Spectra and air-kerma strength for encapsulated 192Ir sources.

The photon spectra in vacuum around four types of 192Ir HDR brachytherapy sources are calculated using the Monte Carlo code EGS4 and the most recent spectral information on 192Ir decay. The air-kerma strengths per unit activity are calculated based on the photon fluence around a bare 192Ir 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.