Experimental Evaluation of the SNR in Counting Detectors under Pile-Up Conditions

As a follow-up of a Signal-to-Noise Ratio (SNR) study presented previously, this work discusses the experimental results obtained for the statistical analysis of photon counting detector measurements. The test device is a hybrid assembly built with a pixelated 400 mu m thick electron collection Si sensor bump-bonded to a SPHIRD test readout ASIC. The analog front-end in each pixel of the ASIC produces a pulse of tens of nanoseconds for each X-ray hit, and its digital circuitry implements both amplitude and time-based pile-up compensation methods, making this device an excellent candidate for this study. Under pile-up conditions, the SNR of the photon counting measurements deviates from Poisson statistics and it has been evaluated by applying the numerical method introduced in a previous work. In addition to standard photon counting operation, two pile-up compensation methods implemented in SPHIRD were tested. The results were evaluated for individual pixels. The standard photon counting results reproduce the simulated behavior, presenting a SNR response that peaks around 30% pile-up, and then drops. Meanwhile, both pile-up compensation methods have presented a comparable effect on improving not only the count rate also the statistical response of the system, for the covered count-rate range of up to 45Mcps/pixel. The obtained results validate the presented methodology to obtain the SNR, also elucidating pile-up detrimental effect on the statistical quality of the data.