Method for Sintering High Density Yttria-Gadolinia Ceramic Scintillators

This application is related to application Ser. Nos. 389,812, 389,814, 389,815, 389,816, 389,817, 389,818, 389,828 and 389,830, all filed on the same date and all assigned to the same assignee as the present invention.

The present invention relates to rare-earth doped ceramic scintillators for computerized tomography (CT) and other x-ray, gamma radiation, and nuclear radiation detecting applications. More specifically, the invention relates to rare-earth-doped, polycrystalline, yttria/gadolinia (Y.sub.2 O.sub.3 /Gd.sub.2 O.sub.3) ceramic scintillators.

Computerized tomography scanners are medical diagnostic instruments in which the subject is exposed to a relatively planar beam or beams of x-ray radiation, the intensity of which varies in direct relationship to the energy absorption along a plurality of subject body paths. By measuring the x-ray intensity (i.e., the x-ray absorption) along these paths from a plurality of different angles or views, an x-ray absorption coefficient can be computed for various areas in any plane of the body through which the radiation passes. These areas typically comprise approximately a square portion of about 1 mm.times.1 mm. The absorption coefficients are used to produce a display of, for example, the bodily organs intersected by the x-ray beam.

An integral and important part of the scanner is the x-ray detector which receives the x-ray radiation which has been modulated by passage through the particular body under study. Generally, the x-ray detector contains a scintillator material which, when excited by the impinging x-ray radiation, emits optical wavelength radiation. In typical medical or industrial applications, the optical output from the scintillator material is made to impinge upon photoelectrically responsive materials in order to produce electrical output signals, the amplitude of which is directly related to the intensity of the impinging x-ray radiation. The electrical signals are digitized for processing by digital computer means which generates the absorption coefficients in a form suitable for display on a cathode ray tube screen or other permanent media.

Due to the specific and demanding computerized tomography requirements, not all scintillator materials which emit optical radiation upon excitation by x-ray or gamma ray radiation are suitable for CT applications. Useful scintillators must be efficient converters of x-ray radiation into optical radiation in those regions of the electromagnetic spectrum (visible and near visible) which are most efficiently detected by photosensors such as photomultipliers or photodiodes. It is also desirable that the scintillator transmit the optical radiation efficiently, avoiding optical trapping, such that optical radiation originating deep in the scintillator body escapes for detection by externally situated photodetectors. This is particularly important in medical diagnostic applications where it is desirable that x-ray dosage be as small as possible to minimize patient exposure, while maintaining adequate quantum detection efficiency and a high signal-to-noise ratio.

Among other desirable scintillator material properties are short afterglow or persistence, low hysteresis, high x-ray stopping power, and spectral linearity. Afterglow is the tendency of the scintillator to continue emitting optical radiation for a time after termination of x-ray excitation, resulting in blurring, with time, of the information-bearing signal. Short afterglow is also highly desirable in applications requiring rapid sequential scanning such as, for example, in imaging moving bodily organs. Hysteresis is the scintillator material property whereby the optical output varies for identical x-ray excitation based on the radiation history of the scintillator. This is undesirable due to the requirement in CT for repeated precise measurements of optical output from each scintillator cell and where the optical output must be substantially identical for identical x-ray radiation exposure impinging on the scintillator body. Typical detecting accuracies are on the order of one part in one thousand for a number of successive measurements taken at relatively high rate. High x-ray stopping power is desirable for efficient x-ray detection. X-rays not absorbed by the scintillator escape detection. Spectral linearity is another important scintillator material property because x-rays impinging thereon have different frequencies. Scintillator response must be substantially uniform at all x-ray frequencies.

Among scintillator phosphors considered for CT use are monocrystalline materials such as cesium iodide (CsI), bismuth germanate (Bi.sub.4 Ge.sub.3 O.sub.12), cadmium tungstate (CdWO.sub.4), and sodium iodide (NaI). Many of the aforementioned materials typically suffer from one or more deficiencies such as excessive afterglow, low light output, cleavage, low mechanical strength, hysteresis, and high cost. Many monocrystalline scintillators are also subject to hygroscopic attack. Known polycrystalline scintillators are efficient and economical. However, due to their polycrystalline nature, such materials are not efficient light propagators and are subject to considerable optical trapping. Internal light paths are extremely long and tortuous, resulting in unacceptable attenuation of optical output.

Fabrication of monocrystalline scintillators from multicomponent powder constituents is typically not economical and frequently impractical. The multicomponent powder composition must be heated to a temperature above its melting point, and ingots of dimensions larger than those of each detector channel are grown from the melt. Considering the size of the bars required and the temperatures involved, the process is difficult in and of itself. In addition, some materials exhibit phase changes while cooling, which would cause the crystals to crack when cooled after the growing process. Furthermore, single crystals tend to be susceptible to the propogation of lattice defects along the crystal planes.

U.S. Pat. No. 4,242,221 issued to D. A. Cusano et al (assigned to the same assignee as the present invention) describes methods for fabricating polycrystalline phosphors into ceramic-like scintillator bodies for use in CT.

The present invention provides improved ceramic scintillators composed of yttria-gadolinia and including a variety of rare earth activators for enhancing luminescent efficiency.

The terms "transparency" and "translucency", as used herein, describe various degrees of optical clarity in the scintillator material. Typically, the inventive scintillator materials exhibit an optical attenuation coefficient of less than 100 cm.sup.-1, as measured by standard spectral transmittance tests (i.e., "narrow" angle transmission) on a polished scintillator material plate, at the luminescent wavelength of the respective ion. The most desirable scintillator materials have lower attenuation coefficients and hence higher optical clarity (transparency).

The invention relates to a method for preparing a yttria-gadolinia ceramic scintillator body. The method includes a step of preparing a multicomponent powder containing between about 5 and 50 mole percent Gd.sub.2 O.sub.3 and between about 0.02 and 12 mole percent of at least one rare earth activator oxide selected from the group consisting of Eu.sub.2 O.sub.3, Nd.sub.2 O.sub.3, Yb.sub.2 O.sub.3, Dy.sub.2 O.sub.3, Tb.sub.2 O.sub.3, and Pr.sub.2 O.sub.3. The powder also includes at least one transparency promoter selected from the group consisting of ThO.sub.2, ZrO.sub.2, and Ta.sub.2 O.sub.5, as well as at least one light output restorer selected from the group consisting of CaO and SrO. The remainder of the multicomponent powder is Y.sub.2 O.sub.3. The multicomponent powder is cold pressed to form powder compacts which are sintered to form the yttria-gadolinia ceramic scintillator body.

It is an object of the invention to provide a sintering process for preparing rare-earth-doped, transparent-to-translucent polycrystalline yttria-gadolinia ceramic scintillators having cubic crystalline structure, high x-ray stopping powder, high density, high uniformity, and high radiant efficiency, and which are useful in radiation detectors such as those used in CT and digital radiography.

It is another object of the invention to provide a sintering process for preparing polycrystalline yttria-gadolinia ceramic scintillators exhibiting low luminescence afterglow.

The features of the invention believed to be novel are set forth with particularity in the appended claims. The invention itself, however, both as to its organization and method of operation, together with further objects and advantages thereof, may best be understood by reference to the following description taken in conjunction with the accompanying drawings.

FIG. 1 depicts graphically the effect of increased thoria (ThO.sub.2) content on the light output and the light output restorative effects of CaO and SrO in a yttria-gadolinia ceramic scintillator containing 3 mole percent Eu.sub.2 O.sub.3.

FIG. 2 is a graphical illustration of the dependence of the relative light output on Eu.sub.2 O.sub.3 activator concentration in an inventive scintillator material containing 35 mole percent Gd.sub.2 O.sub.3, the remainder being Y.sub.2 O.sub.3.

FIG. 3a illustrates graphically the dependence of scintillator efficiency on relative yttria-gadolinia content of an inventive ceramic scintillator containing 3 mole percent Eu.sub.2 O.sub.3 activator.

FIG. 3b is a graph illustrating 73 kev x-ray stopping power versus yttria-gadolinia compositional ratio of an inventive ceramic scintillator.

FIG. 4 is a graphical illustration of the effect of increased Yb.sub.2 O.sub.3 concentrations on scintillator material afterglow.

FIG. 5 depicts graphically the relative light output of an inventive scintillator material with increased Yb.sub.2 O.sub.3 content.

FIG. 6 is a graphical illustration of the effect of increased SrO concentration on scintillator material afterglow.

U.S. Pat. No. 3,640,887, issued to R. C. Anderson and assigned to the same assignee as the present invention, describes the manufacture of transparent polycrystalline ceramic bodies. The bodies include the oxides of thorium, zirconium and hafnium and mixtures thereof with oxides of rare earth elements 58 through 71 of the Periodic Table. The bodies may optionally include yttria. The average ionic radius of the rare earth oxide with or without yttria must not exceed about 0.93 .ANG., and the difference in ionic sizes of the constituent oxides should not exceed 0.22 .ANG.. The polycrystalline ceramics are disclosed as useful in high temperature applications and/or applications requiring light transmission. Exemplary applications include high temperature microscopes, lamp envelopes, laser applications, and furnace windows.

The aforedescribed patent teaches that each polycrystalline ceramic body includes between about 2 to 15 mole percent of thoria (ThO.sub.2), zirconia (ZrO.sub.2), hafnia (HfO.sub.2), or some combination thereof to act as a densifying agent during sintering to promote transparency.

Translucent-to-transparent ceramic scintillator bodies composed of Y.sub.2 O.sub.3, Gd.sub.2 O.sub.3, and one or more rear earth activator oxides, as described and claimed in copending application Serial No. 389,812, filed concurrently herewith on June 18, 1982 by the same inventors as herein and assigned to the same assignee as the present invention, can be produced without the aid of transparency promoters such as ThO.sub.2, ZrO.sub.2, Ta.sub.2 O.sub.5, and SrO. However, the transparency of some yttria-gadolinia ceramic scintillators is improved by the addition of transparency promoters. For example, a test pattern formed on one surface of a scintillator material, including a transparency promoter, approximately 1.5 mm thick can be resolved on the opposite surface of the scintillator material down to 7 line pairs per millimeter. Stated in another way, a series of 3 mil line widths can be read through a 1.5 mm thick scintillator material sample. Without the transparency promoter, a 1.5 mm thick sample will barely resolve one line pair per millimeter, or stated differently, a series of 0.5 mm lines will be barely resolved by the 1.5 mm thick scintillator sample. It is apparent, therefore, that scintillator materials which include transparency promoters have short optical escape paths from within any part of the scintillator body where x-rays are absorbed. Such scintillator materials are particularly desirable for CT use.

Applicants have found, however, that the inclusion of ThO.sub.2, ZrO.sub.2, or HfO.sub.2 in the scintillator materials of the present invention in amounts specified in the aforedescribed Anderson patent (about 2-15 mole percent) results in a material having greatly reduced light output when excited by high energy radiation such as x-rays. The resulting materials are unsuitable for use in CT. It has also been found that Ta.sub.2 O.sub.5 produces a similar reduction in scintillator material light output. These tetravalent (4.sup.+) and pentavalent (5.sup.+) additive species have an inhibiting effect on the light output of the scintillator. It is important to note that light output refers to scintillation resulting from x-ray excitation. This is a significant distinction since some ceramic bodies fluoresce under ultraviolet excitation, but do not scintillate upon x-ray excitation, for example.

Curve H, FIG. 1, illustrates relative light output (vertical axis) of a polycrystalline ceramic composed of about 58.7 mole percent Y.sub.2 O.sub.3, 38 mole percent gadolinia (Gd.sub.2 O.sub.3), 3 mole percent Eu.sub.2 O.sub.3, 0.3 and mole percent Yb.sub.2 O.sub.3, with increasing thoria (ThO.sub.2) content in mole percent (shown on the horizontal axis). As the quantity of ThO.sub.2 is increased, the quantity of Y.sub.2 O.sub.3 is correspondingly decreased. The average ionic radius of the ceramic constituents and the difference between ionic radii are as specified in the Anderson patent. It is evident from Curve H that the light output for a material containing 2 mole percent ThO.sub.2 (the minimum amount specified by Anderson) is only 5 percent of the light output for the same material without thoria. In fact, the addition of as little as 0.5 mole percent ThO.sub.2, well below the lower limit specified in the Anderson patent, reduces the light output to a low value of about 18 percent of that measured for the material without ThO.sub.2. In another example (not shown in FIG. 1), the relative light output of a ceramic body containing 56.5 mole percent Y.sub.2 O.sub.3, 38 mole percent Gd.sub.2 O.sub.3, 3 mole percent Eu.sub.2 O.sub.3, 0.5 mole percent Yb.sub.2 O.sub.3, and 2 mole percent ZrO.sub.2 is only 4 percent of the light output of the same material without ZrO.sub.2.

It has been discovered by the inventors herein that the light reducing effect due to the addition of ThO.sub.2, ZrO.sub.2, and Ta.sub.2 O.sub.5 can be partially reversed by the addition of calcium oxide (CaO) or strontium oxide (SrO). The rejuvenating effect of CaO on the relative light output degraded by the addition of ThO.sub.2 is illustrated by Curve K in FIG. 1. The relative light output of a scintillator material having 38.0 mole percent Gd.sub.2 O.sub.3, 3 mole percent Eu.sub.2 O.sub.3, 0.3 mole percent Yb.sub.2 O.sub.3, and 0.5 mole percent ThO.sub.2, the remainder being Y.sub.2 O.sub.3, was observed to be 18 percent of that of a material without ThO.sub.2 (Curve H, FIG. 1). Curve K illustrates the light output restorative effect of CaO in a similar scintillator composition in which the molar ratio of CaO to ThO.sub.2 is 2:1. Thus, the light output of a composition including 1.0 percent CaO and 0.5 mole percent ThO.sub.2 was restored to about 44 percent of that of a material without ThO.sub.2. It is also evident from Curve K that the addition of 4 mole percent CaO to a composition containing 2 mole percent ThO.sub.2 increased relative light output from about 4 percent to about 13 percent.

A light output restorative effect has also been observed for SrO as illustrated by Curve J, FIG. 1, for the scintillator having the above-described composition and a molar ratio of SrO to ThO.sub.2 of 2:1. For example, the light output was observed to increase from 18 percent for a material containing 0.5 mole percent ThO.sub.2, to about 32 percent for the same material having in addition 1 mole percent SrO.

It has been found that SrO can also function as a transparency promoter in the yttria-gadolinia ceramic scintillator. The preferred mole percentage of SrO when used as a transparency promoter is between about 0.1 and 2, and is between about 0.5 and 3.0 when it is used as a light output restorer. Preferred CaO content is between about 0.2 and 4 mole percent.

Transparency promoters ThO.sub.2, ZrO.sub.2, and Ta.sub.2 O.sub.5 are useful in quantities of up to 0.7 mole percent, 0.7 mole percent, and 0.5 mole percent, respectively. It will be noted from Curve H, FIG. 1, that if the mole percent of ThO.sub.2 is kept below about 0.35 mole percent, the light output is not severely reduced. Hence, for some x-ray applications where maximum light output is not required, light output restoring additives may not be needed. Similarly if the mole percentages of ZrO.sub.2 and Ta.sub.2 O.sub.5 are maintained below 0.35 and 0.3, respectively, light output restoring additives are unnecessary.

The oxides of rare-earth elements europium, neodymium, ytterbium, dysprosium, terbium, and praseodymium (Eu.sub.2 O.sub.3, Nd.sub.2 O.sub.3, Yb.sub.2 O.sub.3, Dy.sub.2 O.sub.3, Tb.sub.2 O.sub.3, and Pr.sub.2 O.sub.3, respectively) are added to the basic yttria-gadolinia phosphor system, including transparency promoters, as activators to enhance scintillator efficiency. It is to be noted that activator efficacy is independent of the relative proportions of Y.sub.2 O.sub.3 and Gd.sub.2 O.sub.3, and of the other scintillator material constituents. Generally, rare earth activator content may range between 0.02 and 12 mole percent.

Yttria-gadolinia scintillators containing Eu.sub.2 O.sub.3 exhibit excellent scintillating efficiency. Optimum concentration of Eu.sub.2 O.sub.3 is between 1 and 6 mole percent. This is illustrated in FIG. 2, which shows that the highest relative light output, indicated on the vertical axis, is observed for Eu.sub.2 O.sub.3 concentrations of between about 1 and 6 mole percent, as indicated on the horizontal axis. The curve depicted in FIG. 2 was obtained by varying Eu.sub.2 O.sub.3 content of a scintillator material containing 25 mole percent Gd.sub.2 O.sub.3, the remainder being Y.sub.2 O.sub.3.

Neodymium oxide (Nd.sub.2 O.sub.3) is preferably added in quantities of between about 0.05 and 1.5 mole percent. Most preferably, however, Nd.sub.2 O.sub.3 is added in concentrations of between about 0.1 and 0.5 mole percent. Preferred terbium oxide (Tb.sub.2 O.sub.3) activator concentration is between about 0.05 and 3 mole percent, while the preferred concentration of dysprosium oxide (Dy.sub.2 O.sub.3) activator is between about 0.03 and 1.0 mole percent. The preferred range of Yb.sub.2 O.sub.3 as an activator is between about 0.1 and 2 mole percent. The preferred mole percentage for Pr.sub.2 O.sub.3 activator is between about 0.02 and 0.05.

Eu.sub.2 O.sub.3 is the preferred activator followed, in order to preference, by Nd.sub.2 O.sub.3 and Dy.sub.2 O.sub.3.

FIG. 3a illustrates the dependence of scintillator efficiency, as measured by relative light output, on the compositional ratio of yttria and gadolinia in a scintillator material containing 3 mole percent Eu.sub.2 O.sub.3. The relative mole percentages of Gd.sub.2 O.sub.3 and Y.sub.2 O.sub.3 are shown on the horizontal axis, while the relative light output is shown on the vertical axis. The dashed line at 50 mole percent Gd.sub.2 O.sub.3 and 50 mole percent Y.sub.2 O.sub.3 indicates the beginning of a gradual crystalline phase transition in the scintillator material structure from the cubic phase to the monoclinic phase. It will be observed that high relative light output is obtained from scintillator materials containing up to about 50 mole percent Gd.sub.2 O.sub.3. Scintillator materials containing between 50 and 65 mole percent Gd.sub.2 O.sub.3 exhibit modest relative light output, but are increasingly subject to grain boundary cracking and reduced relative light output due to progressive transition from cubic to monoclinic crystalline phase.

The cubic crystalline phase is characterized by a high degree of scintillator material structural symmetry. Materials having such structure are particularly desirable for CT applications. Scintillator materials having increasing amounts of monoclinic phase are characterized by lower relative light outputs and poor optical clarity due to grain boundary cracking and nonuniform crystalline structure. Materials having such noncubic structure exhibit appreciable light scattering and reabsorption due to a longer effective relative light path length, thereby reducing the amount of light available for detection by external photosensors.

In considering the usefulness of the scintillator material in CT applications, the x-ray stopping power of the material must also be considered. FIG. 3b illustrates the dependence of 73 kev x-ray stopping length on yttria-gadolinia compositional ratio for transparent and efficient scintillators. Stopping power is measured in terms of x-ray stopping length, i.e., the distance an x-ray photon penetrates into the scintillator prior to its conversion to optical wavelength photons which are detectable by photosensors. X-ray stopping length is primarily dependent on Gd.sub.2 O.sub.3 content and, as shown in FIG. 3b, increases with increased Gd.sub.2 O.sub.3 concentrations. Generally, it is preferred to use between about 5 mole percent and 50 mole percent Gd.sub.2 O.sub.3. Materials containing less than about 5 mole percent Gd.sub.2 O.sub.3 exhibit low x-ray stopping power for most practical design, while materials having more than 50 mole percent are increasing non-cubic and exhibit poor optical clarity. A more preferred range of Gd.sub.2 O.sub.3 content is between 20 and 40 mole percent. The most preferred range of Gd.sub.2 O.sub.3 concentration is between 30 mole percent and 40 mole percent, corresponding to an x-ray stopping length of about 0.45 mm. For a 2 mm thick scintillator material having an x-ray stopping length of 0.45 mm, approximately 99 percent of x-ray photons entering the material are converted to optical wavelength photons.

Certain additives are useful in the yttria-gadolinia scintillator system of the present invention to reduce undesirable scintillator material luminescent afterglow, which may lead to undesirable distortion and the presence of artifacts in reconstructed images. The luminescent afterglow phenomenon is classifiable into primary or fundamental afterglow and secondary afterglow. Primary afterglow is of relatively short duration (up to approximately 3 milliseconds), while secondary afterglow may be several times to much more than several times the primary decay time. Fundamental luminescent afterglow of a phosphor is thought to be inextricably associated with the specific activator identity and the activator local environment in the host matrix (in this case yttria-gadolinia). The secondary, and most objectionable type of afterglow, can be associated with more subtle changes in the activator environment or simply with the presence of additional electron-hole "trapping" centers created by native defects and/or low level impurities at other sites in the host crystal. Both types of afterglow may be reduced by suitable purification or the addition of compensating dopants. The added dopants used to reduce afterglow do so by forming "killer" centers which are believed to compete with the activator centers for electron-hole pairs that otherwise combine radiatively at the activator centers.

Luminescent afterglow of rare-earth doped yttria-gadolinia ceramic scintillators of the present invention can be substantially reduced by two types of additives.

The addition of ytterbium oxide (Yb.sub.2 O.sub.3), itself a luminescent activator in the yttria-gadolinia host if used alone as described heretofore, results in the reduction of undesirable secondary afterglow with only minor sacrifice of luminescent efficiency. If, as depicted in FIG. 4, the mole percentage of Yb.sub.2 O.sub.3 is increased from zero to about 2 mole percent, the primary or fundamental afterglow, .tau., of the scintillator material activated with 3 mole percent of Eu.sub.2 O.sub.3 is reduced from 1.1 to 0.82 milliseconds. An increase in Yb.sub.2 O.sub.3 content from 0 to 2 mole percent is accompanied by the loss of nearly 50 percent of scintillator material luminescent efficiency as graphically depicted in FIG. 5, in which relative light output is shown on the vertical axis, while the Yb.sub.2 O.sub.3 concentration is shown on the horizontal axis.

Curves A, B, C, and D, depicted in FIG. 4, illustrate the fraction of secondary luminescent afterglow (vertical axis) remaining at times greater than 10 milliseconds (horizontal axis) following the cessation of x-ray excition. For a scintillator material having 30 mole percent Gd.sub.2 O.sub.3, 3 mole percent Eu.sub.2 O.sub.3, and 67 mole percent Y.sub.2 O.sub.3, but no Yb.sub.2 O.sub.3, it is evident from Curve A that at the end of 10 milliseconds following x-ray turn-off, about three percent of the luminescence present immediately upon x-ray shut-off remains. Curves B, C, and D depict fractional afterglow for similar scintillator materials which additionally contain 0.2, 0.5, and 2 mole percent Yb.sub.2 O.sub.3, respectively, and corresponding less Y.sub.2 O.sub.3. It is apparent that increasing quantities of Yb.sub.2 O.sub.3 reduce secondary afterglow. For example, at about 10 milliseconds after x-ray turn-off, fractional afterglow for a scintillator material containing 2 mole percent Yb.sub.2 O.sub.3 (Curve D) is approximately only 0.7 percent (7.times.10.sup.-3) of its value immediately upon termination of x-ray excitation as compared to about 3 percent (3.times.10.sup.-2) for a material without Yb.sub.2 O.sub.3 (Curve A). The addition of 0.3 mole percent Yb.sub.2 O.sub.3 to a scintillator composition made up of 66.7 mole percent Y.sub.2 O.sub.3, 30 mole percent Gd.sub.2 O.sub.3, and 3 mole percent Eu.sub.2 O.sub.3 results in an extremely useful CT scintillator material having a fast decay time. Preferred concentration of Yb.sub.2 O.sub.3 for afterglow reduction is between about 0.15 and 0.7 mole percent.

Another additive dopant which is effective in reducing scintillator material luminescent afterglow is strontium oxide (SrO). The addition of SrO results primarily in the reduction of secondary afterglow with relatively little sacrifice of luminescent efficiency. In the yttria-gadolinia scintillator system, the quantity of SrO generally found to be useful in reducing afterglow is between 0.1 and 2 mole percent. It is shown in FIG. 6 that an increase in the quantity of SrO from 0 to 2 mole percent has no appreciable effect on primary afterglow, .tau., 1.08 milliseconds and 1.10 milliseconds, respectively. However, there is appreciable effect on secondary afterglow as depicted by curves E and F. A scintillator material having 30 mole percent Gd.sub.2 O.sub.3, 2 mole percent Eu.sub.2 O.sub.3, 68 mole percent Y.sub.2 O.sub.3, but no SrO (curve E) exhibits, at about 150 milliseconds after x-ray turn-off, about 0.8 percent (8.times.10.sup.-3) of the luminescence present immediately after x-ray shut-off. Scintillator materials having the same composition (Curve F) but including 2 mole percent SrO (and 2 mole percent less Y.sub.2 O.sub.3) exhibit, after the same elapsed time, only about 0.03 percent (3.times.10.sup.-4) fractional afterglow as indicated on the vertical axis in FIG. 6.

The aforedescribed yttria-gadolinia rare-earth-doped ceramic scintillator materials may be prepared by sintering, sintering plus gas hot isostatic pressing, and hot pressing ceramic methods. The ceramic scintillator materials are preferably and most economically fabricated by employing a sintering process.

A preliminary step in the fabrication of the ceramic scintillators, by any of the aforementioned methods, requires the preparation of a suitable powder containing the desired scintillator material constituents. In accordance with a first method for preparing such a powder, submicron-to-micron powders of yttria (Y.sub.2 O.sub.3) and gadolinia (Gd.sub.2 O.sub.3) having purities of, for example, 99.99 percent to 99.9999 percent are mixed with the desired rare earth activators in the form of oxides, oxalates, carbonates, or nitrates and mixtures thereof. The mixing of the selected constituents may be carried out in an agate mortar and pestle or in a ball mill using water, heptane, or an alcohol (such as ethyl alcohol) as liquid vehicles. Dry milling may also be used for both mixing and breakup of powder aggregates. If dry milling is employed, a grinding aid such as 1 to 5 weight percent of stearic acid or oleic acid should be employed to prevent powder packing or sticking inside the ball mill. Transparency promoters such as SrO, Yb.sub.2 O.sub.3, Ta.sub.2 O.sub.5, ZrO.sub.2 and ThO.sub.2 may also be added in the form of oxides, nitrates, carbonates, or oxalates before ball milling. If the various additives are nitrates, carbonates, or oxalates, a calcining step is required to obtain the corresponding oxides prior to fabrication of the ceramic scintillator by any of the methods described hereinafter.

A second approach to obtaining the desired scintillator starting powder employs a wet chemical oxalate method. In this method, the selected molar percentages of the nitrates of predetermined ones of Y, Gd, Eu, Nb, Yb, Dy, Tb, Pr, and Sr are dissolved in water and coprecipitated in oxalic acid to form the respective oxalates. The oxalate precipitation process involves the addition of the aqueous nitrate solution of the desired scintillator material constituents to an oxalic acid solution which is, for example, 80 percent saturated at room temperature. The resulting coprecipitated oxalates are washed, neutralized, filtered, and dried in air at about 100.degree. C. for approximately eight hours. The oxalates are then calcined in air (thermally decomposed) at approximately 700.degree. C. to about 900.degree. C. for a time ranging from one to four hours, to form the corresponding oxides. Typically, heating for one hour at 800.degree. C. is sufficient. Preferably, if either the hot pressing or the sintering method is used to prepare the scintillator, the oxalates and/or the resulting oxides may be milled by one of several methods such as ball, colloid, or fluid energy milling to enhance optical clarity. Milling of the powder for between one-half hour and ten hours has been found to be sufficient. It should be noted, however, that typically the optical clarity of the scintillator is improved by milling the oxalates and/or oxides regardless of the preparation method. Zirconium and tantalum which do not form stable oxalates are added to the calcined oxalates in the form of oxides or nitrates. Other additives introduced as oxides include CaO and ThO.sub.2. Since these additives are already oxides, they are introduced after the calcination step.

Following the preparation of the selected powder composition by one of the methods described above, in accord with the preparation of scintillator materials by sintering, selected amounts of powder composition are formed into powder compacts by either die pressing, or die pressing followed by isostatic pressing to further increase green density. A die material which is inert with respect to the scintillator constituents is preferred to avoid undesired reactions and contaminations. Suitable die materials include alumina, silicon carbide, and metals such as molybdenum, hardened steel, or nickel-based alloys. The powder compacts are formed by die pressing at pressures of between about 3,000 psi and 15,000 psi. Alternatively, the die pressed powder compacts may be isostatically pressed at between about 10,000 and 60,000 psi to further increase powder compact green density. If any grinding aids or compaction aids (lubricants, such as waxes) have been used, an oxidation treatment to remove all organic additives can be employed prior to sintering.

During the sintering phase, the compacts are heated in a high temperature tungsten furnace, for example, in vacuum or a reducing atmosphere such as a wet hydrogen atmosphere (dew point of about 23.degree. C., for example) at a rate of between approximately 100.degree. C. per hour and 700.degree. C. per hour to the sintering temperature of between 1800.degree. C. and 2100.degree. C. The sintering temperature is then held from 1 hour to about 30 hours to cause extensive densification and optical clarity development. Upon termination of the sintering step, the compacts are cooled from the sintering temperature to room temperature over a period of time ranging from about 2 to 10 hours.

Sintered ceramic scintillators may also be prepared by a heating sequence which includes a hold at a temperature lower than the final sintering temperature. Typically, the powder compact is heated at a rate of between 300.degree. C./hr and 400.degree. C./hr to a holding temperature of between about 1600.degree. C. and 1700.degree. C. The holding period may range from 1 hour to 20 hours, following which the temperature is raised to between about 1800.degree. C. and 2100.degree. C. for final sintering for between 1 hour and 10 hours. The increase from the holding temperature to the final sintering temperature is at a rate of between about 25.degree. C./hr and 75.degree. C./hr. A preferred heating sequence comprises heating the powder compact to a holding temperature of 1660.degree. C. in five hours, holding this temperature for ten hours, followed by heating to 1950.degree. C. in 6 hours, and then sintering at 1950.degree. C. for two hours. The preferred heating sequence was used to prepare the aforedescribed ceramic scintillator materials discussed in connection with FIG. 1.

Yttria/gadolinia ceramic scintillators for luminescent applications may also be prepared by a combination of processes involving sintering and gas hot isostatic pressing (GHIP). The starting oxide powder compositions are prepared in accordance with one of the aforedescribed methods. Preferably, the oxalate coprecipitation method is used. By way of example and not limitation, one useful yttria-gadolinia scintillator composition comprised 66.7 mole percent Y.sub.2 O.sub.3, 30 mole percent Gd.sub.2 O.sub.3, 3 mole percent Eu.sub.2 O.sub.3, and 0.3 mole percent Yb.sub.2 O.sub.3. Another useful composition comprised 49.7 mole percent Y.sub.2 O.sub.3, 45 mole percent Gd.sub.2 O.sub.3, 5 mole percent Eu.sub.2 O.sub.3, and 0.3 mole percent Yb.sub.2 O.sub.3. In contrast to the previously described sintering process, which preferably requires milling of the oxalate and/or oxide powders to produce transparent ceramics, the process of sintering combined with GHIP permits the fabrication of transparent ceramics from unmilled powders.

In the fabrication of yttria-gadolinia ceramic scintillators by the combined processes of sintering and gas hot isostatic pressing, following the preparation of a powder having the desired composition, powder compacts are formed by cold pressing at pressures of between 3,000 psi and 10,000 psi, followed by isostatic pressing at pressures of between 15,000 psi and 60,000 psi. The pressed compacts are then presintered to 93 to 98 percent of their theoretical density at temperatures of about 1500.degree. C. to 1700.degree. C. for between 1 and 10 hours. The presintered compacts are then gas hot isostatically pressed with argon gas at pressures of 1,000 psi to 30,000 psi at temperatures between 1500.degree. C. and 1800.degree. C. for 1 to 2 hours.

In accordance with an example of the preparation of a ceramic scintillator employing the sintering and GHIP technique, a powder compact was formed by cold pressing approximately 20 grams of powder in a rectangular die at a pressure of approximately 4,000 psi. The sample was then isostatically pressed at 30,000 psi to increase green density to 49 percent of its theoretical value. The cold pressing of the sample was followed by sintering in a wet hydrogen atmosphere (dew point 23.degree. C.) for two hours at 1660.degree. C. so that closed porosity was developed. The density of the sintered sample, as measured by the water immersion method, at this stage in the fabrication process was determined to be between 93 and 98 percent of its theoretical value. In order to obtain additional densification and optical transparency, the sample was gas hot isostatically pressed in a carbon resistance furnace at 1750.degree. C. for one hour at an argon pressure of 25,000 psi. During gas hot isostatic pressing, the temperature was increased to the final value of 1750.degree. C. in a step-wise manner. The sample was initially heated in one hour to 1400.degree. C. and the temperature raised thence to 1750.degree. C. in another hour. Following a holding period of one hour at 1750.degree. C., the resulting ceramic scintillator had a black appearance due to reduction in the reducing furnace atmosphere. The sample was rendered transparent to visible light by suitable oxidation treatment such as heating in air at a temperature of 1200.degree. C. for thirty-two hours. Comparison of the physical dimensions of the sample before and after the GHIP treatment indicates that the sample shrunk during the GHIP step, indicating further densification. The finished ceramic exhibited a density of greater than 99.9 percent of theoretical value.

Transparent yttria-gadolinia ceramic scintillators may also be prepared by vacuum hot pressing a scintillator material powder prepared, preferably, by the aforedescribed wet oxalate coprecipitation process. In accordance with this method, a selected quantity of the calcined oxalate powder is pressed in a graphite die with molybdenum foil used as spacers between the upper and lower graphite plungers. Alternatively, a boron nitride coated graphite die may be used. A pressure of about 1000 psi to 1200 psi is applied at a temperature between about 600.degree. C. and 700.degree. C. under a vacuum of less than 200 microns and maintained for about one hour. Thereafter, the pressure is increased to approximately between 4000 psi and 10,000 psi and the temperature increased to between 1300.degree. C. and 1600.degree. C. The pressure is released after holding at the elevated temperature for between one-half to four hours and the sample furnace cooled to room temperature.

Ceramic scintillator samples prepared in accordance with the hot-pressing method may be discolored due to surface reaction with the molybdenum spacer. Additional discoloration may be due to oxygen deficiency in the furnace atmosphere during hot pressing. The ceramics, however, can be made optically clear by oxidation in air or an oxygen-containing atmosphere at a temperature of about 800.degree. C. to 1200.degree. C. for between one and twenty hours. Any residual discoloration may be removed by conventional grinding and polishing techniques.

In a specific example of the preparation of a scintillator material by the vacuum hot-pressing method, 10 grams of a scintillator oxide material were obtained from the aforedescribed oxalate coprecipitation process by calcination of the oxalates at 800.degree. C. for one hour in air. The oxides were initially hot pressed in a boron nitride coated graphite die at 700.degree. C. for one hour under a vacuum of about 20 microns and at a pressure of 1200 psi. The temperature and pressure were then increased to 1400.degree. and 6,000 psi, respectively, under a vacuum of approximately 100 microns. These conditions were maintained for two hours, following which the pressure was released and the resulting scintillator material furnace cooled.

The scintillator material was gray to gray-black in color due to the reducing atmosphere created in the hot press. Light grinding of the scintillator surface and heating at 950.degree. C. for four hours removed carbon sticking to the scintillator material. The remainder of the dark coloration was removed by additional oxidation at 1150.degree. C. for two hours in air. The resulting scintillator material was "light tan" in color, translucent to transparent in optical quality, and exhibited good relative light output upon excitation by x-rays.

From the foregoing, it will be appreciated that the invention provides a method for sintering transparent-to-translucent, rare-earth-doped polycrystalline yttria-gadolinia ceramic scintillators having cubic crystalline structure, high x-ray stopping power, high radiant efficiency, high density, high uniformity, and as well as low luminescence afterglow, and which are useful as radiation detectors such as those used in CT and digital radiography.

While certain preferred features of the invention have been shown by way of illustration, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.