Introduction
Two ABCD2 n-type chips mounted on Freiburg kapton hybrid (forward). During irradiation the chips were clocked at 40MHz and read out. All bits in all DACs were set to 1 - this is the method to make the DACs less sensitive to radiation damage for this version of chips.
Irradiation was done in three steps. After each of the irradiation steps a number of threshold scans were done and from them gain, offset and noise was determined. Threshold scans were done with 5 charges: 1.5, 2, 2.5, 3 and 3.5fC and at different preamplifier and shaper bias currents. Measurements were taken after received fluencens of:
- 0 n/cm2
- 1*1013 n/cm2
- 6*1013 n/cm2
- 2*1014 n/cm2
DACs:
Lineaity of DACs was checked before and after irradiation - voltage was measured on test pads with a probe and compared to the set value for three DACs. On the plot one can see that no change in DAC response was observed after 2*1014 n/cm2 neutrons.
Chip performance
As mentioned above, chip performance was evaluated from threshold scans. The examples of threshold scans after each of irradiation steps is shown in the figure . Threshold scans (shown in the figure) were taken with shaper bias current set to 19.2uA but at differnet preamplifier (FE bias) currents.
In the next figure gain for individual channels after each of irradiation steps is shown. One can see that all channels survived the irradiation. Note that measurements were taken at different FE biases. The response curves show that the lineraity stays unchanged up to a fluence of 6*1013 n/cm2. After 2*1014 n/cm2 response is still linear but one can observe drop in gain, especially for chip 1. Again, measurements were taken at different preamplifier biases.
The change in the chip performance caused by radiation can best be seen in the next plot where average gain of all channels in a chip is ploted versus preamplifier bias setting. One can see that as the fluence is increased the premplifier bias setting at which chips work is shifted towards lower values. Before irradiation chips work at preamp bias currents of 200uA and above whereas after 2*1014 n/cm2 the working range is a narrow interval around 40uA. Noise , offset and offset spread are also shown as functions of preamplifiear bias current and received fluences. The measurements were taken with shaper bias current set to 19.2uA.
A simillar set of plots for shaper bias current set to 15.6uA and 9.6uA is listed below.
Plots for Shaper Bias set to 15.6uA
Plots for Shaper Bias set to 9.6uA
Annealing
After irradiation with 2*1014 n/cm2 chips were under bias and clocked. Their performance was checked every few days in order to observe eventual annealing. On the gain vs time after irradiation plot the behaviour of gain is shown. Measurements were taken with preamp bias at 36.8uA. Very little, if any, annealing can be seen. Annealing plots are made also for noise , offset and offset spread .
Gain at different analogue voltages
A scan of chip gain vs preamp bias at different analogue voltages was done after the chips were irradiatied with 2*1014 n/cm2 . The graph of preamp bias at maximum gain versus Vcc shows a small change of best preamp bias setting as one changes the Vcc.
Conclusions