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Reference Dosimetry for An 80 MeV/n Carbon Ion Beam Based On Graphite Calorimetry

S Rossomme

S Rossomme1*, H Palmans2, R Thomas2, N Lee2, D Shipley2, M Bailey2, S Duane2, D Bertrand3, S Vynckier4, (1) Universite catholique de Louvain, Brussels, Belgium, (2) National Physical Laboratory, Teddington ,UK, (3) R&D IBA, Louvain-la-Neuve, Belgium, (4) Cliniques Universitaires St. Luc, Brussels, Belgium

SU-C-137-5 Sunday 1:00PM - 1:55PM Room: 137

Comparison between the response of a primary standard graphite calorimeter and the response of an ionization chamber (IC) allows the estimations of values of the beam quality correction factor, kQ,Q0, for the IC. This factor is used in the IAEA TRS-398 dosimetric protocol to derive the absorbed dose-to-water using an IC. It depends on three parameters, one of which is the average energy required to produce an ion pair in dry air (Wair). The aim of this work is the experimental determination of the Wair-value for an 80 MeV/A carbon ion beam.

The dose measured with a graphite calorimeter, after correction for heat transfer between different parts of the calorimeter, must be converted to dose-to-water. This conversion needs knowledge of the water-to-graphite stopping power ratio and the fluence correction factor. These have been determined experimentally and numerically using Geant4 Monte Carlo simulations. In addition to these calorimetric corrections, the determination of kQ,Q0-values needs the IC reading to be corrected for ion recombination.

The water-to-graphite stopping power ratio has been determined numerically to be 1.115 while the fluence correction factor has been determined experimentally and numerically to vary linearly between 0.9996 at the phantom surface and 1.0063 before the Bragg peak. The analysis of the ion recombination correction measurements is still ongoing. Two experimental campaigns with the graphite calorimeter and several IC's have been performed. Based on recommended data from TRS-398, we derived preliminary Wair-values of 36. J/C and 34.85 J/C for each campaign. These results are under investigation.

These results demonstrate the feasibility of graphite calorimetry in an 80 MeV/A carbon ion beam and are encouraging for our future work: a direct comparison between water and graphite calorimetry in a clinical carbon ion beam.

Funding Support, Disclosures, and Conflict of Interest: Funding support for Severine Rossomme: BioWin program of the Walloon Government (Belgium); Funding support for NPL people: Acoustics and Ionising Radiation Metrology Programme of the National Measurement System (UK)

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