Posted on 15 October 2014
France’s national metrology lab for ionizing radiation, LNHB (Laboratoire national Henri-Becquerel), has designed and built a new graphite calorimeter, the GR11. The device will be used to measure the dose delivered to patients during radiotherapy treatments with small field sizes (less than 1 cm in diameter).
This primary instrument is equipped with a 3-cm-diameter core whose mass and thermal leakage have been rigorously verified. The GR11 has also undergone lab testing with larger beams (2×2 cm2 and 4×4 cm2) to compare its performance to that of other graphite calorimeters developed by LNHB. The new calorimeter was then tested with mini-beams (0.75 cm in diameter) provided by a medical accelerator. For a five-minute irradiation, the calorimeter was able to measure a temperature rise of 0.6 mK, demonstrating its very high sensitivity.
The instrument can measure the energy deposited in a dosimeter whose surface is larger than the very-small-section radiotherapy beam. This measurement unit is called the dose area product (DAP). It is useful in that it gets around the challenges of mini-beams, whose increasingly small sizes make it impossible to use small dosimeters completely irradiated by the beam.
