Magnetic field generator and charged particle beam irradiator

Details Magnetic field generator and charged particle beam irradiator Magnetic field generator and charged particle beam irradiator

1998-09-17

2001-06-12

Berman, Jack (Department: 2881)

Radiant energy

Irradiation of objects or material

Ion or electron beam irradiation

C250S3960ML, C250S374000, C250S309000

Reexamination Certificate

active

06246066

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a magnetic field generator and to a charged particle beam irradiator and, more particularly, to a magnetic field generator for forming a magnetic field by moving a magnetic pair couple in a volume inside a return yoke, and to a charged particle beam irradiator for deflection control of a charged particle beam utilizing a magnetic field formed by the magnetic field generator.
2. Description of the Related Art
A charged particle beam irradiator according to the prior art was disclosed at pages 2055 to 2122, Number 8, Volume 64, 1993
, Review of Scientific Instruments
, by W. T. Chu, et al.
FIG. 1
is a schematic perspective view for explaining an example of the charged particle beam irradiator according to the prior art. A charged particle beam generator
35
generates a charged particle beam and, for example, an accelerator is employed as the charged particle beam generator. A charged particle beam transporter
37
transports the charged particle beam generated by the accelerator
35
. For example, a transporter having an electromagnet is employed as the charged particle transporter to transport the charged particle beam generated by the accelerator
35
. A charged particle beam deflector
39
deflects the charged particle beam
33
transported by the charged particle beam transporter
37
. The charged particle beam deflector
39
may be an electromagnet.
A magnetic field generator
10
generates a magnetic field. The charged particle beam
33
passes through the magnetic field generated by the magnetic field generator. Magnetic poles
3
a
and
3
b
form a magnetic pole pair in which the magnetic pole
3
a
and the magnetic pole
3
b
are opposite each other.
A coil
1
a
mis wound around the magnetic pole
3
a
, and a coil
1
b
is wound around the magnetic pole
3
b
. The coils
1
a
and
1
b
are connected to a power source (not illustrated), and, by supplying a current from the power source, a magnetic field is formed between the magnetic pole
3
a
and the magnetic pole
3
b
. A return yoke
5
is disposed outside the magnetic pole pair
3
a
and
3
b
, and the return yoke
5
and the magnetic poles
3
a
and
3
b
are one solid unit.
The magnetic field generator
10
is fixed to a toothed gear
21
. A toothed gear
22
engages the toothed gear
21
. A driver
11
, for example, a motor, rotationally drives the toothed gear
22
. By driving the motor
11
, the toothed gear
22
is rotated, so the toothed gear
21
and the magnetic field generator
10
are also rotated.
The charged particle beam deflector
39
deflects the charged particle beam
33
to move along a rotation axis
29
of the toothed gear
21
. The charged particle beam
33
travels along the rotation axis of the toothed gear
21
and enters the magnetic field generator
10
.
A magnetic field corresponding to the current flow in the coils
1
a
and
1
b
is generated between the magnetic poles
3
a
and
3
b
, and a force (Lorentz force) is applied to the charged particle beam passing between the magnetic poles
3
a
and
3
b
. This force corresponds to the vector product of the magnetic field and the charged particle velocity. Accordingly, after passing through the magnetic field generator
10
, the direction of the charged particle beam is changed (i.e., deflected).
An irradiated object
15
receives the charged particle beam. When the charged particle beam irradiator is applied to a medical treatment appliance, the irradiated object
15
is a human body.
When the charged particle beam is not deflected by the magnetic field generator
10
, the irradiation location of the charged particle beam
33
corresponds to the position where the rotational axis of the toothed gear
21
intersects the irradiated object
15
. On the other hand, when deflected by the magnetic field generator
10
, the irradiated location moves to a position on a straight line along a direction perpendicular to the magnetic field generated between the magnetic poles
3
a
and
3
b
. The direction of that movement varies, corresponding to the direction of the current flowing in the coils
1
a
and
1
b
, and the magnitude of that movement varies, corresponding to the magnitude of the current flowing in the coils
1
a
and
1
b
. By controlling the current flowing in the coils
1
a
and
1
b
, the irradiated position may be oscillated along a straight line (such an operation is hereinafter referred to as scanning irradiation).
Further, by rotating the toothed gear
21
, the straight line rotates around the rotation axis
29
of the toothed gear
21
, so the direction of scanning irradiation also rotates. Therefore, the entire region within a circle
19
on the irradiated object
15
is irradiated by the charged particle beam. The radius of the circle can be changed by varying the magnitude of the current flowing through the coils
1
a
and
1
b.
The charged particle beam irradiator according to the prior art has several problems. Since the magnetic pole
3
a
, the magnetic pole
3
b
, and the return yoke
5
are a solid unit in the magnetic field generator, to change the direction of scanning irradiation, all of the magnetic pole
3
a
, the magnetic pole
3
b
, and the return yoke
5
must be entirely rotated. However, in using the charged particle beam irradiator as a medical treatment appliance for treating a deep tumor, for example, it is necessary to irradiate the tumor with a heavy charged particle beam, such as a proton beam, a carbon beam, etc., having a high energy (250 MeV-400 MeV per nucleon). In that case, the total weight of the magnetic field generator
10
amounts to several tons.
Accordingly, in the construction according to the prior art, in rotating the magnetic pole pair comprising the magnetic poles
3
a
and
3
b
, it is necessary to rotate the return yoke
5
at the same time, together with the magnetic pole pair, which means that a load on the motor
11
is very large. Further, since a large torque motor
11
is required, it is difficult to rotate the magnetic pole pair at a high speed with high precision. Therefore, it takes a very long time to irradiate all of the area within the circle
19
.
SUMMARY OF THE INVENTION
An object of the present invention is to provide a magnetic field generator, using a motor with a small torque and varying a magnetic field at a high speed with high precision, and a charged particle beam irradiator, shortening irradiation time at a region and using the magnetic field generator.
A magnetic field generator according to the invention comprises a first return yoke having a first internal volume; a magnetic pole pair comprising a pair of magnetic poles disposed opposite each other, disposed in the first internal volume, and movable relative to said first return yoke; and a driver for moving said magnetic pole pair within the first internal volume.
A charged particle beam irradiator according to the invention comprises a charged particle beam generator for generating a charged particle beam; and a magnetic field generator for deflecting the charged particle beam to adjust a position on an irradiated object irradiated by the charged particle beam, wherein said magnetic field generator includes a first return yoke having a first internal volume; a magnetic pole pair comprising a pair of magnetic poles disposed opposite each other, in the first internal volume, and movable relative to said first return yoke; and a driver for moving said magnetic pole pair within the first internal volume.
A charged particle beam irradiator according to the invention includes a charged particle beam generator for generating a charged particle beam, and a magnetic field generator for deflecting the charged particle beam to adjust a position on an irradiated object irradiated by the charged particle beam, herein said magnetic field generator comprises a first magnetic field generator for deflecting the charged particle beam, and a second magnetic field generator for deflecting the charged particle beam deflected by the first m

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