Patent Publication Number: US-2022229205-A1

Title: Apparatus, system and method regarding borehole muon detector for muon radiography and tomography

Description:
FIELD 
     This technology relates generally to tracking cosmic ray muons through an underground sensor in order to develop an image of subsurface density above the sensor (muon radiography), and to use multiple sensors to build a 3D model of density (muon tomography). 
     BACKGROUND 
     Exploitation of underground resources, for example, but not limited to mineral deposits and oil reservoirs, employs varied geophysical methods to detect, image, and monitor underground regions of interest. Many of the devices and systems used are large. 
     There are numerous designs of borehole detectors. For example, U.S. Pat. No. 8,881,808 discloses a method of determining a value indicative of fracture quality with a neutron-gamma tool. At least some of the illustrative embodiments are methods including: obtaining or measuring gas saturation of a formation to create a value indicative of pre-fracture gas saturation; and after a fracturing process measuring gas saturation of the formation to create a value indicative of post-fracture gas saturation; and creating a value indicative of fracture quality based on the value indicative of pre-fracture gas saturation and the value indicative of post-fracture gas saturation. The neutron-gamma tool is a borehole device but does not rely on muon detection. Production of gamma rays is inherently dangerous to human health. 
     Another method is based on a technique known as muon radiography. Muons are elementary particles produced in high energy nuclear interactions that are initiated by cosmic rays in the upper atmosphere. The muons travel at nearly the speed of light and, depending on their energy, can penetrate deep into the earth. The rate at which the muons lose energy in matter is dependent on the properties of the medium, and in particular on the density of matter. Many of the muon detectors are used in industrial and manufacturing settings. For example, U.S. Pat. No. 10,191,180 discloses a detector assembly that includes an insulating substrate, a printed circuit board, a resistive plate, a drilled board, a drift volume, and a cathode. A surface of the printed circuit board exposed to the resistive plate includes printed circuit lines for measuring first and second coordinates of a charge event. A mechanical assembly applies a force between the insulating substrate and the resistive a plate to form an electrical contact between the printed circuit lines on the printed circuit board and the resistive plate without the use of an electrical adhesive. This is a large-scale detector and would not be suitable for boreholes nor would it be suitable for interrogating geographic voids and regions of reduced or enhanced density. 
     U.S. Pat. No. 9,851,311 discloses methods, system and devices for inspecting materials in a vehicle or object. In one aspect, a system for muon tomography detection includes a first and second housing structure each including a first array and second array of muon detection sensors, respectively, the first housing structure positioned opposite the second at a fixed height to form a detection region to contain a target object, in which the muon detection sensors measure positions and directions of muons passing through the first array to the detection region and passing from the detection region through the second array; support structures to position the first housing structure at the fixed height; and a processing unit to receive data from the muon detection sensors and analyze scattering behaviors of the muons in materials of the target object to obtain a tomographic profile or spatial distribution of scattering centers within the detection region. These detectors would not be suitable for boreholes nor would they be suitable for interrogating geographic voids and regions of reduced or enhanced density. 
     U.S. Pat. No. 7,863,571 discloses an economical position-sensing muon detector for muon radiography that is constructed using a pair of glass plates spaced apart by crossed parallel barriers. Smaller detector wires are interspersed between the barriers and an ionizing gas is used to fill the space between the plates. A muon striking near where detector wires cross causes a local momentary current flow. The current flow in two of the detector wires is sensed to determine the coordinates of the muon impact. Such muon detectors can be assembled in modular surface arrays and such arrays can be positioned on spatial surfaces for differential inspection and detection of muons transiting through and emanating from objects placed within the inspection space. Such a detector constitutes a novel and useful invention in providing an inspection device and means for cargo or cargo vehicles that detects muons transiting through and emanating from hazardous materials intended to cause malicious harm. This detector would not be suitable for boreholes and would not be suitable for interrogating geographic voids and regions of reduced or enhanced density. 
     Those directed to detection of geographic features include U.S. Pat. No. 8,384,017, which discloses methods and related systems for use for making subterranean nuclear measurements. The system can include a plurality of elongated scintillator members each generating optical signals in response to ionizing radiation. Optical detector units can be optically coupled to at least one end of each elongated scintillator member so as to detect optical signals from each elongated scintillator member. The system can be suitable for permanent or semi-permanent deployment downhole. For example, the system can operate for more than six months in a subterranean deployment measuring cosmic radiation. The system can be suited to monitor density changes in subterranean regions of interest, for example, density changes brought about by steam injection as part of a steam assisted gravity drainage operation. This system uses an optical detector at each end of a bundle of scintillator fibers. This approach leads to cross talk and unnecessarily increases the complexity of the system. 
     U.S. Pat. No. 7,488,934 discloses a system configured for detecting cosmic ray muon (CRM) flux along a variety of trajectories through a subterranean test region, collecting the muon detection data and processing the data to form a three-dimensional density distribution image corresponding to the test region. The system may be used for identifying concentrations of high (or low) density mineral deposits or other geological structures or formations well below the earth&#39;s (or ocean floor) surface. The system may be utilized for imaging geological materials and structures of higher and/or lower density in a test region having a depth of several kilometers or more. 
     Malmqvist et al (Geophysics Vo. 44 No. 9 pp 1549-1569) discloses the use of a muon detector for determining rock density. The detector has two scintillator plates with an absorber plate between them and a coincident circuit to count the muons as they pass through the plates. 
     Drell et al (http://www.hep.utexas.edu/mayamuon/information.html https://repositories.Iib.utexas.edu/handle/2152/39757) discloses the uses of a muon detector based on the scintillator system from Fermilab (Pla-Dalmau, Bross, and Mellott, “Low-Cost Extruded Plastic Scintillator”) for use in studying pyramids. The muon detector has scintillator strips with wavelength-shifting (WLS) optical fiber located in a groove extruded along a face of the scintillator strips. The WLS fiber re-emits the absorbed scintillator light at a slightly different wavelength; this light is transmitted both directions in the fiber with relatively low loss to photomultipliers (PMTs) at each end. Drell et al arranges the scintillator strips on three adjacent layers. On the two outer layers, the strips form helices of pitch angle ±30° relative to the axis; on the inner layer strips are oriented parallel to the detector axis. The stereo layers make “one-half wrap” around the cylinder from one end to the other. 
     Basset et al (Nuclear Instruments and Methods in Physics Research A 567 (2006) 298-301) discloses a muon detector that has three coaxial PVC cylinders covered with straight scintillating bars or with 2 mm diameter scintillating optical fibers positioned along a clockwise coil on the middle cylinder (158 fibers) and along a counterclockwise coil on the external cylinder (178 fibers). All the scintillating elements are covered to avoid light leak: the bars are covered with mylar sheets and the fibers with Teflon Tape®. There are six photomultiplier tubes, one for each end of each cylinder, hence the probability of cross talk is high. 
     What is needed is a borehole muon detector that is inexpensive to construct, is highly sensitive, is accurate and consumes very little power. It would be preferable if it includes both scintillator fibers and scintillator bars, the latter including a wave-length shifting optical fiber. It would be still more preferable if there was a one to one relationship between at least one end of the wave-length shifting optical fibers and photodetectors. It would be more preferable if there was a one to one relationship between at least one end of the scintillator fibers and the photodetectors. It would be more preferable if there was a first helical bundle of scintillator fibers that define a bore in which the scintillator bars are housed. 
     SUMMARY 
     The present technology is a borehole muon detector that is inexpensive to construct, is highly sensitive, is accurate and consumes very little power. All embodiments include both scintillator fibers and scintillator bars, with the scintillator bars including a wave-length shifting optical fiber. At least one end of each scintillator bar is optically connected to a photodetector via the optical fiber. There is a one to one relationship between at least one end of the scintillator fibers and the photodetectors. There is a first helical bundle of scintillator fibers that define a bore in which the scintillator bars are housed. There is also an oppositely wound helical bundle of scintillator fibers that, with the first helical bundle define the bore in which the scintillator bars are housed. 
     In one embodiment a borehole muon detector for detecting and characterizing geographic regions of interest is provided, the borehole muon detector comprising a housing and sensor, which is housed in the housing, the sensor including: a plurality of photodetector elements; at least one printed circuit board in electrical communication with the plurality of photodetectors; a first helical bundle of scintillator fibers; an oppositely wound helical bundle of scintillator fibers, the oppositely wound helical bundle and the first helical bundle defining an outer cylinder, which includes a first end and a second end and a bore therebetween, each scintillator fiber of each bundle directly optically connected to a photodetector element at least at one end and indirectly optically connected to a photodetector element at no more than one mirrored end; and a plurality of scintillator bars, which are vertically disposed in the bore, each comprising a first end, a second end and an optical fiber extending from the first end to the second end, each optical fiber of the scintillator bar optically directly connected to a photodetector element at least at one end and indirectly optically connected to the photodetector at no more than one mirrored end. 
     In the borehole muon detector, the first helical bundle may comprise at least one winding. 
     In the borehole muon detector, the optical fiber may be a wave-length shifting optical fiber. 
     In the borehole muon detector, the plurality of scintillator bars may define an inner cylinder with a bore therethrough. 
     In the borehole muon detector, each scintillator bar may have a triangular cross section which includes a base and two sides. 
     In the borehole muon detector, the plurality of scintillator bars may include a plurality of first scintillator bars and a plurality of second scintillator bars, and the triangular cross section of the first scintillator bars may be larger than the triangular cross section of the second scintillator bars. 
     In the borehole muon detector, the first scintillator bars may alternate with the second scintillator bars, the bases of the first scintillator bars may face the outer cylinder and the bases of the second scintillator bars may face the inner bore. 
     In the borehole muon detector, each scintillator bar may include a bore in which the wave-length shifting (WLS) optical fiber is housed. 
     In the borehole muon detector, each scintillator fiber of each bundle may be directly optically connected to a photodetector element at each end. 
     In the borehole muon detector each wave-length shifting optical fiber may be directed optically connected to a photodetector element at each end of the scintillator bars. 
     In another embodiment a borehole muon detector for detecting and characterizing geographic regions of interest is provided, the borehole muon detector comprising a housing and a sensor, which is housed in the housing, the sensor including: a plurality of first photodetector elements; at least one first printed circuit board in electronic communication with the plurality of first photodetector elements, the first printed circuit board including an integrated electronic circuit for tracking time; at least a second printed circuit board in electrical communication with the plurality of second photodetector elements, the second printed circuit board including an integrated electronic circuit for tracking time; a helical bundle of scintillator fibers the helical bundle comprising n windings, where n is greater than zero and is not an integer, each scintillator fiber directly optically connected to a photodetector element at each end, the helical bundle defining an outer cylinder, which includes a bore therethrough; and a plurality of scintillator bars, each comprising a first end and a second end and an optical fiber extending from the first end to the second end, the plurality of scintillator bars vertically disposed in the outer cylinder to define an inner cylinder with a bore therethrough, each optical fiber of each scintillator bar optically directly connected to a photodetector element at least at one end and indirectly optically connected to the photodetector at no more than one end. 
     In the borehole muon detector, n may be greater than one. 
     In the borehole muon detector, the scintillator bars may have a triangular cross section which includes a base and two sides. 
     In the borehole muon detector, the plurality of scintillator bars may include a plurality of first scintillator bars and a plurality of second scintillator bars, and the triangular cross section of the first scintillator bars may be larger than the triangular cross section of the second scintillator bars. 
     In the borehole muon detector, the first scintillator bars may alternate with the second scintillator bars, the bases of the first scintillator bars may face the outer cylinder and the bases of the second scintillator bars may face the inner bore. 
     In the borehole muon detector, the optical fibers of the scintillator bars may be wave-length shifting (WLS) optical fibers. 
     The borehole muon detector may further comprise an oppositely wound helical bundle of scintillator fibers, the oppositely wound helical bundle comprising n windings, wherein n is greater than zero and is not an integer. 
     In the borehole muon detector, the oppositely wound helical bundle may comprise at least one winding. 
     In another embodiment, a borehole muon detector for detecting and characterizing a geographic region of interest is provided, the borehole muon detector comprising a housing and sensor, which is housed in the housing, the sensor including: a plurality of photodetector elements; a printed circuit board in electrical communication with the plurality of photodetectors; a plurality of scintillator fibers, each including a first end and a second end, the first end and the second end of each scintillator fiber each optically connected to a photodetector element, the plurality of scintillator fibers arranged as a helical bundle of scintillator fibers, the helical bundle comprising n windings, where n is greater than zero and is not a integer; and a plurality of scintillator bars, each comprising a first end, a second end and an optical fiber extending from the first end to the second end, the plurality of scintillator bars vertically disposed in the bore of the outer cylinder, each optical fiber of the scintillator bar optically directly connected to a photodetector element at least at one end and indirectly optically connected to the photodetector at no more than one end. 
     In the borehole muon detector, one end of each optical fiber in the scintillator bars may include a reflective layer. 
     In the borehole muon detector, both ends of each optical fiber in the scintillator bars may be optically connected to a photodetector element. 
     In yet another embodiment, a method of detecting and characterizing a geographic regions of interest is provided, the method comprising: inserting a muon detector into a borehole, the muon detector including a housing and a sensor, the sensor including at least one helical bundle of scintillator fibers to define a bore, a plurality of scintillator bars disposed along a length of the bore, a plurality of photodetector elements optically connected to the plurality of scintillator fibers and the optical fibers of the plurality of scintillator bars and a printed circuit board electrically connected to the plurality of photodetector elements; in response to a plurality of muons traversing the helical bundle and scintillator bars, the scintillator fibers and scintillator bars that have been traversed generating an optical signal which is detected by photodetector elements; the printed circuit board receiving a plurality of electrical signals from the photodetector elements; and the printed circuit board processing the electrical signals to determine a location of the geographic regions of interest. 
    
    
     
       FIGURES 
         FIG. 1  is a schematic of an embodiment of a muon detector. 
         FIG. 2  is a schematic of the scintillator fibers and scintillator bars of the muon detector of  FIG. 1 . 
         FIG. 3  is a schematic of a cross section of two scintillator bars. 
         FIG. 4  is a schematic of two exemplary scintillator fibers and exemplary scintillator bars describing the scintillation light from a muon passing through an “unrolled” muon detector. 
         FIG. 5  is a schematic of an alternative muon detector. 
         FIG. 6  is a schematic of the scintillator fibers and scintillator bars of the alternative muon detector. 
         FIG. 7  shows a simplified schematic of the muon sensor  10  as a muon strikes. 
         FIG. 8A  is a schematic of an alternative embodiment of  FIG. 2 ; and  FIG. 8B  is a schematic of an alternative embodiment of  FIG. 5 . 
         FIG. 9A  is a schematic of an alternative embodiment of  FIG. 2 ; and  FIG. 9B  is a schematic of an alternative embodiment of  FIG. 5 . 
     
    
    
     DESCRIPTION 
     Except as otherwise expressly provided, the following rules of interpretation apply to this specification (written description and claims): (a) all words used herein shall be construed to be of such gender or number (singular or plural) as the circumstances require; (b) the singular terms “a”, “an”, and “the”, as used in the specification and the appended claims include plural references unless the context clearly dictates otherwise; (c) the antecedent term “about” applied to a recited range or value denotes an approximation within the deviation in the range or value known or expected in the art from the measurements method; (d) the words “herein”, “hereby”, “hereof”, “hereto”, “hereinbefore”, and “hereinafter”, and words of similar import, refer to this specification in its entirety and not to any particular paragraph, claim or other subdivision, unless otherwise specified; (e) descriptive headings are for convenience only and shall not control or affect the meaning or construction of any part of the specification; and (f) “or” and “any” are not exclusive and “include” and “including” are not limiting. Further, the terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. 
     Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. Where a specific range of values is provided, it is understood that each intervening value, to the tenth of the unit of the lower limit unless the context clearly dictates otherwise, between the upper and lower limit of that range and any other stated or intervening value in that stated range, is included therein. All smaller sub ranges are also included. The upper and lower limits of these smaller ranges are also included therein, subject to any specifically excluded limit in the stated range. 
     Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the relevant art. Although any methods and materials similar or equivalent to those described herein can also be used, the acceptable methods and materials are now described. 
     Definitions 
     Photodetector element—in the context of the present technology, a photodetector element may be a channel in a multichannel device or may be a device. 
     Optically connected—in the context of the present invention, optically connected may be direct or indirect. Indirect is via a mirror or mirrored surface or reflective surface. If there are photodetectors at each end of the optical fiber, each end is directly connected. If there is one photodetector at one end of the optical fiber and a mirror or mirrored surface or reflective surface at the other end, the other end is indirectly optically connected. 
     DETAILED DESCRIPTION 
     In an embodiment shown in  FIG. 1 , a muon detector, generally referred to as  10  has a housing  12  and a muon sensor, generally referred to as  14 , which is housed in the housing  12 . The sensor  14  includes photodetector elements  16  which are attached to the one end  18  of scintillator fibers  20  and one end  22  of wave-length shifting optical fibers  76  that are embedded in scintillator bars  24 , in a one to one relationship—one photodetector element  16  to one end  18 ,  22 . The other end  26  of the scintillator fibers  20  is mirrored as is the other end  28  of the wave-length shifting optical fiber  76  in the scintillator bars  24 . Each photodetector element  16  is preferably a single device and is not part of a multichannel photodetector. One or more printed circuit boards (PCBs)  30  are electrically connected to the photodetector elements  16 . The PCBs  30  contains amplifiers, clocks, and/or field programmable gate array(s) (FPGA&#39;s), and/or application specific integrated circuit(s) (ASIC&#39;s), and/or analog to digital converter(s) (ADC&#39;s) that allow signals from the photodetector elements  16  to be digitally analyzed, to determine light yield from the scintillator bars  24  and which of the scintillator fibers  20  emitted scintillation light, and which photodetector elements  16  detected light within a user-specified period of time that may be consistent with the time it takes for a muon to pass through the detector  10  and for scintillation light to be produced, propagated to photodetector elements  16  and detected. The photodetector readouts for the scintillator bars  24  and the scintillator fibers  20  along with auxiliary information such as a global timestamp, comprises the data that is stored or sent to a backend processor+memory for further processing for each candidate muon event. If the data are stored it is periodically retrieved (either by being pushed, or being pulled, over a data network) by an offline system consisting of a processor and memory for further processing. The further processing runs an algorithm to carry out the methodology to determine the muon trajectory for candidate muon events and to ignore candidate events that may not be consistent with the passage of a muon through the detector  10 . 
     The details of the arrangement of the scintillator fibers  20  and scintillator bars  24  are shown in  FIG. 2 . There is a first helical bundle, generally referred to as  52 , of scintillator fibers  20 , which has m clockwise windings along the length, where m is greater than zero and is ideally not an integer value. In one embodiment m is greater than one. The second helical bundle  54  has n counter-clockwise windings along the length, where n is ideally not an integer value and is greater than zero. In one embodiment, n is greater than one. The first helical bundle  52  and the second helical bundle  54  are mounted on a mandrel to form an outer cylinder, generally referred to as  56 . The bundles  52 ,  54  are wound around the mandrel m and n times. m and n are judiciously chosen such that no two of all of the overlaps of any one fiber from the bundle  52  and any one fiber from the bundle  54  occur along a vertically oriented line of the outer cylinder  56 . The outer cylinder  56  has a bore  58 . Housed in the bore  58 , is an inner cylinder  60  of vertically disposed scintillator bars  24 . The inner side  66  of the inner cylinder  60  faces a bore and the outer side  70  of the inner cylinder  60  faces the outer cylinder  56 . 
     As shown in  FIG. 3 , there are two sizes of scintillator bars  24 , both of which have a triangular cross section with two sides  60  and a base  62 . The smaller cross section scintillator bars  64  are on the inner side  66  of the inner cylinder  60  and the larger cross section scintillator bars  68  are on the outer side  70  of the inner cylinder  60  (See  FIG. 2 ). The base  62  of the larger cross section scintillator bars  68  faces the outer cylinder  56  and the base  62  of the smaller cross section scintillator bars  64  face the inner bore  70  of the inner cylinder  60 . This provides for a smooth, regular circular shape. The scintillator bars  64 ,  68  are coated with a reflective coating  72  and have a central bore  74  which houses the wave-length shifting optical fiber  76 . 
     In an alternative embodiment, the wave-length shifting optical fiber is replaced with an optical fiber. 
       FIG. 4  shows a simplified schematic of the muon sensor  10  as a muon strikes. The horizontal width is 2πρ where ρ is the radius of the apparatus, and the vertical height is h, the height of the apparatus. In this schematic only two scintillator fibers  20  are shown, one from each of the counter-wound helical bundles  52 ,  54 . The lines representing the fibers  20  are dashes and dots to distinguish which bundle they are in. In this case, m=4 and n=5. There is an (m+n)-fold ambiguity of crossing positions where a muon could have crossed through in order to create scintillation light in both fibers (the scintillation light is indicated by the star icons, and is measured by photo-detectors on only one side of any fiber). These (m+n) possible locations are indicated by the double lines. The additional inner layer of vertically disposed extruded scintillators  24  with embedded WLS fibers performs an additional measurement. Multiple light yield measurements from this layer (shown by the small star icons), taken from one side of each of the segmented, coated bars, can be used to calculate a barycenter where the muon passed through. This provides an additional measurement with associated uncertainty indicated by the gradient band. If the uncertainty is narrower than the characteristic pitch between the (m+n) possible solutions, then the actual position at which the muon hit one side of the cylindrical system (the black dot) is uniquely determined. 
     Without being bound to theory, since any muon must pass through at least two adjacent bars (or a single bar if the muon passes exactly through the apex of the triangle) in order to pass through the cylinder, then by measuring the relative light yield between adjacent bars the position through which the muon passed in the (x-y) plane can be interpolated to very good precision. The advantages of the design are:
         1. Extruded scintillator bars are very inexpensive and the resolution of the measurement in the x-y plane for the azimuthal coordinate can be done very precisely; this precision allows for superior precision on the z measurement.   2. Instrumentation only needs to be done on one side of the system; only one side of each scintillator element needs to be coupled to a photodetector.   3. No fast timing with picosecond resolution needs to be performed; therefore, simpler and less expensive scintillators and simple and less expensive electronics can be utilized.       

     Method 
     A muon crossing through the outer cylinder  56  will intersect with at least one scintillator fiber  20  in each helical bundle  52 ,  54  upon entering the outer cylinder  56  and will cross through at least one scintillator fiber  20  in each helical bundle  52 ,  54  upon exiting. For a muon crossing event, scintillation light will be created in four scintillator fibers  20  [FO1, FO2, FI1 and FI2(1=inner O=outer)], and possibly more depending on the angle at which the muon impinges on the outer cylinder  56 . 
     The time it takes for the muon to cross the outer cylinder  56  can be as short as 0.15 nanoseconds. Given the time jitter in the evolution of the scintillation light in the scintillator fibers  20  it is not possible to associate the scintillation light measured at one end of each scintillator fiber  20  with the entry or exit of the muon as it passes through the detector. 
     The counter-wound helical bundles  52 ,  54  create crossing points wherein a muon will pass through scintillator fiber pairs, each pair consisting of one scintillator fiber  20  from the inner helical bundle  54  and one scintillator fiber  20  from the outer helical bundle  54 . There will be two possible combinations FI1/FO1, FI2/FO2 and FI1/FO2, FI2/FO1. If the inner and outer helical bundles  52 ,  54  wrap around the outer cylinder  56  m and n times (not necessarily an integer, and not necessarily &gt;1) respectively then for each pair of scintillator fibers  20  FIX and FOY there will be M+N points at which the fibers cross over each other, where M=floor(m) and N=floor(n), if M and N have no common factors. Thus, there are 2×(M+N) possible points along the surface of the outer cylinder  56  at which a muon may have crossed through either on entry or exit. Each of these points will be at a unique azimuthal position. 
     In addition, the muon will cross through at least four (total) scintillator bars  24  in entry and exit. Only events are recorded for offline processing where scintillation light is measured from scintillator bars  24  that are separated by some number of scintillator bars  24 , to ensure that a muon crosses through all layers of the system. 
     By Birk&#39;s law, the amount of scintillation light (photons) emitted by a muon as it passes through a scintillator bar  24  is related approximately linearly to the path length through the scintillator bar  24 . This allows the muon position to be determined with precision far better than the pitch of the scintillator bars  24  in the inner cylinder  60 , by interpolating the position at which the muon passed through neighbouring scintillator bars  24  the inner cylinder  60 . 
     The inner cylinder  60  thus allows two azimuth points to be measured, corresponding to either entry or exit. These azimuth points are determined with precision finer than the minimum separation of candidate entry or exit positions determined from the counter-wound helical bundles  52 ,  54 . Thus, exactly two of the 2×(M+N) candidate points are selected corresponding to either entry or exit. These candidate points also determine a longitudinal position along the inner cylinder  60  for entry or exit. 
     With two longitudinal positions, a zenith angle with respect to vertical can be determined for the muon trajectory. There are two possible combinations for entry and exit. The combination that is consistent with muons arriving from the surface of the earth (opposed to the solution that has muons passing from the far side of the earth) is chosen. Thus, a measurement of the muon azimuth and zenith angles is performed. 
     In an alternative embodiment, the second or other end  26  of the scintillator fibers  20  and the other end  28  of the wave-length shifting optical fiber  76  in the scintillator bars  24  are not mirrored and instead, are attached to a photodetector element  16  as described above (in a one on one relation). The photodetector elements  16  are electrically connected to the PCB  30  as described above. 
     In another alternative embodiment, the second or other end  26  of the scintillator fibers  20  are not mirrored and instead, are attached to a photodetector element  16  as described above (in a one to one relation). The other end  28  of the wave-length shifting optical fiber  76  in the scintillator bars are mirrored. The photodetector elements  16  are electrically connected to the PCB  30  as described above. 
     In yet another embodiment, the second or other end  26  of the scintillator fibers  20  are mirrored. The other end  28  of the wave-length shifting optical fiber  76  in the scintillator bars are attached to a photodetector element  16  as described above. The photodetector elements  16  are electrically connected to the PCB  30  as described above. Still further embodiments include photodetectors at both ends of the scintillator fibers and photodetectors at only one end of the wave-length shifting optical fibers and photodetectors at both ends of the wave-length shifting optical fibers and photodetectors at only one end of the scintillator fibers. 
     In yet another embodiment, shown in  FIG. 5 , a muon detector, generally referred to as  110  has a housing  112  and a muon sensor, generally referred to as  114 , which is housed in the housing  112 . The sensor  114  includes photodetector elements  116  which are attached to the one end  118  of scintillator fibers  120  and one end  122  of the wave-length shifting optical fiber  176  that are embedded in the scintillator bars  124 , in a one to one relationship—one photodetector element  116  to one end  118 ,  122 . The second or other end  126  of the scintillator fibers  120  and the second or other end  128  of the wave-length shifting optical fiber  176  in the scintillator bars  124  are also attached to a photodetector element  116  in a one on one relation. A photodetector element  116  is preferably a single device and is not a channel in a multichannel device. At least one printed circuit board (PCB)  130  is electrically connected to the photodetector elements  116 . The PCB  130  contains amplifiers, clocks, and/or field programmable gate array(s) (FPGA&#39;s), and/or application specific integrated circuit(s) (ASIC&#39;s), and/or analog to digital converter(s) (ADC&#39;s) that allow signals from the photodetector elements  116  to be digitally analyzed, to determine light yield from the scintillator bars  124  and which of the scintillator fibers  120  emitted scintillation light along with the relative detection time of the light at the first and second end of those respective scintillator fibers  120 , and which photodetector elements  116  detected light within a user-specified period of time that may be consistent with the time it takes for a muon to pass through the detector  110  and for scintillation light to be produced, propagated to photodetector elements  116  and detected. The photodetector readouts for the scintillator bars  124  and the scintillator fibers  120  along with auxiliary information such as a global timestamp, comprises the data that is stored or sent to a backend processor+memory for further processing for each candidate muon event. If the data are stored it is periodically retrieved (either by being pushed, or being pulled, over a data network) by an offline system consisting of a processor and memory for further processing. In any case, the further processing runs an algorithm to carry out the methodology to determine the muon trajectory for candidate muon events and to ignore candidate events that may not be consistent with the passage of a muon through the detector  10 . 
     In the preferred embodiment, one end  122  or the other end  128  of each wave-length shifting optical fiber  176  is mirrored and is not attached to photodetector elements  116 . Photodetector elements  116  are attached to the opposite end  122  or  128  of the wave-length shifting optical fiber  176  that are embedded in the scintillator bars  124 . The photodetector elements  116  are electrically connected to the PCB  130  as described above 
     The details of the arrangement of the scintillator fibers  120  and scintillator bars  124  is shown in  FIG. 6 . There is a helical bundle, generally referred to as  152 , of scintillator fibers  120 . The helical bundle  152  has n clockwise or counter-clockwise windings. In one embodiment, n is greater than one. The helical bundle  152  is mounted on a mandrel  153  to form an outer cylinder, generally referred to as  156 . The outer cylinder  156  has a bore  158 . Housed in the bore  158 , is an inner cylinder  160  of vertically disposed scintillator bars  124 . The scintillator bars  124  and their arrangement is exactly as shown in  FIG. 3 . 
       FIG. 7  shows a simplified schematic of the muon sensor  10  as a muon strikes. Only one scintillation fiber  120  is shown. If the scintillation fiber  120  has n windings, there is an N-fold ambiguity (where N=floor(n)) of crossing positions where a muon could have crossed through in order to create scintillation light in the scintillation fiber  120  and within the resolution of the azimuthal position determined by the inner cylinder  160  of triangle scintillator bars  124  (shown by the vertical gray band). Again, the scintillation light is indicated by the star icons. In order to resolve the N-fold ambiguity, the relative arrival time of scintillation light at the photodetectors  116  on either end  118 ,  126  of the scintillation fiber  120  is used. Using this information, an estimate for the position along the whole helical length of the scintillation fiber  120  where the scintillation occurred can be attained (shown by the diagonal gray band). If the uncertainty on this estimate is smaller than the distance along the helical length between any of the N-fold candidate locations, then the actual position at which the muon hit one side of the outer cylinder  156  is uniquely determined. In the layer of extruded scintillator bars  124  with embedded WLS fibers  76 , multiple light yield measurements (shown by the small star icons), taken from one side of each of the coated scintillation bars  124 , are used to calculate a barycenter where the muon passed through. 
     Method 
     Assuming only F1 and F2 scintillator fibers  120  are struck by a muon (and there could be more), the determination of the azimuth for entry and exit of the muon using the inner layer of inscribed n-gon of scintillator bars  124  proceeds in the same way as described in relation to  FIG. 4 . The azimuth position determines two vertical bands B1 and B2 within which the entry and exit of the muon occurred. There are multiple intersections of F1 and F2 with both bands, N points for F1 &amp; B1 and F1 &amp; B2 and N points for F2 &amp; B1 and F2 &amp; B2. By measuring the difference in the arrival &amp; detection time of light at both ends of either F1 and F2, it is possible to estimate the approximate position along F1 and F2 where the muon-initiated scintillation. This determines unique combinations of all possible intersection points of F1 and F2 with the vertical bands B1 &amp; B2. With such a determination a trajectory is determined up to a 180 degree ambiguity in azimuth corresponding to the assignment of entry and exit. The assignment of entry and exit is chosen to be consistent with muons arriving from the surface and not from the far side of the earth. 
     As shown in  FIGS. 8A  and B in another embodiment, the inner cylinders  60 ,  160  are replaced with a bundle of scintillator bars  24 ,  124 . 
     As shown in  FIGS. 9A  and B in another alternative embodiment, the helical bundles  52 ,  54 ,  152  are wound around the inner cylinders  60 ,  160 . 
     While example embodiments have been described in connection with what is presently considered to be an example of a possible most practical and/or suitable embodiment, it is to be understood that the descriptions are not to be limited to the disclosed embodiments, but on the contrary, is intended to cover various modifications and equivalent arrangements included within the spirit and scope of the example embodiment. Those skilled in the art will recognize or be able to ascertain using no more than routine experimentation, many equivalents to the specific example embodiments specifically described herein. Such equivalents are intended to be encompassed in the scope of the claims, if appended hereto or subsequently filed.