Room: AAPM ePoster Library
Purpose: The response of synthetic diamond detectors in external radiotherapy beams has been investigated extensively using Monte Carlo (MC) methods, but there is no consensus on which mass density (?) and mean excitation energy (I-value) should be adopted for mass electronic stopping power (S_el/?) calculations and MC simulations. The present study investigated the change in energy imparted (e) in diamond having different I-value in various beams.
Methods: The PENELOPE MC code was used to calculate e in diamond (r=1.1 mm, t=1 µm (mimicking a commercial diamond detector), 10 µm and 30 µm) with ?=3.52 g/cm³ but I = 81 eV or 88 eV. The investigated beams include diagnostic x-rays (25 - 180 kV), brachytherapy sources (¹°6Ru, ¹²5I, ¹6?Yb, ¹?²Ir), 6°Co, 6 MV photon beam (0.5x0.5 cm², 10x10 cm² fields). The statistical uncertainties were 0.2-0.3% (k=2). Additionally, S_el/? and the density-effect correction term (d) of diamond were calculated assuming ?=3.52 g/cm³ and I=88 eV.
Results: For kV and BT beams, e decreased by 1% in t=1 µm diamond (I=88 eV); no significant change for other thicknesses. For the ¹°6Ru ß-source, 6°Co, and 6 MV (both field sizes), e decreased by 0.5-0.7% regardless of the cavity thickness. S_el/? decreased by down to 1.8% for 10 keV electrons. The change in d was largest below 1 MeV where its contribution to S_el/? is few percent. Above 1 MeV, the difference reduced from 10 to 1% at 12 MeV.
Conclusion: Using the I-value of 88 eV estimated from the diamond energy-loss function instead of I = 81 eV for graphite would not affect the current small-field output correction factors. The impact on other beam qualities is close to negligible. Nevertheless, the use of diamond material properties is important when analyzing diamond detector response, and a distinction between diamond and graphite should be made.