Construction of a large graphite calorimeter for measurements in small fields used in radiotherapy

As the national metrology institute for ionization radiation the LNE-LNHB develops, maintains and transfers dosimetric references for high energy x-ray radiotherapy beams. Through the European project MetrexRT, a new approach has recently been developed to extend the dosimetric references to small fields (less than 2 cm wide). This method is based on the measurement of an absorbed dose on a surface larger than the beam. A new primary dosimeter (a graphite calorimeter) with a large surface has been built and characterized as well as a large surface ionization chamber. Both dosimeters can now be considered operational for measurements in small beams.