Abstract: The structure and performance of the 4 πγ ionization chamber substandardestablished are described. The necessity of selecting source vials in accordance withwall absorption fractions using a ~(125)I needle source which is used as asensitiveprobe is stated. The chamber is calibrated with 26 radionuclides. Theoverall uncertainties (3 σ) of calibration factors are in range of 1.2--4.6%.A efficiency curve of the chamber is derived and it can be used to predict thecalibration factors of those radionuclides for which the chamber isn't calibrated.