Determination of the absorbed dose of epithermal neutrons in a phantom object filled with water

Accurate dosimetric assessment of epithermal neutron irradiation is important for neutron-capture-therapy-related studies. In this work, the spatial distribution of neutron kerma rate was evaluated in a water-filled cylindrical PMMA phantom containing a biological tumor sample and irradiated by the epithermal beam of the VVER-SM reactor. The incident neutron spectrum was taken from previous experimental and MCNP-4C-supported beam-characterization studies, and a kerma-based multigroup scheme was applied using energy-dependent neutron fluxes, macroscopic cross-sections, and kerma factors from the JENDL-4.0u library. The water-filled phantom was used as a first-order brain-equivalent medium, providing a biologically more relevant configuration than the previously studied air-filled phantom. The incident epithermal flux decreased from 4.034 × 10 8 to 1.025 × 10 7 n·cm −2 s −1 after transmission through the phantom. Sample position strongly affected the neutron kerma rate, decreasing from 1.761 × 10 −5 Gy/s at r=(3,0) cm to 1.202 × 10 −6 Gy/s at r=(17,0) cm. The obtained values should be regarded as first-order reference estimates within a simplified effective attenuation model and may serve as a practical basis for in-vitro epithermal-neutron irradiation studies and future Monte Carlo validation.

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