TABLE 1. Mass density and compostion of the materials used in the Monte Carlo simulations Mass density (g/cm3) Composition (element/weight fraction) TABLE 2. Two dimensional geometry function () for GZP6 source number 3 Material: description Cobalt: source core Nickel: source plating Titanium: source capsule Steel pellets: spacers in the source braid Steel: spring cover Air Water: phantom material r(cm) 0.5 0.75 1.5 1 2 3 4 2.5 3.5 4.5 5 0° 1.037 1.016 1.009 1.004 1.002 1.001 1.001 1.001 1.001 1.000 1.000 10° 1.036 1.016 1.009 1.004 1.002 1.001 1.001 1.001 1.001 1.000 1.000 8.85 8.902 4.54 7.9 6.999 0.001205 1 20° 1.031 1.014 1.008 1.003 1.002 1.001 1.001 1.001 1.000 1.000 1.000 The radial dose function, g(r), and anisotropy function, F(r, θ) are defined as: In which denotes the dose rate at the point P(r, θ) from the source. Monte Carlo calculations The MCNP-4C Monte Carlo code was employed to estimate absorbed dose towards obtaining TG- 43U1 parameters.18 Since it has the option of de- fining the mesh grids, the MCNPX version 2.4.0 Monte Carlo code was also utilized to obtain 2D 30° 1.024 1.011 1.006 1.003 1.002 1.001 1.001 1.000 1.000 1.000 1.000 [6] [7] Toossi MTB et al. / GZP6-TG43 173 Co/1 Ni/1 Ti/1 Fe/0.71994, C/0.0005, Si/0.0072, Mn/0.0137, S/0.00011, P/0.00025, Cr/0.17, Ni/0.0822, Mo/0.0013, V/0.0006, Ti/0.0042 Fe/0.7416, Ni/0.069, S/0.0001, Cr/0.167, C/0.0006, Mn/0.0062, Cu/0.0026, Al/0.0062, Mo/0.0015, Si/0.0052 C/0.000124, N/0.7555267, O/0.231781, Ar/0.012827 H/0.111894, O/0.888106 θ (°) 40° 1.016 1.007 1.004 1.002 1.001 1.001 1.000 1.000 1.000 1.000 1.000 50° 1.008 1.003 1.002 1.001 1.000 1.000 1.000 1.000 1.000 1.000 1.000 60° 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 1.000 70° 0.994 0.997 0.998 0.999 1.000 1.000 1.000 1.000 1.000 1.000 1.000 80° 0.990 0.995 0.997 0.999 0.999 1.000 1.000 1.000 1.000 1.000 1.000 90° 0.988 0.995 0.997 0.999 0.999 1.000 1.000 1.000 1.000 1.000 1.000 dose distribution for the source.19 When this option is used it would be easier to score tally values in a large number of scoring volumes. Table 1 lists the mass density and chemical composition of the ma- terials used in our Monte Carlo calculations. In calculations of dose rate constant and 2D dose distributions, the value of air kerma strength was taken from our previous work.20 The source braid was centered in a cylindrical wa- ter of dimensions R=25 cm and L=50 cm in the simu- lations. A cutoff energy of 10 keV was used for both photons and electrons. To speed up the Monte Carlo calculations, the absorbed dose was approximated as kerma to estimate dose at points where the elec- tronic equilibrium exists. For a 60Co source, the dose build up region affects only points in close vicinity from the 60Co pellet. The f6 tally was used to score collision kerma (in MeV/g/photon). At the points in the vicinity of the source, in which the electronic equilibrium may not exist, dose was scored using *f8 Radiol Oncol 2012; 46(2): 170-178.