Electric lamp and discharge devices: systems – Discharge device load with fluent material supply to the... – Electron or ion source
Reexamination Certificate
2001-11-15
2002-08-20
Wong, Don (Department: 2821)
Electric lamp and discharge devices: systems
Discharge device load with fluent material supply to the...
Electron or ion source
C250S283000, C250S385100, C250S492300
Reexamination Certificate
active
06437513
ABSTRACT:
TECHNICAL FIELD
The invention relates to an ionisation chamber for ion beams and to a method of monitoring the intensity of an ion beam according to the preamble of claims
1
and
16
.
BACKGROUND OF THE INVENTION
Such ionisation chambers are also known in the prior art as ion counting tubes. The chamber housing is customarily made from a tube, with one of the two end portions of the tube serving as the beam inlet window and the other end of the tube serving as the beam outlet window. The tube is filled with a counting gas under reduced pressure and has a cylindrical high-voltage cathode that lies coaxially in the counting tube, insulated from the tube wall. Located in the centre of the tube is a cylindrical high-voltage anode which is insulated from the high-voltage cathode and the surrounding tube. In order to operate the ionisation chamber, a voltage is applied between the high-voltage cathode and the high-voltage anode and the current between the cathode and anode is measured. If charged particles, such as ions, are passed through the ionisation chamber or are captured by the ionisation chamber, the current between the cathode and the anode increases in dependence upon the number of ions that pass through the ionisation chamber. More complex cylindrical ionisation chambers have a large number of axially aligned anodes in order, for example, to measure the path of a charged particle or ion through a tubular ionisation chamber by means of anodes distributed axially over the cross-section.
A disadvantage of such cylindrical ionisation chambers is their large axial dimension and the relatively complex construction of the counting anodes. Moreover, the space required by such ionisation chambers in the direction of the diffusion of a beam is relatively large. The space available at the beam outlet in a treatment room in front of patients is, however, very limited. Moreover, with therapy systems in which an ion beam is scanned over the entire extent of a tumour tissue, there must be available an ionisation chamber of hitherto unknown dimensions in terms of breadth and length. Generally, all the measurements in the beam must be carried out in front of the patient in the transmission mode. It is imperative to avoid impairment of the quality of the beam, for example as a result of projectile fragmentation and angular scatter of the beam particles.
SUMMARY OF THE INVENTION
The problem underlying the invention is accordingly to provide an ionisation chamber for ion beams and a method of monitoring the intensity of an ion therapy beam that overcomes the disadvantages of existing ionisation chambers, is suitable for monitoring and controlling patient irradiation in the context of tumour therapy with heavy ions that are concentrated with high energy into a pencil beam, in which the dimensions of the detector in the direction of the beam are small, that enables a high level of safety to be achieved, especially in respect of plasma and spark formation, and can be used in the field of medicine.
For that purpose, the ionisation chamber for ion beams consists of a chamber housing, a beam inlet window and a beam outlet window, a chamber volume filled with counting gas and a high-voltage anode and a high-voltage cathode. The ionisation chamber is constructed flat and sandwich-like from plate-shaped large-surface-area structures of those components, which are aligned orthogonally relative to the axis of the ion beam. A centrally arranged large-surface-area orthogonally aligned plate-shaped counting anode is surrounded on both sides by a large-surface-area plate-shaped high-voltage cathode consisting of two parallel cathode plates. The chamber housing consists of a housing frame which frames a virtually square ionisation chamber volume, and on which frame the beam inlet window and beam outlet window are mounted gas-impermeably. Such a device has the advantage of being easier to maintain since the plate-shaped construction can be removed from the housing frame and replaced by simply dismantling or removing different plate structures. The plates can be replaced easily and suitable numbers thereof can be held in stock. The plate-shaped structure also enables mass production of spare parts and finished ionisation chambers.
In a preferred embodiment of the invention, the counting gas is a gas mixture of argon or krypton and carbon dioxide, preferably having a gas volume mixing ratio of 4:1, which is adapted to the energy and intensity of the ion beam, and is introduced into the ionisation chamber. A counting gas of such composition has the advantage, compared with customary air-filled cylindrical ionisation chambers, that the measurements are easier to reproduce since in this case air humidity does not influence the sensitivity of the ionisation chamber. Such a counting gas ensures a good signal
oise ratio and makes available a high dynamic range in the particle rates. With the preferred counting gas, sufficient dielectric strength is also ensured.
Such a counting gas is preferably of the highest purity since the signal sensitivity, especially in its amplitude and waveform, is impaired by impurities. Moreover, inside the chamber volume there are preferably used for the individual plate elements and other supporting and insulating elements as well as for auxiliary units and sensors materials that do not release gases, or elements and components that do release gases are cast in epoxy resin.
In a further preferred embodiment, the ionisation chamber has sensors that are mounted in the housing frame in through openings that are gas-impermeably sealed and that measure the counting gas pressure and the counting gas temperature. The ionisation chamber is operated with slightly elevated pressure compared with the ambient air, which advantageously makes penetration of extraneous gases more difficult. For that purpose, the extent of the gas reflux from the chambers is monitored by a sensor system in the counting gas outlet region or in the outlet. By measuring gas pressure and gas temperature, it is advantageously possible to monitor the gas density and, if necessary, keep it constant, the gas density being used directly in the determination of the ion beam particle number.
The beam inlet window and the beam outlet window, which are substantially square, preferably consist of radiation-resistant non-polarisable plastics films. These are secured to metal plate-shaped frames, which in turn seal off the ionisation chamber volume gas-impermeably from the beam inlet window and from the beam outlet window by means of O-ring seals in the housing frame. That gas-impermeable construction keeps impurities away from the counting gas and gas exchange of the chamber volume with the environment by diffusion is minimised even when the ionisation chambers are not in operation.
The beam inlet window and the beam outlet window preferably comprise polyimide or polyester films, which has the advantage that exclusively radiation-resistant and non-polarisable materials are exposed to the ion beam, so minimising the effect on the ion beam and the ion beam intensity.
In a further preferred embodiment of the invention, the beam inlet window and the beam outlet window are metallised on the side facing the ionisation chamber volume. Such metallisation of the beam inlet window and the beam outlet window prevents the windows from becoming charged and thus prevents falsification of the measurement values, since charges can be conducted away to the ionisation chamber housing directly by way of the metallisation of the windows and by way of the window frames. The ionisation chamber housing is thus advantageously earthed.
Such metallisation can be achieved by aluminising or nickel-plating one of the sides of the beam inlet window or beam outlet window. Such aluminised films have a conductive layer in order to avoid high electrical field densities as trigger points for gaseous discharges, so that the occurrence of gaseous discharges is minimised. Moreover, metallised films form a smooth surface which also serves to prevent high electrical fie
Stelzer Herbert
Voss Bernd
Frommer Lawrence & Haug
Gesellschaft fuer Schwerionenforschung mbH
Santucci Ronald R.
Tran Thuy Vinh
Wong Don
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