GB2384675A

GB2384675A - Energy degrader or moderator with focus means for a particle beam

Info

Publication number: GB2384675A
Application number: GB0201818A
Authority: GB
Inventors: Francis James Macdonald Farley
Original assignee: Individual
Current assignee: Individual
Priority date: 2002-01-28
Filing date: 2002-01-28
Publication date: 2003-07-30
Anticipated expiration: 2022-01-28
Also published as: GB2384675B; GB0201818D0; WO2003065382A1

Abstract

An energy degrader for particle beams comprising two or more slabs of matter through which the beam passes in succession with focusing means before the first slab and between the slabs to concentrate the beam successively onto the centre of each slab. The slabs may be made of diamond, boron carbide, beryllium or graphite, and the beam is focused and refocused by quadrupole triplets to a small spot at the centre of each slab. By dividing the degrader in this way into several separate slabs with refocusing lenses between the slabs the increase of beam emittance due to lateral scattering is minimized.

Description

ENERGY DEGRADE FOR PARTICLE BEAMS The invention provides a means for reducing the kinetic energy of beams of protons or other charged particles without excessive spread by lateral scattering.

Beams of high energy charged particles such as protons or other atomic nuclei are used for treating tunours and other medical conditions. The depth to which the beam penetrates in the body depends on the kinetic energy of the particles. This should be adjusted appropriately for each treatment. However some particle accelerators deliver particles at a fixed energy. It is then necessary to reduce or degrade the energy by passing the particles through a slab of matter, the thickness of the slab being a function of the amount of energy to be lost. However the particles are also scattered laterally by the slab with the result that the size of the beam and its angular spread are increased and particles will be lost unless subsequent elements in the beam transport system are made larger and cost more. The slab of material used to reduce the beam energy is conventionally called the"degrader". The lateral spread of the beam is conventionally characterised by its emittance which is the product of its typical lateral dimension multiplied by its typical lateral angle. It is desirable to degrade the energy of the beam with the smallest possible increase in beam emittance. This invention provides means for reducing the energy of the beam while minimizing the lateral diffusion of the beam by scattering, that is minimizing the inevitable increase of beam emittance.

According to the invention in its first characteristic the energy degrader comprises two or more slabs of matter interleaved with focusing means and the beam is directed to pass in succession through all the slabs.

According to the invention in its second characteristic focusing means are provided to concentrate the beam to a small spot at the centre of each slab. The beam is initially focused onto the centre of the first slab. The particles emerging from this slab are then refocused onto the centre of the second slab and so on with the effect that the beam emerging from each slab is refocused onto the centre of the next slab until the end. The energy loss then depends on the combined total thickness of all the slabs. However the increase in the emittance of the beam when many slabs are used in this manner is less than the increase in the emittance caused by one slab of the same total thickness. If the degrader is divided into N separate slabs with refocusing means as explained above, the total increase of beam emittance is smaller than the best that can be achieved with a single slab of the same total thickness by approximately the factor N. As this applies in both transverse planes the final number of

2 particles per unit solid angle in the beam is improved by the factor N 2.

A specific embodiment of the invention will now be described by way of example with reference to the accompanying drawing, in which: Figure 1 shows a lateral view of a portion of the energy degrader.

Referring to the drawing the energy degrader comprises two or more slabs of matter, of which two only are illustrated at 4 and 8, with a triplet of quadrupole lenses 1,2 and 3 to focus the beam of particles coming from the left onto the first slab, and a quadrupole triplet 5,6 and 7 to refocus the beam emerging from slab 4 onto slab 8. The degraded beam with particles of reduced energy leaving to the right may then pass through another quadrupole triplet, not

illustrated but substantially identical to 5,6, 7, which refocuses the beam onto a further slab, not illustrated but more or less identical to 8, and so on. Any number of slabs may be added, each preceded by a quadrupole triplet to refocus the beam, or alternatively the degrader may comprise only the two slabs 4 and 8. In the drawing, following the usual conventions, the track followed by the particles in the horizontal plane is shown by the solid line 9 while the track followed by particles in the vertical plane is shown by the dotted line 10. With advantage the angles of convergence of the particles onto the degrading slabs are identical in the two planes. The slabs may be of different materials and different or variable thicknesses, adjustable according to the desired final particle energy.

For minimum increase in beam emittance, the degrading slabs should be of high density but low atomic number, the best materials being diamond, boron carbide, beryllium and graphite but the first is usually not available in the required thickness. The ratio of spot size to beam convergence angle at each focus should be adjusted according to the type of particle in the beam, the degrader material and its thickness, typical values for example without limitation being in the range 0.01 to 0.04 mm per milliradian.

Claims

CLAIMS 1. An energy degrader for particle beams comprising two or more slabs of matter through which the beam passes in succession with focusing means before the first slab and between the slabs to concentrate the beam successively onto a small spot at the centre of each slab.

2 An energy degrader for particle beams as claimed in claim 1 in which the slabs of matter are made of diamond, boron carbide, beryllium or graphite.

3 An energy degrader for particle beams as claimed in claims 1 or 2 in which the focusing means are provided by quadrupole triplets.

10. An energy degrader for particle beams substantially as described and illustrated in Figure 1.

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Amendments to the claims have been filed as follows CLAIMS 1. An energy degrader for particle beams comprising two or more slabs of matter through which the beam passes in succession with focusing means located before the first slab and located between the slabs to concentrate the beam successively onto a small spot inside or close to each slab.
2. An energy degrader as in claim 1, positioned between a particle accelerator and the principal beam transport system which guides the beam to its destination.
3 An energy degrader for particle beams as claimed in claims 1 or 2 in which the slabs of matter are made of diamond, boron carbide, beryllium and/or graphite.
4 An energy degrader for particle beams as claimed in claims 1,2 or 3 in which the focusing means comprise three or more quadrupole lenses.
5 Use of an energy degrader as in any of the preceding claims for purposes of proton therapy.
6. An energy degrader for particle beams substantially as described and illustrated in Figure 1.