Logo of the Physikalisch-Technische Bundesanstalt

Spectrometry of X-ray radiation fields

29.09.2008

The spectrometric determination of the energy dependence of the photon fluence is a substantial part of the characterization of the X-ray fields realized in Department 6.6. For this purpose, the pulse height spectra of the radiation fields are measured and unfolded with an HPGE detector. The unfolding procedure used [1] works in accordance with the Bayes method and requires knowledge of the response (response matrix) of the detector.

In addition to the filtered spectra of the different spectra series, unfiltered spectra were recorded for all anode voltages occurring in the spectra series. To ensure a counting rate adequate for the spectrometer, the unfiltered spectra were measured at low anode currents (a few µA) at different distances from the focus of the X-ray tubes, using small diaphragms (diameter ≤ 1 mm). These unfiltered spectra allow - by subsequent numerical filtering with any filters desired - any desired X-ray spectra to be generated. Figure 1 shows the unfiltered spectrum of an X-ray tube with a tungsten anode operated with 110 kV at a distance of one meter. Figure 2 shows the measured B110 spectrum of the same tube (—) compared to the spectrum of Figure 1 numerically filtered subsequently with the B110 filtering () (2 mm copper and 4 mm aluminium). The two spectra show only small deviations from each other in the range of 80 keV and demonstrate the reliability of the procedure.

Figure 1 : Unfiltered spectrum of an X-ray tube with a tungsten anode, operated with 110 kV at a distance of one meter. ∫dN/dE • dE = 1.

Figure 2 : B110 spectrum (—) and spectrum from Figure 1 filtered subsequently with B110 filtering (). ∫dN/dE • dE = 1.

Literature

  1. Kennett, T. J.; Prestwich, W. V.; Robertson, A.:
    Bayesian deconvolution I: convergent properties,
    NIM 151 (1978), pp. 285-292