Abstract:
A thermal neutron detector includes a planar detector housing having two glass panels spaced apart by a gas-tight seal defining a detection chamber. Lithium foils adhered to inner surfaces of the glass panel emit alpha particles and tritons in response to incident thermal neutrons, and an inert detection gas is ionized to generate drift electrons. A planar array of detection wires is spaced from the lithium foils and extends outside the detection chamber. Electronic bias applies a field voltage between the wires and the lithium foils to establish a drift electric field to attract the drift electrons to the wires and provide a large electric field to cause electron multiplication. Detection circuitry detects electric signals in the wires generated by the drift of positive ions away from the wires and interprets the electrical signals as incidence of thermal neutrons on the detector.

Description:
STATEMENT OF FEDERAL RIGHTS 
     This invention was made with US Government support under the following contracts. The Government has certain rights in the invention. 
     1. CBET-1036066 National Science Foundation 
     2. 2011-DN-077-ARI052-02 Department of Homeland Security 
    
    
     BACKGROUND OF THE INVENTION 
     A major effort to reducing the risk of a nuclear catastrophe is to deploy radiation detectors within a larger security architecture. One important component of radiation detection is neutron detection, since neutron detectors can aid in the detection of fissile material such as plutonium, and are components of systems that perform active interrogation to detect materials such as highly enriched uranium (HEU). The standard method for neutron detection uses  3 He gas within a tube and signals read out by a central high voltage wire. The combination of high probability for the neutron/ 3 He interaction, full energy readout from the reaction, and the simple tube geometry makes the method a gold standard of neutron detection. 
     The limited supply of  3 He and the demand on its use for other activities such as medical imaging and low-temperature research has limited the number of neutron detectors that can be deployed, however one may argue that cost and scalability have not played a large role in the design of  3 He replacement technologies. This may limit the ability to expand to larger deployments of thermal neutron detection beyond the task of replacing  3 He units. The present invention provides cost effective neutron monitoring which can be used at a very large number of trade routes throughout the world, including shipping, air, and rail travel. The goal is to make a large dent in the completion of a global security architecture by increasing the amount of  3 He equivalent neutron detectors by ten to one hundred fold at a fraction of the cost of other technologies. 
     Slow neutrons can be detected with high efficiency by a few special detector materials.  6 Li has a cross section of 940 barns for thermal neutrons in the reaction equation  6 Li+n→α+ 3 H. 4.786 MeV of kinetic energy is given off by the alpha (α) and triton ( 3 H) as they travel outward back to back from the reaction point. The mean free path for thermal neutron capture in enriched lithium is 229 microns, whereas the distances that the alpha and triton particles travel with the 4.786 MeV of kinetic energy received by the reaction are 23.3 and 135 microns respectively. 
     U.S. Pat. No. 4,365,159 by Young et al. shows the use of thin sheets or foils of lithium in a gas-filled multi-wire proportional chamber (MWPC). The lithium foil is sufficiently thin so that large particles created by reactions between incident neutrons and lithium nuclei escape the foil with sufficient energy to cause ionization of a detection gas in the chamber. Drift electrons from the ionizations are attracted to an array of wires in the chamber biased at a high voltage, and the resulting signals induced on the wires are detected and processed by electronic circuitry. 
     U.S. Pat. No. 4,447,727 of Friesenhahn et al. shows a large area neutron proportional counter constructed utilizing a large sealed metal box. The interior walls of the box are coated with  6 Li enriched metal. A multicelled proportional counter structure within the internal space defined by the box is fabricated using a hydrogenous plastic. 
     US patent application publication 2006/0138340 of Ianakev et al. shows detector for detecting neutrons and gamma radiation that includes a cathode that defines an interior surface and an interior volume. A conductive neutron-capturing layer is disposed on the interior surface of the cathode and a plastic housing surrounds the cathode. A plastic lid is attached to the housing and encloses the interior volume of the cathode forming an ionization chamber, into the center of which an anode extends from the plastic lid. A working gas is disposed within the ionization chamber and a high biasing voltage is connected to the cathode. Processing electronics are coupled to the anode and process current pulses which are converted into Gaussian pulses, which are either counted as neutrons or integrated as gammas, in response to whether pulse amplitude crosses a neutron threshold 
     BRIEF SUMMARY OF THE INVENTION 
     Prior slow neutron detectors such as those described above include certain limitations and drawbacks that are preferably avoided. One challenge, for example, is to seal the detection chamber and to secure the lithium within it in such a way that the lithium does not react with the detector materials or otherwise degrade from exposure to other substances. Other challenges relate to the readout circuitry and the need for high discrimination between signals generated by slow neutrons versus noise from a variety of sources, including gamma radiation and microphonic effects from the tensioned detection wires. 
     A disclosed detector employs large, thin sheets or foils of lithium in a gas-filled multi-wire proportional chamber (MWPC). The lithium foil is sufficiently thin that large particles created by reactions between incident neutrons and lithium nuclei escape the foil with sufficient energy to cause ionization of a detection gas (e.g., argon) in the chamber. Drift electrons from the ionizations are attracted to an array of wires in the chamber biased at a high voltage, and the resulting signals induced on the wires are detected and processed by electronic circuitry to infer the incidence of neutrons from patterns of electrical signals appearing on the wires. 
     More particularly, a disclosed thermal neutron detector includes a planar detector housing having two glass panels spaced apart by a gas-tight seal to define a detection chamber, where the detection chamber includes an inert detection gas and one or more lithium foils each adhered to an inner surface of a respective glass panel. Each lithium foil emits alpha particles and tritons into the detection chamber in response to incidence of thermal neutrons, and the inert detection gas is ionized by the particles to generate drift electrons. The detector housing further includes a planar array of wires spaced apart from the lithium foils and having external portions extending outside the detection chamber through the gas-tight seal. 
     Electronic bias and detection circuitry is connected to the external portions of the wires, and it is operative during use of the neutron detector to (a) apply a field voltage between the wires and the lithium foils to establish a drift electric field in the detection chamber effective to attract the drift electrons to the wires, (b) provide a large electric field near the anode wires to cause electron multiplication near the wire resulting in large numbers of electrons and positive ions for each initial electron, and (c) detect electric signals in wires generated by the drift of positive ions away from the wires and interpret the electrical signals as incidence of thermal neutrons on the detector. 
     Among the advantages of the disclosed detector, it employs glass panels and other technology similar to those used in the window industry, and thus can leverage cost, design, durability, performance, and other characteristics of that technology. Additionally, several design features allow for both the long-term (more than one year) use of the argon fill gas without the need for recycling or flushing the gas. The first is the use of low outgassing materials for the construction of the chambers, such as low outgassing epoxy, stainless steel, aluminum, and glass. Secondly, desiccant strips, which collect gas impurities such as water and oxygen, are integrated into the edges of the detector wall. Lastly, designing the proportional gas multiplication to be on the order of 100-200 eliminates the need for quenching additives, which can create impurities over long term use. The creation of gas multiplication of about 100-200 allows for less expensive pre-amplification to be used. For example, within such a gas multiplication environment, using preamplifiers that create a 1 mV signal per 1 fC of charge and a readout circuit which has an overall noise level of 10 keV to 50 keV allows for the proper identification of a neutron events within the chamber for a 100 keV or greater energetic alpha or triton particle exiting the lithium surface into the readout gas. Such a threshold provides discrimination of other ionization events such as those caused by gamma rays that might be a source of false positive indications, and therefore allows for a maximal efficiency of the detector geometry. 
     The present invention provides a number of beneficial aspects including but not limited to the following. 
     An aspect of the invention is a radiation detector that is implemented as a proportional chamber which balances the low-noise requirements of the preamplification circuitry with an amount of gas multiplication such that there is a 100 keV threshold for ionizing radiation detection. 
     Another aspect of the invention is to create a long-term (more than one year) gas multiplication environment using argon fill gas without the need for recycling or flushing the gas by minimizing the voltages and gas gains required to less than 1300V and 250 respectively, so as not to employ age inducing quenching gas additives. 
     Another aspect of the invention is to create a long-term (more than one year) gas multiplication environment using argon fill gas without the need for recycling or flushing the gas by selection of low outgassing materials as the sole contact structures to the lithium and the internal gas volume of the detector. 
     Another aspect of the invention is to create a long-term (more than one year) gas multiplication environment using argon fill gas without the need for recycling or flushing the gas by providing a desiccant chamber in high volume contact with the gas volume as a getter to clean the gas. 
     Another aspect of the invention is to provide alternating ground wires between each high voltage anode wire to shape the ionization events occurring from neutron interactions with the detector so that they are more distinguishable from noise and gamma ray induced signals. 
     Another aspect of the invention is to use the digitized pulse that comes from the summation of preamplified signals or a single preamplified signal to delineate noise and gamma ray induced signals from events induced by neutrons. 
     Another aspect of the invention is the use of spray epoxy to secure the layer of lithium to the flat walls of the detector such that the outgassing of the flat wall does not destroy the lithium. 
     Another aspect of the invention is the use of glass as the material for the flat wall to reduce cost of the detector and provide a more rigid, non-vibrational surface to adhere the lithium. 
     Another aspect of the invention is to use a conductive metallic bus whose internal side is in contact with the lithium and external side is electrically connected to the ground of the circuit. 
     Another aspect of the invention is the use of low outgassing epoxy whose outgassing properties do not react adversely to the lithium. 
     Another aspect of the invention is that this low outgassing epoxy provide the connective substance in the thicknesses appropriate for full set strength of the metal and glass parts that are the boundaries for the gas volume of the detector. 
     Another aspect of the invention is that the epoxy provides the feedthrough for the field shaping and anode wires to pass from the internal gas volume to the external electronics portion of the detector. 
     Another aspect of the invention securing the field shaping and anode wires in a tensioned state with the epoxy that also serves as the gas seal for the detector. 
     Another aspect of the invention is the manufacturing procedure for the detectors that involves stacking, epoxy curing, laying down of pre-tensioned wires that lead to a fully sealed detector. 
     One of the beneficial features of the panel form factor is its scalability. It is not necessary to make a larger single-panel detector to provide detection over a larger area—rather, multiple smaller-size detectors can be arranged to cover the area and their outputs monitored by a central processing component. Additionally, flat glass panels may be more easily used in applications where anonymity of the detector is desired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other objects, features and advantages will be apparent from the following description of particular embodiments of the invention, as illustrated in the accompanying drawings in which like reference characters refer to the same parts throughout the different views. The drawings are not necessarily to scale, emphasis instead being placed upon illustrating the principles of various embodiments of the invention. 
         FIG. 1  is a schematic depiction of the principles of neutron detection according the present invention; 
         FIG. 2  is a quasi-schematic side view of a detection assembly; 
         FIG. 3  is a quasi-schematic top view of the detection assembly; 
         FIG. 4  is a quasi-schematic top view of a detection assembly according to another embodiment; 
         FIG. 5  is a quasi-schematic side view of an edge portion of the detection assembly of  FIG. 4 ; 
         FIG. 6  is a schematic diagram of detection circuitry; 
         FIG. 7  is a flowchart of operation; 
         FIG. 8  is a perspective view of a detector assembly according to one embodiment; 
         FIGS. 9-11  are isometric section views of portions of the detector assembly of  FIG. 8 ; 
         FIG. 12  is a perspective view of a detector assembly according to another embodiment; 
         FIGS. 13-15  are isometric section views of portions of the detector assembly of  FIG. 12 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  is a schematic illustration of the detector and its operation. Two flat pieces of enriched lithium ( 6 Li) metal foil  10  are placed a small distance apart (e.g., 1 cm), with a single element gas like argon between the foils  10 . A plane of wires  12  is positioned in the gas-filled gap between the foils  10 , the wires being spaced apart at a pitch on the order of 5 mm. The wires alternate from ground to high voltage anode collection wires  12   a  and  12   b . This creates a more uniform field around each high voltage wire which provides for a shape for the signals generated from neutron interactions with the detector that is more distinguishable from noise and gamma ray induced signals. In the illustrated embodiment, the wires  12  are alternating ground wires and wires maintained at a high positive voltage, ranging from 750V to 1300V, with respect to the Li foils  10 , such as by a high-voltage DC power supply  14 . 
     In operation, thermal neutrons n strike the foils  10  and react with Li nuclei, each reaction creating an alpha particle (α) and a triton ( 3 H) traveling in opposite directions. One of these particles escapes from the lithium foil  10  and enters the gas-filled gap, where it has ionizing collisions with atoms/molecules of the gas. Drift electrons (e − ) produced by the ionization are attracted to the anode wires  12   b  where they produce electrical signals in the form of small-amplitude voltage pulses. Detection circuitry (DET)  16  amplifies these signals, sums the amplified signals from multiple wires, and performs additional processing on the summed amplified signals to detect a “signature” (predetermined pattern of voltage pulses) of a thermal neutron. The detection circuitry  16  design as well as the localized lithium product ionization allow good discrimination between neutrons and gamma rays. 
     For efficient operation it is important that the foils  10  be sufficiently thin that a high percentage of the particles created by incident neutrons escape the foil  10  and enter the gas region. One of the benefits of lithium is the ability to roll or extrude it to a desired thinness, which for this application is about 100 microns. 
       FIG. 2  is a quasi-schematic side or edge view of a detector assembly  20  built and operable according to the general scheme of  FIG. 1 . It includes two spaced-apart planar glass panels  22  with a gas-tight edge seal  24  at an outer periphery, the seal  24  and panels  24  defining a chamber  26  containing an inert gas such as argon. The chamber  26  may be at 1 atmosphere absolute pressure. The inner surface of each glass panel  22  has a respective lithium foil  10  adhered thereto, and the anode wires  12  extend across the chamber  26  between and spaced from the foils  10 . As shown, endmost portions of the wires  12  extend outside of the chamber  26  through the seal  24  and connect to bias and detection circuitry (BIAS/DETECT)  28  which provide the functions of the power supply  14  and detection circuitry  16  of  FIG. 1 . Additional electric feedthroughs  30  attach to the foils  10  as cathode-side connections (see connections of power supply  14  in  FIG. 1 ). 
       FIG. 3  is a quasi-schematic plan view (e.g., top view) of the detector assembly  20 . As shown, it has a generally rectangular shape. A cutaway view at upper left reveals the seal  24  extending around the periphery, indicated by a dotted line outside the cutaway area. Also revealed are the wires  12 . The bias/detection circuitry  28  and cathode electrical feedthroughs  30  are omitted in the view of  FIG. 3 . 
     The foils  10  may be adhered to the glass panels  22  using a non-reactive, low-outgassing adhesive such as an epoxy. Conventional paste epoxy may be used, or a spray epoxy might be used in the alternative. One undesirable feature of glass is that it typically contains some measure of water, which in this application could adversely react with the lithium foils  10  causing premature degradation. The glass can be “baked out” by heating in a dry environment or stored in a dry environment prior to detector manufacturing in order to minimize water outgassing. In addition, the epoxy adhesive preferably isolates the foil  10  from the glass panel  22  to minimize direct exposure to the glass water outgassing. Also, the assembly  20  preferably employs some type of desiccant to draw water vapor out of the chamber  26 . Desiccant may be incorporated into the seal  24  for example, similar to the arrangement used in double-paned windows. 
     It is generally desirable to avoid use of any reactive materials in the chamber  26  to avoid degradation of the lithium foils  10  and prolong the usable life of the detector  20 . In the illustrated embodiment, the contents of the chamber  26  are limited to non-reactive materials such as the gold-plated wires  12 , glass, stainless steel, aluminum, and low-outgassing epoxy. Specifically excluded from the chamber  26  is the circuitry  28 , which will typically include more reactive materials including the “FR4” material commonly used in printed circuit boards. 
       FIGS. 4 and 5  illustrate a detector assembly  40  generally similar to that of  FIGS. 2 and 3 , except that it employs a rigid non-glass frame  42  around the periphery of the glass panels  22 . The frame  42  provides greater structural robustness as well as providing an electric feed-through  30  through which the cathodic lithium strips are connected to the ground of the circuitry  48 . It may also provide features for mounting the detector assembly  40  as may be required during installation. Generally, a strong and non-reactive metal, such as stainless steel or aluminum, is preferred for the frame  42 . 
       FIG. 5  is a side view illustrating the configuration of a particular embodiment of the detector assembly  40  at one end. The frame  42  has two pieces, an upper frame piece  42 -U and a lower frame piece  42 -L, each having an L-shaped notch or groove extending around its inner edge, and each glass panel  22  is adhered to a respective frame piece  42 -U,  42 -L via a bead of adhesive  44  such as epoxy. A pair of insulating spacers  46 , preferably of glass, are used for passage of the wires  12  to circuitry  48 , which will generally include at least a preamplifier portion of the detection circuitry  16  (see further description below). The adhesive that acts as the seal between the insulating spacers  46  also maintains the tension of wires  12  and acts as a feed-through for the wires  12 . In order to maximize the joint strength between the wires  12  and the insulating spacers  46 , the adhesive must feely flow completely around the wire before curing. This is allowed by etching a groove along the insulating spacers  46  that the wire is flush against. A separate rigid substrate  52 , such as of stainless steel or aluminum, extends away from the frame  42  and provides support for the circuitry  48 . As shown, the spacers  46  and substrate  52  are arranged in a stack, adhered to each other and to the frame pieces  42 -U,  42 -L via beads of adhesive  44 . 
     Although in  FIG. 5  the substrate  52  is shown as an individual element, in an alternative embodiment it may be formed as an extension of the lower frame piece  42 -L. 
     One of the concerns for MWPC detectors with low energy thresholds is so-called “microphonics”, the phenomenon of unwanted electrical noise induced by mechanical vibration (specifically of the wires  12 ). Microphonics can be addressed by proper mechanical design that increases mechanical resonant frequencies above typical ambient vibration frequencies. In one embodiment, the wires  12  may be gold plated tungsten-rhenium wires of approximately 50 microns in diameter. Wires of this type can be safely stretched to a tension of 450 grams without risk of breakage. Analysis shows that 30-cm wires of this type and tension have a fundamental resonant frequency of about 600 Hz, which may provide acceptable performance in normal operating environments. If necessary, the wires  12  can be stabilized using supports placed inside the chamber  26  from materials that do not generate unwanted outgassing. Such stabilization may be particularly desirable for larger detectors using correspondingly longer wires  12 . 
       FIG. 6  illustrates an embodiment of the detection circuitry  16 , also referred to as “readout” circuitry. The readout circuitry  16  generally includes sets  60  of front-end circuitry, each set  60  generating a respective output provided to processing circuitry (PROC CKTRY)  62  which is described below. In many applications, it is not necessary to identify the specific direction or position information of incident neutrons. In these cases, it is not important to know which specific wire(s)  12  produce signals from drift electrons created by the triton or alpha particles entering the readout gas, nor the location of a track of electrons along a wire  12 . The readout circuitry can therefore be designed to maximize the efficiency of detecting neutrons within the MWPC geometry. Neutron detection efficiency is maximized by setting the lowest energy limit, so that all signals above the limit can be attributed to neutrons. The lower energy limit is affected by several types of noise, as well as gamma ray radiation background signals. The noise that contributes to the energy limit is thermal noise, radio frequency noise, amplification circuit noise, and signal digitization noise. The purpose of the set-based organization is to minimize the lower energy limit for a given detector geometry (detector length and width). Additionally, it may be desirable to connect multiple wires  12  together before a first preamplifier of a set  60  (so called “wire ganging”) to reduce pre-amplification and summing amplification noise as well as cost. Drawbacks of wire ganging include increased input capacitance and susceptibility to radio-frequency (RF) noise, which increasing the lower energy limit. Radio-frequency (RF) noise can be addressed through use of known RF shielding techniques including use of a Faraday. Combining more wires together into a single set  60  also reduces cost but increases the lower energy limit due to non-localized gamma-ray ionization events. For each detector size and cost, there will be an optimal arrangement of wire ganging, preamplifier summing, and digitization. 
     Each set  60  receives a number of wire gangs  64 , each having one or more of the wires  12  directly connected together (where the degenerate case of one wire per gang is equivalent to no ganging). The wire gangs  64  are connected to inputs of respective preamplifiers  66 . In the illustrated example, each gang  64  includes three wires  12 , and each set  60  includes four preamplifiers  66  for four wire gangs  64 . Generally, the number of sets  60  is W/(PG), where W is the total number of wires  12  whose signals are to be processed by the processing circuitry  62 , P is the number of preamplifiers  66 , and G is the number of wires per gang  64 . The outputs of the preamplifiers  66  are provided to a summing amplifier  68  whose output is proportional to the sum of the signals from the preamplifiers  66 . The output from the summing amplifier  68  is provided to an analog-to-digital (A/D) converter  70 . Digital signal samples from the A/D converter  70  are provided to the processing circuitry  62 . 
     The processing circuitry  62  may be implemented in a variety of ways. In one embodiment, it can be realized using a complex programmable logic device or CPLD, programmed to perform sample acquisition and front-end signal processing, sending the resulting signal data to a separate processor or computer for higher-level processing. The processing circuitry  62  may utilize a separate memory  72  for storage of data, as well as storage of executable instructions of an operational program in the case that the processing circuitry  62  is a stored-program processor such as a microprocessor or microcontroller. Either the processing circuitry  62  or separate higher-level processor may employ timing and logic circuitry to avoid any undesirable double counting of tracks from the same neutron-induced ionization event. 
       FIG. 7  illustrates the general manner of operation of the detection scheme described herein. At  80 , a suitably high DC bias is applied to the anode wires  12  with respect to the lithium foils  10  to create a sufficiently strong electric field to direct the drift electrons from ionization events to the anode wires  12 . At  82 , the signal pulses produced on the anode wires  12  are received and conditioned at the detection circuitry  16 , where “conditioned” refers to the preamplification, summing, etc. performed by the sets  60 . At  84 , the conditioned pulse signals are analyzed in a desired manner to detect signal patterns corresponding to ionization events, as distinct from gamma ray events or other potential noise, by either pulse height or shape characteristics. 
     The above-discussed discrimination is included in the analysis of step  84 . At  86 , the result of the analysis at  84  is used to assert detection of incident neutrons, such assertion being usable in some system-specified manner. For example, the assertion might be used to activate a security alarm in applications for detecting smuggled radioactive material. 
       FIGS. 8-15  show two alternative arrangements for detector assemblies. Generally these differ from the arrangement of  FIG. 5  in that the frame pieces are located at the inner surfaces of the glass panels rather than at the outer surfaces as in  FIG. 5 . This enables certain advantageous features as described below. 
       FIG. 8  shows an embodiment of a detector assembly  100  in which the glass panels  102  (note the use of -U to designate “upper” and -L to designate “lower”) are on the outside of a rigid metal frame  104  forming the complete peripheral edge. Among the features is a pair of desiccant strips  106  along two edges, which as described below are in fluid communication with the internal chamber to reduce the level of water vapor present in the chamber. Also visible in  FIG. 8  are outer portions of anode wires  111  that extend through a shallow opening at one end of the frame  104 . 
       FIG. 9  shows the detector assembly  100  in exploded view. The frame  104  has two pieces, an upper frame piece  104 -U and a lower frame piece  104 -L, each having a notch or groove  108  extending along two side edges to hold the desiccant strips  106 . Each frame piece  104  also has a slightly depressed front edge section  109  to form a shallow opening for passage of the anode wires  111 . The frame pieces  104  are joined by epoxy  110 . Each glass panel  102  is an outer member adhered to an outer face of a respective frame piece  104  by respective epoxy  112 . To the inside of each glass panel  102  is adhered a respective set of lithium strips  114  in a planar layer (either abutting or with slight overlap among adjacent strips) as shown. 
       FIGS. 10 and 11  show certain additional features of the detector assembly  100 .  FIG. 10  represents a section across a central part of the left edge in  FIG. 8 . As shown, each frame piece  104  has a respective L-shaped inner portion  120  that extends into the chamber and is in direct contact with the respective lithium strips  114 . An alternative embodiment is to replace the L-shaped inner portion  120  with low-outgassing conductive epoxy. Either of these arrangements places the lithium strips  114  at the same electrical potential as the frame  104 . This bias/detection circuitry  28  ( FIG. 2 ) is referenced to the potential of the frame  104 , taken as a system ground. This ensures that the proper bias voltage is developed between the anode wires  111  and the lithium strips  114  in operation. 
       FIG. 11  represents a section along the front edge in  FIG. 8 , showing the shallow opening  130  through which the anode wires  111  pass. The epoxy  110  provides the feedthrough for the anode wires  11  to pass from the internal gas volume to the external electronics portion of the detector. 
       FIG. 12  shows a detector assembly  140  that relies more on the glass panels  142  themselves for structural strength and rigidity. It lacks unitary surrounding frame pieces such as the frame pieces  104  shown in  FIG. 10 , instead relying on metal side frame pieces and glass spacers at front and rear edges. 
       FIG. 13  shows the detector assembly  140  in exploded view. As mentioned, it employs two metal side frame pieces  144  along with front and back glass spacers  150  sandwiched between the glass panels  142 . Each frame piece  144  includes a lengthwise channel  148  to hold a desiccant strip (not shown). The channel  148  is in fluid communication with the chamber via a set of openings  146  extending from the channel  148  to an inner surface of the frame piece  148 . As in the other embodiments, the lithium strips  154  are adhered to the inner surfaces of the glass panels  142 . Epoxy  152  attaches each panel  142  to the frames  148  and glass spacers  150 . 
       FIG. 14  shows a section across a central part of the left edge in  FIG. 13 . The frame piece  144  includes L-shaped portions  160  that extend into the chamber and are in direct contact with the lithium strips  154  for grounding purposes as discussed above for  FIG. 10 . 
       FIG. 15  shows the front edge, specifically the arrangement of the epoxy  152  that bonds the glass spacers  150  to the glass panels  12  and seals the front edge while also providing for passage of the wires  111 . It will be noted that the upper epoxy  152  is slightly thicker and has the respective spacer  150 -U completely embedded in it, leaving a thin section  162  between the two spacers  150  through which the wires  111  extend. 
     The detector assembly of  FIGS. 12-15  may enjoy a cost advantage over that of  FIGS. 8-11  by avoiding the need for the large, machined frame pieces  104  that completely surround the assembly. 
     Detectors according to the present invention may be manufactured in a number of alternative ways, the following being described by way of example and not limitation. 
     The following steps comprise a manufacturing process 
     (a) laying down the first piece of lower glass within a dry environment, 
     (b) spraying epoxy on the top surface of the glass, 
     (c) placing lithium foil on top of the epoxy to form a surface, 
     (d) epoxying glass to the frame; enabling conductive electrical bus feed-through of the frame to make electrical connection with the lithium, 
     (e) performing steps (a)-(d) on top surface as well, 
     (e) placing desiccant strip with molecular sieve desiccant internal into the lower holder of the frame, 
     (f) epoxying circuit board with electronics on outward support, awaiting wires, 
     (f) suspending the wires in proper position above the lower frame so that they are 5 mm away from lower lithium surface and flush with their contact points on the circuit board, 
     (g) epoxying the top frame and bottom frame together, creating the main seal and feedthrough for the wires, 
     (f) soldering wires onto the connective pads on the circuit board and cut wire off of tensioning frame, 
     (g) flowing gas into system through the desiccant strip channels until full epoxy dry, 
     (h) place desiccant strips within the channels, 
     (i) sealing desiccant strip hole with epoxy and metal plate. 
     While various embodiments of the invention have been particularly shown and described, it will be understood by those skilled in the art that various changes in form and details may be made therein without departing from the spirit and scope of the invention as defined by the appended claims.