Scintillators, defined as materials that emit light upon absorbing ionizing radiation or energy from ionizing radiation, are one of the primary means for detecting most kinds of radiation. There is a need among the research, defense and industrial communities for scintillators that demonstrate improved capabilities in terms of light output, detection efficiency, high count rate capability, better time resolution of events, and for neutron scintillators, fewer false counts due to gamma rays.
At this time, the greatest deficiency in the area of scintillators is in neutron scintillators. Neutrons are uncharged particles that can travel through matter without ionizing the matter. Because neutrons travel through matter in such a manner, neutrons are difficult to detect directly. Some other evidence of a neutron event must be detected in order to determine its existence. An indirect method detects the results of a neutron interaction event and not the neutron itself per se. (For example, recoil protons from fast neutrons scattering off hydrogen atoms and alpha and triton particles emitted when 6Li captures a neutron, all give off energy that can be measured.)
Neutron scintillators can be broadly grouped into two categories: intrinsically neutron-sensitive scintillators and composite neutron scintillators. This grouping can also be made on other bases, most notably optical transparency; intrinsically neutron-sensitive scintillators are usually quite transparent and composite scintillators are usually substantially opaque. Intrinsically neutron-sensitive scintillators (e.g., cerium-activated lithium-bearing glass) are very well known and their operating principles are extensively described in the literature. Composite neutron scintillators usually consist of scintillation particles (hereafter referred to as fluorescent dopant particles to avoid confusion) dispersed in a matrix material containing neutron-sensitive atoms (e.g., neutron target material), although other configurations are possible. Neutrons interact with neutron-sensitive atoms (e.g., 6Li, 10B) in the matrix material, producing reaction products (usually charged particles) that travel some distance (typically microns to tens of microns) through the composite scintillator, relinquishing their energy as they go. If the fluorescent dopant particles are present at a sufficient concentration and are not too large, most or all of the reaction products will at some point enter one or more fluorescent dopant particle and deposit energy in them, the fluorescent dopant particles then producing scintillation light emission that is generally proportional to the amount of energy deposited in them. (The actual scintillation light emission intensity is affected by the linear energy transfer value of the reaction products, possible energy transfer effects across matrix-fluorescent dopant particle boundaries, etc.) Variations on this are possible, such as using a matrix material that emits scintillation light (in addition to the light emitted by the fluorescent dopant particles) or encapsulating neutron-sensitive atoms in dopant particles and using a scintillation matrix material. The most common composite neutron scintillators consist of ZnS:Ag particles in matrices containing 6Li, 10B, or H atoms.
There are two common reasons for using composite neutron scintillators in lieu of intrinsically neutron-sensitive scintillators. The first is that, generally speaking, the more intense the light emission by a scintillator, the better, and some non-neutron sensitive fluorescent materials yield much greater light output than intrinsically neutron-sensitive scintillators. For example, an optimized ZnS:AeLiF combination may emit 150,000 photons per neutron capture, whereas a 6Li-bearing cerium-activated glass scintillator may emit around 10,000 photons per neutron capture. The second reason is that some composite scintillators can yield much better neutron-gamma discrimination than intrinsically neutron-sensitive scintillators. (In most neutron detection applications, the ability to detect neutrons without false neutron counts produced by gamma rays is critical.) The same type of scintillation material produces different quantities of light emission per unit of energy deposited in the scintillator depending on the type and energy of charged particle depositing the energy. The ratio of light emission produced by heavy charged particles (e.g., 6Li reaction products) to that produced by electrons (gamma rays relinquish their energy via electrons) per unit of deposited energy for non-neutron sensitive ZnS:Ag, for example, is much higher than that for 6Li-bearing cerium-activated glass. Thus, a composite scintillator based on ZnS:Ag can much more effectively discriminate between neutron- and gamma-induced scintillation events by rejecting low pulse amplitude events, all other things being equal.
The major drawback to composite scintillators is their tendency to be optically opaque, making them useable only in a thin film form, limiting their efficiency for detecting neutrons. A major source (normally the most important source and sometimes the only significant source) of this is a mismatch of optical indices between the materials forming the composite scintillator (e.g., fluorescent dopant particles, matrix material, neutron target dopant particles if the neutron-sensitive material is added that way). Scintillation photons traveling through the composite scintillator scatter or reflect at the material boundaries and thus must travel a much greater distance between reaching the edge of the scintillator and exiting, thus giving them a greater opportunity to be absorbed along the way. As such scintillators are made thicker, the pulse amplitudes from the additional neutron counts thereby gained rapidly decreases. The consequent lack of consistent neutron pulses amplitude, and thus a neutron capture reaction energy peak, makes it very difficult to know where to set specific minimum amplitude for rejection of non-neutron (e.g., gamma) pulses and to know what fraction of gamma and what fraction of neutron pulses are accepted and rejected as a result.
Scintillators may be used for the detection of other kinds of radiation as well. For example, some alpha and beta particle detectors use a thin layer of scintillator covered by a light-tight cover. Scintillators are commonly used for gamma ray spectroscopy, although very high-resolution applications use semiconductor detectors. Gamma scintillators with higher efficiencies and therefore increased light yields will demonstrate better energy resolution, potentially enabling their use in gamma ray spectroscopy applications that are currently restricted to using semiconductor devices. Faster gamma scintillators are also desirable as some of the brightest and most widely used gamma scintillators (for example, thallium-doped sodium iodide, NaI:Tl), have decay times in the hundreds of nanoseconds or longer and this does limit their use in some very high-speed applications.
Neutron radiation is an unequivocal signature of the presence of transuranic elements associated with nuclear power-generated plutonium and enriched uranium and plutonium derived from the disassembly of nuclear weapons. Both passive and active neutron detection methods have been used in such applications, with the latter involving detection of the secondary fission neutrons induced by a brief pulse of neutrons. The prerequisite for neutron scintillators is the presence of neutron target elements (including neutron absorbing elements), which is not required for the fabrication of other radiation scintillators such as beta and alpha radiations. Favored isotopes for neutron target material include 10B, 6Li, 3He, and 235U, all of which have high absorption cross sections for thermal neutrons. In previous work by Wallace et. al. (Nuclear Instruments and Methods in Physics Research A, 2002), a surface barrier detector based on sol-gel technology was reported as part of an active interrogation method for identifying the presence of uranium. Then, Im et al. reported in Applied Physics Letters, March 2004, an approach to neutron scintillator fabrication that employs a room temperature sol-gel processing.
Industries and geological survey agencies who use or need to detect neutrons are interested in the development of new neutron detectors with advantages in detection efficiency (and therefore sensitivity) and versatility over the methods in current use. An improved neutron detector technology could even play a role in national security in screening for insecure fissile weapons materials. Currently, many solid-state neutron scintillators such as 6Li-doped silica glasses are prepared by high-temperature methods. Because of the high temperature employed, these materials are very difficult to integrate as films into electronic devices for neutron detection or to cast as large screens. Furthermore, the high temperature methods eliminate the possibility of using organic scintillators because such organic compounds are seldom stable at elevated temperatures. Thus, the development of efficient solid-state scintillation materials, which will significantly enhance general capabilities for in situ monitoring and imaging of radioactive contaminants in the environment, is demanded.
Neutron detection for monitoring the dose of thermal neutrons given to patients receiving boron neutron-capture therapy has used lithium-6 and a cerium activator in a glass fiber (M. Bliss et. al., IEEE Trans. Nucl. Sci., 1995). Hiller et. al., in U.S. Pat. No. 5,973,328, issued on Oct. 26, 1999, improve this technique by allowing a cerium-activated glass fiber to be coated with fissionable elements. A wet chemistry method of placing radioactive fissile elements into glass—which in the vitrified state does not pose a hazard—as described in the '328 patent using sol-gel based technology, is a significant benefit. The '328 device introduced sol-gel techniques unique in the art of neutron detection. Sol-gel chemistry was first discovered in the late 1800s.
Emission detectors such as microchannel plates, channeltrons, and avalanche photodiodes are commonly used for detecting ultraviolet (UV) light and fissioned charged particles such as electrons or protons. M. Ghioni et. al. (1996) describe an avalanche photodiode implementation for detecting neutron induced ionization and optical pulse detection. Microchannel plates are commercially available and well known in the art. Typically, a microchannel plate is formed from lead glass having a uniform porous structure of millions of tiny holes or microchannels. Each microchannel functions as a channel electron multiplier, relatively independent of adjacent channels. A thin metal electrode is vacuum deposited on both the input and output surfaces to electrically connect channels in parallel. Microchannel plates can be assembled in stacked series to enhance gain and performance. The '328 patent demonstrated the use of a microchannel plate for the detection of neutrons.
The microchannel plates serve to amplify emissions from fissionable material resulting from the bombardment of neutrons. The amplified signal is then detected and recorded. The signal frequency is proportional to the charged particle emissions, which are proportional to the amount of neutrons bombarding the fissionable material.
Typically, due to the exotic materials and sensitivity of the equipment, the neutron detectors currently available are expensive and difficult to maintain. For example, helium-3 is an extremely rare stable isotope which must be separated at considerable expense from the radioactive gas tritium. Furthermore, the use of a gas absorber results in a slower response time than a solid absorber as disclosed herein. The '328 device thus incorporated fissionable material into a sol-gel composition in combination with an emission detector.