Accuracy, precision and reproducibility of ionization chambers for environmental radiation measurement

Regarding the instruments to measure environmental natural radiation, the error arising in ionization chambers is evaluated. Systematic error in calibration is divided into the errors in measurement practice and correction, if the change of radiation before entering a detector is excluded. In measurement practice, the errors based on source accuracy, measurement of the distance between the source and an ionization chamber, and reading out of the record obtained, arise. In correction, the effect from the scattered radiation generated mainly by neighboring objects and earth surface, that of detector volume when the distance between a source and the detector is small as compared with detector size, and that of attenuation in air are observed. Other influences due to the accuracy of measurement system and the reproducibility of instruments and α-emitters in detectors are also expected. Taking an example of a sealed ionization chamber with 4 l volume and wall of 3 mm thickness, the error in measurement practice is max. +-10%, and the error in correction max. +-3% as the systematic error in calibration, and the reproducibility of the instrument (deviation) is max. +-4%, accuracy of measurement system, +-10%; total standard deviation is approx. +-7%. This standard deviation is obtained in careful practice, and normally it is supposed to be 10%. (Wakatsuki, Y.)