(a) Drawing of the large volume ionization chambers designed and built for use at the University of Pennsylvania’s Roberts Proton Therapy Center. (b) Image of the assembled Mg-Ar (top) and TE-TE (bottom) ionization chambers.
Experimental setup (not to scale) showing the measurement points 10 cm downstream from the distal side and 50 cm downstream of the proximal side of either the closed MLC (a) or a 6.5 cm thick brass block placed downstream of the MLC with leaves open to form an 8.7 cm diameter circle (b).
Lateral beam profiles, taken with radiochromic film, along the x- and y-axis for 0 cm (a) and 32 cm (b) depth in solid water plastic, where the 32 cm measurement is at isocenter. The fractional depth dose profile (c) in water measured with the PTW 8.4 cm diameter plane parallel chamber.
Simulated neutron fluence spectra through a 1 cm diameter sphere placed 10 cm from the distal edge of the closed MLC or the brass block (a), and 50 cm from the proximal edge of the MLC or the brass block (b).
Relative chamber sensitivity as a function of incident neutron energy for the large volume Mg-Ar and TE-TE chamber. The sensitivities are calculated with the Geant4 (Ref. 42) Monte Carlo toolkit using detailed geometrical models of the chambers.
The dose to water calibration factors, NDW , for the chambers used in the measurements.
Relative p + γ and neutron sensitivities of the Mg-Ar chamber (hU and kU ) and the TE-TE chamber (hT and kT ), dosimeter response (R’ U and R’ T ), neutron (Dn ) and p + γ (Dp + γ ) absorbed doses to water per Gy of absorbed dose to water delivered at isocenter by a pristine 230 MeV proton beam. Relative uncertainties of approximately 5% for p + γ sensitivities (hU and hT ), 20% for kU , and 6% for kT are inserted into Eqs. (6) and (7) to determine the relative uncertainty of the absorbed dose calculations. The materials used in this experiment are an MLC with tungsten alloy leaves and a 6.5 cm thick brass block used for custom-milled proton collimators. Measurements are at isocenter 10 cm from the distal edge and 50 cm from the proximal edge of the collimating material.
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