The single photon sensitivity of the Adaptive Gain Integrating Pixel Detector

TL;DR

This paper evaluates the single photon sensitivity of the AGIPD detector at XFEL, showing it can detect photons of 8 keV or more with high efficiency, and explores its low-energy performance and noise effects.

Contribution

It provides the first detailed analysis of AGIPD's single photon sensitivity and the impact of detector noise, including charge summing benefits and low-energy detection capabilities.

Findings

AGIPD detects single photons of 8 keV with >50% efficiency

Charge summing improves detection rate and reduces multiple counts

Low-energy detection at 3 keV with a noise floor below 0.035 hits/pixel/frame

Abstract

Single photon sensitivity is an important property of certain detection systems. This work investigated the single photon sensitivity of the Adaptive Gain Integrating Pixel Detector (AGIPD) and its dependence on possible detector noise values. Due to special requirements at the European X-ray Free Electron Laser (XFEL) the AGIPD finds the number of photons absorbed in each pixel by integrating the total signal. Photon counting is done off line on a thresholded data set. It was shown that AGIPD will be sensitive to single photons of 8 keV energy or more (detection efficiency $\gg$ 50%, less than 1 count due to noise per 10$^6$ pixels). Should the final noise be at the lower end of the possible range (200 - 400 electrons) single photon sensitivity can also be achieved at 5 keV beam energy. It was shown that charge summing schemes are beneficial when the noise is sufficiently low. The total detection rate of events is increased and the probability to count a single event multiple times in adjacent pixels is reduced by a factor of up to 40. The entry window of AGIPD allows 3 keV photons to reach the sensitive volume with approximately 70% probability. Therefore the low energy performance of AGIPD was explored, finding a maximum noise floor below 0.035 hits/pixel/frame at 3 keV beam energy. Depending on the noise level and selected threshold this value can be reduced by a factor of approximately 10. Even though single photon sensitivity, as defined in this work, is not given, imaging at this energy is still possible, allowing Poisson noise limited performance for signals significantly above the noise floor.