Spectra and air-kerma strength for encapsulated 192Ir sources

The photon spectra in vacuum around four types of Ir-192 HDR brachytherapy sources are calculated using the Monte Carlo code EGS4 and the most recent spectral information on Ir-192 decay. The air-kerma strengths per unit activity are calculated based on the photon fluence around a bare Ir-192 source and around each of four types of encapsulated sources using recent mass energy-absorption coefficients. For the full spectrum the bare vs encapsulated difference is up to 23% due to the large air-kerma contribution from the unfiltered low-energy photons. For the penetrating part of the photon spectrum (>11.3 keV), the air-kerma strength per unit source activity on the transverse axis for a bare source is 2-15% higher than for the encapsulated sources due to the attenuation and absorption in the core and the encapsulating material. The contribution to the air-kerma strength from photons scattered in the capsule and from bremsstrahlung are calculated to increase the air-kerma strength by 2-4% and 0.2-0.3%, respectively. Air-kerma strengths for a variety of sources agree well with previously reported results for sources from Nucletron International, Best Industries, Inc., and Alpha-Omega Services, Inc. In addition we present air-kerma strengths for the present model of the HDR source from Nucletron International and the source from Varian Associates, Inc. [S0094-2405(99)00211-4].