To check that the IPCs can sustain operating Gas Gains over time without sparking and to check that sparks are not induced in the IPCs by illumination.
Testing Setup and Procedure Spark testing is performed by turning on the Anode or Anti high voltage to attain a specific Gas Gain and sending pulses at 200 Hz to the test pulse inputs of the Anode or the Backanti preamplifiers. The data acquisition software has the capability of monitoring the number of events acquired per second and responding with a message written to the screen and to a text file if the count rate does not fall within a user-specified window. The sensitivity of this method can be estimated as follows. A spark causing the preamplifier to saturate will be observed as a drop in the count rate. The preamplifier saturates at 2.5 V corresponding to 2.5 pC of charge at the preamplifier input (due to the 1 pF capacitance in the amplifier feedback loop). Since capacitance of the Anode and Anti coupling capacitors is 1500 pF, if the voltage at the Anode or Anti wire plane changes by 1.7 mV, the preamp will be saturated. Thus, this method of spark monitoring allows for a sensitivity of a few millivolts.Analysis and Results Tables 32 to 35 summarize the results of the spark tests performed at Columbia with both the Anode and the Anti on for each IPC. During the tests reported in the tables, no sparks were observed.
Spark testing was also performed on both the engineering model IPC and the flight model IPCs to test the response of the IPCs to a high radiation environment similar to that which the IPCs will be exposed to in orbit. While at Metorex, the engineering model IPC was spark tested with and without alpha particle illumination. The alpha particle illumination did not induce sparking. At both Metorex and Columbia, the flight model IPCs were illuminated with radiation from 60Co, an emitter of 316 keV beta particles and 1 MeV gamma rays, at 200 to 300 counts/second. Sparking was not observed during IPC illumination by 60Co.
Table 32: LE1 Spark Test Report| Gas Gain | Cathode HV (V) | Duration (hrs) | |||||||||
| Anode | |||||||||||
| 10,000 | 50 | 19 | |||||||||
| 10,000 | 400 | 20 | |||||||||
| 20,000 | 400 | 3 | |||||||||
| 20,000 | 400 | 2 | |||||||||
| Anti | |||||||||||
| 10,000 | 50 | 24 | |||||||||
| 10,000 | 400 | 15 | |||||||||
| 20,000 | 400 | 18 |
Table 33: LE2 Spark Test Report
| Gas Gain | Cathode HV (V) | Duration (hrs) | |||||||||
| Anode | |||||||||||
| 10,000 | 400 | 16 | |||||||||
| 20,000 | 400 | 1 | |||||||||
| 10,000 | 400 | 26 | |||||||||
| 20,000 | 400 | 17 | |||||||||
| Anti | |||||||||||
| 20,000 | 400 | 2 | |||||||||
| 10,000 | 400 | 18 | |||||||||
| 20,000 | 400 | 2 | |||||||||
| 20,000 | 700 | 3 | |||||||||
| 15,000 | 400 | 19 | |||||||||
| 18,000 | 400 | 3 | |||||||||
| 18,000 | 400 | 4 |
Table 34: HE1 Spark Test Report
| Gas Gain | Cathode HV (V) | Duration (hrs) | |||||||||||
| Anode | |||||||||||||
| 10,000 | 400 | 1 | |||||||||||
| 10,000 | 400 | 16 | |||||||||||
| 20,000 | 400 | 6 | |||||||||||
| 20,000 | 400 | 23 | |||||||||||
| Anti | |||||||||||||
| 5,000 | 400 | 1 | |||||||||||
| 5,000 | 400 | 24 |
Table 35: HE2 Spark Test Report
| Gas Gain | Cathode HV (V) | Duration (hrs) | |||||||||||
| Anode | |||||||||||||
| 5,000 | 400 | 4 | |||||||||||
| 5,000 | 400 | 24 | |||||||||||
| 10,000 | 400 | 24 | |||||||||||
| Anti | |||||||||||||
| 5,000 | 400 | 1 | |||||||||||
| 5,000 | 400 | 24 |
5.8 LE1 "Bright Spot"
During the calibration, counts of unknown origin were detected by LE1. These extra counts became known as the "bright spot" because they are localized within the IPC. Figure 5.8-1 shows an LE1 image with no external source illuminating the IPC. Note that excess counts are observed in two spots. Events in the spot on the right are produced by the 55Fe calibration source, while the spot to the left is the bright spot.
Measurements of the count rate of the bright spot show that it is about 1 count/second with count rate variations consistent with Poisson statistics. The count rate has weak dependence if any on Gas Gain between Gas Gains of 5,000 and 20,000. Figures 5.8-2 and 5.8-3 show Slow Anode pulse height histograms for data collected with no external source illuminating the IPC. Figure 5.8-2 shows the histogram for all the data collected, while Figure 5.8-2 shows only the events from the bright spot. The peak near channel 650 in Figure 5.8-2 is the x-ray peak from the 55Fe calibration source. Note that the 55Fe calibration source x-ray peak marks an energy of 5.9 keV.