Speaker
Description
The National Radiation Standard Laboratory (NRSL/INER) established a primary standard for the reference air kerma rate (RAKR) of HDR Ir-192 brachytherapy sources based on dual graphite-walled spherical cavity ionization chambers combined with a dedicated dual-collimator geometry. The original realization achieved an expanded uncertainty of 0.92% (k = 2) and demonstrated consistency in the APMP.RI(I)-K8 key comparison.
The measurement system employs a bidirectional collimation arrangement together with a shadow-shield technique to minimize room scatter, reduce alignment-induced solid-angle uncertainties, and suppress perturbation effects from wall and stem scatter. Experimental verification confirmed that the dual-collimator configuration reduces the scatter-to-primary ratio to approximately 0.01%, providing improved beam definition under near-minimal scatter conditions. Dual-chamber averaging further enhances internal consistency and long-term stability.
Following the publication of ICRU Report 90, key dosimetric parameters including the stopping power ratio s_(g,air), photon mass energy-absorption coefficients, and crystalline graphite density corrections required re-evaluation. Updated Monte Carlo simulations using MCNP6 for Ir-192 photon spectra and EGSnrc user codes were performed to recalculate correction factors. The revised value of s_(g,air) changed from 1.0082 to 1.0058, corresponding to a relative reduction of 0.24%. The overall correction product decreased from 1.0516 to 1.0462, resulting in a net reduction of approximately 0.51% in the Ir-192 RAKR standard.
After incorporating revised constants and uncertainty components, the expanded uncertainty increased from 0.92% to 1.2% (k = 2). Despite this systematic shift, the degree of equivalence with international comparisons remains within stated uncertainties. The updated standard has been formally implemented since 1 January 2024, confirming the robustness and international consistency of the NRSL/INER Ir-192 primary standard.