Dosimetric modeling of the microselectron high-dose rate Ir-192 source by the multigroup discrete ordinates method

The DANTSYS multigroup discrete ordinates computer code is applied to quant itatively estimate the absorbed dose rate distributions in the vicinity of a microSelectron Ir-192 high-dose-rate (HDR) source in two-dimensional cyli ndrical R-Z geometry. The source is modeled in a cylindrical water phantom of diameter 20 cm and height 20 cm. The results are also used for evaluatio n of the Task Group 43 (TG-43) dosimetric quantities. The DANTSYS accuracy is estimated by direct comparisons with corresponding Monte Carlo results. Our 210-group photon cross section library developed previously, together w ith angular quadratures consisting of 36 (S-16) to 210 (S-40) directions an d associated weights; per octant, are used in the DANTSYS simulations. Stro ng ray effects are observed but are significantly mitigated through the use of DANTSYS's stochastic ray-tracing first collision source algorithm. The DANTSYS simulations closely approximate Monte Carlo estimates of both direc t dose calculations and TG-43 dosimetric quantities. The discrepancies with S-20 angular quadrature (55 directions and weights per octant) or higher a re shown to be less than +/-5% (about 2.5 standard deviations of Monte Carl o calculations) everywhere except for limited regions along the Z axis of r otational symmetry, where technical limitations in the DANTSYS first collis ion source implementation makes adequate suppression of ray effects difficu lt to achieve. The efficiency of DANTSYS Simulations is compared with that of the EGS4 Monte Carlo code. It is demonstrated that even with the 210-gro up cross section library, DANTSYS achieves two-fold efficiency gains using the the S-20 quadrature set. The potential of discrete ordinates method for further efficiency improvements is also discussed. [S0094-2405(00)01510-8] .