For unknown radionuclide identification, it is important that a high purity germanium (HPGe) spectrometry system be calibrated correctly for energy. The energy calibration of an HPGe spectrometry system will drift over time due to a variety of factors including the ambient temperature, the line voltage applied to the system, variation in the electronics, and other possible influences. In order to better understand the nature of this energy calibration drift, calibration spectra were collected over a period of several months from a laboratory HPGe spectrometry system. System parameters, including detector voltage, amplifier gain, and preamplifier gain, were not deliberately modified during the course of the experiment. The system was calibrated routinely over the 90 days, and the results of the calibrations were compared in order to assess the drift in the energy calibration of the detector over time. The analysis of a 36% high purity germanium system demonstrated the energy calibration drifted air average of 0.014 keV d(-1) to 0.041 keV d(-1) depending upon energy. At 1,332 keV, one day after calibration, it was shown that tip to half of the total error in energy calibration was as a result of calibration drift.
