  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.
