Fifty-sixth annual meeting of the American association of physicists in medicine
SU-E-T-408: Determination of KQ,Q0-Factors From Water and Graphite Calorimetry in a 60 MeV Proton Beam
To reduce the uncertainty of the beam quality correction factor k<sub>Q,Q0</sub>, for scattered proton beams (SPB). This factor is used in dosimetry protocols, to determine absorbed dose-to-water with ionization chambers. For the Roos plane parallel chambers (RPPICs), the IAEA TRS-398 protocol estimates k<sub>Q,Q0</sub>-factor to be 1.004(for a beam quality Rres=2 g.cm2), with an uncertainty of 2.1%.
A graphite calorimeter (GCal), a water calorimeter (WCal) and RPPICs were exposed, in a single experiment, to a 60 MeV non-modulated SPB. RPPICs were calibrated in terms of absorbed dose-to-water in a 20 MeV electron beam. The calibration coefficient is traceable to NPL's absorbed dose standards. Chamber measurements were corrected for environmental conditions, recombination and polarity. The WCal corrections include heat loss, heat defect and vessel perturbation. The GCal corrections include heat loss and absorbed dose conversion. Except for heat loss correction and its uncertainty in the WCal system, all major corrections were included in the analysis. Other minor corrections, such as beam profile non-uniformity, are still to be evaluated. Experimental k<sub>Q,Q0</sub>-factors were derived by comparing the results obtained with both calorimeters and ionometry.
The absorbed dose-to-water from both calorimeters was found to be within 1.3% with an uncertainty of 1.2%. k<sub>Q,Q0</sub>-factor for a RPPIC was found to be 0.998 and 1.011, with a standard uncertainty of 1.4% and 0.9% when the dose is based on the GCal and the WCal, respectively.
Results suggest the possibility to determine k<sub>Q,Q0</sub>-values for PPICs in SPB with a lower uncertainty than specified in the TRS-398 thereby helping to reduce uncertainty on absorbed dose-to-water. The agreement between calorimeters confirms the possibility to use GCal or WCal as primary standard in SPB. Because of the dose conversion, the use of GCal may lead to slightly higher uncertainty, but is, at present, considerably easier to operate.