Patent Publication Number: US-7915596-B2

Title: Integrated neutron-gamma radiation detector with optical waveguide and neutron scintillating material

Description:
This application is a continuation of application Ser. No. 12/027,828, filed Feb. 7, 2008, the entire contents of which are incorporated herein by reference. 
     CROSS-NOTING TO RELATED APPLICATION 
     This application is related to application Ser. No. xx/xxx,xxx, filed May 17, 2010, entitled “RADIATION DETECTOR WITH OPTICAL WAVEGUIDE AND NEUTRON SCINTILLATING MATERIAL,” the entire contents of which are incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The teachings herein relate to a detector of ionizing radiation and more particularly to a detector for detecting a gamma component and a neutron component. 
     2. Description of the Related Art 
     Detection of radioactive materials, particularly those illicitly hidden in the stream of commerce, requires the availability of a variety of radiation detection equipment. In particular, Hand-Held RadioIsotope Identification Devices (HHRIID) are needed in the field to quickly determine the presence of special nuclear material and distinguish it from the presence of medical and industrial radioisotopes, as well as from normally occurring radioactive material. One possible embodiment of an HHRIID consists of two optically separated radiation sensors that emit light and are coupled to a common photodetector. The first radiation sensor is a neutron sensing component that contains atomic nuclei with a high neutron cross section, such as  6 Li in a chemical compound, such as  6 LiF, surrounded by particles of a scintillator material, for example, ZnS:Ag, and bound together in an epoxy matrix. The second radiation sensor is a gamma sensing component and consists of a scintillator crystal with enhanced gamma energy resolution, high gamma stopping power, and an atomic composition with very low neutron absorption cross section. The two radiation sensors are optically separated in such a manner that the light emitted by one sensor does not reach the other sensor in order to avoid optical crosstalk. The HHRIID may include a pulse shape discrimination circuit that identifies the source of light emitted (either by the neutron sensing component or the gamma sensing component based on the difference in scintillation light decay times.) 
     In the detection of neutrons via solid-state scintillation, perhaps the most highly-utilized material stems from a granular mixture of  6 LiF and ZnS:Ag. Each component in this mixture represents “best-of-class” performance (i.e., respectively, neutron capture and luminescence). For neutron capture, the LiF crystal structure offers one of the highest Li atom densities in solid-state form and maximizes the probability of neutron interaction, especially if in addition it is enriched in  6 Li. Furthermore, the absorption of thermal neutrons by  6 Li induces direct disintegration into alpha and triton particles with no other secondary radiation. The absence of multiple reaction pathways and/or radiation by-products enables one to optimize the corresponding phosphor to a single secondary radiation type (i.e., heavy charged particles). For luminescence, ZnS:Ag is one of the brightest phosphors known and remains unparalleled in its emission under alpha and triton exposure. 
     A crucial metric in determining the performance of a neutron scintillator is neutron sensitivity, the number of neutron events registered per incoming neutron flux. This measurement requires the collection and counting of photons from the neutron scintillator. However, the light output of  6 LiF/ZnS:Ag materials is limited by two factors: [1] self-absorption of the emitted light by the ZnS:Ag phosphor, and [2] optical attenuation of the emission photons via scattering. The latter arises due to the granular nature of the material (i.e., the multitude of interfaces with index-of-refraction mismatches). The end result is a threshold in thickness beyond which further (useful) light output is unachievable. 
     Conventional neutron detection approaches typically rely on the optical coupling of a thin disk of  6 LiF/ZnS:Ag composite (&lt;1 mm) to the flat, circular face of a photosensor. For reasons stated above, the neutron sensitivity of this design cannot be improved by increasing the thickness of the disk. Instead, multiple layers of  6 LiF/ZnS:Ag composite must be employed, which in turn, create substantial difficulties in transporting the resulting additional light to the photosensor(s). Furthermore, a flat disk may not be the desired shape for neutron capture. If an application requires the moderation of neutron energies (i.e., reduction to ambient, thermal energies), cylindrical shells are preferable to disks. For this geometry, the challenge of light transport becomes even more acute. 
     In order to improve the total neutron sensitivity of the detector while providing a design that reduces both the size and weight of the detector, an optimal integrated gamma/neutron detector must address the issue of packaging a larger area of neutron sensing composite. 
     BRIEF SUMMARY OF THE INVENTION 
     In one aspect, a radiation detector comprises a neutron sensing element comprising a neutron scintillating material at least partially surrounded by an optical waveguide material; and a photosensing element optically coupled to the neutron sensing element, wherein photons emitted from the neutron sensing element are collected and channeled through the optical waveguide material and into the photosensing element. 
     In another aspect, a method for detecting radiation comprises: 
     disposing a neutron scintillating material within an optical waveguide material to form a neutron sensing element; and 
     optically coupling the neutron sensing element to a photosensing element, 
     whereby photons emitted from the neutron sensing element are collected and channeled through the optical waveguide material and into a photosensing element. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an exploded view of an integrated neutron/gamma detector including a neutron sensing element with an optical waveguide and neutron scintillating material according to an embodiment of the invention; 
         FIG. 2  is an exploded view of the neutron sensing element of the neutron/gamma detector of  FIG. 1  according to an embodiment of the invention; 
         FIG. 3  is a perspective view of the integrated neutron/gamma detector of  FIG. 1  when assembled; 
         FIG. 4  is an exploded view of an integrated neutron/gamma detector including a neutron sensing element with an optical waveguide and neutron scintillating material according to another embodiment of the invention; 
         FIG. 5  is an exploded view of the neutron sensing element of the neutron/gamma detector of  FIG. 4  according to another embodiment of the invention; 
         FIG. 6  is a perspective view of the integrated neutron/gamma detector of  FIG. 4  when assembled; 
         FIG. 7  is a cross-sectional view of strands of the neutron sensing element according to an alternate embodiment of the invention; and 
         FIG. 8  is a cross-sectional view of strands of the neutron sensing element according to another alternate embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to  FIGS. 1-3 , an integrated neutron-gamma radiation detector is shown generally at 10 according to an embodiment of the invention. At one end, the detector  10  includes a neutron moderator  12  that includes a material that slows down fast neutrons entering the moderator  12 , but allowing thermal neutrons and gamma rays to easily pass therethrough. For example, the neutron moderator  12  may include hydrogen, and the like. The moderator  12  includes a cavity  14  that can be lined with an optical reflector material  16 . A gamma sensing element  18  is disposed within the cavity  14  of the neutron moderator  12  and surrounded by another layer of the optical reflector  16  to increase the optical efficiency of the detector  10 . In one embodiment, the gamma sensing element  18  comprises a scintillator crystal that emits a light pulse having a decay time, τ, when a gamma ray collides with the gamma sensing element  18 . Typical materials for the scintillator crystal include, without limitation, crystalline materials with high energy resolution (3% or better at 662 keV) from the Lanthanum halides class (LaBr 3 , LaCl 3 , LaI 3 ), as well as solid solutions of these materials. Other dense, bright and fast scintillator materials are useful for incorporation into the gamma sensor  18  as well. For example, the scintillator crystal may be made of a mixed lanthanum halide LaX 3 :Ce (X═Cl, Br, I) gamma scintillator material that emits a light pulse having a decay time, τ 1 , of about 20 nanoseconds. The mixed lanthanum halides LaX 3 :Ce (X═Cl, Br, I) gamma scintillator material has outstanding energy resolution that will consequently enable high-performance room temperature detectors at considerably lower cost when compared to current technologies, such as cryogenically cooled high purity Germanium (HP Ge). 
     The detector  10  includes a neutron sensing element  20  that is disposed within the cavity  14  of the neutron moderator  12 . In the illustrated embodiment, the neutron sensing element  20  is disposed proximate the gamma sensing element  18  such that at least a portion of the gamma sensing element  18  is disposed within the neutron sensing element  20  for a spatially-compact design, as shown in  FIG. 3 . The optical reflector material  16  is used in layers that surround both the neutron sensing element  20  and the gamma sensing element  18  to provide an optically efficient design. 
     The detector  10  also includes a photosensing element or array of photosensing elements  24 , such as a photodiode or photomultiplier tube (PMT), and the like, which is optically coupled to the gamma sensing element  18  and the neutron sensing element  20 . In the illustrated embodiment, a light collector  22  is disposed between the photosensing element  24  and the gamma sensing element  18  and the neutron sensing element  20 . The collector  22  is made of an optically transparent material, such as epoxy, plastic, glass, and the like. The purpose of the collector  22  is to act as a lens and funnel the photons emitted from the gamma sensing element  18  and neutron sensing element  20 , which have a relatively large diameter, into the photosensing element  24 , which typically has a relatively smaller diameter, and increase the optical efficiency of the detector  10 . The collector  22  may be surrounded by an optical reflector (not shown), if necessary, to further increase the optical efficiency of the detector  10 . It will be appreciated that the collector  22  can be eliminated when the relative diameters of the gamma and neutron sensing elements  18 ,  20  are approximately equal to the diameter of the photosensing element  24 . 
     It will be appreciated that the invention can be practiced with any suitable photosensor acting as the photosensing element, and that the use herein of the photodiode or PMT as a photosensor is merely illustrative and non-limiting. The photosensing element  24  outputs a signal, S, indicative by its decay time characteristic of the two different types of photons emitted by the gamma sensing element  18  and the neutron sensing element  20 . Thus, detector  10  needs only a single photosensing element  24  to detect the two different types of light pulses emitted by the gamma and neutron sensing elements  18 ,  20 . 
     Although in the illustrated embodiment the integrated detector  10  includes an array of photodiodes as the photosensing element  24 , the detector  10  may comprise other photosensitive devices. For example, other embodiments of the detector  10  may include a photomultiplier tube, a PIN photodiode, an avalanche photodiode, a Geiger-mode operating photodiode, a hybrid photodetector and other similar devices, operating either individually or grouped as an array. In short, the photosensing element  24  is designed to receive and interpret a signal from each of the gamma sensing element  18  and the neutron sensor element  20  (each of the gamma sensing element  18  and the neutron sensing element  20  being a scintillator and providing and optical output in response to a radiation interaction). To interpret such signals, the detector  10  may also include a pulse shaping and processing electronics package (not shown) of a type well-known in the art that processes the signal, S, from the photosensing element  24  to determine whether a given photon-emitting event is indicative of radiation interaction in the gamma sensing element  18  or in the neutron sensor element  20 . In the case of gamma rays, the electronics also determine the energy of the gamma ray based on the amount of charge generated in the photosensing element  24  and a calibration procedure with known gamma ray energies from radioisotopic sources. For example, the pulse shape and process electronics package may include an analog-to-digital converter (ADC) and also a charge [Q] to digital converter (QDC) (not shown) that receive the signal, S, from the photosensing element  24 . Each signal, S, is indicative of a radiation interaction in one of the gamma sensing element  18  and the neutron sensing element  20 , and has a signal amplitude V 0 . 
     Referring now to  FIG. 2 , one aspect of the invention is that the neutron sensing element  20  comprises an optical waveguide material  20   a  surrounding a neutron scintillating material  20   b , and a central aperture  20   c  of sufficient dimension such that at least a portion of the gamma sensing element  18  is capable of being disposed therein. In an embodiment, the optical waveguide material  20   a  may be made of, for example, fused/glassy silica, transparent plastic, and the like. For example, the optical waveguide material  20   a  may comprise an optical waveguide of a type well-known in the art. The neutron scintillating material  20   b  may comprise, for example, a mixture of particles of a neutron capture material, for example,  6 Li in a chemical compound form, such as  6 LiF, and a scintillator material, such as ZnS:Ag, both in an optically transparent epoxy matrix. 
     The neutron scintillator material has a relatively large cross section (940 barns per Li 6 atom) for thermal neutrons. Upon absorption of a thermal neutron,  6 Li decays into  3 H and emits an alpha particle, both charged particles with a total kinetic energy of about 4.8 MeV. The alpha particle and the triton are absorbed by the scintillator material, such as ZnS:Ag, which emits a 450 nm photon having a decay time, τ 2 , of about 110 nanoseconds, which is different than the decay time, τ 1 , of the photons emitted from the scintillator crystal of the gamma sensing element  18 . An optimum radius or thickness is utilized to maximize the photon flux exiting the surface area of the neutron scintillating material  20   b . For example, the radius of a very thin shell of  6 LiF/ZnS:Ag composite material would be selected to maximize internal light generation, and the thickness of the cylindrical shell would be chosen to optimize emissions from the exterior and interior surfaces. 
     It will be appreciated that the invention is not limited to using  6 LiF/ZnS:Ag as the neutron scintillating material, and that other neutron scintillating materials can be disposed within the optical waveguide. These materials may include compositions in which both the neutron capture and luminescence functions are embodied within a single composition (i.e., not two as in the case of  6 LiF and ZnS:Ag). For example, liquid or plastic neutron scintillating materials containing  6 Li,  10 B,  157 Gd, and the like, are suitable candidates as neutron scintillating materials. 
     As described above, interaction of the alpha particle and triton with the scintillator material, such as ZnS:Ag, provides for photon emission from the neutron scintillating material  20   b . Accordingly, although other phenomena may be included or potentially influence signals generated by the LiF/ZnS:Ag component, it should be recognized that the use of “neutron sensor” accounts for the various aspects and mechanisms that provide for or are attendant with neutron detection, and therefore the term “neutron sensor” is not to be limited by the various aspects and mechanisms. 
     In the illustrated embodiment, the neutron sensing element  20  comprises a plurality of concentric cylindrical rings or shells of alternating optical waveguide and neutron scintillating material. The rings or shells of material are extended along and are substantially aligned with a longitudinal axis  26  of the detector  10 . However, it will be appreciated that the invention is not limited to a particular configuration for the neutron sensing element  20  and that many possible configurations for the neutron sensing element  20  are within the scope of the invention. In one example, the neutron sensing element  20  has an innermost shell of optical waveguide material  20   a  followed by alternating shells of neutron scintillating material  20   b  and waveguide material  20   a , and an outermost shell of waveguide material  20   a . In another example, the neutron sensing element  20  has an innermost solid “core” of either optical waveguide material  20   a  or neutron scintillating material  20   b  followed by alternating shells of optical waveguide material  20   a  and waveguide material  20   b , and an outermost shell of waveguide material  20   a  (i.e., without the central aperature of  20   c ); in this latter case, it is not required that the gamma sensing element  18  be disposed within the central aperture  20   c  of the neutron sensing element  20 . 
     It will also be appreciated that the invention is not limited by the number and thickness of the alternating shells of waveguide material and neutron scintillating material, and that the number and thickness of the shells can be varied as needed. For example, the neutron sensing element  20  may comprise of only three shells of material, e.g., a shell of neutron scintillating material  20   b  disposed between an innermost and an outermost shell of optical waveguide material  20   a . It will also be appreciated that the invention is not limited by the use of alternating layers, and that the invention can be practiced with non-alternating layers of optical waveguide material and neutron scintillating material. 
     Because the optical waveguide  20   a  surrounds the neutron scintillation material  20   b , the photon emitted by the neutron sensing material  20   b  is collected and channeled to the end of the optical waveguide material  20   a . The photosensing element  24 , which is optically coupled to the neutron sensing element  20 , then detects the photon as a neutron event. To enhance internal reflection of the photon, the optical waveguide material  20   a  should have (ideally) a refractive index greater than that of the neutron scintillating material  20   b . If needed, the surfaces of the optical waveguide material  20   a  may be coated with a thin reflective layer (not shown) to induce a “one-way mirror” effect to improve internal reflection. Ideally, the optical waveguide material  20   a  should also have a very high transmission (&gt;90%) at the wavelength of the neutron scintillating material  20   b . It is appreciated that the thickness of the optical waveguide material  20   a  and the thickness and diameter of the core of neutron scintillating material  20   b  can be optimized to maximize the amount of photons collected by the waveguide material  20   a  and ultimately interpreted as an event. 
     Referring now to  FIGS. 4-6 , another configuration of the neutron sensing element  20  of the detector  10  is shown. In this embodiment, the neutron sensing element  20  comprises a plurality of “strands” of the neutron scintillating material  20   b  disposed within the interior of an annular or tubular optical waveguide material  20   a . The “strands” extend along and are substantially aligned with the longitudinal axis  26  of the detector  10  to form a multi-layered structure having the central aperture  20   c  such that at least a portion of the gamma sensing element  18  can be disposed therein, similar to the cylindrical shell of the neutron sensing element  20  shown in  FIGS. 1-3 . The “strands” may also form the neutron sensing element  20  with a solid core (without the central aperture  20   c ) in the case where it is not required that the gamma sensing element  18  be disposed within the central aperture  20   c  of the neutron sensing element  20 . 
     It will be appreciated that the invention is not limited by the number of layers of “strands” to form the neutron sensing element  20 , and that the invention can be practiced with an optimal number of layers for photon collection and transmission to the photosensing element  24 . 
     In the embodiment shown in  FIG. 5 , each “strand” of the neutron sensing element  20  comprises a cylindrical tube of optical waveguide material  20   a  and a solid “core” of neutron scintillating material  20   b  disposed within the cylindrical tube of optical waveguide material  20   a . However, it should be understood that the invention is not limited to the configuration for each “strand” of the neutron sensing element  20  and that many configurations are within the scope of the invention. For example, the “strands” of neutron sensing element  20  can comprise an outer, cylindrical shell of optical waveguide material  20   a  and an innermost concentric solid “core” of optical waveguide material  20   a  wherein a neutron scintillating material  20   b  is disposed between the shell and “core”, as shown in  FIG. 7 . In an alternate embodiment, the “strands” of neutron sensing element  20  can comprise two concentric, cylindrical shells of optical waveguide material  20   a  and a neutron scintillating material  20   b  disposed between the two shells, as shown in  FIG. 8 . 
     While the invention has been described with reference to an exemplary embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims.