Measurement of diagnostic x‐ray spectra using a silicon photodiode

Diagnostic x‐ray spectra (66–103 kV, 1–2 mA) were measured without pinhole collimator or liquid nitrogen cooling by using a silicon p – i – n photodiode. An x‐ray count rate was low (≊400 photons/s) at distances of 2–3 m away from an x‐ray target, because the i layer of the photodiode was very small (1.5 mm 2 in area and 75 μm in thickness). The junction reverse current of the photodiode was 50–70 pA at room temperature and energy resolution for 59.5‐keV γ rays was 2.0 keV (full width at half‐maximum), which was limited by the electric noise of a preamplifier. Measured spectra were corrected for detector distortion. The corrected spectra were similar to those determined with a Ge detector. The i ‐layer thickness of the photodiode was an important parameter for the correction. In particular, when the thickness was thin, the fraction of photoelectron escape became large; this fraction was estimated analytically for various i ‐layer thicknesses. The angular dependence of the full‐energy peak efficiency was measured; for the photons whose energies were above 17.8 keV the angular dependence was isotropic except for angles greater than 60° off the photodiode axis. This method is therefore applicable to the measurement of scattered x rays.