CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of and priority to U.S. Provisional Application No. 62/019,093 filed June 30, 2014, the entirety of which are incorporated by reference herein.

FIELD OF THE INVENTION

In general, the invention relates to devices and methods for the detection of neutrons.

More specifically, the invention relates to devices and methods related to large area flat panel neutron detectors utilizing sheets of lithium.

BACKGROUND OF THE INVENTION

Governments mobilize radiation detectors to stop the illicit movement of nuclear material such as plutonium and uranium. Previous approaches to neutron detection used an isotope of helium gas, He-3, a limited resource which has begun to show signs of a global shortage. Due to increasing He-3 shortages and the resulting increase in associated costs, neutron detectors utilizing He-3 cannot be deployed economically at scales. Efforts to develop replacement technologies have been initiated; however, none of these efforts have produced a cost effective, scalable solution.

The lack of scalable technology has limited the evolution of existing systems to meet evolving threats. Specifically, current modeling efforts show that the deployment of a large, networked array of detection technologies— where the detectors are placed at potential points of attack, material source locations, and discreetly at randomized points of transportation pathways— will lead to the greatest increase of overall security against nuclear threats.

Despite many advances in the area of radiation detection, there exists an urgent need for an inexpensive radiation detector that can be incorporated into large scale detection networks. SUMMARY OF THE INVENTION

The invention, in various embodiments, features methods and devices for the detection of radiation, using large area, flat panel detectors. The flat panel detectors are inexpensive and can detect thermal neutrons, making them a suitable building block for large-area networked scanning and other future solutions for nuclear threat reduction. Given the simple, flat-panel form factor, the invention can be applied to mobile, transportable, and fixed detectors— meeting both the policy requirements for scale, and end-user requirements for operational applications.

In one aspect, the invention features a radiation detector. The radiation detector includes at least two rigid components. The radiation detector also includes a polymeric sealant providing a seal between the at least two rigid components wherein the at least two rigid components and polymeric sealant form a radiation detection chamber. The radiation detector also includes a gas encapsulated by the chamber.

In some embodiments, the polymeric sealant has a non-zero permeability to gas entering or exiting the chamber. In some embodiments, the polymeric sealant allows less than 3% mass percentage exchange of gas over a time period of at least 10 years. In some embodiments, the readout gas includes at least one of Oxygen, Nitrogen, or Argon. In some embodiments, surfaces of the rigid structure contacting the polymeric material have mechanical tolerances of up to 50 thousandths of an inch. In some embodiments, surfaces of the rigid structure contacting the polymeric material have mechanical tolerances greater than 10 thousandths of an inch and less than 50 thousandths of an inch. In some embodiments, the polymeric seal comprises polyisobutylene. In some embodiments, the chamber is a proportional chamber. In some embodiments, the chamber is a multi-wire proportional chamber. In some embodiments, the chamber is rectangular in shape, having a length, width, and height. In some embodiments, the product of the length and the width is greater than the square of the height. In some

embodiments, the length and the width are at least ten times greater than the height. In some embodiments, a surface area of the radiation detector is greater than 14000 cm ² and less than 20000 cm ² . In some embodiments, the volume of gas inside the chamber is greater than 14000 cm ³ and less than 100000 cm ³ . In some embodiments, the polymeric sealant allows less than 1% mass percentage exchange of gas over a time period of at least 10 years. In some embodiments, the polymeric sealant allows less than 3% mass percentage exchange of gas over a time period of at least 30 years. In some embodiments, the polymeric sealant allows less than 1 % mass percentage exchange of gas over a time period of at least 30 years.

In another aspect, the invention features a radiation detector. The radiation detector includes a top plate. The radiation detector also includes a bottom plate. The radiation detector also includes a spacer positioned between the top plate and the bottom plate. The radiation detector also includes a polymeric sealant providing a seal between the top plate, bottom plate, and spacer, with the top plate, bottom plate, and the first polymeric sealant forming a radiation detection chamber. The radiation detector also includes a readout gas encapsulated by the chamber. In some embodiments, the polymeric sealant has a non-zero permeability to gas entering or exiting the radiation detection chamber. In some embodiments, the polymeric sealant allows less than 3% mass percentage exchange of gas over a time period of 1 to 30 years. In some embodiments, the readout gas includes at least one of Oxygen, Nitrogen, or Argon. In some embodiments, surfaces of the top plate, bottom plate, and spacer have mechanical tolerances of up to 50 thousandths of an inch. In some embodiments, surfaces of the top plate, bottom plate, and spacer have mechanical tolerances greater than 10 thousandths of an inch and less than 50 thousandths of an inch. In some embodiments, the polymeric sealant comprises polyisobutylene. In some embodiments, the radiation detection chamber is a proportional counter. In some embodiments, the radiation detection chamber is a multi-wire proportional counter. In some embodiments, the radiation detection chamber is rectangular in shape, having a length, width, and height. In some embodiments, the product of the length and the width is greater than the square of the height. In some embodiments, the length and the width are at least ten times greater than the height. In some embodiments, a surface area of the radiation detector is greater than 14000 cm ² and less than 20000 cm ² . In some embodiments, the volume of gas inside the chamber is greater than 14000 cm ³ and less than 100000 cm ³ .

In yet another aspect, the invention features a radiation detector. The radiation detector includes a radiation detection chamber housing that encapsulates a quantity of detector gas, said chamber housing including at least two housing structures that are arranged such that surface portions thereof of are facing one another to define at least one sealing perimeter as a gap between said surface portions. The radiation detector also includes a polymeric sealant disposed within said gap and in sealed contact with said surface portions, the polymeric sealant being selected from a group of polymer materials that are sufficiently impermeable to gases outside the radiation detection chamber such that for a period of at least 10 years a total percentage of outside gas permeating through said seal, from outside the radiation detection chamber to the interior of the radiation detection chamber, is less than a predetermined quantity.

In some embodiments, at least one of the at least two housing structures is a plate. In some embodiments, at least one of the at least two housing structures is a frame. In some embodiments, the at least two housing structures include two plates and a frame disposed therebetween. In some embodiments, the at least one sealing perimeter has a mechanical tolerance of up to 50 thousandths of an inch. In some embodiments, the at least one sealing perimeter has a mechanical tolerance greater than 10 thousandths of an inch and less than 50 thousandths of an inch. In some embodiments, one or more of the at least two housing structures is composed of steel. In some embodiments, one or more of the at least two housing structures is composed of glass. In some embodiments, the predetermined quantity of outside gas is a sufficiently low fraction of the quantity of detector gas such that efficiency of the radiation detector is reduced by less than 20%. In some embodiments, the predetermined quantity of outside gas is 3% of the quantity of detector gas such that efficiency of the radiation detector is reduced by less than 30%. In some embodiments, the radiation detector is an avalanche detector having an efficiency based in part on the selection of a particular detector gas, the detector gas including a quantity of outside gas in sufficiently low proportions such that the efficiency of detection is reduced by an amount that is less than 30%. In some embodiments, the gap has a width which varies by approximately .015 inches across a length of the radiation detector. In some embodiments, the predetermined quantity is 1 % of the quantity of detector gas. In some embodiments, the predetermined quantity is 3%o of the quantity of detector gas.

In yet another aspect, the invention features a radiation detector. The radiation detector includes a top plate. The radiation detector also includes a bottom plate. The radiation detector also includes a spacer positioned between the top plate and the bottom plate to form a chamber. The radiation detector also includes a plurality of feedthroughs disposed along an outer surface of the spacer. The radiation detector also includes a lithium sheet disposed within the chamber. The radiation detector also includes a plurality of non-conductive pillars contained within the chamber, each of the pillars contacting an inner face of the top plate and an inner face of the bottom plate, the plurality of non-conductive pillars providing a rigid mechanical connection between the top plate and the bottom plate, thereby reducing vibrations between the top and bottom plate.

In some embodiments, the plurality of non-conductive pillars include slots. In some embodiments, the radiation detector also includes a plurality of wires traversing the chamber, with each of the plurality of wires passing through the slot of at least one of the non-conductive pillars. In some embodiments, the plurality of non-conductive pillars form a rigid connection between the top and bottom plate and define an electric field for the detection of radiation. In some embodiments, the chamber has a rectangular shape with a surface area greater than 14000 cm ² . In some embodiments, the plurality of non-conductive pillars form a rigid connection between the chamber walls and the wires, the formed rigid connection reducing mechanical vibrations of the wires and defining an electric field for the detection of radiation. In some embodiments, the chamber is a proportional counter.

In yet another aspect, the invention features a radiation detector. The radiation detector includes a top plate. The radiation detector also includes a bottom plate, a perimeter of the bottom plate connected along a perimeter of the top plate and forming a chamber. The radiation detector also includes a lithium sheet disposed within the chamber. The radiation detector also includes a plurality of non-conductive pillars contained within the chamber, each of the pillars contacting an inner face of the top plate and an inner face of the bottom plate, the plurality of non-conductive pillars providing a rigid mechanical connection between the top plate and the bottom plate, thereby reducing vibrations between the top and bottom plate.

In some embodiments, the plurality of non-conductive pillars include slots. In some embodiments, the radiation detector also includes a plurality of wires traversing the chamber, with each of the plurality of wires passing through the slot of at least one of the non-conductive pillars. In some embodiments, the plurality of non-conductive pillars form a rigid connection between the top and bottom plate and define an electric field for the detection of radiation. In some embodiments, the chamber has a rectangular shape with a surface area greater than 14000 cm ² . In some embodiments, the plurality of non-conductive pillars form a rigid connection between the chamber walls and the wires, the formed rigid connection reducing mechanical vibrations of the wires and defining an electric field for the detection of radiation. In some embodiments, the chamber is a proportional counter

In yet another aspect, the invention features a method of radiation detection in a radiation detector comprising a chamber, readout wires, readout electronics, and lithium foils. The method involves collecting and amplifying the movement of charges in the radiation detector. The method also involves measuring gamma-ray and neutron signals. The method also involves providing a readout gas adjacent to the lithium foils, the readout gas causing the measured neutron signals to increase relative to the measured gamma-ray signals. The method also involves distinguishing between gamma-ray and neutron events based on the level of charge collected in the detector.

In some embodiments, the method also involves providing a voltage bias between the lithium foils and the readout wires to increase the signal-to-noise ratio of both the measured gamma-ray and neutron signals. In some embodiments, the method also involves grouping the readout wires, with each group having a designated amplifier, the grouping of the readout wires causing the signal-to-noise ratio of the measured gamma-ray signals to increase. In some embodiments, the readout gas extends between 1 and 3 cm from the lithium foils. In some embodiments, the voltage bias applied between the lithium foils and the readout wires is about 1500 volts. In some embodiments, the voltage bias applied between the lithium foils and the readout wires is between 900 and 2000 volts. In some embodiments, the chamber is a proportional counter. In some embodiments, the radiation detection chamber is a multi-wire proportional counter. In yet another aspect, the invention features a radiation detector. The radiation detector includes a chamber containing a gas, the gas producing charged particles in the presence of gamma rays. The radiation detector also includes a lithium sheet disposed within the chamber, the lithium sheet cooperating with the gas to convert neutrons to charged particles. The radiation detector also includes a plurality of wires for collecting the charged particles. The radiation detector also includes readout electronics coupled to at least one of the plurality of wires distinguishing between gamma ray events and neutron events based on the amount of charge collected.

In some embodiments, the gas is adjacent to the lithium sheet and extends between 1 and 3 cm from the lithium sheet, the thickness of the gas causing the charge collected from neutron events and gamma ray events to be distinguishable.

In some embodiments, the radiation detector includes grouped readout wires, with each group having a designated amplifier, the grouping of the readout wires causing a signal-to-noise ratio of a measured gamma-ray signal to increase. In some embodiments, the chamber is a proportional counter. In some embodiments, the radiation detection chamber is a multi-wire proportional counter.

As used herein, the terms "approximately," "roughly," and "substantially" mean ±10%, and in some embodiments, +5%. Reference throughout this specification to "one example," "an example," "one embodiment," or "an embodiment" means that a particular feature, structure, or characteristic described in connection with the example is included in at least one example of the present technology. Thus, the occurrences of the phrases "in one example," "in an example," "one embodiment," or "an embodiment" in various places throughout this specification are not necessarily all referring to the same example. Furthermore, the particular features, structures, routines, steps, or characteristics may be combined in any suitable manner in one or more examples of the technology. The headings provided herein are for convenience only and are not intended to limit or interpret the scope or meaning of the claimed technology.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages of the invention described above, together with further advantages, may be better understood by referring to the following description taken in conjunction with the accompanying drawings. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention.

FIGS. 1 A- I D are diagrams of a flat panel neutron detector according to an illustrative embodiment of the invention.

FIGS. 2A-2B are diagrams of a flat panel neutron detector according to an illustrative embodiment of the invention.

FIGS. 3A-3D are diagrams showing histogram outputs associated with a gamma ray and neutron detector according to an illustrative embodiment of the invention.

DETAILED DESCRIPTION

FIGS. 1 A-1 D show a flat panel neutron detector 100. The flat panel neutron detector 100 includes a top plate 104, a bottom plate 108, lithium foils 1 12, wires 1 16, a spacer frame 120, a seal 124, wire feedthroughs 128, front electronics board 132, and back electronics board 136. The front electronics board 132 and back electronics board 136 include readout electronics 140. A lithium foil 1 12 is attached to both the top plate 104 and the bottom plate 108. The top plate 104 and the bottom plate 108 are attached to the spacer frame 120 as shown in FIG. 1 to form a chamber containing a readout gas. The seal 124 provides a seal between the top plate 104, spacer frame 120, and bottom plate 108 that greatly reduces readout gas from exiting the chamber, as well as greatly reduces environmental air from entering the chamber. The wires 1 16 are fed through wire feedthroughs 128 located on a front and back side of the spacer frame 120. The wires are electrically connected to the front electronics board 132 and the back electronics board 136. The readout electronics 140 provide a voltage bias between the high voltage wires 1 16 and the lithium foils 1 12. In some embodiments, the lithium can be in the range of 20 microns to 200 microns in thickness. Additionally, the readout electronics 140 can provide decoupling of the signals received from the high voltage wires 1 16, provide the amplification of the signals received from the wires 1 16, and host post-digitization and further computer and wireless interfacing to share information relating to the collected signals with user applications.

During operation, neutrons and gamma rays impinge upon the flat panel neutron detector 1 00. Neutrons impinging upon the detector and passing through the top plate 104 or the bottom plate 108 to the lithium foils 1 12 can be captured by one of the Li-6 atoms contained in the lithium foils 1 12. The capture of the neutron by the Li-6 atom results in a Li-7 atom that decays into two daughter particles, an alpha particle and a triton. The triton and alpha particle travel in opposite directions, and lose energy as they travel through the lithium foil. Upon exiting the lithium foil, tritons or alpha particles having sufficient kinetic energy will ionize atoms in the readout gas. Electrons produced during the ionization of readout gas atoms will drift towards the high voltage wires 1 16 and the ions produced during the ionization of readout gas atoms will drift towards the lithium foils 1 12. Drifting electrons within a predetermined distance of roughly 5 times the radius of the readout wire 1 16 (i.e., the Townsend avalanche region) experience an electric field that accelerates the drifting electrons fast enough to cause further ionization of the readout gas. The further ionization of the readout gas creates additional electrons, which drift toward the high voltage wires and cause even further ionization of the readout gas. This process that occurs within the Townsend avalanche is called gas multiplication. Ionized readout gas atoms within the Townsend avalanche region that move towards the lithium foils 1 12 cause a movement of charge along the readout wire 1 16. The charge moving along the readout wire 1 16 is collected by the readout electronics 140 and amplified with a pulse-mode, charge-sensitive preamplifier to produce a voltage output signal. Pulse height discrimination circuitry included within the readout electronics 140 then compares the voltage output signal to a first

predetermined threshold and determines if a neutron event has been detected (e.g., for a gas multiplication of roughly 100, and an amplification circuitry gain of l fC/mV, pulse heights greater than 250keV volt can indicate the presence of a neutron). In some embodiments, the false positive detection rate of neutrons based on the first predetermined threshold is less than 10 ^{" 5} for a gamma ray exposure rate of l OOmR/hr. A second predetermined threshold can be selected, the second predetermined threshold being lower than the first predetermined threshold. Voltage output signals below the second predetermined threshold are deemed to be very low ionizing gamma ray events or movements of charge in the flat panel neutron detector 100 induced by another source (e.g., thermal heat, radio frequency electromagnetic radiation, and changes in the relative position of the readout wires 1 16 and the lithium foils 1 12 - known as microphonics). Voltage output signals below the first predetermined threshold and above the second predetermined threshold are indicative of gamma ray events. The detected rate of neutrons and gamma rays impinging upon the detector can be used in radiation detection methodologies (e.g., detect the presence of a nuclear weapon or unauthorized nuclear device).

It is important that the composition of the readout gas remains relatively constant over time to avoid deterioration of the gamma ray and neutron detection process. Changes greater than 1 % in the composition of the readout gas can affect the Townsend avalanche process. For example, nitrogen, oxygen, or water molecules that leak into the chamber do not ionize as well as the argon gas in the amplification region near the readout wires 1 16, and therefore can reduce the Townsend avalanche process near the readout wires 1 16 when introduced into the readout gas. This reduces the ability of the readout electronics 140 to distinguish between noise, gamma ray, and neutron events, thereby decreasing the efficiency of the flat panel neutron detector 100. A 1 % change in the readout gas composition can lead to an 8% change in the voltage output signal. In order for the readout gas composition to not be changed in an adverse way, it is desirable that the following processes be reduced to about 1% changes over the lifespan of the chamber: (1 ) the leakage of readout gas from the chamber; and (2) the leakage of air constituents (nitrogen, oxygen, carbon dioxide), water, and other airborne molecules into the chamber.

In some embodiments, the seal 124 can be formed from polyisobutylene to maintain the readout gas composition over long periods of time (e.g., 30 years). The polyisobutylene seal 124 can conform to the region between the top plate 104 or bottom plate 108 and the spacer frame 120, filling any gaps due to imperfections in the surface quality of the top plate 104, the bottom plate 108, and the spacer frame 120. In some embodiments, the surface quality of the top plate 104, bottom plate 108, and spacer frame 120 can be chosen to generate uniform electric fields near the readout wires 1 16 of the flat panel neutron detector 100 (e.g., the variance in the dimensions of the top plate 104, the bottom plate 108, and the spacer frame 120 can be approximately 0.020" in) , with no regard for sealing of the top plate 104, bottom plate 108, and spacer frame 120, since a polyisobutylene seal can accommodate such fluctuations in the seal. The polyisobutylene seal 124 is also compatible with a low temperature manufacturing process that eliminates welding or brazing steps to seal the chamber, thereby reducing warping and bending of the components of the flat panel neutron detector 100. The polyisobutylene seal 124 also accommodates thermal expansion/contraction in the chamber components, thereby allowing a greater number of material choices for the top plate 104, bottom plate 108, and spacer frame 120 such as glass, aluminum, or stainless steel. The polyisobutylene seal 124 can have a thickness in the range of about 0.001 " to 0.050" and a width in the range of about 1 to 5 cm to provide a less than 1 % leakage of an argon-methane readout gas out of the chamber, and less than 1 % leakage of oxygen into the chamber over a 30 year period for a chamber having a length of about 0.5 m, a width of about 1 m, and a height of about 1 cm. In some embodiments, the polyisobutylene seal 124 has a thickness of 1.5 cm, a total surface area of 30cm ² , and maintains an oxygen leak rate into the chamber of about 1.3E- 10 cm3.cm/(s.cm2.cm-Hg). A leak rate of about 1 .3E- 10 cm3.cm/(s.cm2.cm-Hg) leads to an oxygen concentration of about 0.75% by volume for a 5000 cm ³ volume chamber after 30 years of operation. In some embodiments, the seal 124 can be any non-rigid polymeric material that can seal the chamber and maintain the readout gas composition over a period of 30 years.

In some embodiments, the spacer frame 120 can be made of stainless steel. In some embodiments, the spacer frame 120 can be made of aluminum. In some embodiments, the spacer frame 120 can have a lip on a top and bottom side that presses up against the top plate 104 and the bottom plate 108. In some embodiments, the readout gas is 100% argon. In some embodiments, the readout gas is 90% argon and 10% quenching gas such as carbon dioxide or methane. In some embodiments, the voltage bias of the wires 1 16 is increased over time to compensate for changes in readout gas composition, with a 1 % change of the bias voltage causing a 15% change in the voltage output signal.

In some embodiments, the lithium foils 1 12 are suspended on a mesh inside of the detector. In some embodiments, the height of the spacer can be 1 cm, the length and width of the top plate and the bottom plate can be 48 inches and 30 inches, respectively, and the wires can be spaced apart by about 4 cm from one another. In some embodiments, the wires 1 16 pass through the top plate 104 and the bottom plate 108 and not the spacer frame 120. The top plate 104 and the bottom plate 108 can be made of glass such as soda-lime or borosilicate glass.

In some embodiments, the bottom plate 108 is pressed into a tray-like form and sealed to the top plate 104 to form a chamber, without the use of the spacer frame 120. The removal of the spacer frame 120 from the chamber removes one sealing surface, thereby reducing the geometrical requirements of the seal 124.

FIGS. 2A-2B shows a flat panel neutron detector 200. Flat panel neutron detector 200 includes similar elements as flat panel neutron detector 100 (e.g., flat panel neutron detector 200 includes a spacer frame 220 which is similar to the spacer frame 120 of flat panel neutron detector 100). Flat panel neutron detector 200 also includes an array of elongate structural members 250. The elongate structural members 250 have a top side 264, a bottom side 268, and a slot 272. The elongate structural members 250 extend between the top plate 204 and the bottom plate 208, providing structural support to reduce mechanical vibrations. For example, a tungsten wire having a 100 cm length and a 30 micron diameter, stretched with 250g of force has a first vibrational frequency of about 200 Hz, which corresponds to significant vibrations generated by vehicles. By placing a single structural support near the middle of the tungsten wire, the first vibrational frequency increases to 420 Hz, thereby reducing by a factor of 100 or more vibrations induced by vehicular movement.

Reducing vibrations in radiation detectors with surface areas greater than 1000 cm ² is important because as the surface area of the chamber and the length of the readout wires is increased, the increased dimensions can lead to vibrations that cause changes in the relative position of the 21 6 readout wires and the lithium foils 212. Relative changes in position between the readout wires 216 and the lithium foils 212 can cause movement of charge in the flat panel neutron detector 200 which produces voltage output signals that cannot be distinguished from gamma ray or neutron signals - this phenomenon is known as microphonics. The elongate structural members 250 can reduce mechanical vibrations of the top plate 204 and the bottom plate 208 by providing a mechanical connection therebetween. For example, adding a support at the center of a 1 x 1 meter plate increases the resonance frequency by more than a factor of two and reduces the maximum amplitude of the vibrations by a factor of two compared to when the plate is supported only at its edges. The shape of the elongate structural members can be chosen to minimize vibrations between the top plate 204 and the bottom plate 208 (e.g., the cross section of the elongate structural members 250 can be a "T", an "I", an "L", or a "0")· In some embodiments, the cross section of the elongate structural members can be rectangular. Each of the readout wires 216 can pass through the slot 272 of at least one elongate structural members 250 to reduce mechanical vibrations in the readout wires 216. The slots 272 can provide mechanical support for the readout wires 216. In some embodiments, the slot 272 is located near a side edge of the elongate structural member 250. A wire 216 traversing the chamber can pass through multiple slots 272. The elongate structural members 250 can be positioned within the chamber such that the slots 272 alternate sides as the wire 216 traverses the chamber. For example, the wire can pass through a first slot located on the right side of a first elongate structural member, a second slot located on the left side of a second elongate structural member, and a third slot located on the left side of a third elongate structural member. In some embodiments, the elongate structural members 250 are non-conductive. In some embodiments, the slot 272 can be positioned to provide an upward or downward force on the wire 216. In some embodiments, the wires 216 are supported by a structural member that attaches to the top plate 204 or the bottom plate 208, but not both. In some embodiments, the elongate structural members 250 contact the top plate 204 and the bottom plate 208 but do not include a slot 272 and are displaced from the readout wires 216 so as to not cause a mechanical interference.

FIGS. 3A-3D show histogram outputs associated with a gamma ray and neutron detector (e.g., detector 100 or detector 200) over a period of time (e.g., a 30% thermal neutron efficient 1 m ² detector collecting background radiation for 30 minutes). FIG. 3A shows a number of counted events plotted against output voltage pulse height. FIGS. 3B-3D shows a number of counted events plotted against readout channel, with each readout channel corresponding to a different pulse height. Each readout channel corresponds to a grouping of one or more readout wires, together with amplification circuitry (e.g., each grouping can include two readout wires and an amplifier).

FIG. 3A shows a combination of three types of events inside of the chamber, separated into individual histograms as shown in FIGS. 3B-3D. When combined, the histograms shown in FIGS. 3B-3D form the histogram shown in FIG. 3A. FIG. 3B shows noise events created by movements of charge in the neutron detector induced by a source other than a radiation event in the detector, including thermal heat noise in the circuit, impinging radio frequency

electromagnetic radiation, and changes in the relative position of the wire and the lithium foil surface— known as microphonics. These events correspond to pulse heights below pulse height 305 as shown in FIG. 3A. FIG. 3C shows events created by gamma rays interactions with the walls of the chamber and the readout gas through Compton scattering, the photoelectric effect, / and pair production. These events correspond to pulse heights greater than pulse height 305 and less than pulse height 306 as shown in FIG. 3A. FIG. 3D shows events created by neutron interactions with the lithium foils, and the resulting triton and alpha particles ionizing the readout gas. These events correspond to pulse heights greater than pulse height 307 and less than pulse height 308 as shown in FIG. 3A. The measured noise, gamma ray, and neutron responses can be maximally separated so as to maximize the count rate of the measurements of gamma rays and neutrons, and to minimize the overlap error in the measurements by proper design of the neutron detector as described below. Increasing the voltage bias on the readout wires will move the pulse heights of both the gamma ray and neutron signal away from the electronics noise. For example, for a 1 cm readout gas thickness, a readout gas comprising 90% Argon and 10% methane, and 50 micron diameter readout wire, a voltage of 1400V applied between the lithium foils and readout wires is acceptable to measure gamma ray induced events at an efficiency above 1 %, and to measure neutron events with a gamma ray rejection ratio of below 10 ^"5 .

Additionally, by reducing the number of readout wires that are read out on a given amplification channel, the electrical noise is reduced to further separate the gamma ray signals from the noise events. For example, reading out 800 cm ² of detector area, which can be performed by ganging 4 wires into a single amplification channel where the wires are spaced 2 cm apart and spanning 100 cm of detector length, provides an acceptable amount (approximately 50-100 pF) of input capacitance to delineate gamma ray events from noise events. Furthermore, by increasing the height of the gas region, a larger amount of ionization from tritons and alphas exiting the lithium foils occur, with a non-equivalent increase in gamma ray ionization, thereby providing greater separation between the gamma ray and neutron signals. For example, a 1 atmosphere, 1 cm wide readout gas composed of 90% Argon and 10% Methane located between the lithium foils is acceptable to delineate gamma ray events from neutron events with a false positive detection rate of neutrons of less than 10 ^"5 for a gamma ray exposure rate of l OOmR/hr.

The terminology used herein is for the purpose of describing particular embodiments and is not intended to be limiting of the inventive concepts. It will be understood that, although the terms first, second, third etc. are used herein to describe various elements, components, regions, layers and/or sections, these elements, components, regions, layers and/or sections should not be limited by these terms. These terms are only used to distinguish one element, component, region, layer or section from another element, component, region, layer or section. Thus, a first element, component, region, layer or section discussed below could be termed a second element, component, region, layer or section without departing from the teachings of the present application.

[001 ] While the present inventive concepts have been particularly shown and described above with reference to exemplary embodiments thereof, it will be understood by those of ordinary skill in the art, that various changes in form and detail can be made without departing from the spirit and scope of the present inventive concepts described and defined by the following claims.