Patent Publication Number: US-2010111487-A1

Title: Phosphate Glasses Suitable for Neutron Detection and Fibers Utilizing Such Glasses

Description:
This application claims the benefit of, and priority to U.S. Provisional Patent Application 61/197,853 filed on Oct. 31, 2008 entitled, “Phosphate Glasses Suitable for Neutron Detection and Fibers Utilizing Such Glasses”, the content of which is relied upon and incorporated herein by reference in its entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates generally to cerium aluminophosphate glasses that include tin and/or antimony, for example lithium-cerium aluminophosphate glasses; and to fibers utilizing such glasses, and more particularly to glasses and fibers suitable for neutron detection. 
     2. Technical Background 
     Traditional neutron detectors are typically  3 He or BF 3  gas-filled counter tubes and scintillating materials such as plastics, organics, semiconductors, or doped glass. 
     Neutrons are usually “invisible” to standard detectors. When neutrons undergo an interaction with the nucleus of an absorber material, e.g.,  6 Li, the energy and momentum of the neutron is drastically changed. If the neutron is captured, the capture reaction gives rise to secondary radiation and charged particles. These reaction products can be detected by conventional Coulombic interactions within a detector through excitation or ionization. 
     Neutron-sensitive scintillating glasses and fibers are an excellent alternative to traditional gas-filled detectors, since they are all solid state and they offer much better use of weight allowances. In addition, signal brightness due to the fluorescence of scintillating glasses can be used to separate the signal of neutrons from that of gamma rays, thereby preventing the detection of gamma ray radiation from innocent sources, such as medical equipment that uses isotopes, tiles made from quarry stone, or even foods, etc. Scintillating fibers also offer remote monitoring and video integration capability over networks, plus rugged designs for harsh vibration environments and temperatures, and would significantly reduce the cost of existing radiation portal monitors. For all of these reasons, efficient neutron sensors made from scintillating glasses and/or fibers have attracted special attention for homeland security, nuclear power generation and military applications. 
     However, current scintillating glass and fiber technology is focused on cerium-activated lithium aluminosilicate glasses and there are significant challenges associated with the use of these glasses and fibers made of these glasses. The amount of lithium oxide that can be practically incorporated in aluminosilicate glasses is limited to less than about 3 wt %, because cerium-activated lithium aluminosilicate glasses are prone to phase separation and/or devitrification at higher lithium concentrations. Accordingly, fibers made from such glasses must be fabricated by a rapid quenching double-crucible draw method, which is relatively expensive and not suitable for large commercial production. Oxidation state control in scintillating cerium-activated lithium aluminosilicate glasses must be strict in order to maximize the concentration of Ce 3+  and, therefore, to avoid the formation of Ce 4+ . The presence of cerium in the latter state results in self-absorption, i.e. the absorption by Ce 4+  of the fluorescence that arises from the interaction of neutrons with Ce 3+ , so that neutron detection is not possible. This requirement for oxidation control further increases glass cost. Moreover, useful fiber lengths are typically limited due to large attenuation losses arising from the presence of Ce 4+ . Altogether, these problems limit the overall neutron detection efficiency of cerium-activated lithium aluminosilicate scintillating glasses and fibers. 
     Cerium-lithium aluminophosphate glasses are known and have been used in applications other than scintillating glasses for neutron detectors. In these cerium-lithium aluminophosphate glasses Ce is largely present in the form of Ce 4+ , which causes self-absorption, making them unsuitable for use in neutron detectors. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the invention, an aluminophosphate glass comprises: (i) 45 to 75 mole % of P 2 O 5 ; (ii) at least one material selected from the group consisting of: Li 2 O, B 2 O 3 , CdO, Gd 2 O 3 ; (iii) 1 to 25 mole % of Al 2 O 3 ; (iv) 0.25 to 15 mole % Ce 2 O 3 ; and (iv) at least 0.25 mole % of SnO and/or Sb 2 O 3 . According to some embodiments, the glass comprises 0.5 to 25 mole % Li 2 O. Preferably, the amount of Li 2 O is at least 3 mole %, more preferably at least 5 mole %, and even more preferably at least 10 mole %. According to embodiments of the present invention, the glass includes Ce ions in the 3+ state. Preferably, more than 90% of Ce ions are in the 3+ state. 
     The tin and/or antimony doped cerium-lithium aluminophosphate glasses of this invention are ideally suited for scintillating glass and fiber applications. These glasses are capable of having lithium and cerium contents that are substantially higher than those that of aluminosilicate glasses, which significantly improves neutron detection. The tin and/or antimony doped cerium-lithium aluminophosphate glasses are not prone to phase separation and/or devitrification problems as are lithium aluminosilicates. In these tin and/or antimony doped cerium-lithium aluminophosphate glasses Sn and/or Sb beneficially act as reducing agents that offer excellent oxidation state control. In addition, cerium absorption in these glasses is shifted to shorter wavelengths relative to that in aluminosilicate glasses, thereby offering better optical transmittance in visible range. 
     Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings. 
     It is to be understood that both the foregoing general description and the following detailed description present embodiments of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated into and constitute a part of this specification. The drawings illustrate various embodiments of the invention and together with the description serve to explain the principles and operations of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view of one embodiment of the fiber according to the present invention. 
         FIG. 2  illustrates cerium fluorescence in lithium aluminophosphate and lithium aluminosilicate glasses. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Reference will now be made in detail to the present preferred embodiment(s) of the invention, examples of which are illustrated in the accompanying drawings. Whenever possible, the same reference numerals will be used throughout the drawings to refer to the same or like parts. 
     A glass suitable for neutron detection should include a neutron absorbing material and a scintillator. A neutron absorbing material combined with a scintillator is capable of interacting with incident neutrons to produce photons. According to some embodiments of the invention, an aluminophosphate glass  10  comprises: (i) 45 to 75 mole % of P 2 O 5 ; (ii) at least one neutron absorbing material selected from the group consisting of: Li 2 O, B 2 O 3 , CdO, Gd 2 O 3 ; (iii) 1 to 25 mole % of Al 2 O 3 ; (iv) 0.25 to 15 mole % Ce 2 O 3 ; and (iv) at least 0.25 mole % of SnO and/or Sb 2 O 3 . According to some embodiments the glass  10  contains no more than 70 mole % P 2 O 5 . According to some embodiments the glass  10  contains 50 to 68 mole % P 2 O 5 . According to some embodiments the glass  10  contains 52 to 65 mole % P 2 O 5 . 
     The preferred neutron absorbing isotopes are  6 Li and/or  10 B. Because  6 Li ions have a very high cross-section for thermal neutron capture and release a large amount of kinetic energy (4.79 MeV),  6 Li is the preferred neutron absorbing material. As the natural abundance of  6 Li in Li 2 O is about 7.5%, higher concentrations of Li 2 O will provide higher amounts of  6 Li. According to some embodiments, the glass comprises 0.5 to 30 mole % Li 2 O. For example, the amount of Li 2 O in the glass may be 3, 3.5, 4, 5, 10, 12, 14, 15, 16, 18, 20, 22 or 25 mole %. According to some embodiments, the glass comprises 0.5 to 25 mole % Li 2 O. It is noted that the glass  10  may be further enriched with  6 Li ions. 
     Preferably, the amount of Li 2 O is at least 10 mole %. Preferably the glass has at least 0.5 mole %, and more preferably at least 1 mole % of Ce 2 O 3 . The scintillation property of the glass  10  is due to the presence of Ce. The high lithium (Li) and cerium (Ce) contents in the glass advantageously provide significantly improved cross-section and scintillation, and thus increased neutron detection efficiency. 
     All glasses of the exemplary embodiments of the invention have been batched using commercially available raw materials. Lithium and aluminum were incorporated as lithium phosphate and aluminum metaphosphate, respectively. In some glass compositions, lithium and aluminum were batched as lithium carbonate and alumina, respectively. Phosphorus was added either as the metal phosphates or as ammonium phosphate. In a few exemplary glass compositions in which the amount of phosphorus contributed by the lithium and aluminum phosphate materials was less than the desired total concentration, the difference was batched as phosphorus pentoxide. Tin and antimony were batched as SnO and Sb 2 O 3 , respectively. Alternatively, tin and antimony can be batched in a different manner, for example, tin can be batched as Sn oxalate. Cerium was typically added as cerium oxalate, in which Ce is nominally present in the 3+ oxidation state. However, similar results in terms of glass coloration and, hence, Ce oxidation state were achieved when CeO 2  (in which Ce is nominally present in the 4+ oxidation state), was used instead—surprisingly, most of the cerium was reduced to the 3 30  oxidation state (during glass melting) due to the presence of Sn and/or Sb. The batches were mixed, charged into silica crucibles and melted at temperatures of 1200-1450° C. for 2-3 hours. The resultant melts were poured in cylindrical molds to form glass rods suitable for fiber draw. 
     Thus, according to embodiments of the present invention, glass  10  includes Ce ions that are in the 3+ state. Preferably, more than 90% of Ce ions are in the 3+ state, more preferably more than 98% or 99% of Ce ions are in the 3+ state. Preferably, the aluminophosphate glass  10  contains less than 1% of Ce ions in 4+ state, because of the undesirable self-absorption properties of these Ce 4+  ions. It is SnO and/or Sb 2 O 3  that act as reducing agents that provide excellent oxidation state control and minimize or eliminate the formation of Ce 4+  ions and promote formation of Ce 3+  ions. Advantageously, while glasses  10  are scintillating glasses, they are not prone to phase separation and/or crystallization problems typical of the lithium aluminosilicate glasses. It is noteworthy that aluminophosphate glasses  10  can provide very high doping of lithium and cerium but do not need to be made under reducing conditions. 
     Those skilled in art would understand that, in order to tailor certain glass properties such as thermal expansion or viscosity, lithium and cerium can be partially replaced by other oxides RO x , where R is selected from the group consisting of alkali metals (Na, K, Rb, Cs), alkaline earths (Be, Mg, Ca, Sr, Ba), zinc, boron, gallium, yttrium, lanthanum, and combinations thereof Thus, glass  10  may also include these materials. However, glass  10  should contain at least 0.5 mole % of Li 2 O, at least 0.25 mole % Ce 2 O 3 , and at least 0.25 mole % of SnO and/or Sb 2 O 3 . Therefore, according to some embodiments, glass  10  contains 0 to 10 mole % of RO x , where R is: sodium, potassium, rubidium, cesium, beryllium, magnesium, calcium, strontium, barium, zinc, boron, gallium, yttrium, lanthanum and combinations thereof. 
     One embodiment of the fiber according to the present invention is shown in  FIG. 1 , and is designated generally throughout by the reference numeral  20 . As embodied herein and depicted in  FIG. 1 , fiber  20  includes a fiber core  22 , surrounded by the cladding  14 . The fiber core  22  is made from glass  10  and comprises (i) 45 to 75 mole % of P 2 O 5 ; (ii) 0.5 to 30 mole % of at least one material selected from the group consisting of: Li 2 O, B 2 O 3 , CdO, Gd 2 O 3 ; (iii) 1 to 25 mole % of Al 2 O 3 ; (iv) 0.25 to 15 mole % Ce 2 O 3 ; and (iv) at 0.25 mole % of SnO and/or Sb 2 O 3 . Core  22  is scintillating. The scintillation property of the core  22  is due to the presence of Ce 3+  ions. Preferably, the neutron absorbing component is Li 2 O, and the amount of Li 2 O in the core  22  is least 10 mole %. According to embodiments of the present invention the core glass includes Ce ions in the 3+ state. According to the embodiments of the present invention, the glass core  22  has an outer diameter of 10 μm to 500 μm, and the cladding  24  has an outer diameter of 25 μm to 750 μm. According to some embodiments the core  22  has an outer diameter of 25 μm to 250 μm and the cladding  24  has an outer diameter of 50 μm to 300 μm. Preferably, the core  22  has an outer diameter of 100 μm to 200 μm to provide adequate cross-sectional area for neutron detection. Preferably, the cladding  24  is polymer and has an outer diameter of 125 μm to 300 μm. Preferably the cladding  24  is made of fluorinated coating material, such as fluorinated plastic. 
     The fiber core preforms of the fibers  20  can be made by a conventional redraw process (instead of a double-crucible draw process with rapid quenching), and then overclad with polymer (e.g., a polymer or acrylate) cladding. The plastic cladding  24  has a lower index of refraction than the glass core  22 . The resulting fibers have good mechanical properties and good durability. The neutron capturing reaction by a  6 Li ion produces an alpha particle and  3 H. The alpha particle and  3 H interact with the glass to produce a series of ionization processes by electronic excitations that excite Ce 3+  ions. After the relaxation of excited Ce 3   30  ions to the ground state, optical photons are emitted. These represent cerium fluorescence. In other words, the alpha particle and  3 H interaction with the glass results in the excitation of a cerium electron and the resulting de-excitation produces a photon with a wavelength around 400 nm Such scintillation further propagates through the glass core  22  of fiber  20 , which acts as a wave guide, especially when clad with a lower refractive index coating. The fiber(s)  20  may be optically coupled to a photo-multiplier tube, where the photon is multiplied and converted to an electronic pulse that can be processed and counted. 
     The tin and/or antimony doped cerium-lithium aluminophosphate core(s)  22  of the fiber(s)  20  preferably have very high lithium and cerium contents for significantly improved cross-section for neutron detection. Both Sn and Sb act as reducing agents that offer excellent oxidation state control, therefore the +3 oxidation state of cerium is much easier maintained than in silica-based scintillating glasses. Moreover, the cerium absorption peak in glasses  10  is shifted to shorter wavelengths, thus offering better optical transmittance in visible range. 
     Examples 
     The invention will be further clarified by the following glass examples. The amount of materials is provided in mole %. The inventive glasses D-J are colorless or only weakly tinted, which indicates the absence or very low amounts of Ce 4+  ions. 
     
       
         
           
               
               
               
               
               
               
               
               
               
               
               
             
               
                   
               
               
                 Oxide 
                 Glass A 
                 Glass B 
                 Glass C 
                 Glass D 
                 Glass E 
                 Glass F 
                 Glass G 
                 Glass H 
                 Glass I 
                 Glass J 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
               
               
               
               
               
            
               
                 P 2 O 5   
                 60.5 
                 66.7 
                 55.3 
                 59.5 
                 55.3 
                 60.5 
                 60.5 
                 60.5 
                 60.5 
                 59.9 
               
               
                 Al 2 O 3   
                 18.4 
                 15.4 
                 24.1 
                 18.4 
                 18.4 
                 13.2 
                 15.3 
                 17.4 
                 15.8 
                 15.1 
               
               
                 Li 2 O 
                 15.8 
                 15.4 
                 20.1 
                 15.8 
                 15.8 
                 15.8 
                 15.8 
                 15.8 
                 15.8 
                 15.6 
               
               
                 Ce 2 O 3   
                 5.26 
                 2.56 
                 0.5 
                 5.26 
                 5.26 
                 5.26 
                 5.26 
                 5.26 
                 5.26 
                 5.21 
               
               
                 SnO 
                 0 
                 0 
                 0 
                 1.05 
                 5.26 
                 5.26 
                 3.16 
                 0 
                 0 
                 3.12 
               
               
                 Sb 2 O 3   
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 0 
                 1.05 
                 2.63 
                 1.04 
               
               
                 Glass 
                 Yellow 
                 Yellow 
                 Yellow 
                 Yellowish 
                 Colorless 
                 Colorless 
                 Colorless 
                 Pale yellow 
                 Yellowish 
                 Colorless 
               
               
                 color 
               
               
                   
               
            
           
         
       
     
     More specifically, Table 1 lists the compositions and color of some manufactured glass embodiments. A yellow color indicates the presence of a significant concentration of Ce 4+  ions, and such a glass can not be used for scintillating applications, unless it was melted in a strictly controlled environment under reducing conditions. A pale yellow tint qualitatively indicates a lesser number of Ce 4+  ions, while a yellowish color reveals the least number of Ce 4   30  present in the glass. A colorless appearance indicates that virtually all cerium ions in the glass are in the 3+ oxidation state. The Table 1 data illustrate that both tin (Sn) and antimony (Sb) act as reducing agents in these glasses and that preferably that at least 1.5 mol % of SnO or at least 3 mol % of Sb 2 O 3  is needed in cerium lithium aluminophosphate glasses to maintain sufficient Ce in the desired 3+ state for scintillating applications. These glasses  10  (glasses D-J) were drawn into glass fibers for splicing into typical glass optical fiber systems using standard techniques. Fibers  20  included glass core and polymer cladding. Their mechanical properties and chemical durability were excellent. 
     The optical glasses made according to the embodiments of the present invention provide better optical transmittance in the visible wavelength range as compared to the lithium aluminosilicate scintillating glasses commonly used for neutron detection, because the cerium absorption peak in phosphate glasses  10  is shifted toward shorter wavelengths relative to that in aluminosilicate glasses. In addition, a comparison of cerium fluorescence in lithium aluminophosphate and lithium aluminosilicate glasses for the excitation at 310 nm, shows stronger fluorescence of aluminophosphate glasses, as indicated in  FIG. 2 . The fact that the cerium fluorescence peak in a phosphate glass  10  is shifted toward shorter wavelengths implies a comparable shift in the cerium absorption peak towards shorter wavelengths (and thus less absorption in visible wavelengths), which therefore provides better optical transmittance in the visible range. 
     It will be apparent to those skilled in the art that various modifications and variations can be made to the present invention without departing from the spirit and scope of the invention. Thus, it is intended that the present invention cover the modifications and variations of this invention provided they come within the scope of the appended claims and their equivalents.