Deviations in the control of the monitor calibration of a circular accelerator

The operation of an accelerator requires to perform at regular intervals a control measurement of the basic calibration. This measurement showed a deviation growing from week to week. Finally, the test measurements gave occasion to suppose a defect of the used ionizing chamber, but this was not confirmed by measurements at a gamma irradiation unit. When the chamber had been repaired by the manufacturer, there were no more deviations with respect to the basic measurement.