Patent Application
20070102647
This invention relates to sensor materials and a method for detecting ionizing radiation and neutrons by converting the incident radiation to visible light that is detected with an optical detector such as a camera or silicon photodiode.
The materials that are the subject of this invention are substantially glass such as those disclosed in U.S. Pat. No. 6,352,949, issued to Williams et al. Mar. 5, 2002, the entire disclosure of which is incorporated by reference, that is doped with sensitizers including ions of the rare earth element such as europium as well as others as will be disclosed and ions of elements that are sensitive to neutrons such a isotopically enriched lithium-six. The materials can be fabricated into glasses that can be made in various forms including plates, arrays of plates and shaped articles such as hollow cylinders or solid rods. The sensors are of use in medical imaging where high resolution is required, in detecting radiation sources for security applications such as cargo and freight inspection and scanning individuals at ports of entry, airports, etc. In addition the materials are of use in imaging and data acquisition systems for beamlines and testing facilities that use radiation as a probe tool.
Accordingly, it is an object of the invention to provide a radiation detector, comprising a glass emitting photons by scintillation in response to incident neutrons and/or electromagnetic radiation of at least about 1 keV, and a system associated with the glass for detecting the presence of photons emitted by scintillation.
Another object of the invention is to provide a glass ceramic material capable of scintillation upon incident neutrons and/or electromagnetic energy of at least 1 keV comprising a fluoride glass matrix having nano-crystalline particles distributed therein substantially all of which are in a phase that scintillates.
A final object of the invention is to provide a method for making a glass-ceramic material containing nano-crystalline particles with average diameters of less than about 100 nm in a fluorozirconate matrix, comprising mixing ZrF4, an alkali fluoride, an alkaline earth fluoride, a fluoride of a tri-valent metal selected from the group consisting of transition metal ions, rare earth metal ions, In ions, Ga ions, Tl ions, Pb ions and mixtures thereof, together with a bromide/chloride compound selected from the group consisting of alkali and alkaline earth bromides/chlorides, such that zirconium fluoride is present in a concentration of at least 35 mole % and bromide/chloride ions are present in a concentration of at least 5 mole % in the glass-ceramic, heat treating the fluorozirconate matrix at a temperature and for a time sufficient such that substantially all of the nano-crystalline particles are in a phase that scintillates, and thereafter cooling the mixture to room temperature.
The invention consists of certain novel features and a combination of parts hereinafter fully described, illustrated in the accompanying drawings, and particularly pointed out in the appended claims, it being understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.
FIGS. 4(a) and (b) are Eu-doped glass (4b) and commercial single crystal cadmium tungstate (4a). The dark lines result from blocking of the x-rays by gold lines on the test mask. The widths of the lines are 100, 50, 20 10, 5, 2 and 1 micron marked on the image;
FIGS. 5(a) and (b) are mouse foot joints taken by Eu-doped glass ceramic (5b) and commercial single-crystal cadmium tungstate (5a) under identical x-ray imaging conditions; and
This invention relates to solid-state radiation detectors that are sensitive to a wide range of electromagnetic radiation energies from x-ray to gamma ray (approximately 1 keV to 10 MeV or more) and/or sensitive to neutrons. More specifically the invention relates to low-cost solid-state radiation detectors that are based on glass. Solid state detectors have been developed that are separately sensitive to narrow ranges of the electromagnetic spectrum or to neutrons, but none is available that have a wide sensitivity range which may include neutrons. This invention further relates to the use of glass as the medium for the active detector materials that can be incorporated separately or together. The glass can be formed into sheets that can be fabricated even into large area detectors of the types that are envisioned for scanning of large objects such as trucks and cargo containers for security applications.
The invention includes a solid-state detector that is simultaneously sensitive to electromagnetic radiation over a wide spectrum of energies and/or sensitive to neutrons. The invention is a new class of high-resolution imaging detectors based on glass-ceramic plates that emit visible light in response to irradiation by ionizing radiation such as x-rays or gamma rays and/or irradiation by neutrons. The emitted light can be detected using silicon-based photodetectors or video imaging that is inexpensive and can be made using either standard or custom components. An important difference between the invention and the prior art is that the present invention discloses that the light may be directly emitted by scintillation, rather a stored and then released by photostimulated luminescence (PSL). The use of scintillation enables a simpler detector system that does not require a light source to develop PSL and does not require “bleaching” to eliminate residual PSL before the plate is ready to detect again.
The detectors of this invention have applications as medical x-ray detectors in that they provide fine resolution because the material is optically homogeneous. In medical applications, the use of these detectors can permit the detection of abnormalities in the body at an early stage when they are very small, having a strong impact on preventive medicine. These detectors also have a place in homeland security because of the ease of scaling up the detectors to cover a large area, either by forming large sheets of glass or tiling multiple flat or contoured pieces. These detectors are also of use in connection with scientific research applications of electromagnetic radiation and particles such as x-ray tomography for examination of structural components.
The invention combines desirable features of glass products—ease of fabrication, low production cost, scalablity, and detectivity and sensitivity to various types of radiation.
X-ray detection technology includes, film, image intensified video cameras, flat panel detectors based on semiconductors or selenium, sodium iodide doped with thallium, mercuric iodide-based sensors, and PSL storage phosphors based on polycrystalline materials. The current approaches have well recognized limitations: film is one-use, expensive, time consuming and subject to improper exposure. Video camera chains are subject to improper alignment and susceptible to image distortion and have limited sensitivity ranges. Amorphous silicon flat panels have poor resolution and are susceptible to image distortion. Photoconductor flat panel detectors such as systems based on amorphous selenium have proved to be unstable, have low x-ray sensitivity and must be operated at extremely high voltage. Mercuric iodide has high x-ray sensitivity but must be deposited on a substrate as a single crystal, which is time consuming, expensive and limits the size of detector that is practical. Further, the prior art techniques are limited to narrow ranges of detectivity that limit their application in cases where the energies are not predetermined and fixed.
In an effort to extend capabilities for x-ray imaging, storage phosphors based on polycrystalline materials and on glass-based materials have been widely investigated. In these products, the storage principle is that incident x-rays generate electron-hole pairs, which are separately trapped and are stable at room temperature. Recombination of the electron and the hole can be initiated by exposure of the material to a visible wavelength corresponding to the absorption band of the electron. The recombination energy is transferred to a doped activator, such as a rare earth ion, and a higher energy luminescence of the activator is observed. Exposure of the detector to a light source of appropriate frequency could neutralize the detector allowing for reuse. The invention utilizes a fluoride-based proprietary glass composition described in the '949 patent to which various metallic ions have been added to increase sensitivity to a range of electromagnetic energies. For example test samples of about 2% Eu2+ and Br/Cl-doped zirconium fluoride-based formulations have been shown to detect x-rays in the 15 keV range. The effectiveness of the glass as a detector is controlled via the composition and processing method including heat treatment temperature and times as described herein. These detectors may be extended to allow detection of higher energy radiation such as gamma rays.
Glass-based detectors of the invention can detect neutrons by the addition of sensitizers that respond to neutron irradiation by emission of visible light, by doping with lithium-six.
One embodiment of the invention is shown in
In response to the incident radiation, the sensor glass emits a photon or photons that are characteristic of the detecting medium. The emitted photons are indicated as 5 and 6. The emitted photons are detected by an optical system that may be a photon counting device or a spatially resolved imaging device such as a video camera, 7. The camera was a Photometric Coolsnap cooled CCD camera equipped with a Zeiss lens that enables resolution of ca. 1 micron in imaging the scintillation. The mask used to test resolution consisted of a custom-made gold on silicon plate with the thinnest line being 1 micron.
The detection signal is provided to an operator either as a raw signal or may be automatically processed.
A glass sensor plate, in which the glass is simultaneously sensitized to radiation over a broad range of energies by using multiple sensitizers in a single layer of glass.
A large area glass sensor that comprises more than one smaller piece of glass that is tiled in a “mosaic” to form a larger sensitive area. Each tile may be single layer or multi-layered. Combinations of single and multi-layered tiles may be put together to achieve spatially differentiated sensitivity on a single detector.
A glass sensor made from a glass comprising a mixture of metal halides that include at least some europium difluoride (EuF2) that is sensitive to x-ray stimulation. Examples of composition that may be used are given in Table 1 below.
A glass plate that is sensitive to x-rays and gamma rays where the glass has been processed to accomplish one or more of the following:
1. Eu (II) doped ZBLAN heavy metal fluoride glass (see the '949 patent) with partial replacement of F with Br.
2. Eu (II) doped ZBLAN heavy metal fluoride glass with partial replacement of F with Cl.
3. Eu (II) doped ZBLAN heavy metal fluoride glass with replacement of Na with Cs and Li and partial replacement of F with Br and/or Cl.
Additional sensitizers that may be added singly or additively to achieve, enhance or modify the response to radiation are all the metals listed hereafter in Table II, but preferably In+, Ga+, Tl+, Sm3+, Li+, and/or Ce3+. The sensitizers may be added singly or in groups that enable enhanced detection ranges.
A specific example of the preparation of a glass that includes a specialized heat treatment to achieve the desirable response of scintillation properties is given below
The glass melt comprised 2.937 g of zirconium tetrafluoride, 0.237 g of lanthanum fluoride, 0.084 g of aluminum fluoride and 0.278 g of sodium fluoride are placed in a platinum crucible. 0.057 g of indium fluoride is added in order to control the reduction of zirconium (IV) species to zirconium (III). The crucible is then placed in a suitable furnace under an atmosphere of high purity argon. The fluoride powders are firstly dried at 400 degrees Celsius for 1 hour and then the temperature of the furnace is raised to 800 degrees Celsius. The fluoride powders are held at 800 degrees Celsius for a further three quarters of an hour in the high purity argon atmosphere. The melt is then oxidized by changing the argon atmosphere from pure argon to 25% oxygen in argon. The melt is oxidized in the oxygen/argon atmosphere for a further 15 minutes. The crucible is then removed from the furnace and quenched. A white heavily crystallized disc is obtained. 1.380 g of barium chloride and 0.037 g of europium (II) chloride is then added to the platinum crucible containing the heavy metal fluoride disc. These masses give a target composition expressed in mole percent of 53 mol % ZrF4, 20 mol % BaCl2, 3.65 mol % LaF3, 3 mol % AlF3, 20 mol % NaF and 0.35 mol % EuCl2. The temperature of the furnace is then lowered to 750 degrees Celsius and the platinum crucible is placed back into the furnace. The combined fluoride/chloride melt is held at 750 degrees Celsius for 1 hour in a high purity argon atmosphere. The melt is then removed from the furnace and cast into a brass mould preheated to 200 degrees Celsius. The glass is now annealed at 200 degrees Celsius for 3 hours after which the temperature of the mould is raised to 235 degrees Celsius. The glass is held at 235 degrees Celsius for 10 hours and then cooled to room temperature over 15 hours.
In the double anneal the first step at 200 degrees Celsius can be for any period of time. This temperature is lower than the glass transition temperature and Tx3, the crystallization event that gives rise to the new crystalline phase in these materials. The temperature is then quickly raised to 235 degrees Celsius. The time at 235 degrees Celsius can be from 30 minutes to more than 40 hours in order to grow the hexagonal BaCl2. There is a time/size dependence as well as an intensity/time dependence at this stage. The particles grow with increasing time up to 20 hours and the number of particles increases up to 10 hours and then becomes constant.
As long as the temperature is kept below 235 degrees Celsius in order to prevent the transformation of hexagonal to orthorhombic, the glass can be annealed in any number of ways. For example, 200 degrees Celsius from about 1 hour to 30 hours, 210 degrees Celsius or 220 degrees Celsius or 230 degrees Celsius from about 2 hours to 40 hours and 235 degrees Celsius from about 30 minutes to 40 hours.
The heat treatment of the glass achieves desirable changes in the structure of the materials and “ripens” the components of the glass that achieve scintillation by interaction of the europium fluoride with x-rays or gamma rays.
An example of the response of a scintillator glass processed as described above is shown in
The glasses of example 4 wherein at least part of the glass formulation has been replaced with 6Li2F (lithium fluoride made with isotopically enriched lithium 6, such materials are available from Cambridge Isotope Laboratories) that can interact with neutrons to produce an excited helium atom, shown as He* in the equation below.
6Li+1n=3H+4á+2e−[He*]
He*+Ce3+=He+Ce3+*=He+Ce3++hí
Visible light is produced by interactions between excited helium atoms and trivalent cerium ions. The glass can readily incorporate the cerium ions that would be added as CeF3.
In order to detect neutrons, it is necessary that they are “stopped” by the sensitizer and interact to result in light. It is possible for very high energy, fast neutrons to penetrate the material without interacting to produce evidence of their presence. In order to slow neutrons down, a moderator such as a polythene film can be used.
The inventive glasses exhibit high scintillation efficiency comparable to that of single crystal materials in the x-ray energy range from about 1-20 keV. The output of light increases exponentially with increasing x-ray photon energy. Thus output from high-energy radiation, such as short wavelength, high energy x-rays and gamma rays, is large, enabling high sensitivity to the radiation sources that emit in this band. Glasses of the invention should emit photons by scintillation for energies up to about 100 MeV and from neutrons of at least 0.0025 eV.
FIGS. 5(a) and 5(b) compare imaging resolution for the prior art single crystal, 5(a), and the invention, 5(b). The figures show that these two x-ray scintillators have almost the same imaging resolution. The Eu-doped glass-ceramic scintillator has some other advantages such as being sensitive to neutron radiation. It's potential for arbitrary shapes and sizes at a much lower cost than cadmium tungstate also makes it attractive. Glasses can easily accommodate other components that can alter spectral range and lifetime of the rare earth activator. The inventive glass has comparable performance to cadmium tungstate and a lot more versatility.
In addition to the metals listed in Table 1, a variety of other metals may be incorporated into the inventive glass in a wide variety of mixtures or singly, so long as the phases maintain scintillation.
Table II sets forth all the known metals useful as sensitizes for scintillation glasses.
As seen therefore, there has been described a glass-ceramic material in which nanocrystalline particles having average diameters of less than about 100 nm and preferably less than about 20 nm have been disclosed. The nanocrystalline particles are substantially in a phase that maintains scintillation, which for BaCl2
in the hexagonal phase, and produce photons in response to incident neutrons of at least 0.0025 eV and/or electromagnetic radiation of at least 1 keV and up to about 100 MeV, while incident electromagnetic radiation of up to about 20 keV is common.
While there has been disclosed what is considered to be the preferred embodiments of the present invention, it is understood that various changes in the details may be made without departing from the spirit, or sacrificing any of the advantages of the present invention.
The United States Government has rights in this invention pursuant to Contract No. W-31-109-ENG-38 between the U.S. Department of Energy and The University of Chicago representing Argonne National Laboratory.
