Abstract:
A neutron detection apparatus is provided which includes a selected numberf flat surfaces of lithium-6 foil, and which further includes a gas mixture in contact with each of the flat surfaces for selectively reacting to charged particles emitted by or radiated from the lithium foil. A container is provided to seal the lithium foil and the gas mixture in a volume from which water vapor and atmospheric gases are excluded, the container having one or more walls which are transmissive to neutrons. Monitoring equipment in contact with the gas mixture detects reactions taking place in the gas mixture, and, in response to such reactions, provides notice of the flux of neutrons passing through the volume of the detector.

Description:
STATEMENT OF GOVERNMENT INTEREST 
     The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor. 
    
    
     BACKGROUND OF THE INVENTION 
     The invention disclosed and claimed herein pertains to the field of neutron detection devices of the type which employ lithium-6, in a solid form, to respond to neutrons by radiating charged particles into an ionizable counting gas. More specifically, the invention pertains to detection devices of the above type wherein the lithium radiator is configured to provide one or more flat surfaces of lithium-6 foil in contact with ionizable gas, each surface being in spaced parallel relationship with an array of high voltage count wires which indirectly measure neutron activity by sensing pulses of gas ionization. Even more particularly, the invention pertains to detection devices of the above type wherein a plurality of flat lithium-6 foil surfaces may be stacked in parallel layers with one another, and with arrays of counting wires, in order to provide a neutron detector which is of very high sensitivity and which is yet capable of being contained in a package which is extremely portable, compact, and durable. 
     At present, most high-sensitivity neutron detectors of the radiator-ionizable gas type employ either  10  BF 3  or  3  He, in a gaseous state, as the radiator medium for the detector, i.e., for the detector component which receives neutrons and which radiates ionizing particles in response thereto.  3  He is always in a gaseous state at practical temperatures and pressures.  10  BF 3  must be employed in a gaseous state, since the principal ionizing particle which results from the reaction between a neutron and a boron nucleus of  10  BF 3  is an alpha particle, which is of extremely short range (e.g., 5×10 -3  mm). If a reaction generating an alpha particle were to take place within a solid material, the dimensions of the material would have to be extremely small, to prevent the alpha particles from being trapped therewithin. 
     Because of the low density of  10  BF 3  and  3  He at ordinary pressures, they must be contained in chambers of large volume in order to be used as the radiator component in a neutron detector. Consequently, such detectors tend to be comparatively large or bulky. While neutron detectors are available which have used a solid layer of  10  B as an alpha particle radiator, the layer must be kept very thin, as aforementioned, (e.g., 10 -2  mm) and it may still be necessary to supplement the  10  B radiator with one of the above gaseous radiator components. 
     In the past, solid lithium-6 ( 6  Li) has been used as the neutron sensitive component in a radiator-ionizable gas neutron detector, wherein the  6  L i  is coated upon the curved inner surface of a cylinder. Note, for example, U.S. Pat. No. 2,721,944, issued Sept. 9, 1950, which discloses a neutron detector for use in geological exploration of oil fields. However, the Applicant has found that if, instead of such curved arrangement, a number of flat sheets of  6  L i  are employed in a neutron detector, it becomes possible to provide a substantial reduction in detector size and to increase the ruggedness thereof, and yet provide high detection sensitivity. The Applicant uses flat sheets of  6  L i  so that the sheets may be stacked in parallel layers within a thin, flat container. By enabling a number of sheets to be enclosed in the container, high sensitivity to neutron detection is provided since the probablility that a neutron entering the container will encounter a lithium-6 nucleus is optimized. By making the detector flat and thin, a neutron striking the detector from almost any angle will pass into at least one of the  6  L i  sheets. Also, by stacking sheets in parallel relationship, the space between sheets, which is filled with ionizable counting gas, may be made so small that gamma rays generated within the detector can be prevented from being registered as neutron counts. High-sensitivity neutron detectors employing the principles of the present invention have been fabricated which are small enough to be held in an operator&#39;s hand, and to be carried in a coat pocket. 
     SUMMARY OF THE INVENTION 
     In the present invention, a neutron detection apparatus is provided which includes a selected number of flat surfaces of lithium-6 foil, and further includes a gas mixture in contact with each of the surfaces for selectively reacting to charged particles radiated from the lithium foil. A container means is provided for sealing the lithium foil and the reacting gas mixture within a volume from which water vapor and atmospheric gases are excluded, the container means having walls which are transmissive to neutrons. A monitoring means in contact with the gas mixture detects reactions in the gas mixture, and in response to detected reactions provides an output which represents the flux of neutrons passing through the detector volume. 
     Preferably, the lithium-6 foil comprises one or more flat sheets of lithium-6, the thickness of each sheet being substantially less than the range of a triton particle when such particle is traveling through lithium. The gas mixture comprises a counting gas which is readily ionized when charged particles pass through it, and the lithium foil and the counting gas are enclosed together within a flat, hermetically sealed container. Preferably also, the monitoring means includes an array of count wires which are maintained at a selected high voltage within the container, such array being in spaced parallel relationship with one of the sheets of lithium foil. An electronic processing means, located externally to the container, is coupled to the array of count wires. 
     It has been found that a flat sheet of lithium foil may be enclosed within the container by bonding it to a flat inner wall of the container, whereby one side of the lithium sheet is exposed to the counting gas and to the interior of the container. Alternatively, the sheet may be suspended within the interior of the container, so that both sides of the sheet are exposed to the counting gas. In a preferred embodiment of the invention, a neutron detector is formed by enclosing a plurality of flat lithium sheets within a flat container, some of the sheets being bonded to container walls and others being suspended. The sheets are stacked in spaced parallel relationship, an array of counting wires being positioned in spaced parallel relationship with each lithium surface which is exposed to counting gas. It has been found that by so configuring the lithium sheets and wire arrays, a neutron detector may be provided which has a very high neutron sensitivity and a low gamma-ray sensitivity, and which is yet enclosed in a container having a thickness of less than one inch. 
     OBJECTS OF THE INVENTION 
     An object of the present invention is to provide a lighter, more compact neutron detector of high sensitivity. 
     Another object is to provide a neutron detector which employs one or more flat sheets of lithium-6 foil to radiate ionizing particles into a counting gas in response to neutrons received by the lithium sheets. 
     Another object is to provide a detecting device which is of high sensitivity to neutrons and low sensitivity to gamma rays which may be contained in a small, flat package having a thickness of less than one inch. 
     Another object is to provide a detecting device of the above type in which flat sheets of lithium-6 foil are spaced apart in parallel relationship so that the thickness of a layer of counting gas between them is very small, whereby there is substantially less ionization of the gas from gamma-ray passing therethrough than from a charged particle generated by a neutron. 
     Other objects and many of the attendant advantages of this invention will be readily appreciated as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is an exploded perspective view showing an embodiment of the invention. 
     FIG. 2 is a schematic view for illustrating the principle of operation of the embodiment of FIG. 1. 
     FIG. 3 is an overhead view showing an array of count wires for the embodiment of FIG. 1. 
     FIG. 4 is a perspective view showing the embodiment of FIG. 1 fully assembled and coupled to monitoring electronics. 
     FIGS. 5 and 6 are schematic views which illustrate modifications of the embodiment of FIG. 1. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
     Referring to FIG. 1, there are shown various components which may be assembled to form three-layer lithium-6 neutron detector 10. Detector 10 includes flat top and bottom plates 12 and 14, respectively, which are usefully formed of brass or other material which is highly transmissive to neutrons n in proximity to detector 10. Detector 10 further includes upper and lower support frame sections 16a and 16b, respectively, which are likewise formed of brass, and which are provided with slots for respectively retaining ceramic strips 18a and 18b. 
     By means of a silk-screening and high-temperature firing process, for example, strips 18a and 18b are metallized. A number of count wires 20a (e.g., 44) are tensioned between and soft-soldered to metallized strips 18a in parallel relationship with one another, at a spacing which usefully is 1/8 inch. A like number of count wires 20b are similarly tensioned between and soft-soldered to ceramic strips 18b. Consequently, when detector 10 is assembled, upper and lower frame sections 16a and 16b support arrays of count wires 20a and 20b, respectively, in parallel relationship with one another and with top plate 12 and bottom plate 14. 
     In order to apply a selected high positive voltage (e.g. 1,000 V) to each count wire 20a, a cable 22a passes through frame section 16a, through a coupling 24a, and is connected to one of the metallized strips 18a. Similarly, a cable 22b passes through a coupling 24b to apply a high positive voltage to each count wire 20b. 
     Referring further to FIG. 1, there is shown a flat sheet or layer 26 of lithium-6 metal bonded to the under surface of top plate 12, and there is also shown a flat sheet 28 of lithium-6 bonded to the upper surface of bottom plate 14. Sheets 26 and 28 may be bonded to their respective plates by heating the plate to just below the melting point of lithium, and then rubbing lithium foil onto the surface of the plate with the aid of a large soldering iron. A quantity of  6  L i  foil is applied to a plate which is sufficient to cover the surface of the plate to a depth of 50 microns (0.002 inches). By keeping the brass plate horizontal while a soldering iron is vigorously worked back and forth over the surface of the plate, it has been found that a lithium sheet of fairly uniform thickness may be bonded to the plate. 
     In addition to the array of count wires 20a, lower frame section 16b supports a grid of foil support wires 30 which are tensioned between and soft-soldered to opposing upper edges of lower frame section 16b. The grid formed by wires 30 is provided to support a layer or sheet of rolled  6  L i  foil 32 of 50 microns thickness. A similar grid of wires (not shown) is attached to the lower edges of upper frame section 16a, so that if sections 16a and 16b are joined together along their respective lower and upper edges, for example, by means of epoxy glue,  6  L i  foil layer 32 is immovably sandwiched therebetween. 
     Referring once more to FIG. 1, there is shown a counting gas mixture 34, which is sealed within an enclosed volume formed by hermetically joining sections 16a and b, top plate 12 and bottom plate 14. Counting gas 34 usefully comprises a mixture of 90% argon and 10% methane or 80% argon and 20% isobutane. 
     In order to assemble respective components of detector 10, all of the components are carefully cleaned and then placed into a glove box without being exposed to the atmosphere. As is well known, a glove box is a device which enables mechanical operations to be manually performed upon various work pieces or components while the components are isolated from both atmospheric gases and from water vapor. It is essential that lithium-6 be kept isolated therefrom because of its extremely reactive nature. The glove box may be filled with pure argon, an inert gas, to prevent any contact between lithium and elements or substances with which the lithium would react in such way that the lithium would be severely damaged. 
     It has been found that an epoxy glue may be employed to bond top plate 12 to the upper edges of frame section 16a and bottom plate 14 to the lower edges of frame section 16b, the count wires,  6  L i  flat sheets, and counting gas being hermetically sealed in the chamber formed thereby. After assembly, purge tubes 36, which penetrate to the interior of detector 10, are employed to introduce counting gas 34 into the chamber, purge tubes 36 being provided with shutoff valves 38. 
     It has been found that frame sections 16a and 16b may each have a thickness of no more than one-quarter inch, so that detector 10 is very thin and flat. Most neutrons penetrating into detector 10 therefore pass into at least one of the lithium-6 foil sheets. 
     Referring to FIG. 2, there is shown a neutron n entering one of the sheets of  6  L i  foil enclosed within detector 10. Because the foil is in a solid rather than a gaseous state, the density of lithium nuclei therein is very high and there is a very high probability that the neutron will react with, or be absorbed by, a  6  L i  nucleus N. When a neutron reacts with a  6  L i  nucleus, the following reaction occurs: 
       6  L i  +n→ 3  H+ 4  He+4.78 Mev. 
     As is well known,  3  H is a triton particle. As is also well known, the range of a triton particle traveling through  6  L i  is comparatively long (e.g. 0.135 millimeters). Consequently, in excess of 70% of the triton particles resulting from the reaction between a neutron and a  6  L i  nucleus are able to escape from a layer of lithium of 50 microns thickness. By providing lithium foil layer 32, which is suspended within the chamber of detector 10, four lithium surfaces are provided from which triton particles T can be emitted into the chamber, and come into contact with counting gas 34. 
     Emitted tritons (or alpha particles which are able to escape the lithium) cause counting gas which they encounter to become ionized, generating electrons e. Since each of the count wires 20a and 20b of the count wire arrays is maintained at a high positive voltage, as aforementioned, released electrons are attracted thereto. When attracted electrons come within a range R of a count wire, they enter a region of avalanche multiplication, wherein they interact with counting gas to substantially increase the level of counting gas ionization. Sufficient electrons are released by counting gas in the avalanche multiplication regions of respective count wires to generate millivolt-size pulses thereupon. Such pulses may be readily detected and measured by electronic apparatus external to detector 10 to provide a quantitative indication of neutron activity. 
     While using detector 10 to monitor neutrons, it may be very important to prevent gamma rays occurring in the detector from being registered as neutron counts. By providing the aforementioned one-quarter inch spacing between lithium-6 sheets, the layer of counting gas 34 between adjacent sheets, or between a sheet and a wall, is too thin to enable significant ionization of the gas by a gamma-ray. The pulse generated by a gamma-ray is therefore detectably less than the pulse generated by a neutron in detector 10, and may therefore be readily distinguished from a neutron pulse. 
     Referring to FIG. 3, there is shown an overhead or plan view of upper frame section 16a, together with useful inner and outer dimensions therefor. Lower frame support section 16b is similar or identical thereto, so that frame sections 16a and 16b may be joined, as aforementioned, to form integrated sidewalls for the chamber of detector 10. If detector 10 is intended to be used in situations where small size and compactness are important, it may have an outer cross-section on the order of 61/2&#34;×61/2&#34;. Its inner dimensions, the dimensions of the chamber which contains the counting gas, flat  6  L i  sheets and count wires, may be 6&#34;×51/4&#34;. 
     Referring to FIG. 4, there is shown detector 10 in a fully assembled form, conductors 22a and b being connected to monitoring electronics 40. Monitoring electronics 40 usefully includes a source of high voltage, a pulse height discriminator, and a scaler, the latter two devices being well known in the art of charged particle proportional counting systems. Low-level pulses coupled through conductors 22a and b, which are caused by noise or gamma-ray absorption, are rejected by the pulse height discriminator. Other pulses, which are sufficiently high that they may be presumed to result from the reaction between a neutron and a lithium nucleus included in one of the lithium-6 sheets of detector 10, are recorded by the scaler as neutron counts. 
     Referring to FIG. 5, there is shown a first modification of the invention, wherein only a single flat sheet of  6  L i  foil 32 is provided to radiate charged particles in relation to adjacent neutron activity level. It will be noted that by suspending the sheet in the detector, tritons T may be radiated from either surface thereof into counting gas contained within the detector chamber. An array of parallel count wires 42 is maintained in spaced parallel relationship with each surface of the  6  L i  foil. 
     Referring to FIG. 6, there is shown a second modification of the invention, wherein three flat sheets of  6  L i  foil 32 are suspended within the chamber of a neutron detector, to provide maximum opportunity for a neutron entering the detector to collide with a lithium-6 nucleus. An array of parallel count wires 44 is maintained in spaced parallel relationship with each surface of each sheet of  6  L i  foil. 
     Obviously, many other modifications and variations of the present invention are possible in the light of the above teachings. It is therefore to be understood that within the scope of the appended claims the invention may be practiced otherwise than as specifically described.