A precise knowledge of the ionising radiation dose is vital in the field of medical radiotherapy. METAS calibrates measurement instruments for radiotherapy and nuclear medicine, thereby contributing to the safety of patients and hospital personnel. The Ordinance of the FDJP on the measurement equipment for ionising radiation (StMmV) regulates the marketing and the utilisation of this equipment in the domains of radiation protection and medicine.
The measuring equipment requires pattern approval and initial verification. To maintain the measurement stability periodic verifications are also mandatory.
METAS and two verification laboratories authorized by him, the Paul Scherrer Institute (PSI) in Villigen, and the Institute of Radiation Physics (IRA) in Lausanne, provide the services required by the ordinance.
METAS checks dosimeters of high energy radiotherapy facilities. METAS has an accelerator for high-energy electron and photons beams and a 60Co irradiation unit.
The dose is disseminated the SI unit Gray (Gy = J / kg).
METAS is also responsible for the verification of activimeters used in nuclear medicine for the dosage of radiopharma-ceuticals.
The activity is disseminated in the SI unit Becquerel (Bq = 1/s).
To ensure traceability of dose measurements at the Swiss level and remain at the forefront in its field of competence, the laboratory "Radiotherapy" develops and maintains two primary standards for the realization of the absorbed dose to water: water calorimeter and Fricke dosimeter.
The technical field "Radiotherapy" has an electron accelerator and a 60Co irradiation unit. The electron accelerator allows to generate high-energy electron and photon beams of variable radiation qualities (energies).
To determine the calibration coefficient for absorbed dose to water of a dosimeter belonging to a client, the dosimeter is compared with a secondary standard ionisation chamber in a water phantom under well defined conditions. The secondary standard ionization chambers are calibrated against the primary standards.
METAS offers the following verifications:
• Absorbed dose to water for photon radiation: tissue phantom ratio TPR2010 between 0.639 and 0.802
(4 MV to 21 MV)
• Absorbed dose to water for electron radiation: half-value depth R50 between 1.75 g/cm2 and 8.54 g/cm2 (5 MeV to 22 MeV)
• Absorbed dose to water for 60Co radiation
• Absorbed dose to water for soft X-rays up to 100 kV
• Air kerma for 60Co radiation
• Air kerma for soft X-rays up to 100 kV
• Verification of activimeters
The mandated verification laboratories offer the following verifications:
• Radiotherapy dosimeters for X-rays from 150 kV to 300 kV (IRA)
• Radiology dosimeters (IRA)
• Surface contamination monitors (PSI, IRA)
• Measuring devices for ambient dose rate (PSI, IRA)
• Deep well ionization chambers (IRA)
• Measuring devices for radon gas (PSI)
Research and development
Primary standard: water calorimeter
With the water calorimeter, the absorbed dose in water Dw is determined by the highly accurate measurement of a temperature difference. The temperature increase induced by the absorbed dose of radiation is measured by very high sensitive thermometers in a closed glass vessel that is filled with ultra-pure water at 4 ° C, the temperature for optimum thermal stability. This primary standard is primarily used for radiation of high energy photons.
Primary standard: Fricke dosimetry
The Fricke dosimeter is a chemical type dosimeter, using the oxidant effect of ionizing radiation on a solution containing Fe2+. The dose of radiation absorbed in a solution of Fricke (iron(II)-ammonium-sulfate) converts the Fe2+ ions to Fe3+ ions, their concentration being measured by UV absorption spectrometry. The Fricke dosimeter is particularly suited to the primary standard role for electron radiation.
The primary standards are checked regularly through international comparison measurements.
Development of a new protocol for dose-area-product dosimetry for scanning proton beams using plane-parallel ionization chambers, in collaboration with the Paul Scherrer Institute (PSI).
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