Electrometers

ARPANSA calibrates electrometers which are used for radiotherapy dosimetry

The calibration is usually performed with an ionisation chamber in Cobalt-60 radiation. The electrical current produced by the chamber is measured with a calibrated electrometer, and a timer is used to deliver a known charge to the electrometer being tested. ARPANSA has another procedure which uses a known voltage applied to a standard resistor to set up a reference current. Both methods can be used with therapy electrometers, provided they are of the floating design (the outer electrode is ground) and the polarising voltage can be set to zero. If the electrometer is calibrated with an ionisation chamber, the calibration will be performed in Cobalt-60.

The electrometer calibration is in terms of charge or current. ARPANSA does not recommend that electrometers be used with units other than coulombs or amperes because of the risk of accidentally applying the wrong calibration coefficient.

The calibration is traceable to Australian standards of voltage, resistance and time.

By default, ARPANSA will attempt to calibrate all the ranges of the electrometer and will do so using both the internal timer and an external timer (ARPANSA’s Cobalt-60 source timer). Some high ranges (above 1 µA) require a special setup and we may consult with the customer to determine if these ranges need calibration.