Abstract:
The present invention is a method and apparatus for operating boron-coated straw detectors in sealed mode, without the need for a continuous flow of gas. Sealed-mode operation is necessary when using the boron-coated straw detectors in the field, where access to a continuous flow of the required gas mixture is not practical. Also, sealed-mode operation is necessary when the straw detectors are used as portable instruments, that must be moved from one location to the next swiftly, or that must be operated while in motion.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     Claims priority to provisional application 61/334,362 filed May 13, 2010. 
    
    
     STATEMENTS REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     Not applicable. 
     REFERENCE TO A MICROFICHE APPENDIX 
     Not applicable. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     This invention relates to radiation detection. More particularly, the invention relates to a method and apparatus for passive detection of neutron emitting materials with applications in homeland security and nuclear safeguards. 
     2. Description of the Related Art 
     US government plans to equip major seaports with large area neutron detectors, in an effort to intercept the smuggling of nuclear materials, have precipitated a critical shortage of  3 He gas. It is estimated that the annual demand of  3 He for US security applications alone is 22 kiloliters, more than the worldwide supply. This is strongly limiting the prospects of neutron science, safeguards, and other applications that rely heavily on  3 He-based detectors. Clearly, alternate neutron detection technologies that can support large sensitive areas, and have low gamma sensitivity and low cost must be developed. 
     The applicant has previously developed and patented a technology based on close-packed arrays of long aluminum or copper tubes (straws), 4 mm in diameter, coated on the inside with a thin layer of  10 B-enriched boron carbide ( 10 B 4 C). In addition to the high abundance of boron on Earth and low cost of  10 B enrichment, the boron-coated straw (BCS) detector offers distinct advantages over conventional  3 He-based detectors, including faster signals, short recovery time (ion drift), low weight, safety for portable use (no pressurization), and low production cost. 
     The background to the present invention and related art is best understood by reference to Applicant&#39;s own prior work, including in particularly, U.S. Pat. No. 7,002,159 B2 (the &#39;159) entitled “Boron Coated Straw Neutron Detector” which issued Feb. 21, 2006. The &#39;159 is hereby incorporated by reference in its entirety, for all purposes, including, but not limited to, supplying background and enabling those skilled in the art to understand, make and use in Applicant&#39;s present invention. 
     The background to the present invention and related art is best understood by reference to Applicant&#39;s own work. Applicant&#39;s issued patents and pending applications that may be relevant, including; (1) U.S. Pat. No. 5,573,747 entitled, “Method for Preparing a Physiological Isotonic Pet Radiopharmaceutical of  62 CU; (2) U.S. Pat. No. 6,078,039 entitled, “Segmental Tube Array High Pressure Gas Proportional Detector for Nuclear Medicine Imaging”; (3) U.S. Pat. No. 6,264,597 entitled, “Intravascular Radiotherapy Employing a Safe Liquid Suspended Short-Lived Source”; (4) U.S. Pat. No. 6,483,114 D1 entitled, “Positron Camera”; (5) U.S. Pat. No. 6,486,468 entitled, “High Resolution, High Pressure Xenon Gamma Rays Spectroscopy Using Primary and Stimulated Light Emissions”; (6) U.S. Pat. No. 7,002,159 B2 (the &#39;159) entitled “Boron Coated Straw Neutron Detector”; (7) U.S. Pat. No. 7,078,704 entitled, “Cylindrical Ionization Detector with a Resistive Cathode and External Readout”; (8) U.S. patent application Ser. No. 10/571,202, entitled, “Miniaturized  62 Zn/ 62 CU Generator for High Concentration and Clinical Deliveries of  62 CU Kit Formulation for the Facile Preparation of Radiolabeled Cu-bis(thiosemicarbazone) Compound”; (9) U.S. patent application Ser. No. 12/483,771 entitled “Long Range Neutron-Gamma Point Source Detection and Imaging Using Rotating Detector”; (10) U.S. Patent Application No. 61/183,106 entitled “Optimized Detection of Fission Neutrons Using Boron Coated Straw Detectors Distributed in Moderator Material”; (11) U.S. Patent Application No. 61/333,990 entitled “Neutron Detectors for Active Interrogation”; and (12) U.S. Patent Application No. 61/334,015 entitled “Nanogenerator.” Each of these listed patents are hereby incorporated by reference in their entirety for all purposes, including, but not limited to, supplying background and enabling those skilled in the art to understand, make and use in Applicant&#39;s present invention. 
     BRIEF SUMMARY OF THE INVENTION 
     The present invention is a method and apparatus for operating boron-coated straw detectors in sealed mode, without the need for a continuous flow of gas. Boron-coated straw detectors are described in Applicant&#39;s prior U.S. Pat. No. 7,002,159 B2 entitled “Boron Coated Straw Neutron Detector” which issued Feb. 21, 2006. The present invention includes an apparatus for detecting radiation comprising at least one boron-coated straw, a thin wall tube having a diameter sized no larger than necessary to accommodate said boron-coated straw(s); and a gas mixture sealed within said thin wall tube. Another embodiment of the present invention includes an apparatus for detecting radiation comprising multiple boron-coated straws, a thin wall tube having a diameter sized no larger than necessary to accommodate said multiple boron-coated straws; and a gas mixture sealed within said thin wall tube wherein said straws are arranged in close-packed, hexagonal configurations with the following number of tubes 
             N   =     1   +       ∑     k   =   0       B   -   1       ⁢     6   ⁢   k               
wherein N=the number of boron coated straws in a detector; B=the number of layers of straws in a detector, i.e. single straw is one layer, and k=positive integers.
 
     The gas contained within the straw detectors is a specified gas mixture, of high purity and specified pressure, and it is critical to the successful operation of the straw detectors. Straw detectors can operate either with a continuous flow of the specified gas mixture, or in sealed mode as presented here. When operated in sealed mode, proper sealing of the straw detectors is crucial for stable operation. Embodiments of the present invention include those where the gas mixture is composed of any noble gas combined with a quench gas, including CO2, CH4, CF4, C2H6, N2, H2, H2O, for absorbing photon emissions and increasing the drift velocity of electrons. Additional embodiments of the present invention include gas mixtures comprising: (1) Ar/CO2 with CO2 content in the range 1% to 20%; (2) Ar/CH4 with CH4 content in the range 1% to 20%; (3) Xe/CO2 with CO2 content in the range 1% to 20%; (4) Xe/CH4 with CH4 content in the range 1% to 20%; (5) He/CH4 with CH4 content in the range 1% to 20%; and (6) He/CO2 with CO2 content in the range 1% to 20%. An embodiment of the present invention includes having the gas mixture is maintained at an absolute pressure less than 2 atm. 
     Sealed-mode operation is necessary when using the boron-coated straw detectors in the field, where access to a continuous flow of the required gas mixture is not practical. 
     Also, sealed-mode operation is necessary when the straw detectors are used as portable instruments that must be moved from one location to the next swiftly or that must be operated while in motion. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a drawing of the present invention. 
         FIG. 2  shows the variation of gas gain over time, measured in prototype detectors that were sealed according to the present invention. 
         FIG. 3  shows the variation of gas gain over temperature, measured in prototype detectors that were sealed according to the present invention. 
         FIG. 4   a  through  FIG. 4   h  shows design examples of boron-coated straw detectors grouped together to form bundles that are sealed inside a single external tube. 
         FIG. 5  is the predicted thermal neutron sensitivity (per unit length) of boron-coated straw bundles as a function of the number of straws making the bundle. 
         FIG. 6  shows the variation of gas gain over temperature, measured in a 7-straw bundle that was sealed according to the present invention. 
         FIG. 7  illustrates the readout circuit for seven 7-straw bundles (49 straws). 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to  FIG. 1 , the apparatus comprises combining a thin walled aluminum or stainless steel (or similar material) tube  2 , and a fitting  3 , at either end of the tube. Fitting  3  can be composed of aluminum or other material that is easy to machine and bonds well with the other materials attached to it. The boron coated straw  1  fits entirely within the tube and is secured in place with the two end fittings. Embodiments include end fitting configured to receive and position the boron-coated straw(s) centrally within the thin wall tube and to receive and position an anode wire. The end fittings incorporate a central hole through which a ceramic feed-through tube  4  is positioned. A crimping tube  5  is positioned inside the ceramic (or other electrical insulator) tube  4 . Embodiments of crimping tube  5  are composed of copper. Crimping tubes can have an inner diameter large enough to accommodate a thin metallic wire up to 50 μm in diameter and are capable of crimping around the wire to securely retain high tension in the wire. A thin metallic wire  6  passes through the tube  5 . The wire  6  is tensioned, then crimped in place. A gold-plated pin  8  connects to the crimp tube  5 . The wire forms the anode electrode that connects, through the crimp tube  5  and pin  8 , to a high voltage supply, and to a preamplifier through a coupling capacitor. A plastic collar  7  is used to provide additional electrical insulation between the anode and fitting  3 . For embodiment including multiple boron coated straws in a single bundle, end fittings are provided with accurately positioned insulating feed-throughs capable of receiving and positioning all associated wires. Embodiments may include fitting having multiple holes through which feed-through insulating tubes are fitted. Tube  9  serves as a gas port, used to purge the volume inside tube  2 , and to fill the volume inside tube  2  with a specified gas mixture. A grounding collar  10  connects the tube  2  (cathode) to an electrical ground. The ceramic tube  4 , the crimping tube  5 , the plastic collar  7 , the gas port tube  9  and the end fittings  3  are fixed with epoxy. 
     Several boron-coated straw detectors were sealed using the present invention. Initially, the gas port  9  of the sealed detector was connected to a supply of a gas mixture of argon/CO2. The detector was then purged with a continuous flow of this gas mixture, while heated to 60° C. for a period of 18-24 hours. Using valves, the gas flow was stopped, then the detector was allowed to cool to room temperature. The detector was then connected to a vacuum pump, and evacuated to a pressure of 0.7 atm. The gas port  9  was then crimp sealed. In an embodiment of the present invention the gas port  9  fits inside the off-center hole of the fitting  3  and can be connected to an external vacuum and gas filling system. Embodiments of the present invention include those wherein the gas port  9  is composed of a ductile metal such as copper, stainless steel, nickel, or aluminum and capable of being sealed using pinch off technique. 
     In order to gage the seal quality and the resulting gas purity, the amplitude of signals corresponding to a single radiation energy were tracked in the sealed detectors over a period of time. Gas purity is essential to maintaining stable operation and an adequate signal-to-noise ratio. Gas contamination over long periods of time (due to materials outgassing, for instance) may alter the amplitude of signals, which in turn will affect the performance of the detector. 
     A pulse height spectrum was collected using a  241 Am gamma ray source. Photons emitted by this isotope, primarily with an energy of 60 keV, interact with the copper walls of the straw detector. At this energy, most interactions in copper are of the photoelectric kind, resulting in the absorption of the incident photon, and prompt emission of a characteristic 8 keV X-ray photon. This 8 keV X-ray photon may subsequently escape into the gas volume, and interact with argon atoms, depositing all of its energy. As a result, an 8 keV energy peak appears in the pulse-height spectrum. 
     The location of the characteristic X-ray peak in the gamma energy spectrum was used to track gas purity as shown in  FIG. 2 . 
     Temperature cycling tests were also carried out to evaluate the ability of the sealed straw detector to maintain stable operation at extreme environments.  FIG. 3  shows the measured variation in the neutron counts recorded in a sealed straw detector during operation inside a chamber, where the temperature was varied from +60 C to −40 C. 
     EXAMPLES 
     The proposed invention is illustrated in  FIGS. 4   a  to  4   h . Each figure shows a standalone detector. The detector is a close-packed bundle of straws, where each straw detector is 4 mm in diameter and of length equal to the bundle length. Embodiments of the present invention include those wherein the thin wall tube and the straw(s) are approximately equal in length. The length may vary from a few centimeters to several meters. In the embodiments of  FIGS. 4   a  to  4   h  the straws are arranged in close-packed, hexagonal configurations with the following number of tubes 
             N   =     1   +       ∑     k   =   0       B   -   1       ⁢     6   ⁢   k               
wherein N=the number of boron coated straws in a detector; B=the number of layers of straws in a detector, i.e. single straw is one layer, and k=positive integers. The bundle is housed inside a sealed aluminum or stainless steel tube fitted with a fitting of appropriate design. Embodiments of the invention include those where the thin wall tube is composed of other materials which minimizes scattering of low energy neutrons and/or low Z material to minimize the sensitivity for gamma ray interactions Depending on the number of straws bundled, the dimensions and neutron sensitivity of the tube will vary, as shown in Table 1.
 
     The anode electrodes of all BCS detectors within the bundle are connected together and read out with a single amplifier, using common electronics typically used to read out  3 He tubes. Although the overall capacitance presented to the amplifier will be higher than that presented by a single tube of large diameter, the signals generated in the straw detectors are several times larger than those generated in  3 He tubes, and thus, the signal-to-noise ratio is not affected by the larger capacitance. 
     The detection efficiencies of the straw bundles were estimated in Monte Carlo simulations implemented in MCNP5 and are listed in Table 1. A parallel beam of monoenergetic neutrons was directed normally over the entire side of the bundle. The computed sensitivity (per unit length) is also plotted in  FIG. 5  as a function of the number of straws. In all cases, a  10 B 4 C coating thickness of 1 μm was assumed. 
     The thermal neutron sensitivity of a  3 He tube, with a 5.08 cm diameter (2 inches), pressurized to 2.5 atm, is ˜3.4 cps/nv/cm, equivalent to that obtained with the 187-straw bundle, whose diameter is only slightly larger at 6.36 cm. The sensitivity of the BCS bundle can be further improved by optimizing the thickness of the  10 B 4 C coating. 
     The gain stability of the 7-straw bundle was also measured over the course of 255 days, as shown in  FIG. 6 . The gain variation was less than ±4%. 
     Readout. When several straw detectors are grouped together in a bundle, reading them out separately would require a number of pre-amplifiers equal to the number of straws. Significant savings can be achieved with a readout scheme based on delay lines, offering the capability to decode the identity of the firing straw with only 2 pre-amplifiers.  FIG. 7  illustrates the readout circuit for seven 7-straw bundles (49 straws). On one end of the bundles, all straws with the same index across different bundles are connected together, then to a different tap on delay line  1 . On the other end of the bundles, all straws within the same bundle, are connected together, then to a tap on delay line  2 . In this scheme, delay line  1  identifies the straw index within a single bundle, and delay line  2  identifies the specific bundle among the 7 bundles. 
     
       
         
               
             
               
               
               
             
               
               
               
               
             
               
               
               
               
             
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Boron-coated straw bundle dimensions 
               
               
                 and thermal neutron sensitivity. 
               
             
          
           
               
                   
                 Detection 
                 Thermal neutron 
               
             
          
           
               
                   
                 Bundle 
                 efficiency 
                 sensitivity 
               
             
          
           
               
                 Number of straws 
                 Diameter 
                 for thermal 
                 per unit length 
               
               
                 in bundle 
                 (cm) 
                 neutrons (%) 
                 [(cps/nv)/cm] 
               
               
                   
               
             
          
           
               
                 1 
                 (FIG. 4a) 
                 0.4 
                 9.0 
                 0.036 
               
               
                 7 
                 (FIG. 4b) 
                 1.27 
                 18.4 
                 0.234 
               
               
                 19 
                 (FIG. 4c) 
                 2.12 
                 26.6 
                 0.564 
               
               
                 37 
                 (FIG. 4d) 
                 2.97 
                 33.3 
                 0.989 
               
               
                 61 
                 (FIG. 4e) 
                 3.82 
                 38.8 
                 1.48 
               
               
                 91 
                 (FIG. 4f) 
                 4.67 
                 43.3 
                 2.02 
               
               
                 127 
                 (FIG. 4g) 
                 5.52 
                 47.0 
                 2.59 
               
               
                 187 
                 (FIG. 4h) 
                 6.36 
                 50.0 
                 3.4