Electric lamp and discharge devices: systems – High energy particle accelerator tube – Magnetic field acceleration means
Reexamination Certificate
2001-02-26
2002-10-29
Anderson, Bruce (Department: 2881)
Electric lamp and discharge devices: systems
High energy particle accelerator tube
Magnetic field acceleration means
C315S507000, C250S492300, C250S505100, C250S3960ML
Reexamination Certificate
active
06472834
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to an accelerator for accelerating charged-particle beam and producing the beam to be used, a method of producing the beam, and a medical system using the beam.
A conventional accelerator system and method of producing the charged particle beam from the accelerator system are described in JP No. 2,596,292.
As in the publication Ser. No. 2,596,292, the charged particle beam from a preaccelerator is made incident to the following-stage accelerator. The following-stage accelerator accelerates the charged particle beam up to the energy to be necessary for treatment, and produces the beam. The charged particles circulate while vibrating left and right or up and down. There are called betatron oscillations. The number of vibrations per orbit of the betatron oscillation is called tune. Two four-pole electromagnets for convergence and for divergence are used, making the tune close to an integer+⅓ or an integer+⅔ or an integer+½. At the same time, a multiple-pole electromagnet for causing resonance provided on the circular orbit is excited, thereby suddenly increasing the amplitude of the betatron oscillations of the charged particles having more than a certain betatron oscillation amplitude, of a large number of the charged particles that go round. This sudden amplitude increase phenomenon is called resonance of betatron oscillation. The threshold of the amplitude of the betatron oscillations at which the resonance occurs is called stability limit, the value of which changes depending on the relation between the intensities of the resonance generating multi-pole magnetic field and the four-pole magnetic field. The resonance caused when the tune made close to an integer+½ is called second order resonance, and the resonance when the tune made close to an integer+⅓ or+⅔ is called third order resonance. A description will hereinafter be made of a case in which the tune is made close to an integer+⅓ at the third order resonance. The value of the stability limit of resonance decreases as the deviation of tune from an integer+⅓ diminishes. Thus, in the prior art, while the intensity of the resonance generating multi-pole electromagnet is kept constant, the tune is first approached to an integer+⅓, and made constant, namely, the field intensity of the four-pole magnet is maintained constant as well as the intensities of the deflecting electromagnet and resonance generating multi-pole electromagnet are kept constant. Then, a high-frequency electromagnetic field having a plurality of different frequency components or a frequency band is applied to the beam, increasing the betatron oscillation amplitude to generate resonance. The beam is produced from the extracting deflector by making use of the increase of betatron oscillation due to the resonance. The extracted ion beam is transported by use of an electromagnet of an ion beam transport system to a treatment room.
An extracting-purpose high-frequency source used in the conventional accelerator is described in JP-A-7-14,699. The charged particle beam has its tune changed depending on the betatron oscillation amplitude under the action of the resonance generating multi-pole electromagnet. Therefore, the high frequency for beam extraction is required to have a frequency band, or a plurality of different frequency components. In the prior art, such high frequencies, are applied to the charged particle beam, as to have a frequency band of about several tens of kHz including the product of the tune's decimal fraction and revolution frequency of the charged particle beam extracted from the cyclic type accelerator.
The charged particle beam emitted from the accelerator, as described in JP-A-10-118,204, is transported to a treatment room where an irradiator for treatment is provided. The irradiator has a scatterer for increasing the beam diameter, and a beam scanning magnet for making the diameter-increased beam circularly scan. The circular scanning of the beam increased in its diameter by this scatterer acts to flatten the integrated beam intensity inside the locus of the scanning beam center. The beam with the intensity distribution flattened is made coincident in its shape with the diseased part by a patient collimator before being irradiated on the patient.
In addition, though different from the above, a small-diameter beam may be used and scanned for its shape to comply with the diseased part by use of the beam scanning electromagnet. In this small-diameter beam scanning method, the current to the beam scanning electromagnet is controlled to irradiate the beam at a predetermined position. The high frequencies are stopped from being applied to the beam after confirming the application of a certain amount of irradiation by a beam intensity monitor, thus the beam being stopped from emission. After the stopping of beam irradiation, the current to the beam scanning electromagnet is changed to change the irradiation position, and the beam is again irradiated in a repeating manner.
Thus, in the conventional medical accelerator system, before being irradiated, the beam is increased in its diameter by the scatterer and circularly deflected to scan so that the integrated intensity distribution in the region inside the scan circle can be flattened. In this beam scanning irradiation, to flatten the intensity distribution, it is desired to reduce the change of the beam intensity, and particularly to decrease the frequency components ranging from about tens of Hz to tens of kHz. However, in the conventional medical accelerator system, since the high frequencies to be applied to the charged particle beam have a frequency band, or a plurality of different frequencies for the emission, the beam emitted from the accelerator has frequency components ranging from about tens of Hz to tens of kHz, and the intensity thereof is changed with lapse of time. Therefore, in order to obtain a uniform irradiation intensity distribution, it is necessary to properly select the circular scanning speed according to the change of beam intensity with time, or to flatten the irradiation intensity distribution by selecting a scanning frequency deviated from the frequency of the beam intensity change. The beam intensity change problem can be solved by much increasing the circular scanning frequency, but the cost of the scanning electromagnets and power supply is greatly increased. Moreover, when the beam intensity change with time is great, the conditions such as reproducibility and stability of the current to the scanning electromagnet, which are necessary to suppress the change of the irradiation field intensity distribution to within an allowable range, are severer than in the case where the beam intensity change with time is small.
Moreover, in the prior art, even though the canning beam diameter is large or small, the beam intensity change with time makes it necessary to increase the time resolution of the beam intensity monitor to confirm a predetermined irradiation intensity distribution.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the invention to provide an accelerator capable of suppressing the change of the emitted beam current of, particularly, frequencies from about tens of Hz to tens of kHz, a medical accelerator system using that accelerator and a method of operating the system.
According to one aspect of the invention to achieve the above object, there is provided a circular type accelerator having deflecting electromagnets and four-pole electromagnets for making a charged particle beam circulate, a multi-pole electromagnet for generating a stability limit of resonance of betatron oscillation in order to produce the charged particle beam, and a high-frequency source for applying a high-frequency electromagnetic field to the charged particle beam to move the charged particle beam to the outside of the stability limit and thereby to excite resonance in the betatron oscillation, characterize
Hiramoto Kazuo
Nishiuchi Hideaki
Mattingly Stanger & Malur, P.C.
Wells Nikita
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