector  803 . The shields  802  may include shield protrusion portions  812  (small grid-hatch) comprising a different material than the rest of the shields  802 . For example, the shields  802  may be lead and the protrusion portions  812  may be depleted uranium, thereby providing extra attenuation for gammas from various directions. 
     Also shown are a photodiode light sensor  804  viewing the middle detector  803 , and phototubes  814  viewing each of the side detectors  801 . The phototubes  814  are shown mounted toward the back of the system, while the photodiode  804  views the middle detector  803  from the top surface. The fourth detector  827  is read out by an appropriate sensor  824 . If the fourth detector  827  is a scintillator, the sensor  824  may be a light sensor; and if the fourth detector  827  is a semiconductor or gaseous ionization detector, the sensor  824  may be an amplifier circuit. 
     In some embodiments, the fourth detector  827  may comprise a spectroscopic or energy-resolving detector configured to measure the energy of incoming particles that pass through the thin middle detector  803 . Due to the collimation effect of the shields  802  and shield protrusions  812 , the energy-resolving fourth detector  827  may detect source particles primarily when the system is aimed at the source. The energy spectrum measured by the energy-resolving fourth detector  827  may thereby identify the source isotope, based on the energies observed. According to some embodiments, the energy-resolving fourth detector  827  may be, for example, a slab of NaI or other scintillator, or an HPGe or GeLi semiconductor detector, or a gaseous ionization chamber with linear charge collection. Preferably the energy-resolving fourth detector  827  has an energy uncertainty of 10% or less, wherein the energy uncertainty may be, for example, the full-width-at-half-maximum of a full-energy peak in the energy spectrum. 
     In other embodiments, the fourth detector  827  may be configured to detect the same type of particle as the middle detector  803 , or a different particle type. For example, the side and middle detectors  801  and  803  may be configured to detect neutrons, and thereby to determine the location of a neutron source, while the fourth detector  827  may be configured to detect gamma rays and measure the energy spectrum of the gamma rays, thereby providing additional information about the source. 
     As a further option, the fourth detector  827  may comprise a material that emits a first signal when traversed or partially traversed by a lightly-ionizing particle such as an energetic electron, and a second signal different from the first signal when traversed or partially traversed by a heavily-ionizing particle such as an energetic ion. For example, the material of the fourth detector  827  may comprise a scintillator such as a PSD or pulse-shape discriminating scintillator, configured to produce a first light pulse upon detecting a Compton electron, and a second light pulse different from the first light pulse upon detecting a recoil proton or an alpha or triton from a neutron-capture event. Thus the initial particle may be identified as a neutron or a gamma ray according to the ionization density of tracks within the material. Exemplary PSD scintillators include CsI(Tl) and certain plastic scintillators with a special fluor, or alternatively ZnS which is mainly sensitive to energetic ions. Such a PSD scintillator can discriminate between gamma ray events and neutron events, since recoil protons and ions from neutron capture events have a high ionization density, whereas gamma-generated electrons have a low ionization density. The pulse properties of each event in the fourth detector  827  can indicate whether the event was due to a gamma ray or a neutron. The fourth detector  827  can thus reveal a neutron source if present, while the other detectors can determine the location of the source. 
       FIG. 9  is a cross-section sketch of an embodiment of the system wherein the middle detector is a member of a plurality of similar front-facing middle detectors  903 , mounted sequentially between the shields  902  and closer to the front of the system than the back. Each of the plurality of middle detectors  903  may be a slab-shaped form, perpendicular to the aiming plane and to the midplane, parallel to the front of the system, and configured to detect the particles and to produce a responsive signal. A purpose of using a plurality of middle detectors  903  may be to obtain higher detection efficiency. Each detector of the plurality of middle detectors  903  can contribute data for determining the source angle. 
     The shields  902  in this example are configured to block only 20% of the orthogonally incident particles, which nevertheless provides sufficient isolation between the side detectors  901  because, in the intended application, most particles are expected to arrive from the front and therefore experience a tangentially longer pathlength in each shield  902 . 
     In other embodiments, the system may include a plurality of middle detectors positioned toward the front, and a plurality of back-facing detectors positioned toward the back, and all oriented perpendicular to the aiming plane and perpendicular to the midplane. Such pluralities of middle detectors and back-facing detectors may provide enhanced detection efficiency for particles arriving from the back as well as the front. 
       FIG. 10  shows an embodiment of the system in which the side detectors  1001  are energy-resolving scintillators (in cross hatch) surrounding the shields  1002 , and wherein the middle detector is a member of a plurality of similar front-facing middle detectors  1003 , each configured to detect the particles and emit a signal. The plurality of middle detectors  1003 , here comprising three middle detectors  1003 , are mounted between the shields  1002 , closer to the front of the system than the back, perpendicular to the aiming plane and to the midplane, parallel to the front of the system, and configured to detect the particles. As indicated by different stipple densities in the figure, each member of the plurality of middle detectors  1003  comprises a different scintillator material, each such material being configured to produce a different pulse shape; hence by pulse shape analysis, each detection event can be allocated to whichever of the plurality of middle detectors  1003  was active. As shown, the middle detectors  1003  are optically coupled to each other in a “phoswich” arrangement, which is optically coupled to a light guide  1010 . Light sensors  1014  may be connected to the light guide  1010  and to each side detector  1001 . Thus the system can determine the source angle by analyzing signals from each of the plurality of middle detectors  1003  separately. 
     The side detectors  1001  in the depicted example are energy-resolving scintillators, as mentioned. Energy spectra from the side detectors  1001  may thus be used to identify the source isotope. As a further option, the light guide  1010  may be made of a transparent high-density material such as leaded glass, thereby providing additional shielding. 
       FIG. 11  is a cross-section sketch of an embodiment of the system in which the shields  1102  are truncated, or cut short, relative to the back surfaces of the side detectors  1101 . That is, the side detectors  1101  extend rearward beyond the shields  1102 . A purpose of truncating the shields  1102  may be to save weight. The truncation distance  1121  of the shields  1102  is shown as the distance from the back ends of the shields  1102  to the back ends of the side detectors  1101 . Preferably the shield truncation distance  1121  is large enough to save substantial weight, but not so large that particles can readily pass behind the shields  1102  and strike the downstream side detector  1101 . For example, the truncation distance  1121  may be at least as large as the shield separation distance  1122 , but no greater than the side detector separation distance  1123 . With those limitations, the particle isolation between the two side detectors  1101  is nearly unaffected by the truncation, at least for source angles less than about 60 degrees. For higher source angles, such as 90 degrees, the particles can occasionally pass through and hit the wrong side detector  1101 , but this is an infrequent effect that can be accounted for in the predetermined angular correlation function. 
     The sketch further shows an additional shield element comprising a central shield slug  1132  positioned between the shields  1102  and behind the middle detector  1103 . In a preferable embodiment, the shield slug  1132  may be configured to block most of the particles orthogonally incident on it. A purpose of the shield slug  1132  is to block particles arriving from the back, so that the system can more readily determine, from the middle detector  1103  counting rates, whether the source is in front or behind. The counting rates in the side detectors  1101  are nearly the same whether the source is located in front or behind the system, and therefore the side detectors  1101  alone cannot determine which half-space the source is in. But the longitudinal ambiguity is broken by the frontward position of the middle detector  1103 , as well as the shield slug  1132  which shields the middle detector  1103  from the back. 
     Operationally, the front-versus-back source position can be determined by dividing the counting rate of the middle detector  1103  by the sum of the two side detector  1101  counting rates according to some embodiments. If that ratio is anomalously low, the source may be determined to be behind the system. Then, using the same data, the sign of the source angle can be determined by subtracting one of the side detector  1101  counting rates from the other, thereby obtaining a differential, wherein the sign of the source angle corresponds to the left-versus-right position of the source. Then, continuing with the same data, the magnitude of the source angle can be determined by dividing the differential by the middle detector  1103  counting rate, and comparing that ratio to a predetermined angular correlation function. Thus, the system can determine the full source angle, with sign and magnitude, from the detection data. 
       FIG. 12  depicts an embodiment with an alternative shield configuration. Here the shields  1202  are shaped as triangles in cross-section, as viewed from the top. In the depicted case, each shield  1202  is a right triangle with one sharply pointed edge directed toward the back. Alternatively, the shields  1202  may comprise blunt triangles or narrow trapezoidal shapes or other tapered shapes, configured to tailor the amount of shielding at the back end. Allowing some amount of shielding at the back may be useful in applications where the source is expected to be at high angles relative to the aiming plane. In a preferred embodiment, the thickest region of the shields  1202  can be sufficiently thick that most of the orthogonally incident gamma rays are blocked or attenuated by the thickest region. Such “tapered” shields  1202  can provide extra shielding toward the front where it is needed for high signal contrast, and reduced thickness toward the back where it is not needed, thereby resulting in a weight reduction of about half in some embodiments. The tapered shields  1202  are most applicable in inspections where the source is unlikely to be behind the system. Side detectors  1201  and a middle detector  1203  are also shown. 
       FIG. 13  depicts an embodiment in which the shields  1302  are made of detector material (shown in dark stipple) and are configured to emit signals upon detecting particles from the source. The two detector-shields  1302  may be configured to emit distinct signals wherein, as mentioned, distinct signals may comprise signals that indicate which detector produced each signal according to an electrical or optical property. For example, the signals may be carried on two separate conductors or cables corresponding to the two detector-shields  1302  respectively, or the signals may have a different shape or other feature corresponding to each detector-shield  1302 . Such detector-shields  1302  may comprise a scintillator material that provides both shielding and detection of the particles. For gamma ray detection and shielding, the detector-shields  1302  may be a high-Z, high-density, transparent scintillating material such as BGO, LYSO, CdWO 4 , or the like. For neutrons, the detector-shields  1302  may comprise a transparent material incorporating hydrogen and preferably a neutron-reaction nuclide such as lithium or boron, wherein the scintillator may be configured as microbeads of lithiated glass scintillator or borosilicate scintillator embedded in a polymer such as PMMA. Also shown are side detectors  1301  and a middle detector  1303  which may also be scintillators as in the example depicted. One advantage of such detector-shields  1302  may be that they can detect a weak or shielded source with high sensitivity due to their ample volume typically. The detector-shields  1302  may be thick enough to block more than 50% of the orthogonally incident particles, thereby providing sufficient particle isolation between the side detectors  1301 . The detector-shields  1302  and the side detectors  1301  may be viewed by separate light sensors  1304 , or they may be viewed together by a single light sensor  1314 . In the latter case, the detector-shields  1302  and the side detectors  1301  preferably comprise different scintillator materials with different pulse shapes so that their signals can be separated in analysis. 
     In a further embodiment, the middle detector  1303  may be optically coupled to a light guide  1310  (shown in cross-hatch) which itself may comprise an energy-resolving light-guide-scintillator  1310 , so long as the pulse properties of the middle detector  1303  are detectably different from pulses emitted by the light-guide-scintillator  1310 . Energy-dependent signals from the light-guide-scintillator  1310  may enable isotopic identification of the gamma ray source material. As a further benefit, the light-guide-scintillator  1310  may provide extra shielding. The high-Z detector-shields  1302  and the large energy-resolving light-guide-scintillator  1310  together comprise a sensitive, high-efficiency detector combination. When aligned with the source location, the detection signals can reveal even a well-shielded weapon based on counting rates and spectroscopy alone. 
     In a further embodiment, the light-guide-scintillator  1310  may comprise material that produces a different pulse according to the ionization density of the particle, thereby discriminating between gamma-generated electrons and neutron-generated ions. Such an ionization-dependent light-guide-scintillator  1310  may contain hydrogen and detect fast neutrons by proton recoil detection, or it may contain Li or B compounds and detect slow neutrons according to the alphas or triton ions emitted by slow neutron capture reactions. In this way, neutron and gamma ray sources and backgrounds can be determined independently. 
     In some embodiments, the shields  1302  may be configured to detect the opposite type of particle from that detected by the middle and side detectors  1303  and  1301 . More specifically, the middle and side detectors  1303  and  1301  may be configured to detect a first particle type, and the detector-shields  1302  may be configured to detect a second particle type different from the first particle type. For example, a detector-shield  1302  may detect neutrons while the side and middle detectors may detect gammas, or vice-versa. Such detector-shields  1302  and complementary side detectors  1301  may thereby enable detection of both neutrons and gammas simultaneously, while determining the source angle according to one of the particle types. 
     In other embodiments, the detector-shields  1302  may comprise an energy-resolving scintillator or semiconductor which provides information about the particle energy spectrum. The large volume of such energy-resolving detector-shields  1302  may provide sufficient data to identify the source nuclide rapidly, while at the same time the side detectors  1301  and the middle detector  1303  determine its location. 
       FIG. 14  depicts an embodiment of the system with a rear-facing detector  1413  which is mounted toward the back of the system, positioned between shields  1402  and orthogonal to the side detectors  1403  and parallel to the middle detector  1403 . In a preferable embodiment, the rear-facing detector  1413  may be similar in shape and performance to the middle detector  1403 , perpendicular to the aiming plane and to the midplane, parallel to the front of the system, and may be configured to detect the particles and emit signals responsively. The rear-facing detector  1413  may be recessed from the back of the shields  1402  by substantially the same distance that the middle detector  1403  is recessed from the front. Also, the shields  1402  may protrude beyond the side detectors  1401  in both the front and back directions, preferably by the same protrusion distance. A shield slug  1422  may be centrally positioned. Thus, the front and back of the system can be symmetrical. 
     The middle detector  1403  and the rear-facing detector  1413  may, in cooperation, enable simultaneous scanning of particles from the front and back, and thereby determine the source location regardless of whether the source is in front or behind the system. The middle detector  1403  and the rear-facing detector  1413  may be viewed by two sensors  1404  and  1414 , comprising photodiodes if the middle and rear-facing detectors  1403  and  1413  are scintillators, or amplifiers if the middle and rear-facing detectors  1403  and  1413  are semiconductor or gaseous ionization detectors. The sensors  1404  and  1414  may be placed in the space between the middle and rear-facing detectors  1403  and  1413 , thereby avoiding placing material in the way of incoming particles from either direction. Alternatively, the sensors  1414  and  1404  may be mounted on the top and/or bottom edges of the middle and rear-facing detectors  1403  and  1413 . The depicted configuration may be especially useful in applications where a source is equally likely to be anywhere around the system. 
       FIG. 15  is a perspective sketch of an embodiment configured to determine whether the source is above or below or substantially on the midplane of the system, without having to turn the system on its side. In the sketch, the system is shown pointed toward the viewer&#39;s right side and out of the page. Each side detector may be split into two portions, an upper side portion  1501  and a lower side portion  1511 , abutting edgewise at the midplane  1530  which is shown in dash. The upper and lower side portions  1501  and  1511  may be configured to emit signals upon detecting the particles from the source. According to some embodiments, if the source is above the midplane  1530 , the upper side portions  1501  count higher than the lower side portions  1511 , and vice-versa if the source is below the system. If the source is substantially on the midplane  1530 , then the upper and lower side portions  1501  and  1511  may count at substantially the same rate. Thus, the system can compare signals from the upper and lower side portions  1501  and  1511  and thereby determine whether the source is above, below, or substantially on the midplane  1530 , according to the ratio or difference of counts in the upper side portions  1501  versus the lower side portions  1511 . As used herein, the source is substantially on a plane if the angle between the plane and the source is less than a predetermined small angle such as one degree or two degrees. As used herein, the counting rates of two detectors are substantially the same if they differ by no more than an expected variation such as one or two times the statistical uncertainty. 
     The embodiment shown can also determine the source angle by adding together the signals from the upper and lower side portions  1501  and  1511  on each side, so that the combined signals or counting rates effectively act as an unsplit left side detector and an unsplit right side detector. Then, the left sum may be subtracted from the right sum, thereby obtaining a differential. The differential may then be divided by the counting rate for the middle detector  1503 , thereby obtaining a ratio. The ratio may then be compared to the predetermined angular correlation function to determine the horizontal component of the source location. In addition, the system may be configured to determine, when the counting rates in all four side detector portions  1501  and  1511  are substantially equal, that the system is aimed at the source both horizontally and vertically. 
     Alternatively, or in addition, the shields may be configured to detect the particles, as discussed with reference to  FIG. 13 , wherein each shield may be split into an upper shield portion  1502  and a lower shield portion  1512 , abutting at the midplane  1530  in some embodiments. Detection data from the upper and lower shield portions  1502  and  1512  may then be compared or subtracted to determine whether the source is above, or below, or substantially on the midplane  1530 . Thus the system may compare detection data of the upper shield portions  1502  with detection data of the lower shield portions  1512 , and thereby determine whether the source is above or below or substantially on the midplane. 
     Alternatively, or in addition, the system may include a fourth detector, positioned between the shields  1502  and  1512 , behind the middle detector  1503  as discussed with reference to  FIG. 8 , and may be split into an upper detector  1507  and a lower detector  1517  abutting at the midplane  1530  in some embodiments. The upper and lower detectors  1507  and  1517  may be configured to detect the particles and responsively emit signals. In the figure, the upper and lower detectors  1507  and  1517  are shown separated from the system by an arrow, and also shown positioned within the system, so as to show construction details. The system may be configured to compare or subtract detection data from the upper and lower detectors  1507  and  1517 , thereby determining whether the source is above, or below, or substantially on the midplane  1530 . 
     The system disclosed herein enables numerous greatly improved radiation detection applications. In some embodiments, the system may be incorporated into a cargo inspection station of the type used for scanning trucks and maritime shipping containers to detect radiation sources. By indicating the longitudinal position of any sources detected, the system can greatly accelerate the inspection process. In a second embodiment, the system may comprise an advanced hand-held radiation detection meter that indicates the direction of the source. In a third embodiment, the system may be configured to detect radioactive contamination on personnel in a walk-through portal. In a fourth embodiment, the system may be incorporated in a mobile threat scanner of the type that measures radiation and detects threat materials while being driven through a region such as city streets. 
       FIG. 16  is a perspective sketch of an exemplary vehicle and cargo scanner array comprising a large number of the present systems  1601  arrayed around an inspection object such as a shipping container  1602 . The systems  1601  may be arranged on one side of the inspection zone, or both sides as shown, or above and below, or all around the inspection zone for maximum sensitivity. Each of the systems  1601  may be configured to detect gamma rays or neutrons from the container  1602  individually and to determine the horizontal angle of the source relative to each of the systems  1601 , according to some embodiments. Alternatively, the detection data from all the systems  1601  may be analyzed together so as to fit the data to a source distribution model or other global analysis for example. Such a global analysis is typically more sensitive and more accurate than simply analyzing the source angle determined by each system  1601  separately. Such a global analysis may be particularly advantageous when the source is shielded. 
     In a further embodiment, the array may include some systems  1601  mounted vertically, as depicted, to determine the horizontal position of the source, and others oriented horizontally to measure the elevation angle of the source. Then the data from all the detectors in the array may be input to a source model or other fitting routine which can determine the most likely location of the source or sources in three dimensions. 
     In a further embodiment, some of the systems  1601  may be configured to detect gamma rays, while others may be configured to detect neutrons. Alternatively, each of the systems  1601  may include ionization-density-dependent scintillators that discriminate between neutron and gamma events. In either case, the array can thereby determine (a) whether a gamma ray source is present in the cargo, (b) if so, where the gamma source is, (c) whether a neutron source is present in the cargo, and (d) if so, where the neutron source is. This information can enable much faster scanning and a much more reliable threat localization than non-directional detectors. 
     In a further embodiment, vehicles being inspected may pass through the inspection zone without stopping, although preferably at a low speed. The exact position of the vehicles may be measured continuously using a position sensing device such as an optical, RF, magnetic or another type of position sensor. The detector data can then be fit to a moving-source model. 
     In a further embodiment, a cosmic ray scattering-type vehicle inspection system may be provided, comprising two cosmic ray tracking chambers  1603  (shown in dash) positioned above and below the shipping container  1602 . The cosmic ray tracking chambers  1603  may be configured to measure the amount of scattering of cosmic ray particles that pass through the shipping container  1602 , and thereby reveal nuclear materials or shielding materials that produce anomalously high amounts of scattering. To account for cosmic rays that pass through the systems  1601 , each track that intersects one of the systems  1601  may be corrected for extra scattering that may occur in passing through the systems  1601 . Alternatively, the track analysis may select only those cosmic rays that do not pass through any of the systems  1601 . As a further option, any events may be rejected if any of the systems  1601  is active at the same time as the tracking chambers  1603  detect a cosmic ray. As an even simpler alternative, the systems  1601  may be positioned outside the field of view of the tracking chambers  1603 , thereby eliminating any interference. 
     The combination of a cosmic ray scattering inspection with a directional radiation detection array can provide many advantages. An adversary wishing to reduce the radiation signal may add more shielding around a weapon, but this would increase the amount of cosmic ray scattering and would thereby reveal the threat. Likewise the adversary could reduce the shielding to reduce the scattering signature, but this would greatly increase the amount of radiation detected. Thus the two types of inspections, working in cooperation, leave an adversary no available design space for concealing the threat. 
       FIG. 17A  is a perspective sketch of an embodiment of a portable radiation survey meter  1701  that can detect a radioactive source and also indicate the direction of the source. The meter  1701  may include a system  1702  (hidden, shown in dash) and a processor  1705 . In some embodiments, the meter  1701  may also include a first display  1703  and a first handle  1723  mounted on a first surface of the meter  1701 , and a second display  1704  and a second handle  1724  mounted on a second surface of the meter  1701  which is orthogonal to the first surface. Thus the second handle  1724  and the second display  1704  are orthogonal to the first display and first handle  1703  and  1723 . The orthogonal handles  1723  and  1724  thereby allow an operator to conveniently turn the meter  1701  on its side, and the orthogonal displays  1703  and  1704  allow the operator to observe the detection results during a horizontal angle scan or a vertical angle scan. 
     In some embodiments, the meter  1701  may be configured to display a rotatable icon  1713  pointing toward the calculated source location, preferably presented on whichever display  1703  or  1704  is currently on the uppermost surface of the meter  1701 . The depicted meter  1701  is useful for measuring the horizontal angle of a source and indicating its horizontal location to the operator using the first display  1703 . When rotated 90 degrees on its side, the depicted meter  1701  is also useful for measuring the vertical angle of a source and indicating its vertical location to the operator using the second display  1704 . The meter  1701  may be further configured to blank or disable or otherwise deactivate whichever of the displays  1703  and  1704  is currently not on top. Thus when the meter  1701  is oriented to measure horizontal angles, the first display  1703  may be activated while the second display  1704  may be disabled, and when the meter  1701  is turned on its side and oriented to measure vertical angles, the second display  1704  may be activated and the first display  1703  disabled. 
     In some embodiments, the meter  1701  may include an electronic compass  1707 , and/or a GPS receiver  1708 , and/or a multi-axis accelerometer  1709 . The compass  1707  can measure the geographical bearing of the meter  1701  which may enable the processor  1705  to determine when the meter  1701  is rotated horizontally, and can thereby relate the source angle to real-world coordinates of the detected source. The GPS receiver  1708  can determine the spatial position of the meter  1701 , which can then be recorded internally or transmitted to an external receiver. Using the GPS data, the position of the source can be determined by triangulation from two measurement locations. The accelerometer  1709  can detect when the meter  1701  is turned on its side, thereby enabling the processor  1705  to allocate measurements to horizontal or vertical source angles accordingly. The accelerometer  1709  may also detect when the meter  1701  is moved or rotated quickly, thereby enabling the displayed icon  1713  to be erased until new data is accumulated. For example, when the meter  1701  is rotated or moved quickly, the processor  1705  may blank the displays  1703  and  1704  to avoid misleading the operator, or the displays  1703  and  1704  can be changed to show a busy-icon for example. Alternatively, the processor  1705  may be configured to correct the display  1703  or  1704  according to the rotation, for example by subtracting the rotation angle from the angle of the directional icon  1713 , so that the icon  1713  continues to point toward the source after the rotation. In either case, as soon as sufficient additional detection data are acquired to enable an updated source angle determination, the displays  1703  and  1704  can be updated to again show an icon  1713  pointing toward the source. 
     In some embodiments, the meter  1701  may also include a microphone  1710 , and/or a speaker  1711 , and/or a hold button  1712 . The microphone  1710  can enable the operator to record comments made during the scan in real-time. The speaker  1711  can provide acoustical alarm data to warn the operator of a high radiation environment, and/or other alarms. The speaker  1711  can also provide special sounds indicating that the source is to the right or left of the aiming plane, and yet another sound when the aiming plane is directly aligned with the source, thereby assisting the operator in localizing the source without looking away from the scene. The hold button  1712  may allow the operator to freeze the displays  1703  and  1704  with the accumulated radiation and directional information displayed, so that the operator can then read them at a later time. The hold button  1712  may be a press-to-run button, a press-to-hold button, a run-hold toggle switch, or any other manually-operable control component. In a further embodiment, the holding and running modes may be controlled by spoken commands, such as the operator saying “hold” or “stop” to freeze the display, and “go” or “run” to resume updating the display by speaking one of the commands, which can be reliably discerned with low-cost microcontrollers in some embodiments. 
     The portable meter  1701  may be operated by a human operator, or by a robot operator with grasping means to manipulate the meter  1701 . The robot may include viewing means to read the display  1703 , or a wireless link to receive detection and angular data. 
     In some embodiments, the processor  1705  may continuously calculate the source angle relative to the current aiming plane, and then may update the display  1703  or  1704  to show the calculated source angle (preferably including both sign and magnitude of the source angle) in real-time. The processor  1705  may be configured to update the source angle incrementally, with the oldest data being discarded or attenuated relative to the most recent acquisitions. Emphasis on the newest data may be accomplished using a ring buffer, or by weighting the most recent data above the older data, or by incremental averaging, or other means to reflect the latest angular results while discarding or attenuating the older results. Continuous updating of the calculated source location may help the operator to rapidly locate the source. 
     The display  1703  or  1704  may be configured to show a directional display icon  1713  indicating the source direction. In a first embodiment of the display icon  1713 , the meter  1701  may be configured to display a left or right arrow indicating the left or right directionality of the source, wherein the length or other feature of the arrow may be configured to indicate the magnitude of the source angle. The operator may then rotate the meter  1701  left or right in the direction indicated by the display icon  1713 , preferably rotating more rapidly or by a larger amount if the indicated source angle is large or rotating more slowly and by a smaller amount if the indicated source angle is small. By these steps, the operator can rapidly converge on the source direction. 
     In a second embodiment, the icon  1713  may be rotatable and configured to point directly toward the source location according to the currently determined source angle. After viewing the rotatable display icon  1713 , the operator may simply turn the meter  1701  according to the display icon  1713 , thereby bringing the system directly into alignment with the source in one step, without having to gradually or iteratively converge on it. The processor may then detect the actual amount of rotation of the meter  1701 , according to signals from the compass  1707  for example, and may then adjust the display icon  1713  accordingly. For example, the processor  1705  may subtract the rotation angle from the current angle of the display icon  1713 . In that way the displayed icon  1713  continues to point toward the source even as the meter  1701  is rotated. In a preferable embodiment, the processor  1705  may then acquire additional detection data at the (rotated) second orientation, and may recalculate the source angle based on the new data, and recalculate the source angle, and adjust the direction of the displayed icon  1713  according to the updated calculation. 
     In some embodiments, the meter  1701  may be configured to determine when the aiming plane becomes aligned with the source. For example, the meter  1701  may use the “calculated angle” criterion to determine the alignment, wherein the aiming plane is aligned with the source when the calculated source angle is less than some threshold such as 1 degree or 2 degrees. Alternatively, the meter  1701  may use the “equal-rates” criterion, wherein the side detector rates are equal within expected statistical errors when the aiming plane and the source are aligned. As a further option, the meter  1701  may use a combination of the two criteria. Additionally, when the meter  1701  is aligned with the source, the display  1703  or  1704  may indicate that the aiming plane is aligned with the source using, for example, a special icon, thereby enabling the operator to localize the source more easily and more rapidly than possible with directional meters that fail to determine the magnitude of the source angle. 
     Embodiments of the meter  1701  may include a light beam transmitter  1725  emitting a light beam  1726 . In some embodiments, the light beam  1726  may be directed along the aiming plane, thereby showing exactly where the meter  1701  is aimed. In addition, the light beam  1726  may be configured to indicate the left or right direction toward the source, for example with an asymmetric beam shape such as a wedge pointing left or right. The beam shape may be further configured to indicate the magnitude of the source angle as well as the left-right direction, for example being elongated when the magnitude of the source angle is large or foreshortened when the source angle is small. In addition, the beam shape may be reduced to a circular spot, or caused to flicker, or otherwise visibly modulated, when the source angle is very small or zero, thereby indicating the exact source location visually when the meter  1701  is aligned with the source. Such a variable light beam shape, indicating the direction of the source as well as the magnitude of the source angle, greatly assists the operator in locating a source quickly. 
     In another embodiment, the meter  1701  may be configured to redirect the light beam  1726  directly toward the calculated source location as soon as its location is determined. In some embodiments, the meter  1701  may be configured to rotate the light beam transmitter  1725  itself according to the calculated source angle. Alternatively, the meter  1701  may be configured to redirect the light beam  1726 , using a rotatable mirror for example. In both cases, the redirected light beam  1726  may bathe the source location in light and thereby provide an unambiguous visual indication of the source location. The light beam  1726  may be flickered or modulated in shape or otherwise modulated to enhance visibility. As a particular advantage, the light beam  1726  may appear to remain “locked on” to the calculated source location while the meter  1701  is moved around and rotated. For example, the processor  1705  may be configured to detect any rotation of the meter  1701 , using signals from the compass  1707  or otherwise, and thereby adjust the light beam  1726  direction accordingly. In addition the processor  1705  may be configured to recalculate the source angle repeatedly and to adjust the light beam  1726  angle in near-real-time. By these means, the light beam  1726  may remain continuously and persistently directed toward the source location, thereby revealing the source to the operator in a compelling and intuitive visual manner. 
     Embodiments of the meter  1701  may include a camera  1727  or other imaging device to record images of the scene. The camera  1727  may be aligned with the aiming plane, in which case an icon or other indicator may be added to the image at the calculated source location. In addition to recording the scene and the source location, the camera  1727  may also be useful for determining a rotation angle of the meter  1701 . For example, as the meter  1701  is rotated to different orientations, the image shifts accordingly, and the processor  1705  can then perform image analysis by comparing sequential images, and thereby determine how far the meter  1701  has been rotated. The processor  1705  can then use that rotation angle, along with the detector counting rates acquired both before and after the rotation, to localize the source using, for example, interpolation. 
     In an alternative embodiment, the system  1701  may be configured to redirect the image so that it is centered on the source location. For example, the system  1701  may be configured to rotate the camera  1727  according to the calculated source angle, or the system  1701  may use a rotating mirror, or other optical means, to cause the image to be centered on the calculated source location. In addition, the camera  1727  may be configured to vary a zoom lens or equivalent, and thereby acquire both wide-angle and magnified images centered on the source location. For example, the source-centered scene may be magnified successively in various images, such as acquiring a new image whenever the angular uncertainty in the source angle is improved with further data. In addition, the angular uncertainty may be indicated on each image by a numerical or graphical overlay for example. The camera  1727  may be activated upon the start or end of each period of detector data acquisition, or manually by an operator, or continuously, or periodically, or upon a computer command, or whenever the meter  1701  is rotated, or otherwise. 
     In some embodiments, the meter  1701  may be configured to accumulate detector data for a particular time interval termed the “integration time,” and then may analyze the accumulated data to determine the source angle. For example, the integration time may be set to a default value such as one second or ten seconds, or it may be adjustable manually by the operator using an integration time control  1728  such as a knob, or the integration time may be adjusted automatically by the processor  1705 . A short integration time may be sufficient to localize the source quickly if the radiation level is high, but if the source is small or well-shielded, the rates are likely to be much lower or barely above background, in which case a longer integration time may be preferable. As a further option, the integration time may be adjusted dynamically in real-time according to the detection rate obtained, or according to the angular uncertainty so far obtained, or other criteria. The processor  1705  may be configured to perform a method comprising first checking the overall radiation level, for example by checking the accumulated counts in the middle detector and/or by adding the counting rates of the two side detectors. The processor  1705  may be configured to then adjust the integration time so as to obtain sufficient counts for a satisfactory determination of the source angle. 
     In another embodiment, the processor  1705  may be configured to follow a sequential acquisition program comprising first acquiring detector data for a short integration time, then obtaining an early indication of the presence of a source based on the sum of the side detector rates being above background levels, then continuing to acquire data for a second integration time to obtain sufficient data to determine the sign of the source angle based on the difference between the two side detector counting rates, then continuing to acquire data for a third integration time to obtain sufficient data to determine the magnitude of the source angle based on a comparison of the middle detector and side detector rates, and then acquiring data for a fourth integration time to obtain sufficient data to reduce the uncertainty in the source angle to a predetermined level. 
     In some embodiments, the processor  1705  may be configured to analyze the detector data and calculate a best-fit source angle continuously while further data is being accumulated. The processor  1705  may be configured to update the source angle determination after every detection, or at preset intervals such as once per second, using whatever data has been accumulated so far. In one embodiment, the processor  1705  may be configured to determine when the meter  1701  has been rotated, and to then delete the accumulated detector data, and to then start over with new data, thereby avoiding showing an outdated result to the operator. In a second embodiment, the processor  1705  may be configured to determine how far the meter  1701  has been rotated, and then subtract that rotation from the current estimate of the source angle before continuing to accumulate further detection data, thereby continuing to provide the best estimate of the source angle to the operator throughout the rotation. 
       FIG. 17B  is an exemplary sketch in perspective of a wearable health and safety monitor  1740  which may be worn by the operator of the portable meter  1701  of  FIG. 17A . The monitor  1740  may be wirelessly linked to the meter  1701 , using Bluetooth or Wi-Fi or other communication technology for example. Embodiments of the monitor  1740  can be worn by the operator using an attachment  1744  such as a belt clip, harness, neck strap, or the like. The monitor  1740  can continuously monitor the health and safety status of the operator, and can communicate any detected problems automatically to the meter  1701 , which can pass the alarm to a central facility and/or peer nodes in the LAN. Alternatively, the monitor  1740  may transmit the alarm directly to an external receiver such as an emergency response facility. The monitor  1740  may include a 3-axis accelerometer  1741  or other means to determine whether the operator remains upright or has fallen, a microphone  1742  configured to receive verbal data from the operator, and a wireless transceiver  1743  configured to communicate with the processor  1705  or other receiver. The monitor  1740  may further include biometric devices such as a respiration sensor  1745 , a pulse timer  1746 , and a blood pressure sensor  1747  as well as other health and safety related diagnostics. Thus, the meter  1701 , with its linked health and safety monitor  1740 , can enable a near-instantaneous rescue response when the operator experiences an emergency situation in the field. 
       FIG. 17C  shows an exemplary display  1751  that provides information about the source angle as well as the uncertainty in the source angle. In some embodiments, the processor  1705  of  FIG. 17A  may be configured to execute two parallel analyses with two different integration times. For example, the processor  1705  may be configured to carry out a first analysis using a short integration time, which thereby provides a rapidly updated value of the source angle, albeit with large uncertainties due to the limited number of detections observable in that short integration time. The processor  1705  may be further configured to carry out a second analysis concurrently, using a much longer integration time, which thereby provides a more reliable measure of the source angle, but more slowly. Both the fast and slow results may be displayed and updated continuously, or periodically, so that the operator can assess the results visually in real time. For example, the fast results with lower resolution may be displayed using a broad directional icon  1752 , thereby suggesting a general source direction with a relatively wide range of angles, overlain by a sharper and more stable directional icon  1753  showing the slower, high-resolution angle result. With such a composite display, the operator can evaluate the source location in real-time while moving through a clutter field and other variations, by attending either the fast low-resolution icon  1752  or the slow high-resolution icon  1753  according to the current inspection conditions. 
       FIG. 18  shows in perspective, partially cut-away, an embodiment of a walk-through portal  1801  in which a plurality of the present systems  1802  are mounted. In this application, it may be advantageous to mount the systems  1802  with their aiming planes horizontal so that each system  1802  can measure the vertical location of the source on a person. Systems  1802  may be mounted in the walls and ceiling of the portal  1801 . In another embodiment, further systems  1802  may be mounted under the floor  1803  of the portal  1801  as well. Alternatively, the floor  1803  may comprise an automatic weighing scale configured to determine when a person is in the portal  1801 , and to sound an alarm if the person tries to pass through the portal  1801  too quickly. By localizing the source, the portal  1801  can indicate where the source is concealed on a worker&#39;s clothes, toolbox, hair, shoes, etc. 
       FIG. 19  shows in perspective an embodiment of a mobile radiation scanner  1901  comprising a truck or van configured to detect hidden sources in, for example, an urban environment. The mobile scanner  1901 , partially cut-away, may include an array of the present systems  1903 , of which some are shown in dash and others are behind the shell of the mobile scanner  1901 . Preferably the array of systems  1903  nearly fills the central region of the mobile scanner  1901  so as to maximize the detection area when viewed from the side. The systems  1903  may be of the bidirectional type as depicted in  FIG. 15  to detect particles arriving from either the left or right side of the mobile scanner  1901 . Alternatively, the systems  1903  may be of the unidirectional type as depicted in  FIG. 1  to detect particles arriving from one side, such as the curb side, of the mobile scanner  1901 . 
     Some embodiments also include a cosmic ray veto counter  1905 , comprising a plastic scintillator for example, configured to reject events that include a cosmic ray signal. An optional neutron shield  1906  may also be added to the ceiling to block naturally occurring low-energy neutrons, and a second neutron shield  1907  may be mounted on the floor to block ground-effect neutrons. The neutron shields  1906  and  1907  may each comprise a layer of LiF in HDPE, for example. 
     By recording the detection rates in each of the systems  1903  and  1904  as well as the GPS coordinates and bearing, the mobile scanner  1901  can prepare a two-dimensional map of radiation sources in the environment. High sensitivity and high specificity can be achieved in the radiation map due to the directionality of each system  1903  and  1904 . Any future changes to the radiation distribution would then be a cause for alarm. 
     In some embodiments, some of the systems  1903  or  1904  may comprise neutron-sensitive gamma-blind detector material, while others may comprise gamma-sensitive neutron-blind material, thereby simultaneously providing a map of the neutron radiation distribution and a separate map of gamma sources. Likewise the systems  1903  and  1904  may be configured to detect high energy neutrons and to reject low energy neutrons, or vice-versa. Most naturally-occurring background neutrons have low energy due to multiple scattering in the atmosphere, whereas neutrons from weapon materials are generally high energy of 1 MeV to a few MeV depending on composition. In some embodiments, detecting even a few high energy neutrons would be suspicious, particularly if they all come from a particular location. 
       FIG. 20  is a graph showing the results of an MCNP6 simulation, in which gamma rays from a source were detected in a simulated system configured according to the present disclosure. The graph shows the detection rates in the side detectors, versus the source angle, using a simulated configuration such as that of  FIG. 2 . The data were obtained assuming PVT side detectors with a thickness of 25 mm, and Pb shields with a thickness of 15 mm. The source was a 1.0 MeV isotropic gamma source. The aiming plane was held constant at zero degrees in the simulation, while the source was moved around the system from −90 to +90 degrees relative to the aiming plane. The curve shown with O&#39;s is the counting rate in arbitrary units, for one of the side detectors. It exhibits a high counting rate when the source is at +90 degrees, and drops to a low counting rate when the source moves around to −90 degrees relative to the aiming plane. This is expected since the shields block the gammas from reaching that side detector when the source is on the opposite side. The other side detector, marked with X&#39;s, shows a high rate when the source is at −90 degrees, and a low rate at +90 degrees due to the shielding. 
       FIG. 21  is a graph showing the calculated differential between the two side detector counting rates for the simulation of  FIG. 20 . The differential equals the counting rate of the first side detector, marked with O&#39;s, minus the second side detector rate, marked with X&#39;s. The differential curve is a smooth antisymmetric distribution centered on the aiming plane at zero degrees. 
       FIG. 22  is a graph from the same simulation as  FIG. 20 , showing the counting rate for the middle detector versus source angle. The simulated middle detector was CdWO 4 , 3 mm thick and 30 mm wide, mounted with a 15 mm recess from the front ends of the shields. As expected, the middle detector had a high detection rate when the system was aimed at the source. 
       FIG. 23  is a graph showing the angular correlation function, derived from the detection data versus source angle in the simulation. The curve of  FIG. 23  is the angular correlation function relating the source angle to the ratio R=D/S3 where D is the differential between the two side detector counting rates as shown in  FIG. 21 , and S3 is the middle detector counting rate of  FIG. 22 . The angular sensitivities of the side detectors and the middle detector are sufficiently different that the function graphed can determine the source angle, both sign and magnitude, from a single set of detector rates. 
     In operation, according to some embodiments, the counting rates may first be acquired, then the differential may be found by subtracting the counting rate of the first side detector from the second side detector. Then the differential may be divided by the middle detector rate, thereby obtaining the ratio R. Then, reading across the horizontal axis of  FIG. 23  to the calculated value of R, the curve value at that point may indicate the corresponding source angle on the vertical axis. The correlation is monotonic, meaning that a unique source angle can be found from R. For example, using the angular correlation function of  FIG. 23 , the simulated detection data with a ratio of R=5.6 (dashed line) corresponds to a source angle of about 59 degrees (dotted horizontal line), in close agreement with the actual value of 60 degrees. 
       FIG. 24  is a flowchart showing the steps of an exemplary method for determining the source angle according to some embodiments. First  2401  the side detector counting rates and the middle detector counting rates may be measured. Then,  2402  the detection rates for one side detector may be subtracted from the other side detector, thereby obtaining a differential. Then, the ratio R may be calculated  2403  by dividing the differential by the middle detector rate. R may then be compared  2404  to the predetermined angular correlation function, and the source angle may be determined  2405  as the particular angle that matches the angular correlation function at the calculated value of R. Then,  2406  the calculated angle, both sign and magnitude, may be displayed or transmitted or otherwise reported. 
     More specifically, the system may include non-transient computer-readable media containing instructions that, when executed by a computer or processor, carry out a method to determine the source angle from detection data. The method may include measuring detection rates S1 and S2 of the side detectors and S3 if the middle detector, calculating a differential D equal to the difference between the side detector counting rates or D=S1−S2, and dividing the differential by the middle detector counting rate to obtain a ratio R=D/S3. Alternatively and equivalently, the side detector rates may be divided by the middle detector rate first and then subtracted, as in R=(S1/S3)−(S2/S3). The method may then include comparing R (or its inverse) to the predetermined angular correlation function. Or, equivalently, the value of R may be provided as input to the predetermined angular correlation function. Since the correlation function is configured to relate the source angle to the particle detection rates (or to the ratio R), the angular correlation function thereby provides the value of the source angle as output. The method may include determining both the sign and magnitude of the source angle from the comparing of R to the angular correlation function, thereby directly obtaining the angle between the source direction and the aiming plane. Positive and negative values of the differential may correspond to positive and negative values of the source angle respectively, while large and small magnitudes of the ratio may correspond to large and small magnitudes of the source angle. Preferably, the counting rates of each detector may be corrected for the detection efficiency and normal background rate of each detector before the differential is calculated. In one embodiment, the analysis may use a different measure of the detector activity, other than the counting rates, such as the integrated signal current or the accumulated charge from each detector, or other measure associated with particle detection in each detector, so long as the resulting ratio is related to the source angle by a specific, and preferably monotonic, relationship. 
       FIG. 25  is a flowchart showing the steps of an exemplary method for aligning the system with the source. Many applications require that the system be rotated into alignment with the source to localize the source more precisely, or to acquire spectral data on the source using an energy-resolving fourth detector positioned between the shields for example, or for other reasons. To align the aiming plane with the source location, the side detector counting rates and the middle detector counting rates may first be measured  2501 . Then,  2502  the detection rates for one side detector may be subtracted from the other side detector, thereby obtaining a differential. Then,  2503  the differential may be tested for being at zero or within some limit of zero. If so, then the task is done  2508 , since the aiming plane is aligned with the source. If the aiming plane is not yet aligned with the source, the ratio R may be calculated  2504  by dividing the differential by the middle detector rate. R may then be compared  2505  to the predetermined angular correlation function, and the source angle may be determined  2506  as the particular angle that matches the angular correlation function at the calculated value of R. Then,  2507  if the magnitude of the calculated angle is below a threshold value, the aiming plane is sufficiently aligned with the source  2508  and the task is done. If the calculated angle is not below the threshold value, then  2509  the system may be rotated according to the calculated angle, including both sign and magnitude of the calculated angle, thereby bringing the aiming plane closer into alignment with the source. The method then returns to the beginning  2501  to acquire detection data at the new orientation. 
     In some embodiments, the system can arrive at or close to the source location in a single rotation according to the calculated source angle. Preferred embodiments can then verify the alignment by comparing the side detector counting rates, which are substantially identical (within statistical uncertainties) when the source is aligned with the aiming plane. By determining the magnitude of the source angle as well as its sign, embodiments of the system can avoid the time-consuming, error-prone, skill-intensive, iterative hunt-and-peck process employed by conventional detectors to locate a source. 
       FIG. 26  is a flowchart showing an exemplary method for determining the source angle in incremental stages according to some embodiments. In this method, successive details of the source location are revealed sequentially, each detail being obtained with an additional set of detector data. The simplest results are obtained first, then the full source location is determined at the end. First  2601 , signals may be acquired from the side detectors and the middle detector during a first integration time, and counting rates may be calculated. Then  2602 , the side detector rates may be added and the sum compared  2603  to a normal background rate. If the summed signals are not significantly above background levels, the system may indicate  2604  that no source is present, or at least no source is yet detected. But if the sum of the side detector rates is above the normal backgrounds, then a source is known to be present, although its location is not yet determined. In that case, additional data may be acquired  2605  in a second integration period to enable a more reliable angular analysis. Then  2606 , the differential may be calculated by subtracting one of the side detector rates from the other, and the differential may be compared  2607  to a predetermined threshold. If the differential is small or below a threshold, the task is done  2608  since the aiming plane is now aligned with the source. If the differential is not zero, or is not below a predetermined threshold, then additional detector data may then be acquired  2609  during a third integration time, to accumulate sufficient counts in the middle detector. For example, additional data may be required if the middle detector counting rate is still comparable to its statistical uncertainty, since in that case there are not enough middle detector counts to determine the magnitude of the source angle. After such additional acquisition, if necessary, the source angle may be calculated by dividing the differential by the middle detector rate  2610  and comparing the ratio to the predetermined angular correlation function  2611 . The source location or source angle may then be reported  2612  by displaying it on a screen, transmitting it to a facility computer, storing it in media, or otherwise responding according to the application needs. 
     Optionally  2613  a fourth acquisition interval may be added to acquire sufficient data to reduce the angular uncertainty in the result. If a source is present, it is usually worth taking some extra data to obtain the best angle determination. 
     According to some embodiments, the predetermined angular correlation function may be a table of detection ratios versus source angle. The table values can be linearly interpolated whenever an intermediate value is needed between the table entries. The system can determine the sign of the source angle from the sign of the difference between the counting rates of the two side detectors, and then determine the magnitude of the source angle from the counting rate in the middle detector according to the predetermined angular correlation function. 
       FIG. 27  is a flowchart of an exemplary method to continuously or periodically update the source angle as conditions change, thereby causing the light beam or camera view or displayed icon to remain persistently aimed at the source location even as the system is rotated. The method of  FIG. 27  may be particularly applicable to a portable radiation survey meter such as that disclosed in  FIG. 17 . First  2701 , detection data may be acquired from the side and middle detectors. Then  2702  a calculated source angle value may be determined from the detection data, using the angular correlation method for example. Then  2703  the meter orientation may be checked to determine if the system has been rotated. This step may involve signals from a compass or accelerometer configured to sense rotations, or it may involve image processing to detect a change in direction, or other means for determining if the system has been rotated and by how much. If no such rotation is detected, then  2704  the source angle may be updated by combining the currently calculated value with the previously determined source angle results, so as to obtain a new value that reflects both the recent measurement and the past data. In some embodiments, combining the old and new data may include discarding or otherwise attenuating the oldest data while emphasizing the more recent results. For example, the processor may be configured to store sequential calculated values of the source angle in a ring buffer, such that each newest result overwrites the oldest value in the buffer, and then the entire set of values may be averaged to obtain a best-fit or maximum likelihood or time-average value of the source angle. Alternatively, the current result may be averaged incrementally with the prior average, wherein the current result and the prior average are weighted to obtain an updated average source angle. For example, in a particular embodiment, an updated average may be calculated by adding the old average times 0.9, plus the new value times 0.1. The resulting updated average thereby incorporates the new results incrementally into the running average while gradually attenuating the oldest measurements. Then  2705 , using the updated average source angle so obtained, the system may redirect a light beam, and/or reorient the view of a camera, and/or redisplay a directional icon pointing toward the source according to the updated average source angle. If, on the other hand, the meter has been found to be rotated, then  2706  the display icon and/or light beam and/or camera angle may be revised according to the rotation angle (that is, adjusted opposite to the rotation angle), thereby causing the light beam or camera or icon to continue pointing toward the source location after the rotation. Thus the light beam or camera or icon may be adjusted according to the updated source angle if the system is not rotated, and if the system is rotated the items may be adjusted opposite to the rotation angle, so as to persistently remain centered on the source location. In addition, according to the exemplary flowchart, whenever such a rotation occurs, the most recent detection data may be discarded, since there is no way to know what orientation of the meter corresponded to while the meter was being rotated. In every case, the flow returns to the initial step  2701  of acquiring additional data. By causing the light beam or camera view or display icon to remain persistently locked onto the source location, while the source angle is repeatedly refined and while the meter is arbitrarily rotated, the system greatly assists operators in finding the source. 
     Embodiments of the system as described herein can provide many advantages over conventional detectors. (a) Embodiments can determine the full sign and magnitude of the angle between the aiming plane and the source direction. (b) Embodiments can determine the source angle from a single set of detector data without rotations or iteration. (c) In applications requiring that the system be aimed at the source, the system can be rotated according to the calculated angle and thereby converge on the source direction in only one rotation. (d) The system can achieve high detection efficiency because the detectors have an unobstructed view of the source particles regardless of the orientation of the system, the shields being almost completely surrounded by the side detectors. (e) Embodiments of the system can be compact, low-cost, easy to use in an inspection environment, suitable for a variety of important security scanning applications, and virtually immune to defeat by conventional shielding or obfuscation techniques. 
     The ability to localize a clandestine radioactive source rapidly is enabling for many applications in nuclear threat detection. Advanced radiation detection systems like those disclosed herein will be needed in the coming decades to protect innocent people from the threat of nuclear and radiological terrorism. 
     The embodiments and examples provided herein illustrate the principles of the invention and its practical application, thereby enabling one of ordinary skill in the art to best utilize the invention. Many other variations and modifications and other uses will become apparent to those skilled in the art, without departing from the scope of the invention, which is defined by the appended claims.