Measurement of peak kilovoltage across x-ray tubes by ionization methods Public Deposited

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  • Voltage calibration methods used with X-ray tubes are either electrical procedures requiring direct electrical connection or ionization techniques which require no electrical connection to the generating system. One major restriction on the use of electrical methods for measuring the voltage applied to X-ray tubes is that special electrical adaptors are required to bring the conductors out of the grounded tube housing or high-voltage cables. X-ray installations are not normally equipped with these special adaptors and in most cases the use of the X-ray tube would be restricted by the presence of the external high-voltage adaptor. High voltage measuring methods which do not require electrical connection to the X-ray generating system employ direct or indirect ionization techniques to measure the applied voltage. The K-edge calibration method, used in this project, is a direct ionization method based upon the detection of the K-series fluorescent radiation from various secondary radiators. The two main advantages of this method are accuracy and the elimination of electrical connections to the X-ray generating system. Published information on this method lacked data in the region extending from 40 kV to 67 kV. The lack of calibration data in this voltage range was due to the unavailability of secondary radiator elements of suitable purity. Certain rare earth elements were used to provide calibration measurements within this voltage range. The oxides of cerium, samarium, gadolinium, holmium and ytterbium were used and calibration data was obtained at 40.43 kV, 46.85 kV, 50.23 kV, 55.61 kV, and 61.31 kV, respectively. The K-edge calibrations for tin (29.2 kV) and tantalum (67.5 kV) were used to provide an experimental base line. These data, combined with previous reference values, permit the use of the K-edge ionization calibration method throughout the entire voltage range commonly used in diagnostic X-ray procedures. Reproducible results can be obtained using this kilovoltage calibration method if certain variables are rigidly controlled. The geometrical arrangement of all the equipment must remain constant throughout the entire period of time during which the measurements are taken at the K-edge. Changes in geometry alter the amount of scattered radiation and the effective thickness of the secondary radiator. The oxide secondary radiator must not be subjected to mechanical shock or vibration after final positioning in the beam and its moisture content must be held constant throughout the entire measurement period. Changes in any of these parameters result in errors in the ionization chamber readings. The accuracy and wide voltage range make the K-edge ionization method of X-ray tube kilovoltage measurement a reliable procedure for use in X-ray physics. Most other kilovoltage measuring methods encounter problems at high tube currents and short exposure times. The ionization method is restricted only by the intensity limits of the ionization chambers and if these are selected properly then this method may be used at tube currents and exposure times where most other methods fail.
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