GENERATING NEUTRONS USING A ROTATING NEUTRON SOURCE MATERIAL

CROSS-REFERENCE TO RELATED APPLICATION

The present application claims the benefit of U.S. Patent Application No. 62/064,257 filed October 15, 2014 and U.S. Patent Application No. 62/157,652 filed May 6, 2015, which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention.

[0001] The present invention relates to the methods and systems for generating neutrons using a proton accelerator and a neutron source material.

2. Description of the Related Art.

[0002] Accelerator-based neutron sources have many potential applications, including medical treatments, isotope production, explosive/fissile materials detection, assaying of precious metal ores, imaging, and others. A particular area of interest boron neutron capture therapy (BNCT), which is a cancer treatment technique in which boron is preferentially concentrated in a patient's malignant tumor and a neutron beam is aimed through the patient at the boron-containing tumor. When the boron atoms capture a neutron, particles are produced having sufficient energy to cause severe damage to the tissue in which it is present. The effect is highly localized, and, as a result, this technique can be used as a highly selective cancer treatment method, effecting only specifically targeted cells.

[0003] Many of these activities are presently carried out at nuclear research reactors where neutrons are plentiful. However, many practical issues make this approach challenging, including safety, nuclear materials handling, and the approach of end-of life and decommissioning of many research reactors. Accelerator-based neutron sources can also be used as a relatively low-cost, compact alternative. A small, relatively inexpensive linear accelerator can be used to accelerate ions, such as protons, which can then be focused on a target capable of generating neutrons.

[0004] One of the most commonly proposed target materials is lithium, which reacts upon treatment with protons to produce neutrons through the reaction 7 Li(p,n) 7 Be. This reaction has a high neutron yield and produces neutrons of modest energy, both desirable for many applications. The reaction requires a source of protons having an energy of at least 1.88 MeV. Neutron yield and energy increase with higher proton energy. A commonly proposed operating point with high yield is at a proton energy of -2.4-2.7 MeV. High proton current corresponds to higher neutron generation rate, and, thus a high current proton accelerator is desirable. Such systems are known, particularly those operating with a proton current of 10-50 mA and a proton energy of 1.9-3.0 MeV and would be suitable for such an application.

[0005] In addition to the proton accelerator, an accelerator-based neutron source requires a target which presents the source material, such as lithium, to the proton beam. The primary challenge of this aspect of the technology is the handling of the high power which is imparted to the target by the beam. The energy of the proton beam is dissipated as heat in the target and must be removed before the target is destroyed. Two primary approaches have been proposed for heat removal. The first is a stationary solid target, intensively cooled from the backside. The second is a liquid target in which the beam impinges on a flowing jet of liquid source material. However, both of these approaches have significant drawbacks, particularly when lithium is used as the neutron source. Lithium has a relatively low melting temperature (180°C) and a relatively low thermal conductivity, which makes it very challenging to remove the heat from a solid target without overheating and melting the surface. In addition, exposure to intense proton beams quickly leads to blistering of the solid lithium, requiring frequent target replacement. Solutions have been proposed for these problems, but no fully productive solid target has yet been demonstrated. While liquid targets solutions have been described, these, in general suffer from slow heat-up times and potential solidification of flowing lithium if the temperature in the circuit drops too low, causing the charge of lithium to be inadvertently diverted into the target chamber. Flowing liquid lithium approaches also require a large amount of lithium to fill up the circuit, pump and heat exchanger, which leads to both high cost and a significant safety hazard from the highly reactive lithium.

[0006] Therefore, while neutron beam generation using proton accelerators have significant potential, there is a need for a lithium target that solves the problems of blistering and heat removal without using a high volume of lithium.

SUMMARY OF THE INVENTION

[0007] The present invention relates to a system for generating neutrons comprising a rotatable structure comprising a neutron source material and a proton beam generator. The structure is rotatable about an axis of rotation, and the proton beam generator is configured to direct a proton beam at the neutron source material as it rotates, thereby generating neutrons. The system can be used in various applications, including boron neutron capture therapy.

[0008] In one embodiment, the rotatable structure is a disk-shaped structure having a base comprising a rotatable hub centrally positioned within the base, the base further comprising at least one base segment having an outwardly-facing exterior surface configured to contain the neutron source material, particularly a layer of solid neutron source material, such as lithium. The hub comprises at least one coolant line extending to the base segments. The proton beam generator is configured to direct the proton beam at the neutron source material contained on the base segments. Preferably the disk-shaped structure is rotatable about an axis of rotation, and the proton beam is directed along a beam path that is substantially parallel to the axis of rotation.

[0009] In another embodiment, the rotatable structure is a cylindrical structure having a base connected to a substantially perpendicular outer wall. The outer wall comprises at least one wall segment having an inwardly-facing exterior surface configured to contain a film of a liquid neutron source material, such as lithium. The proton beam generator is configured to direct the proton beam at the film of liquid neutron source material contained on the wall segments. Preferably, the cylindrical structure is rotatable about an axis of rotation and has a horizontal base connected to a substantially vertical outer wall, and the proton beam is directed along a beam path that is substantially perpendicular to the axis of rotation.

[0010] The present invention further relates to a method of generating neutrons, particularly using the system of the present invention. Thus, in the present method a rotatable structure is provided comprising a neutron source material. The structure is rotated about an axis of rotation, and a proton beam provided by a proton beam generator is directed at the neutron source material as it rotates, thereby generating neutrons. The method can be used in various applications, including boron neutron capture therapy.

[0011] In one embodiment, the rotatable structure is a disk-shaped structure having a base comprising a rotatable hub centrally positioned within the base, the base comprising at least one base segment having an outwardly-facing exterior surface comprising a layer of solid neutron source material. The hub comprises at least one coolant line extending to the base segments. As the disk- shaped structure is rotated about the axis of rotation, the proton beam provided by the proton beam generator is directed at the layer of solid neutron source material along a beam path that is substantially parallel to the axis of rotation, thereby generating neutrons.

[0012] In another embodiment, the rotatable structure is a cylindrical structure having a base connected to a substantially perpendicular outer wall, the outer wall comprising at least one wall segment having an inwardly-facing exterior surface configured to contain a film of a liquid neutron source material. A solid neutron source material is provided on the base, and liquid neutron source material is formed by melting the solid neutron source material. As the cylindrical structure is rotated about the axis of rotation, liquid neutron source material flows from the base to the inwardly-facing exterior surface of the wall segment, thereby forming the film. The proton beam provided by the proton beam generator is directed at the film of the liquid neutron source material along a beam path that is substantially perpendicular to the axis of rotation, thereby generating neutrons.

[0013] It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are intended to provide further explanation of the present invention, as claimed. BRIEF DESCRIPTION OF THE DRAWINGS

[0014] FIG 1 and FIG 2 show a specific embodiment of the rotatable cylindrical structure that can be used in the system and method of the present invention.

[0015] FIG 3a-3c show various stages of formation of a film of liquid neutron source in a specific embodiment of the system and method of the present invention.

[0016] FIG 4a-d show specific embodiments of the rotatable disk-shaped structure that can be used in the system and method of the present invention.

[0017] FIG 5 shows an embodiment of a BNCT system of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

[0018] The present invention relates to a method and system for generating neutrons.

[0019] The system of the present invention comprises a rotatable structure, such as a platform or stage, which comprises a neutron source material, and a proton beam generator configured to direct a proton beam at the neutron source material on the rotatable structure as it rotates, thereby generating neutrons. The neutron source material can be any neutron generating material known, in the art, including, for example, lithium, and can be positioned anywhere on the rotatable structure using a variety of known techniques depending, for example, on the type and form of the source material and the design of the rotatable structure. Preferably, the rotatable structure comprises neutron source material positioned on an exterior, outwardly facing surface of the rotatable structure that can be readily exposed to the directed proton beam.

[0020] The structure is rotatable about an axis of rotation and can have a variety of different overall shapes, such as disk-shaped (including circular), annular, or cylindrical, depending, for example, on the overall system design requirements. Preferably, the rotatable structure is symmetrical, having an axis of rotation perpendicular to and in the center of the structure. The rotatable structure may be contained within an exterior housing, as desired, depending on the targeted application. In addition, the rotatable structure may be formed using a variety of different materials, depending, for example, on the chemical reactivity of the neutron source material, the conditions needed to contain the source material in its desired form, and cost. For example, the rotatable structure may comprise stainless steel or molybdenum.

[0021] The rotatable structure comprises a base that is generally flat but may further include various additional components or features, as desired, in order to, for example, contain a solid and/or liquid form of the neutron source material. The base may further comprise a means for rotation, such as a motor and axel. Preferably, the base comprises a central rotatable hub comprising various means of delivering heat transfer agents, such as heating fluids or coolants, to various portions or components of the base and/or of the rotatable structure. Channels can also be provided in the base as well as in various components of the rotatable structure to assist in delivering these fluids. Furthermore, the base may comprise at least one neutron source material containment section which is configured to hold the neutron source material in a targeted position. The shape, size, location, and number of containment sections will depend, for example, on the type and form of the neutron source material, the method in which the source material is provided onto the base, as well as the design of the rotatable structure.

[0022] In one embodiment of the present invention, the rotatable structure comprises a base having at least one base segment that comprises the neutron source material, which is preferably a solid. The base may be positioned vertically or horizontally depending on the configuration and position of the proton beam generator. The overall shape of the rotatable structure can vary but is preferably disk-shaped, comprising a base that is annular, circular, or nearly circular (having a polygonal shape approximating a circular disk). Thus, preferably the rotatable structure is symmetrical, having an axis of rotation perpendicular to and in the center of the base. Also, for this embodiment, the base can be generally flat, or, may comprise an annular stepped or angled region comprising the base segments, depending on, for example, the overall design and shape of the system. Preferably, the base segments, comprising the source material, are substantially flat.

[0023] The base segment of the rotatable structure can be any portion within or on the base and comprises an outwardly-facing exterior surface (i.e., facing towards the proton beam generator) that is configured to contain the neutron source material, and, in particular, a layer of solid neutron source material, such as lithium. Thus, the base may be divided into various base segments, separated, for example, by raised separators, or may be one continuous surface. For example, the rotatable structure may comprise a circular base comprising annular base segments or comprising pie-shaped or partial pie-shaped base segments. In this way, the exterior surface may comprise one continuous layer of neutron source material or may comprise layers positioned in various targeted segments or sections along the exterior outwardly-facing surface. Preferably, the exterior surface of the base segment is perpendicular to the proton beam directed at the neutron source material by the proton beam generator. However, the segments may be tipped or angled in order to increase the surface area of the solid source material that becomes contacted by the proton beam. Alternatively, the beam itself can be directed to strike the neutron source material at an angle, thereby increasing the contacted area.

[0024] The rotatable structure for this embodiment may be formed using a variety of different materials, depending, for example, on the chemical reactivity of the neutron source material, the conditions needed to produce the layer of solid source material, and cost. For example, the rotatable structure may comprise stainless steel or molybdenum. The base segments preferably comprise high conductivity materials, such as copper, aluminum, or molybdenum. The solid layer of source material may be provided on the exterior surface of the base segments using any method known in the art. For example, the layer can be deposited or coated directly onto the outwardly-facing surface using known methods. Preformed layers may also be provided and subsequently placed or positioned directly onto the exterior surface of the base segments. In addition, the exterior surface may comprise one or more neutron source material containment sections, and solid neutron source material, in non-layer form (such as flakes, pieces, or pellets) may be provided within these sections, melted, and cooled to form the layer of solid source material. While the solid source layer may be provided directly onto the base segment surface, one or more intermediate layers may also be used, particularly to improve bonding, and thereby the thermal contact, of the layer to the segment surface. This layer may also provide a physical barrier to prevent chemical interactions between the source material and to the segment. For example, for a solid lithium target, an intervening layer of copper may be used to provide improved bonding of the lithium target to an aluminum base segment while preventing amalgamation of the aluminum by the lithium. Intermediate layers may also be used to increase the dose threshold of blistering in the target. When exposed to high fluences of protons, most materials eventually blister due to the accumulation of hydrogen gas and damage at the end of range of the particles in the material. If the thickness of the source material is chosen to be less than the range of the protons in that material, then the protons will stop in a deeper layer. This deeper layer may be made of a material that is chosen for its ability to resist blistering, such as iron, tantalum, or others known in the art. This blister stop layer may comprise a bonding or barrier layer, or may be in addition to any bonding or barrier layers. Other methods would be known by one of ordinary skill in the art. The thickness of the solid layer of source material can vary, depending, for example, on the targeted application, the power of the proton beam, and the exposure time of the layer. Generally the solid layer is 2 mm in thickness or less, such as from about 0.1 to about 1 mm or from about 0.05 mm to about 0.5 mm.

[0025] Furthermore, for this embodiment, the base comprises a rotatable hub centrally positioned within the base, along with various means of delivering heat transfer agents, such as heating fluids or coolants, to various portions of the base. As a specific example, the base can include at least one coolant line extending from a central rotatable hub unit to the base segment, which comprises the solid neutron source material. In this way, coolant can be delivered to cool the outwardly-facing exterior surface of the base, such as through channels provided therein that enable thermal communication of the coolant with the exterior surface, particularly while the proton beam is focused and reacting with the layer of solid neutron source material.

[0026] In another embodiment of the present invention, the rotatable structure comprises a base connected to an outer wall, and the outer wall comprises the neutron source material. Preferably the rotatable structure is symmetrical, having an axis of rotation perpendicular to and in the center of the base. The overall shape of the structure can vary but is preferably cylindrical or nearly cylindrical, comprising a base connected to a substantially perpendicular outer wall. For example, the rotatable structure can be a horizontal base, such as a circular base, connected to a vertical or substantially vertical wall along its outer circumference.

[0027] The outer wall of the rotatable structure comprises an inwardly-facing exterior surface (i.e., facing towards the center of the base) that is configured to contain the neutron source material, and, in particular, a film of a liquid neutron source material, such as lithium. A variety of different techniques can be used to contain the film, and preferred examples are described below. In addition, the outer wall may be angled or tipped a few degrees (such as 1-2 degrees) outwardly from vertical (i.e., away from the center of the base) and is therefore not precisely parallel with the axis of rotation, in order to assist in formation of the film of the liquid neutron source material.

[0028] For this embodiment, the outer wall of the rotatable structure may be a continuous circular ring, having one continuous inwardly-facing exterior surface, or may be segmented into a plurality of distinct wall segments, thereby separating the inwardly-facing exterior surface into various sections configured to each contain a film of a targeted amount of the liquid neutron source material. For example, the outer wall may comprise a plurality of wall segments, each having a similar shape and size. The wall segments may have a curved shape, forming arc segments of the overall circular cross- sectional shape of the outer wall, or may be flat, thereby approximating the overall circular ring shape of the outer wall. The inwardly-facing exterior surface of the outer wall can be segmented into wall segments using a variety of methods, including, for example, by providing raised separators attached to the exterior surface of the outer wall, by forming indented pockets or depressions within the outer wall, or by physically separating and dividing the outer wall into detachable wall segment pieces.

[0029] The base of the rotatable structure is preferably contiguous with the outer wall. Thus, the base and wall may be formed as one unit or, alternatively, may be separate components connected or bonded together. The base can have a variety of different shapes but is preferably annular, circular, or nearly circular (having a polygonal shape approximating a circle). For this embodiment, the base is generally flat and comprises a rotatable hub centrally positioned within the base as well as various means of delivering heat transfer agents, such as heating fluids or coolants to various portions of the base and/or the outer wall. As a specific example, the base can include at least one coolant line extending from a central hub unit to one or more wall segments of the outer wall, thereby delivering coolant to cool the inwardly-facing exterior surface of the outer wall, such as through channels provided therein that enable thermal communication of the coolant with the exterior surface. [0030] In addition, for this embodiment, the base may further comprise at least one neutron source material containment section, such as a trough or well, in which solid neutron source material, such as lithium, can be placed and held, particularly as the base is rotated. The volume of the trough is preferably greater than the volume of the neutron source material to be positioned therein, and, as such, would be sufficient to contain the source as a melt. The total volume of is sufficient to hold the total volume of source needed to produce the desired neutron beam. The trough can be located anywhere within or on the base but is most conveniently positioned at the junction between the base and the outer wall, permitting fluid communication between the trough and the outer wall. While one continuous trough or well can be used, it is preferred that a plurality of troughs are provided and that each trough is separated from a neighboring trough by a raised separator or divider. In this way, liquid neutron source material formed within the trough or well can be contained in discreet portions at specific locations along the base, and, in particular, at the junction between the base and the outer wall. Preferably, when the base comprises a plurality of troughs, the outer wall also comprises a plurality of wall segments, with each trough being in fluid communication with one or more, preferably one, wall segment. Thus, the discreet portion of liquid neutron source material will be in fluid communication with a wall segment having an interior surface which is configured to contain a film of the source material.

[0031] When the base comprises one or more troughs, the base can also include at least one feeder line extending from a central hub unit to one or more of the troughs, thereby delivering heat transfer fluid to heat and melt solid neutron source material positioned in the trough, such as through channels provided in the base near or beneath the trough to enable thermal communication of the fluid with the trough. In this way, the neutron source material can be converted to a liquid form. Alternatively, the system can comprise at least one heat source, such as one or more internal heaters or heat lamps, positioned and configured to heat the solid neutron source material. Such heating can be applied to one trough at a time or to all troughs simultaneously (such as by rotating the structure) as desired to form the liquid neutron source material. In addition, the proton beam to be directed at the film of neutron source material, generated as discussed below, may be re-directed onto the trough, with the beam power being used to assist in initiating the melting of the neutron source material.

[0032] The rotatable structure for this embodiment may be formed using a variety of different materials, depending, for example, on the chemical reactivity of the neutron source material, the conditions needed to contain the source material in liquid form, and cost. For example, the rotatable structure may comprise stainless steel or molybdenum. Surprisingly, in the present invention, the ability of the liquid neutron source material to wet the material used to form the rotatable structure material is not necessarily a deciding factor. For example, it is known that lithium has a relatively high surface tension (approximately 400 dynes/cm at 200°C) and a relatively low density (approximately 0.5 g/cm ), which leads to a very high tendency of lithium to "ball-up" or contract into thick puddles on a flat surface, making it challenging to create a thin, uniform film of liquid lithium. To address this problem, the rotatable structure used in the present invention can be formed with a material with a surface that is readily wetted by lithium. However, this would make it more challenging to remove unused or spent lithium for replacement or system maintenance. In the system and method of the present invention, materials that are not well wetted by lithium at the desired operating temperatures (such as below 300°C) can be used, providing both economic and functional advantages.

[0033] As noted above, the system of the present invention further comprises a proton beam generator. Any source of a proton beam known in the art can be used, including, for example, a proton beam generator comprising a proton accelerator, depending, for example, on the proton beam target and the desired application of the resulting neutron beam. For example, for neutron production from a lithum target ( 7 Li(p,n) 7 Be), the reaction requires a source of protons with an energy from at least 1.88 MeV to about 2.4-2.7 MeV. A high current proton accelerator is preferred, such as a proton accelerator operating with a proton current of 30-50 mA and a proton energy of 1.9-2.7 MeV. The proton beam from the proton beam generator can be focused on neutron source material (either as a liquid film or solid layer, as described above) thereby generating neutrons.

[0034] Furthermore, in the present system and method, the beam can be monitored and profiled during neutron production. The rotatable structure is segmented in the circumferential direction such that there are many depressions each containing a film or layer of lithium. A single, small hole can be drilled in the base segments or vertical outer wall between each pair of depression such that the plurality of holes would form, for example, a helical pattern on the inside of the outer wall. A faraday cup can be placed behind the rotating structure such that the beam impinges on it when a hole passes in front of the beam. The data collected from the faraday cup, combined with timing information from the rotation structure can be used to reconstruct a two-dimensional profile of the beam at each revolution, without interrupting neutron production. This information would be useful in ensuring that the desired beam profile, location and intensity are maintained.

[0035] Thus, the present invention further relates to a method of generating neutrons using the system of the present invention. Any of the components described above can be included, as needed. Thus, in one embodiment, the method comprises the step of providing a disk-shaped rotatable structure having a base comprising a rotatable hub centrally positioned within the base, the base comprising at least one base segment having an outwardly-facing exterior surface configured to contain a layer of solid neutron source material, such as lithium, and the hub comprising at least one coolant line extending to the base segments. As the disk-shaped structure is rotated about the axis of rotation, the proton beam provided by the proton beam generator is directed at the layer of solid neutron source material, thereby generating neutrons. Preferably, the proton beam is directed along a beam path that is substantially parallel to the axis of rotation.

[0036] In another embodiment, the rotatable structure is a cylindrical structure having a base connected to a substantially perpendicular outer wall. Preferably, the outer wall comprises at least one wall segment having an inwardly-facing exterior surface configured to contain a film of a liquid neutron source material, such as lithium. In this embodiment of the method of the present invention, a solid neutron source material is provided on the base, preferably in one or more troughs or wells positioned at the junction between the base and the outer wall, permitting fluid communication between the trough and the outer wall, and liquid neutron source material is formed by melting the solid neutron source material. As the cylindrical structure is rotated about the axis of rotation, liquid neutron source material flows from the base to the inwardly-facing exterior surface of the wall segment, thereby forming the film. The proton beam provided by the proton beam generator is directed at the film of the liquid neutron source material, thereby generating neutrons. Preferably, the proton beam is directed along a path that is substantially perpendicular to the axis of rotation (for example, forming an angle with the axis of rotation of from about 80° to about 100°, including from about 85° to about 95°)

[0037] For this embodiment, melting of the solid neutron source material can occur prior to or simultaneously with the rotation of the rotatable structure, depending, for example, on the speed of rotation and the relative rate of heating and melting of the source. For example, the rotational speed can be about 500 rpm, generating a centrifugal force of at least about 150 gs. The thickness of the liquid film can vary, depending, for example, on the targeted application, the power of the proton beam, and the exposure time of the film. Generally the liquid film is 5 mm in thickness or less, such as from about 1 mm to about 3 mm. Conditions for forming the film will depend on the properties of the liquid source material and the exterior surface of the outer wall. Sufficient centrifugal force should be applied to the liquid source in order to produce a flat substantially continuous film having the desired thickness. For example, for a lithium source, a rotation frequency of approximately 600 rpm can be used, corresponding to a centrifugal force of approximately 200 gs, which would be expected to be sufficient to produce a lithium film of approximately 1.25 mm in the case of a segment material that is not wetted by the lithium. Use of a volume of lithium sufficient to provide a thicker film, such as 2mm thick, would ensure that the behavior of the puddle will not be dominated by surface tension, and it will expand to cover the entire depression in the pedestal.

[0038] Specific embodiments of the system and method of the present invention are described below and shown in FIG 1-5. However, it should be apparent to those skilled in the art that this is merely illustrative in nature and not limiting, being presented by way of example only. Numerous modifications and other embodiments are within the scope of one of ordinary skill in the art and are contemplated as falling within the scope of the present invention. In addition, those skilled in the art should appreciate that the specific conditions and configurations are exemplary and that actual conditions and configurations will depend on the specific system. Those skilled in the art will also be able to recognize and identify equivalents to the specific elements shown, using no more than routine experimentation. [0039] A specific example of one embodiment of a neutron generation system of the present invention is shown FIG 1. As shown, system 100 comprises rotatable structure 110 having axis of rotation X contained within external housing 125. Rotatable structure 110 has an overall cylindrical shape and comprises horizontal base 130 connected to vertical outer wall 140, which is segmented in a plurality of wall segments, which are more clearly seen in FIG 2. System 100 further comprises rotatable hub 160 centrally positioned within base 130 that includes coolant lines and/or feeder lines for heat transfer fluids as needed. Motor 180 rotates structure 110. As shown in this specific example, proton beam 190 enters structure 110 through opening 191 which is above the top edge of outer wall 140 and passes on to the opposite side, striking the liquid neutron source film as it rotates past. Neutrons are generated and exit structure 110 through aperture 192. Thus, for this cylindrical structure, the proton beam is directed along a beam path that is substantially perpendicular to the axis of rotation, deviating slightly from perpendicular only by the height of outer wall 140. Adding a slight outward tip of a few degrees may be desirable in order to assist in film formation, and would bring the beam path closer to perpendicular.

[0040] FIG 2 shows further details concerning rotatable structure 110. As shown, the horizontal base portion of structure 110 is divided into a plurality of troughs 210 by raised base separators 215. In addition, outer wall portion of structure 110 is also divided into a plurality of wall segments 220 by raised wall separators 225. As shown, each trough 210 is positioned adjacent to a corresponding wall segment 220 at the junction of the base and the outer wall.

[0041] A specific example of the formation of a film of a liquid neutron source material, such as lithium, is shown in FIG 3a-3c. As shown in FIG 3a, solid lithium 300 in the form of tubes (although other forms and shapes are possible) is positioned within troughs 210 of structure 110. Heat is provided to the troughs, either through channels in the horizontal base connected to feeder lines in the central rotatable hub or by an internal heater, such as heat lamps, positioned near the troughs. As shown in FIG 3b, heating melts solid lithium 300 to produce liquid lithium 310. Troughs 210 have a volume higher than the volume of liquid lithium 310, and thus the liquid neutron source material is contained therein. Simultaneously with the melting step, or subsequently, structure 110 is rotated at sufficient speed to cause liquid lithium 310 to climb up and into wall segments 220 (indicated by arrow A) thereby forming liquid lithium film 320 (shown if FIG 3c). Wall segment 220 is properly configured to contain film 320, having the proper volume, surface properties, and separator heights for the given rotational speed and conditions.

[0042] A proton beam directed at the lithium film target as it rotates through the beam would be expected to produce the desired stream of neutrons. This method has the benefit of minimizing the volume of liquid lithium required as the neutron source target while also avoiding the problem of blistering, expected for solid neutron targets or liquid targets that prematurely solidify. In addition, because the liquid source (i.e, lithium) does not need to flow, there is no significant concern if unintentional solidification does occur. In fact, in the present method and system, lithium could be maintained at or below its melting point, operating as a two-phase system. Much of the beam energy would be absorbed by the phase change of the lithium, minimizing any temperature spike as the target passes through the beam. This would be expected to reduce the risk of boiling lithium, allowing operation at lower temperatures while still eliminating the problem of blistering. Furthermore, heat removal is improved compared to methods using a stationary target since the heat generated is spread over a large area as the target rotates, while also maintaining the neutron source in a small region, as is desirable for most applications. Addition heat removal can be provide by circulating coolant through coolant lines extending from a central hub unit in the horizontal base to one or more wall segments of the outer wall, thereby delivering coolant to be in thermal communication with the inwardly-facing exterior surface of the outer wall, such as through channels provided therein. Also, heat transfer fluid used to melt the lithium may also be circulated to be in thermal communication with the inwardly-facing exterior surface, such as through connecting channels, maintaining the temperature of the liquid film.

[0043] Additional expected benefits of the present method and system include fast, robotic removal of the neutron source material for replacement or system maintenance. For example, a lithium target could be allowed to solidify in the trough of the horizontal base. If the trough is prepared using a material having a suitable anti-stick surface for lithium, pellets of lithium would form, which could be removed directly or, alternatively, a removable trough can be used. This would minimize downtime for maintenance or lithium replacement and would also significantly reduce the radiation hazard to maintenance personnel associated with the radioactive beryllium reaction product contained in the lithium. Additional benefits are also possible, given the benefit of the present disclosure.

[0044] A specific example of another embodiment of a neutron generation system of the present invention is shown FIG 4a. As shown, system 400 comprises rotatable structure 410 having axis of rotation X contained within external housing 425 and surrounded by neutron reflector 426. Rotatable structure 410 has an overall disk shape and comprises vertical base 430 which has an annular stepped region comprising base segment 420 upon which solid lithium layer 435 is provided. Base 430 further comprises rotatable hub 460 (only partially visible in FIG 4a) and includes coolant lines and/or feeder lines for heat transfer fluids as needed to cool solid lithium layer 435 while rotating. Rotatable structure 410 is rotated about axis X, and proton beam 490 strikes solid lithium layer 435 as it rotates past, thereby generating neutrons, which exit through moderator 491 and collimator 492. Thus, for this disk-shaped rotatable structure, the proton beam is directed along a path that is substantially parallel to the axis of rotation.

[0045] An additional specific example of this embodiment is shown in FIG 4b-4d. Regarding FIG 4b, system 401 is shown comprising rotatable structure 411 having an overall disk shape, which further comprises vertical base 431 having base segments 421 upon which solid lithium layer 436 is provided. This is more clearly seen in FIG 4c, which is a front view of rotatable structure 411. Base 431 comprises rotatable hub 461 which includes a plurality of coolant lines 470 and feeder lines 475 connecting hub 461 to base segments 421 to deliver coolant to solid lithium layer 436 as structure 411 is rotated about axis Y by motor assembly 450. The hub and associated lines or channels are more clearly seen in FIG 4d, which is a back view of rotatable structure 411. Also, segment 422 is shown in FIG 4c and FIG 4d in cutaway view showing channels in the segment to deliver coolant behind lithium layer 436. Rotatable structure 411 is rotated about axis Y, and proton beam 491 strikes solid lithium layer 436 as it rotates past, thereby generating neutrons, which exit through the back of the target. Thus, for this disk- shaped rotatable structure, the proton beam is directed along a path that is substantially parallel to the axis of rotation.

[0046] These examples also have the benefit of minimizing the amount of lithium needed as the neutron source target. Since coolant is circulated through lines or channels extending from the central hub to the base segments, heat is thereby removed from the solid target as it rotates, minimizing overheating and blistering and allowing thinner solid targets to be used compared to methods using a stationary target. The heat generated is spread over a large area as the target rotates, while also maintaining the neutron source in a small region, as is desirable for most applications. In addition, the rotatable disk-shaped structure can be positioned vertically, horizontally, or at any angle desired, depending on the position of the target of the generated neutron beam. This provides the present system with considerable design flexibility. Furthermore, the base segments may be individually removable from the rotatable structure for fast, robotic removal of the neutron source material for replacement or system maintenance. For example, a segmented solid lithium target could be used on separate yet attached base segments, as shown in FIG 4c and FIG 4d. Detachment of the segments from, for example, the portion of the base comprising coolant and feeder channels allows for quick and easy removal and replacement, minimizing downtime for maintenance or neutron source replacement and would also significantly reduce the radiation hazard to maintenance personnel. Additional benefits are also possible, given the benefit of the present disclosure.

[0047] The neutrons produced by the systems and methods of the present invention can be used in a variety of different applications. For example, the resulting neutrons can be used for isotope production, explosive and/or fissile materials detection, for assaying of precious metal ores, or in various imaging and medical techniques. As a specific example, the neutrons can be included as part of a boron neutron capture therapy (BNCT) for treatment of cancer.

[0048] Thus, the present invention further relates to both a BNCT system as well as a BNCT method. A general schematic of an embodiment of the present BNCT system and method is shown, FIG 5 as well as, in part, in FIG 4. For example, referring to FIG 5, which is not drawn to scale, BNCT system 500 comprises neutron generating system 550 and patient positioning and treatment system 580. Neutron generating system 550 comprises proton beam generator 510 and neutron source target 520, which is provided on a rotatable structure (not shown). Any of the rotatable structures of the present invention and described above can be used. Proton beam generator 510 can be provided in a variety of different positions relative to neutron source target 520, depending upon, for example, the size and design of the facility in which they are placed. Various known bending or focusing magnets can be used to direct the generated proton beam to the target.

[0049] Proton beam 590, produced by proton beam generator 510, passes through beam transport system 515, which may include, for example, various types of focusing magnets, and reacts with neutron source target 520, thereby generating neutrons, which are generally produced in multiple directions around the source depending on their energy - higher energy neutrons moving forward from the target and lower energy neutrons scattering perpendicular to or back from the source. To generate neutron beam 570 having the desired energy and direction for BNCT treatment, neutron generating system 550 further comprises reflector 526, beam moderator 591, and beam collimator 592. Any neutron beam reflector, moderator, or beam collimator/delimiter known in the art can be used, and each can be positioned around the target as desired in order to capture neutrons having the desired energy range. For example, reflector 526 can be positioned around the sides and behind the target, as shown in FIG 4 and FIG 5, and can comprise any material known in the art that is relatively non-absorbent to neutrons, such as high atomic number material (including lead, bismuth, or alumina), or carbonaceous materials (including graphite). In this way, low energy back-scattered neutrons are reflected back into the system, thereby protecting or shielding surrounding components as well as patient 599. The forward-directed, relatively higher energy neutrons can be captured by moderator 591 (also comprising materials that are relatively non-absorbent to neutrons), in order to reduce their energy to a desired epithermal range. In this way, for example, neutrons having an initial energy of approximately 500 keV can be reduced to a final energy of from about leV to about lOkeV, which is a range desirable for BNCT treatment. Suitable moderator materials are known in the art and include, for example, D ₂ 0, MgF, LiF, A1F ₃ , Al, Teflon, and mixtures thereof. Finally, as shown, beam collimator 592 can be positioned after moderator 591 to produce and focus the desired neutron beam onto target 598 in patient 599. [0050] As shown in FIG 5, BNCT system 500 further comprises patient positioning and treatment system 580 which includes equipment and controls for delivering the neutron beam to the patient. For example, a boron delivery system and protocol are used in which the chosen boron-containing treating agent is delivered to patient 599 at the prescribed dose in order to produce target 598. Control systems are used to accurately position the target to coincide with expected neutron beam path, and such control systems would be known to one skilled in the art. Additional equipment and components can also be used as needed and would also be well known in the field.

[0051] The foregoing description of preferred embodiments of the present invention has been presented for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed. Modifications and variations are possible in light of the above teachings, or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.

[0052] What is claimed is: