LET dependence of GafChromic films and an ion chamber in low-energy proton dosimetry

Dosimetry using a PMMA phantom was performed in 15 and 29 MeV proton beams from the Birmingham cyclotron, with a Markus parallel-plate ionization chamber and GafChromic EBT and MD-V2-55 film. Simulations of the depth dose curves were performed with FLUKA 2008.3 and MCNPX 2.5.0, which agreed almost perfectly with each other in range and only differed by 2% in the Bragg peak (BP) region. FLUKA was also used to calculate k(Q) factors for Markus chamber measurements as an improvement to the IAEA TRS-398 values in low-energy beams. FLUKA depth-dose simulations overestimate the BP height measured by ion chamber by about 10%, where the initial proton energy spread was estimated by fitting to the slope of the measured BP distal edge. Both GafChromic films showed an under-response in the BP compared to ion chamber; however, EBT exhibits this effect at lower energies than MD-V2-55. A possible reason for this is attributed to the shape and arrangement of the monomer particles being different in the active components of EBT and MD-V2-55. Relative effectiveness (RE) of both films is presented as functions of residual range R-res in water and peak proton energy determined by FLUKA, with considerations for the spatial separation of the two active layers in each film. The proton energies at which RE reduces to 90% of maximum film response are 6.7 and 3.2 MeV for MD-V2-55 and EBT, respectively. Additionally, a beam quality correction factor (g(Q,Q0)) is suggested for both GafChromic films, involving water-to-film stopping power ratios evaluated using ICRU recommendations, and a polymer yield factor G(Q0)/G(Q). RE in this work is equated to the reciprocal of the polymer yield factor. The calculated values of (s(w,film))(Q)/(s(w,film))(Q0) are constant within 2.1% and 1.2% across the proton energy range of 1-300 MeV for EBT and MD-V2-55, respectively, so it is concluded that the polymer yield factor is the dominant factor causing the LET quenching effect.