Impact Classification

Ejecta produced during a primary dust impact can produce further impact plasma elsewhere in the detector. Therefore it is common that both targets show signals after an impact. Figure 3 shows for the calibration data the charges liberated on the large and the small target. Figure 3 suggests that from the ratio of the target charges it can uniquely be decided whether the primary impact took place at the large or the small target. However, it can also be seen in Figure 3 that an impact on the large target can cause as much charge on the small target as it produced on the large one. Therefore it must be suspected that impacts anywhere inside CDA can produce signals at the targets. Therefore there are three instead of two possibilities for the place of the primary impact (large target, small target and none of the targets) and the target ratio cannot uniquely decide the place of the primary impact. The target ratio can at most rule out one of the targets as the place of the primary impact:

not small target < 0.32 not large target (1)

To decide whether the primary impact occurred at the remaining target or the wall another criterion is required. From the calibration data, it was found that the charge measured at the grid is at most 40% of the charge measured at the target, where the primary impact occurred. Therefore, if the ratio of charges measured at the grid and the target exceeds 0.4 the impact did not occur at this target. With this criterion, for 6 out of the 27 measured impacts, the targets are ruled out as the place of the primary impact. For one of these 6 events the ions arrive at the ion collector grid before a signal is observed at the target. This confirms independently that the primary impact did not hit one if the targets.

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