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.

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.

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.

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 Detection Efficiency

xreff1

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.

xreff2
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).

CdTe Detection Efficiency - Log-log plot of interaction probability between 1 keV and 1 MeV

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

cdte_15
Figure 4. Linear plot of interaction probability between 10 keV and 250 keV.

si_cdte_7
Figure 5. 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

cdte_si_pb_xrf

Figure 6. Pb XRF from 57Co (log).

cdte_si_pb_xrf_linear

Figure 7. Pb XRF from 57Co (linear).

Pb Resolution (eV FWHM)

Line Si-PIN CdTe
Pb La1 (10.55 keV) 237 418
Pb Ka1 (74.96 keV) 532 704

57Co Spectrum

cdte_si_57co

Figure 8. 57Co (log).

cdte_si_57co_linear

Figure 9. 57Co (linear).


241Am Spectrum

cdte_si_241am

Figure 10. 241Am (log).

cdte_si_241am_linear

Figure 11. 241Am (linear).

241Am Resolution (eV FWHM)

Line Si-PIN CdTe
13.95 keV 240 400
59.54 keV 400 600

55Fe Spectrum

cdte_si_55fe

Figure 12. 55Fe (log).

cdte_si_55fe_linear

Figure 13. 55Fe (linear).

55Fe Performance

Parameter Si-PIN CdTe
5.9 keV Resolution (eV FWHM) 140 240
Peak to Background (P/B) Ratio 19,000:1 100:1

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

minix50kv_cdte_si
Figure 14. Mini-X X-Ray Tube (Ag) Output Spectrum taken with a 500 µm thick Si-PIN and a 1 mm thick CdTe.

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