US3837882A - Optical bodies with non-epitaxially grown crystals on surface - Google Patents

Abstract

This is a process for growing non-epitaxially a different crystal upon a base crystal surface and thereafter destroying said different crystal, thereby generating a uniformly random arrangement of surface irregularities which are independent of lattice orientation in the base crystal. The texture of the treated surface is unlike prior art surfaces, whether produced chemically or mechanically, and specular reflections are essentially eliminated.

Description

United States Patent 191 Swinehart et al.

[ Sept. 24, 1974 OPTICAL BODIES WITH NON-EPITAXIALLY GROWN CRYSTALS ON SURFACE [75] Inventors: Carl F. Swinehart, University Heights; James T. Lindow, Cleveland Heights, both of Ohio [73] Assignee: Kewanee Oil Company, Bryn Mawr,

22 Filedz Sept. 2, 1971 [21] Appl.No.: 177,447

[52] US. Cl 117/16, 117/47 R, 117/118, 156/2, 156/5, 156/7, 252/301.4 H [51] Int. Cl B44d 5/02, C09k 1/06 [58] Field of Search 117/47 R, 118, 16; 156/2, 156/4, 5, 7; 250/71.5 R; 252/3014 H [56] References Cited UNITED STATES PATENTS 1,482,793 2/1924 Hartmann 156/2 2,666,145 6/1949 Eversole et al. 250/71.5 R 2,822,250 2/1958 de Nobel 156/17 UX 3,068,359 11/1962 Carlson 260/751 R 3,089,793 5/1963 Jordan et al. 156/17 UX 3,102,955 9/1963 Carlson 250/75.l R 3,235,426 2/1966 Bruner 156/2 3,483,048 11/1969 Lenz et al. 156/2 3,490,982 1/1970 Saveniere et al. 117/118 X 3,595,718 7/1971 Fishman et al .1 156/2 FOREIGN PATENTS OR APPLICATIONS 757,814 9/1956 Great Britain 156/2 Primary ExaminerWilliam D. Martin Assistant ExaminerWilliam H. Schmidt Attorney, Agent, or Firm-Cain and Lobo [57] ABSTRACT 14 Claims, N0 Drawings OPTICAL BODIES WITH NON-EPITAXIALLY GROWN CRYSTALS ON SURFACE BACKGROUND OF THE INVENTION Crystal growth has a history of interest with respect to the epitaxial growth of crystals and much work has been done in this regard. The instant invention is oppositely directed to a process for producing growth on a crystal non-epitaxially. By epitaxy is generally meant that the growth structure is determined by the crystal structure and oriented by the underlying substrate. The dictionary defines epitaxy as the oriented overgrowth of one crystalline substance on a substrate of different chemical composition. In a strict crystallographic sense, epitaxy is confined to the oriented growth of a crystalline substance in the same lattice configurations as a continuation of the structure of the base crystal. In whatever sense it is considered, the instant invention is concerned with the disoriented growth of a different and destroyable form of the base crystal capable of generating uniformly random surface irregularities. More particularly, it is concerned with non-epitaxial growth on preselected surfaces of a scintillation crystal, which is a necessary intermediate step in the improvement of its resolution.

In general, scintillation crystals, also known as scintillation phosphors or scintillators are classified in four categories as used for alpha, beta, and gamma-ray detection: inorganic crystals, organic crystals, plastic phosphors, and liquid phosphors. Typical organic scintillation phosphors are thallium-activated sodium io' dide, europium-activated lithium iodide, thalliumactivated cesium iodide, sodium-activated cesium iodide, and thallium-activated potassium iodide; typical organic scintillation phosphors are anthracene and trans-stilbene crystals. The instant invention is concerned most particularly with inorganic scintillators. The primary advantage of the inorganic crystals over the other types is their higher density, which is mainly responsible for the higher stopping power and thus the greater counting efficiency for gamma-rays. Since the alkali halides, and in particular thallium-activated sodium iodide, exhibit such desirable characteristics as high density, high light output, and transparency, it is this material which is normally used for camera plates and spectrometer units, which require excellent resolution.

It has been observed that the resolution of a scintillator having a diffuse surface is superior to one having a polished surface. This is true both for plastic and crystalline scintillators, the effect increasing as the index of refraction increases and being most pronounced for symmetrical shapes. (Applied Gamma-Ray Spectrometry, p. 57, edited by C. E. Crouthamel, Pergamon Press 1960). Diffuse-reflection of light at the surface of a scintillation phosphor is of critical importance in the performance of crystal scintillation detectors. To improve resolution of a crystal and compensate for geometrical shape, its surfaces are usually sanded, scratched, ground, or polished in various areas in particular ways, mostly according to unwritten rules learned by trial over an extended period of time. The results of such mechanical treatment are judged by experimental measurement of the energy resolution of the crystal as a gamma-ray spectrometer.

Theoretically, the best resolutions, i.e., the smallest ratio of the width of the peak at half maximum to the energy of the peaks midpoint, should be obtained if all the light from each event in a scintillation crystal reached the sensing surface of a light detector and was absorbed by it. This could only result if there were no light absorption in the crystal and one hundred per cent reflection at all surfaces except where the sensing element is attached. Also the sensor should respond equally to light across its surface and for any angle of incidence. Even in thin discs, the foregoing conditions cannot be realized in actual detectors. A second approach to obtain optimum resolution is equal sampling of light from events in all parts of the crystal, assuming the electronic circuitry is able to accommodate the weakened signal. Since it is generally accepted that none of the above theoretical conditions can be met in practice, compromise techniques must be used depending upon the geometry of the crystal. For example, it may be desirable to roughen some areas more than others, even resorting to polishing some areas to let light out, as described in more detail in US. Pat. No. 3,102,955.

Modern scintillation scanning devices are disclosed in US. Pat. No. Re. 26,014 and in US. Pat. No. 3,159,744, in which scanners are described in detail, along with the circuitry required. Generally speaking, the scintillation detector is a device which is used in medical science to monitor the level of the radiation emanating from a patient suspected of having a tumor, cancerous condition, or other physical problem after administering a radioactive tracer. The patient is positioned below a scintillation detector and the energy emanating from the patient is detected by it. Energy detected by the scintillation detector or probe is converted to light and then transformed by phototubes to electrical signals, which are then selectively conducted to a suitable pulse height analyzer which stores and sorts the data. Depending upon the particular circuitry of the measuring device, an image of the intensity distribution emanating from the patient may be obtained. Such images are composed of dots, which vary in concentration according to the activity detected so that the physician is able to determine not only the area in which the activity is present but its distribution.

Isotopes currently used in the biomedical field are in the below-350 KEV range; typically, KEV from Technitium 99. At this relatively low energy level the camera image quality is limited by the photon statistics of the relatively few photons produced by a scintillation crystal. Image resolution improves inversely as the square root of the number of photons.

The degree of surface roughening of a scintillation crystal affects the light output of the crystal as a whole and the desirability of surface treatments has been recognized for some time (see US. Pat. No. 3,102,955 and Journal of the Optical Society of America, Vol. 40, No. 11, pp. 748-750, November 1950). It was found that in a finished optical crystal the destroyed surface layer, by cutting and polishing, has a depth of several microns, depending upon the hardness of the crystal, the load, and the length of time of optical working. The grinding process produces further destruction of the crystal surface. In addition to the material being removed by fine chipping on the surface, plastic deformation takes place in the form of flow-lines which extend into the crystal. Since the optimum conditions for polishing can only be obtained by trial and error, with specific pastes and creams of particular abrasives found to provide the desired effects, known processes to provide the desired diffusion surface on a crystal leave much to be desired because the orientation of the crystal influences the working of its surface. The instant process provides a way of producing a uniform, random surface of precisely the right texture, obviating the necessity of the trial and error grinding and polishing by experienced hands. It has been found that the number of the grains per unit area, and to some extent their size, which are grown on the surface of the base crystal may be controlled depending upon the wave-length of the light which the scintillation crystal surface will be required to diffuse essentially completely.

In the production of known hygroscopic, ionic crystal scintillation detectors, such as sodium iodide doped with thallium, or lithium iodide doped with europium, it has been found that a section of a crystal cut from crystal ingot, when left in an atmosphere not entirely free of moisture, develops a surface which has a blotchy appearance. Each blotch develops from a cen ter of crystal growth, wherein a hydrate of the crystal has formed. The depth of the growth effected by the localized hydration is difficult to control, and usually results in the crystal surfaces having to be reworked.

The manner in which sodium iodide and most other crystalline bodies accept scratches in machining and sanding depends both on the crystal plane of the surface and the direction in which it is scratched. The edges of scratches consist of minute cleavage facets which account for specular reflections and high transmissions in particular directions because the cleavage facets are oriented by the base crystal structure. Grinding with coarse enough grits to leave a rough surface, even with random motion of the lap, shows texture and specular reflection, oriented by the base crystal. Because large crystal ingots are composed of several differently oriented optically integral components, large ground surfaces invariably show areas of different texture. The lattice alignment of components does not affect light emission or transmission within the body, but it affects the way in which the surface responds to optical working and thus the behavior of light at the surface. The use of finer grits, to give smoother surfaces, still produces texture and oriented specular reflections. The instant process provides a nearly smooth surface with uniformly random irregularities, essentially free of oriented texture and specular reflections, which enables a scintillation crystal to perform with surprisingly high resolution unobtainable by any other method. Crystals, other than scintillation crystals, with a surface provided by the instant invention have found utility in numerous applications which require special surfaces.

SUMMARY OF THE INVENTION It has been discovered that, with the aid of a crystal nidus agent, another crystal different from the base crystal may be grown non-epitaxially on said base crystal in predictably uniform but random manner. This is most easily done where the base crystal material is in contact with a finely divided solid which provides a multiplicity of crystal growth centers. Thereafter, said another crystal is destroyed, leaving the base crystal surface roughened, but nearly smooth, with irregularities randomly arranged, or, if stable, the added crystal layer may be left intact, its thickness and optical prop erties being controlled by the process.

A scintillation phosphor with surprisingly good energy resolution in gamma-ray measurement is prepared by selectively treating at least one surface of the phosphor, and preferably at least three, to impart to each treated surface diffusion-reflection and diffusiontransmission characteristics unmatched in any prior art crystal phosphor.

It has been discovered that a crystal scintillation phosphor treated as described hereinabove forms a thin, roughened layer on the surface of the phosphor which affects diffusion and reflection of the light from a plurality of events in a crystal in such a way as to permit the construction of a scintillation detector with higher resolution. The instant process obviates the necessity of mechanically grinding the crystal surface with a grit to provide the desirable roughened crystal surface and the damage to scintillation efficiency near the surface which this work entails.

PREFERRED EMBODIMENT OF THE INVENTION Some of the applications for optical systems in which light diffusing surfaces are useful are as follows:

l. to create uniform light intensity, such as is desired in sensing scintillations in a crystal to compensate for irregularities in detector response;

2. to display a real image, as when focusing an optical system;

3. to compare flux density for opposing beams;

4. to accept light from wide angles; and

5. to reduce ghosts or unwanted images by treating the ends of prisms and edges of lenses or beamsplitting elements. In each of the foregoing applications, an even-textured surface, free from oriented specular reflections, is desired, and the instant nonepitaxial growth process, with or without the subsequent destruction of new growth on the crystal, provides a desirably roughened surface which gives superior performance.

The instant invention will be described in detail with respect to roughening of at least one crystal surface of an inorganic crystal scintillation phosphor used in a gamma-ray image-type camera, though it will be apparent that the process may be used on any crystal where the particular physical characteristics of the instant sur face are desired. The scintillation detector is one of the oldest methods of nuclear radiation detection, but the modern scintillation detector came into being only after the evolution of special photomultiplier tubes for the purpose. Surface treatments by the instant invention are contributors to a new generation of high performance detectors. Generally speaking, a scintillation detector comprises a light reflector which encases a scintillator crystal such as an optically integral, fully dense, single crystal, multiple crystal, or polycrystalline body of thallium-activated sodium iodide. A light pipe channels the light from the scintillator into a photocathode of a photomultiplier tube, which is powered by a high voltage power supply. The output of the photomultiplier tube is conducted to a preamplifier, thence to a discriminator and pulse shaper, and lastly to an electronic counter or means for presentation of the measurements in visual form. When charged particles or gamma rays are stopped by scintillators, excited states are produced which, during their return to the normal states, produce discrete light flashes of short duration (less than 10 microseconds) or scintillations.

5 By optically coupling the scintillator with a photomultiplier tube, a pulse or charge can be passed into an electronic system, making counting possible.

Though various scintillators are available, including crystals of inorganic and organic materials, liquids, powders, and plastics, the instant invention is directed to crystals, particularly inorganic crystals, in which the resolution is to be improved. This is done by providing a thin layer of crystal growth on the surface, thereafter destroying the growth to produce a controlledly roughened crystal surface which appears nearly smooth. The scintillation phosphor in a camera may be a large, thallium-activated sodium iodide, Nal (Tl) crystal cut from a multiple crystal ingot. Generally, the crystal section cut is about 13 inches in diameter and the thickness, depending upon the energy of the radiation for which it is to be used, is in the range from about 0.5 in. to about 0.75 in. thick. Prior to treatment for new growth, the crystal surfaces may be polished or, alternatively,

ground with a fine grit of aluminum oxide to produce a mechanically scratched surface. The new growth on the surface of the crystal must be thin, normally in the range from about 0.1 to l.O'microns, and 0.1 to 50 microns in the longest dimension.

The controllable thickness of the new growth layer is obtained by pressing a preselected prepared crystal face against a crystal nidus agent, a mass of which is contained in a suitably large receptacle. The crystal nidus agent seeds the surface of the crystal in a uniformly random manner, i.e., the seeding is uniform with respect to the number of seeds for new crystal growth which are deposited on the crystal surface, but it is random in that the orientation of grains is not related to the lattice of the base crystal. The crystal nidus agent may be any finely divided solid with which is associated a transferable compound capable of initiating nonepitaxial growth on the surface of the base crystal.

Choice of a crystal nidus agent depends upon the base crystal material. For example, where the base crystal forms a hydrate with water, one may use a crystal nidus agent with water of crystallization associated with it, such as calcium sulfate (CaSO,.2H O), magnesium sulfate (MgSO .7H O), barium iodide (Bal .2- H O), sodium bromide (NaBr.2H O), and the like. Where base crystal does not unite with water, a crystal nidus agent must be found capable of providing a moiety which can initiate non-epitaxial growth on the base crystal. For example, where lithium fluoride or cesium iodide is the base crystal material, boron fluoride or some of its complexes may be used with potassium fluoborate or other powdered solids as a crystal nidus agent. A desirable crystalnidus agent is a finely divided solid particulate form of the compound to be formed non-epitaxially on the base crystal material. For example, where sodium iodide is the base crystal material, small crystals less than 80 Tyler mesh in size, of the hydrated form of sodium iodide, namely NaI.2H O, may be used to provide the crystal nidus agent. Where lithium iodide is used, hydrated lithium iodide (LiI.3H O) may be used as the crystal nidus agent. It is immaterial whether or not the crystal nidus agent is a scintillator.

It is also required that the transferable moiety be present under process conditions which permit its transfer to the surface of the base crystal, which unites with the base crystal to supply material for nonepitaxial crystalline growth, the orientation of which is unrelated to the orientation of the grains in. the base crystal. Where the transferable moiety is water, it may be present as water crystallization in the crystal nidus agent provided that sufficient water is present in this form to feed the new growth until it reaches a predetermined thickness. Where the concentration of said moiety is insufficient to provide the extent of new growth desired, additional amounts of the moiety may be introduced in a transferable form as required.

Crystal nidus agents which may be used successfully when associated with a transferable amount of moisture are finely divided solids such as aluminum oxide silica, magnesium oxide, magnesium fluoride, calcium carbonate, and the like, which are available or easily comminuted to the desired particle size by grinding, and which are easily removed after the desired amount of growth has been effected. The transferable moiety such as water may be provided either in liquid or gaseous form. Carbon black and some inorganic pigments will also function as seeds, but they should be avoided where the end product must have a white surface. Other crystal nidus agents which are advantageously used are the finely divided hydrate forms of the base crystal, For example, Nal.2H O in a size range from about 325 to about 40 Tyler mesh, in the presence of additional water, will grow an excellent layer of hydrated crystals on the base crystal surface of Nal. Still other crystal nidus agents which may be used are macroporous and microporous alkali metal silicates and alkaline earth metal silicates, such as those silicates commonly known as molecular sieves and which may have water, sulfur dioxide, ammonia, boron fluoride, and other non-epitaxial growth forming moieties associated with them in a physical manner only.

Most uniform results are obtained with a crystal nidus agent in finely divided powder form having a size range from less than 325 Tyler mesh to about 40 mesh. Though it is not essential that the crystal nidus agent be a powder of uniform size, it will be found that the uniformity of random growth is better when the powder is of relatively uniform size in that its flow properties play a part in its application so that it may contact the surface uniformly.

The crystal nidus agents may be applied to the surface of the base crystal material by any mechanical means such as, for example, by brushing, dusting, or packing the finely divided powdered material on the surfaces of the crystal on which the distortion is to be generated, or by applying a suspension in an inert vehicle, such as silicone oil, mineral spirits, etc., of the appropriate crystal nidus agent.

Still another method of applying crystal nidus agents to the crystal surface is to subject the surface to a fluidized stream carrying the crystal nidus agents along with an appropriate amount of transferable moiety for formation of non-epitaxial growth. The fluid conveying of crystal nidus agents is advantageous for application on massive crystals which weigh so much as to make movement of the crystals difficult. For example, where a crystal weighs half a ton or more, the desired amount of distortion may be generated on its surfaces by flow control of a fluid carrying a predetermined amount of crystal nidus agents associated with a growthsustaining, transferable moiety in sufficient concentration for a predetermined period of time.

In each case, the base crystal material is contacted with the finely divided solid crystal nidus agents on all surfaces on which a controlled distortion is to be produced. In each case, the growth of new crystal material generated upon the surface of the base crystal is of a compound different from that of the base crystal, and grows on the surface of the base crystal with a different lattice structure. In each case, the growth of the new crystalline material is unequivocally not oriented by the orientation of the base crystal. Contact with the crystal nidus agents may be momentary. The base crystal ma terial may then be removed and set aside, awaiting sufficient formation of new crystal growth on the surface. in general, a suitable period of time for contacting the base crystal material with the crystal nidus agents ranges from about 0.05 second to about hours, depending upon the depth of the diffusion surface to be generated on the crystal surface and the rate of supply for the transferable moiety. After the desired amount of time has elapsed, the crystal nidus agent is mechanically removed from the surface of the crystal and the crystal is treated so as to disproportionate the newly formed covering of crystal material by removing essentially all of the chemically different growth-sustaining moiety. Where the crystal nidus agent with the transferable moiety generates a hydrate, after a predetermined period of time during which the new crystalline mate rial is grown non-epitaxially, the base crystal is placed in a very dry box, preferably under vacuum, so as to dehydrate the surface of the crystal. Selective solvents can also be found for some combinations of materials which will remove the covering growth without attack upon the base crystal.

Though it is preferred to use a mass of finely divided solid crystal nidus agents, non-epitaxial growth may be effected directly by exposing the base crystal surface to a vapor or finely divided liquid spray in the apparent absence of the agents. As has been mentioned hereinabove, non-epitaxial growth of the base crystal hydrate is often formed accidentally on the surface of a base crystal which is left in a humid atmosphere for too long. Characteristically, such accidental or deliberate exposure to a humid atmosphere results in an uneven, blotchy distortion of the crystal surface, some areas being favored with non-epitaxial growth, while others are not. The exact reason for this behavior is not known, but it is known that such uncontrollably uneven and non-uniform hydration on a crystal surface is detrimental. Of course, if a base crystal material sensitive to hydration is left exposed to a humid atmosphere for very long, the entire crystal surface becomes hydrated to such an extent as to make the crystal unusable. For example, in the case of the thallium-activated sodium iodide crystal, such accidental overhydration results in the formation of a yellow crust on the surface of the base crystal material and extreme amounts of water produce a partially liquid coating.

The non-epitaxial growth of another crystalline material on the surface of the base crystal occurs so long as the fugacity of the growth-sustaining moiety, for example, water, exceeds the partial pressure of the nonepitaxially growing crystal. For example, where sodium iodide dihydrate is to be grown, the relative humidity must exceed 38 per cent at 25 C. for satisfactory growth with a crystal nidus agent essentially free of transferable water of crystallization. Depending upon the particular texture desired on a specific base crystal material, a mixture of various crystal nidus agents may be used.

Most interestingly, microscopic examination of the surface of a base crystal material treated with the instant process shows certain features in the texture of the surface produced that differ from surfaces generated by scratching, grinding, or sand-blasting and a conspicuous absence of specular reflections in specific crystal directions. Between crossed polarizers, on sodium iodide or lithium iodide. the nonepitaxially grown crystal material is easily recognizable, but after disproportionation and removal of the new growthforming moiety, the angular image remains both in the base crystal surface and in the remaining portion of the new crystalline overgrowth.

The following examples will serve more clearly to illustrate the instant invention.

EXAMPLE 1 A machined disc of thallium activated sodium iodide crystal approximately 13 in. in diameter and 0.5 in. thick is ground on one face in a dry atmosphere (dew point below 40 C.) with 400 grit in a light silicone oil using a scored glass daub or grinding tool. The disc is then polished with an organic solvent, such as a primary alcohol or a ketone, such as methanol or acetone. The clean, bright surface is then pressed into a 0.5 in. deep bed of plaster-of-Paris of the rapid setting dental variety for about 20 minutes. The powder is then brushed away and the plate allowed to stand for about one-half hour in a very dry atmosphere, dew point below C. After again cleaning with a jet of dry air or dusting, but not rubbing, with calcined alumina or magnesia powder, a transparent interface coupling fluid is applied and pressed out with a glass window of larger diameter. After the coupling fluid is polymerized, adhesively bonding crystal to glass, the excess is removed.

Exposed side and back surfaces of the crystal are then ground and solvent-polished, as above. Plaster-of- Paris powder is added to cover the brightly polished surfaces and allowed to remain 20 minutes. After removing the powder and cleaning as above in a very dry atmosphere, the encapsulation of the crystal is completed. This involves sealing the glass plate into a heavy metal support flange that also carries a thin aluminum back cover through which gamma rays will enter. The inner surface of the aluminum back is covered with white reflector paint. The encapsulated crystal is used as a plate for an Anger camera. In use, it is coupled to a complex light pipe which carries about 19 phototubes, the signal from which tubes are converted to an image showing the location of gamma rays causing scintillations within the sodium iodide crystal.

EXAMPLE 2 A disc of thallium-activated sodium iodide is processed as described in Example 1 hereinabove. The exposed sides and back face of the crystal are then roughened mechanically by sanding in the usual manner, and the encapsulation is completed, as described hereinabove. The completed encapsulated crystal gives 17 per cent more output than units made without the nonepitaxial growth and subsequent destruction of that growth on the crystal surface. When this unit is installed in an Anger camera, it gives better resolution for the image produced than any plate made by a prior art method.

EXAMPLE 3 A camera plate of thallium-activated sodium iodide is made as described in Example 1 hereinabove, except that after polishing the second side of the crystal, it is lightly roughened by rubbing with a sanding material called Scotch Brite produced by Minnesota Mining & Manufacturing Company, which consists of a fine silica grit in matted nylon fiber, more completely described in US. Pat. No. 2,958,593. The rough surface produced by hand-rubbing in such a way as to have minimum contrast in texture between components is then covered with plaster-of-Paris, allowed to stand minutes, cleaned, dried for about one-half hour, and then finished as described hereinabove in Example 1. The light output of this unit is excellent and better than the units produced in Examples 1 and 2 hereinabove.

EXAMPLE 4 A crystal of sodium-activated cesium iodide l in. in diameter and 1 in. thick is polished with a dampened paper tissue, then treated in a dry atmosphere (dew point below 40 C.) with a paste consisting of 10 parts potassium fluoborate powder and 2 parts boron fluoride-ethyl ether mixture. After a short period of time, ranging from about 1 to about 10 minutes, the paste is removed and the surface cleaned with alumina powder, by dusting without rubbing. The crystal with a surface roughened by the instant process is encapsulated in the usual way, as described hereinabove, with one end optically coupled to the glass window and the circumference and the back surface packed in alumina reflector powder. The resolution for cesium 137 gamma radiation is 8.5 percent. The same crystal with a polished end coupled to the glass window and its other surfaces mechanically roughened according to prior art processes when encapsulated as described above gives 90 percent resolution. An improvement of 5.56 percent results with the new crystal. This improvement depends upon the ratio of photocathode area to crystal surface area, getting substantially better as this ratio gets smaller, i.e., with large crystal surfaces for the same photocathodes.

EXAMPLE 5 A crystal of thallium-activated cesium iodide l in. in diameter and l in. thick is treated in the same manner as in Example 4 hereinabove and shows a comparable improvement in resolution when compared to the same crystal treated with prior art methods.

EXAMPLE 6 Potassium bromide crystals are treated in the same manner as the cesium iodide crystals in the examples hereinabove. The disfigured surface has a texture comparable with that of crystals treated in the previous examples.

EXAMPLE 7 A polished lithium fluoride crystal is treated with a paste of potassium fluoborate powder and boron fluoride-ethyl ether, as in Example 4 hereinabove, and after being in contact for about 30 minutes, is washed with a saturated solution of lithium fluoride. The crystal surface shows a randomly oriented, disfigured texture.

EXAMPLE 8 A polished lithium fluoride crystal is coated with a thin layer of an equal mixture of metaboric acid and potassium fluoborate powders using as a volatile vehicle a primary alcohol, such as methanol. The dry coated crystal is then exposed to a hydrochloric acid vapor for about one-half hour by being disposed in a glass dessicator above 35 per cent hydrochloric solution. The crystal is then washed with saturated lithium fluoride solution. The crystal surface has a randomly oriented disfigured surface with a roughened texture essentially free of specular reflections.

EXAMPLE 9 A ground cylinder of barium fluoride crystal approximately 1 in. in diameter and 0.5 in. thick is polished on one end after testing as a scintillator to cesium 137 radiation, and is then coated with a mixture of 99 per cent potassium fluoborate and l per cent gypsum powder as the thin layer, using methanol as the vehicle. It is then exposed to boron fluoride gas for about an hour and then washed with water saturated with barium fluoride. The polished surface is found to be roughened with a non-oriented texture and the specular reflections are essentially eliminated. As a scintillator in the same mounting configuration as before, the output was increased and resolution improved. Considering that the refractive index of barium fluoride is a near match to the coupling media, this is a surprising and unexpected improvement. Crystals of barium fluoride, calcium fluoride, strontium fluoride and magnesium fluoride are treated as described hereinabove, giving them about 2 hours exposure to boron fluoride gas, after which the crystals are washed. The crystals exhibit a disfigured surface instead of the original smooth surface, and the crystals are essentially free of specular reflections.

EXAMPLE 10 A ground disc approximately 1 in. in diameter and 0.25 in. thick of a europium-activated calcium fluoride crystal is polished on one face; after testing for scintillation output it is treated as in Example 9 hereinabove and retested as a scintillator. The output is found to increase from 35 percent to about 40 percent relative to Nal (Tl) as 100 percent.

We claim:

1. A process for controllably disfiguring a surface of a crystalline material comprising contacting said crystalline material with a finely divided solid crystal nidus agent having a size range from about less than 325 Tyler mesh to about Tyler mesh for a predetermined period of time, said crystal nidus agent being associated with a sufficient amount of a transferable moiety capable of generating non-epitaxial crystalline growth on the surface of the base crystal by reaction with the base crystal, said moiety having a fugacity exceeding the partial pressure of said non-epitaxial growth, and said growth occuring in a substantially uniform yet random manner irrespective of the crystallographic orientation of the base crystal.

2. The process of claim 1, wherein said crystal nidus agent is suspended in an inert fluid.

3. The process of claim 1, including in addition mechanically removing substantially all of said crystal nidus agent after said predetermined period of time.

4. The process of claim 3, including in addition dis proportionating said non-epitaxial growth by removing substantially all of said moiety from the surface of said base crystal so as to render the surface essentially free of specular reflections.

5. A scintillator crystal with at least one preselected surface controlledly disfigured to a preselected grain density in a uniformly random manner, irrespective of the particular submicroscopic orientation of particular portions of said scintillator crystal, as a result of the destruction of a non-epitaxial growth of crystalline material generated by a finely divided crystal nidus agent in association with a transferable, non-epitaxial growth forming moiety, said surface being characterized by a freedom of specular reflections.

6. The article of claim wherein said crystal is an inorganic scintillation phosphor.

7. The article of claim 6 wherein said inorganic scintillation phosphor is thallium-activated sodium iodide, europium-activated lithium iodide, thallium-activated cesium iodide, sodium-activated cesium iodide, or thallium-activated potassium iodide.

8. The article of claim 6 wherein said inorganic scintillation phosphor is a hygroscopic ionic crystal.

9. The article of claim 8, wherein said finely divided crystal nidus agent has water of crystallization associated therewith.

10. The article of claim 9 wherein said crystal nidus agent is calcium sulfate, magnesium sulfate, barium iodide. sodium iodide. sodium bromide, alkali metal silicates or alkaline earth metal silicates.

11. The article of claim 6 wherein said inorganic scintillation phosphor does not unite with water 12. The article of claim 11 wherein said finely divided crystal nidus agent is potassium fluoborate and the transferable moiety is boron fluoride or complexes thereof.

13. A crystalline optical body having at least one preselected surface controlledly disfigured to a preselected grain density in a uniformly random manner, irrespective of the particular submicroscopic orientation of particular portions of said optical body. as a result of non-epitaxial growth of crystalline material generated by a finely divided crystal nidus agent in association with a transferable. non-epitaxial growth-forming moiety. said surface being characterized by a freedom of specular reflections.

14. The crystalline body of claim 13 wherein said non-epitaxial growth of crystalline material is destroyed.

Claims (13)

1. 2. The process of claim 1, wherein said crystal nidus agent is suspended in an inert fluid.
2. 3. The process of claim 1, including in addition mEchanically removing substantially all of said crystal nidus agent after said predetermined period of time.
3. 4. The process of claim 3, including in addition disproportionating said non-epitaxial growth by removing substantially all of said moiety from the surface of said base crystal so as to render the surface essentially free of specular reflections.
4. 5. A scintillator crystal with at least one preselected surface controlledly disfigured to a preselected grain density in a uniformly random manner, irrespective of the particular submicroscopic orientation of particular portions of said scintillator crystal, as a result of the destruction of a non-epitaxial growth of crystalline material generated by a finely divided crystal nidus agent in association with a transferable, non-epitaxial growth forming moiety, said surface being characterized by a freedom of specular reflections.
5. 6. The article of claim 5 wherein said crystal is an inorganic scintillation phosphor.
6. 7. The article of claim 6 wherein said inorganic scintillation phosphor is thallium-activated sodium iodide, europium-activated lithium iodide, thallium-activated cesium iodide, sodium-activated cesium iodide, or thallium-activated potassium iodide.
7. 8. The article of claim 6 wherein said inorganic scintillation phosphor is a hygroscopic ionic crystal.
8. 9. The article of claim 8, wherein said finely divided crystal nidus agent has water of crystallization associated therewith.
9. 10. The article of claim 9 wherein said crystal nidus agent is calcium sulfate, magnesium sulfate, barium iodide, sodium iodide, sodium bromide, alkali metal silicates or alkaline earth metal silicates.
10. 11. The article of claim 6 wherein said inorganic scintillation phosphor does not unite with water.
11. 12. The article of claim 11 wherein said finely divided crystal nidus agent is potassium fluoborate and the transferable moiety is boron fluoride or complexes thereof.
12. 13. A crystalline optical body having at least one preselected surface controlledly disfigured to a preselected grain density in a uniformly random manner, irrespective of the particular submicroscopic orientation of particular portions of said optical body, as a result of non-epitaxial growth of crystalline material generated by a finely divided crystal nidus agent in association with a transferable, non-epitaxial growth-forming moiety, said surface being characterized by a freedom of specular reflections.
13. 14. The crystalline body of claim 13 wherein said non-epitaxial growth of crystalline material is destroyed.