The Research Of Air-kerma Rate Measurement For 25kV Mammography X-ray

Mammography screening is recognised as the early detection method of breast cancer with the greatest impact in reducing mortality,and with the popolarity of this technology,the quality assurance for the mammography device is important.To make sure the dosemeters could be calibated and traceable to national facility,it has to eatablish the mammography X-ray field and radiation quality,and determine the value of incident air-kerma.A two-dimensional matrix ionization chamber was used to confirm the coaxality of the beam and rail in the radiation field and set the positioning plane at 600 mm.The radiation field size at different distances is measured by using the finger ionization chamber,and the effective radiation field above 99% at the plate of the 600 mm location is 50 mm,it suit for the demands of the experiment.According to IEC-61267,series RQR-M and other 14 radiation quality was established based on exponential fitting method.And use the monitor chamber to supervise the stability of the equipments and ionization chamber.A parallete free-air ionization chamber is designed and assembed,and test the leakage current and high voltage saturation curve after the completion of the chamber.The leakage level compared with the usual measurement,the effects of less than 0.01%,meet the needs of the measurement.Get the correction factors by means of experimental measurement and Monte Carlo method,and with the correction factors,using the free-air ionization chamber to get the values of the airkerma rate and calibration factors of transfer chamber RC6 M of RQR-M series.The same transfer chamber with two series of calibration factors,those two series calibration factors compared with each other,the deviation is between 0.04% ~ 0.09%.The low energy standard ionization chamber was used to measure the air-kerma rate of the same series radiaton quality and the calibration factors of transfer chamber,the deviation range of those two calibration factors was 0%~0.21%.The uncertainty of the air-kerma rate measured by the free-air ionization chamber and calibration factors by the transfer chamber were analyzed in detail.The uncertainty of the air kerma rate the relative standard uncertainty was 0.42%(k=1),the calibration factors relative expanded uncertainty was 0.88%(k=2).