Calibration coefficients of epitaxial diodes used in diagnostic radiology and computed tomography beams

Abstract

The assessment of the calibration coefficients of an epitaxial diode, previously characterized for diagnostic radiology (RQR qualities) and computed tomography (RQT qualities) dosimetry, is reported in this work. The diode, with an n-type epitaxial layer (50 µm) grown on a thick (300 µm) Czochralski silicon substrate, is directly connected to an electrometer Keithley 6517B in the photovoltaic mode and exposed to X-ray beams from a Pantak/Seifert generator, model Isovolt 160 HS. Under this operational condition, the dosimetric quantity is the dose rate correlated with the output current signal from the diode when exposed to radiation. The corresponding collected charge (the integral of the current signal) is proportional to the dose. The repeatability of the current signals and the dose response of the diode are investigated in several RQR and RQT beam qualities spanning from 50 kV to 150 kV. The calibration coefficients (Nk) are assessed by linking the diode readings to those from the standard ionization chambers for reference beam qualities (RQR-5 and RQT-9) following the formalism recommended by Technical Reports Series No. 457. To take into account the variations of the X-ray beams related to the correspondent reference beam qualities, the correction factors are also calculated. Despite being close to 1, they evidence the energy dependence of the EPI diode for voltages above 100 kV.