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Si-PIN vs. CdTe Comparison

Amptek manufactures X-ray detectors using both Si-PIN and CdTe sensors, each of which has advantages in certain applications. The plots below show detailed comparisons of the efficiency and resolution performance of Amptek Si-PIN and CdTe X-ray detectors.

The key points are:

1. For the lowest energies, generally below 25 keV, Si-PIN is the detector of choice. Si-PIN has better energy resolution than CdTe at all energies, lower background counts, and has good efficiency up to 25 keV. The efficiency variation depends on the detector thickness, but is near 100% up to 10 keV or so and is 20% or better to 25 keV.
   Example: For detecting the L-lines of Pb (10.55 keV, 12.61 keV) the XR-100CR with Si-PIN detector is recommended.
2. For higher energy X-rays, generally above 25 keV, CdTe is the detector of choice. It has better stopping power, with efficiency near 100% up to 50 keV and 50% at 100 keV. Its resolution is slightly worse and its background is higher.
   Example: For detecting the K-lines of Pb (74.96 keV, 84.92 keV) the XR-100T-CdTe with CdTe detector is recommended.
3. There will be some overlap and some trade-offs, so the best choice depends on the details of the application. Si-PIN detectors will almost always have better spectral characteristics: better resolution, better peak to background ratios. CdTe detectors will have better efficiency and also operate at shorter shaping times, which is helpful at high count rates.

Si-PIN and CdTe Efficiency Curves

Si-PIN Detection Efficiency

Figure 1 (linear). Shows the intrinsic full energy detection efficiency for Si-PIN detectors. This efficiency corresponds to the probability that an X-ray will enter the front of the detector and deposit all of its energy inside the detector via the photoelectric effect.

Figure 2 (log). Shows the probability of a photon undergoing any interaction, along with the probability of a photoelectric interaction which results in total energy deposition. As shown, the photoelectric effect is dominant at low energies but at higher energies above about 40 keV the photons undergo Compton scattering, depositing less than the full energy in the detector.

Both figures above combine the effects of transmission through the Beryllium window (including the protective coating), and interaction in the silicon detector. The low energy portion of the curves is dominated by the thickness of the Beryllium window, while the high energy portion is dominated by the thickness of the active depth of the Si detector. Depending on the window chosen, 90% of the incident photons reach the detector at energies ranging from 2 to 3 keV. Depending on the detector chosen, 90% of the photons are detected at energies up to 9 to 12 keV.

CdTe Detection Efficiency

For 1 mm thick CdTe (Be window dominates low energy response).

Figure 3. Log-log plot of interaction probability between 1 keV and 1 MeV.

Figure 4. Linear plot of interaction probability between 10 keV and 250 keV.

Figure 4a. Log plot of interaction probability for Si-PIN and CdTe up to 200 keV.

For more information on the efficiency of the CdTe detector see the ANCZT-1 application note. This note includes the numeric table of efficiencies that created the figures above .

Click here to download the Amptek Efficiency Package.

Pb (lead) Fluorescence from ⁵⁷Co

Figure 5.

⁵⁷Co Spectrum

Figure 6.

²⁴¹Am Spectrum

Figure 7. Logarithmic Scale.

Figure 8. Linear Scale.

⁵⁵Fe Spectrum

Figure 9. Logarithmic scale.

Figure 10. Linear scale.

Mini-X Silver (Ag) 50 kV X-Ray Tube Output Spectrum

Figure 11. Mini-X X-Ray Tube (Ag) Output Spectrum taken with a 500 µm thick Si-PIN and a 1 mm thick CdTe.

Revised May 4, 2011