Detector resolution
The typical resolution for Si(Li) detectors of approximately 140 eV for energies of 5.9 keV is only one indicator of the quality of semiconductor detectors. Such characteristics as the maximum count rate or the presence of background may be more important in analytical methods.
Si PIN detectors have a resolution in the range of 165 – 220 eV. While for SDD detectors the FWHM is < 150 eV. It is worth noting that the detector resolution largely depends on the corresponding settings of the detector pulse processor.
Fig. 1. K-series spectrum of Mn obtained with a Si(Li) detector.
Typical resolution for Si(Li) detectors can be better than 140 eV for energies of 5.9 keV. However, this is only one indicator of the quality of semiconductor detectors, the maximum count rate, or the presence of background - these aspects can be more important in analytical methods. Peltier cooled Si PIN detectors have a resolution in the range of 165 - 220 eV. While for SDD detectors FWHM < 150 eV. It is worth noting that the resolution of the detector largely depends on the appropriate settings of the pulse processor of the detector.
Neglecting the natural width of the X-ray spectral line, the energy resolution of a semiconductor detector is a function of two independent factors. One of these factors is the design properties of the detector itself, the other depends on the pulse processing system used in the spectrometer.
The measured FWHM for an X-ray line is the square root of the sum of the detector contributions and the contributions associated with the electrical pulse processing system:
The detector component is determined by the statistics of the charge formation process in the diode volume. The average number of electron-hole pairs produced by an incident photon can be calculated as the total photon energy divided by the average energy required to form one electron-hole pair. If the fluctuations in this process are governed by Poisson statistics, the standard deviation can be calculated as
In semiconductor devices, the details of the energy loss process are such that individual events are not strictly independent and deviate from Poisson statistics. This deviation from statistics can be accounted for by the Fano factor
Taking:
we get:
where ε is the average energy required to form a free electron-hole pair, E is the incident photon energy, F is the Fano factor, and the factor 2.35 accounts for the standard deviation of the FWHM from Poisson statistics. The characteristic values of the detector contributions to the spectrometer resolution, usually called dispersion, are ~ 120.
For comparison, the values of various detector characteristics are given in the table:
