The XRS provides a pulseheight measurement of each X-ray photon detected. It turns out to be a rather complicated procedure to convert that number to the actual energy of the photon.
Several things can cause the system response to vary as a function of time.
We handle this by including a calibration source near the detectors. This source emits X-rays of a few fixed, known energies. Nonlinearity Ideally, the pulseheight would be proportional to the energy of the X-ray photon. In reality, the detector responds less to higher energy photons, leading to a curvature in the relationship between energy and pulseheight. The graph shows the linear relationship that we wish the detectors had, and an example of a nonlinear relationship, which is closer to reality.