Using measured 30-150 kVp polychromatic tungsten x-ray spectra to determine ion chamber calibration factors, N(x) (Gy C -1 )

Two methods for determining ion chamber calibration factors (N x ) are presented for polychromatic tungsten x-ray beams whose spectra differ from beams with known N x . Both methods take advantage of known x-ray fluence and kerma spectral distributions. In the first method, the x-ray tube potential is unchanged and spectra of differing filtration are measured. A primary standard ion chamber with known N x for one beam is used to calculate the x-ray fluence spectrum of a second beam. Accurate air energy absorption coefficients are applied to the x-ray fluence spectra of the second beam to calculate actual air kerma and N x . In the second method, two beams of differing tube potential and filtration with known N x are used to bracket a beam of unknown N x . A heuristically derived N x interpolation scheme based on spectral characteristics of all three beams is described. Both methods are validated. Both methods improve accuracy over the current half value layer N x estimating technique.