Three effects have been observed to occur in the spark-chamber throughout the mission: long term degeneration, odd-even gap effects and corner sparks.
Long term degeneration is a result of the spark-chamber gas composition being altered by the spark-discharges and possibly by deposition of cracking products onto the wires. A supply of gas was carried on board which allowed the periodic renewal (flushing) of the used gas. This operation was performed 22 times during the COS-B mission. Initially at 6 week intervals stretching to 20 weeks at the end of the mission. The decision to flush the chamber was based subjectively upon the apparent quality of the spark-chamber pictures. Although initially flushing restored the chamber to its previously high efficiency, the improvement is never seen as a step function in the experiments sensitive area. Altogether a continuous overall reduction of the experiment is observed over the entire mission.
This is taken account of in the relative sensitivities given in the database. However the shorter term effects of the flushings remain in the data and must be taken into account.
The odd-even gap-loss effect was not anticipated and occurred during several periods particularly in the second year of the mission. Repeatedly for a period of time, ranging from minutes to days, the spark-chamber pictures were missing either the odd-gap or even-gap positional information. This was a consequence of an undue delay in one of the two spark gaps which were triggering the two subsets of spark-chamber modules into which the spark-chamber was divided for redundancy reasons.
After several curing attempts were made (either by performing a burn-in procedure or by switching to redundant trigger units), the problem was practically reduced to a level which no longer affected the efficiency over long periods. The sporadic reappearance over relatively short time intervals has been observed and must be kept in mind when time variability is investigated. The loss in efficiency is corrected for on an observational period basis within the database.
Corner sparks are parasitic sparks which appear in one or two corners of the spark-chamber when the gas has deteriorated or, more generally, towards the end of the mission. These have been removed from the data by rejecting events having origins in the affected areas. The remaining contamination is negligible. The related reduction in efficiency is at the 1% level and is compensated by the corrections already described.