US 4,695,731 AGranted

Ionization chamber

AssigneeU.S. Philips Corporation

InventorKeith J. Larkin

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Abstract

To enable an ionization chamber used for measuring the intensity of a beam of ionizing radiation, for example an electron beam produced by a linear accelerator and used for radiotherapy, both to give an output signal which is independent of ambient pressure and temperature and to present a low weight of scattering material per unit area to the beam, the chamber is of flexible construction so that the volume of gas in it adapts to ambient pressure and temperature, and such that the weight of gas in the active region between the electrodes per unit area remains substantially constant. Suitably, the electrodes are conductive layers on flexible plastics sheets, an outer annular portion of one sheet providing a flexible connection between two opposed chamber wall portions which remain substantially planar and parallel; the proportional change (.DELTA.V.sub.1 /V.sub.1) in a volume bounded by the opposed wall portions and including the active region equals the proportional change (.DELTA.V.sub.2 /V.sub.2) in the remainder (V.sub.2) of the internal volume.

Claims14

Dependent claims

A device for measuring intensity of a beam of ionizing radiation comprising

a closed chamber having at least one flexible wall through which ionizing radiation is passed,
an ionizable gas completely contained in said closed chamber at an ambient pressure, said gas having a total volume of V.sub.T, and
two opposing electrodes contained in said closed chamber, one of said two electrodes being disposed on said flexible wall, said two electrodes having an applied potential difference for producing an ionization current from said ionizing radiation,
said two electrodes defining an active region therebetween having a volume V.sub.A, said volume V.sub.A being less than V.sub.T, said ionization current flowing in said active region,
wherein said volume V.sub.T and said volume V.sub.A both vary with changes in ambient pressure and in ambient temperature, said changes in ambient pressure and ambient temperature producing changes .DELTA.V.sub.T and .DELTA.V.sub.A in V.sub.T and V.sub.A respectively,
wherein a proportional change .DELTA.V.sub.A /V.sub.A in said volume of said active region is substantially equal to a proportional change .DELTA.V.sub.T /V.sub.T in said total volume within operating ranges of said ambient pressure and said ambient temperature, and
wherein said ionizable gas in said active region has a weight per unit area remaining substantially constant when measured in a plane normal to a line intersecting said electrodes.

4 more elements

Dependencies are inferred from the claim text.