Abstract:
A radiation detection system and method suitable for use by a first responder to detect a radiological source. The system includes a container that encloses a chamber containing a pressurized inert gas. Incident gamma rays pass through walls of the container to interact with inert gas atoms within the chamber. Wavelength-shifting fiber elements are disposed within scintillator bars oriented parallel to and radially spaced from the chamber axis. At least one sensor is interconnected with the fiber elements to receive first signals therefrom in response to the scattered gamma rays. An electrically-charged wire is disposed within the container along the axis thereof. The wire is adapted to attract electrons released from atoms of the inert gas that are ionized from being impacted by an incident gamma ray, and then produce second signals in response to the released electrons. Electronic circuitry collects the first and second signals, and a processor acquires and analyzes the first and second signals to produce an output based thereon.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the benefit of U.S. Provisional Application No. 60/883,259, filed Jan. 3, 2007, the contents of which are incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention generally relates to systems and methods for detecting radiation. More particularly, this invention relates to a system and method suitable for use by first responders to quickly detect radiation and locate its source. 
         [0003]    The role of a “first responder” is to quickly arrive on the scene of an emergency, accident, natural or human-made disaster, or similar event, tend to the injured, and assess any existing risks. Because of the possibility for harmful radiation levels in such events, there is a need to equip first responders with radiation detectors. However, existing radiation detectors are not well suited for use by first responders due to the inherent intricacies and complex technologies associated with high efficiency detection and identification of gamma rays. For example, sophisticated analysis and identification of radiation sources have conventionally required expensive detectors, elaborate electronics and cooling equipment, and specially trained personnel to ensure proper function and use of the equipment. Consequently, existing radiation detection equipment are generally too heavy and costly for practical use by first responders. 
         [0004]    Existing equipment are also not well suited for quickly detecting radiological sources at distances (distance undefined), which is desirable to enable a first responder to “sweep” a building, facility, etc., from the outside or otherwise at a sufficient distance to avoid risks that might arise from entering a confined space with a potential radiological source. Furthermore, existing radiation detection equipment typically provide little if any spatial resolution capability. This shortcoming hinders the ability of a first responder to quickly identify the location of radiological materials, which is a key primary mission of first responders in that the ability to spatially resolve radiological sources is an important step in limiting a population&#39;s exposure to a radiological threat. At the same time, a first responder is typically not required to analyze or identify the specific source of radiation in any refined manner. Instead, the first responder&#39;s mission is to quickly determine whether harmful radiation levels are present, and if so determine the location of the radiation source. Such a capability is greatly enhanced if the detection equipment were to provide spatial resolution and a simple TRUE/FALSE indication, and did not depend on precision instrumentation often found in radiation detection equipment. 
         [0005]    In view of the above, there is an existing need for lowcost radiation detection equipment that is simple to operate, accurate, and direction-sensitive, and as such provides a direction-sensitive radiation detection capability for first responders as well as other situations where there is a desire to quickly determine whether radiation is present and spatially locate its source. 
       BRIEF SUMMARY OF THE INVENTION 
       [0006]    The present invention provides a radiation detection system and method suitable for use by a first responder to detect the presence and location of a radiological source. 
         [0007]    The system includes a container that encloses a sealed chamber containing an inert gas at an elevated pressure. The container has a longitudinal axis, and walls through which incident gamma rays are able to pass and enter the chamber before interacting with atoms of the inert gas within the chamber. The container further has a plurality of scintillator bars oriented parallel to and radially spaced from the axis of the container. Wavelength-shifting fiber elements are disposed within the scintillator bars and oriented parallel to the axis. The composition of the fiber elements render them responsive to gamma rays scattered by atoms of the inert gas within the container. At least one sensor is interconnected with the fiber elements to receive first signals therefrom in response to the scattered gamma rays. An electrically-charged wire is disposed within the container along the axis thereof, and is being adapted to attract electrons released from atoms of the inert gas that are ionized from being impacted by an incident gamma ray. The wire produces second signals in response to the released electrons. The system further includes electronic circuitry adapted to collect the first signals of the fiber elements and the second signals of the electrically-charged wire, and a processor to acquire and analyze the first and second signals and produce an output based thereon. 
         [0008]    In view of the above, it can be seen that the system utilizes a gas-filled chamber to measure both the energy and arrival time of incident gamma rays that are Compton-scattered and then absorbed. The container can be formed of plastic scintillator bars and the electronic circuitry can be uncomplicated, allowing for a lowcost implementation of a practical radiation detector suitable for use by first responders without the need for specialized training. 
         [0009]    The directional sensitivity of the radiation detection system of this invention provides for the capability of spatially resolving radiological sources, and the overall sensitivity of the system provides for the capability of detecting radiological sources at distances (distance undefined). These capabilities enable a first responder to sweep a building, facility, etc., from the outside or exterior to spatially locate a radiological source, and therefore avoid risks that might arise from entering a confined space with a potential radiological source. Use of the system can also be extended to manned and unmanned aircraft for detecting radiological sources on the ground, and therefore finds applications in the military and security forces. 
         [0010]    Other objects and advantages of this invention will be better appreciated from the following detailed description. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0011]      FIG. 1  is a schematic perspective view of a direction-sensitive radiation detector that utilizes a gas-filled chamber enclosed by a scintillator in accordance with an embodiment of this invention. 
           [0012]      FIG. 2  represents a diametrical cross-section of the chamber and scintillator of  FIG. 1 . 
           [0013]      FIG. 3  represents a longitudinal cross-section of the chamber and scintillator of  FIG. 1 , and identifies nomenclature used to correlate gamma ray energy and scattering angle of a representative photon following a collision with a gas atom within the chamber. 
           [0014]      FIG. 4  is a schematic of electronic components of the radiation detector of  FIG. 1 . 
           [0015]      FIG. 5  is a graph indicating the angular acceptance of signals generated by a scattered gamma ray based on the energy ratio of incident and scattered photons sensed by the chamber and the scintillator. 
           [0016]      FIG. 6  is a graph representing contours for Compton scattering as a function of energy and angle. 
           [0017]      FIGS. 7A and 7B  illustrate two situations in which the radiation detector detects gamma rays entering through the sides (scintillator) of the detector. 
           [0018]      FIG. 8  is a graph plotting the acceptance of the detector to Compton scattering as a function of energy. 
           [0019]      FIG. 9  is a graph plotting the measured energy spectrum of gamma rays from a Cf-252 source. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0020]      FIG. 1  represents a radiation detector system  10  within the scope of the invention. The system  10  is preferably sized to be capable of being handheld, and makes use of a Compton camera  12  (shown in partial section along its longitudinal axis in  FIG. 1 ). In the present invention, the camera  12  is represented as comprising a gas-filled chamber  14  enclosed by a tubular-shaped container  16 . As known in the art, the operation of a Compton camera is based on the Compton Effect and the phenomenon of Compton scattering, which describes the behavior of photons when they interact with matter. Compton cameras employ a first detector designed to maximize the likelihood of a collision between a photon and matter (Compton scattering), and a second detector designed to absorb the energy remaining after the Compton scatter. The direction of incident gamma rays can be ascertained based on locations of the interactions and the energies detected by the scatterer and absorber. Compton cameras do not require the use of collimators or shielding. 
         [0021]    In the present invention, the gas-filled chamber  14  is employed as the photon scatterer and the container  16  is employed as the photon absorber, which together cooperate to measure both the energy and arrival time of a Compton-scattered gamma ray or the absorption of a gamma ray. The chamber  14  is filled with an inert gas at an elevated pressure. The inert gas is preferably xenon or argon at a pressure of about five to about ten atmospheres, more preferably about ten atmospheres, though it is within the scope of this invention to use different inert gases at a variety of pressures. 
         [0022]    The container  16  preferably has a high Compton-scattering cross-section due to the high electron density of the high-pressure inert gas. In the embodiment represented in  FIGS. 1 and 2 , the container  16  comprises a cylindrical outer wall made up of a number of segmented scintillator bars  18  arranged parallel to the axis of the container  16 , with the longitudinal ends of the container  16  being closed by end walls  17  to define the gas-tight chamber  14 . A wire  20  coupled to a high voltage source is disposed along the axis of the container  16  and serves as an anode to the container  16 , which serves as a cathode. A preferred material for the wire  20  is gold-plated tungsten, though it is foreseeable that other materials could be used, such as a stainless steel. With the arrangement shown, any electrons that are released by the impact of a photon with an inert gas atom within the container  16  are attracted to the wire  20  in a direction away from the container  16 , thereby generating what will be referred to herein as a chamber signal associated with photon interactions with the inert gas. 
         [0023]    Generally speaking, any number of scintillator bars  18  may be used to form the container  16 . From a performance standpoint, the number of bars  18  is chosen to attain good light collection, whereas from a practical standpoint it is preferred that the number of bars  18  is chosen to be compatible with a commonly-available multi-anode photomultiplier. For these reasons, a suitable number of bars  18  is believed to be thirty-two. A preferred construction for each bar  18  involves one or more photon-energy absorbing, wavelength-shifting fiber elements  22  embedded in a plastic body  24 . Suitable materials for the fiber elements  22  are multi-mode scintillating fibers of the type commercially available under the designation BCF-91A from Saint-Gobain Crystals and under the designation Y7 from Kuraray. Suitable plastic materials for the body  24  include types commercially available under the designation BC-400 from Saint-Gobain Crystals and under the designation SCSN  81  from Kuraray. The bars  18  and end walls  17  may be held together by bonding or with mechanical means (not shown) to form the container  16 . 
         [0024]    As will become evident from the following discussion, the resulting mixed technology utilizing a noble gas and plastic construction is desirable for minimizing the cost of manufacturing and implementing the radiation detection system  10 , while also achieving a directional-sensing capability desired by first responders without the need for specialized training. 
         [0025]    In the presence of a radiological source, gamma rays enter the chamber  14  through the bars  18  or “front face” end wall  17  (seen in  FIGS. 1 and 3 ) of the container  16  and interact with the inert gas molecules within the chamber  14 . Gamma ray photons are deflected (Compton scatter) and subsequently absorbed by the scintillator bars  18 , generating what will be termed herein a scintillator signal, and impacted inert gas atoms are ionized and release electrons that are attracted to the wire (anode)  20 , generating a chamber signal. The fiber elements  22  convert the absorbed energy to light of a wavelength which can be detected by a suitable detector. In  FIG. 1 , the light signals developed in the scintillator bars  18  are transported to one or more miniature light sensors, such as two sixteen-channel multi-anode photomultiplier tubes (PMTs)  26 , where the scintillator signals are used to measure both the energy and arrival time of the first or second scattering of gamma rays. The total signal rate observed by the wire  20  of the gas-filled chamber  14  and the fiber elements  22  of the bars  18  is used to determine the radiation source strength, while the combined information in both is used to determine the source direction, preferably within about ten to about fifteen degrees. 
         [0026]    In addition to the Compton camera  12  and PMTs  26 ,  FIG. 1  shows the system  10  as further including an interface plate  28  (shown in partial section) to hold the fiber elements  22  to the face of the PMTs  26 , a keyboard and directional display unit  30 , and an electronic housing  32  that contains electronic circuitry ( 38  in  FIG. 4 ), at least one processor ( 40  in  FIG. 4 ), one or more high voltage sources that power the chamber wire  20  and PMTs  26 , low voltage sources that power the electronic circuits, and one or more batteries. The system  10  also includes a handle  36  for convenience when holding and using the camera  12 . The circuitry  38  preferably contains separate circuits to collect the total charge produced by each scintillator and chamber signal and relative arrival time between the scintillator signals and the chamber signals. The processor  40  acquires and analyzes this electronic information, produces the effective total gamma ray energy in each signal, and places appropriate restrictions on the observed signal patterns before processing them for directional information. The display unit  30  provides the operator with means to control the system  10  and provide a visual readout of the system operation, including directional information regarding the presence and location of a radiological source. 
         [0027]      FIG. 3  depicts an occurrence of Compton scattering within the chamber  14 , and correlates the gamma ray energy and scattering angle. The direction of the incident gamma ray is within a cone whose axis is along the direction of the scattered gamma ray and whose apex is at the position of the interaction with an atom of inert gas. The gamma ray is first scattered on impact with the inert gas atom to an angle θ and has an energy loss directly correlated to the scattering angle θ by the equation 
         [0000]      ƒ E   =E   scattered (γ)/ E   γ =[1 +E   γ   /m   electron   c   2 (1−cos(θ))] −1    
         [0000]    where E γ  is the energy of the incident gamma ray, E scattered  (γ) is the energy of the scattered gamma ray, m electron  is the mass of an electron, and c is the speed of light. The scattered gamma ray is then absorbed in one of the scintillator bars  18 , and an electron released from the impacted gas atom causes additional ionization within the gas. The released electrons travel to the chamber wire  20 . 
         [0028]      FIG. 4  represents a schematic of electronic circuitry  38  suitable for use with the system  10 . Timing and pulse height information received from both the wire  20  and the scintillator bar  18  are collected and stored for further analysis if the following conditions are met: the pulse height in the scintillator bar  18  is above a predetermined threshold set by a pre-amplifier (PreAmp 1 ); the pulse height in the wire  20  is above a predetermined threshold set by a second pre-amplifier (PreAmp 2 ); and the pulse from the wire  20  is observed within a timing gate set by a gate generator and initiated by the scintillator signal. If these conditions are met, the processor  40  collects the information from analog-to-digital converters (ADCs) and performs the calculations in Table 1. The variables in Table I are as follows: E γ  is the total energy of an incoming gamma ray; ƒ γ  is the fraction of the energy of the gamma ray that is absorbed by a scintillator bar  18 ; T bar-wire  is the electron drift time relative to the clock start time produced by the scintillator bar  18 ; E wire  is the estimated energy the gamma ray deposits in the gas from the pulse produced from the gas chamber  14 ; E bar  is the estimated energy the scattered gamma ray deposits in the scintillator bar  18 ; Σ (left-right)  is the left-right scintillator bar asymmetry; Σ (up-down)  is the up-down scintillator bar asymmetry; P wire  is the pulse observed in the gas chamber corrected to give an estimate of the energy deposited there by the gamma ray; P bar  is the pulse observed in the scintillator bar  18  corrected to give an estimate of the energy deposited there by the scattered gamma ray; t wire  is the time difference between a start time generated by the scintillator pulse and a stop time generated by the gas chamber pulse; E threshold  is a pre-set parameter representing the minimum energy accepted by the system  10  due to a pulse from the gas chamber  14  or a scintillator bar  18 ;          E γ            is the average energy observed as a running value over the last set (e.g., forty) events; N left  is the number of times the left half of the scintillator bars  18  have recorded hits; N right  is the number of times the right half of the scintillator bars  18  have recorded hits; N up  is the number of times the upper half of the scintillator bars  18  have recorded hits; and N down  is the number of times the lower half of the scintillator bars  18  have recorded hits. 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE I 
               
               
                   
               
               
                 Variable Measured 
                 Calculation 
                 Comment 
               
               
                   
               
             
             
               
                 E Y   
                 P wire  + P bar   
                 Scaled pulse heights 
               
               
                 f Y   
                 P bar /(P wire  + P bar ) 
                 Scaled pulse height 
               
               
                   
                   
                 ratio 
               
               
                 T bar-wire   
                 t wire   
                 Electron drift time 
               
               
                   
                   
                 relative to 
               
               
                   
                   
                 scintillator start 
               
               
                 E wire   
                 &gt;E threshold   
                 Scaled pulse height 
               
               
                 E bar   
                 &gt;E threshold   
                 Scaled pulse height 
               
               
                 Σ (left-right)   
                 &lt;E Y &gt; (N left -N right ) 
                 Left-right scintillator 
               
               
                   
                   
                 bar asymmetry 
               
               
                 Σ (up-down)   
                 &lt;E Y &gt; (N up -N down ) 
                 Up-down scintillator 
               
               
                   
                   
                 bar asymmetry 
               
               
                   
               
             
          
         
       
     
         [0029]    The energy resolution of the system components is sufficient to eliminate low energy gamma ray events. The combined threshold setting is preferably between about 300 keV and about 500 keV. Once the ADCs are read, the energy ratio between the scintillator and chamber signals is determined. The ratio is preferably approximately 0.5, which when the energy resolution is taken into account corresponds to a true energy ratio of 0.3&lt;ƒ E &lt;0.7 for the energy ratio between the scattered and incident photon. Because of the energy and angular correlation in Compton scattering, the scattered gamma ray lies in the angular region shown in  FIG. 5  as a function of incident energy. As such, angular acceptance of interaction events accepted for analysis lie between the two dotted lines in  FIG. 5 . 
         [0030]    It should be recognized that gamma rays may enter the chamber  14  through the front face wall  17  and the scintillator bars  18  of the Compton camera  12 .  FIG. 5  represents contours for Compton scattering as a function of energy and angle. Starting with the outer contour and moving inward, the incident gamma ray energy is 0.3, 0.5, 1.0 and 5.0 MeV. A line from the center of the plot to the contour edge gives the relative number of gamma rays scattering into that angular region. In  FIG. 5 , for gamma rays that enter the front face wall  17  of the camera  12 , it can be seen that there is a strong preference for these gamma rays to forward-scatter off the gas atoms into the scintillator bars  18  opposite the source of radiation, as represented in  FIG. 3 . By averaging over the hit locations of a scintillator bar  18  of between, for example, thirty and forty events, a directional detection sensitivity is obtained notifying the operator to turn the camera  12  in either, or both, a left-right direction and an up-down direction to face the radiation source. As the camera  12  is moved to nearly a face-on orientation with the source, two things occur. The first is that the average energy of the accepted events drops, as can be seen by examining  FIGS. 5 and 6 . This occurs because the higher energy events scatter forward along the axis of the chamber  14  to the rear of the container  14 , which is not equipped with scintillators such that these interaction events are not accepted by the trigger logic. (Note that the overall source strength is measured by the singles rates of the scintillators  18  and wire  20  and is unaffected by the trigger logic used in the directionality measurement.) The second is that the size of the asymmetry measurement made with the scintillator bars  18  is limited to statistical fluctuations, which in the left-right asymmetry is given by: 
         [0000]        N   fluctuation =2( N   L   N   R /( N   L   +N   R )) 1/2    
         [0000]    and in the up-down asymmetry is given by: 
         [0000]        N   fluctuation =2( N   U   N   D /( N   U   +N   D )) 1/2    
         [0000]    where N L  is the number of events observed in the left half of the scintillator bars  18 , N R  is the number of events observed in the right half of the scintillator bars  18 , N U  is the number of events observed in the top half of the scintillator bars  18 , and N D  is the number of events observed in the bottom half of the scintillator bars  18 . 
         [0031]    The asymmetry measurement can be obtained by dividing the N total  (thirty-two scintillator bars  18 ) into two sets of sixteen left-side bars and sixteen right-side bars to assess the left-right asymmetry and, similarly, two sets of sixteen top-side bars and sixteen bottom-side bars for the up-down asymmetry. Based on these considerations, it is expected that the angular resolution of the system  10  when averaged over forty events is 
         [0000]      δθ Resolution =         θ scattering             E-1MeV ( N   fluctuation   /N   total )           E   on-axis             /             E   off-axis           =about 10° 
         [0000]    where δθ Resolution  is the estimated angular resolution of the system,          θ scattering             E-1MeV  is the gamma ray scattering angle for gamma rays of approximately 1 MeV, N total  is the total number of events used in the estimate,          E on-axis            is the average gamma ray source energy as measured when the source is on-axis (of the container  14 ), and          E off-axis            is the average gamma ray source energy as measured when the source is off-axis (of the container  14 ). 
         [0032]    While the above discussion indicates excellent spatial resolution for gamma rays that enter the front face wall  17  of the detector system, an operator is not likely to initially orient the camera  12  in nearly the correct direction of a radiation source. For this reason, gamma rays that enter the chamber  14  through the sides of the camera  12  (i.e., via the scintillator bars  18 ) must also be considered. There are two classes of events that can trigger the system  10  when gamma rays enter from the sides. In the first event class shown in  FIG. 7A , a gamma ray first excites a scintillator bar  18 , followed by interaction with a gas atom.  FIG. 7A  depicts the maximum forward scattering angles allowed by the trigger for two different energies, 0.5 MeV and 1 MeV. Because of the energy ratio requirement found using the ADC information, the scattered gamma ray can&#39;t scatter at a more forward angle than the angle of the lines shown in  FIG. 7A  and be accepted into the calculation. In this case, the acceptance is limited and can be further restricted if these events, having long drift times, are disallowed by the timing gate.  FIG. 7B  represents the second event class, in which a gamma ray first interacts with a gas atom and then interacts with a scintillator bar  18 . Minimum and maximum scattering angles are shown for the gamma ray. This type of event has a more uniform timing distribution and, as shown in  FIG. 7B , preferentially excites scintillator bars  18  on the opposite side of the container  16  to the gamma ray entry point. When both these classes of events are combined in the running average, a strong signal is produced to notify the user that the camera  12  must be turned in a direction toward the source of radiation. 
         [0033]    In order to achieve rapid results, it is important that the detector system  10  have good acceptance for both the scattered and absorbed gamma rays. To accomplish this, a suitable length and diameter for the chamber  14  when filled with xenon gas at a pressure of about five to ten atmospheres is about 40 cm and about 10 cm, respectively, and the scintillator bars  18  may have a radial thickness of about 2.5 cm. With these conditions, the probability of a 1 MeV gamma ray traveling along the axis of the chamber  14  to Compton scatter is about 60% (calculated from the Klein-Nishina formula).  FIG. 8  shows the estimated acceptance of the system  10  with the foregoing dimensions for Compton events of gamma rays directed along the axis of the chamber  14  as a function of energy. As shown, the efficiency of the system  10  peaks at approximately 20% for 1 MeV gamma rays. The efficiency curve is well matched to locate radiation sources, as 1 MeV is a typical gamma ray energy emitted by radioactive sources. For comparison, the energy spectrum for gamma radiation from a Californium-252 source is shown in  FIG. 9 . The plotted energy spectrum in  FIG. 9  is also typical of uranium. 
         [0034]    While the invention has been described in terms of a specific embodiment, it is apparent that other forms could be adopted by one skilled in the art. For example, the physical configuration of the system  10  could differ from that shown, and materials and processes other than those noted could be used. Therefore, the scope of the invention is to be limited only by the following claims.