CERAMIC RADIATION DETECTOR DEVICE AND METHOD

Abstract

A ceramic lithium indium diselenide or like radiation detector device formed as a pressed material that exhibits scintillation properties substantially identical to a corresponding single crystal growth radiation detector device, exhibiting the intrinsic property of the chemical compound, with an acceptable decrease in light output, but at a markedly lower cost due to the time savings associated with pressing versus single crystal growth.

Claims

1. A method for forming a ceramic radiation detector material, comprising: receiving a source material comprising a powder, wherein the source material comprises a chalcopyrite; applying a pressure to the powder for a predetermined period of time; holding the powder at an elevated temperature below the melting temperature of the powder for the predetermined period of time, wherein the elevated temperature is between 100° C. and 400° C.; and annealing a resulting pressed pellet formed from the powder, wherein the pressed pellet comprises a plurality of crystals with different orientations that collectively exhibit a scintillation behavior of a single crystal of the source material.

2. The method of claim 1, wherein the powder is loaded into a die or mould to which the pressure is applied.

3. The method of claim 1, wherein the pressure is between 1500 psi and 4500 psi.

4. The method of claim 1, wherein the pressure is applied to the powder in a vacuum of less than 0.1 atm.

5. The method of claim 4, wherein the pressure and vacuum are held constant while the pressed pellet is allowed to cool to room temperature.

6. The method of claim 1, wherein the pressed pellet is annealed in an inert atmosphere for 6 hours or more at 400° C.

7. The method of claim 1, wherein, prior to applying the pressure, the powder is first packed into a forming mould and compressed at room temperature to increase the density of the powder to about 80% of the original density.

8. The method of claim 1, wherein the elevated temperature is achieved by injecting a heated inert gas into the powder.

9. The method of claim 1, wherein the annealing step is initiated by ramping down the pressure while maintaining the elevated temperature.

10. A method for forming a ceramic radiation detector material, comprising: receiving a source material comprising a powder, wherein the source material comprises a chalcopyrite; applying a pressure to the powder for a predetermined period of time, wherein the predetermined period of time is between 6 hours and 24 hours; holding the powder at an elevated temperature below the melting temperature of the powder for the predetermined period of time; and annealing a resulting pressed pellet formed from the powder, wherein the pressed pellet comprises a plurality of crystals with different orientations that collectively exhibit a scintillation behavior of a single crystal of the source material.

11. The method of claim 10, wherein the powder is loaded into a die or mould to which the pressure is applied.

12. The method of claim 10, wherein the pressure is between 1500 psi and 4500 psi.

13. The method of claim 10, wherein the pressure is applied to the powder in a vacuum of less than 0.1 atm.

14. The method of claim 13, wherein the pressure and vacuum are held constant while the pressed pellet is allowed to cool to room temperature.

15. The method of claim 10, wherein the pressed pellet is annealed in an inert atmosphere for 6 hours or more at 400° C.

16. The method of claim 10, wherein, prior to applying the pressure, the powder is first packed into a forming mould and compressed at room temperature to increase the density of the powder to about 80% of the original density.

17. The method of claim 10, wherein the elevated temperature is achieved by injecting a heated inert gas into the powder.

18. The method of claim 10, wherein the annealing step is initiated by ramping down the pressure while maintaining the elevated temperature.

19. A ceramic radiation detector material, comprising: a pressed pellet formed from a powder comprising a plurality of crystals with different orientations that collectively exhibit a scintillation behavior of a single crystal of a source material, wherein the source material comprises a chalcopyrite.

20. The ceramic radiation detector material of claim 19, wherein the pressed pellet is formed by a process comprising: receiving the source material comprising the powder; applying a pressure to the powder for a predetermined period of time; holding the powder at an elevated temperature below the melting temperature of the powder for the predetermined period of time; and annealing the resulting pressed pellet formed from the powder.

21. The ceramic radiation detector material of claim 20, wherein the elevated temperature is between 100° C. and 400° C.

22. The ceramic radiation detector material of claim 20, wherein the predetermined period of time is between 6 hours and 24 hours.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0012] The present disclosure is illustrated and described herein with reference to the various drawings, in which like reference numbers are used to denote like method steps/device components, as appropriate, and in which:

[0013] FIG. 1 is a flowchart illustrating a die press process for the formation and pressing of a lithium indium diselenide or like ceramic material, in accordance with the present disclosure;

[0014] FIG. 2 is a flowchart illustrating a HIP process for the formation and pressing of a lithium indium diselenide or like ceramic material, in accordance with the present disclosure;

[0015] FIG. 3 is a schematic diagram illustrating an exemplary die apparatus that may be used in the die press process of the present disclosure; and

[0016] FIG. 4 is a schematic diagram illustrating an exemplary die/die press apparatus that may be used in the die press process of the present disclosure.

DETAILED DESCRIPTION OF THE DISCLOSURE

[0017] Several processes may be used to form lithium indium diselenide (LiInSe.sub.2) crystals or ceramic pressed material or pellets. Disclosed herein are two exemplary methods for rapidly creating pressed ceramic compounds of LiInSe.sub.2 and the like that advantageously scintillate substantially identically to the single crystal forms. LiInSe.sub.2 powder may be compacted into ceramic compounds using die press or HIP processes. Although the pressed ceramic forms of the source materials exhibit decreased light output as compared with the single crystal forms, the time required to form the pressed materials is much less than the slower growth process for the single crystals.

[0018] In a die press process, such as the example provided in FIG. 1, pressure is applied through hydraulic or other means (mechanical, fluid, etc.) to the top and bottom of the die, where the sides of the die are effectively rigid and inflexible under the pressure. An exemplary die and die press are illustrated in FIGS. 3 and 4. In general, the die may be heated below the melting point of the material such that the chemical structure does not segregate into InSe.sub.3 or other possible alternative components. The minimum temperature depends on the pressure, but preferably the temperature is greater than about 200° C. The die is then held under a pressure ranging from about 1500 psi (10342 kpa) to about 4500 psi (31026 kpa) at the elevated temperature for a given time period. The temperature may range from about 200° C. to about 400° C. The time period may range from about 6 to about 24 hours. A vacuum may remove the majority of oxygen from surrounding areas and voids within the material. Thus, the die may be kept at vacuum of less than about 0.1 atm to prevent the constituents from oxidizing at the elevated temperatures. The vacuum may be maintained while heat is applied. The pressure and vacuum may be held constant while the die cools to room temperature for removal. The pressure may be independent while the material heats up, but preferably pressure is maintained during cooling to restrict the possibility of relaxation or axial growth. The material may be removed from the die as a pressed pellet. The pressed pellet may then be annealed in an atmosphere of an inert gas for more than about 6 hours at 400° C. Any inert gas may be used, but to prevent filling any voids within the material—such as helium might—a heavier, larger atom size may be preferable, such as argon, for example.

[0019] Referring to FIGS. 1, 3, and 4 specifically, the method 10 and forming apparatus 100 include: first, receiving a polycrystalline LiInSe.sub.2 material 102 pulverized or otherwise formed into a fine powder and loading the material 102 into a die 104 including a top portion 106, a bottom portion 108, a top plunger 110, and a bottom plug 112 (12); applying pressure to the die 104 using a top press 114 that acts on the top plunger 110 and a bottom press 116 that supports the bottom plug 112 (14); heating the die 104 below the melting point of the material 102 so that the chemical structure does not segregate (>200° C.) using a heating element 118 and thermal insulation 120 coupled to the die 104 (16); holding the die 104 under a pressure of between 1500 psi (10342 kpa) and 4500 psi (31026 kpa) at the elevated temperature of between 200° C. and 400° C. for between 6 and 24 hours with the die kept at vacuum of <0.1 atm (18); holding the pressure and vacuum constant while the die 104 cools to room temperature for removal (20); and, finally, annealing the pressed pellet in an inert atmosphere (such as argon) for >6 hours at 400° C. (22). It will be readily apparent to those of ordinary skill in the art that this process may be varied slightly and that any suitable die/press assemblies may be utilized equally.

[0020] In a hot isostatic press (HIP) process, such as the example provided in FIG. 2, a forming mould may be packed tightly with powder to control the shape of the resulting ceramic. The material may be compressed in a press at room temperature such that the density is increased to about 80% of the original density. Again, an exemplary die and die press are illustrated in FIGS. 3 and 4. Thin, flat, or slab shapes may be preferred for neutron detection, since the lithium absorber will effectively self-shield after some thickness. The grain size diameter of the powder may range from about 1 μm to about 1 mm. The form may be loaded into a HIP and pressurized to more than about 1500 psi (10342 kpa). The pressure and temperature may be ramped up through the addition and heating of an inert gas (such as argon). The temperature may range from about 100° C. to about 400° C. The pressure and temperature may be held constant for a time period ranging from about 6 hours to about 24 hours. Ramping down of the pressure may occur first under the elevated temperature as the direct entry into an annealing phase of approximately 6 hours, which relaxes the polycrystalline lattice and avoids cracking.

[0021] Referring to FIGS. 2-4 specifically, the method 30 and forming apparatus 100 include: first, receiving a polycrystalline LiInSe.sub.2 material pulverized or otherwise formed into a fine powder and packing the material into a forming mould, where material is compressed at room temperature to increase the density to ˜80% of the original density (32); loading the form into the HIP 104 and pressurizing the form to >1500 psi (10342 kpa) (34); ramping up the pressure and temperature through the addition and heating of an inert gas (such as argon) (36); holding the pressure and temperature (between 100° C. and 400° C.) constant for 6 to 24 hours (38); and, finally, ramping down the pressure—first under elevated temperature as the direct entry into an annealing phase of ˜6 hours, which relaxes the polycrystalline lattice and avoids cracking (40). Again, it will be readily apparent to those of ordinary skill in the art that this process may be varied slightly and that any suitable die/press assemblies may be utilized equally.

[0022] Thus, the ceramic material of the present disclosure may be formed utilizing source material. The source material may be produced by various methods where the source material stoichiometry is controlled and synthesized into the correct chemical formula, including the method described in U.S. Pat. No. 9,334,581, for example, the disclosure of which is fully incorporated herein by reference. Similar ceramic source materials may also be used, incorporating various co-dopants, etc. Material formed as a boule of polycrystalline LiInSe.sub.2 material may be pulverized or otherwise formed into a fine powder. The powder may then be loaded into either the die or forming mould.

[0023] The scintillation properties are intrinsic to the chemical makeup of LiInSe.sub.2, so heating does not induce this. However, the heating and post-annealing phases serve to relax mechanical stresses that can inhibit light transmission. In terms of each granule, individuals scintillate with the exact same light creation properties as a large single crystal, and the aggregation of many individual granules is preferred for an increase in “absolute efficiency” with respect to the light readout method.

[0024] Absolute efficiency is defined as the number of particles that pass through the detector per the number of particles emitted from a given source. The optimization of this absolute efficiency is gained by increasing detector surface area. Detector efficiency is determined by absolute efficiency and by the “intrinsic efficiency.” Intrinsic efficiency is defined as the number of interactions within the volume per the number of particles traversing the volume. Intrinsic efficiency is bounded by the volume of material, where self-shielding of the lithium compound restricts the usefulness of large thicknesses. So the ability to utilize this method to press very large, thin surface areas is a unique and beneficial feature over prior single crystal-based methods.

[0025] No binder agent is needed or ordinarily utilized, whereas some ceramics do utilize an epoxy. Multiple grain sizes are often mixed to create a heterogeneous structure that exhibits some cohesive property. Here, no binder agent is necessarily beneficial, but its use cannot be ruled out.

[0026] Detectors produced in the manner described herein need to reach a density ˜98% of theoretical maximum density (4.47 g) in order to be efficient as a scintillator. This production method does not result in the semiconductor detection mechanism form of LiInSe.sub.2.

[0027] Fast neutron detectors measure the energy deposited in the material during a scattering event. LiInSe.sub.2 is able to utilize lithium metal as the scattering mechanism given the low atomic masses of Li-6 and Li-7. Higher energy neutrons deposit energy through elastic or inelastic scattering, and the Li-6 or Li-7 dissipates this energy through a scintillation mechanism. Higher energies emit greater amounts of light that allow the material to act as a neutron spectrometer above the 150-200 keV lower limit. For large volumes (>0.5 cm thick), the material of the present invention will provide a very compact neutron detector for a CCD array, pixelated array of SiPMs, segmented PMT, and other fast neutron imaging modalities.

[0028] Because the pressed ceramic material of the present invention is cost effective to produce (˜20% of the cost of single crystal growth) and may assume a small form factor (e.g., 1 cm.sup.3 or less), it advantageously enables less costly production of large tiled detector arrays and the like, opening up novel imaging and detection applications at a distance, for example.

[0029] Although the present invention is illustrated and described herein with reference to preferred embodiments and specific examples thereof, it will be readily apparent to those of ordinary skill in the art that other embodiments and examples may perform similar functions and/or achieve like results. All such equivalent embodiments and examples are within the spirit and scope of the present invention, are contemplated thereby, and are intended to be covered by the following non-limiting claims for all purposes.