Throughout the mission, and especially in the early months when tuning of the instrument took place, the triggering criteria varied. For several observations the trigger pulse from the energy calorimeter was required to be observed in coincidence with the triggering telescope. This obviously affected the trigger rate and possibly the detection efficiency. As this trigger pulse failed in the latter part of the mission, it was decided to normalize all trigger conditions by software to the equivalent triggering thresholds, so that the instrument may be considered to have a single mode.
This change to software thresholds might have made the sensitivity more susceptible to the influence of occasional electromagnetic interferences, created by the spark-chamber discharge currents, by modifying the content of the counters temporarily stored in the experiment electronics for readout. As a consequence an increasing fraction of events might have been lost in the late phases of the mission.
Alternately the observed long-term reduction of sensitivity, especially in the second part of the mission, could be due at least partially to the slowly decreasing efficiency of the spark-chamber, possibly connected to cracking products of the quenching agent contained in the spark-chamber gas which are produced by the spark discharges and are deposited on the spark-chamber wires.
The gain changes in the energy calorimeter were corrected using in-flight proton data and may therefore be disregarded.
During Jan 1978 a veto PMT failed giving an increased trigger rate in the detector temporarily. Although the dead time factor is correctly calculated, a reduction in efficiency was observed during this interval. A comparable effect occurred in June 1979 when a coincidence flag from the energy calorimeter failed. In this case the rate of triggers was reduced for a short period until a solution could be implemented. The efficiency was thereby reduced during this approximately 3-day interval.