Abstract:
Methods and apparatus for a detector system to detect gamma and neutron radiation. In one embodiment, a detector comprises a tank to hold a liquid, a plurality of tubes adjacent the tank to detect neutrons, and a plurality of photon detectors to detect Cherenkov light generated by gamma radiation in the liquid. The tank is configured to contain the liquid so that the liquid generates the Cherenkov light and moderates the neutrons.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH 
     This invention was made with government support under Contract No. HDTRA1-10-C-0002 awarded by the Defense Threat Reduction Agency. The government has certain rights in the invention. 
    
    
     BACKGROUND 
     As is known in the art, a Cherenkov detector is a particle detector using the mass-dependent threshold energy of Cherenkov radiation to allow discrimination between a lighter particle, which does radiate, and a heavier particle, which does not radiate. A particle passing through a material at a velocity greater than that at which light can travel through the material emits light in the form of Cherenkov radiation. This light is emitted in a cone about the direction in which the particle is moving. The angle of the cone, θ c , is a direct measure of the particle&#39;s velocity through the formula 
                 cos   ⁢           ⁢     θ   c       =     c   nv       ,         
where c is the speed of light, and n is the refractive index of the medium.
 
     Gamma radiation or gamma rays refer to electromagnetic radiation of high frequency (very short wavelength) produced by decay of high energy states in atomic nuclei and high energy sub-atomic particle interactions in natural and man-made processes, such as electron-positron annihilation, neutral pion decay, fusion, fission, lightning strike and terrestrial gamma-ray flash, and astronomical sources in which high-energy electrons are produced. These electrons then produce secondary gamma rays by the mechanisms of bremsstrahlung, inverse Compton scattering and synchrotron radiation. Gamma ray events range from production of a single gamma photon in nuclear decay processes, to explosive bursts of gamma rays in the universe. 
     As a rule of thumb, gamma rays typically have frequencies above 10 exahertz (&gt;10 19  Hz), with energies above 100 keV and wavelength less than 10 picometers, i.e., less than the diameter of an atom. Gamma rays from radioactive decay have energies of a few hundred keV, and almost always less than 10 MeV. Energies from astronomical sources can be much higher, ranging over 10 TeV. 
     Gamma rays and x-rays are typically distinguished by their origin with X-rays emitted by electrons outside the nucleus, and gamma rays emitted by the nucleus. Exceptions include high energy processes other than radioactive decay, which are still referred to as sources of gamma radiation, such as extremely powerful bursts of high-energy radiation known as long duration gamma ray bursts resulting from the collapse of stars called hypernovas. 
     A variety of Cherenkov detectors are known. For example, glass scintillators for thermal neutron detection use Li-loaded silicate glass or Li-loaded glass fibers for neutron detection. One disadvantage with Li-loaded glass scintillators is that the neutron response is not well distinguishable from the gamma response when there is a high gamma flux. 
     Cherenkov light is emitted when a charged particle, such as an electron or a positron, moves faster than the speed of light in a medium. Gamma and x rays of sufficient energy can produce Cherenkov light indirectly by liberating electrons from atoms in the medium (Compton scattering and photoelectric effect) and by generating positrons (pair production). 
     Detection of Cherenkov light is desirable in high-energy physics applications, such as muon detection and very high-energy (&gt;1 GeV) particle detection. Known detectors include ring imaging Cherenkov detectors made with glass gels of various index of refraction butted together and ordered according to index of refraction so that the Cherenkov cone developed in each section of gel is superimposed on all the others to form a ring of light that is indicative of the energy of the particle passing through the assembly. 
     Other known Cherenkov detectors use water to detect Cherenkov light caused by the interactions of neutrinos with electrons or nucleons. These interactions result in high-energy electrons that produce Cherenkov light. 
     In another conventional water Cherenkov detector, the water includes a neutron absorbing material. Neutrons passing through the water are captured by the neutron-absorbing material resulting in the emission of prompt gamma rays, which energize electrons to such an extent that the electrons produce Cherenkov light within the water. 
     U.S. Pat. No. 7,629,588 to Bell at al., which is incorporated herein by references, discloses an activator detector that detects neutrons using Cherenkov light. U.S. Patent Application Publication No. 2010/0265078 of Friedman, which is incorporated herein by reference, discloses a plasma panel based ionizing particle radiation detector. 
     To detect gamma radiation and perform neutron moderation in a conventional manner requires separate detection systems. While there are known liquid scintillators sensitive to both neutrons and photons, they are difficult to deploy due to toxicity and flammability. For example, most liquid scintillator detectors are xylene-based. Further, liquid scintillators are not scalable and quite expensive. 
     SUMMARY 
     The present invention provides methods and apparatus for an integrated gamma/neutron detector having a common volume of liquid, such as conditioned water, in which Cherenkov light is generated and neutrons are moderated. With this arrangement, a cost-effective and efficient gamma/neutron detector is provided with the use of toxic or flammable materials used in scintillator-type detectors. While exemplary embodiments of the invention are shown and described in conjunction with particular tank configurations and energy thresholds, it is understood that the invention is applicable to high energy detectors in general in which it is desirable to detect gamma and neutron energy. 
     In one aspect of the invention, a detector system comprises a tank to hold a liquid, a plurality of tubes adjacent the tank to detect neutrons, and a plurality of photon detectors to detect Cherenkov light generated by gamma radiation in the liquid, wherein the tank is configured to contain the liquid so that the liquid generates the Cherenkov light and moderates the neutrons. 
     The system can further include one or more of the following features: the plurality of tubes are contained in the tank, the liquid comprises conditioned water, the water is contained in the tank, the plurality of tubes comprises B-10 lined proportional tubes, the photon detectors include photo multipliers, and/or the detector has an energy threshold of about 300 keV. 
     In another aspect of the invention, a detector system comprises a means for detecting neutrons comprising tubes, and a means for detecting Cherenkov light generated by gamma radiation comprising liquid in a tank. The system can further include tubes contained in the tank. 
     In a further aspect of the invention, a method comprises using a combined volume of conditioned water for gamma detection and neutron moderation. The method can further include containing the water in a tank, employing a plurality of tubes adjacent the tank to detect neutrons, and/or employing a plurality of photon detectors to detect Cherenkov light generated by gamma radiation in the water. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing features of this invention, as well as the invention itself, may be more fully understood from the following description of the drawings in which: 
         FIG. 1  is a schematic representation of a gamma/neutron detector in accordance with exemplary embodiments of the invention; 
         FIG. 2  is a schematic representation of a neutron detector portion of the detector of  FIG. 1 , 
         FIG. 3  is a pictorial representation of Cherenkov light generation; 
         FIG. 4  is an exploded view of a gamma/neutron detector showing internal tubes to detect neutrons; 
         FIG. 5  is an exploded view of a gamma/neutron detector showing external tubes to detect neutrons; 
         FIG. 6  is a schematic representation of a gamma/neutron detector having a data acquisition system; 
         FIG. 6A  is a schematic representation of a frame to support a reflective material forming a portion of an exemplary gamma/neutron detector; 
         FIG. 7  is a top view of a frame within a tank of a gamma/neutron detector; 
         FIG. 7A  is a schematic representation of a base member to support PMTs forming a part of a of a gamma/neutron detector; 
         FIG. 8A  is a graphical representation of gamma radiation and  FIG. 8B  is a graphical representation of gamma and neutron radiation; 
         FIG. 9  is a tabular representation of gamma and neutron detection information; 
         FIG. 10A  is a graphical representation of 3 MeV gamma radiation and  FIG. 10B  is a graphical representation of gamma/neutron radiation; and 
         FIG. 11  is a high level block diagram of a radiation interrogation system having a gamma/neutron detector. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 1  shows an exemplary neutron/gamma detector  100  in accordance with exemplary embodiments of the invention. The detector  100  includes a gamma detector  102  and a neutron detector  104 . In one embodiment, the gamma detector  102  includes a water Cherenkov detector and the neutron detector  104  includes Boron-lined (B-10) proportional tubes  106 . Water  108  contained in a tank  109  results in the production of Cherenkov light from gamma radiation and performs neutron moderation. By using the water for gamma detection and neutron moderation, a relatively compact, cost effective, and scalable unit is provided. 
     In general, the integrated neutron/gamma detector  100  utilizes a large volume water Cherenkov detector  102  to collect gamma signatures through the generation of Cherenkov light in the water. High energy gamma-rays above 300 keV generate high energy electrons that emit Cherenkov light in the water  108 . These visible photons are collected by a set of six, for example, photomultiplier (PMT) tubes  110  at the top of the tank. It is understood that any practical number of PMTs  110  can be used to meet the needs of a particular application. While the illustrative embodiments include the use of PMTs, it is further understood that any suitable sensor can be used to detect the Cherenkov light. 
     The B-10 lined proportional tubes  106  mounted to the front of the detector serve as the neutron detector  104 . Because they are mounted in the front of the water  108  volume, the bulk of the required neutron moderation comes from the water Cherenkov detector  102 . 
     The water Cherenkov detector  102  has a natural lower energy threshold of about 300 keV to provide natural filtering of the low energy gamma-rays for eliminating or reducing the possibility of pile-up due to background. The integrated B-10 detectors  106  utilize the water of the Cherenkov detector  102  as the primary moderator to reduce the overall size and weight of the detector, as compared to separate conventional detectors. 
     In one embodiment, the neutron detector  104  includes pre-amplifier devices  116  coupled to each of the Boron-lined tubes  106 . It is understood that the Boron-lined tubes can be so-called B-10 type tubes. Other suitable tubes for detecting neutrons will be apparent to one of ordinary skill in the art. The tubes  106  can be secured in place with polyethylene or other suitable material. 
     As shown in  FIG. 3 , when a charged particle (usually an electron) passes through a medium, such as water, at a speed greater than the speed of light in that medium, that particle will radiate from a cone behind itself, rather than in front, in a manner similar to that of a sonic wave created when breaking the sound barrier. The resultant Cherenkov light can be detected by photomultipliers or other detector types. 
     In one embodiment shown in  FIG. 4 , the boron tubes  106  are internal to, i.e., contained within, the tank  109 . In an alternative embodiment shown in  FIG. 5 , the boron tubes are external to the tank. In general, internal tubes conserve overall detector volume at the expense of some gamma sensitivity while external tubes maximize gamma sensitivity at the expense of higher detector volume. 
       FIG. 6  shows an exemplary gamma/neutron detector  200  having a tank  202  to hold a liquid, e.g., conditioned water, in accordance with exemplary embodiments of the invention. The detector  200  can include a data acquisition system (DAS)  210  to receive and process data from the amplifiers coupled to the boron tubes and from the PMTs. In one embodiment, the data acquisition system comprises a digital waveform digitizer of 12 bits, 75 Msps. Each waveform from the PMTs are digitized and time stamped. It is understood that processing the information can be performed in a manner well known to one of ordinary skill in the art. 
       FIG. 6A  shows an exemplary frame  250  comprising a series of sleeves  252  and posts  254 . In one embodiment, the frame  250  includes a solid bottom  256 . A circulation hole  258  enables water to flow through the sleeves  252 . Any practical number of circulation holes  258  can be provided. The frame  250  fits inside the tank  202 , as shown in  FIG. 7 , which includes exemplary dimensions. Circulation of the water through the detector conditions the water for maximum performance. Circulation rates of about 1 GPM through a 0.1 micron particulate filter and deionizing resin minimizes light absorption in the tank. 
     In an exemplary embodiment, the sleeves  252  and/or the posts  254  comprise polypropylene. The polypropylene frame, which can be fabricated with thin stock, e.g., ⅛″, was used since it is relatively easy to fabricate and serve as a backing on which to mount the reflective material  260 . While the polypropylene frame may provide relatively little structural strength, it prevents the reflective material  260  from floating in the water. The bottom of the tank can be angled to allow the water to be drained fully from the tank. 
     In one embodiment, a reflective material  260  is disposed on an inner surface of the sleeves  252 . The reflective material  260  reflects light to enhance detection by the PMTs. In one particular embodiment, the reflective material  260  comprises expanded PTFE (ePTFE) to provide a diffuse reflector that has a reflectance of over 99% at the wavelength of interest for Cherenkov light and PMTs. It is understood that the thickness of the reflective material can vary to meet the needs of a particular application. An exemplary thickness for the reflective material ranges from about 0.5 mm to 3 mm. 
     In one particular embodiment, a base member  262  is secured to the sleeve for securing a PMT assembly  270 , as shown in  FIG. 7A   
     While exemplary dimensions are shown for the illustrative detector, it is understood that the dimensions of the tank and other components can be varied to meet the needs of a particular embodiment. The exemplary modular unit is readily scalable to any desired geometry. 
     In one embodiment, the gamma detection has a selected threshold, e.g., 300 keV. That is, the detector is insensitive to photons below about 300 keV. Intrinsic efficiencies are in the order of 10-20 percent for neutrons and 20-50 percent for gammas. 
     It is understood that the tank can comprise any suitably rigid material. Exemplary materials include aluminum, stainless steel, and polypropylene. It is understood that materials generating rust, e.g., ferrous materials, should be avoided since rust will significantly degrade the transmission of light through the water. In addition, the detector  100  can be fabricated from non-toxic materials, unlike scintillator-type detectors. 
     Exemplary embodiments of the gamma/neutron detector can be compared against a conventional Cherenkov detector. As can be seen in  FIGS. 8A and 8B , for 10 MeV gammas, while the efficiency for gamma/neutron and gamma alone is the same, the number of Cherenkov photons created is smaller (see Table 1 below) due to loss of energy when going through the neutron detector. 
     
       
         
               
               
               
             
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
               
                   
                 gamma 
                 gamma/neutron 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 623.958 
                 478.7715 
               
               
                   
                 1166.267 
                 959.5701 
               
               
                   
                 1673.898 
                 1401.103 
               
               
                   
                 2365.423 
                 2092.515 
               
               
                   
                   
               
             
          
         
       
     
     The spectra (frequency vs. number of PMT hits per gamma) indicate that the gamma/neutron, as compared to gamma, shifts the spectrum towards “fewer photons per gamma,” i.e., to the left. Those gammas are still detectable because photons are still produced, but fewer per gamma.  FIG. 9  shows further comparison information and  FIGS. 10A and 10B  show a comparison of 3 MeV gammas. 
     It is understood that exemplary embodiments of the invention are applicable to a wide variety of applications. For example, active and passive interrogation systems use radiation to interrogate an object, person or environment for objects of interest. As will be readily appreciate, it would be highly desirable to achieve gamma detection and neutron moderation in a cost efficient manner using systems that do not require toxic or flammable materials. 
       FIG. 11  shows an exemplary gamma/neutron detector  300  proximate an interrogation system  302  for irradiating an object of interest  304 . Interrogation systems can be active or passive. The detector  300  can detect radiation in the interrogation  302  vicinity in order to determine the type of material being irradiated. Because the emitted radiation comprises either gammas or neutrons, or even combined fields of gammas and neutrons, it is highly desirable to detect both gammas and neutrons in order to accurately determine the nature of the inspected object. 
     Having described exemplary embodiments of the invention, it will now become apparent to one of ordinary skill in the art that other embodiments incorporating their concepts may also be used. The embodiments contained herein should not be limited to disclosed embodiments but rather should be limited only by the spirit and scope of the appended claims. All publications and references cited herein are expressly incorporated herein by reference in their entirety.