The X-ray response of InP: Part B, synchrotron radiation measurements

In this, the second part of a detailed study into the X-ray response of InP, we present results of a series of X-ray measurements on a 3.142 mm² × 180 μm thick semi-insulating InP detector at the BESSY II and HASYLAB synchrotron radiation research facilities. Photon metrology was carried out at energies ranging from 8 to 100 keV. Additional measurements were made using radioactive and fluorescent target sources. At -60°C, under full-area illumination, the FWHM energy resolution was 2.4 keV at 5.9 keV rising to 8.5 keV at 59.54 keV. Under pencil-beam illumination, the measured resolutions were generally less, being 2 keV FWHM at 8 keV rising approximately linearly to 5 keV at 100 keV. Analysis of the energy resolution function indicates that poor charge transport presently limit the performance of InP detectors and specifically hole trapping. This is borne out by the observed low-energy tailing of the pulse height spectra at intermediate and high energies. At very low count rates, it was found that the device was linear to within 1%. However, for count rates above ∼ 100 s^-1 and energies above ∼50 keV, the detector began to display time variable gain shifts indicative of polarization effects. The spatial uniformity of the detector was investigated by raster scanning a 50 × 50 μm², 15 keV monoenergetic X-ray beam across the active area. Apart from a few localized areas, the detector response was found to be reasonably uniform at the 10% level, although inconsistent with that expected from counting statistics alone (2%).