Patent Publication Number: US-9894747-B2

Title: Radio-frequency electrode and cyclotron configured to reduce radiation exposure

Description:
BACKGROUND 
     The subject matter herein relates generally to isotope production systems and, more specifically, to components that have neutral particles incident thereon within an acceleration chamber of the isotope production system. 
     Radioisotopes (also called radionuclides) have several applications in medical therapy, imaging, and research, as well as other applications that are not medically related. Systems that produce radioisotopes typically include a particle accelerator, such as a cyclotron, that accelerates a beam of charged particles (e.g., H-ions) and directs the beam into a target material to generate the isotopes. The cyclotron includes an ion source that provides negative ions into an acceleration chamber of the cyclotron. The cyclotron uses electrical and magnetic fields to accelerate and guide the negative ions along a predetermined orbit within the acceleration chamber. The magnetic fields are provided by electromagnets and a magnet yoke that surrounds the acceleration chamber. The electrical fields are generated by a pair of radio frequency (RF) electrodes (or dees) that are located within the acceleration chamber. The RF electrodes are electrically coupled to an RF power generator that energizes the RF electrodes to provide the electrical field. The electrical and magnetic fields cause the negative ions to take a spiral-like orbit that has an increasing radius. When the negative ions reach an outer portion of the orbit, the negative ions are stripped of their electrons and form a particle beam that is directed toward the target material for isotope production. 
     As the negative ions are guided along the orbit, however, a portion of the negative ions may collide with other particles, such as residual gas molecules from the ion source. A negative ion may become a neutral particle upon colliding with the other particle. The neutral particle has a trajectory that is essentially tangent to the point in the orbit at which the negative ion collided with the other particle. The neutral particle then collides with other surfaces in the acceleration chamber, such as the RF electrodes. RF electrodes often comprise copper (or other conductive material). When a proton or a neutral hydrogen collides with copper, a relatively large amount of gamma and neutron radiation is generated and long-lived isotopes (e.g., Zn-65) may be generated. This is often the primary source of radiation within an acceleration chamber. Due to the geometry of the cyclotron in the acceleration chamber, the RF electrodes are particularly exposed to the neutral particles. 
     The accumulation of induced by-products from unwanted irradiation is a hazard to individuals. When service personnel open the acceleration chamber, the personnel are exposed to the activated parts. Moreover, the health risk created by the prompt radiation is often addressed by increasing the amount of shielding that surrounds the acceleration chamber. This can increase the cost of the cyclotron and require a larger space. 
     BRIEF DESCRIPTION 
     In an embodiment, a radio-frequency (RF) electrode for a cyclotron is provided. The RF electrode includes a hollowed dee having first and second surfaces that oppose each other and define a gap therebetween. The hollowed dee is configured to be electrically controlled to direct a beam of charged particles through the gap and along an orbit plane of the cyclotron. The orbit plane extends parallel to the first and second surfaces through the gap. The RF electrode also includes a bridge structure that is coupled to and extends away from the hollowed dee. The bridge structure includes a side wall that defines an interior cavity of the bridge structure. The side wall has a particle opening therethrough that coincides with or is proximate to the orbit plane such that the particle opening receives neutral particles from an orbit of the charged particles. 
     In an embodiment, a cyclotron is provided that includes an electrical field system and a magnetic field system configured to direct charged particles along an orbit plane within an acceleration chamber. The magnetic field system includes a pair of pole tops that oppose each other across a central region of the acceleration chamber. The orbit plane extends between and generally parallel to the pole tops. The electrical field system includes a plurality of RF electrodes having hollowed dees that are positioned between the pole tops. At least one of the RF electrodes includes a bridge structure that is coupled to and extends away from the corresponding hollowed dee. The bridge structure includes a side wall that defines an interior cavity of the bridge structure. The side wall has a particle opening therethrough that coincides with or is proximate to the orbit plane. The particle opening is located to receive neutral particles generated within the acceleration chamber that project through the particle opening along the orbit plane. The cyclotron also includes an interception panel that is positioned to receive the neutral particles that project through the particle opening along the orbit plane. 
     In an embodiment, a method is provided that includes providing an RF electrode that has a hollowed dee having first and second surfaces that oppose each other and define a gap therebetween. The RF electrode also includes a bridge structure that is coupled to and extends away from the hollowed dee. The bridge structure includes a side wall that defines an interior cavity of the bridge structure. The side wall has a particle opening therethrough. The method also includes positioning the RF electrode within an acceleration chamber of a cyclotron. The cyclotron is configured to direct charged particles along an orbit plane within the acceleration chamber. The RF electrode is positioned such that the orbit plane extends between the first and second surfaces and extends through or proximate to the particle opening of the side wall. The particle opening is configured to receive neutral particles that project along the orbit plane during operation of the cyclotron. The method also includes positioning an interception panel within the acceleration chamber to receive the neutral particles that project through the particle opening along the orbit plane. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram of a cyclotron in accordance with one embodiment. 
         FIG. 2  is a schematic side view of a cyclotron in accordance with one embodiment. 
         FIG. 3  is a perspective view of a portion of a yoke and pole section that may be used with a cyclotron in accordance with one embodiment. 
         FIG. 4  is a plan view of a yoke section that may be used with a cyclotron formed in accordance with an embodiment. 
         FIG. 5  is an enlarged plan view of the yoke section of  FIG. 4  illustrating an orbit of charged particles in accordance with an embodiment. 
         FIG. 6  is a side perspective view of an RF electrode formed in accordance with an embodiment. 
         FIG. 7  is a sectional view of the RF electrode illustrating an interior cavity and interception panel in greater detail. 
         FIG. 8  is a flow chart of a method in accordance with an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The following detailed description of certain embodiments will be better understood when read in conjunction with the appended drawings. To the extent that the figures illustrate diagrams of the functional blocks of various embodiments, the functional blocks are not necessarily indicative of the division between hardware circuitry. For example, one or more of the functional blocks (e.g., processors, memories) may be implemented in a single piece of hardware (e.g., a general purpose signal processor or a block of random access memory, hard disk, or the like) or multiple pieces of hardware. Similarly, the programs may be stand alone programs, may be incorporated as subroutines in an operating system, may be functions in an installed software package, and the like. It should be understood that the various embodiments are not limited to the arrangements and instrumentality shown in the drawings. 
     As used herein, an element or step recited in the singular and proceeded with the word “a” or “an” should be understood as not excluding plural of said elements or steps, unless such exclusion is explicitly stated. Furthermore, references to “one embodiment” are not intended to be interpreted as excluding the existence of additional embodiments that also incorporate the recited features. Moreover, unless explicitly stated to the contrary, embodiments “comprising” or “having” an element or a plurality of elements having a particular property may include additional such elements not having that property. 
     A technical effect of one or more embodiments may be that individuals working with or near a cyclotron will be less exposed to radiation and/or harmful by-products. For example, an individual may be able to work a greater amount of time in or around the cyclotron before reaching his or her maximum yearly dose limit. Service personnel may be able to install more systems and/or repair more systems before reaching his or her maximum yearly dose limit. Another technical effect of one or more embodiments may include a smaller and/or more efficient radiation shield. As such, the cyclotrons may be constructed smaller than other known cyclotrons. 
       FIG. 1  is a block diagram of an isotope production system  100  formed in accordance with one embodiment. The system  100  includes a cyclotron  102  that has several sub-systems including an ion source system  104 , an electrical field system  106 , a magnetic field system  108 , and a vacuum system  110 . The cyclotron  102  may be, for example, an isochronous cyclotron. The cyclotron  102  may include an acceleration chamber  103 . The acceleration chamber  103  may be defined by a housing or other portions of the cyclotron and have an evacuated state or condition during operation. The cyclotron  102  shown in  FIG. 1  has at least portions of the sub-systems  104 ,  106 ,  108 , and  110  located in the acceleration chamber  103 . 
     During operation of the cyclotron  102 , charged particles are placed within or injected into the acceleration chamber  103  through the ion source system  104 . For example, the ion source system  104  may include a grounded ion source tube (not shown) positioned between two cathodes (not shown) that are negatively biased. Hydrogen (H 2 ) gas may be flowed through the ion source tube. A voltage difference between the cathodes and the ion source tube may cause a plasma discharge in the hydrogen gas, thereby generating positive hydrogen ions (protons) and negative hydrogen ions (H−). The negative hydrogen ions may then be extracted from the ion source tube and provided into the acceleration chamber  103 . Although the foregoing describes one example of an ion source system, it should be understood that other methods and configurations may be used to provide charged particles to the acceleration chamber  103 . 
     The magnetic field system  108  and the electrical field system  106  are configured to generate respective fields that cooperate in producing a particle beam  112  of the charged particles. The charged particles are accelerated and guided within the acceleration chamber  103  along a predetermined or desired path. The predetermined path may be referred to herein as an orbit and occurs generally along an orbit plane. As described herein, the charged particles may collide with other particles in the acceleration chamber, such as residual gas molecules. These collisions may create neutral particles that are not controlled by the electrical field system  106 . The neutral particles exit the orbit and typically collide with other material within the acceleration chamber. 
     During operation of the cyclotron  102 , the acceleration chamber  103  may be in a vacuum (or evacuated) state and experience a large magnetic flux. For example, an average magnetic field strength between pole tops in the acceleration chamber  103  may be at least  1  Tesla. After the particle beam  112  is generated, the pressure of the acceleration chamber  103  may be approximately 2×10 −5  millibar. However, the above values are only examples and various embodiments may operate within different parameters. 
     Also shown in  FIG. 1 , the system  100  has an extraction system  115  and a target system  114  that includes a target material  116 . In the illustrated embodiment, the target system  114  is positioned adjacent to the cyclotron  102 . To generate isotopes, the particle beam  112  is directed by the cyclotron  102  through the extraction system  115  along a beam transport path or beam passage  117  and into the target system  114  so that the particle beam  112  is incident upon the target material  116  located at a corresponding target location  120 . When the target material  116  is irradiated with the particle beam  112 , radiation from neutrons and gamma rays may be generated. In alternative embodiments, the system  100  may have a target system located within or directly attached to the accelerator chamber  103 . 
     The system  100  may have multiple target locations  120 A-C where separate target materials  116 A-C are located. A shifting device or system (not shown) may be used to shift the target locations  120 A-C with respect to the particle beam  112  so that the particle beam  112  is incident upon a different target material  116 . A vacuum may be maintained during the shifting process as well. Alternatively, the cyclotron  102  and the extraction system  115  may not direct the particle beam  112  along only one path, but may direct the particle beam  112  along a unique path for each different target location  120 A-C. Furthermore, the beam passage  117  may be substantially linear from the cyclotron  102  to the target location  120  or, alternatively, the beam passage  117  may curve or turn at one or more points therealong. For example, magnets positioned alongside the beam passage  117  may be configured to redirect the particle beam  112  along a different path. 
     The system  100  is configured to produce radioisotopes (also called radionuclides) that may be used in medical imaging, research, and therapy, but also for other applications that are not medically related, such as scientific research or analysis. When used for medical purposes, such as in Nuclear Medicine (NM) imaging or Positron Emission Tomography (PET) imaging, the radioisotopes may also be called tracers. By way of example, the system  100  may generate protons to make  18 F −  isotopes in liquid form,  11 C isotopes as CO 2 , and  13 N isotopes as NH 3 . The target material  116  used to make these isotopes may be enriched  18 O water, natural  14 N 2  gas,  16 O-water. The system  100  may also generate protons or deuterons in order to produce  15 O gases (oxygen, carbon dioxide, and carbon monoxide) and  15 O labeled water. 
     In particular embodiments, the system  100  uses  1 H −  technology that brings the charged particles to a designated energy and creates a designated beam current. For example, the system  100  may bring the charged particles to a low energy (e.g., about 9.6 MeV) with a beam current of approximately 10-30 μA. The negative hydrogen ions may be accelerated and guided through the cyclotron  102  and into the extraction system  115 . The negative hydrogen ions may then hit a stripping foil (not shown) of the extraction system  115  thereby removing the pair of electrons and making the particle a positive ion,  1 H + . It is contemplated that other embodiments may use deuterium or helium. 
     The system  100  may include a cooling system  122  that transports a cooling or working fluid to various components of the different systems in order to absorb heat generated by the respective components. The system  100  may also include a control system  118  that may be used by a technician to control the operation of the various systems and components. The control system  118  may include one or more user-interfaces that are located proximate to or remotely from the cyclotron  102  and the target system  114 . Although not shown in  FIG. 1 , the system  100  may also include one or more radiation and/or magnetic shields for the cyclotron  102  and the target system  114 . 
     The system  100  may also be configured to accelerate the charged particles to a predetermined energy level. For example, some embodiments described herein accelerate the charged particles to an energy of approximately 18 MeV or less. In other embodiments, the system  100  accelerates the charged particles to an energy of approximately 16.5 MeV or less. In particular embodiments, the system  100  accelerates the charged particles to an energy of approximately 9.6 MeV or less. In more particular embodiments, the system  100  accelerates the charged particles to an energy of approximately 7.8 MeV or less. However, embodiments describe herein may also have an energy above 18 MeV. For example, embodiments may have an energy above 100 MeV, 500 MeV or more. 
       FIG. 2  is a side view of a cyclotron  200  formed in accordance with one embodiment. Although the following description is with respect to the cyclotron  200 , it is understood that embodiments may include other cyclotrons and methods involving the same. As shown in  FIG. 2 , the cyclotron  200  includes a magnet yoke  202  having a yoke body  204  that surrounds an acceleration chamber  206 . In alternative embodiments, the acceleration chamber may be surrounded or defined by components other than a magnet yoke, such as a housing or shield. The yoke body  204  has opposite side faces  208  and  210  with a thickness T 1  extending therebetween and also has top and bottom ends  212  and  214  with a length L extending therebetween. In the exemplary embodiment, the yoke body  204  has a substantially circular cross-section and, as such, the length L may represent a diameter of the yoke body  204 . The yoke body  204  may be manufactured from iron and be sized and shaped to produce a desired magnetic field when the cyclotron  200  is in operation. The yoke body  204  may also be sized and shaped to shield prompt radiation generated within the yoke body  204 . 
     The yoke body  204  may have opposing yoke sections  228  and  230  that define the acceleration chamber  206  therebetween. The yoke sections  228  and  230  are configured to be positioned adjacent to one another along a mid-plane  232  of the magnet yoke  202 . The charged particles are configured to move along the mid-plane  232  in a predetermined orbit. Accordingly, the mid-plane  232  is hereinafter referred to as an orbit plane  232 . 
     As shown, the cyclotron  200  may be oriented vertically (with respect to gravity) such that the orbit plane  232  extends perpendicular to a horizontal platform  220  supporting the weight of the cyclotron  200 . The cyclotron  200  has a central axis  236  that extends horizontally between and through the yoke sections  228  and  230  (and corresponding side faces  210  and  208 , respectively). The central axis  236  extends perpendicular to the orbit plane  232  through a center of the yoke body  204 . The acceleration chamber  206  has a central region  238  located at an intersection of the orbit plane  232  and the central axis  236 . In some embodiments, the central region  238  is at a geometric center of the acceleration chamber  206 . 
     The yoke sections  228  and  230  include poles  248  and  250 , respectively, that oppose each other across the orbit plane  232  within the acceleration chamber  206 . The poles  248  and  250  may be separated from each other by a pole gap G. The pole  248  includes a pole top  252  and the pole  250  includes a pole top  254  that opposes the pole top  252 . The poles  248  and  250  and the pole gap G therebetween are sized and shaped to produce a desired magnetic field when the cyclotron  200  is in operation. In some embodiments, the poles  248  and  250  include hills and valleys such that the pole gap G varies. 
     The cyclotron  200  also includes a magnet assembly  260  located within or proximate to the acceleration chamber  206 . The magnet assembly  260  is configured to facilitate producing the magnetic field with the poles  248  and  250  to direct charged particles along a desired beam path. The magnet assembly  260  includes an opposing pair of magnet coils  264  and  266  that are spaced apart from each other across the orbit plane  232  at a distance D 1 . The magnet coils may be substantially circular and extend about the central axis  236 . The yoke sections  228  and  230  may form magnet coil cavities  268  and  270 , respectively, that are sized and shaped to receive the corresponding magnet coils  264  and  266 , respectively. Also shown in  FIG. 2 , the cyclotron  200  may include chamber walls  272  and  274  that separate the magnet coils  264  and  266  from the acceleration chamber  206  and facilitate holding the magnet coils  264  and  266  in position. 
     The acceleration chamber  206  is configured to allow charged particles, such as  1 H −  ions, to be accelerated therein along a predetermined orbit. The orbit wraps in a spiral manner about the central axis  236  and remains generally along the orbit plane  232 . The charged particles are initially positioned proximate to the central region  238 . When the cyclotron  200  is activated, the orbit of the charged particles spirals around the central axis  236 . In the illustrated embodiment, the cyclotron  200  is an isochronous cyclotron and, as such, the orbit of the charged particles has portions that curve about the central axis  236  and portions that are more linear. However, embodiments described herein are not limited to isochronous cyclotrons, but also includes other types of cyclotrons and particle accelerators. As shown in  FIG. 2 , when the charged particles orbit around the central axis  236 , the charged particles may project out of the page of the acceleration chamber  206  and extend into the page of the acceleration chamber  206 . As the charged particles orbit around the central axis  236 , a radius R that extends between the orbit of the charged particles and the central region  238  increases. When the charged particles reach a predetermined location along the orbit, the charged particles are directed into or through an extraction system (not shown) and out of the cyclotron  200 . For example, the charged particles may be stripped of their electrons by a foil. 
     The acceleration chamber  206  may be in an evacuated state before and during the forming of the particle beam  112 . For example, before the particle beam is created, a pressure of the acceleration chamber  206  may be approximately 1×10 −7  millibars. When the particle beam is activated and H 2  gas is flowing through an ion source (not shown) located at the central region  238 , the pressure of the acceleration chamber  206  may be approximately 2×10 −5  millibar. As such, the cyclotron  200  may include a vacuum pump  276  that may be proximate to the orbit plane  232 . The vacuum pump  276  may include a portion that projects radially outward from the end  214  of the yoke body  204 . 
     In some embodiments, the yoke sections  228  and  230  may be moveable toward and away from each other so that the acceleration chamber  206  may be accessed (e.g., for repair or maintenance). For example, the yoke sections  228  and  230  may be joined by a hinge (not shown) that extends alongside the yoke sections  228  and  230 . Either or both of the yoke sections  228  and  230  may be opened by pivoting the corresponding yoke section(s) about an axis of the hinge. As another example, the yoke sections  228  and  230  may be separated from each other by laterally moving one of the yoke sections linearly away from the other. However, in alternative embodiments, the yoke sections  228  and  230  may be integrally formed or remain sealed together when the acceleration chamber  206  is accessed (e.g., through a hole or opening of the magnet yoke  202  that leads into the acceleration chamber  206 ). In alternative embodiments, the yoke body  204  may have sections that are not evenly divided and/or may include more than two sections. 
     The acceleration chamber  206  may include passages that lead radially outward away from the outer spatial region  243 , such as a passage that extends through the yoke body  204  to a target system or a passage for cables. The acceleration chamber  206  may also have a shape that extends along and is substantially symmetrical about the orbit plane  232 . For instance, the acceleration chamber  206  may be substantially disc-shaped and include an inner spatial region  241  defined between the pole tops  252  and  254  and an outer spatial region  243  defined between the chamber walls  272  and  274 . As used herein, an element may “define” a space without the element entirely defining or enclosing the space. For example, the pole top  252  only defines one boundary of the inner spatial region  241 , and the pole top  254  defines an opposite boundary of the inner spatial region  241 . The inner spatial region  241  has an undefined boundary and opens to the outer spatial region  243 . The inner spatial region  241  represents the space at which the pole tops  252 ,  254  directly face each other. The inner spatial region  241  includes the orbit of charged particles. 
     The poles  248  and  250  (or, more specifically, the pole tops  252  and  254 ) are separated by the inner spatial region  241  where the charged particles are directed along the designated orbit. The magnet coils  264  and  266  may be separated by the outer spatial region  243 . In particular, the chamber walls  272  and  274  may have the spatial region  243  therebetween. Furthermore, a periphery of the spatial region  243  may be defined by a wall surface  255  that also defines a periphery of the acceleration chamber  206 . The wall surface  255  may extend circumferentially about the central axis  236 . As shown, the inner spatial region  241  extends a distance equal to a pole gap G along the central axis  236 , and the outer spatial region  243  extends the distance D 1  along the central axis  236 . 
     As shown in  FIG. 2 , the outer spatial region  243  surrounds the inner spatial region  241  about the central axis  236 . The inner and outer spatial regions  241  and  243  may collectively form the acceleration chamber  206 . Accordingly, in the illustrated embodiment, the cyclotron  200  does not include a separate tank or wall that only surrounds the spatial region  241  thereby defining the spatial region  241  as the acceleration chamber of the cyclotron. The vacuum pump  276  may be fluidly coupled to the spatial region  241  through the spatial region  243 . Gas entering the spatial region  241  may be evacuated from the spatial region  241  through the spatial region  243 . In the illustrated embodiment, the vacuum pump  276  is fluidly coupled to and located adjacent to the spatial region  243 . In addition, an RF electrode (not shown), such as RF electrodes  416 ,  418  (shown in  FIG. 4 ), may have bridge structures that extend through the outer spatial region  243  to locate hollowed dees within the inner spatial region  241 . The RF electrodes may be directly or indirectly coupled to the magnet yoke  202 . In particular embodiments, the RF electrodes extend radially inward from the wall surface  255 . 
       FIG. 3  is a partial perspective view of a yoke section  330  formed in accordance with one embodiment. The yoke section  330  may oppose another yoke section (not shown). When the opposing yoke section and the yoke section  330  are sealed together, an acceleration chamber may be formed therebetween. When sealed, the two yoke sections may constitute the magnet yoke of a cyclotron, such as the magnet yoke  202  of the cyclotron  200  described above. The yoke section  330  may have similar components and features as described with respect to the yoke sections  228  and  230  ( FIG. 2 ). 
     As shown, the yoke section  330  includes a ring portion  321  that defines an open-sided cavity  320  having a magnet pole  350  located therein. The open-sided cavity  320  may include portions of inner and outer spatial regions (not shown) of the acceleration chamber, such as the inner and outer spatial regions  241  and  243  discussed above. The ring portion  321  may include a mating surface  324  that is configured to engage a mating surface of the opposing yoke section during operation of the cyclotron. The yoke section  330  includes a yoke or beam passage  349 . As indicated by dashed lines, the beam passage  349  extends through the ring portion  321  and provides a path for a particle beam of stripped particles to exit the acceleration chamber. 
     In some embodiments, a pole top  354  of the pole  350  may include hills  331 - 334  and valleys  336 - 339 . The hills  331 - 334  and valleys  336 - 339  may facilitate directing the charged particles by varying the magnetic field experienced by the charged particles. The yoke section  330  may also include radio frequency (RF) electrodes  340  and  342  that extend radially inward toward each other and toward a center  344  of the pole  350  (or acceleration chamber). The RF electrodes  340  and  342  may include hollowed dees  341  and  343 , respectively, that extend from bridge structures  345  and  347 , respectively. The hollowed dees  341  and  343  are located within the valleys  336  and  338 , respectively. The bridge structures  345  and  347  may be coupled to an interior wall surface  322  of the ring portion  321 . The bridge structures  345 ,  347  extend through an outer spatial region of the acceleration chamber when the yoke section  330  and the opposing yoke section are closed. 
     Also shown, the yoke section  330  may include interception panels  371  and  372  arranged about the pole  350 . The interception panels  371  and  372  are positioned to intercept lost particles within the acceleration chamber. The interception panels  371  and  372  may comprise aluminum or other material, such as graphite or tungsten. Although only two interception panels  371  and  372  are shown in  FIG. 3 , embodiments described herein may include additional interception panels. For example, interception panels may be disposed relative to or within the bridge structures  345 ,  347  as described below. 
     The RF electrodes  340  and  342  may form at least a portion of an RF electrode system  370 , such as the electrical field system  106  described with reference to  FIG. 1 , in which the RF electrodes  340  and  342  accelerate the charged particles within the acceleration chamber. The RF electrodes  340  and  342  cooperate with each other and form a resonant system that includes inductive and capacitive elements tuned to a predetermined frequency of, for example, at least 30 MHz. In one particular embodiment, the resonant system operates at 100 MHz. The RF electrode system  370  may have a high frequency power generator (not shown) that may include a frequency oscillator in communication with one or more amplifiers. The RF electrode system  370  creates an alternating electrical potential between the RF electrodes  340 ,  342  and grounded structures (e.g., pole tops) thereby accelerating the charged particles. The magnetic fields generated by the yoke sections and electromagnetic coils may facilitate in guiding the charged particles. 
     Also shown in  FIG. 3 , a plurality of movable mechanical devices may be disposed within the acceleration chamber. For example, a stripping assembly  380  may be mounted to the pole  350  and a diagnostic probe assembly  382  may also be mounted to the pole  350 . In addition to the stripping and probe assemblies  380  and  382 , embodiments described may include other movable mechanical devices within the acceleration chamber. The movable mechanical devices may be configured to move during operation of the cyclotron and/or when the magnet yoke is sealed. More specifically, the mechanical devices may be configured to repeatedly operate (e.g., move back and forth between different positions) while within a vacuum state and while sustaining a large magnetic flux. 
       FIG. 4  is a plan view of a sub-assembly  400  in accordance with an embodiment. The sub-assembly  400  is configured to mate with an opposing yoke section (not shown) to form a magnet yoke, such as the magnet yoke  202  ( FIG. 2 ), and/or a cyclotron, such as the cyclotron  200  ( FIG. 2 ). The sub-assembly  400  may be similar or identical to the yoke section  330  ( FIG. 3 ). As shown, the sub-assembly  400  includes a yoke section  402  having an inner wall  404 . The yoke section  402  may comprise, for example, iron. The inner wall  404  has a radially-inward surface  406 . When the yoke section  402  is mated with another yoke section, an acceleration chamber is formed therebetween. The acceleration chamber is indicated as  408  in  FIG. 4 , but it should be understood that only a portion of the acceleration chamber is shown in  FIG. 4 . The acceleration chamber  408  includes an inner spatial region  409 A and an outer spatial region  409 B that surrounds the inner spatial region  409 A about a central axis  414  of the cyclotron. 
     The sub-assembly  400  also includes a pole  410  having a pole top  412 . The central axis  414  extends through a center of the pole top  412 . The central axis  414  may extend through a central region  415  around which a particle beam is directed. An ion source (not shown) may be configured to provide the charged particles within or proximate to the central region  415 . The sub-assembly  400  also includes a pair of RF electrodes  416 ,  418 . Each of the RF electrodes  416 ,  418  includes a hollowed dee  420  and a bridge structure  422 . The bridge structures  422  are mounted to the inner wall  404  of the yokes section  402  and extend radially inward toward the central axis  414  from the radially-inward surface  406 . The bridge structures  422  extend through the outer spatial region  409 B. 
     The acceleration chamber  408  is defined by a chamber surface  411 . The chamber surface  411  may collectively include multiple surfaces of the yoke sections that define the acceleration chamber  408 . For example, the chamber surface  411  may include the radially-inward surface  406 , an axial surface  413 A along the yoke section, and an axial surface  415 A along the pole  410 . The axial surfaces  413 A,  415 A face in a direction that is generally along the central axis  414 . It should be understood that other surfaces that define the acceleration chamber  408  may be portions or areas of the chamber surface  411 . 
     In  FIG. 4 , the hollowed dees  420  are positioned adjacent to the pole top  412 . More specifically, the hollowed dees  420  of the RF electrodes  416 ,  418  are positioned within valleys  426 ,  428 , respectively, of the pole top  412 . When the cyclotron is fully assembled, the hollowed dees  420  are positioned between the pole top  412  and the pole top of the opposing yoke section. Also shown, the pole top  412  includes valleys  430 ,  432 . Optionally, the valleys  430 ,  432  may provide room for additional sub-systems. 
       FIG. 5  is an enlarged plan view of the sub-assembly  400 . During operation of the cyclotron, an orbit  440  of the particle beam is created. As the charged particles are introduced into the acceleration chamber  408  near the central region  415 , the RF electrodes  416 ,  418  are electrically controlled to direct the charged particles along the designated orbit  440 . The designated orbit  440  is generally parallel to an orbit plane (or mid-plane)  444  (shown in  FIG. 6 ) that divides the inner spatial region  409 A between the pole tops. As used herein, the phrase “generally parallel to the orbit plane” includes being positioned at least partially above or below the orbit plane or coinciding with the orbit plane. It should be understood that the orbit  440  shown in  FIG. 4  is illustrative only and may have different dimensions or qualities in practice. For example, the orbit  440  may include a different number of wraps or turns than those shown in  FIG. 4 . 
     The orbit  440  wraps about the central axis  414  and includes first curved portions  434  and second curved portions  436 . The curvature of the orbit  440  is a function of the strength of the magnetic field between the pole tops. The first curved portions  434  of the orbit  440  correspond to regions that have a stronger magnetic field, and the second curved portions  436  of the orbit  440  correspond to regions that have a weaker magnetic field (i.e., compared to the stronger magnetic fields). In the illustrated embodiment, the weaker magnetic fields occur because there is a greater gap between the pole tops within the valleys  426 ,  428 ,  430 ,  432 . As such, the stronger magnetic fields cause the first curved portions  434  of the orbit  440  to have sharper or tighter curvatures than the second curved portions  436 , which may be more linear. As the charged particles are directed along the orbit  440 , the charged particles may collide with other particles (e.g., gas molecules) that transform the charged particles into neutral particles. At this moment, the electrical fields generated by the RF electrodes  416 ,  418  may no longer control the particles. As such, the trajectory of the neutral particles may be tangent to the point at which the corresponding charged particle and other molecule collided. During operation of the cyclotron, the neutral particles are essentially sprayed radially outward along a periphery of the acceleration chambers. This is indicated by the rays  442  shown in  FIG. 5 . It is noted that only a three rays  442  are shown in  FIG. 5  to demonstrate examples of trajectories that the neutral particles may take. It should be understood that the neutral particles are sprayed throughout the acceleration chamber in a direction that is generally parallel to the orbit plane. 
     The neutral particles may collide with surfaces within the acceleration chamber. For example, the neutral particles may propagate into the outer spatial region and collide with the bridge structures of the RF electrodes. When the neutral particles collide with the interior surfaces, secondary gamma radiation is generated and radioisotopes may be created. For example, if the neutral particles collide with copper, the long-lived isotope of Zinc-65 may be generated in addition to the prompt radiation. Prompt radiation may be characterized as radiation that results directly/instantaneously from a nuclear reaction. For example, the prompt radiation may be gamma radiation that results from a proton hitting certain material (e.g., copper). To ameliorate these undesirable events, interception panels may be positioned within the acceleration chamber. 
       FIG. 6  is a side perspective view of the RF electrode  416 . The RF electrode  418  ( FIG. 4 ) may be similar or identical to the RF electrode  416 . The RF electrode  416  includes the hollowed dee  420  and the bridge structure  422 . The bridge structure  422  is configured to couple to the chamber surface  411  ( FIG. 4 ) and hold the hollowed dee  420  at a designated position in the inner spatial region  409 A ( FIG. 4 ). In some embodiments, the bridge structure  422  couples to an area of the chamber surface  411  that defines the outer spatial region  409 B ( FIG. 4 ), such as the radially-inward surface  406  ( FIG. 4 ) or the axial surface  413 A ( FIG. 4 ). At least a portion of the bridge structure  422  is disposed within or proximate to the orbit plane  444 . The orbit plane  444  is indicated as a dashed line along surfaces of the RF electrode  416  in  FIG. 6 . 
     For example, in the illustrated embodiment, the bridge structure  422  includes an elongated stem  450  and an optional base panel  452 . When positioned in the acceleration chamber  408  ( FIG. 4 ), the elongated stem  450  is configured to extend in a radial direction between the base panel  452  (or the radially-inward surface  406 ) and the hollowed dee  420 . The elongated stem  450  may extend radially away from the central region  415  ( FIG. 4 ) and extend away from the hollowed dee  420 . At least a portion of the elongated stem  450  is disposed within or proximate to the orbit plane  444  such that neutral particles may collide with surfaces of the elongated stem  450  (or would collide with surfaces if the particle opening did not exist). As shown, the orbit plane  444  intersects the elongated stem  450  along an entire length  454  of the elongated stem  450 . A portion of the elongated stem  450  may be located immediately above or immediately below the orbit plane  444  at a position where neutral particles may collide with the elongated stem  450 . 
     The base panel  452  is configured to directly couple to the radially-inward surface  406  ( FIG. 4 ). For example, the base panel  452  has a mating surface  456  and a panel surface  458  that face in opposite directions and a thickness extending therebetween. The base panel  452  may also include thru-holes  460  for receiving fasteners that couple the base panel  452  to another structure. The base panel  452  may be secured directly to the chamber surface  411  or the radially-inward surface  406 . In other embodiments, the base panel  452  does not exist within the acceleration chamber  408  and the elongated stem  450  or other portion of the bridge structure  422  may extend through a passage of the magnet yoke. 
     It is contemplated, however, that the base panel  452  or other portion of the bridge structure  422  may couple to the axial surface  413 A ( FIG. 4 ). In such embodiments, the bridge structure  422  or the elongated stem  450  may have a non-linear or non-planar shape. For example, the bridge structure  422  or the elongated stem  450  may be L-shaped in which one leg is disposed within or proximate to the orbit plane  444  and another leg extends in an axial direction and coupes to the axial surface  413 A. Accordingly, the bridge structure  422  may have shapes and or dimensions that differ from the shapes and dimensions shown in  FIG. 6 . 
     The hollowed dee  420  includes first and second plate sections  464 ,  466  that include first and second inner surfaces  465 ,  467 , respectively. The first and second plate sections  464 ,  466  have respective outer edges  468  that define triangular or pie-shaped profiles. For example, the profiles of the first and second plate sections  464 ,  466  may forms sectors of a circle having arcs that equal between about 30° and about 40° of the circle. The outer edges  468  may each define a point, wherein the points of the first and second plate sections  464 ,  466  may collectively represent a distal end  470  of the RF electrode  416 . In other embodiments, the profiles of the first and second plate sections  464 ,  466  may be semi-circular (e.g., half circles). 
     As shown, the first and second plate sections  464 ,  466  oppose each other and define a gap  472  therebetween. The gap  472  is sized and shaped to permit the beam of the charged particles to be directed therethrough along the orbit  440  ( FIG. 5 ). Also shown in  FIG. 6 , the elongated stem  450  includes a plurality of side walls  481 - 484  that define an interior cavity  474  of the elongated stem  450 , or the bridge structure  422  more generally. In the illustrated embodiment, the interior cavity  474  is completely surrounded by the side walls  481 - 484 . In other embodiments, the interior cavity  474  may be open-sided channel. For example, one or more portions of the side wall  481  and/or of the side wall  483  may be removed or the entire side walls  481 ,  483  may be removed. 
       FIG. 7  is a sectional view of the RF electrode  416 . The side walls  482 ,  484  oppose each other with the interior cavity  474  therebetween, and the side walls  481 ,  483  ( FIG. 6 ) oppose each other with the interior cavity  474  therebetween. The interior cavity  474  has a cavity opening  485  that opens to the gap  472 . Only the plate section  466  is shown in  FIG. 7 . In the illustrated embodiment, the orbit plane  444  generally intersects the side walls  482 ,  484  and may extend parallel to and between the side walls  481 ,  483 . The orbit plane  444  may also extend through the opening  485 . 
     As shown, the side wall  484  has a single elongated particle opening  490  therethrough that extends generally parallel to the orbit plane  444 . The particle opening  490  is positioned to coincide with or be proximate to the orbit plane  444 . As shown, the particle opening  490  has a first dimension  492  measured radially with respect to the central axis  414  (or measured parallel to the orbit plane  444 ) and a second dimension  494  that is measured perpendicular to the orbit plane  444  (or parallel to the central axis  414  ( FIG. 4 )). The first dimension  492  may be at least two times (2×), at least three times (3×), at least five times (5×), or at least ten times (10×) the second dimension  494 . The first dimension  492  may be at least 50%, at least 60%, at least 70%, or at least 80% of the length  454  ( FIG. 6 ) of the elongated stem  450 . In some embodiments, the side wall  484  has a cavity edge  487  that defines the cavity opening  485 . The side wall  484  may have a support section  489  that includes the cavity edge  487  and defines an end of the particle opening  490 . The support section  489  may have a relatively small dimension measured parallel to the orbit plane  444 . For example, the support section  489  may be one, two, or three centimeters (cm) in some embodiments. 
     However, it should be noted that although the side wall  484  includes only one elongated particle opening  490  in  FIG. 7 , other embodiments may include a plurality of particle openings having various dimensions. The particle opening  490  is located to receive the neutral particles that project from the orbit  440  of the particle beam. The trajectories of the neutral particles may be along the orbit plane (e.g., generally parallel to the orbit plane). In addition to the particle opening  490 , the cavity opening  485  may receive neutral particles therethrough. 
     Embodiments set forth herein include interception panels that are configured to receive the neutral particles so that the neutral particles collide with the interception panels instead of other surfaces within the acceleration chamber. In some embodiments, the interception panel are positioned within a volume of space that is substantially free of electromagnetic fields, compared to the surrounding volume of space in the acceleration chamber. For example, the interception panels may be disposed within the interior cavity  474  of the bridge structure  422 , which may function similar to a Faraday cage that surrounds the interception panel  500  and substantially excludes electromagnetic fields. Electromagnetic fields may induce a current that generates heat through electromagnetic losses. This heat may negatively affect the performance of the cyclotron. However, it should be understood that the interception panels are not required to be positioned within the interior cavity  474  and may have different positions in other embodiments. 
     In the illustrated embodiment, the RF electrode  416  includes an interception panel  500  that is positioned within the interior cavity  474 . More specifically, the interception panel  500  faces the particle opening  490  with the interior cavity  474  extending between the particle opening  490  (or the side wall  484 ) and the interception panel  500 . The interception panel  500  is positioned so that neutral particles propagating through the particle opening  490  will collide with the interception panel  500 . The interception panel  500  is also positioned so that neutral particles propagating through the cavity opening  485  will collide with the interception panel  500 . In  FIG. 7 , the interception panel  500  is a single continuous structure. In other embodiments, the interception panel  500  may be multiple discrete structures or multiple interception panels  500  may be used. 
     As shown, the interception panel  500  is secured directly to the bridge structure  422 . In particular, the interception panel  500  is secured directly to the side wall  482 . To this end, the side wall  482  and the interception panel  500  may be configured to permit securing the interception panel  500  thereto. For example, the interception panel  500  and the side wall  482  may include thru-holes  502  that are configured to receive hardware (e.g., screws). In some embodiments, the interception panel  500  may be removed and replaced with another interception panel  500 . 
     However, it is contemplated that the interception panel  500  may have other positions within the interior cavity  474 . For example, the interception panel  500  may be secured to an interior side of the side wall  484  such that the neutral particles move through the particle opening  490  and immediately collide with the interception panel  500 . Optionally, a portion of the interception panel  500  may extend into and, optionally, through the particle opening  490 . In another configuration, the interception panel  500  may be spaced apart from each of the side walls  482 ,  484 . For example, the interception panel  500  may be positioned about half-way between the side wall  482  and the side wall  484 . 
     As shown, the interception panel  500  has a planar, rectangular structure that extends parallel to the side walls  482 ,  484 . However, the interception panel  500  may have other shapes and other orientations within the interior cavity  474  in alternative embodiments. The dimension of the interception panel  500  may be configured to increase the likelihood that neutral particles will collide with the interception panel  500  instead of the conductive material of the bridge structure  422 . 
     Yet in other embodiments, the interception panel  500  may be positioned outside of the RF electrode  416 . For example, the side wall  482  may also include a particle opening (not shown). The neutral particles may propagate through the particle opening  490  and the particle opening of the side wall  482  such that the neutral particles travel entirely through the bridge structure  422 . In such embodiments, the interception panel  500  may be outside of the RF electrode  416  and positioned and shaped to intercept the neutral particles. 
     The bridge structure  422  and the hollowed dee  420  comprise a conductive material, such as copper. The interception panel  500  comprises a blocking material that has a different composition than the conductive material. The blocking material and the conductive material may both be capable of generating respective radioisotopes when the neutral particles are incident thereon. For example, Zn-65 isotopes may be generated when neutral hydrogen particles are incident on copper. The blocking material, however, may be a material that generates radioisotopes having shorter half-lives than the radioisotopes that are generated by the conductive material if the charged particles are incident thereon. By way of example, the blocking material of the interception panel  500  may include at least one of graphite (e.g., electro-graphite) or tungsten. In some embodiments, the composition of the interception panel  500  consists essentially of graphite or tungsten or is pure graphite or pure tungsten. 
     As used herein, a “long-lived radioisotope” is a radioisotope that has a half-life that is at least one day. As used herein, a “short-lived radioisotope” is a radioisotope that has a half-life that is at most 10 hours. In more particular embodiments, a long-lived radioisotope may have a half-life that is greater than 18 hours or greater than 10 hours. 
     It should be noted that the particular arrangement of components (e.g., the number, types, placement, or the like) of the illustrated embodiments may be modified in various alternative embodiments. In various embodiments, different numbers of a given element may be employed, a different type or types of a given element may be employed, a given element may be added, or a given element may be omitted. 
       FIG. 8  is a flow chart of a method  550  in accordance with an embodiment. The method  550  may be, for example, a method of manufacturing a cyclotron, assembling a cyclotron, or maintaining a cyclotron. For example, the method  550  may be performed when replacing and/or cleaning elements within an acceleration chamber. The method  550  may employ structures or aspects of various embodiments (e.g., systems and/or methods) discussed herein. In various embodiments, certain steps may be omitted or added, certain steps may be combined, certain steps may be performed simultaneously, certain steps may be performed concurrently, certain steps may be split into multiple steps, certain steps may be performed in a different order, or certain steps or series of steps may be re-performed in an iterative fashion. 
     The method  550  includes providing, at  552 , an RF electrode that has a hollowed dee having first and second surfaces that oppose each other and define a gap therebetween. The RF electrode may be similar to, for example, the RF electrode  416  ( FIG. 4 ). The RF electrode also includes a bridge structure that is coupled to and extends away from the hollowed dee. The bridge structure includes a side wall that defines an interior cavity of the bridge structure. The side wall has a particle opening therethrough. In particular embodiments, the bridge structure includes an elongated stem having a length that is greater than a width or height of the elongates stem. 
     The method  550  also includes positioning, at  554 , the RF electrode within an acceleration chamber of a cyclotron. The cyclotron is configured to direct charged particles along an orbit plane within the acceleration chamber. The RF electrode may be positioned such that the orbit plane extends between the first and second surfaces and extends through or proximate to the particle opening of the side wall. The particle opening is configured to receive neutral particles that project along the orbit plane during operation of the cyclotron. 
     The method  550  may also include positioning, at  556 , an interception panel within the acceleration chamber to receive the neutral particles that project through the particle opening along the orbit plane. In some embodiments, the positioning at  554  and the positioning at  556  occur simultaneously. For example, the RF electrode may be fully assembled with the interception panel attached thereto. Thus, when the RF electrode is operably positioned within the acceleration chamber, the interception panel is also positioned. As described herein, the interception panel may comprise a blocking material. 
     It is to be understood that the above description is intended to be illustrative, and not restrictive. For example, the above-described embodiments (and/or aspects thereof) may be used in combination with each other. In addition, many modifications may be made to adapt a particular situation or material to the teachings of the inventive subject matter without departing from its scope. Dimensions, types of materials, orientations of the various components, and the number and positions of the various components described herein are intended to define parameters of certain embodiments, and are by no means limiting and are merely exemplary embodiments. Many other embodiments and modifications within the spirit and scope of the claims will be apparent to those of skill in the art upon reviewing the above description. The scope of the inventive subject matter should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled. In the appended claims, the terms “including” and “in which” are used as the plain-English equivalents of the respective terms “comprising” and “wherein.” Moreover, in the following claims, the terms “first,” “second,” and “third,” etc. are used merely as labels, and are not intended to impose numerical requirements on their objects. Further, the limitations of the following claims are not written in means-plus-function format and are not intended to be interpreted based on 35 U.S.C. §112(f) unless and until such claim limitations expressly use the phrase “means for” followed by a statement of function void of further structure. 
     This written description uses examples to disclose the various embodiments, and also to enable a person having ordinary skill in the art to practice the various embodiments, including making and using any devices or systems and performing any incorporated methods. The patentable scope of the various embodiments is defined by the claims, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if the examples have structural elements that do not differ from the literal language of the claims, or the examples include equivalent structural elements with insubstantial differences from the literal languages of the claims. 
     The foregoing description of certain embodiments of the present inventive subject matter will be better understood when read in conjunction with the appended drawings. To the extent that the figures illustrate diagrams of the functional blocks of various embodiments, the functional blocks are not necessarily indicative of the division between hardware circuitry. Thus, for example, one or more of the functional blocks (for example, processors or memories) may be implemented in a single piece of hardware (for example, a general purpose signal processor, microcontroller, random access memory, hard disk, or the like). Similarly, the programs may be stand alone programs, may be incorporated as subroutines in an operating system, may be functions in an installed software package, or the like. The various embodiments are not limited to the arrangements and instrumentality shown in the drawings.