The Research Of Air Kerma Absolute Measurements For 300kV X-ray

Air kerma is one of the important Physics of measurement and transfer in radiation system.At present,many countries have established?10-300?kV X-rays air kerma primary standard.Our country also has established?10-250?kV X-rays air kerma primary standard and?250-600?kV X-rays air kerma primary standard is in research.The existing X-rays air kerma primary standards all use free air chamber for absolute measurements present.But the energy of the X-rays is higher than the usual in"?250-600?kV X-rays air kerma primary standard research”,we will need a free air chamber in large volume and quality of tons for the low measurement uncertainty.It is difficult to make and preserve such a big ionization chamber.So we choose graphite cavity chamber in the research.Graphite cavity chambers have good energy responses to high energy rays and fine in precision and stability,which is widely used in ¹⁹²Ir,¹³⁷Cs and ⁶⁰Co gamma rays air kerma absolute measurements.This paper uses free air chamber and graphite cavity chamber for 300 kV X-rays air kerma absolute measurements.The results of the free air chamber and graphite cavity chamber for the300kV X-ray absolute measurements are compared to verify the results and methods mutually.The main research results and innovation of this paper are as follows:?1?The narrow spectrum 300 kV X-ray standard radiation field is established according to ISO 4037 standard?N represents narrow spectrum in this paper?.In standard radiation field establishing,we use the curve fitting method to measure the half-value-layers.The result is within 5%to standard recommended value.Using BEAMnrc program package make the spectral simulation and result is in good agreement with theoretical spectrum,so the radiation field that established in this paper conforms to standards.?2?Make the absolute measurement of N300 kV X-ray air kerma with free air chamber.In the process:measure the compound loss correction factor by change the tube current of the X-rays machine and the high voltage of the free air chamber,measure the wall penetration correction factor by adding and removing the lead plug,measure the polar correction factor by changing high voltage polarity of the free air chamber,measure air attenuation correction factor?aperture edge penetration correction factor?fluorescence and scattering and electron loss correction factors by EGSnrc Monte Carlo simulation program,evaluate the electric field distortion correction factor and cite air humidity correction factor,finally obtain 0.9320 total correction factor of free air chamber.Based on measurements and uncertainty evaluations of correction factors,bremsstrahlung,air ionization energy,air density,ionization currents,temperatures,pressures and effective volume,6.99 x 10^-3?mGy/s?is got for free air chamber in the absolute measurement of N300 kV X-ray air kerma,and the uncertainty is 0.69%?0.85%when the uncertainty is 0.15%?0.35%for air ionization energy respectively.?3?Make the absolute measurement of N300 kV X-ray air kerma with graphite cavity chamber.In the process:measure the compound loss correction factor by double voltage method,measure the radial and axial non-uniform correction factors by changing the position of graphite cavity chamber,measure the stem scattering correction factor by adding virtual stem,measure the wall correction factor by EGSnrc simulation,cite air humidity correction factor,finally obtain 0.9320 total correction factor of graphite cavity chamber.Based on measurements and uncertainty evaluations of correction factors,stopping-power ratio of graphite to air,ratio of photo mass energy coefficients air to graphite and cavity volume,6.99 x 10^-3?mGy/s?is got for graphite cavity chamber in the absolute measurement of N300kV X-ray air kerma,the uncertainty is 0.38%?0.52%when the uncertainty is0.15%?0.35%for air ionization energy respectively.The deviation of N300kV X-ray air kerm measured by graphite cavity chamber and free air chamber is 0.57%,and that is in the free air chamber's uncertainty,so verified the results and measurement methods generally.Graphite cavity ionization chamber was used to measure ⁶⁰Co and¹³⁷Cs gamma rays'air kermas.Deviations of measurement results and the standards values are 0.32%and 0.18%respectively.The results are very near the standards'values,and verified the measurement methods used in this paper.This paper realizes absolute measurements of 300kV X-ray air kerma in free air chamber and graphite cavity chamber,improves X-rays air kerma measurement ability and offers methods for X-rays air kerma measurements in graphite cavity chambers.It offers references for the study of X-rays air kerma measurement.At the same time,it offers references for standard radiation fields establish,some physical parameters study,correction factors study in free air chamber and graphite cavity chamber and measurement uncertainties evaluation.