Patent Publication Number: US-2018043188-A1

Title: System for adjusting the energy level of a proton beam provided by a cyclotron, a cyclotron target holder assembly with a removable degrader, a removable degrader for use in a cyclotron target holder assembly, and methods of use thereof

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application is a continuation application of U.S. patent application Ser. No. 15/253,277, which was filed Aug. 31, 2016, issued on Oct. 17, 2017 as U.S. Pat. No. 9,789,341, and claims the benefit, under 35 U.S.C. §119(e), of U.S. Provisional Pat. App. Ser. No. 62/212,238, filed Aug. 31, 2015, all of which are hereby incorporated herein by reference in their entireties. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a system for adjusting the energy level of a proton beam provided by a cyclotron, a cyclotron target holder assembly, a removable degrader, and methods of using the same. 
     BACKGROUND 
     Particle accelerators are used to provide particle beams of charged particles. Cyclotrons commonly provide particle beams that have a fixed energy level. For example, the PETtrace 880 cyclotron manufactured by GE Healthcare (Husbyborg, Sweden) has a fixed energy of 16.5 meV. Some cyclotrons are capable of producing particle beams where the energy is variable, for example the ACS PET trace system. 
     SUMMARY 
     The cyclotrons capable of producing beams with variable energy cannot be adjusted to produce energy below 14 meV. However, there may be instances where a lower meV is desired, for example as discussed in U.S. Provisional Pat. App. Ser. No. 62/171,453, the entirety of which is incorporated by reference herein. Furthermore, commercially available cyclotron instruments allowing for variable energy are much more complicated than fixed-energy cyclotrons. These differences result in additional cost and downtime related to maintenance and repairs of variable energy cyclotrons compared to fixed-energy cyclotrons. Finally, commercially available cyclotrons having variable energy beams do not allow for independent variation of energy level at each of the targets where there is more than one target. This is because the energy of the beam is adjusted by intrinsically changing the energy level at the beam source. 
     The present invention provides a target holder assembly configured to hold a target material for bombardment of the target by a particle beam of a fixed-energy cyclotron including: a target body including a housing with a slot for receiving a removable degrader to place an attenuation disc of the degrader in a path of the particle beam of the cyclotron to reduce an energy level of the particle beam prior to the beam reaching the target material, wherein the degrader can be inserted into, and removed from, the target holder assembly without removal of the target holder assembly from the fixed-energy cyclotron. 
     According to an example embodiment of the present invention, the target body includes a first target body section separate from a second body section, the first body section being configured to hold the target material. According to an example embodiment of the present invention, the second target body section is configured to house the removable degrader. According to an example embodiment of the present invention, the target body further includes a third target body section, and the second and third target body sections are configured to attach to the fixed-energy cyclotron without attachment of the first target body section. 
     According to an example embodiment of the present invention, a removable degrader for use in a fixed-energy cyclotron includes a frame, an attenuation disc, a first circular channel and a second circular channel, where the first and second circular channels circumscribe the attenuation disc on respective sides of the attenuation disc, the removable degrader is configured for insertion into the cyclotron such that, when inserted, the attenuation disc is in a path of the particle beam and is configured to allow for removal and/or replacement of the attenuation disc from the path of the particle beam without removal of a target holder assembly housing the removable degrader, and the attenuation disc is configured to reduce the energy level of the particle beam that passes through the attenuation disc to reach a target. 
     In an example embodiment of the present invention, the removable degrader further includes a first metal O-ring within the first circular channel and a second metal O-ring within the second circular channel. 
     According to an example embodiment of the present invention, a target holder assembly for use in a fixed-energy cyclotron includes a body and a removable degrader according to any one of the above embodiments, where the removable degrader can be fitted into the body of the target holder assembly to place an attenuation disc in a path of the particle beam, and can be removed and/or replaced from the body of the target holder assembly without removal of the target holder assembly from the fixed-energy cyclotron, and the attenuation disc is configured to reduce the energy level of the particle beam that reaches a target. 
     In an example embodiment, the body includes a first target body, a second target body, a third target body, where the first target body is configured to hold a target material, the second target body is configured to house a removable degrader, and the second and third target bodies are configured to attach to the fixed-energy cyclotron without attachment of the first target body. 
     The present invention also provides a removable degrader for use in a fixed-energy cyclotron including a frame; an attenuation disc; a first circular channel at a first side of the attenuation disc and a second circular channel at a second side of the attenuation disc; wherein the first and second circular channels circumscribe the attenuation disc; the removable degrader is configured for removable insertion of the degrader into a target holder, without removal of the target holder from the fixed cyclotron and so that, when the removable degrader is removably inserted into a target holder, the attenuation disc is positioned in a path of a particle beam, thereby reducing an energy level of the particle beam before the particle beam reaches a target held by the target holder; and the target holder into which the removable degrader is adapted to be inserted includes a first housing section with a slot for receiving the removable degrader and a second housing section that is configured to hold the target and to be shifted towards the first housing section to form an air tight seal of the attenuation disc of the degrader, when the degrader is positioned in the slot. According to an example embodiment of the present invention, the removable degrader further includes a first metal O-ring within the first circular channel and a second metal O-ring within the second circular channel. 
     The present invention also provides a system for adjusting the energy level of a particle beam provided by a fixed-energy cyclotron, the system including the fixed-energy cyclotron and a target holder assembly according to any one of the above target holder assemblies. 
     The present invention also provides a method for reducing an energy level of a particle beam of a fixed-energy cyclotron, the method including providing a target holder assembly with a removable degrader in the path of the particle beam of the fixed-energy cyclotron, where the removable degrader is configured to be removed and/or replaced without removal of the target holder assembly from the cyclotron. 
     The present invention also provides a method for independently reducing an energy level of particle beams reaching at least two targets of a fixed-energy cyclotron, the method including providing two or more target holder assemblies each with a removable degrader in the path of the particle beam of the fixed-energy cyclotron before their respective targets, where each removable degrader includes an attenuation disc, the thickness of each removable degrader is independently selected, and each of the removable degraders is configured to be removed and/or replaced without removal of its respective target holder assembly from the cyclotron. 
     These and other embodiments of the invention will be better appreciated and understood when considered in conjunction with the following description and the accompanying drawings. However, the following description, while indicating various embodiments of the invention and numerous specific details thereof, is given by way of illustration and not of limitation. Many substitutions, modifications, additions and/or rearrangements can be made within the scope of the invention without departing from the spirit thereof, and the invention includes all such substitutions, modifications, additions and/or rearrangements. 
     The drawings accompanying and forming part of this specification are included to depict certain aspects of the invention. A clearer conception of the invention and of the components and operation of systems provided with the invention will become more readily apparent by referring to the exemplary, and therefore non-limiting, embodiments illustrated in the drawings, where like reference numerals (if they occur in more than one view) designate the same or similar elements. The invention may be better understood by reference to one or more of these drawings in combination with the description presented herein. The features illustrated in the drawings are not necessarily drawn to scale. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exploded side view of a schematic illustrating a target holder assembly according to an example embodiment of the present invention for a solid target with a removable degrader placed inside a degrader receiving window. 
         FIG. 2  shows a first exploded perspective view of a schematic illustrating the target holder assembly, according to an example embodiment of the present invention, with a generalized first target body and with the removable degrader pulled out of the degrader receiving window. 
         FIG. 3  shows a second exploded perspective view of the schematic illustrating the target holder assembly of  FIG. 2 , according to an example embodiment of the present invention. 
         FIG. 4  shows an exploded cross-sectioned perspective view of a schematic illustrating the target holder assembly, according to an example embodiment of the present invention. 
         FIG. 5  shows a cross-sectioned perspective view of a schematic illustrating the removable degrader, according to an example embodiment of the present invention. 
         FIG. 6  shows an exploded perspective view of a schematic illustrating the removable degrader along with its accompanying metal O-rings, according to an example embodiment of the present invention. 
         FIG. 7  shows a schematic illustrating a second target body portion of the target holder assembly, without the removable degrader, according to an example embodiment of the present invention. 
         FIG. 8  shows a cross-sectional perspective view of a schematic illustrating the second target body without the removable degrader. 
         FIG. 9  shows another perspective view of the schematic illustrating the second target body without the removable degrader. 
     
    
    
     DETAILED DESCRIPTION 
     Example embodiments of the present invention provide a cyclotron system, a target holder assembly, a removable degrader, and methods of use thereof that allow for adjusting the energy level of a fixed-energy cyclotron beam. According to an example embodiment of the present invention, a cyclotron system includes a target holder assembly, a removable degrader, and a fixed-energy cyclotron, where the energy level of the proton beam of the fixed-energy cyclotron is adjusted by providing a removable degrader into the path of the proton beam in a target holder assembly. According to an example embodiment of the present invention, the cyclotron system includes multiple targets, where the energy of the proton beam is independently adjusted for each target by independently providing a removable degrader including an attenuation disc with a respective thickness (or material) for each target holder assembly of the fixed-energy cyclotron. According to an example embodiment of the present invention, the target holder assembly and the removable degrader are configured to allow for removal and/or replacement of the removable degrader from the path of the proton beam without removal of the target holder assembly from the cyclotron. According to an example embodiment of the present invention, the energy level of the proton beam produced by the fixed-energy cyclotron is adjusted by replacing a removable degrader including a first thickness (or material) with another removable degrader including a second thickness (or material). A method, according to an example embodiment of the present invention, of adjusting the beam energy using the removable degrader, substantially reduces the adjustment time by advantageously doing away with the need for powering down the cyclotron, removing the target holder assembly, reassembling the target holder assembly, and powering on the cyclotron. By allowing for the removable degrader to be removed and/or replaced from the body of the target holder assembly without removal of the target holder assembly from the fixed-energy cyclotron, the cyclotron is able to remain powered on and in particular the vacuum created by the cyclotron is maintained to decrease the downtime of the cyclotron. The method, according to an example embodiment of the present invention, of adjusting the beam energy using the removable degrader, advantageously reduces the resulting exposure of service personnel to radiation thereby increasing operating safety. The compact design of the target holder assembly, according to an embodiment of the present invention, allows for a direct bolt-on replacement of conventional target holder assemblies without the need for additional modification to the cyclotron. 
     Cyclotrons are generally used to irradiate target materials with a proton beam in order to cause a nuclear reaction in the target material. For example, commercially available proton beam cyclotrons such as the PETtrace 880 cyclotron manufactured by GE Healthcare (Husbyborg, Sweden), can be used to create radioisotopes. The target material can be liquid, gas, foam, or solid. The target material is held or contained in a target holder assembly using the appropriate assembly configuration for the target material type. 
     In an example embodiment, a fixed-energy cyclotron is provided with a configuration that facilitates reduction of the energy level of the proton beam from its factory setting.  FIG. 1  shows a target holder assembly  100  and removable degrader  200  according to an example embodiment present invention. In an example embodiment, the target holder assembly includes a first target body  110 , a second target body  120 , and a third target body  130 . The third target body  130  is configured to mate with a housing of the cyclotron (not shown). This particular target holder assembly  100  is configured for irradiating a solid target  102 . However, the target holder assembly  100  can be modified by replacing solid target components  101 ,  102 , and  110  with a modified first target body that is configured for a liquid, gas, or foam target. 
     In an example embodiment, as shown in  FIGS. 2 and 3 , the target holder assembly  100  allows for the second target body  120  and third target body  130  to be attached to the cyclotron housing independently from the first target body  110  (which includes the target material, outlined in the figure, and is to be bombarded by the proton beam). This allows for the pressure between the first target body  110  and the second target body  120  to be adjusted independently from the pressure supplied between the second target body  120  and third target body  130  and independently from the pressure supplied between the third target body  130  and the cyclotron housing. For example, the pressure between the first target body  110  and the second target body  120  may be provided by a pneumatic system (not shown). The pressure applied by the pneumatic system is independent from the means of providing pressure between the second target body  120  and the third target body  130 , as well as the pressure between the third target body  130  and the cyclotron housing (not shown). The vacuum foil  131  between the second target body  120  and the third target body  130  allows for negative pressure to be created inside the chamber of the third target body  130  by a vacuum generated by the cyclotron. During normal operation the cyclotron provides enough of a vacuum to generate a deep space vacuum (10 −6  to &lt;3×10 −17  Torr). The negative pressure provided by this vacuum is sufficient to maintain attachment of the third target body  130  to the cyclotron housing. The third target body  130  may alternatively or additionally be attached to the cyclotron housing by mechanical means known in the art. The second target body  120  may be attached to the third target body  130  by mechanical means known in the art, for example using holts  123 - 126 . In this example configuration, the second target body  120  and third target body  130  remain attached to the cyclotron housing at least due to the vacuum being generated by the cyclotron. In this manner, the pressure applied upon the removable degrader  200  by the first target body  110  can be adjusted to allow for removal and/or replacement of the removable degrader  200  without removal of the target holder assembly  100 . Therefore, by reducing only the pressure applied on the removable degrader  200  by the first target body  110 , the removable degrader  200  may be removed and/or replaced with another removable degrader that includes an attenuation disc having a different thickness (or material), without needing to disassemble and/or remove any of the target bodies. 
       FIG. 4  shows the position of the attenuation disc  203 , which is part of the removable degrader  200 , when introduced into the beam path  300 . The surface of the attenuation disc  203  is perpendicular to the beam path  300  in order to reduce the MeV of the cyclotron. The first target body  110  can be tightened to press lip  111 , which is inserted into bore  122  of target body  120 , against metal O-ring  204   a,  which provides pressure to the removable degrader  200  and the oilier metal O-ring  204   b.  This results in an air tight seal around the attenuation disc  203 , so that the cyclotron can then be activated to provide a positive pressure flow of cooled helium gas to both sides of the attenuation disc  203  without gas leakage. 
     In  FIG. 5 , a cross section view of the removable degrader  200  is shown. The removable degrader  200  includes a frame  201  that surrounds an attenuation disc  203  and two O-ring channels  205   a,    205   b  that hold and support metal O-rings  204   a,    204   b  between concentric lips of the O-ring channels  205   a,    205   b  provided by the frame  201  and inner ring  202 . The frame  201  and the attenuation disc  203  can be made from distinct components or fabricated as a single integral piece. Preferably, the entire removable degrader is made from a single piece of aluminum. Preferably, a thickness of the attenuation disc  203  is uniform and depends on the desired level of beam energy reduction. In an example embodiment, the attenuation disc  203  is cooled by a flow of cooled helium gas on both surfaces of the attenuation disc  203  to minimize production of byproducts and other impurities. 
     In  FIG. 6 , the configuration of the metal O-rings  204   a,    204   b  and the O-ring channel(s)  205   a  (and  205   b ) are shown. The removable degrader  200  and metal O-rings  204   a,    204   b  are structured for insertion in the second target body  120  so that the attenuation disc  203  is between the vacuum foil  131  and the target material  102  in the beam path (as shown in  FIG. 4 ). A vacuum is then created in a beam channel along the beam path  300  upstream of the vacuum foil  131  with respect to the beam flow direction  300 . Using the removable degrader  200 , the energy of the provided proton beam downstream of the attenuation disc  203  is adjusted, prior to reaching the target material, from the default setting. 
     In an example embodiment, the thickness of the attenuation disc  203  in the removable degrader is about 0.6 to 0.9 mm and is used to reduce the energy to about 11-12 MeV from the factory default energy level of 16.5 MeV. 
       FIGS. 7 to 9  show the second target body  120  without the removable degrader  200 . A receiving window  121  is structured for an accurate fit and alignment of the frame  201  of removable degrader  200  in second target body  120 . Channels  133 - 136  are provided to allow for connection of the second target body  120  to the third target body  130  for example using bolts  123 - 126 , or other affixation components. Raised ridge  128  is provided to contact metal O-ring  204   b  to provide an air tight seal once sufficient pressure is applied. 
     The present invention is not limited to only the PETtrace 880 cyclotron, but can be equally implemented using other cyclotron models to provide reduced beam energies. Fixed-energy accelerators can be modified by introducing degraders into the pathway of the proton beam. The degraders, in example embodiments of the present invention, work according to the principle according to which any particle passing through a block of material undergoes a decrease in its energy by an amount which is, for particles of a given type, a function of the thickness and/or intrinsic characteristics of the material through which the particle passes. Therefore, the energy of the beam can also be adjusted to other energy levels by, for example, changing the thickness of the energy attenuation disc because the greater the thickness of the energy attenuation disc, the greater the reduction in energy. In an embodiment, the energy level of the beam is reduced using the removable degrader by about 0.5 MeV to about 8 MeV. In another embodiment, the energy level of the beam is reduced using the removable degrader by about 3 MeV to 7 MeV. In an embodiment, the energy level of the beam is reduced using the removable degrader by about 5 MeV. In one embodiment, a beam current of at least 30-60 μA is used. 
     EXAMPLE 
     A PETtrace 880 cyclotron was modified using a target holder assembly and removable degrader according to an example embodiment of the present invention. The energy attenuation disc was placed in the proton beam path, downstream of a vacuum foil of a solid target, and upstream of the target material. The removable degrader had a 0.6 mm thick aluminum attenuation disc in order, and reduced the MeV of the GE PETtrace 880 from 16.5 MeV to about 11-12 MeV at the target. 
     The above description is intended to be illustrative, and not restrictive. Those skilled in the art can appreciate from the foregoing description that the present invention may be implemented in a variety of forms, and that the various embodiments may be implemented alone or in combination. Therefore, while the embodiments of the present invention have been described in connection with particular examples thereof, the true scope of the embodiments and/or methods of the present invention should not be so limited since other modifications will become apparent to the skilled practitioner upon a study of the drawings, specification, and the following.