Long-term stability of liquid ionization chambers with regard to their qualification as local reference dosimeters for low dose-rate absorbed dose measurements in water

The long-term sensitivity and calibration stability of liquid ionization ch ambers (LICs) has been studied at a local and a secondary standards dosimet ry laboratory over a period of 3 years. The chambers were transported sever al times by mail between the two laboratories for measurements. The LICs us ed in this work are designed For absorbed dose measurements in the dose rat e region of 0.1-100 mGy min(-1) and have a liquid layer thickness of 1 mm a nd a sensitive volume of 16.2 mm(3). The liquids used as sensitive media in the chambers are mixtures of isooctane (C8H18) and tetramethylsilane (Si(C H3)(4)) in different proportions (about 2 to 1). Operating at a polarizing voltage of 300 V the leakage current of the chambers was stable acid never exceeded 3% of the observable current at a dose rate of about 1 mGy min(-1) . The volume sensitivity of the chambers was measured to be of the order of 10(-9) C Gy(-1) mm(-3). No systematic changes in the absorbed dose to wate r calibration was observed for any of the chambers during the test period ( sigma < 0.2%). Variations in chamber dose response with small changes in th e polarizing voltage as well as sensitivity changes with accumulated absorb ed dose were also investigated. Measurements showed that the LIC response v aries by 0.15% per 1% change in applied voltage around 300 V. No significan t change could be observed in the LIC sensitivity after a single absorbed d ose of 15 kCy. The results indicate that the LIC can be made to serve as a calibration transfer instrument and a reference detector for absorbed dose to water determinations providing good precision and long-term reproducibil ity.