Dynamic dose determination in a gamma irradiator using PHITS Monte Carlo code : Transit dose

Nafisah, Intan and Saputro, Bimo and Firmansyah, Okky Agassy and Zhang, Zhongming and Prastowo, Dian Adi and Fendinugroho, Fendinugroho and Nurfiana, Fifi and Yusmaman, Waringin Margi (2026) Dynamic dose determination in a gamma irradiator using PHITS Monte Carlo code : Transit dose. Radiation Measurements, 197: 107751. ISSN 1350-4487

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Abstract

Gamma irradiation facilities have been widely utilised for various sectors, including healthcare, agriculture, and material science. Therefore, it is crucial to understand the dose rate distribution within the chamber to ensure the samples receive the prescribed dose. PHITS Monte Carlo code was employed to simulate a self-shielded gamma irradiator category I. The facility features 36 pencil sources of Co-60 that are arranged around the sample chamber. A three-dimensional model of the gamma irradiator was developed using the PHITS Monte Carlo Code to simulate photon transport and dose deposition, while dynamic dose components during chamber movement were estimated using Simpson's numerical integration. Validation using alanine dosimetry yielded a good agreement (2.4% relative difference), confirming the model's accuracy and reliability. The dose map revealed the heterogeneity for both radial and axial distribution along the central axis. The highest differences in dose rate distribution were∼ 40% radially and ∼45% axially. Therefore, to achieve a more uniform dose distribution (ratio ∼1.1), it is suggested that the samples be placed at a height between 8 and 21 cm. The Simpson's integration method was implemented to bridge the gap between discrete simulations and dynamic physical processes for transit dose determination. The simulation calculated a transit dose of approximately 1.9 Gy with an uncertainty of 0.16% at the chamber's centre-bottom position for one irradiation cycle.