Ion chamber for radiation measurement

1. An ion chamber comprising:

a chamber having an interior volume;

a first electrode and a second electrode in the chamber and separated by a gap; and

a collector electrode positioned between the first electrode and the second electrode, the collector electrode shaped to occlude a portion of the first electrode from the second electrode.

2. An ion chamber as in claim 1 , further comprising an entrance window with a thickness sufficient to stop nearly all electrons from an upstream linear accelerator.

3. An ion chamber as in claim 1 , wherein the first electrode and the second electrode are plates.

4. An ion chamber as in claim 1 , further comprising a plurality of collector electrodes positioned in a symmetrical pattern around an axis of the ion chamber.

5. An ion chamber as in claim 4 , wherein the plurality of collector electrodes comprises four annular segments positioned in a single collecting plane covering an annular area parallel to the first electrode and the second electrode.

6. An ion chamber as in claim 4 , wherein the plurality of collector electrodes further comprise an annular ring electrode positioned to cover an annular area parallel to the first electrode and the second electrode.

7. An ion chamber as in claim 4 , wherein the plurality of collector electrodes are rectangular plates.

8. An ion chamber as in claim 4 , wherein the plurality of collector electrodes further comprise four wedge plates positioned in a single collecting plane covering a circular area centered on an axis of the ion chamber, wherein the four wedge plates are separated from each other by gaps with an area substantially smaller than the circular area.

9. A method comprising:

filling, with a gas, an interior volume of an ion chamber comprising:

a first electrode and a second electrode separated by a gap; and

a collector electrode positioned between the first electrode and the second electrode, the collector electrode shaped to occlude a portion of the first electrode from the second electrode;

measuring, at a current detector, a first current received from the collector electrode, the first current representative of a first charge generated by collisions between a radiation beam and the gas in the ion chamber, the first charge corresponding to a first portion of the radiation beam aligned with the collector electrode; and

generating, by at least one programmable processor, a radiation beam profile based on the first current and a shape and a position of the collector electrode.

10. A method as in claim 9 , further comprising:

measuring, at the current detector, a second current received from the first electrode and the second electrode, the second current representative of a second charge generated by collisions between the beam and the gas in the ion chamber, the second charge corresponding to a second portion of the radiation beam that is not aligned with the collector electrode; and

determining, by the at least one programmable processor, a radiation beam profile normalization based on the second current; and

generating, by the at least on programmable processor, a normalized radiation beam flux based on the first current and the normalization.

11. A method as in claim 9 , further comprising:

measuring, at the current detector, a plurality of currents each corresponding to a plurality of collector electrodes positioned in a symmetrical pattern around an axis of the ion chamber; and

generating, by the at least one programmable processor, a second beam profile based on a plurality of currents received from the plurality of collector electrodes, shapes of the plurality of collector electrodes, and positions of the plurality of collector electrodes.

12. A method as in claim 9 , further comprising:

measuring, at the current detector, a plurality of currents each corresponding to a plurality of collector electrodes positioned in a symmetrical pattern around an axis of the ion chamber;

calculating, based on the plurality of currents, a ratio of currents received at the plurality of collector electrodes; and

calculating, based on the ratio of currents, a radiation beam energy.

13. A method as in claim 12 , further comprising:

monitoring, with the ion chamber, the radiation beam energy during operation of a linear accelerator that generates a radiation beam through the ion chamber; and

ceasing operation of the linear accelerator when the radiation beam energy is outside of an allowable range.

14. A method as in claim 9 , further comprising:

applying an electric field or a magnetic field separate from a main MRI magnetic field, based on the beam profile to move the beam profile to align the radiation beam with an axis of the ion chamber.

15. A computer program product comprising a non-transient, machine-readable medium storing instructions which, when executed by at least one programmable processor, cause the at least one programmable processor to perform operations comprising: measuring, at a current detector, a first current received from a collector electrode, positioned between a first electrode and a second electrode, the collector electrode shaped to occlude a portion of the first electrode from the second electrode, the first current representative of a first charge generated by collisions between a radiation beam and a gas in an ion chamber, the first charge corresponding to a first portion of the radiation beam aligned with the collector electrode; and generating, by the at least one programmable processor, a radiation beam profile based on the first current and a shape and a position of the collector electrode.

16. A computer program product as in claim 15 , further comprising:

measuring, at the current detector, a second current received from the first electrode and the second electrode, the second current representative of a second charge generated by collisions between the beam and the gas in the ion chamber, the second charge corresponding to a second portion of the radiation beam that is not aligned with the collector electrode; and

determining, by the at least one programmable processor, a radiation beam profile normalization based on the second current; and

generating, by the at least one programmable processor, a normalized radiation beam flux based on the first current and the normalization.

17. A computer program product as in claim 15 , further comprising:

measuring, at the current detector, a plurality of currents each corresponding to a plurality of collector electrodes positioned in a symmetrical pattern around an axis of the ion chamber; and

generating, by the at least one programmable processor, a second beam profile based on a plurality of currents received from the plurality of collector electrodes, shapes of the plurality of collector electrodes, and positions of the plurality of collector electrodes.

18. A computer program product as in claim 15 , further comprising:

measuring, at the current detector, a plurality of currents each corresponding to a plurality of collector electrodes positioned in a symmetrical pattern around an axis of the ion chamber; and

calculating, based on the plurality of currents, a ratio of currents received at the plurality of collector electrodes; and

calculating, based on the ratio of currents, a radiation beam energy.

19. A computer program product as in claim 18 , further comprising:

monitoring, with the ion chamber, the radiation beam energy during operation of a linear accelerator that generates a radiation beam directed through the ion chamber; and

ceasing operation of the linear accelerator when the radiation beam energy is outside an allowable range.

20. A computer program product as in claim 15 , further comprising:

applying an electric field or a magnetic field separate from a main MRI magnetic field, based on the radiation beam profile to, to align the radiation beam with an axis of the ion chamber.