The response of the FBX aqueous chemical dosimeter to 10 MeV electron and 100 MeV proton beams was investigated, and the results were compared with those obtained for cobalt-60 gamma rays over a dose range of 0–30 Gy. The sensitivities of the FBX dosimeter in which the absorbance values were measured at the wavelength of maximum absorption (λmax) and 548 nm are approximately the same and amount to 0.091, 0.088, and 0.081 Gy-1 for 10 MeV electrons, cobalt 60 γ-rays, and 100 MeV proton beams, respectively. The corresponding calibration coefficients are 10.99, 11.36, and 12.35 Gy, in their respective order. The molar absorption coefficients that were calculated for cobalt-60 gamma rays amount to 17824 ± 560, 16417 ± 248, and 17694 ± 437 M− 1cm− 1 at the wavelengths λmax, 540 nm, and 548 nm, respectively. The radiation chemical yield-G (Fe3+) at the wavelengths of λmax and 548 nm has approximately the same values and amounts to 51.1 × 10− 7, 49.4 × 10− 7, and 45.4 × 10− 7 mol.J− 1 for 10 MeV electrons, cobalt-60 γ-rays, and 100 MeV proton beams, respectively, and both of them are lower by ~ 8% than that obtained at the wavelength of 540 nm. A linear response of the FBX dosimeters prepared in the current study was achieved up to 12 Gy, compared with the linear response up to 5 Gy reported in the previous literature. Furthermore, by measuring the absorbance values at the wavelength of the isobestic point, i.e., 555 nm, the linearity of the FBX dosimetric solution may be extended to cover the entire dose range used, i.e., 0.2–30 Gy. Consequently, with good reproducibility (less than 1%), the prepared FBX dosimeter can be utilized for radiotherapy research over the dose range of 0.2–30 Gy.