Spectral resolution in pixel detectors with single photon processing

Abstract

Pixel detectors based on photon counting or single photon processing readout are becoming popular for spectral X-ray imaging. The detector is based on deep submicron electronics with functions to determine the energy of each individual photon in every pixel. The system is virtually noiseless when it comes to the number of the detected photons. However noise and variations in system parameters affect the determination of the photon energy.Several factors affect the energy resolution in the system. In the readout electronics the most important factors are the threshold dispersion, the gain variation and the electronic noise. In the sensor contributions come from charge sharing, variations in the charge collection efficiency, leakage current and the statistical nature of the charge generation, as described by the Fano factor.The MEDIPIX technology offers a powerful tool for investigating these effects since energy spectra can be captured in each pixel. In addition the TIMEPIX chip, when operated in Time over Threshold mode, offers an opportunity to analyze individual photon interactions, thus addressing charge sharing and fluorescence. Effects of charge sharing and the properties of charge summing can be investigated using MEDIPIX3RX. Experiments are performed using both Si and CdTe detectors.In this paper we discuss the various contributions to the spectral noise and how they affect detector response. The statements are supported with experimental data from MEDIPIX-type detectors.

Energy Calibration of the Pixels of Spectral X-ray Detectors

“…The threshold equalization is likely to be more effective by using a higher reference energy or equivalent test pulse rather than using the noise floor. However, when using test pulse, the variations in the charge injection capacitor across the pixel matrix can distort the spectroscopic response [27]. So, using higher photon reference energy close to the energy of interest in threshold equalization is the most promising solution even though it takes longer time.…”

“…Similarly, the mean and median gain of whole pixel matrix are 2.08 threshold DAC/keV and 2.10 threshold DAC/keV respectively in each counter. The gain is mainly controlled by the feedback capacitor in the analogue circuit and this can be cancelled to a large extent with narrow energy range by performing the energy calibration of the detector at the energy of interest [27]. …”

“…The most important factors that affect the spectral resolution in readout electronics are threshold dispersion, gain variation and electronic noise. Similarly, charge sharing, variation in the charge collection efficiency, leakage current and Fano factors are other major factors from sensor that contribute to spectral resolution of the detector [27]. Fig.…”

Full-field X-ray fluorescence imaging with a straight polycapillary X-ray collimator

“…It should be noted that the electronics noise of the front end is one of the main factors deciding the energy resolution of the system. However, various sources, such as thermal noise at room temperature and sensor leakage current contribute to the noise [30], resulting in a photon counts tail at the low energy range. In this work, no suppression of the sources is made.…”