Introduction

The present invention relates to a system for focussing gamma-rays. Gamma-rays have energies greater than around 100 keV. In particular, the present invention relates to a gamma-ray focussing system that can be used for cancer treatment.

Background

Gamma-ray photons have a very high energy, in the range of a few hundred keV. The properties of these high energy photons are used in numerous applications including sterilization (via irradiation), medical imaging (PET scan), cancer treatment and high density matter imaging. The high energy of gamma-ray photons renders the manipulation of gamma-ray beams, in particular focussing, very challenging as no suitable refractive optical material exists. Optics for gamma-ray focusing, such as Fresnel and Laue lenses, are complex, expensive and have a long focal length, of the order of tens of meters. This sets a limit to their application, which is mainly restricted to astronomy.

Currently, the treatment of cancer via non invasive gamma-ray surgery requires the use of a large number of collimated gamma-ray beams converging on one or more tumours. This so called gamma knife technology uses hundreds of radioactive sources distributed in a hemisphere. A disadvantage of this technology is that complex collimation is required, and radioactive sources have to be handled, loaded and shielded. Also, the effectiveness of the treatment relies on a complex alignment procedure, which limits its application to brain and neck treatment.

GB1546363 describes a system for generating X-rays by stopping an electron beam in an anode to produce a converging x-ray beam. A problem with this approach is that the stopped electrons can cause scattering, which may result in defocusing of the X-ray beam.

US5473661 describes a system for X-ray or gamma-ray production. This uses a crystal lattice to produce channelling radiation. The crystal that produces the channelling radiation is bent into a curved shape to produce channelling radiation directed towards a real focal position to produce focussed radiation or away from a virtual focal position to produce diverging radiation. In this case, the electron beam needs to be aligned to the crystal plane to produce X-rays. This means that the target must also be bent and a very high quality electron beam must be used.

Summary of the invention

According to the present invention, there is provided a system for generating a focused gamma ray beam having: means for generating an electron beam; a lens for focusing the electron beam to provide a converging electron beam and a target positioned between the lens and the focal point of the electron beam, wherein the electron beam and target are selected and arranged, so that collision of the converging electron beam with the target material causes bremsstrahlung radiation that forms a converging gamma ray beam. The inventors have found that a converging high energy electron beam that is incident on a target can be used to create a naturally converging gamma ray beam without requiring any complex gamma ray focusing optics. This is a significant technical advantage. The properties of the converging gamma ray beam are determined by the electron beam and the target properties. In particular, the electron beam energy, the material of the target, the position and thickness of the target and the point at which the converging electron beam focuses can have an impact on the characteristics of the converging gamma ray beam. These can be selected or varied depending on the application requirements. Equally, these can be tuned or varied to create a converging gamma ray beam having particular properties.

The energy of the electron beam may be more than 100 MeV. The electron beam may be generated using a laser plasma wakefield accelerator.

The target may be made of a material with an atomic number Z greater than 12. For example, the target may be made of a metal, such as Aluminium, Copper, Indium, or Tungsten. The minimum energy of the electron beam required to produce a converging gamma- ray beam, scales with Z and the material thickness. For a 1 mm thick Al target the electron beam energy needs to be greater than 100 MeV. This limit is higher for higher Z and thicker targets.

The means for generating a converging electron beam comprise an electron beam source and a charged particle beam lens, for example a magnetic lens.

Brief Description of the Drawings

Various aspects of the invention will now be described by way of example only and with reference to the accompanying drawings, of which:

Figure 1 is a schematic representation of a system for converging gamma-rays;

Figure 2 shows a list of parameters values used in the gamma-ray beam profile simulations;

Figure 3 shows the gamma-ray beam r.m.s. radius as a function of increasing distance from the target, simulated for different electron beam energies;

Figure 4 shows the gamma-ray beam r.m.s. radius as a function of increasing distance from the target, simulated for different target thickness;

Figure 5 shows the gamma-ray beam r.m.s. radius as a function of increasing distance from the target, simulated for different magnetic lens focal lengths spanning from 10 to 30 cm;

Figure 6 shows the gamma-ray beam r.m.s. radius as a function of increasing distance from the target, simulated for different target materials: Aluminium, Indium, Copper and Tungsten;

Figure 7 shows the gamma-ray beam r.m.s. radius as a function of increasing distance from the lens, simulated for three distances between the magnetic lens and the target: 1 cm, 3 cm and 5 cm;

Figure 8 is a simulation of the gamma-ray focal position obtained as a function of a) target atomic number, b) magnetic lens focal length, and c) target thickness.

Detailed description of the invention

Figure 1 shows a system 10 for producing a focused a gamma-ray. The system 10 includes an electron beam source 1 1 for generating an electron beam, a magnetic lens 12 and a target 14. In a preferred example, the electron beam source is a laser driven accelerator, such as a Laser-plasma Wakefield Accelerator (LWFA). The electron beam may be a beam of relativistic electrons. The target 14 is placed at a distance d1 from the magnetic lens on the converging portion of electron beam path 22 and before the focal point of the lens. The target and its position are selected to ensure that when the converging electron beam passes through it, interaction between electrons in the beam and the target causes emission of bremsstrahlung radiation in the form of a converging gamma ray beam.

In use, electron beam 16 passes through the magnetic lens 12 and is focused to a minimum size at a focal point 18 at a distance d2 from the magnetic lens on the electron beam propagation axis 20. Interaction between the electron beam and the target generates bremsstrahlung radiation resulting in a converging gamma-ray beam 24 propagating along a converging path similar to the path followed by the electron beam 16. The gamma-ray beam focuses to a minimum r.m.s. radius at a focal point 26 a distance d3 from the target. In the example shown in Figure 1 , the focal point 26 of the gamma ray beam is beyond the focal point 18 of the electron beam.

Generation of the converging gamma ray beam is caused by the interaction between high energy electrons colliding with nuclei in the target. Deceleration of the incoming electrons on the target results in high energy gamma-ray bremsstrahlung radiation emission. This radiation propagates in a narrow on-axis cone that converges in the direction of propagation with a solid angle that depends on the electron energy and target properties. Where relativisitic electrons are used, the gamma-rays are emitted in a narrow cone with an angle of approximately 1 /γ, where γ is the relativistic Lorentz factor of the beam. This is a simple and effective technique for naturally focusing gamma-rays without requiring the use of complex gamma-ray optics.

The properties of the converging gamma ray beam are a function of the target material, the position and thickness of the target and the point at which the converging electron beam focuses, as well as the energy of the electron beam. These can be selected or varied depending on the application requirements.

The electron beam shape and properties can be modified by varying the electron beam energy and energy spread, divergence, spot size and focal length of the electron beam transport properties. Transport also depends on the beam emittance, which is a measure of the transverse momentum spread of the beam and is the analogue of the wavelength of light (thus shorter wavelength light or lower emittance beams can be focussed to smaller spots). Ideally, the electron beam is substantially collimated, with a beam emittance that is preferably less than, for example, 5 π mm mrad, in order to achieve small spot size at the focal point.

Simulations of the system of Figure 1 were carried out using the GEANT4 computer code to reproduce a mono-energetic electron beam passing through a metal target. The beam profile was investigated as a function of different parameters: the electron beam energy, the magnetic lens focal length, the atomic number (Z) of the target material, the target thickness, the distance (d1 ) between the magnetic lens and the target and the propagation medium. The values of the parameters used in the simulations are listed in Figure 2.

Figure 3 shows the gamma-ray r.m.s beam r.m.s. radius as a function of increasing distance from the target, simulated for different electron beam energies. For this set of simulations an electron beam was made to converge with a magnetic lens of 10 cm focal length and passed through a 1 mm thick aluminium target placed 5 cm from the lens, and then passed through air collinearly with the emerging bremsstrahlung gamma-ray beam. For a low electron beam energy (50 MeV) the emerging gamma ray beam diverges, while for higher energy electron beams it converges. Low energy electrons are more strongly deviated from the converging trajectories following interaction with the aluminium foil and thus do not produce a converging gamma-ray beam. As an example, a 1 mm aluminium foil requires an electron beam with energy greater than 100 MeV. This minimum energy will increase as either the atomic number of the target or its thickness is increased. Thus, a converging gamma-ray beam can only be produced using the bremsstrahlung process using a converging high energy electron beam.

Figure 4 shows the gamma-ray beam r.m.s. radius as a function of increasing distance from the target, simulated for different target thicknesses. The simulation was carried out for a 250 MeV electron beam, which is sufficiently high to produce a converging gamma-ray beam, colliding on an aluminium target. The results of the simulation show that, under these conditions, the maximum thickness allowable for producing a converging gamma-ray beam is 1 cm. This maximum thickness depends on electron beam energy, focal length and target material. Figure 5 shows the gamma-ray beam size as a function of increasing distance from the target, simulated for different magnetic lens focal lengths of 10 cm (red), 15 cm (blue), 20 cm (black) and 30 cm (green). For this simulation the electron beam energy was set to 250 MeV with an electron beam r.m.s. radius of 1 mm and the distance d1 between the magnetic lens and the target was set to 5 cm. The results of the simulation show that the gamma-ray beam focal position and spot r.m.s. radius can be varied by adjusting the electron beam focal length. The gamma-ray beam focal position may vary relative to the electron beam focal length. For example, when the focal length of the magnetic lens is greater than 10 cm, the focal position of the gamma-ray beam is 5 cm after the electron beam focus.

Figure 6 shows the gamma-ray beam size as a function of increasing distance from the target, simulated for different target materials: Aluminium (red), Indium (green), Copper (black) and Tungsten (grey). The simulation was conducted using the same parameters as in Figure 5. The results displayed in Figure 6 show that the spot r.m.s. radius at focus is strongly influenced by the target material. The gamma-ray beam convergence and spot r.m.s. radius increase concomitantly with increasing atomic number. At very high atomic number (tungsten), the gamma-ray beam is diverging. However, in this latter case, a converging gamma-ray beam could still be produced using higher energy electron beams.

Figure 7 shows the gamma-ray beam r.m.s. radius as a function of increasing distance from the target, simulated for three distances d1 between the magnetic lens and the target of 1 cm, 3 cm and 5 cm, respectively. The beam r.m.s. radius is observed to decrease with increasing distance between the magnetic lens and the target.

Figure 8 shows various simulations based on gamma-ray focal position. Figure 8(a) is a simulation of gamma-ray focal position as a function of target atomic number. Figure 8(b) is a simulation of gamma-ray focal position (expressed as a percentage of magnetic lens focal length) as a function of magnetic lens focal length. Figure 8(c) is a simulation of gamma-ray focal position as a function of target thickness. For these simulations the electron beam focal length was fixed at 10 cm. The result of these simulations show that the distance d3 between the target and the gamma-ray focal position decreases with increasing target atomic number and increasing target thickness.

A skilled person will appreciate that variations of the disclosed arrangements are possible without departing from the invention. Accordingly, the above description of the specific embodiment is made by way of example only and not for the purposes of limitation. It will be clear to the skilled person that minor modifications may be made without significant changes to the operation described.