Neutron detector, method for manufacturing a neutron detector and a method of use thereof

Abstract

A neutron detector includes: a hollow shell; a layer of scintillator on an inner surface of the hollow shell; a light guide; a wavelength shifting paint coated on an outer surface of the light guide, wherein the light guide is positioned within the hollow shell, wherein an air gap is present between the inner surface of the hollow shell and the outer surface of the light guide; at least one photo detector coupled to an end of the light guide, wherein the at least one photo detector is adapted to generate a detector pulse; and a processing system adapted to count the detector pulse if a duration of the detector pulse is longer than a predetermined pulse width criterion or reject the detector pulse if the duration of the detector pulse is shorter than the predetermined pulse width criterion.

Description

NEUTRON DETECTOR, METHOD FOR MANUFACTURING A NEUTRON DETECTOR AND A METHOD OF USE THEREOFCROSS-REFERENCE

[0001] The present specification relies on United States Provisional Patent Application Number 63 / 659,937, titled “Neutron Detector, Method for Manufacturing a Neutron Detector and a Method of Use Thereof’, filed on June 14, 2024, for priority and United States Provisional Patent Application Number 63 / 600,212, titled “Neutron Detector, Method for Manufacturing a Neutron Detector and a Method of Use Thereof’, filed on November 17, 2023, for priority. The above- mentioned applications are herein incorporated by reference in their entirety.FIELD

[0002] The present specification is related generally to the field of neutron detection. More specifically, the present specification relates to a neutron detector that may be used in place of a He-3 (Helium-3) neutron detector. The detector of the present specification has performance characteristics similar to that of a He-3 (Helium-3) neutron detector, is a low-cost alternative to a He-3 (Helium-3) neutron detector and has a high detection accuracy of neutron events.BACKGROUND

[0003] The measurement and counting of neutrons are critical in many applications such as, for example, neutron spectroscopy, thermal neutron monitoring of nuclear reactors, thermal neutron monitoring close to aircraft fuel tanks, and personal dosimetry. Neutrons may be emitted by special nuclear materials such as weapons-grade plutonium, making neutron detection an important capability in security applications such as within radiation portal monitors (RPM) at ports of entry.

[0004] The accepted standard in neutron detection is based on the use of Helium-3 (He-3). A disadvantage of conventional neutron detectors based on Helium-3 is that there is currently a very limited supply of Helium-3. The limited supply of Helium-3 increases the cost substantially for systems using neutron detectors based on it. Traditionally, Helium-3 based detectors have offered a high level of gamma rejection. Gamma rejection refers to the ability of the detector to avoid counting gamma interactions within the detector which could lead to a falsely elevated neutron count rate reading. This can result in too many false positive neutron alarms, which may be indicative that neutron particles have been detected when in reality gamma particles were detected.Many neutron detectors suffer from a lower gamma rejection performance than helium-3 neutron detectors. Many deployed Helium-3 neutron detectors used in industry, and particularly within radiation portal monitors, have a form factor of a right circular cylinder of approximately 2 inches diameter with a length dependent on the application requirements.

[0005] Accordingly, there is a need for a neutron detector that has a form factor that is compatible for use as a He-3 (Helium-3) neutron detector replacement. There is also a need for the neutron detector to have performance similar to that of the He-3 (Helium-3) neutron detector while at the same time being a low cost alternative to the He-3 (Helium-3) neutron detector. There is a further need for the neutron detector to accurately detect neutrons, without false detection of gamma rays and / or dark noise as neutrons.SUMMARY

[0006] The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods, which are meant to be exemplary and illustrative, and not limiting in scope. The present application discloses numerous embodiments.

[0007] The present specification discloses a radiation detector for detection of neutrons, comprising: a first hollow cylinder; a scintillator layer provided on an inner surface of the first hollow cylinder; a second cylinder, wherein the second cylinder is positioned within the first hollow cylinder such that the scintillator layer is sandwiched between an outer surface of the second cylinder and the inner surface of the first hollow cylinder; a wavelength shifting material on the outer surface of the second cylinder; at least one photodetector optically coupled to an end of the second cylinder, wherein the at least one photodetector is adapted to generate a detector pulse representative of at least one of interactions of neutrons in the scintillation layer, interactions of gamma radiation in the scintillation layer and dark noise; and a processing system adapted to count the detector pulse based on a first determination that the detector pulse corresponds to the interactions of neutrons in the scintillation layer or reject the detector pulse based on a second determination that the detector pulse corresponds to at least one of interactions of gamma radiation in the scintillation layer and dark noise.

[0008] Optionally, the second cylinder is solid.

[0009] Optionally, the radiation detector further comprises an air gap between the scintillator layer and the outer surface of the second cylinder.

[0010] Optionally, the scintillator layer comprises scintillator material deposited upon a binder material.

[0011] Optionally, the radiation detector further comprises a light reflecting layer between the scintillator layer and the inner surface of the first hollow cylinder.

[0012] The radiation detector of claim 1, wherein the processing system is configured to generate the first determination and the second determination based on a predetermined pulse width criterion. Optionally, the processing system is configured to determine the detector pulse corresponds to the interactions of neutrons in the scintillation layer when the detector pulse has a duration longer than the predetermined pulse width criterion. Optionally, the processing system is configured to determine the detector pulse corresponds to at least one of the interactions of gamma radiation in the scintillation layer and dark noise when the detector pulse has a duration shorter than the predetermined pulse width criterion.

[0013] Optionally, the processing system includes a trans-impedance amplifier, a comparator and a processor. Optionally, the at least one photodetector comprises photo-multiplier tubes. Optionally, the processing system is configured to optimize at least one performance parameter of the radiation detector by modifying one or more of a time constant of the trans-impedance amplifier, a bias voltage of the photo-multiplier tubes, a threshold voltage of the comparator, or a pulse width criterion.

[0014] Optionally, the first hollow cylinder comprises aluminum.

[0015] Optionally, the second solid cylinder comprises acrylic.

[0016] Optionally, the wavelength shifting material is deposited directly onto the outer surface of the second cylinder by combining of6LiF :ZnS(Ag) with a polymer, creating a mixture, and coating the mixture on the outer surface of the second cylinder.

[0017] Optionally, the wavelength shifting material is layered onto the outer surface of the second cylinder by wrapping a sheet of the wavelength shifting material on the outer surface of the second cylinder.

[0018] The present specification also discloses a method of using a radiation detector for detecting neutrons, wherein the radiation detector comprises a first hollow cylinder, a scintillator layer provided on an inner surface of the first hollow cylinder, a second cylinder, wherein the second cylinder is positioned within the first hollow cylinder such that the scintillator layer is between an outer surface of the second cylinder and the inner surface of the first hollow cylinder, a wavelengthshifting material on the outer surface of the second cylinder, and at least one photodetector optically coupled to an end of the second cylinder, the method comprising: generating a detector pulse using the at least one photodetector, wherein the detector pulse is representative of at least one of interactions of neutrons in the scintillation layer, interactions of gamma radiation in the scintillation layer and dark noise; and counting the detector pulse, using the processing system, based on a first determination that the detector pulse corresponds to the interactions of neutrons in the scintillation layer or rejecting the detector pulse based on a second determination that the detector pulse corresponds to at least one of interactions of gamma radiation in the scintillation layer and dark noise.

[0019] Optionally, the radiation detector further comprises an air gap between the scintillator layer and the outer surface of the second cylinder.

[0020] Optionally, the scintillator layer of the radiation detector comprises scintillator material deposited upon a binder material.

[0021] Optionally, the radiation detector further comprises a light reflecting layer positioned between the scintillator layer and the inner surface of the first hollow cylinder.

[0022] Optionally, the processing system is configured to generate the first determination and the second determination based on a predetermined pulse width criterion. Optionally, the processing system is configured to determine the detector pulse corresponds to the interactions of neutrons in the scintillation layer when the detector pulse has a duration longer than the predetermined pulse width criterion. Optionally, the processing system is configured to determine the detector pulse corresponds to at least one of the interactions of gamma radiation in the scintillation layer and dark noise when the detector pulse has a duration shorter than the predetermined pulse width criterion.

[0023] Optionally, the processing system includes a trans-impedance amplifier, a comparator and a processor and the at least one photodetector comprises photo-multiplier tubes. Optionally, the processing system is configured to optimize at least one performance parameter of the radiation detector by modifying one or more of a time constant of the trans-impedance amplifier, a bias voltage of the photo-multiplier tubes, a threshold voltage of the comparator, or a pulse width criterion.

[0024] Optionally, the first hollow cylinder comprises aluminum.

[0025] Optionally, the second cylinder is solid and comprises acrylic.

[0026] Optionally, the at least one photodetector is a light sensor.

[0027] Optionally, the wavelength shifting material is directly deposited onto the outer surface of the second cylinder by combining of6LiF :ZnS(Ag) with a polymer, creating a mixture, and coating the mixture on the outer surface of the second cylinder.

[0028] Optionally, the wavelength shifting material is layered onto the outer surface of the second cylinder by wrapping a sheet of the wavelength shifting material on the outer surface of the second cylinder.

[0029] The present specification discloses a radiation detector for detection of neutrons, comprising: a first hollow cylinder; a scintillator layer coated on an inner surface of the first hollow cylinder; a second solid cylinder; a wavelength shifting paint coated on an outer surface of the second solid cylinder, wherein the second sold cylinder is positioned within the first hollow cylinder, and wherein an air gap is present between the inner surface of the first hollow cylinder and the outer surface of the second solid cylinder; at least one photo detector optically coupled to an end of the second solid cylinder, wherein the at least one photo detector is adapted to generate a detector pulse corresponding to interaction of neutrons in the scintillation layer, interaction of gamma radiation in the scintillation layer and / or dark noise; and a processing system adapted to count the detector pulse based on a first determination that the detector pulse corresponds to the interaction of neutrons in the scintillation layer or reject the detector pulse based on a second determination that the detector pulse corresponds to interaction of gamma radiation in the scintillation layer and / or dark noise.

[0030] Optionally, the first and second determinations are based on a predetermined pulse width criterion. Optionally, when the detector pulse has a duration longer than the predetermined pulse width criterion, the detector pulse is determined to correspond to the interaction of neutrons in the scintillation layer. Optionally, when the detector pulse has a duration shorter than the predetermined pulse width criterion, the detector pulse is determined to correspond to the interaction of gamma radiation in the scintillation layer and / or dark noise.

[0031] Optionally, the processing system includes a trans-impedance amplifier, a comparator and a processor. Optionally, the at least one photo detector is a photo-multiplier tube. Optionally, at least one performance parameter of the radiation detector is optimized by modifying one or more of a time constant of the trans-impedance amplifier, a bias voltage of the photo-multiplier tubes, a threshold voltage of the comparator, or a pulse width criterion.

[0032] Optionally, a binder material is used with the scintillator in order to produce a sheet of the scintillator adapted to wrap on the inner surface of the first hollow cylinder.

[0033] Optionally, the first hollow cylinder is fabricated from aluminum.

[0034] Optionally, the second solid cylinder is fabricated from acrylic.

[0035] The present specification also discloses a method of using a radiation detector for detecting neutrons, wherein the radiation detector includes a first hollow cylinder having a scintillator layer coated on an inner surface of the first hollow cylinder, a second solid cylinder having a wavelength shifting paint coated on an outer surface of the second solid cylinder, wherein the second solid cylinder is positioned within the first hollow cylinder, and wherein an air gap is present between the inner surface of the first hollow cylinder and the outer surface of the second solid cylinder, the method comprising: generating a detector pulse by at least one photo detector optically coupled to an end of the second solid cylinder; determining, by a processing system, if the detector pulse has a duration shorter or longer than a predetermined pulse width criterion; and counting the detector pulse if the duration is longer than the predetermined pulse width criterion or rejecting the detector pulse if the duration is shorter than the predetermined pulse width criterion.

[0036] Optionally, the processing system includes a trans-impedance amplifier, a comparator and a processor. Optionally, the at least one photo detector is a photo-multiplier tube. Optionally, at least one performance parameter of the radiation detector is optimized by modifying one or more of a time constant of the trans-impedance amplifier, a bias voltage of the photo-multiplier tubes, a threshold voltage of the comparator, or the pulse width criterion.

[0037] Optionally, a binder material is used with the scintillator in order to produce a sheet of the scintillator adapted for wrapping on the inner surface of the first hollow cylinder.

[0038] Optionally, the first hollow cylinder is fabricated from aluminum.

[0039] Optionally, the second solid cylinder is fabricated from acrylic.

[0040] The present specification also discloses a radiation detection system for detection of neutrons, comprising: a hollow shell; a layer of scintillator coated on an inner surface of the hollow shell; a light guide; a wavelength shifting paint coated on an outer surface of the light guide, wherein the light guide is positioned within the hollow shell, and wherein an air gap is present between the inner surface of the hollow shell and the outer surface of the light guide; a photo detector optically coupled to an end of the light guide, wherein the photo detector is adapted to generate a detector pulse corresponding to interaction of neutrons in the scintillation layer,interaction of gamma radiation in the scintillation layer and / or dark noise; and a processing system adapted to count the detector pulse if a duration of the detector pulse is longer than a predetermined pulse width criterion or reject the detector pulse if the duration of the detector pulse is shorter than the predetermined pulse width criterion.

[0041] Optionally, a cross-section of the light guide is a square, rectangle, twisted hexagonal or polygon.

[0042] Optionally, when the detector pulse has a duration shorter than the predetermined pulse width criterion, the detector pulse is determined to correspond to the interaction of gamma radiation in the scintillation layer and / or dark noise.

[0043] Optionally, when the detector pulse has a duration longer than the predetermined pulse width criterion, the detector pulse is determined to correspond to the interaction of neutrons in the scintillation layer.

[0044] Optionally, the processing system includes a trans-impedance amplifier, a comparator and a processor. Optionally, the detector is a photo-multiplier tube. Optionally, at least one performance parameter of the radiation detector is optimized by modifying one or more of a time constant of the trans-impedance amplifier, a bias voltage of the photo-multiplier tubes, a threshold voltage of the comparator, or the pulse width criterion.

[0045] Optionally, a binder material is used with the scintillator in order to produce a sheet of the scintillator adapted for wrapping on the inner surface of the hollow shell.

[0046] Optionally, the hollow shell is fabricated from aluminum.

[0047] Optionally, the light guide is fabricated from acrylic.

[0048] Optionally, the hollow shell has a length ranging from 30cm to 300cm, an outer diameter ranging from 1cm to 10cm, a wall thickness ranging from 1mm to 2mm and the scintillator layer has a thickness ranging from 200pm to 600 pm, wherein the light guide has a length less than or equal to that of the hollow shell, a diameter ranging from 1cm to 10cm, and the wavelength shifting paint has a thickness ranging from 20pm to 1mm, and wherein the air gap has a thickness of up to 10mm.

[0049] The aforementioned and other embodiments of the present specification shall be described in greater depth in the drawings and detailed description provided below.BRIEF DESCRIPTION OF THE DRAWINGS

[0050] The accompanying drawings illustrate various embodiments of systems, methods, and various other aspects of the disclosure. Any person with ordinary skills in the art will appreciate that the illustrated element boundaries (e.g., boxes, groups of boxes, or other shapes) in the figures represent one example of the boundaries. In some examples one element may be designed as multiple elements or that multiple elements may be designed as one element. In some examples, an element shown as an internal component of one element may be implemented as an external component in another and vice versa. Furthermore, elements may not be drawn to scale. Nonlimiting and non-exhaustive descriptions are described with reference to the following drawings. The components in the figures are not necessarily to scale, emphasis instead being placed upon illustrating principles.

[0051] FIG. 1 A is a perspective view of a neutron detector, in accordance with some embodiments of the present specification;

[0052] FIG. IB is a cross-sectional view of the neutron detector of FIG. 1A, in accordance with some embodiments of the present specification;

[0053] FIG. 1C is an exploded view of the cross-section shown in FIG. IB, in accordance with some embodiments of the present specification;

[0054] FIG. ID is a cross-sectional view of another neutron detector, in accordance with some embodiments of the present specification;

[0055] FIG. 2 is a block diagram of a data acquisition, storage and processing system, in accordance with some embodiments of the present specification;

[0056] FIG. 3A illustrates a first oscilloscope waveform trace displaying exemplary pulse data, over a time period, corresponding to dark noise or gamma events, in accordance with some embodiments of the present specification;

[0057] FIG. 3B illustrates a second oscilloscope waveform trace displaying exemplary pulse data, over a time period, corresponding to a neutron pulse or event, in accordance with some embodiments of the present specification;

[0058] FIG. 3C illustrates a third oscilloscope waveform trace displaying exemplary pulse data, over a time period, corresponding to a neutron pulse or event, in accordance with some embodiments of the present specification;

[0059] FIG. 4 illustrates an oscilloscope waveform capture displaying atypical neutron waveform and comparator signal corresponding to a neutron event, in accordance with some embodiments of the present specification;

[0060] FIG. 5 is a flowchart of a method of using the neutron detector of FIG. 1A for detecting neutrons, in accordance with some embodiments of the present specification;

[0061] FIG. 6 shows a first plot of a first neutron count rate for a first neutron detector and a second plot of a second neutron count rate for a second neutron detector over a predetermined range of temperature, in accordance with some embodiments of the present specification; and

[0062] FIG. 7 is a flowchart of a method of manufacturing a prototype of the neutron detector of FIG. 1A, in accordance with some embodiments of the present specification.DETAILED DESCRIPTION

[0063] The present specification is directed towards multiple embodiments. The following disclosure is provided in order to enable a person having ordinary skill in the art to practice the invention. Language used in this specification should not be interpreted as a general disavowal of any one specific embodiment or used to limit the claims beyond the meaning of the terms used therein. The general principles defined herein may be applied to other embodiments and applications without departing from the spirit and scope of the invention. Also, the terminology and phraseology used is for the purpose of describing exemplary embodiments and should not be considered limiting. Thus, the present invention is to be accorded the widest scope encompassing numerous alternatives, modifications and equivalents consistent with the principles and features disclosed. For purpose of clarity, details relating to technical material that is known in the technical fields related to the invention have not been described in detail so as not to unnecessarily obscure the present invention.

[0064] In the description and claims of the application, each of the words “comprise”, “include”, “have”, “contain”, and forms thereof, are not necessarily limited to members in a list with which the words may be associated. Thus, they are intended to be equivalent in meaning and be open- ended in that an item or items following any one of these words is not meant to be an exhaustive listing of such item or items, or meant to be limited to only the listed item or items. It should be noted herein that any feature or component described in association with a specific embodiment may be used and implemented with any other embodiment unless clearly indicated otherwise.

[0065] It must also be noted that as used herein and in the appended claims, the singular forms “a,” “an,” and “the” include plural references unless the context dictates otherwise. Although any systems and methods similar or equivalent to those described herein can be used in the practice or testing of embodiments of the present disclosure, the preferred, systems and methods are now described.

[0066] Overview

[0067] FIG. 1A is a perspective view of a neutron detector 100, FIG. IB shows a cross-sectional view of the neutron detector 100 shown in FIG. 1A, FIG. 1C shows an enlarged section of the cross-sectional view of FIG. IB, and FIG. ID shows a cross-sectional view of another embodiment of a neutron detector in accordance with some embodiments of the present specification. Referring now to FIGS. 1A, IB, 1C, and ID neutron detector 100 is cylindrical having a circular crosssection. Detector 100 has a hollow outer shell 102 of a length ‘T’ and an outer diameter ‘D’. In some embodiments, the outer shell 102 is fabricated from aluminum. In alternate embodiments, the outer shell 102 could also be fabricated using material such as, for example, steel or plastic; or almost any other rigid material that prevents ambient light from entering the detector 100. In some embodiments, the inner surface of the shell 102 is polished, to be a specular reflector, in order to improve light collection. In some embodiments, the outer surface of the shell 102 may be smooth. A light guide 106 that, in some embodiments, is in the form of a solid rod, cylinder or pipe having a circular cross-section is positioned within the outer shell 102.

[0068] In embodiments, a scintillator layer 104 is provided on the inside of the outer shell 102. Referring now to FIG. ID, a cross-sectional view of a neutron detector 100, in accordance with some embodiments of the present specification, is shown. In an embodiment, the scintillator layer comprises a thin neutron scintillator material 103 that is deposited upon a transparent backing film 107, such as a polymer binder or agent, forming a sheet. In embodiments, scintillator materials may be deposited onto backing film 107 using various methods, including, but not limited to spreading by using a doctor blade technique, screen printing, spray-on deposition, dip-coating, or using a roller or paint brush.

[0069] The transparent backing film 107 and scintillator material 103 forming the scintillator layer / sheet 104 is sufficiently flexible such that it can be applied within the inner surface of the shell 102 or wrapped onto the outer surface of the light guide 106. In embodiments, the neutronscintillator sheet is loosely wrapped onto the outer surface of the light guide 106. In one embodiment, the neutron scintillating sheet can be bent to a radius of approximately 0.5 cm without cracking or breaking. In this embodiment, the inner surface of the shell 102 is coated with a light reflecting layer 105 to improve light reflection. In embodiments, the light reflecting layer 105 is a diffuse reflector layer and is deposited on top of the scintillator sheet. In embodiments, the light reflecting later 105 is fabricated from a PTFE tape / wrap. In other embodiments, other materials such as but not limited to expanded PTFE (ePTFE) sheets, aluminum or silver coatings or sheets, and / or sheets made with or coated with diffuse reflective white powders such as TiCE or BaSCE may be employed.

[0070] Referring back to FIGS. IB and 1C, in an alternate embodiment, an inner surface of the outer shell 102 includes a coating of a thin layer 104 of neutron scintillator containing small particle powders of6LiF and ZnS(Ag). In various embodiments, the thin layer 104 of neutron scintillator includes, but is not limited to, any suspension formulation of6LiF :ZnS(Ag). In an embodiment, in order to deposit the scintillator layer 104 is directly onto the inner surface of outer shell 102, the6LiF:ZnS(Ag) is mixed into a polymer and coated onto the inner surface using a rotating mechanical arrangement in which the scintillator / polymer mixture is dispersed on the inner surface by rotating the outer shell until the polymer has hardened.

[0071] Referring again to FIGS. 1 A, IB, and 1C, and ID, light guide 106 functions as a waveguide. In some embodiments, a first central longitudinal axis 108a of shell 102 coincides with a second central longitudinal axis 108b of the light guide 106. In some embodiments, the light guide 106 is made of acrylic and forms a clear acrylic cylinder or rod. The rod, cylinder or pipe of the light guide 106 has a length ‘t’ and a diameter ‘d’. The outer surface of the light guide 106 is coated with a layer of a wavelength shifting (WLS) coating 110. In embodiments, the WLS layer 110 comprises a coating with a paint such as Eljen EJ-298G. However, in various embodiments, any WLS paint may be used to achieve the objectives of the present specification and any uses of a specific paint should not be construed as limiting. An air gap 112 exists between the layer 104 of6LiF:ZnS(Ag) scintillator and the WLS layer or coating 110.

[0072] In embodiments, the light guide 106 is made of a material (such as plastic, and more specifically, acrylic) containing neutron moderating elements (hydrogen, carbon) since such a material may offer an advantage in thermalizing neutrons. In alternate embodiments, the light guide 106 is made of an optically transparent material such as, for example, glass. In embodiments,if the WLS layer 110 is coated onto the surface of the light guide material it will adhere well and have a closely matching index of refraction to that of the waveguide material.

[0073] In some embodiments, the surface of the light guide 106 functions as an optically smooth internal reflective surface. Optically smooth internal reflective surfaces work better than rough surfaces, because light ray paths that undergo total internal reflection propagate longer distances in the light guide 106 without escaping than those that scatter from rough surfaces at randomized angles. Light guide geometries that allow for fewer internal reflections prior to reaching the photo detectors or light sensors are preferred, to maximize light collection.

[0074] In embodiments, the6LiF :ZnS(Ag) scintillator layer 104 emits light (~ 420 nm wavelength) in all directions, captured by the WLS layer or coating 110 of the light guide 106 and is re-emitted as light (at ~ 530 nm wavelength) within the light guide 106, thereby, improving propagation of light to at least one photodetector positioned at an end of the light guide 106. Also, the WLS layer 110 and light guide 106 have very similar or matched indices of refraction, with the air gap 112 serving to promote total internal reflection within the light guide 106 of the WLS emitted light. It should be noted that internal reflection can occur between the light guide 106 and the WLS layer 110 or between the WLS layer 110 and the air gap 112.

[0075] In various embodiments, the length of the outer shell 102 is at least 1 cm and preferably ranges from 30 cm to 300 cm and the diameter ranges from 1 cm to 10 cm. In one embodiment, the length of the outer shell 102 is 100 cm and the diameter is 5.08 cm. In some embodiments, the outer shell 102 wall has a thickness ranging from 1 mm to 2 mm. In one embodiment, the outer shell 102 wall has a thickness of 1.6 mm.

[0076] In some embodiments, the length of the light guide 106 is less than or equal to the length of the outer shell 102. In some embodiments, the length of the light guide 106 is 100 cm. In some embodiments, the diameter of the light guide 106 ranges from 1 cm to 10 cm. In one embodiment, the diameter of the light guide 106 is 3.81 cm.

[0077] In some embodiments, the6LiF:ZnS(Ag) scintillator layer 104 has a thickness ranging from 200 pm to 600 pm. In one embodiment, the6LiF:ZnS(Ag) scintillator layer 104 has a thickness of 350 pm.

[0078] In some embodiments, the WLS layer 110 has a thickness ranging from 20 pm to 1 mm. In one embodiment, the WLS layer 110 has a thickness of 60 pm.

[0079] In some embodiments, the air gap 112 has a thickness ranging from direct contact (with a tiny, but uncontrolled air gap) to 10 mm. In one embodiment, the air gap 112 has a thickness of 4.35 mm.

[0080] In some embodiments, where an objective is to have a neutron detector with a form factor compatible as a He-3 (Helium-3 proportional counter) neutron detector replacement (since He-3 detectors are usually cylindrical detectors), it is preferred that the geometrical configuration of the neutron detector is also cylindrical - that is, the light guide 106 is cylindrical. However, in alternate embodiments the light guide 106 may have non-circular cross-sections such as, but not limited to, square, rectangular, oval, twisted hexagonal or polygonal. In some embodiments, the WLS coated light guide 106 could also be hollow internally, i.e. a round or square tube, for example. It should be appreciated that, in various embodiments, the cross-section of the outer shell 102 as well as other components of the detector 100 would assume the same cross-section as that of the light guide 106.

[0081] Data Acquisition, Storage and Processing System

[0082] FIG. 2 is a block diagram of a data acquisition, storage and processing system 200, in accordance with some embodiments of the present specification. Referring now to FIGS. 1 A and 2, in embodiments, photodetectors or light sensors 202 are optically coupled to one or both ends of the light guide 106 that is coated with the WLS layer 110 of the neutron detector 100. In various embodiments, the photo detectors or light sensors 202 can comprise photodiodes, silicon photomultipliers (SiPM), photomultiplier tubes (PMT), or hybrid PMTs that include a photocathode and a semiconductor electron sensor.

[0083] During operation, neutrons pass through the outer shell 102 and are captured via interactions with the enriched lithium (Li-6) in the scintillator layer 104. These interactions of the neutrons are converted into a plurality of photons (of the order of 15,000 to 50,000 photons per neutron interaction) within the scintillator layer 104 that are emitted in a wavelength range of about 400 nm to 540 nm, with a peak emission of 450 nm. Some of this light is absorbed in the6LiF:ZnS(Ag) and binder material and lost, while a portion of the light escapes the inner surface of the scintillator layer 104, passes through the airgap 112, and is absorbed within the WLS coating layer 110 on the light guide 106.

[0084] The WLS layer 110 converts a portion of the absorbed scintillation light to a longer wavelength which is re-emitted isotropically within the light guide 106. A portion of the reemitted, wavelength-shifted light is captured in and propagates toward the ends of the light guide 106, via total internal reflection, where it is detected by the photodetectors or light sensors 202, at one or both ends of the light guide 106, and converted into an electrical current, which is delivered to a trans-impedance amplifier 204.

[0085] It should be appreciated that some neutrons would pass through the thin6LiF:ZnS(Ag) scintillator layer 104 without reacting with the6Li. Many of these would be expected to scatter and lose energy as they pass through light guide 106. At lower energies, these neutrons would have a higher probability of interaction with the scintillator layer 104 on the “opposite side” of the detector 100 (since, some neutrons may pass through the6LiF:ZnS(Ag) surface 104, WLS layer 110 and light guide 106 and be captured by the6LiF:ZnS(Ag) surface 104 on the “opposite side” from where they entered the detector 100) thereby contributing to the sensitivity of the detector 100. These ‘opposite side’ interacting neutrons also have an advantage in that they enter the scintillator layer 104 facing the WLS layer 110 (paint coating surface) and, therefore, higher light output would be expected when these neutrons interact with6Li. While the light guide 106 can be fabricated from glass, using a hydrogen-rich material such as acrylic, in some embodiments, is beneficial for its neutron moderating properties. In embodiments, the presence of H atoms may help thermalize (lower) the energy of neurons, which, in turn, makes them more likely to be captured by the 6-Li in the6LiF:ZnS(Ag) layer.

[0086] Referring back to FIG. 2, the trans-impedance amplifier 204 amplifies the electrical current pulses, received from the photodetector 202, and converts these to voltage pulses 220. The voltage pulses 220 from the trans-impedance amplifier 204 are discriminated by a voltage comparator 206 against a predetermined threshold voltage in order to output digital pulses 222. The digital pulses 222 are further processed by a neutron signal processor 208 to count only those pulses 222 (also referred to as ‘neutron event generated pulses’) that arise from neutron events and provide as an output only a pulse 224 for each neutron event. In accordance with embodiments of the present specification, pulses that are below a predetermined, yet customizable, duration, time width or pulse width are determined, by the neutron signal processor 208, to be originating from dark current noise corresponding to photodetectors or light sensors 202 or gamma interactions with various materials and rejected by the neutron signal processor 208. Pulses 222 above thepredetermined, yet customizable, duration, time width or pulse width are accepted and counted as originating from neutron captures and result in a neutron pulse 224.

[0087] In some embodiments, pulses that have a duration of 360 nanoseconds or longer are counted as neutron events. The width of the pulses 222 is dependent on whether the event was from a neutron, the comparator threshold level, the amount of gain of the trans-impedance amplifier, the amount of light collected per event, and / or the gain of the photodetector (which is dependent on photodetector bias). In some embodiments, these stated parameters are interdependent. Some neutron events are 10's of microseconds in duration, but some neutron pulses also have a longer "tail" to the waveform 220 shape that will appear to be a neutron, but if less light was collected, then the pulse will be a low amplitude so that it will not cross the threshold long enough to have a long duration. In alternate embodiments, pulse shape discrimination algorithms and machine learning algorithms may be used to classify the nonsymmetrical shape of the neutron event pulses.

[0088] The rejected pulses are generally of a shorter duration (that is, shorter time width or pulse width) than the pulses accepted as occurring due to neutron events. In one non-limiting example scenario, the trans-impedance amplifier 204 with a time constant of 22 nanoseconds typically exhibits fast pulses that have a duration of under 100 nanoseconds for most dark noise or gamma interactions. The neutron events may appear as a burst of fast pulses that merge together at the beginning of the event so that it appears to have a burst of events that gradually reduce in frequency. A longer trans-impedance amplifier 204 time constant of 220 nanoseconds still exhibits a faster single pulse from dark noise or gamma events that may last for a few hundred nanoseconds, while neutron events have a visible longer “tail” duration of several hundred nanoseconds to tens of microseconds.

[0089] An oscilloscope was used to visualize the behavior of pulses generated by the data acquisition, storage and processing system 200, coupled to the neutron detector 100, and display the pulse voltage over a time period. More specifically, an oscilloscope with multiple channels was used to view the trans-impedance amplifier 204 output 220, the comparator 206 output 222, and the neutron processor 208 output 224. FIG. 3 A shows a first oscilloscope waveform trace 300a displaying exemplary pulse data, over a time period, corresponding to dark noise or gamma events, in accordance with some embodiments of the present specification. The trans-impedance amplifier signal 302a is the result of averaging many pulses together with no neutron source. Signal 306ais indicative of no neutron present. Signals 302a and 306a represent the average appearance of pulses due to dark noise or gamma events with an exemplary trans-impedance amplifier time constant of 22 nanoseconds . The comparator signal 304a is the result of applying a discriminator with a threshold of 10 millivolts to one of the pulses forming the average of the trace or signal 302a. Trace or signal 302a has a pulse width under 100 nanoseconds. This narrow pulse width drives the decision to reject this pulse.

[0090] FIG. 3B shows a second oscilloscope waveform trace 300b displaying exemplary pulse data, over a time period, corresponding to a neutron pulse or event, in accordance with some embodiments of the present specification. The trans-impedance amplifier signal 302b corresponds to a neutron pulse 306b with a trans-impedance amplifier time constant of 22 nanoseconds. As can be observed, there are a series of narrower fast spikes merging together to form a decreasing tail 310b after the initial larger event 312b. The comparator signal 304b is the result of applying a discriminator to the trace of signal 302b with a threshold of 10 millivolts. It has many events with pulse width of approximately 750 nanoseconds. This wider pulse width drives a decision to accept this as a neutron event.

[0091] FIG. 3C shows a third oscilloscope waveform trace 300c displaying exemplary pulse data, over a time period, corresponding to a neutron pulse or event, in accordance with some embodiments of the present specification. In this example, the trans-impedance amplifier time constant is set to 220 nanoseconds. The trans-impedance amplifier signal 302c corresponds to a neutron pulse 306c, but due to the longer time constant of the trans-impedance amplifier, there is increased integration of the individual photon events resulting in a smoother, long-lasting tail 310c to the pulse. The comparator signal 304c shows the result of discriminating the trace of signal 302c against a threshold value of 10 millivolts where the trace of signal 304c lasts approximately 2.5 microseconds. This is a clearer example of an event that should be accepted as a neutron event.

[0092] In embodiments, one or more performance parameters of the neutron detector of the present specification are affected by modifying or adjusting one or more of a plurality of optimization parameters such as, but not limited to, trans-impedance amplifier time constant, PMT or photodetector bias voltage, comparator threshold voltage, and pulse width criteria. In various embodiments, the one or more performance parameters of the neutron detector include parameters such as, but not limited to, signal-to-noise ratio (SNR), gamma rejection ratio, and neutron count rate. Additional performance parameters may include but is not limited to variation in neutrondetection rate versus total neutron emission at different neutron energies and operating temperatures. The neutron rate that is acceptable, in some embodiments, depends on the background rate, source used, how far away the source is placed, and other details of the set-up that is used to evaluate a detector. The detector size, in embodiments, is chosen to ensure that it has the sensitivity needed for the requirements of a given application, for example, detect a specific neutron source type with a given activity range with some allowed false-alarm rate.

[0093] It should be appreciated that customization or modification of the pulse width criteria, for example, allows the plurality of optimization parameters to be adjusted. For example, in FIGS. 3B and 3C, the trans-impedance amplifier time constant was modified from 22 nanoseconds to 220 nanoseconds, respectively, affecting the resulting neutron events as fast individual “spikes” to form a longer tail 310b, 310c thereby integrating into a more consistent longer tail pulse. The width of the pulses that results became longer as the time constant was increased, and so the customizable pulse width may be increased to include the neutron events with longer duration pulse widths while rejecting shorter duration pulses likely arising from dark counts or gamma events.

[0094] In addition, there are other parameters that may be adjusted in the system that make a customizable pulse width criteria useful, at least initially for a given set of detector parameters. As stated earlier in the specification, these additional parameters include, for example, the PMT / SiPM or photodetector bias voltage (use of SiPM would be aided by tapering the light guide ends into a parabolic light concentrator shape), the comparator threshold voltage, and any other circuit changes that modify the gain of the front-end trans-impedance amplifier. If the bias voltage is increased, then the gain increases at the front-end trans-impedance amplifier. Trans-impedance amplifier analog component values can also be adjusted to increase or decrease the gain. If the gain is increased, then the analog trans-impedance amplifier pulses shown in the traces 302a, 302b, 302c of FIGS. 3A, 3B and 3C, respectively, could increase in amplitude, or decrease in amplitude if gain is decreased. Relative to the comparator threshold voltage level, this would make the resulting comparator pulses wider when gain is increased, or narrower if gain is decreased. Increasing the threshold will result in a narrower comparator pulse and lowering the threshold can result in a wider comparator pulse. Stated differently, a customizable pulse width criterion allows tuning to achieve a desired performance of the neutron detector if the other optimization parameters are adjusted.

[0095] It should be appreciated that a decision corresponding to accepting an event as a neutron event (or rejecting the event) requires experimentation with various parameters involved, including an analysis of the resulting neutron count rates which occurred when a) no sources were present, b) a Cf-252 neutron source was present, and c) a strong Cs-137 gamma source was present. There are a multitude of settings which may enable good operation of the neutron detector. As an example, the lower the comparator threshold setting, or the higher the photodetector bias setting, or the longer the time constant of the amplifier, then the wider the duration of all pulses, thereby resulting in using a longer duration as the neutron decision. If the comparator threshold is raised, then pulse durations become shorter with only the tallest neutron events remaining, but many neutron events may be missed thereby lowering the usefulness of the neutron detector.

[0096] In general, the goal is to use a high enough photodetector bias to provide good front gain from the photodetector, combined with a time constant for the trans-impedance amplifier to provide fast response so that dark current events and gamma events remain narrow and short in duration relative to the neutron events, and a low comparator threshold to allow many low level events to be processed (less light making it from the far ends of the detector may result in lower amplitude pulses, yet with a longer tail remaining for the neutron events than the gamma events). Finally, the pulse duration is used to determine which of these parameters is adjusted to provide a suitably low "background" neutron rate while responding well to neutron sources and retaining a background count rate response when strong gamma sources are present (gamma rejection remains acceptable). Therefore, in one embodiment, the comparator threshold is a function of, and therefore set at least partially based on, a value of the photodetector bias, a value for the time constant of an amplifier, and / or a pulse duration. In one embodiment, the value of the photodetector bias is a function of, and therefore set at least partially based on, the comparator threshold value, a value for the time constant of an amplifier, and / or a pulse duration. In one embodiment, a value for the time constant of an amplifier is a function of, and therefore set at least partially based on, the comparator threshold value, a value for the photodetector bias, and / or a pulse duration. In one embodiment, the pulse duration is a function of, and therefore set at least partially based on, the comparator threshold value, a value for the photodetector bias, and / or a value for the time constant of an amplifier.

[0097] Also, if a geometrical aspect of the neutron detector is modulated, for example if the length is increased, then the performance parameters of the detector would be affected since a longerlength detector would be expected to have less light arrive at the photodetector than a shorter length detector. Since the plurality of optimization parameters are related to electronic components of the data acquisition, storage, and processing system 200, the same electronic components are usable for a variety of implementations of the neutron detector and allow "calibration" for any specific variations from nominal.

[0098] FIG. 5 is a flowchart of a method 500 of using the neutron detector 100 for detecting neutrons, in accordance with some embodiments of the present specification. Referring now to FIGS.1 A, 2 and 5, at step 502, a detector pulse is generated by at least one photo detector 202 optically coupled to an end of the light guide 106. In some embodiments, the at least one detector 202 is a photo-multiplier tube. In alternate embodiments, two photodetectors 202 may be optically coupled to respective first and second ends of the light guide 106. Also, in various embodiments, the photodetectors 202 include photodiodes, silicon photomultipliers (SiPM), photomultiplier tubes (PMT), or hybrid PMTs that include a photocathode and a semiconductor electron sensor.

[0099] At step 504, the processing system 200 is configured to determine if the detector pulse has a duration shorter or longer than a predetermined pulse width criterion.

[0100] At step 506, the processing system is further configured to count the detector pulse if the duration is longer than the predetermined pulse width criterion or rejects the detector pulse if the duration is shorter than the predetermined pulse width criterion. In embodiments, at least one performance parameter of the neutron detector 100 is optimized by modifying one or more of a time constant of the trans-impedance amplifier, a bias voltage of the photodetector(s), a threshold voltage of the comparator and the pulse width criterion.

[0101] Exemplary Use Case of a First Embodiment of the Neutron Detector: Performance Modeling

[0102] Referring back to FIG. IB and FIG. 1C, in one scenario, performance modeling was carried out for a neutron detector comprised of a cylindrical outer shell 102 of aluminum with a 5.08 cm outer diameter, a length of 100 cm and a wall thickness of 1.6 mm. In addition, the inner surface of the shell 102 was coated with a6LiF:ZnS(Ag) scintillator layer of 350pm thickness. A 100 cm long WLS-coated acrylic cylinder (light guide 106) having a diameter of 3.81 cm was positioned within the outer shell 102 such that an air gap of 4.35 mm thickness exists between the WLS coating layer and the scintillator layer. The WLS coating layer had a thickness of 60pm.

[0103] MCNP (Monte Carlo N-Particle Transport) was used to model neutron capture from6LiF:ZnS(Ag) scintillator layer 104. Detect 2000 (the ray-tracing Monte Carlo software program) was used to model light propagation to ends of the acrylic light guide 106. MCNP5 estimated the fraction of neutrons interacting in the6LiF:ZnS(Ag) scintillator layer 104 using a point neutron source centered at 50 cm (that is, the midpoint) from both ends of the light guide 106 of length 100 cm. A3He proportional counter was included on the opposite side of the source for comparison. No added materials were used in the model.

[0104] Fractions of neutrons emitted from the neutron point source that interacted with6Li and3He were predicted for moderated232Cf, unmoderated232Cf, and thermal neutrons. Detect 2000 was used to predict the light output from the scintillator layer 104 that reached the WLS layer 110.

[0105] A ‘1 / e’ absorption distance of 0.153mm was calculated using known absorption curves. It would be expected that 50% of the light would reach the WLS layer 110 if there was no absorption, however, Detect 2000 showed that at best about 10% of the light escaped and interacted with the WLS layer 110. Detect 2000 does not process ‘bursts’ of photons but rather randomly locates single photons in selected source material and follows them until they are absorbed or lost. This was not an exact prediction of what a photo / light sensor would see from a photon ‘burst’ but was sufficient. Finally, Detect 2000 was used to predict the number of photons reaching the light sensors 202 at the ends of light guide 106 for ‘photons’ originating in the WLS layer 110 at random locations and in random directions. Here, about 50% of the photons were detected. The detected photons would be counted in a real detector, provided there were a sufficient number above some threshold.

[0106] The fraction of emitted neutrons counted = (neutrons interacting with6Li) x (scintillation per interaction) x (photons emitted by wavelength shifter) x (photons transmitted along light guides).Table 1 Modeled detection efficiency

[0107] Table 1 shows the ‘worst case’ results for single photon detection as modeled by system of the present specification, and the ‘best case’ that was observed for photon bursts typical of6Li- ZnS(Ag) neutron detection, in embodiments of the system described in the present specification.

[0108] It was concluded that the predicted neutron detector performance is comparable to that of a 3 atm3He proportional counter.

[0109] Exemplary Use Case of a Second Embodiment of the Neutron Detector: Performance Modeling

[0110] Referring back to FIG. IB, in another embodiment, performance modeling was carried out for a neutron detector having an outer shell 102 made of aluminum and configured as a cylinder having an outer diameter of 5.08 cm with the inside surface coated with6LiF:ZnS(Ag) scintillator (in embodiments, having a thickness of 350 pm). A 172.72 cm long WLS coated acrylic cylinder (light guide 106 having an outer diameter of 3.81 cm, in embodiments) was positioned within the outer shell 102 such that an air gap (having a thickness of 4.35 mm, in embodiments) exists between the WLS coating (having a thickness of 60 pm, in embodiments) and the scintillator. The data acquisition, storage and processing system 200 (FIG. 2) was coupled to the neutron detector in order to detect and process light at both ends of the light guide 106, which acted as a waveguide. MCNP was used to model neutron capture from6LiF:ZnS(Ag). Detect 2000 was used to model light propagation to ends of the acrylic waveguide. The values provided are only exemplary and not to be construed as limiting.

[0111] A neutron decision pulse provided by the data acquisition, storage and processing system 200 was monitored by an oscilloscope and used to derive neutron count rate measurements. FIG. 4 shows a typical neutron waveform 402 and comparator signal 404 corresponding to a neutron event 406. Various performance optimization parameters such as, but not limited to, transimpedance amplifier time constant, PMT bias voltage, comparator threshold voltage, and width of pulses were adjusted to begin optimizing performance. A 40 inches He-3 tube was used with the same neutron source geometry and results scaled up by xl.7 to estimate 68 inches He-3 tube performance to serve as goals for the WLS coated acrylic cylinder detector. Ideally, the WLS coated acrylic cylinder detector would have the following performance parameter objectives: lower background noise, higher count rate in counts per second (CPS) for Cf-252 (Cf-252 is a neutron source typically used to characterize neutron detectors), and higher SNR (signal-to-noise ratio) FOM (figure of merit) than the 68 inches He-3. The parameter SNR FOM is a relative way to compare performance for the detector and uses the signal counts or count rate from a neutron source and the background neutron counts or count rate with no source. It is defined as: FOM = Net / sqrt(B ackground). The term "Net" is a shorthand way to describe (Source - Background). For unmoderated detector, the WLS coated acrylic cylinder detector met these objectives for several optimization parameter combinations, as can be observed from Table 2 below: Table 2

[0112] Table 3 below includes performance data trend (background noise, Cf-252 and SNR (signal-to-noise ratio), wherein SNR = (count rate from Cf-252 - Background) / sqrt(background))corresponding to a plurality of setting combinations for performance optimization parameters such as, for example, the trans-impedance amplifier front end capacitance (C55), PMT bias voltage (HV), comparator threshold voltage and pulse width. As shown, the SNR values of 19.6, 19. 3 and 19.5 were the best that were achieved. Cs-137 is a gamma emitting source. It is used to evaluate the parameters of the detector and if they are sufficient for the detector to reject counting the gamma events as neutron events (gamma rejection).Table 3

[0113] FIG. 7 is a flowchart of a method of manufacturing a prototype of the neutron detector 100 of FIG. ID, in accordance with some embodiments of the present specification. At step 702, a WLS layer is applied to a clear acrylic cylindrical light guide. At step 702a, the WLS coating is applied to the outer diameter of the acrylic light guide, by first wiping clean both the ends as well the outer diameter of the rod and attaching the upper end face to a supporting wire. At step 702b, the acrylic light guide is coated with WLS paint by filling a coating reservoir with WLS fluid ‘paint’ and causing a motor to draw the light guide through the coating reservoir via the attached wire at a constant, controlled speed. The resulting coating layer thickness depends on the draw speed, as well as the WLS paint viscosity. At step 702c, the WLS coated light guide is dried in an oven at a predetermined temperature and subsequently allowed to cool to room temperature.

[0114] At step 704, the WLS coated light guide is wrapped with a6LiF / ZnS(Ag) sheet. An overlayer of a diffuse reflector sheet is applied on top of the6LiF / ZnS(Ag) sheet.

[0115] As discussed with reference to FIGS. 1A, IB, 1C, and ID, in embodiments, an air gap 112 exists between the central light guide 106 and the WLS layer 110. This air gap 112 may be used to cause ‘total internal reflection’ of light traveling inside the light guide 106 so that light is reflected at the edge of the guide, eventually reaching a light sensor at an end of the central light guide 106. To aid manufacturing, in some embodiments, a ‘porous’ material such as, but not limited to Tyvek is used to “fill” the gap 112. While not a ‘perfect’ gap, such a ‘porous’ material filled gap is cost effective and causes little degradation of the total internal reflection. Thus, air gap 112 may be filled with a ‘porous’ material in scenarios where a trade-off between manufacturing cost and reduced internal reflection may be explored.

[0116] During the testing and development phase, two 50 cm long acrylic rods were fabricated for use in thermal cycle testing. Thermal cycle testing over a -30°C to +60°C temperature range was completed for the two short rods coupled to photo sensors and processing circuitry. The test began at 20°Cat neutron background of approximately 0.4 CPS (counts per second). A Cf-252 source was introduced for the remainder of the test. FIG. 6 shows a first plot 602 of a first neutron count rate for the first acrylic rod and a second plot 604 of a second neutron count rate for the second acrylic rod. As shown, the first and second plots 602, 604 show a slight decrease in count rate with increasing temperature, which was consistent.

[0117] The above examples are merely illustrative of the many applications of the systems and methods of the present specification. Although only a few embodiments of the present inventionhave been described herein, it should be understood that the present invention might be embodied in many other specific forms without departing from the spirit or scope of the invention. Therefore, the present examples and embodiments are to be considered as illustrative and not restrictive, and the invention may be modified within the scope of the appended claims.

Claims

CLAIMSWhat is claimed is:

1. A radiation detector for detection of neutrons, comprising: a first hollow cylinder; a scintillator layer provided on an inner surface of the first hollow cylinder; a second cylinder, wherein the second cylinder is positioned within the first hollow cylinder such that the scintillator layer is sandwiched between an outer surface of the second cylinder and the inner surface of the first hollow cylinder; a wavelength shifting material on the outer surface of the second cylinder; at least one photodetector optically coupled to an end of the second cylinder, wherein the at least one photodetector is adapted to generate a detector pulse representative of at least one of interactions of neutrons in the scintillation layer, interactions of gamma radiation in the scintillation layer and dark noise; and a processing system adapted to count the detector pulse based on a first determination that the detector pulse corresponds to the interactions of neutrons in the scintillation layer or reject the detector pulse based on a second determination that the detector pulse corresponds to at least one of interactions of gamma radiation in the scintillation layer and dark noise.

2. The radiation detector of claim 1, wherein the second cylinder is solid.

3. The radiation detector of claim 1, further comprising an air gap between the scintillator layer and the outer surface of the second cylinder.

4. The radiation detector of claim 1, wherein the scintillator layer comprises scintillator material deposited upon a binder material.

5. The radiation detector of claim 1, further comprising a light reflecting layer between the scintillator layer and the inner surface of the first hollow cylinder.

6. The radiation detector of claim 1, wherein the processing system is configured to generate the first determination and the second determination based on a predetermined pulse width criterion.

7. The radiation detector of claim 5, wherein the processing system is configured to determine the detector pulse corresponds to the interactions of neutrons in the scintillation layer when the detector pulse has a duration longer than the predetermined pulse width criterion.

8. The radiation detector of claim 5, wherein the processing system is configured to determine the detector pulse corresponds to at least one of the interactions of gamma radiation in the scintillation layer and dark noise when the detector pulse has a duration shorter than the predetermined pulse width criterion.

9. The radiation detector of claim 1, wherein the processing system includes a trans-impedance amplifier, a comparator and a processor.

10. The radiation detector of claim 8, wherein the at least one photodetector comprises a photomultiplier tubes.

11. The radiation detector of claim 9, wherein the processing system is configured to optimize at least one performance parameter of the radiation detector by modifying one or more of a time constant of the trans-impedance amplifier, a bias voltage of the photo-multiplier tubes, a threshold voltage of the comparator, or a pulse width criterion.

12. The radiation detector of claim 1, wherein the first hollow cylinder comprises aluminum.

13. The radiation detector of claim 1, wherein the second solid cylinder comprises acrylic.

14. The radiation detector of claim 1, wherein the wavelength shifting material is deposited directly onto the outer surface of the second cylinder by combining of6LiF:ZnS(Ag) with a polymer, creating a mixture, and coating the mixture on the outer surface of the second cylinder.

15. The radiation detector of claim 1, wherein the wavelength shifting material is layered onto the outer surface of the second cylinder by wrapping a sheet of the wavelength shifting material on the outer surface of the second cylinder.

16. A method of using a radiation detector for detecting neutrons, wherein the radiation detector comprises a first hollow cylinder, a scintillator layer provided on an inner surface of the first hollow cylinder, a second cylinder, wherein the second cylinder is positioned within the first hollow cylinder such that the scintillator layer is between an outer surface of the second cylinder and the inner surface of the first hollow cylinder, a wavelength shifting material on the outer surface of the second cylinder, and at least one photodetector optically coupled to an end of the second cylinder, the method comprising: generating a detector pulse using the at least one photodetector, wherein the detector pulse is representative of at least one of interactions of neutrons in the scintillation layer, interactions of gamma radiation in the scintillation layer and dark noise; andcounting the detector pulse, using the processing system, based on a first determination that the detector pulse corresponds to the interactions of neutrons in the scintillation layer or rejecting the detector pulse based on a second determination that the detector pulse corresponds to at least one of interactions of gamma radiation in the scintillation layer and dark noise.

17. The method of claim 16, further comprising an air gap between the scintillator layer and the outer surface of the second cylinder.

18. The method of claim 16, wherein the scintillator layer of the radiation detector comprises scintillator material deposited upon a binder material.

19. The method of claim 16, wherein the radiation detector further comprises a light reflecting layer positioned between the scintillator layer and the inner surface of the first hollow cylinder.

20. The method of claim 16, wherein the processing system is configured to generate the first determination and the second determination based on a predetermined pulse width criterion.

21. The method of claim 20, wherein the processing system is configured to determine the detector pulse corresponds to the interactions of neutrons in the scintillation layer when the detector pulse has a duration longer than the predetermined pulse width criterion.

22. The method of claim 20, wherein the processing system is configured to determine the detector pulse corresponds to at least one of the interactions of gamma radiation in the scintillation layer and dark noise when the detector pulse has a duration shorter than the predetermined pulse width criterion.

23. The method of claim 16, wherein the processing system includes a trans-impedance amplifier, a comparator and a processor and the at least one photodetector comprises photo-multiplier tubes.

24. The method of claim 23, wherein the processing system is configured to optimize at least one performance parameter of the radiation detector by modifying one or more of a time constant of the trans-impedance amplifier, a bias voltage of the photo-multiplier tubes, a threshold voltage of the comparator, or a pulse width criterion.

25. The method of claim 16, wherein the first hollow cylinder comprises aluminum.

26. The method of claim 16, wherein the second cylinder is solid and comprises acrylic.

27. The method of claim 16, wherein the at least one photodetector is a light sensor.

28. The method of claim 16, further comprising directly depositing the wavelength shifting material onto the outer surface of the second cylinder by combining of6LiF:ZnS(Ag) with a polymer, creating a mixture, and coating the mixture on the outer surface of the second cylinder.

29. The method of claim 16, further comprising layering the wavelength shifting material onto the outer surface of the second cylinder by wrapping a sheet of the wavelength shifting material on the outer surface of the second cylinder.

Images