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The Thermoluminescence Dosimeter TLD-400 (CaF₂:Mn) exhibits high sensitivity to ionizing radiation and is widely used in radiotherapy and space dosimetry. A key characteristic of TLD-400 is its beam quality–dependent relative efficiency (η(λ,γ)), where thermoluminescence output varies among radiation types even at identical absorbed doses.
In this work, relative efficiencies were measured using a 6 MV photon beam as reference and compared with a 10 MV photon beam, 6 and 9 MeV electron beams, and proton beams of 70, 110, 150, 190, and 230 MeV. The measured efficiency was 0.98 for photons, ranged from 0.85 to 0.87 for electrons, and from 0.83 to 0.94 for protons. These relative efficiencies were also calculated using a microdosimetric approach. Monte Carlo microdosimetric simulations were performed to quantify stochastic energy deposition within micro and nanometric sensitive targets in TLD-400. Both a microdosimetric one-hit detector model characterized by a saturation parameter α and an extended formulation including an additional β interaction term were evaluated.
The calculated relative efficiencies are compared with experimental measurements in the accompanying figure. The results indicate that inclusion of the β parameter does not significantly improve agreement. The relative efficiency behavior across all investigated radiation modalities is adequately described by the one-hit detector model alone, with a fitted saturation parameter α = 0.01668 Gy⁻¹ and an effective sensitive target size of approximately 100 nm.
These findings provide a microdosimetric basis for the beam quality dependence of TLD-400 and support its application as a radiation quality-sensitive dosimeter in particle therapy and space environments.