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Radiation Standards for Absorbed Dose to Water in Photon and Electron Beams

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Radiation Standards for Absorbed Dose to Water in Photon and Electron Beams

A 3-D scanner is used to measure the absorbed dose distribution in water prior to establishing the dose absolutely using water calorimetry. The detector, a small ionization chamber, is being aligned with the central axis of the radiation field by Michel Lalonde,a co-op student working with the IRS group. The high-energy x-ray beams are generated using an Elekta clinical accelerator, which produces radiation beams similar to those, used for radiation therapy.

Accurate measurement of the absorbed dose to water in radiation beams is a critical component of cancer radiotherapy. The IRS group has devoted considerable effort to the development of water calorimetry as a standard for the absorbed dose to water. This work led to the development of our 60Co absorbed dose calibration service, which is used by most Canadian radiation therapy centres.

During the past year, we have used our water calorimeter to establish the absorbed dose in the x-ray fields provided by our new Elekta accelerator. An intercomparison is underway to compare our work with that of several European laboratories. We intend to offer Canadian users calibration services in high-energy x-ray beams, a service similar to those offered by several European standards laboratories.

Radiation-induced chemical reactions can affect the response of the calorimeter by several percent. In the case of pure water or certain well-defined aqueous systems, the radiation chemistry can be accurately modeled. However, unknown impurities can also affect the response, and it is therefore important to control the water purity. We constructed a vessel in which the only material contacting the water is glass. Initial measurements indicate that this system's response remains stable over long periods of time.

Accurate measurement of the absorbed dose to water in radiation beams is a critical component of cancer radiotherapy. The IRS group has devoted considerable effort to the development of water calorimetry as a standard for the absorbed dose to water. This work led to the development of our 60Co absorbed dose calibration service, which is used by most Canadian radiation therapy centres.

During the past year, we have used our water calorimeter to establish the absorbed dose in the x-ray fields provided by our new Elekta accelerator. An intercomparison is underway to compare our work with that of several European laboratories. We intend to offer Canadian users calibration services in high-energy x-ray beams, a service similar to those offered by several European standards laboratories.

Radiation-induced chemical reactions can affect the response of the calorimeter by several percent. In the case of pure water or certain well-defined aqueous systems, the radiation chemistry can be accurately modeled. However, unknown impurities can also affect the response, and it is therefore important to control the water purity. We constructed a vessel in which the only material contacting the water is glass. Initial measurements indicate that this system's response remains stable over long periods of time.