An ionization chamber as a secondary standard for activity

A commercially-available radionuclide calibrator is used to maintain the unit of activity in the Slovak Institute of Metrology (SMU) and to check and verify activity meters in the field of nuclear medicine. The calibration of the instrument is described. The long-term stability is checked at regular intervals by means of 137Cs and 226Ra reference sources. A slow decrease of efficiency of about 2% per year has been observed, which has to be corrected for. An energy-dependent efficiency curve of the ionization chamber is established by transfer of calibration factors from a similar instrument at the Physikalisch-Technische Bundesanstalt (PTB). The curve has been checked by measurements with activity standards of 60Co, 54Mn, 22Na, 85Sr, 65Zn, 88Y, 57Co, 139Ce, 137Cs, 125I, 241Am, 210Pb and 109Cd. The established curve permits one to calculate calibration factors for additional short-lived radionuclides used in nuclear medicine. An overall uncertainty was obtained for activity measurements with calculated calibration factors under standard measurement conditions of better than 1.8%.