CROSS-REFERENCE TO RELATED PATENT APPLICATIONS

[0001] This application claims priority to U.S. Provisional Application No. 62/726,760, filed on September 4, 2018, the content of which is hereby incorporated by reference in its entirety.

STATEMENT OF GOVERNMENT INTEREST

[0002] This invention was made with Government support under Contract No. FA8702-15- D-0001 awarded by the U.S. Air Force. The Government has certain rights in the invention.

BACKGROUND

[0003] Hand-held radiation detectors offer the capability of source identification and can help locate sources at close range.

BRIEF SUMMARY

[0004] This disclosure includes a radiation detector system comprising a plurality of scintillating optical fibers arraigned in a bundle, the bundle having a first end and a second end. Each of the plurality of scintillating optical fibers are arraigned to: receive incident radiation associated with a radiation source, emit photons in response to the incident radiation, and transport the photons to a first end and a second end of a respective one of the plurality of scintillating optical fibers. The radiation detector system can also include an asynchronous detector array positioned relative to one of the first end or second end of the bundle to detect the photons. The radiation detector system can also include direction circuitry operatively coupled to the asynchronous detector array, the direction circuitry programmable or configurable to calculate a direction of travel of an electron or other charged particle associated with the incident radiation, a total energy deposited by the electron or other charged particle associated with the incident radiation, or a position of the electron or other charged particle associated with the incident radiation within the asynchronous detector array, and calculate a direction of the radiation source relative to the radiation detector system based at least in part on the direction of travel of the electron or other charged particle associated with the incident radiation source. [0005] The disclosure also includes a method, the method comprising receiving incident radiation associated with a radiation source. The method further comprises emitting photons in response to the incident radiation, and transporting the photons to a first end and a second end of a respective one of the plurality of scintillating optical fibers. The method can further comprise calculating a direction of travel of an electron associated with the incident radiation, a total energy deposited by the electron associated with the incident radiation, or a position of the electron associated with the incident radiation within the asynchronous detector array.

The method can further comprise calculating a direction of the radiation source relative to the radiation detector system based at least in part on the direction of travel of the electron associated with the incident radiation source.

BRIEF DESCRIPTION OF THE DRAWINGS

[0006] To assist those of skill in the art in making and using exemplary embodiments of the present disclosure, reference is made to the accompanying figures. The accompanying figures, which are incorporated in and constitute a part of this specification, illustrate one or more embodiments of the present disclosure and, together with the description, help to explain the present disclosure. Illustrative embodiments are shown by way of example in the

accompanying drawings and should not be considered as limiting. In the figures:

[0007] FIG. 1 illustrates an exemplary environment in which a hand-held radiation detector as taught herein can detect and locate a radiation source, according to an exemplary embodiment;

[0008] FIG. 2 illustrates a scintillating optical fiber bundle, according to an exemplary embodiment;

[0009] FIG. 3 illustrates another scintillating optical fiber bundle, according to an exemplary embodiment;

[0010] FIG. 4 illustrates a single scintillating optical fiber bundle, according to an exemplary embodiment;

[0011] FIG. 5 illustrates a silicon multiplier with a number of macro-pixels each made up of a plurality of Geiger-mode Avalanche Photodiodes (GM-APDs) sub-pixels arrayed such that the macro-pixels form a plurality of stripes across the entire detector face, according to an exemplary embodiment ; [0012] FIG. 6 illustrates a schematic of a silicon photomultiplier, according to an exemplary embodiment;

[0013] FIG. 7 illustrates a scintillating optical fiber bundle, according to an exemplary embodiment;

[0014] FIG. 8 illustrates a scintillating optical fiber bundle, according to an exemplary embodiment;

[0015] FIG. 9 illustrates a scintillating optical fiber, according to an exemplary embodiment;

[0016] FIG. 10 illustrates a read out integrated circuit (ROIC), according to an exemplary embodiment;

[0017] FIG. 11 is a block diagram for directional detection of a gamma particle;

[0018] FIG. 12 illustrates a scintillating optical fiber bundle with orthogonally layered fibers coupled with a detector array, according to an exemplary embodiment;

[0019] FIG. 13 illustrates a collection of scintillating optical fiber canes with each fiber cane formed by a plurality of scintillating optical fibers and each fiber cane individually coupled to a detector, according to an exemplary embodiment;

[0020] FIG. 14 is an avalanche photodiode array, according to an exemplary embodiment;

[0021] FIG. 15 is an exemplary flow of radiation detection system, according to an exemplary embodiment.

DETAILED DESCRIPTION

[0022] Embodiments taught herein teach directional radiation detector systems. The detector systems taught herein are lightweight and portable. They do not employ shielding, for example, lead shielding, so they advantageously provide omnidirectional receive capability in addition to directionality. The detector systems taught herein leverage the use of scintillating optical fibers to reconstruct a charged particle path through the fibers to determine the location, directionality and time of arrival of a radioactive event, for example, a gamma ray, a fast neutron or beta particle. [0023] Applications for directional detection systems as taught herein, include hand-held radiation detection with enhanced source location, portal (drive-through) radiation monitors for shipping containers, trucks, and other vehicles, and nuclear medical imaging.

[0024] In optical fiber-based directional detection of gamma radiation, gamma rays (1 - 3MeV) are highly penetrating, but can scatter from electrons within materials. When properly designed, single scatter events far outnumber multiple scatter events. When a gamma ray interacts with a free electron, the gamma ray creates one or more recoil electrons that leave a track of 4-10 mm in length. A direction of the incident gamma ray can be partially inferred from the direction of the recoil electron. Analyzing a small collection of recoil electrons improves the estimate on the source location. Scintillating optical fibers fluoresce after the passage of an energetic charged particle. An electron track can be resolved by determining through which fibers the electron has passed. As taught herein, the directional detector detects the small collection of photons that are emitted by each fiber tip to resolve the recoil electron track. It should be noted that an approach can also discriminate fast neutrons from gamma rays and beta particles, which is a difficult problem in other detection methods.

[0025] The sensitivity of a gamma-ray detector is governed by the mass of the active detection material, and the signal from a source scales with the source activity times the inverse square of the source range. Shielding can further attenuate the signal. Thus, even for strong sources, at some range the radiation produced becomes comparable to the natural radiation background, making source localization a very difficult problem in many cases. Embodiments taught herein, do not rely on masks or apertures for detection purposes and thus advantageously provide a higher sensitivity level to conventional gamma-ray detectors.

[0026] A particularly difficult and important problem is detection of Special Nuclear

Materials (SNMs, e.g., Highly Enriched Uranium, HEU) within shipping containers, trucks, etc. With thousands of tons of SNMs produced worldwide to date, such materials can slip out of state control and become available to non-state actors. Also, although such materials are capable of releasing very large amounts of radiation when detonated, their natural radiation rates are very low. As such, such materials are often difficult to detect especially if shielded. Embodiments taught herein, provide a directional detection system capable of detecting radiation sources based on their natural radiation rates, adding source localization to improve discrimination of the source within natural backgrounds. [0027] Medium-energy gamma rays (0.5-2MeV) are highly penetrating in materials and, being uncharged, leave no discernible ionization track. The primary interaction of such gamma rays with materials is via Compton scatter events from individual electrons within the detector active volume. Once such an event occurs, the (charged) recoil electron leaves an ionization track which, if resolved, gives information about the original direction of the gamma ray. The trajectory of the recoil electron, to the degree that it can be measured, determines the angle of the initial gamma ray to within a cone of possible values. A small collection of events from a point source can then be used to reveal the existence and direction of, for example, a point source within a uniform background.

[0028] FIG. 1 illustrates an exemplary environment in which a directional radiation detector as taught herein can detect and locate a radiation source, according to an exemplary embodiment. Exemplary environment 100 includes a radiation source 102 that is emitting gamma rays within its immediate vicinity. The directional radiation detector 101 can detect gamma rays throughout the volume of one or more bundles of scintillating optical fibers that are connected to a detector, which, in turn is coupled to one or more integrated circuits that determine, at least, the location of the radiation source 102. In exemplary environment 100, if a user has a hand-held embodiment of the directional radiation detector 101, the user can determine the direction of the radiation source 102 even when the source is hidden from view. This is accomplished through analysis of the radiation events detected within the directional radiation detector 101, as will be discussed in more detail below. In some embodiments, the directional radiation detector 101 is a hand held device and can be secured to an article of clothing, or is mountable to a fixed or mobile platform.

[0029] FIG. 2 illustrates a scintillating optical fiber bundle, according to an exemplary embodiment. Scintillating optical fiber bundle 200 can be a cutaway volume of a portion of the entire scintillating optical fiber bundle that runs the length and width of the directional detector 101. Gamma particle 201 can enter scintillating optical fiber bundle 200 on side 204, and may scatter through the volume of scintillating optical fiber bundle 200 and eject as scattered gamma particle 202 from side 205. As gamma particle 201 travels through the volume of scintillating optical fiber bundle 200, and interacts with the individual fibers in the scintillating optical fiber bundle 200, one or more electrons, for example, recoil electrons can be generated via Compton scattering. The electrons leave an ionization track which can be used to determine the direction from whence the gamma particle 201 came from relative to side 204. When a plurality of gamma particles strike side 204, the collection of electrons can be traced to through the scintillating optical fibers back to side 204 thereby providing an average direction from which the source of gamma particle 201 are being emitted outside of the directional detector 101.

[0030] As the electron passes through each fiber, it causes the fiber to locally fluoresce, and the fluorescence photons are transported to the ends of the fibers, revealing the track of the electron. The track may be resolved in three dimensions by using the alternating orthogonal fiber structure as shown in FIG. 2. The trajectory of the electron as it moves through the scintillating optical fiber bundle 200 can be projected simultaneously onto two surfaces, from which the track, or path that the electron has traveled through the scintillating optical fiber bundle 200, can be resolved in 3 -dimensions. The orientation of the track of the electron can be determined can be based at least in part on energy loss of the electron as it travels through scintillating optical fiber bundle 200 and stops at a particle location of scintillating optical fiber bundle 200. As the recoil electron begins to lose momentum and slows down, the energy loss rate (dE/dx) or the change in energy as it moves throughout the volume of the scintillating optical fiber bundle 200 increases. Thus the end of the track is brighter than the start. Detector arrays can be affixed to two faces of the structure where the fibers ends are exposed (e.g., fiber ends 207 and fiber ends 205) and thus can detect photons emitted by the fibers, thereby enabling the detector array, and more specifically an integrated circuit, to determine the trajectory of the gamma ray based on the light emitted by the fibers that the electron 203 interacted with as it traveled through the scintillating optical fiber bundle 200. It should be noted that, in this embodiment, the useful volume of the detector array can scale as the square root of the number of detector elements within the readout array(s).

[0031] FIG. 3 illustrates a scintillating optical fiber bundle 400, according to an exemplary embodiment. In some embodiments, directional radiation detector 101 comprises a plurality of scintillating optical fibers 401 that are arranged in a parallel fashion, and the ends of each of the plurality of scintillating optical fibers 401 are affixed to silicon photomultiplier (SiPM) arrays 403 and 405. As taught herein, a SiPM is a single macro-pixel that can be formed into an array.

Not illustrated in Fig. 4, but discussed in detail below are readout integrated circuits (ROICs), which are communicatively coupled to the arrays 403 and 405 The ROIC is an integrated circuit specifically designed to identify a track of a recoil electron, and determine certain parameters associated with the recoil electron such as the time of the event, the track length, the track orientation, and the total track brightness. This is accomplished by the ROIC recording and analyzing the signals provided to it by the SiPM array affixed to one or both ends of the fiber array. In some embodiments, the ROIC is a separate circuit from each SiPM array. In some embodiments, the SiPM array and ROIC are combined into a single integrated circuit.

[0032] When a gamma ray interacts with the scintillating optical fibers 401, a corresponding recoil electron can be generated that travels along exemplary path or track 407. The interaction of a number of energetic electrons (beta particles) as they penetrate a plastic material such as a collection of scintillating optical fibers results in the scattering of these particles by the material. The material can cause low-energy escaping x-rays to be emitted in response to the interaction of the energetic electrons interacting with the plastic material. It should be noted that the recoil electron can travel at any angle relative to the scintillating optical fibers 401. As the recoil electron passes through the scintillating optical fibers 401, it creates fluorescence photons that are transported to the ends of the scintillating optical fibers 401, revealing a projection of the electron track into the plane 500. This projection of the electron path is sufficient to determine, although with an acceptable level of uncertainty, its direction perpendicular to the fiber array and within the plane 500. The direction also leverages the fact that the track end can be brighter than the track start. In addition, some out- of-plane directionality can be determined by comparing the apparent track length to the track brightness. Tracks that pass obliquely through the fibers have a longer path length within each individual fiber and thus leave a brighter track than those passing perpendicularly to the fibers.

[0033] As the recoil electron travels along track 407 each of the scintillating optical fibers 401 fluoresce with photons. These photons can be detected by pixelated SiPM detector arrays 403, 405 at one or both ends of the fiber bundle 400 that measure the total number of photons in each fiber. In one embodiment, the photons are detected by SiPMs designed with a striped macro-pixel geometry, each macro-pixel containing a plurality of GM-APD detectors (sub pixels). The striped macro-pixels can extend for the width of the detector array matched to the width of the fiber bundle. Two striped SiPMs can be used, one at each end of the fiber bundle 400. One side can have a vertical SiPM and the other a horizontal SiPM. The SiPM arrays 403, 405 are communicatively coupled to the ROICs that can partially resolve the projected tracks 402 and 404 of the photons as the recoil electron travels through scintillating optical fibers 401. Although the track is only partially resolved, analysis of the signals from the vertical 403 and horizontal 405 striped SiPMs are sufficient to determine the trajectory of the electron, observing the fact that the end point of the track is brighter than the start point.

In some embodiments, the SiPM and the ROIC are integrated as an integrated circuit. In some embodiments, the SiPM and the ROIC are separate integrated circuits operatively coupled to each other. Vertical SiPM 403 and horizontal SiPM 405 can be striped readouts which provide sufficient information to partially resolve the electron track as projected into the plane perpendicular to the fibers 500. Alternatively, in some embodiments, each fiber could be connected to a separate SiPM macro pixel of a SiPM array located at one end of the fiber bundle, which also allows resolving the electron track within the perpendicular plane 500. In some embodiments, the other end of the fiber bundle could be coated with a reflective material that effectively increases the number of photons impinging on the SiPM array.

[0034] Striped SiPM arrays, as opposed to high-count square-pixel SiPM arrays, provide sufficient information to resolve the track direction as projected into the plane perpendicular to the fibers, but at greatly reduced channel count, potentially reducing the complexity of the system. That is, a vertical SiPM 402 plus an ROIC and horizontal SiPM 405 plus an ROIC resolve the track 407 direction in the same plane that Vertical SiPM 403 and horizontal SiPM 405 lie in. The resolution of track 407 as projected onto the plane that SiPM 403 and SiPM 405 lie in corresponds to projected tracks 402 associated with Vertical SiPM 403 and projected track 404 associated with horizontal SiPM 405.

[0035] In some embodiments, a cane, which is made up of a plurality of unidirectional scintillating optical fibers and optically coupled with SiPM array detectors as shown in FIG.

4 and, the SiPM array detectors communicatively coupled to ROICs can be one of many modules within the directional radiation detector system 101. In some embodiments, for example, as illustrated in FIG. 14, multiple canes or modules can be combined together, improving both overall system sensitivity and directional capability. When a plurality of gamma rays impinge on a multi-module system as shown in FIG. 14 from a radiation source, the horizontal modules 1402 find the source direction in one plane while the vertical modules

1401 find the source in a perpendicular plane. Combining the directionality of horizontal

1402 and vertical 1402 modules thus reveals the source direction with an acceptable statistical uncertainty that reduces as the number of detected events increases. [0036] FIG. 4 illustrates a single scintillating optical fiber bundle and the directional information provided in a plane perpendicular to the single fiber bundle, according to an exemplary embodiment. Plane 500 is a plane that illustrates how information is obtained from the SiPM arrays 403 and 405 and ROICs. The track, or directional information, is said to be projected onto the plane that is perpendicular to the orientation of the parallel scintillating optical fiber bundle because the SiPM arrays detect photons received on the scintillating optical fibers and the SiPM arrays are arranged in a striped orientation that is perpendicular to the orientation of the parallel scintillating optical fiber bundle in one embodiment. In another embodiment, the SiPMs arrays read out each individual fiber, however, the striped

embodiment reduces the complexity of the system.

[0037] FIG. 5 illustrates a plurality of stripes of a silicon photomultiplier (SiPM) array 600 formed from macro-pixels that form a plurality of narrow stripes 602 that cross the full face of the array, according to an exemplary embodiment. Each stripe (macro-pixel) contains a plurality of GM-APD sub-pixels 603 that are combined into a single channel connected to the ROIC. Thus, the number of channels is equal to the number of stripes. The striped SiPM array 600 plus ROIC can detect photons generated by a recoil electron traversing a plurality of scintillating optical fibers The stripped SiPM array 600 is made up of a plurality of striped SiPMs, for example, SiPM 602, 607. The number of striped SiPMs forming the stripped SiPM 600 can vary to match the size of the scintillating optical fiber bundle. Each SiPM 602, 607 can be a pixel that can convert photons emitted by the scintillating optical fiber bundle into an electrical pulse thereby enabling the ROIC to determine a track, or path, of the recoil electron which can be used to determine a point source of one or more gamma particles external to the directional detector device 101.

[0038] SiPM 602 can be a macro-pixel that generates an electrical pulse in response to detecting a photon that traverses one of the scintillating optical fibers. SiPM 602 can detect the photons anywhere along the length of SiPM 602. Likewise, SiPM 607 is a striped SiPM that can detect a photon over its entire length. Exemplary SiPMs 602, 607 can include a plurality of Geiger mode avalanche photo diodes (GM-APDs) 603 that form sub-pixels 606 that make up the macro-pixel SiPMs 602, 607. GM-APD 603 is an exemplary GM-APD.

Each SiPM 602, 607 can include thousands of GM-APDs. [0039] In some embodiments, an exemplary width 604 of each SiPM forming the striped SiPM 600 can be equal to or approximately equal to the diameter of a fiber in the scintillating optical fiber bundle, for example, 200-500 um. In some embodiments, an exemplary length 605 of each SiPM forming the striped SiPM 600 can be the full length of the striped SiPM 600. In some embodiments, an exemplary length is 25 mm. In exemplary embodiments, no fibers extend beyond the top of striped SiPM 600 or beyond the bottom of striped SiPM 600. In some embodiments, the striped SiPM 600 are tiled or stacked to accommodate larger size scintillating optical fiber bundles.

[0040] GM-APDs 609 and 608 detect photons as a result of the recoil electron interacting with a scintillating optical fiber that terminates on GM-APDs 609 and 608. More specifically, the recoil electron can interact with a first scintillating optical fiber that terminates on GM- APD 609 thereby producing the portion of curve 606 in the area occupied by GM-APD 609. The recoil electron can also interact with a second scintillating optical fiber that terminates on GM-APD 608 thereby producing the portion of the curve 606 in the area occupied by GM- APD 609.

[0041] In some embodiments, each GM-APD 603 forming a striped SiPM can have a square shape or rectangular shape have a length between 25-100 microns and a width between 25- 100 microns. GM-APD 603 can be square sub-pixels with a size less than 100 microns. The SiPM macro-pixel stripes can have a width matched to the fiber diameter of 100 - 1000 microns. The stripe length can extend across the width of the entire detector, which could be 25 millimeters or more. Ellipsis 606 indicates that the length 605 can extend to be equal to the width of the hand-held radiation detector.

[0042] FIG. 6 illustrates a schematic of an exemplary SiPM macro-pixel, according to some embodiments. A plurality of GM-APDs sub-pixels 701-705 are coupled in parallel to form the SiPM macro-pixel. Each of the GM-APD sub-pixels 701-705 includes an individual quenching resistor that automatically resets the GM-APD following detection of a photon. Thus, SiPM macro-pixel is considered asynchronous detector. When a plurality of photons impinges on the SiPM macro-pixel, several of the GM-APD sub-pixels activate at once, creating a detectible signal above the noise caused by random activations (“dark noise”). The SiPM output provides a signal to a single channel input on the ROIC, thus effectively combining the signals of many individual detectors. This enables the ROIC to read the outputs of GM-APDs 701-705 after GM-APDs 701-705 convert the detected photons into electrical pulses across terminals 706 and 707.

[0043] FIG. 7 illustrates another view of the scintillating optical fiber bundle 401, according to an exemplary embodiment. Scintillating optical fiber bundle 401 is uni-directional arrangement of scintillating optical fibers arranged in such a way that they are parallel to one another. Although illustrated as having a square cross sectional shape, other cross sectional shapes are possible, for example, rectangular, round, oval, etc. Generally, the cross section of the scintillating optical fiber bundle 401 is matched to a focal plane of the detector. When a GM-APD senses a photon, it goes from the“off’ to“on” state. To reset the GM-APD, the bias voltage must be temporarily reduced. This can be accomplished either by a circuit (active quenching) or by placing the GM-APD in series with a resistor (passive quenching).

[0044] The expected number of photons per scintillating optical fiber has been calculated to be approximately 30 for the case of a lMeV recoil electron travelling perpendicular to the fiber across its axis. This calculation presumes a fiber diameter of 200 microns. Using a larger fiber increases the number of photons per fiber approximately linearly, but lowers the resolution with which the electron track can be reconstructed. The choice of the fiber diameter also affects the number of readout channels required. It can be varied to fit a range of applications as indicated.

[0045] FIG. 8 illustrates the scintillating optical fiber bundle 401, according to an exemplary embodiment. The scintillating optical scintillating optical fiber bundle 401 in volume 901 illustrates that a uni-directional scintillating optical fiber bundle in combination with SiPM ROICs 403 or 405, can resolve the azimuthal angle 902 that a recoil electron travels and as well as the azimuthal angle 903 relative to a coordinate with an origin at a place of entry of the gamma particle to scintillating optical fibers 401. In some embodiments, for example, embodiments deploying the scintillating optical fiber bundle 200 can provide source direction over multiple recoil electrons that traverse scintillating optical fibers from a point source of gamma particles.

[0046] By orienting scintillating optical fibers in a uni-directional parallel orientation, SiPM ROICs 403 and 405 can provide information about whether the track is in the same plane as SiPM ROICs 403 and 405 based at least in part on the number of photons generated (which is proportional to the energy of the event) and the length of the track (which is also a function of the energy of the event). When the track length and number of photons match, SiPM ROICs 403 and 405 can determine that the track that the recoil electron traveled in response to an event is close to being in a plane perpendicular to the scintillating optical fibers. When the track length is not longer than a certain threshold amount for the number of photons detected, SiPM ROICs 403 and 405 can determine that the track that the recoil electron traveled in response to an event occurred out of the perpendicular plane. Using two perpendicular scintillating optical fiber canes, as illustrated in FIG. 14, a collection of events interacting with the two perpendicular scintillating optical fibers can be analyzed to locate a source in 3- dimensions. This uni-directional geometry has the advantage of being easier to fabricate. It also has a simplified coupling to a detector array, and efficiently utilizes all available pixels. Finally, the useful length of the assembly is only limited by the optical transmission of the fibers (2-4 meters depending on the fiber choice). In some embodiments, a hand-held radiation detector can be realized using a single scintillating optical fiber cane whereas in other embodiments two or more scintillating optical fiber canes.

[0047] FIG. 9 illustrates an exemplary scintillating optical fiber, according to an exemplary embodiment. FIG. 10 illustrates what happens when a recoil electron strikes the outside of the scintillating optical fiber, for example, a scintillating optical fiber from bundle 201 or 401.

For example, a recoil electron can transverse scintillating optical fiber 1000 along a path designated by arrow 1001. As the recoil electron interacts with the materials in the scintillating optical fiber, a plurality of photons can be emitted and travel in different directions internal to scintillating optical fiber 1000. For instance, as the recoil electron traverses axis 1008 of scintillating optical fiber 1000, a first photon can be emitted that travels in the direction of arrow 1003, a second photon can be emitted that travels in the direction of arrow 1004, a third photon can be emitted that travels in the direction of arrow 1005, and a fourth photon can be emitted that travels in the direction of arrow 1006. The fourth photon can reflect along the inside of scintillating optical fiber 1000 and travel along the direction of arrow 1007 and travel along the length of scintillating optical fiber 1000. The left hand side and right hand side of scintillating optical fiber can be optically coupled to a striped SiPM such as the SiPM 607. The photons emitted in the scintillating optical fiber 1000 can be detected by the SiPM and read by an ROIC. [0048] FIG. 10 illustrates a scintillating optical fiber bundle as taught herein optically coupled to a focal plane of a detector, for example, an asynchronous SiPM array detector.

The SiPM array can be an array of macro-pixels, where each SiPM array detector can comprise a plurality of asynchronous GM-APDs. Scintillating optical fibers 201, 401 can be optically coupled directly to an asynchronous GM-APD focal plane 1103. Asynchronous GM-APD focal plane 1103 can be directly communicatively coupled to a readout integrated circuit (ROIC) 1105 such as a field programmable gate array (FPGA). Bus 1107 can communicatively couple asynchronous GM-APD focal plane 1103 to ROIC 1105. Due to the scintillating optical fibers having a high numerical aperture, direct communicative coupling can be used couple the scintillating optical fibers directly to a detector array GM-APDs. As explained above when a photon is emitted in a scintillating optical fiber, a GM-APD detects the photon and generates an electrical pulse. That electrical pulse is detected by ROIC 1105. Because the GM-APDs disclosed herein are asynchronous, the macro-pixels will not saturate due to the fact that each GM-APD is reset over time at approximately a rate at which a flux of photons strike the GM-APD. In some embodiments, the GM-APD arrays can reset at a rate of every 0.5 microseconds. In other embodiments, the GM-APDs can reset at a rate that is slower of faster depending on what type of particle the GM-APDs have been configured to detect.

[0049] A plurality of GM-APDs can be used to detect photons on a single fiber. In some embodiments, the electrical pulses generated by the GM-APDs can be read out across all of the GM-APDs at the same time. In other embodiments, the electrical pulses generated by the GM-APDs can be read out from each of the GM-APDs in series. In some embodiments, it is advantageous to know how many photons are generated in each fiber or collection of fibers. This requires using multiple GM-APDs, since each GM-APD can sense at most a single photon. So long as there are more GM-APDs than the maximum number of photons to be detected (for example, twice as many), the photons can be counted with approximate accuracy. Note that the individual GM-APDs could be sensed individually or combined into a single SiPM macro-pixel with a single readout channel.

[0050] The GM-APD or SiPM detector array can be near single-photon sensitive and can be capable of event read-out rates in excess of 10 MHz. The individual GM-APDs can spontaneously trigger due to thermal fluctuations within the detector itself, causing what are called“dark events.” The dark event rate depends on the GM-APD characteristics and the sensor temperature. The dark event rate can be reduced by cooling the detector with, for example, a Peltier cooler. Dark events are uncorrelated, whereas actual radiation events produce bursts of photons that are time correlated to within -10 nanoseconds. Because of this, only a few detected photons are needed to discriminate the true events from dark events.

Therefore the requirement for the dark current is that it not interfere with the interpretation of the event to the degree necessary. Accordingly, the GM-APDs as disclosed herein can detect photons commensurate with the above mentioned event rate, and can tolerate a relatively high dark event rate (e.g., compared to charged couple devices (CCDs), Intensified Charge- Coupled Devices (ICCDs), and complimentary metal oxide (CMOS) devices) due to its fast temporal response.

[0051] ICCDs are complex and do not scale as well as GM-APD arrays, which can be fabricated entirely from silicon and they do not implement a read out time window that is fast enough to accommodate the event rate mentioned above. CMOS detectors are more prone to readout noise and have a limited range of pixel sizes (which are typically lOnm or less). CCD detectors do not implement a read out time window that can accommodate the speed of the event rate, and they are prone to noise as they read out the electrical pulses corresponding to the detection of a photon. Accordingly, the GM-APD arrays as disclosed herein provide an advantageous solution for reading out scintillating fiber bundles.

[0052] FIG. 11 is a flowchart for directional detection of a gamma particle, according to an exemplary embodiments where each GM-APD is read independently or where they are connected into SiPM macro-pixels. Detection circuitry 1208 can include asynchronous readout 1204, determine spatially/termporally correlated events 1205, and calculate track parameters 1206. Readout integrated circuit 1209 can include detection circuitry 1208 and upstream decision logic 1207. Particle 1200 can be detected by scintillating optical fiber bundle 1201. Particle 1200 can be a radiation particle, and can be detected by scintillating optical fiber bundle 1201. Scintillating optical fiber bundle 1201 can be implemented as scintillating optical fiber bundle 400. Detectors 1202 can be on the ends of scintillating optical fiber 1201 which can detect photons associated with the movement of particle 1201 through scintillating optical fiber bundle 400. ROIC 1105 has the structure and operation to perform the functions in process flow 1200. The ROIC 1105 can be programmed or configured, or both to perform the functions in process flow 1200. In some embodiments ROIC 1105 can be replaced with a computer processor, or any other integrated circuit that can execute computer executable instruction sets. In some embodiments, all of the functional blocks in FIG. 12 can be combined into a single integrated circuit. In other embodiments, detectors 1202, which can be implemented as GM-APDs can be separate from the functional blocks that determine the spatial and temporal pairs associated with events in the scintillating optical fibers, and determine track parameters associated with the event.

[0053] A GM-APD or SiPM array can detect photons from a fiber bundle induced by radiation 1201, at functional block 1202 and the GM-APD or SiPM array can detect a photon in functional block 1203 corresponding to example event 1208 of a recoil electron traversing a plurality of scintillating optical fibers as described above. The ROIC can asynchronously readout an electrical pulse generated by the GM-APD or SiPM array at functional block 1204. The ROIC can then resolve the electrical pulse into a temporal and spatial tuple (time, location) based on the electrical pulse, and then determine any spatially and temporally correlated events at functional block 1205. The ROIC can determine this correlation by accessing a memory (random access memory) and/or local storage device (solid state device) storing information associated with previous events. This information can include time, total energy, location, and direction information associated with other exemplary events. The correlation can be performed based on one or more photon events happening within a certain period of time of one another. This period of time can be a fraction of a nanosecond to a microsecond. If the number of photon events happening is above a certain threshold given the 10 nanosecond difference in time between the different events, then a radiation event is said to have occurred. The SiPM can determine that a radiation event has occurred because the peak height of the electrical pulse associated with the one or more photon events represents the number of sub-pixels that have been triggered within the 10 nanosecond response time. If an asynchronous GM-APD is used in place of a SiPM, the asynchronous GM-APD can work with the ROIC to determine that a radiation event has occurred. The ROIC can then determine that there are a cluster of events that have occurred that are tightly correlated in time and location, by comparing the tuple associated with each new example event to the tuples stored in memory associated with past example events. The ROIC can calculate track or path parameters associated with the collection of tuple information associated with the example events at functional block 1206. The track parameters can include the timing information associated with the recoil electrons associated with the example events traveling through the scintillating optical fibers, the length or distance that the recoil electrons traveled in the volume of the scintillating optical fibers, the direction that the recoil electrons traveled throughout the volume of the scintillating optical fibers, the gradient total brightness (a directional change in the intensity of the recoil electrons as they travel through the volume of the scintillating optical fibers). As the gradient total brightness increases the energy decreases thereby causing the scintillating optical fiber that the recoil electron interacts with at the end of its track to glow brighter than any other scintillating optical fibers that the recoil electron interacted with along its track up to the end of its track. The ROIC can perform one or more logical decisions at functional block 1207. The one or more logical decisions can include determining based on the derived track parameters, the location of a point source of radiation associated with incident light 1201.

[0054] FIG. 12 illustrates an orthogonally overlapping scintillating optical fibers bundle optically coupled to a detector array, according to an exemplary embodiment. In this embodiment, crossed layers of fibers are combined into a slab geometry 1300. Detector arrays cover two edges of the slab (1302, 1303). Detector arrays for this embodiment can be designed to sense light from each individual fiber, allowing for high-fidelity reconstruction of recoil electron tracks within the slab. Block 1300 can be a cross-section of a portion of the inside of a hand held radiation detector responsible for detecting a radiation point source (e.g., gamma particles emitted by the radiation point source). Block 1300 comprises orthogonally overlapping scintillating optical fibers as illustrated by a portion of the slab shown in 1301. Scintillating optical fiber volume 1301 can be similar to scintillating optical fiber bundle 200 in function and design, and may be approximately 0.15 inches (0.15”) per side in a square orientation. Block 1300 also includes a plurality of detector arrays 1302 and 1301 that are on two edges of block 1300. Detector arrays 1302 and 1301 can be GM-APD detector arrays as explained above, and the corresponding ROICs connected to each of the detector arrays can unambiguously resolve any events that occur within block 1300 in three dimensions. By canvasing block 1300 with detector arrays the track of a recoil electron can be resolved in three dimensions.

[0055] In some embodiments, block 1300 can be a square that is ten inches by ten inches (10” x 10”). Each detector array can be 1 inch by 1 inch (1” x 1”).

[0056] FIG. 13 illustrates uni-directional orthogonal scintillating optical fiber canes with each fiber cane individually coupled to a detector, according to an exemplary embodiment. Each cane is formed from a plurality of uni-directional scintillating optical fibers. FIG. 4 illustrates how a bundle of scintillating optical fibers can be packaged together as a scintillating optical fiber cane. The scintillating optical fibers are all uni-directional and lie along the same axis. There can be a first group of scintillating optical fiber canes (scintillating optical fiber canes 1402) oriented in a first direction and a second group of scintillating optical fibers canes (scintillating optical fiber canes 1401) are oriented in a second direction, where the first group is placed on top of the second group in such a way that the orientation of the first group of scintillating optical fiber canes is orthogonal to the second group of scintillating optical fiber canes.

[0057] By stacking groups of scintillating optical fiber canes on top of one another with an orthogonal orientation, can improve the coupling of the scintillating optical fibers to the focal plane detector arrays. A hand held radiation detector comprising a plurality of scintillating optical fiber canes can greatly improve the sensitivity of the radiation detector because multiple events that occur in alternate canes can help better resolve the location of the point source of the radiation. The dimensions of the cross section of each of the scintillating optical fiber canes corresponds to the dimension of the detector array. As mentioned above with regard to SiPM ROIC 600, the width of each scintillating optical fiber cane can be equal to the diameter of the scintillating optical fibers and the length of each of the scintillating optical fiber canes can be equal to the of the stacked parallel scintillating optical fibers in the scintillating optical fiber cane. By adding more canes the sensitivity of the radiation detector can be increased significantly because there is greater detector mass and increased probability of interaction with an impinging gamma ray. The length of each scintillating optical fiber cane can be upwards of three meters in length, and may only be limited by the optical transmission of each scintillating optical fiber. The detector arrays are communicatively coupled to at least one end of each scintillating optical fiber cane.

[0058] Stacking uni-directional scintillating optical fiber canes as in FIG. 14 can improve the coupling of a scintillating optical fiber bundle to the focal plane detector arrays. The uni directional scintillating optical fiber canes may only be communicatively coupled to one end of the scintillating optical fiber cane.

[0059] In some embodiments, one way to do this is to use what is called a passive-quenching circuit shown in FIG. 15. In passive quenching circuit 1502 GM-APD 1501 can be charged up to some bias above a breakdown voltage and then it is left open circuited. In response to GM-APD 1501 detecting a photon, it will turn on and discharge a capacitance until it is no longer above the breakdown voltage, at which point the avalanche of electron and hole pairs ceases. Circuit 1502 is a schematic of a first-order circuit model that describes the behavior of the GM-APD as it is discharging.

[0060] In other embodiments, the avalanche can be quenched using an active-quenching circuit. An active-quenching circuit can detect when a GM-APD starts to discharge, and then can expeditious discharge the GM-APD below the breakdown voltage my utilizing a shunting switch. After a period of time has lapsed thereby giving the GM-APD enough time to curb the avalanche, the switch can the active-quenching circuit recharges the APD using the switch.

[0061] FIG. 14 is an avalanche photodiode array, according to an exemplary embodiment. Avalanche photodiode array 1600 can be a 6 inch diameter silicon wafer on which one or more APD arrays can be fabricated. There can be a 4 by 4 array of APDs or a 32 by 32 array of APDs with pixel-to-pixel spacing of 100 micrometers and 150 micrometers. The active area diameters of the APDs can vary between 30 micrometers and 50 micrometers.

[0062] FIG. 15 is an exemplary flow of radiation detection system, according to an exemplary embodiment. Exemplary flow 1500 can begin at block 1502 the radiation detection system can receive incident radiation from a radiation source. The incident radiation can be gamma rays associated with radiation source. The incident radiation can scatter from an electron within the detector volume made up a plurality of scintillating optical fibers, that impart energy to the electron. The electron can traverse a number of scintillating optical fibers, slowing down as it travels and depositing energy within each fiber that it interacts with. In some embodiments, the radiation source can emit gamma particles. At block 1504, the radiation detection system can emit a photon in response to the electron. More specifically, a scintillating optical fiber in the radiation detection system can emit the photon in response to the electron. As each scintillating optical fiber is excited, it emits photons in proportion to the energy deposited. Each scintillating optical fiber captures a proportion of the photons generated within it, transporting these photons to the ends of the fibers.

[0063] At block 1506, the radiation detection system can transport the photon to a first end and a second end of a respective one of a plurality of scintillating optical fibers. At block 1508 the radiation detection system can detect the photon at a first asynchronous array, and at block 1510 detect the photon at a second asynchronous array. At block 1512 the radiation detection system can calculate a direction of travel of the electron based at least in part on which of the plurality of scintillating optical fibers emits a photon. The trajectory of the electron can be determined by analyzing which fibers are excited, and the initial incident radiation trajectories can be determined by analyzing the electron trajectory.

[0064] In some embodiments, the incident radiation can be an energetic (fast) neutron that scatters from a proton within the plurality of scintillating optical fibers leaving a direct track from a passage of the proton. In other embodiments, a species of the incident radiation can be determined on a track length, a track brightness, and a track curvature of a recoil electron or recoil proton.

[0065] The description is presented to enable a person skilled in the art to create and use a hand-held, vehicle mounted, or portal directional radiation detector system in accordance with the related method and systems for determining a direction of travel of recoil electrons introduced into scintillating optical fibers in response to absorbing one or more radiation particles. Various modifications to the example embodiments will be readily apparent to those skilled in the art, and the generic principles defined herein can be applied to other embodiments and applications without departing from the spirit and scope of the invention. Moreover, in the following description, numerous details are set forth for the purpose of explanation. However, one of ordinary skill in the art will realize that the invention can be practiced without the use of these specific details. In other instances, well-known structures and processes are shown in block diagram form in order not to obscure the description of the invention with unnecessary detail. Thus, the present disclosure is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features disclosed herein.

[0066] In describing exemplary embodiments, specific terminology is used for the sake of clarity. For purposes of description, each specific term is intended to at least include all technical and functional equivalents that operate in a similar manner to accomplish a similar purpose. Additionally, in some instances where a particular exemplary embodiment includes multiple system elements, device components or method steps, those elements, components or steps can be replaced with a single element, component or step. Likewise, a single element, component or step can be replaced with multiple elements, components or steps that serve the same purpose. Moreover, while exemplary embodiments have been shown and described with references to particular embodiments thereof, those of ordinary skill in the art will understand that various substitutions and alterations in form and detail can be made therein without departing from the scope of the invention. Further still, other aspects, functions and advantages are also within the scope of the invention.

[0067] Exemplary flowcharts have been provided herein for illustrative purposes and are non-limiting examples of methods. One of ordinary skill in the art will recognize that exemplary methods can include more or fewer steps than those illustrated in the exemplary flowcharts, and that the steps in the exemplary flowcharts can be performed in a different order than the order shown in the illustrative flowcharts.

[0068] Having described certain embodiments, which serve to illustrate various concepts, structures, and techniques sought to be protected herein, it will be apparent to those of ordinary skill in the art that other embodiments incorporating these concepts, structures, and techniques can be used. Elements of different embodiments described hereinabove can be combined to form other embodiments not specifically set forth above and, further, elements described in the context of a single embodiment can be provided separately or in any suitable sub-combination. Accordingly, it is submitted that the scope of protection sought herein should not be limited to the described embodiments but rather should be limited only by the spirit and scope of the following claims.