Non-InvasiveTransfer System to Calibrate kVp Meters

INTRODUCTION Quality assurance programmes in diagnostic radiology have been increased in the last years due to the fact that the extensive use of X-rays in medicine for diagnosis of injuries and diseases represents the largest manmade source of public exposure to ionizing radiation (1). The assessment and control of the performance characteristics of X-ray generators and tubes is an essential part of a quality assurance programme. The high voltage peak is an important measurable quantity to ensure optimal image quality and thus accurate diagnoses. Consequently the calibration of the kilovoltage peak (kVp) meters is very important. Since 1980 instruments are being calibrated at the Calibration Laboratory of IPEN at radiotherapy and radiation protection levels and recently a few qualities at the diagnostic radiology level were established to test some instruments (2). A commercial non-invasive kVp meter, usually, has two diode detectors partially shielded by different filters to provide a ratio that can be used to determine the tube voltage. An additional ionization chamber is necessary to measure air kerma. To calibrate these devices a method using the measurement of the spectrum endpoint has been established with a portable gamma and X-ray spectrometer. The objective of this work is to use as non-invasive transfer system two identical ionization chambers developed at IPEN. The chambers differ only by the electrode material, one is made of aluminium and the other is made of graphite. The good behaviour of the graphite electrode chamber provides the air kerma rates. The transfer system can be calibrated against a spectrometer and against a standard ionization chamber which is traceable to the German Primary Laboratory (Physikalisch-Technische Bundesanstalt PTB). Thus it is possible to use only one reference system to determine kVp and air kerma values.