EP0037051B1 - Linear accelerator for charged particles - Google Patents

Linear accelerator for charged particles Download PDF

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Publication number
EP0037051B1
EP0037051B1 EP81102176A EP81102176A EP0037051B1 EP 0037051 B1 EP0037051 B1 EP 0037051B1 EP 81102176 A EP81102176 A EP 81102176A EP 81102176 A EP81102176 A EP 81102176A EP 0037051 B1 EP0037051 B1 EP 0037051B1
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EP
European Patent Office
Prior art keywords
accelerator tube
accelerator
charged particles
magnetic coils
magnetic
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired
Application number
EP81102176A
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German (de)
French (fr)
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EP0037051A1 (en
Inventor
Volker Adolf Stieber
Current Assignee (The listed assignees may be inaccurate. Google has not performed a legal analysis and makes no representation or warranty as to the accuracy of the list.)
Siemens AG
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Siemens AG
Priority date (The priority date is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the date listed.)
Filing date
Publication date
Priority to US06/135,300 priority Critical patent/US4293772A/en
Priority to US135300 priority
Application filed by Siemens AG filed Critical Siemens AG
Publication of EP0037051A1 publication Critical patent/EP0037051A1/en
Application granted granted Critical
Publication of EP0037051B1 publication Critical patent/EP0037051B1/en
Expired legal-status Critical Current

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    • HELECTRICITY
    • H05ELECTRIC TECHNIQUES NOT OTHERWISE PROVIDED FOR
    • H05HPLASMA TECHNIQUE; PRODUCTION OF ACCELERATED ELECTRICALLY-CHARGED PARTICLES OR OF NEUTRONS; PRODUCTION OR ACCELERATION OF NEUTRAL MOLECULAR OR ATOMIC BEAMS
    • H05H9/00Linear accelerators

Description

  • The invention relates to a linear accelerator for charged particles intended for therapy, with an evacuated accelerator tube with walls made of non-ferromagnetic material, with a device for accelerating the charged particles in the forward direction, the particles forming radiation pulses with a predeterminable pulse frequency, and with a vacuum-tight tube final exit window for the accelerated particles.
  • Accelerators for charged particles, predominantly electron accelerators, sometimes also proton accelerators, are mainly used in medical radiation therapy, but less frequently for purposes of radiation screening and sterilization of all kinds of samples. They create a tightly focused beam of accelerated charged particles. However, electron accelerators, mostly linear accelerators, more rarely circular accelerators (betatrons), are also used to generate X-rays with a target exposed to the electron beam. This mostly very hard X-ray radiation is mostly used for medical radiation therapy, but occasionally also for the sterilization of samples of all kinds.
  • The charged particles are accelerated inside an evacuated accelerator tube. For application, the charged particles or the X-rays of the target exposed to the particle beam must pass through an exit window that seals the accelerator tube in a vacuum-tight manner. The exit window generally consists of a thin metal foil. The beam of charged particles has energy of around 4 MeV in common electron accelerators. When the particle beam hits the metal foil, secondary electrons are knocked out of the metal foil. In addition, the exit window is heated at the point of impact of the particle beam. Some of the secondary electrons are directed backwards, i. H. inside the accelerator tube, made of the material of the radiation exit window. These secondary electrons are accelerated backwards in the electrical field of the accelerator tube and hit the end of the accelerator tube opposite the exit window with the full acceleration energy. There they in turn generate secondary electrons and above all hard X-rays. To shield them, strong and heavy radiation protective clothing is also required at this end of the acceleration tube. Finally, with a high beam power, there is also the risk of the exit window overheating at the point of impact of the particle beam. This leads to an increase in the generation rate of the secondary electrons and in extreme cases is also associated with the risk of the exit window melting through, with the result that the accelerator tube is destroyed.
  • Linear accelerators of the type mentioned at the outset, which are intended for therapy, are available on the market.
  • An accelerator for charged particles, in particular for electrons, with an evacuated accelerator tube, a device for accelerating the charged particles in the forward direction and with an exit window for the particles is known from US Pat. No. 3,222,558. The end piece of the accelerator tube widens and the exit window is elongated. In this reference it is stated that it is customary to arrange magnetic deflection coils at the beginning of the widening end piece in order to scan the beam at high frequency over the (narrow) window width and at low frequency over the (larger) window height. This leads to a more uniform beam distribution on the product to be irradiated and to an increase in the area that is covered by the beam. Apart from the fact that this is not a therapy device, it must be noted that nothing is said in this document about the exact construction of the magnetic deflection unit.
  • DE-A-2 040 158 describes a method for increasing the output power of an electron accelerator, by means of which the risk of window overheating is reduced. The direction of the electron beam is controlled by four electromagnetic scanning coils, which are arranged at 90 ° to one another outside the acceleration container. The electron beam is deflected on the exit window on a closed rectangular path. This requires a relatively complex control device.
  • Finally, CH-A-363 272 discloses an electron accelerator in which an electron beam deflected by a main deflection system passes through an acceleration tube. The heating is distributed over the entire exit window by additionally subjecting the electron beam to a transverse deflection perpendicular to the main deflection. It is assumed that electromagnetic windings cannot be used. Instead, the transverse deflection system has two conductors connected in series and through which current flows in the opposite direction, which are fitted inside the tube on opposite sides and parallel to the axis thereof. Such a solution requires expensive bushings for the two conductors if vacuum problems are not to arise.
  • The invention is based, to build a linear accelerator of the type mentioned smaller, lighter and safer and at the same time the radiation protection and the task improve driving safety.
  • In the case of a linear accelerator of the type mentioned at the outset, this object is achieved according to the invention in that an electromagnetic device known per se for repeated deflection of the beam of charged particles is arranged in the vicinity of the exit window, in that the device comprises three magnetic coils, that the three magnetic coils against one another by 120 ° are arranged around the beam of charged particles around the outer circumference of the walls of the accelerator tube, which walls are made of non-ferromagnetic material, the axes of the three magnet coils intersecting at a common point lying on the axis of symmetry of the accelerator tube, and the three magnet coils on are connected to a three-phase network, the frequency of which is lower than the aforementioned pulse frequency of the radiation pulses, as a result of which the device generates an alternating magnetic field which circularly deflects the point of incidence of the beam of the charged particles on the exit window.
  • This electromagnetic device has the advantage that the thermal secondary electrons emitted at the exit window are also deflected. However, due to their lower energy, they are deflected far more than the accelerated primary electrons. The consequence of this is that the secondary electrons emitted by the exit window are deflected onto the wall of the accelerator tube surrounding the exit window, while the accelerated primary electrons experience only a very slight deflection at the same time. The secondary electrons striking the wall of the accelerator tube can no longer be accelerated backwards and can no longer trigger X-ray quanta at the end of the accelerator tube opposite the exit window. The radiation protection measures in this area can therefore be largely reduced. In addition, the impact area of the accelerated electrons on the exit window increases on average over time, so that the local thermal load is reduced. As a result, the yield of secondary electrons is reduced and, as a side effect, the maximum thermally permissible beam power is increased.
  • Since the device deflects the beam of charged particles in a circular manner, this has the consequence that the deflecting field is always non-zero and that secondary electrons generated at the exit window cannot be accelerated backwards at any time interval. In addition, the area of impact of the particle beam on the window is increased with the least possible deflection force. The changing magnetic field can be generated differently than a deflecting electric field outside the accelerator tube and can be brought into effect inside the accelerator tube without bushings or other internals inside the accelerator tube.
  • A particularly expedient construction results if the electromagnetic device according to a development of the invention is arranged at that end of the accelerator tube which faces the exit window. This has the advantage that it is located in the immediate vicinity of the point of origin of the secondary electrons and that it detects the secondary electrons before it passes through the first cavity resonator of the accelerator tube, i.e. H. with the least possible energy, distracts from the wall of the accelerator tube. As a result, the deflecting forces can be kept particularly small, and the deflection of the beam of accelerated particles - the primary radiation - is kept small.
  • Further details of the invention are explained with reference to an embodiment shown in the figures. It shows
    • 1 is a schematic representation of an accelerator tube,
    • 1 shows a section along the line 11-11 of FIG. 1,
    • 3 is a plan view of the point of impact of the beam of charged particles on the exit window with the solenoids switched off and on,
    • 4 is a plan view of the point of impact of the beam of charged particles on the exit window with a single magnet coil switched on,
    • Fig. A switching arrangement for the magnetic coils of Fig. 2 and
    • Fig. 6 shows a switching arrangement for changing the voltage applied to the three coils of Fig. 2.
  • 1 and 2 show a highly schematic representation of a linear accelerator 1 as used for medical purposes. Its accelerator tube 2 carries a particle source 4 at one end and a radiation exit window 8 at its other end. In the case of the present exemplary embodiment, electrons are emitted from the particle source 4 into the interior of the acceleration tube 2. These electrons are accelerated by the electric fields generated inside the accelerator tube. For this purpose, the accelerator tube consists of a series of mutually coupled cavity resonators 5, to which an electromagnetic wave, whether as a standing wave or as a traveling wave, is coupled in a manner not shown here. The accelerator tube 2 of a linear accelerator is essentially rotationally symmetrical and has a straight axis of symmetry 6. It is evacuated. Such an accelerator tube is known, for example, under the type designation "Los Alamos".
  • The electrons injected into the accelerator tube 2 by the particle source 4, a hot cathode with a downstream pre-acceleration path (not shown), along the axis of symmetry 6 of the loading accelerator tube 2 accelerated in time with the coupled high frequency. A pulsed electron beam therefore strikes the exit window 8. The exit window consists of a thin metal foil that seals the accelerator tube in a vacuum-tight manner. The metal foil in particle accelerators should be as thin as possible in order to weaken the particle beam as little as possible. The electron beam striking the beam exit window has a diameter of approx. 0.5 mm. 3 and 4, the point of incidence of the undeflected electron beam on the exit window is designated by 10.
  • The accelerated electrons leave the exit window 8 as an electron beam 12. In the exemplary embodiment, the electrons have an energy of 4 MeV. This emerging electron beam 12 can also strike a target 13 that is brought into its path if necessary, in order to generate X-ray pulses. Either the emerging electron beam 12 or the X-rays emitted by the target are used in radiation therapy.
  • 1 and 2 show, a magnetic deflection device 14, 16, 18 for repeated deflection of the beam on the accelerator tube is arranged in a plane immediately in front of the beam exit window 8. In the exemplary embodiment, the wall of the accelerator tube consists of non-ferromagnetic material, preferably of copper. The effect of such a deflection device is that it deflects the secondary electrons emerging from the exit window into the interior of the accelerator tube 2 against the wall of the accelerator tube. In addition, it increases the impact area 10 of the accelerated electrons on the exit window averaged over time and thus reduces its local thermal load. In FIGS. 3 and 4, the impact surface 10a of the particle beam on the exit window 8, which is enlarged by periodic deflection over time, is shown in an enlarged representation.
  • In the exemplary embodiment of FIGS. 1 and 2, the magnetic deflection device has three magnetic coils 14, 16, 18. In order for these to bring their variable magnetic field as close as possible to the impact surface of the electrons on the exit window, these magnetic coils are on the outside of the accelerator tube The radiation direction is arranged somewhat in front of the radiation exit window 3. The three magnetic coils are arranged offset by 120 ° relative to one another about the axis of symmetry 6 of the accelerator tube 2 and thus at the same time also about the electron beam accelerated along the axis of symmetry. The three axes of symmetry 24, 26, 28 of the magnetic coils 14, 16, 18 are aligned perpendicular to the direction of the electron beam. They meet at a common point on the axis of symmetry of the accelerator tube.
  • The solenoids 14, 16, 18 are connected to AC voltage. Three-phase current is best suited as an AC voltage source. As shown in Fig. 5, the solenoids can be connected to the poles U, V and W of the three-phase source. When the current is switched on, each of the three coils generates a magnetic field that has a force component directed at right angles to the axis of symmetry of the accelerator tube. The magnetic field of the magnetic coil 14 is drawn out in FIG. 2 and designated 25. 2 shows that the magnetic coils 14, 16 and 18 of the exemplary embodiment are adapted to the circular circumference of the accelerator tube 2. In this way, a better transition of the magnetic field is achieved than with straight coils. Solenoid coils without a core have proven their worth.
  • With the help of the three magnetic coils 14, 16 and 18, a rotating field is generated. Provided that the three coils have the same dimension and the same three-phase current flows through them, the electron beam is shifted in a circle around the point of impact of the axis of symmetry 6 on the beam exit window 8. This is shown in Figure 3. There, in an enlarged representation, both the impact surface 10 of the electron beam when the magnet coils are switched off and the impact surface 10a of the circularly deflected electron beam on the exit window when the magnet coils are switched on are shown. If only one of the three magnetic coils 14, 16, 18 is switched on, only a linear displacement of the point of impact of the electron beam on the exit window is achieved, as is shown in FIG. 4.
  • It is also possible for the three magnetic coils to be arranged at different distances from the accelerator tube 2 if, for example, other components are attached to the accelerator tube at one point. In such a case, the uniform rotating field can be achieved by applying the one coil further away to a higher voltage via a matching transformer. Such a matching transformer 30 in a delta connection is shown in FIG. 6. It serves to adjust the magnet coil current with different numbers of turns or different turn diameters so that the magnetic field is the same size in the interior of the accelerator tube despite different coil dimensions and / or coil spacing. Even if one coil has to be kept smaller than the other coil, for example for reasons of space, this can be compensated for by a corresponding adjustment of the current through this coil. The transformer 30 is connected on one side to the connections U, V, W of a three-phase supply and on the other side to the magnet coil or coils. The easiest way to do this is to use three-phase current from the public grid.
  • If the electrons with a pulse frequency are accelerated by 300 pulses per second and the mains frequency of the three-phase current is 50 or 60 Hz, the point of impact 15 on the circuit 10a rotates about 50 or 60 times per second. Five to six electron pulses will strike the radiation exit window during a single revolution. As a result, the thermal energy generated when the electrons impact the radiation exit window is distributed over a much larger cross section. For example, the original impact area can be increased from 0.5 mm 2 to 2 mm 2 . As a result, the local heating and thus the emission of secondary electrons itself is reduced. As a side effect, the risk of the radiation exit window blowing through is also reduced.
  • While the primary electrons are accelerated to energies of around 4 MeV and are deflected only very slightly by the circular magnetic field, the secondary electrons have lower, so-called thermal energy. They would be accelerated without the coils 14, 16, 18 along the axis of symmetry of the accelerator tube in the opposite direction to the electron source 4 and would generate high-energy X-rays when they hit the wall there or the particle source 4 used in the wall. This, in turn, would require a complex, heavy and space-consuming shield. This undesired backward directed hard X-ray radiation is designated by 44 in FIG. 1. However, the magnetic fields of the switched-on magnetic coils 14, 16, 18 deflect these slow-moving thermal electrons from their original direction at the exit window and let them hit the inner walls of the accelerator tube. In this way, the effort can be used for radiation lenabschirmun g significantly reduced.
  • The effort could be further reduced if, instead of the three magnetic coils, only one magnetic coil were attached to the accelerator tube and the emission of the particle source 4 is controlled as a function of the current through the magnetic coil so that it can only take place when the magnetic field is built up. It would also be possible to use a constant magnetic field, for example a permanent magnet, instead of the magnetic coils through which alternating current flows. In this case, the secondary electrons would be intercepted in the same way, only the thermal load on the radiation exit window would not be reduced.

Claims (6)

1. A linear accelerator (1) for charged particles for X-ray therapy, with an evacuated accelerator tube (2) which possesses walls of non-ferromagnetic material, with a device (5) which serves to accelerate the charged particles in a forwards direction, the particles thereby forming a particle beam of radiation pulses with a predetermined pulse frequency, and with an outlet window (8) for the accelerated particles, which window seals the accelerator tube (2) in vacuum-tight fashion, characterised in that in the vicinity of the outlet window (8) there is arranged an electromagnetic device known per se which serves to repeatedly deflect the beam of charged particles, that the device comprises three magnetic coils (14, 16, 18) which are offset relative to one another by 120° and are arranged around the beam of charged particles at the outer periphery of the walls of the accelerator tube (2) which consist of non-ferromagnetic material, where the axes (24, 26, 28) of the three magnetic coils (14, 16, 18) intersect one another at a common point on the axis of symmetry (6) of the accelerator tube (2), and that the three magnetic coils (14, 16, 18) are connected to a three-phase mains supply (U, V, W) whose frequency is lower than the aforementioned pulse frequency of the radiation pulses, as a result of which the electromagnetic device generates a changing magnetic field which deflects the point of impact (15) of the beam of charged particles on the outlet window (8) in a circular fashion.
2. A linear accelerator as claimed in claim 1, characterised in that the electromagnetic device is arranged at that end of the accelerator tube (2) which faces towards the outlet window (8).
3. A linear accelerator as claimed in claim 1 or 2, characterised in that where the interval between the magnetic coils (14, 16, 18) is non-uniform relative to the axis of symmetry (6) of the accelerator tube (2), each magnetic field (25) of these three magnetic coils (14, 16, 18) is maintained at the same value in the region of the outlet window (8) by virtue of the setting of the connected voltage and/or the selection of a different coil size.
4. A linear accelerator as claimed in one of claims 1 to 3, characterised in that the magnetic coils (14, 16, 18) are electrically connected in a triangle (fig. 6).
5. A linear accelerator as claimed in one of claims 1 to 4, characterised in that the three-phase mains supply (U, V, W) is the public mains with a frequency of 50 or 60 Hz.
6. A linear accelerator as claimed in one of claims 1 to 5, characterised in that the form of the magnetic coils (14, 16, 18) is adapted to the shape of the accelerator tube (2) (fig. 2).
EP81102176A 1980-03-31 1981-03-23 Linear accelerator for charged particles Expired EP0037051B1 (en)

Priority Applications (2)

Application Number Priority Date Filing Date Title
US06/135,300 US4293772A (en) 1980-03-31 1980-03-31 Wobbling device for a charged particle accelerator
US135300 1980-03-31

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EP0037051B1 true EP0037051B1 (en) 1985-01-23

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DE3168429D1 (en) 1985-03-07
US4293772A (en) 1981-10-06
JPH0356440B2 (en) 1991-08-28
JPS56152199A (en) 1981-11-25
EP0037051A1 (en) 1981-10-07

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