Design of modified flattening filter for International Organization for Standardization recommended 147Pm beta source: a Monte Carlo approach.

A modified beam-flattening filter design for the 147Pm beta source of the irradiation facility Beta Secondary Standard BSS-2 has been developed to achieve substantially improved dose-rate uniformity, targeting ±5% or better over the required phantom area. The study was performed using the Monte Carlo simulation codes EGSnrc and PHITS. The proposed filter design demonstrates a significant enhancement in dose-rate homogeneity at a calibration distance of 20 cm compared with the existing International Organization for Standardization (ISO)-recommended filter (ISO 6980 standard series). The dose-rate uniformity improved from approximately ±8% to nearly ±1% within a circular region of radius 7.5 cm, while simultaneously increasing the absolute central dose-rate by ~30%.