EP0853867B1 - Method for sweeping charged particles out of an isochronous cyclotron, and device therefor - Google Patents

Abstract

PCT No. PCT/BE96/00101 Sec. 371 Date Apr. 3, 1998 Sec. 102(e) Date Apr. 3, 1998 PCT Filed Sep. 25, 1996 PCT Pub. No. WO97/14279 PCT Pub. Date Apr. 17, 1997A method for extracting a charged particle beam out of an isochronous cyclotron (1) comprising an electromagnet forming a magnetic circuit that includes at least a number of sectors (3, 3') known as "hills" where the air-gap is reduced, and separated by sector-shaped spaces (4) known as "valleys" where the air-gap is larger. According to the extraction method, the particle beam is extracted without using an extraction device as the magnetic field has a special arrangement produced by designing the electromagnet air-gap at the "hills" (3, 3') of the isochronous cyclotron in such a way that the aspect ratio between the electromagnet air-gap at the "hills" in the region of the maximum radius, and the radius gain per turn of the particles accelerated by the cyclotron at said radius is less than 20.

Description

Subject of the invention.

The present invention relates to a method for extracting charged particles from a cyclotron isochronous in which the particle beam is focused by sectors.

The present invention also relates to said isochronous cyclotron applying this method of extraction of charged particles.

The present invention also relates to compact isochronous cyclotrons than focused cyclotrons by sectors. Likewise, the present invention relates to isochronous cyclotrons known as superconductive or not superconductors.

State of the art.

Cyclotrons are particle accelerators used in particular for the production of isotopes radioactive. These cyclotrons usually consist of two separate main sets, consisting on the one hand by the electromagnet and on the other hand by the high resonator frequency.

The electromagnet guides the particles loaded on a trajectory presenting approximately a spiral of increasing radius around the acceleration. In the modern isochronous cyclotrons, the poles of electromagnets are divided into sectors alternately presenting a reduced air gap and a larger air gap. Variation azimuth of the resulting magnetic field has the effect ensure the vertical and horizontal focus of the beam during acceleration.

Among the isochronous cyclotrons, it is advisable to distinguish the compact type cyclotrons, which are energized by at least one main circular coil, and the so-called separate sector cyclotrons, where the structure magnetic is divided into completely separate units autonomous.

The second set consists of the electrodes accelerators, frequently called "gods" for reasons historical. A voltage is thus applied to the electrodes alternative of several tens of kilovolts at the frequency of rotation of the particles in the magnet, or alternatively at a frequency which is an exact multiple of the frequency of rotation of the particles in the magnet. This has the effect to accelerate the particles of the rotating beam in the cyclotron.

For many applications using a cyclotron, it is necessary to extract the beam from particles accelerated out of the cyclotron, and to guide it to a target where you want to use it. This operation beam extraction is considered by those skilled in the art as the most difficult step in producing a particle beam accelerated by a cyclotron. This operation consists in bringing the beam from the part from the magnetic field where it is accelerated to where the magnetic field can no longer hold the beam. In this case, the beam is free to escape the action of the field and is extracted from the cyclotron.

In the case of cyclotrons accelerating particles positively charged, we know the use of a electrostatic deflector whose role is to draw the particles outside the magnetic field as a device extraction. To achieve such an effect, it is necessary to interpose on the particle path an electrode called the septum, which will intercept some of these particles. Therefore, the extraction yield is relatively limited, and particle loss in the septum will notably contribute to making the cyclotron strongly radioactive.

It is also known to extract particles negatively charged by converting ions negatives into positive ions by passing them through a sheet which has the function to strip the ions negatives of their electrons. This technique allows extraction yields close to 100% and also allows using a device that is significantly less complex than the one described above. However, the acceleration of negative particles presents difficulties important. The main drawback is that that the negative ions are fragile, and are therefore easily dissociated by residual gas molecules or by excessive magnetic fields crossed at high energy and present in the cyclotron. The transmission of beam in the accelerator is therefore limited, which also contributes to the activation of the latter.

In contrast, cyclotrons accelerating positive particles produce higher beam current intensities, and increase the reliability of the system, and while allowing strong reduction in size and weight of the machine.

It is also known by the document "The review of Scientist Instruments, 27 (1956), n ° 7 "and by the document "Nuclear Instruments and Methods 18, 19 (1962), pp. 41-45" by J. Reginald Richardson, a technique whereby the particle beam could have been extracted from the cyclotron without the use of an extraction device. The requirements for this self-extraction are special conditions for resonating the movement of particles in the magnetic field.

However, this described method is particularly difficult to achieve, and would have given a beam whose optical qualities were so bad that in practice it has never been applied.

Document US-A-0324379 relates to a cyclotron type device intended to accelerate particles which has magnetic means being essentially independent of the azimuth angle. This means that it is a non-isochronous cyclotron. In addition, it should be noted that the cyclotron described has beam extraction means which consist of "regenerators" and "compressors", which allow, in disturbing the magnetic field, to obtain an extraction of the beam of particles.

Document WO-93/10651 in the name of the Applicant describes a compact isochronous cyclotron with an air gap located between two basically shaped hills elliptical and tending to close completely at the radial end of the hills on the median plane. The device described in this document also includes conventional means of beam extraction which are a electrostatic deflector in this case.

Aims of the present invention.

The present invention aims to propose a method for extracting charged particles from a cyclotron isochronous avoiding the use of devices extraction as described above.

An additional aim of the present invention is to therefore to propose an isochronous cyclotron which is of simpler and more economical design than those usually used.

The present invention also aims to increase the extraction efficiency of the particle beam, and particularly in the case of extraction of positive particles.

Main characteristic elements of this invention.

The present invention relates to a method for extracting charged particles from a cyclotron isochronous comprising an electromagnet constituting the circuit magnetic which includes a number of pairs of sectors called "hills" where the air gap is reduced, separated by spaces in the form of sectors called "valleys" where the air gap is larger; this method being characterized by the fact that a cyclotron is produced isochronous with a magnet gap between the hills including the dimensions are chosen so that the minimum value of this air gap in the vicinity of the maximum radius between the hills be less than twenty times the gain in radius by round of particles accelerated by the cyclotron at this radius.

According to this particular configuration, we will observe that the ions can be extracted from the influence of the magnetic field without the help of any device extraction.

It should be noted that for cyclotrons isochronous of the state of the art, the air gap that the magnet is in generally between 5 and 20 cm, while the gain in radius per revolution is approximately 1 mm. In this case, the report of the air gap in radius gain per turn is greater than 50.

We observe that by applying the characteristic principal of the present invention, the magnetic field decreases very abruptly near the limit of the magnet pole, so that the point of self-extraction is reached before the phase shift of the particles relative to the accelerating voltage does not reach 90 degrees. In this way, the particles automatically exit the magnetic field without intervention of any device extraction.

According to a particular embodiment preferred of the present invention, one can consider draw an air gap with an elliptical profile which has tendency to close at the radial end of the hills, as described in patent W093 / 10651.

According to a preferred embodiment of this invention, the extraction of particles is concentrated on a sector thanks to a dissymmetry deliberately brought to the shape or magnetic field of said sector.

According to another preferred embodiment of the present invention, the angle of one of the sectors is reduced at the polar radius to allow movement of the orbits and thus get the extraction of the whole beam on this side, so, for example, to be able to irradiate a large volume target.

According to another preferred embodiment of the present invention, a particular distribution is carried out of the particle beam so as to simultaneously irradiate several targets mounted side by side on the trajectory of the beam.

The present invention advantageously makes it possible to be used for proton therapy or the production of radioisotopes, and more particularly of radioisotopes intended to positron emission tomography (PET).

Brief description of the figures.

Figures 1 and 2

represent the magnetic profiles of a isochronous cyclotron according to the state of the technique and an isochronous cyclotron using the extraction method according to the present invention.

Figure 3

schematically represents a view burst of the main elements constituting a isochronous cyclotron.

Figure 4

represents a section view of a cyclotron isochronous.

Description of a preferred embodiment of the invention.

The profile of the magnetic field in an isochronous cyclotron is such that the frequency of rotation of the particles must be constant and independent of their energy. To compensate for the relativistic increase in mass of the particles, the magnetic field must therefore increase with the radius to ensure this condition of isochronism. To describe this relation, the field index is defined by the following relation: not = dB B · R dR in which dB / B and dR / R are respectively the relative variations of the magnetic field and the radius to the radius R.

It should be noted that it is impossible to maintain the isochronism condition near the radius maximum of the pole. Indeed, at this moment, the field stops increase with the radius. It has reached a maximum and then begins to decrease more and more quickly.

Figure 1 illustrates the variation of the field in function of the radius in a classical isochronous cyclotron. A increasing phase shift between the frequency of rotation of particles and the resonant frequency of the electrodes accelerators. When this phase shift reaches 90 degrees, the particles stop being accelerated and they cannot exceed this radius.

Figure 2 illustrates the variation of the field in function of the radius in an isochronous cyclotron using the extraction method according to the present invention. In choosing precisely the dimensions of the air gap of the magnet between the hills, so that it is reduces the shelf gain to less than twenty times per turn, we observe a profile of the magnetic field such that shown in figure 2.

In this case, the magnetic field decreases so very brutal near the limit of the magnet pole, in such a way that the self-extraction point defined by the field index n = -1 is reached before the phase shift particles with respect to the accelerating voltage does not reach 90 degrees.

From this moment, the particles come out automatically from the magnetic field without any intervention extractor device.

An isochronous cyclotron as used in the method of extracting charged particles according to the present invention is shown schematically in the figures 3 and 4. This cyclotron is a compact isochronous cyclotron intended for the acceleration of positive particles, and more especially protons.

• deux plaques de base appelées culasses 2 et 2',
• au moins trois secteurs 3 supérieurs appelés collines et un même nombre de secteurs inférieurs 3' situés symétriquement par rapport à un plan de symétrie 10 dit plan médian aux secteurs supérieurs 3, et qui sont séparés par un faible entrefer 8,
• entre deux collines consécutives, il existe un espace où l'entrefer est de dimension plus élevée et est qui appelé vallée 4,
• au moins un retour de flux 5 réunissant de façon rigide la culasse inférieure 2 à la culasse supérieure 2',

The magnetic structure 1 of the cyclotron consists of a number of elements 2, 3, 4 and 5 made of a ferro-magnetic material and of coils 6 preferably made of a conductive or superconductive material. The ferro-magnetic structure conventionally comprises:

• two base plates called cylinder heads 2 and 2 ',
• at least three upper sectors 3 called hills and the same number of lower sectors 3 'situated symmetrically with respect to a plane of symmetry 10 said median plane to the upper sectors 3, and which are separated by a small air gap 8,
• between two consecutive hills, there is a space where the air gap is of higher dimension and which is called valley 4,
• at least one flow return 5 rigidly joining the lower cylinder head 2 to the upper cylinder head 2 ',

The coils 6 are essentially shaped circular, and are located in the annular space left between sectors 3 or 3 'and flow returns 5.

The central duct is intended to receive at least part of the source of particles 7 to be accelerated. These particles are injected into the center of the device by means known per se.

For an isochronous cyclotron accelerating a beam of protons up to an energy of 11 MeV, the magnet is drawn, according to the present invention, with an air gap of 10 mm for a magnetic field of 2 teslas on the sectors magnetic 3 and 3 '. The accelerating voltage is 80 kilovolts so as to obtain a radius gain of 1.5 mm at maximum radius.

This unusual choice of parameters allows that the radial end of the hills, we observe a decreasing extremely fast external induction which allows to self-extract the particle beam before the limit of acceleration, which is more particularly represented in Figure 2.

According to a first preferred embodiment, one reduces the angle of one of the sectors at the polar radius so as to allow us to move the orbits and obtain the extraction of the whole beam on this side (see figure 4).

The extracted particle beam is then axially focused and radially defocused.

According to another preferred embodiment, use this beam profile for simultaneous irradiation of four targets located between the two coils 6 mounted side by side on the beam path.

Claims (7)

1. Method of extracting a beam of charged particles from an isochronous cyclotron (1) having an electromagnet constituting the magnetic circuit which includes at least a certain number of sectors (3, 3'), referred to as "hills", where the air gap is reduced, these being separated by spaces in the form of sectors (4), referred to as "valleys", where the air gap is of larger size, the extraction method being characterized in that the particle beam is extracted by a particular arrangement of the magnetic field, without resorting to an extraction device, this arrangement being obtained by designing the air gap of the magnet at the hills (3, 3') of the isochronous cyclotron in such a way that the ratio of the dimension of the air gap of the magnet at the hills in the vicinity of the maximum radius to the gain in radius per circuit of the particles accelerated by the cyclotron at this radius is less than 20.
2. Isochronous cyclotron in which a particle beam is focused by sectors and which has an electromagnet constituting a magnetic circuit which includes at least a certain number of sectors (3, 3'), referred to as "hills", where an air gap is reduced, these being separated by spaces in the form of sectors (4), referred to as "valleys", where an air gap is of larger size, characterized in that the air gap of the magnet at the hills (3, 3') is designed in such a way that the ratio of the dimension of the air gap of the magnet at the hills in the vicinity of the maximum radius to the gain in radius per circuit of the particles accelerated by the cyclotron at this radius is less than 20.
3. Isochronous cyclotron according to Claim 2, characterized in that the profile of the air gap of the magnet at the hills is an elliptical profile tending to close on itself at the radial end of the hills.
4. Cyclotron according to Claim 2 or 3, characterized in that at least one sector has a shape or a magnetic field that is asymmetric with respect to the other sectors.
5. Cyclotron according to any one of Claims 2 to 4, characterized in that the angle of one of the sectors is reduced at the pole radius.
6. Cyclotron according to any one of Claims 2 to 4, characterized in that a particular distribution of the particle beam is produced so as simultaneously to irradiate a plurality of targets mounted side by side on the path of the beam.
7. Use of the particle extraction method according to Claim 1 or of the device according to any one of Claims 2 to 6 for proton therapy or for the production of radioisotopes, and in particular for the production of radioisotopes which are intended for positron emission tomography.

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