Poster — Thur Eve — 25: Improving the Accuracy of Electrometer Calibrations

For many years, the Ionizing Radiation Standards (IRS) group, NRC, has maintained the capacity to calibrate high‐quality electrometers used for radiation therapy. In particular, electrometer calibrations have now become a necessary part of the recently introduced MV X‐ray calibration service. With a growing interest in electrometer calibrations, the IRS electrometer calibration facility was due for revamping and upgrading. In 2008, instability in the IRS electrometer calibration system was observed during a period of high humidity. Troubleshooting suggested that the high humidity was causing high leakage currents, but it was difficult to obtain evidence of leakage occurring at any specific location since the leakage currents were small (pA or even fA). Desiccation of the calibrated standard capacitors dramatically improved the overall system performance, so the eventual solution involved isolation of the calibrated standard capacitors using a humidity controlled cabinet. Additional improvements included changing most of the connectors in the system from coaxial BNC to two‐lug triaxial bayonet type (TRB), and designing a 3‐stage filter to allow calibrations of newer current‐based electrometers. The result is an electrometer calibration facility that is unaffected by humidity and very stable, allowing year‐round calibrations for any of the electrometer models commonly encountered.