Studies of the diamond-like coating
5.2 Testing the radiation-hardness of the DLC
5.2.1 Experimental procedure
The HERA-B-MSGC substrates are equipped with a high resistivity diamond-like surface coating (DLC) (see Section5.1). During high-rate tests of the Heidelberg-Siegen-Zurich group shorts between anodes and cathodes emerged. Initially it was not clear, whether the shorts were an inherent problem of the diamond-like surface coating or whether they were induced by sparks.
If a spark occurs in a MSGC material is deposited in a region of about 1 mm radius around the impact point as can be seen in Figure5.3. Most of the deposit probably comes from sputtered gold, the cathode and anode material. Alternatively also a small film of con-ducting graphite could be produced in the spark plasma which leads to shorts. It is also conceivable that a structural change in the coating leads to shorts or lower resistivity. To help clarifying these issues we tested the behavior of the DLC under high-rate irradiation in a hadronic beam.
In order to separate the effects in the coating from spark induced shorts we had to avoid or at least minimize gas-amplification. Nevertheless high voltage should be applied for two reasons:
• It is possible that certain destructive effects only occur in the combination of hadronic irradiation and an applied voltage or a current flowing, respectively.
• By permanently measuring the current flowing through the coating, it is possible to relate sudden changes in the resistivity to external occurrences like switching on and off the beam.
To fulfill these conditions, we used a special chamber without any drift-electrode. The gas gain was minimized by usingN2as counting gas and applying moderate high voltage of the wrong polarity only.
The current flowing through the substrate coating was measured by a nA-meter and digitized in a CAEN-SY127-module. A Labview program running on a Sun workstation recorded the data and controlled the high-voltage. The rate was measured using two scintillators in coincidence.
The test took place in theπM1-area at PSI. We used the same chamber (equipped with a new wafer) and readout-electronics as used during the high-voltage resistance test (see Section5.1.2), supplemented by a beam telescope.
The particle rate and the beam profile were measured by two 5×5 mm2 scintillators.
They were mounted onto a xy-scanner, in a distance of 30 mm of the chamber
perpen-5.2 Testing the radiation-hardness of the DLC 61 time [s] date time comment
0 22. 9. 19:30 test beginning 140000 24. 9. 11:40 no beam 162000 24. 9. 16:30 beam back
220000 25. 9. 9:00 beam-tuning, -scans 252000 25. 9. 17:30 beam back
317000 26. 9. 11:30 beam-scans 332000 26. 9. 16:00 beam back 480000 28. 9. 9:00 beam-scans
512000 28. 9. 18:00 reversion of polarity 590000 29. 9. 15:30 beam back
650000 30. 9. 8:00 test ending
Table 5.1: Protocol of activities during the beam test period.
dicular to each other. That way we could measure the particle rate at each point of the chamber.
100E3 200E3 300E3 400E3 500E3 600E3 t [s]
250E3
100E3 200E3 300E3 400E3 500E3 600E3
t [s]
Figure 5.6: The rate was measured by two scintillators (5×5mm2) in coincidence (Figure (a)).
Figure (b) shows the integrated rate.
We measured the current flowing through the seven cathode groups during one week of heavy hadronic irradiation. The goal was to observe the changes in the resistance.
Table5.1and Figure5.6show the time structure of the irradiation, the instantaneous and the integrated rate.
Except for the last two days, we usedN2as counting gas and applied 200 V of the wrong polarity. To get as many heavy ionizing particles as possible, we used a mixed 350 MeV/c proton pion beam. Figure5.7shows the beam profile. The particle rate was successively enhanced from 0.64 kHz/mm2to 69 kHz/mm2. For the last two days we used Ar/DME in a ratio of 50:50 as counting gas and switched the polarity of the cathode voltage to
−200 V. On the last day we used−350 V.
-150 -100 -50 0 50 100 150
-150 -100 -50 0 50 100 [mm]
[mm]
150
Figure 5.7: Beam profile used during the irradiation tests. This picture was recorded with the scintillator beam telescope, and shows, that a large part of the chamber was illuminated.
5.2.2 Results
Figures 5.8 and 5.9 show the time evolution of the measured currents and the result-ing resistance. The timescale is in seconds and begins with switchresult-ing on the beam (see Table5.1). Interruptions in the irradiation corresponds to beam off periods or other ac-tivities like beam-scans or scintillator optimization. During these times, the values of the currents are interpolated by straight lines and the rate is set to zero, even though the substrate could have been slightly irradiated.
The lines in the plots (see Figures 5.8 and5.9) show the resistance of the two 15×16 and 3×16 cathode bundles. These measurements at a cathode voltage of+200 V show that the resistance of the coating rises during a fairly long period (around 3 hours) of irradiation. The total increase is between 2.5% and 5%. Afterwards it remains constant.
After an interruption of the beam for several hours the resistivity falls back to its initial value (see Figures5.8and5.9). A resistance of 500 MΩcorresponds to surface resistivity of 2·1014Ω/2=500 MΩ·3·16·250 mm·2/60µm.
After switching the beam on again, the substrate showed the same behavior. The rise time is in the order of several hours. The value of the maximal resistivity decreased with increasing beam-intensity. The mean value of the resistivity stayed more or less constant, in addition there was no significant change of the resistivity in correlation with the accumulated charge. After a longer beam break (t>6000000) switching the polarity and increasing the high-voltage to−350 V resulted in a resistivity drop of about 10%. It recovered a little bit during the massive irradiation.
5.2 Testing the radiation-hardness of the DLC 63 R [M ]
100000 200000 300000 400000 500000 600000 t [s]
100 200 300 400 500 600
Ω
Figure 5.8: The resistance of the different HV-channels over the whole testing period is depicted.
The topmost five lines show the resistance of the HV-channels supplying 48(=3×16)cathodes.
The two lower lines indicate the resistance of the bundles of 240 (=15×16) cathodes. The shadowed histograms show the irradiation rate.
280000 300000 320000 340000 360000 380000 400000
t [s]
R [M ]Ω
100 200 300 400 500 600
Figure 5.9: Time expanded view of the results shown in Figure5.8, demonstrating reproducible rises of the resistivity (solid lines) during irradiation (shadowed histogram).
During the last day we read out 128 bundled anodes, amplified by an Ortec Vt100 pream-plifier, with an oscilloscope. Already at a voltage of VC = −300 V, well below the
ex-pected operating voltage of VC=−400 V, we observed sparks. The electrostatic configu-ration of our test chamber differs from that of a complete chamber, because of the missing drift cathode.
(a)Fork-like structure and wide swellings (b) Fork-like structure and point-like shad-ows
(c)Excrescence reaching the anode (d)Darkfield picture of (c)
Figure 5.10: After our high-rate irradiation test at the PSI we discovered these cancer-like struc-tures on the MSGC-substrate. They could be first symptoms of irradiation damages.
Although the resistivity did not change significantly during the test, an optical inspection of the test plate exhibited interesting cancer-like structures (see Figure5.10). Three types have been observed:
• Regular fork-like patterns growing from the cathodes towards the anodes (see Fig-ures5.10(a) & (b)).
• Wide swellings along the anodes (see Figures5.10(a) & (c)).
5.2 Testing the radiation-hardness of the DLC 65
• Small, point-like shadows along the anodes (see Figure5.10(b)).
It is not clear whether these mutations originate from irradiation during the first part of the test (i.e. without gas amplification) or from the last two test days with gas ampli-fication. The darkfield picture (Figure 5.10(c)) shows small deposits also on top of the anodes. This could be an indication that these mutations emerge from the gas amplifica-tion. The typical damage introduced by sparks are not seen in these regions which lets us conclude that we deal with first symptoms of irradiation damages.
5.2.3 Conclusion
We measured the behavior of diamond-like coating under heavy irradiation conditions.
As irradiation source we used a heavy ionizing proton and pion beam with a momentum of 350 MeV/c. To prevent damages induced by the gas amplification, we inhibited gas amplification at all: N2was used as counting gas, and only a cathode voltage VCof 200 V of the wrong polarity was applied.
The measurements of the resistivity of the diamond-like coating showed no dependency on the total irradiation dose. Small changes of the resistivity stabilized gradually and can be explained by temperature changes. Thus from the resistivity measurements we have no hint for radiation damages. However in an optical inspection after the tests, we found cancer-like structures emerging from anodes and cathodes. This could be an indication that the coating is not as radiation hard as our measurement of the resistivity suggests.
Overall we can deduce, that the emerging shorts between anodes and cathodes found in other high rate tests are most probably due to effects induced by the gas amplification in the detector and not due to permanent changes or damages in the diamond-like coating.