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.