Abstract:
Reactive gas is released through a crystal source material or melt to react with impurities and carry the impurities away as gaseous products or as precipitates or in light or heavy form. The gaseous products are removed by vacuum and the heavy products fall to the bottom of the melt. Light products rise to the top of the melt. After purifying, dopants are added to the melt. The melt moves away from the heater and the crystal is formed. Subsequent heating zones re-melt and refine the crystal, and a dopant is added in a final heating zone. The crystal is divided, and divided portions of the crystal are re-heated for heat treating and annealing.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    This invention relates to the purifying of crystal material, the doping of the material and the growth of crystals.  
           [0002]    Bridgeman, Bridgeman-Stockbarger, Czochralski and variations have been used for crystal growth. Depending on the crystal growth method, the crystal type and the crystal size, one has to overcome sets of problems. This invention relates to the purification of the crystal material and the crystal growth process itself.  
           [0003]    Crystal size and the quality of the crystal starting material play important roles in the production of scintillation crystals. The starting material labeled “scintillation grade” is of five 9&#39;s purity 0.9999%. Often the starting material has poor stoichiometry ratio. Growing crystals in a closed type system that have large diameters and up to over 2000 pounds in weight result in crystals that have poor crystal quality. Crystal purity, dopant distribution, defect density and distribution and built-in stress imposed on the crystal during the crystal growth process and the crucible removal may be at unacceptable levels. With the exception of small crystal portions grown at the beginning of the crystal growth, crystals may have lower purity than the starting material. Dopant concentration varies dramatically. That in turn creates uneven light output and decreases the energy resolution of scintillation crystals. When handling large size crystals during the hot transfer, the crystals release large portions of iodine and thallium iodine vapors. Exposure to ambient temperature creates various defects and defect densities in the hot crystals.  
           [0004]    The current practices where large barrel-shaped crystals are grown for all applications, regardless of the fact that most applications use rectangular shapes, makes the yields rather low. Scaling up crystal plate sizes from 0.5-1 inch thick slabs cut perpendicular to the crystal length of a barrel-shaped crystal requires large financial investments. At the same time increasing slab geometry increases the crystal production cost by decreasing the growth rate and lowers the crystal quality and yield.  
           [0005]    Existing purification methods include supplying a gaseous medium to a surface of a melt carried in a crucible. Those methods require extended times for purification, up in the range of 96 hours. Those methods also ineffectively cure the melt, as lower portions of the melt are never purified.  
           [0006]    During melt purification, impurities react with the gas molecules and exit the melt in a gaseous phase. Some impurities react and precipitate from the melt as a sludge. Other reacted impurities float to the surface.  
           [0007]    Needs exist for purification systems that remove impurities faster and more efficiently.  
           [0008]    These problems and many more remain in the present practices. Needs exist for new approaches for crystal material purification and the crystal growth processes.  
           [0009]    Purifying of crystals by reactant gas contact in current systems results in delays and adds significant times to the crystal growth process.  
         SUMMARY OF THE INVENTION  
         [0010]    Reactive gas is released through a crystal source material or melt to react with impurities and carry the impurities away as gaseous products or as precipitates or in light or heavy form. The gaseous products are removed by vacuum and the heavy products fall to the bottom of the melt. Light products rise to the top of the melt. After purifying, dopants are added to the melt. The melt moves away from the heater and the crystals formed. Subsequent heating zones re-melt and refine the crystal, and a dopant is added in a final heating zone. The crystal is divided, and divided portions of the crystal are re-heated under pressure for heat treating and annealing.  
           [0011]    The invention provides multi zone plate crystal growth and purifying.  
           [0012]    The new continuous feed multi-zone crystal grower is capable of growing crystals with very large dimensions under reactive atmospheres. The invention produces high purity crystals with very uniform doping concentrations regardless of the crystal size. The dopant level and the residual impurities are controlled in situ within the crystal feed chamber and during the crystal growth process. Crystal applications include nuclear medicine, high energy physics, optics and others where economical production of high purity and large size crystals are required.  
           [0013]    The invention provides horizontal (or inclined under some angle) continuous crystal growth process for plates of any dimensions.  
           [0014]    Reactive gas permeates start-up material, crystal powder or polycrystalline material or a crystal melt.  
           [0015]    Stoichiometry control or “repair” of start-up material is achieved using the present invention.  
           [0016]    Multi-zone traveling, stationary immersed and non-immersed heaters, resistive and RF heating elements, or other type heaters are used. This allows controlled gradient crystal growth of any size crystals.  
           [0017]    A traveling crucible or crystal slab can be used if the heaters are stationary.  
           [0018]    The present invention can be attached as a module to heaters for in situ purification and dopant control.  
           [0019]    Dopant concentration control can be achieved by adding dopant in solid or gaseous form. If excess dopant has to be controlled, the excess is either neutralized via chemical reaction or by dilution with pure melt.  
           [0020]    For very high purity crystals or crystals with very large sizes, residual impurities control can be achieved by removing the melt from one of the molten zones via vacuum suction and melt draining.  
           [0021]    High temperature and high pressure annealing of the plates in final sizes enhances the crystal quality properties.  
           [0022]    The invention eliminates cutting of at least one dimension of the crystal before further processing.  
           [0023]    A preferred continuous crystal plate growth apparatus has a source of starter material. A valve supplies material from the starter material source. A first, hot zone communicates with the valve for heating the material. A dopant source and a dopant controller are connected to the hot zone for supplying dopant into the material in the hot zone. A second reduced heat zone beyond the hot zone reduces heat in the material, which forms a solid plate. A receiver receives the solid plate from the second, reduced heat zone and advances the solid plate. A lowered temperature heating zone adjacent the receiver lowers temperature of the solid crystal plate on the receiver. An enclosure encloses the zones and the solid crystal plate in a controlled gaseous environment.  
           [0024]    A large heater overlies the small heater. The large heater has first and second zones, and the small heater has the first hot and second reduced heat zones. Baffles separate the first and second zones of the heaters.  
           [0025]    The first zone of the small heater produces a crystal melt temperature higher than a crystal melting temperature in the material. The second zone of the small heater produces a temperature lower than the melting temperature. The temperature in the material at the small heater baffle is about the melting temperature. The large heater first zone provides heat below the melting temperature, and the large heater second zone provides a lower heat.  
           [0026]    Preferably the receiver is a conveyor which moves at a speed equal to a crystal growth rate.  
           [0027]    A second source of starter material and a second valve are connected to the hot zone for flowing material from the second source to the hot zone.  
           [0028]    The crystal melt or starter material is purified in a chamber having a bottom and sides. A lid covers the chamber. An opening introduces liquid or solid material into the chamber. An outlet near the bottom of the chamber releases crystal melt or starter material from the chamber. A shut-off valve opens and closes the outlet. A source of reactive gas is connected to the chamber and extends into a bottom of the chamber. A reactive gas release barrier near the bottom of the chamber slowly releases reactive gas into the crystal starter material. A gas space is located at the top of the chamber above the crystal melt or starter material. An exhaust line is connected to the space at the top of the chamber for withdrawing gas from the top of the chamber. A heater adjacent the chamber heats the chamber and the crystal melt or starter material within the chamber.  
           [0029]    The heater has heating elements around sides of the chamber and along the walls of the chamber.  
           [0030]    The shut-off valve is a thermally activated or a mechanical or electromechanical valve.  
           [0031]    An inlet conduit is connected to the lid. A source of reactive liquid or solid is connected to the inlet conduit. A valve is connected between the source of reactive liquid or solid. A plug is connected to the conduit for plugging the conduit after adding reactive liquid or solid to the chamber.  
           [0032]    Preferably a vacuum pump is connected to the exhaust line. A preferred barrier is a porous plate.  
           [0033]    In one heating and purifying embodiment, a chamber has an inlet and an outlet. A purified material discharge is connected to the outlet. An enclosure has side walls, a bottom and a top. A reactive gas source is connected to a gas inlet tube. A gas distributor is mounted in the chamber near the bottom. A gas releasing plate connected to the gas distributor releases the reactive gas from the inlet tube and the distributor into the material in the feeding and purifying chamber. A heater heats material in the chamber. A gas exhaust exhausts gas from an upper portion of the chamber.  
           [0034]    A preferred casing has a cover and side walls, and the casing side walls include the chamber side walls.  
           [0035]    In one embodiment, an upper heater has heating elements across a top of the chamber.  
           [0036]    The apparatus moves with respect to a stationary base for supporting a growing crystal.  
           [0037]    Preferred crystal growth embodiments have a support for supporting a growing crystal. A first zone heater adjacent the growing crystal heat and liquefies the growing crystal. A second zone heater spaced from the first zone heater along the growing crystal re-liquefies the growing crystal. Preferably multiple zone heaters are spaced from each other along the growing crystal for sequentially liquefying the growing crystal. Preferably the first zone heater further includes heating and purifying apparatus for purifying the crystal melt. A preferred first zone heater includes a reactive gas distributor for distributing reactive gas from near a bottom of the crystal melt.  
           [0038]    A liquid or solid adaptive substance source releases liquid or solid reactive substance into the melt.  
           [0039]    A source of dopant is connected to the last zone heater for supplying dopant into the crystal melt.  
           [0040]    In one embodiment the support is a movable support for moving the liquid crystal along zone heaters. Alternatively, the zone heaters move along the crystal.  
           [0041]    One crystal growth embodiment has a chamber for holding a crystal melt. A crystal support holds a crystal movable with respect to the chamber for forming a bottom of the chamber with the crystal. A first heater adjacent the chamber heats and maintains a crystal melt within the chamber. A baffle is connected to the first heater adjacent a bottom of the chamber. A second heater is connected to the baffle beyond the first heater. A source of reactive gas feeds a gas tube connected to a controller. A distributor is connected to the gas tube and is mounted in the chamber for positioning within the crystal melt. A gas releaser connected to the distributor releases reactive gas into the crystal melt. A gas exhaust is connected to the chamber exhausts gas from the chamber above the crystal melt. An inlet tube and a controller release reactant substance into the chamber and into the crystal melt. A dopant conduit and a dopant source provide a dopant from the source through the conduit to the chamber. The reactive substance and the reactive gas control the dopant.  
           [0042]    These and further and other objects and features of the invention are apparent in the disclosure, which includes the above and ongoing written specification, with the claims and the drawings. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0043]    [0043]FIG. 1 shows an apparatus and process for continuous crystal plate growth.  
         [0044]    [0044]FIG. 2 shows a chamber for purifying crystal material and liquid or solid scavengers. The crystal material may be a starter powder or a crystal melt. The purifier in FIG. 2 may be used to supply the continuous crystal plate growth apparatus and process shown in FIG. 1.  
         [0045]    [0045]FIG. 3 shows a crystal melt purifier for use in a continuous crystal plate growth apparatus and process, such as shown in FIG. 1.  
         [0046]    [0046]FIG. 4 shows a continuous crystal plate growth apparatus and process using multiple zone heating and purifying.  
         [0047]    [0047]FIG. 5 is a detail of sides of the crystal growth apparatus in the melt zones.  
         [0048]    [0048]FIG. 6 shows varied heaters for use in the continuous heat crystal growth process.  
         [0049]    [0049]FIG. 7 shows the use of the present purifying apparatus and process in a vertical Bridgeman crystal growth system.  
         [0050]    [0050]FIG. 8 is a schematic representation of the heat treating and annealing of a cut crystal before use. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0051]    Referring to the drawings, a crystal  1  is grown in a continuous process by first purifying a crystal source material, which is a crystal melt or powder, in a purification station  3 , as later will be described. A second purification station  5  may be provided so that the crystal melt or powder may be prepared in a batch process within alternating stations, which may number several stations.  
         [0052]    Valves  7  control the flow of purified crystal melt or purified crystal source powder  9  to a first hot zone  11  of a first heater  13 . The first hot zone  11  has a temperature which is above the melt temperature of the crystal. A boat-shaped container holds the liquefied crystal  15 .  
         [0053]    A dopant source  17  has a controller  19  which controls the dopant added to the liquefied crystal  15 .  
         [0054]    The first heater  21  surrounding the first hot zone  11  produces a heat above the melting temperature of the crystal. A baffle  23  next to the first heater separates the first heat zone  11  from the second heat zone  25 . The second heater  27  which surrounds the second zone produces a temperature in the second zone which is below the melt temperature of the crystal, so that a crystal solid interface  29  exists in the vicinity of the baffle between the liquefied crystal  15  and the formed crystal  1 . The liquefied crystal, the liquid solid interface and the first portion of the crystal are supported in a boat-shaped crucible container with a bottom  31  and side walls which support the crystal. As the crystal leaves the support plate  31  it passes on to a conveyor  33  with supporting rollers  35 , which continually move the crystal away from the first heater. The crystal moves within an enclosure  43 , which has a noble gas or noble gas and reactant gas atmosphere  45 .  
         [0055]    A large heater has a first zone  37  which heats the initial part of the crystal apparatus to a temperature below the melt temperature, and a second zone  39  which maintains the crystal at a lower temperature.  
         [0056]    A purified and doped crystal emerges from the enclosure.  
         [0057]    In one example, as shown in the chart at the bottom of FIGS.  1 , when using the continuous crystal growth apparatus and process to grow a doped sodium iodide crystal, the first hot zone is maintained at about 700° C. The temperature at the baffle is maintained at the melting point of the material, which in the case of the sodium iodide crystal is about 661° C., or cesium iodide about 621° C. The second zone of the first heater maintains a temperature of about 550° C., or below the temperature of melting. The larger heater  41  has two zones  37  and  39 , which provide heat below the temperature of zone  25 , or at about 450° C. and about 400° C. respectively, so that the crystal uniformly cools as it proceeds.  
         [0058]    As shown in FIG. 2, a crystal purifying apparatus and process is generally referred to by the numeral  51 . The apparatus has a chamber  53 , which is preferably a quartz chamber, for holding a crystal melt  55 , or alternatively for holding crystal-forming powder used to create a crystal melt. The chamber has a lid  57 , which may be a quartz lid, which tightly seals with upper edges of the walls  59  of the chamber  53 . A reactive gas source  61  is controlled by a valve  63 , which supplies reactant gas to a pipe  65 . Tubes  67  conduct the reactant gas to a distributor  69  at or near the bottom  71  of the chamber  53 . As shown in FIG. 2, the distributor may be a plenum. Gas is released from the plenum through a gas release plate  73 , which in this case may be a porous quartz plate. Positive reactant gas pressure is maintained within the plenum  69  so that the gas flows upward through the port plate  73 . A suitable reactant gas, for example, may be bromine mixed with argon or helium or a noble gas. The entire gas mixture is called the reactant gas, although only the bromine may be actually reactant. Bromine, for example, may form gaseous bromides which are removed as gases from the melt or powder  55 .  
         [0059]    The flow of gas through the melt or powder is represented by the gas pockets or bubbles  75 , which move upward. The flow of gas also entrains any water in the crystal material and carries the water from the heated crystal material as gaseous water vapors which are removed from the space  77  at the top of the chamber through a reduced pressure line  79  or vacuum line, which is connected to a source of reduced pressure or a vacuum  81 , as controlled by a valve  83 . The vacuum line  79  withdraws water vapor and reacted gas products. Solid impurities fall to the bottom of the material  55  when the material is in melt, and light solid impurities migrate upward to float on the top of the melt. Heaters, generally indicated by the numeral  85 , surround the chamber.  
         [0060]    The heaters  85  heat the powder material or maintain the high temperature necessary for melting and maintaining the melt  55 . At the top of the heaters a large insulating block  89  is placed to maintain the uniform temperature within the apparatus.  
         [0061]    A source  91  of liquid or solid reactant substance is controlled by a controller  93  for supply to a conduit  95 , which extends through the insulation  89  and lid  57  to an opening  97 , which is controlled by a removable plug  99 , so that the appropriate scavenging liquid or solid may be added to the melt  55 .  
         [0062]    The purified liquid or powder is removed through an outlet  101  in a side wall of the chamber  53  slightly above the bottom.  
         [0063]    A shut-off valve is used in the supply line  101 . The shutoff valve may be a mechanical valve or an electromechanical solenoid operated valve, or a thermally operated valve  103 , such as shown in FIG. 2. The thermally operated valve is a series of cooling and heating coils which freeze or melt the crystal and allow flow of liquid crystal through the conduit  101 .  
         [0064]    [0064]FIG. 3 shows an alternate heating and purifying apparatus and process in which a crystal melt  55  is held between side walls  105  and the base  107  of a casing  109 , which has a cover  111 . An upper heater  113  encloses the crystal melt. Insulation layers  115  above the upper heater  113  concentrate and reduce outward flow of the heat.  
         [0065]    Reactant gas from a source  61  is admitted through a control valve  33  to a reactant gas tube  67 , and from there into a distributor plenum  69  within a distributor housing  117 . A porous quartz plate  73  covers the distributor and releases gas in the form of bubbles  75  through the melt  55 .  
         [0066]    Gaseous reactant products and water vapor escape through small openings  119 , which extend through the heater  113 , the insulation  115  and the cover  111 . Large openings  119  may be supplied for the addition of liquid or solid reactant substances or dopants.  
         [0067]    [0067]FIG. 4 shows a multiple heater arrangement  121  for zone heating and liquefaction  123 ,  125  and  127  as the crystal  1  moves in the direction  131  with respect to the zone heaters  133 ,  135  and  137 . The sequential melting of the crystal further purifies the crystal. In the final melting operation, such as in heating and purifying apparatus  137 , the dopant is added to the crystal.  
         [0068]    The crystal may move through the assembly of heating and purifying apparatus such as on a support  141 , which is part of a conveyor  143  supported by rollers schematically indicated at  145 . Preferably, as shown in FIG. 5, in the areas of the melt zones  123 ,  125  and  127 , the liquefied crystal is supported within a boat-shaped trough  147  with a base  148  and side walls  149 , which are formed of quartz or ceramic. As the molten material solidifies and crystallizes, the individual crystal portions may be picked up by conveyors, or the entire crystal  1  may move along a rigid and smooth quartz or ceramic surface of a support  141 .  
         [0069]    Alternatively, the heating and purifying assemblies  133 ,  135  and  137  may be constructed for movement along a stationary crystal.  
         [0070]    [0070]FIG. 6 shows three configurations of heating and purifying apparatus shown melting and purifying a crystal. The heaters may be used sequentially as different heaters, or each of the heaters in a sequence may be identical. Heating element  151  has an upper heater  153  and a lower heater  155 , which melt the crystal  1  as it flows between the heating elements. The heating and purifying apparatus  157  has an upper heater  158  and side heaters  159 . The bottom  161  may be opened so that the crystal or heater may move and so that the melted crystal may be uniformly supported through the heating area. Alternatively, the heating elements may extend entirely around the liquid crystal area.  
         [0071]    Heater  163  radiates heat downward from a thermal radiator  165 , such as a quartz heating element or a wide laser beam, or a series of laser beams, or simply a strong standard heater. The heat flux  167  heats and melts the crystal material.  
         [0072]    As shown in heating and purifying elements  151  and  157 , the height of the heater openings may be equal or larger than the melt thickness. Alternatively, the opening  169  may be smaller than the melt thickness so that the crystal moves faster through the melt zone than through the approach. In one example, such as when melting and purifying a sodium iodide crystal in multiple melting zones, the crystal may move at a speed of slightly less than one foot per day.  
         [0073]    As shown in the FIG. 7, the present purifying apparatus and process may be used in a standard Bridgeman crystal growth apparatus  171 . An upper heater  173  heats a zone  175  around melt chamber  177  to a temperature above the melting temperature. A baffle below heater  173  separates heat zone  175  from heat zone  179 , in which heat from a heater  181  is below the melting temperature of the crystal. A liquid-solid interface  183  of the crystal occurs at about the position of the baffle  185 . Reactant purifying gas is admitted to the melt  187  through a source  61  and a control  63 , and a tube  65  leading to a distributor  69 , which releases reactant gas through a porous plate into the crystal melt  187 . Gasified impurities are removed through vacuum line  179 , as controlled by valve  83  to a source of reduced pressure  81 . The crystal  191  is contained in a platinum or quartz crucible  193 .  
         [0074]    As shown in FIG. 8, the final product, which is a crystal  201  which has been cut from the long crystal plate, is placed in a support  203  on a substrate  205 , and sides and end surfaces are covered by plates  207  and a cover  209  is placed over the crystal. All of the entire system is enclosed. The entire system is enclosed in a crystal furnace  211  that provides the necessary temperature for the heat treating an annealing process while force is applied to the crystal  201  through the cover and walls  209  and  207 .  
         [0075]    While the invention has been described with reference to specific embodiments, modifications and variations of the invention may be constructed without departing from the scope of the invention, which is defined in the following claims.