Long term comparison of methods to sustain energy calibration in low level gamma-ray spectroscopy and investigation of possible sources for drift.

Two methods to compensate the gain drift of low level gamma-ray spectrometers are compared. For a period of 4 years, source injection and pulser method have been applied in parallel. The long term stability of a reference peak, injected by a precision pulser, has been validated by periodical source injection to sustain the energy calibration. The pros and cons of both methods and the advantage of active vs. passive energy calibration are discussed. The main cause for gain drift and the critical components have been identified.