Other Results

No irregularities were observed on the supply voltages and currents. In fact, this would only be the case with a single event latchup or gate rupture, when a conductive path between supply rails was created.

Figure: Amplitude of the internal calibration pulse vs. the total fluence. The data within the shaded areas were measured at room temperature, while the central part was obtained at $-10^{\circ}\,\rm C$.

Internal calibration measurements were performed periodically when the beam was off. Fig.  shows the amplitude of the internal calibration signals in both peak and deconvolution mode and the noise over the accumulated fluence. The APV chips revealed a small temperature dependence, which is common for semiconductors. Apart from that, a minor decrease in the amplitude of about $-15\%$ relative to the initial value was obtained at the final fluence of $1.87 \cdot 10^{14}\,\rm\pi^+\,cm^{-2}$.

The noise values however show the same development for both fluence and temperature, such that the signal-to-noise remains unaffected. Thus, this effect purely is a matter of gain. Oxide charging is the suspected reason of the gain degradation, but this is not yet confirmed and needs further investigation.