Angular dependence ofH_p(3): horizontal vs. vertical rotation

Area of Science:

• Medical Physics
• Radiation Dosimetry
• Radiological Protection

Background:

• Operational quantities like H_p(3) are crucial for radiation protection dosimetry.
• Standard measurement protocols often assume specific phantom rotation axes (e.g., vertical).
• Understanding the impact of different rotation axes on dose quantities is essential for accurate radiation assessment.

Purpose of the Study:

• To investigate the impact of horizontal versus vertical phantom rotation on the operational quantity H_p(3) under oblique radiation incidence.
• To compare the changes in H_p(3) with the equivalent dose to the lens of the eye (H_lens).
• To provide data that may inform updates to radiation protection standards and measurement procedures.

Main Methods:

• Calculated conversion coefficients (h_pK(3) and h_pD(3)) for photons and betas using both vertical and horizontal cylinder phantom rotations.
• Simulated mono-energetic photons (2 keV–10 MeV) and reference beta-particle fields (0.54–1.2 MeV).
• Determined ratios of H_p(3) values for horizontal versus vertical rotations and compared with H_lens ratios.

Main Results:

• For mono-energetic photons, H_p(3) was significantly lower with horizontal rotation, especially at low energies and high incidence angles, due to increased pathlength.
• For beta particles, differences between horizontal and vertical rotations were generally below 10% due to broader angular distributions.
• H_lens showed larger differences than H_p(3) for both photons and betas, irrespective of rotation axis.

Conclusions:

• Horizontal phantom rotation significantly alters H_p(3) for photons, necessitating consideration for revised dosimetry definitions.
• Beta particle H_p(3) is less sensitive to rotation axis changes compared to photons.
• The substantial differences in H_lens highlight the need for standardized procedures that account for various irradiation geometries and phantom orientations.