Performance of a thin epitaxial diode as a relative dosimeter in standard radioprotection beams

Abstract

The performance of a radiation-hard epitaxial (EPI) diode as a relative dosimeter is investigated for standard radioprotection beams (N-60, N-80, N-100, and N-150 qualities). The diode operates in short-circuit current mode, in which the net output current is linearly proportional to the dose rate. The corresponding collected charge, calculated as the integral of the current signal, is proportional to the dose. The EPI diode features a thin n-type epitaxial layer (50 μm) grown on a 300 μm-thick n-type Czochralski Si substrate. The p+-n junction, with a 25 mm2 active area, is formed by a highly doped p-type silicon layer (≤1 μm) on the front side of the diode. This structure, which maintains a constant active volume with a thin entrance window and negligible dark current, may enhance its suitability for dosimetry applications in continuous low-energy spectra and low dose rates typical of standard radioprotection beam qualities. Irradiations are performed using the Pantak-Seifert 160HS Isovolt X-ray generator, with radiation beams previously standardized using a Radcal 10X5-180 ionization chamber. Data collected with N-60, N-80, N-100, and N-150 beam qualities confirm a linear relationship between current and dose rate, as well as between charge and dose. The lowest current measured, 1 pA, and the corresponding limit of detection in terms of dose rate, 8.5 μGy/min (for N-60 beam quality), are slightly lower than those disclosed by a few commercially available Si diode-based dosimeters. However, the associated current and charge sensitivities show a significant energy dependence for photons above 100 kV. Regardless of beam energy, most dosimetric parameters align with international protocol recommendations, including repeatability, long-term stability, angular response, dose-rate dependence, and signal-to-noise ratio. The results indicate that these radiation-hard epitaxial diodes can be used for the first time to monitor radiation protection beams in real time.