CdTe detectors are often used to measure the spectrum emitted from an X-ray tube. A 1 mm thick
CdTe detector has excellent sensitivity up to 100 keV, with a photopeak efficiency >90% to 65 keV. This is
far superior to Si detectors, where the efficiency drops off by 20 keV. But the response function of the CdTe
detector differs from that of Si or Ge detectors and these differences must be addressed to obtain
quantitative results. Escape of the characteristic Cd and Te X-rays causes particularly noticeable effects.
Figure 1 shows a typical X-ray tube spectrum measured with an XR100T-CdTe detector. This was an
80 kVp tube with a tungsten anode and a 15 keV aluminum filter. Escape of the characteristic X-rays leads
to two features which are clearly visible. First, there are discontinuities in the spectrum at 27 and 32 keV.
Second, although the filter should stop essentially all X-rays incident on the detector below 15 keV, there is a
clear edge at this point, many lower energy events are seen.
Both of these features are due to escape eventsi
. The Cd and Te K edges are 26.7 and 31.8 keV
respectively. Photons with incident energy just above these edges undergo photoelectric interactions,
leaving the Cd and Te atoms in an excited state. When the atoms transition to the ground level they often
emit a characteristic X-ray at one of the energies shown in Table 1. Depending on the direction, this X-ray
may leave the CdTe volume so only a small amount of the incident energy is deposited. Such events lead to
fewer full energy events (causing the absorption edges) and more low energy events (below the filter value)
than would otherwise occur. This process is responsible for the escape peaks which are well known in
spectroscopy, but with a tube there is a continuum of escape events. Escape events are more important in
CdTe than in Si or Ge due to the much higher energies of the characteristic X-rays. Fortunately, algorithms
exist to “clean up” this artifact and are available in software sold by Amptek, Inc.