An ionization chamber as a secondary standard for activity

A commercially-available radionuclide calibrator is used to maintain the un it 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 Cs-137 and Ra-226 reference sources. A slow decrease of efficiency of about 2% per year has been observed, which has to be corr ected for. An energy-dependent efficiency curve of the ionization chamber i s established by transfer of calibration factors from a similar instrument at the Physikalisch-Technische Bundesanstalt (PTB). The curve has been chec ked by measurements with activity standards of Co-60, Mn-54, Na-22, Sr-85, Zn-65, Y-88, Co-57, Ce-139, Cs-137, I-125, Am-241, Pb-210 and Cd-109. The e stablished curve permits one to calculate calibration factors for additiona l short-lived radionuclides used in nuclear medicine. An overall uncertaint y was obtained for activity measurements with calculated calibration factor s under standard measurement conditions of better than 1.8%. (C) 2000 Elsev ier Science Ltd. All rights reserved.