Noise analysis of gallium arsenide pixel X-ray detectors coupled to ultra-low noise electronics

The X-ray spectroscopic performance and the noise analysis of gallium arsenide pixel detectors coupled to ultra-low noise front-end electronics is presented. The pixel junctions have areas of 200 × 200 μm² and they are fabricated on an epitaxial gallium arsenide layer 40 μm thick. The front-end electronics consists of a forward bias field effect transistor amplifier with an equivalent noise of 16 electrons root mean square at room temperature (139-eV full-width at half-maximum (FWHM) silicon equivalent, 159-eV FWHM GaAs equivalent). The whole system shows intrinsic energy resolutions of 242-eV FWHM at room temperature and 163-eV FWHM when cooled to -30 °C. On the 59.5-keV line of ²⁴¹Am, widths of 501-eV FWHM at room temperature and 465-eV FWHM at -30 °C have been measured, close to the Fano limit. The different noise sources affecting the system have been disentangled and analyzed. It was found that the dielectric noise is the dominant component both at room temperature and when the system is cooled down. No excess line broadening due to signal charge loss was observed and a Fano factor of 0.124 ± 0.004 has been derived from the width of the 59.5-keV spectral line of ²⁴¹Am.