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Model XR-100T-CdTe-Stack is a new high performance X-Ray and Gamma Ray Detector, Preamplifier, and Cooler system using a stack of three 5 x 5 x 0.75 mm3 Cadmium Telluride (CdTe) diode detectors mounted on a thermoelectric cooler. Also on the cooler are the input FET and feedback components to the charge sensitive preamplifier. The internal components are kept at approximately -30°C, and can be monitored by a temperature sensitive integrated circuit. The hermetic TO-8 package of the detector has a light tight, vacuum tight 4 mil (100 µm) Beryllium window. All the critical connections between the detector and preamplifier have been made internally to the XR-100T-CdTe-Stack to ensure quick, first time operation by the user. The XR-100T-CdTe-Stack is provided complete with BNC connectors and power cable.
In order to facilitate the use of the detector, the PX4 was developed to provide a shaping amplifier using digital pulse processing technology, and integrated multichannel analyzer, and power supplies.
Figure 3. 137Cs Spectrum. Typical resolutiuon between 0.75% and 1.5% @ 662 keV.
X-Rays and Gamma Rays interact with CdTe atoms to create an average of one electron/hole pair for every 4.43 eV of energy lost in the CdTe. Depending on the energy of the incoming radiation, this energy loss is dominated by either the photoelectric effect or Compton scattering. The probability or efficiency of the detector to “stop” the incoming radiation and create electron/hole pairs increases with the thickness of CdTe. See Figure 3.
In order to facilitate the electron/hole collection process in the CdTe-diode detector, a bias of up to 1500 Volts is applied. This voltage is too high for operation at room temperature, as it will cause excessive leakage and eventually a breakdown. Since the detectors in the XR-100TCdTe-Stack are cooled, the leakage current is reduced considerably, thus permitting the high bias voltage.
To efficiently detect gamma rays with energies of hundreds of keV, Amptek has developed a “stack” detector, using multiple detector elements, each 0.75 mm thick and 25 mm2 area. The detector stack is mounted on a thermoelectric cooler and packaged using Amptek’s successful XR-100 technology.
The advantage of this detector stack over a single detector of the same volume is the significant improvement in charge transport. The charges are transported across 0.75 mm, so the hole tailing is equal to that seen with a single 0.75 mm thick detector, but for radiation interaction purposes, the entire volume is used. Charge collection efficiency is much higher than in a single, planar element of equal volume since the travel distance to the collecting electrode is short.
The intrinsic efficiency of the stacked detectors was measured by comparison with a 1" NaI detector. The intrinsic efficiency was measured to be within 1% of that computed from the volume of the detector, implying that the full volume is used and contributes to the photopeak. See Figures 4 and 5.
The thermoelectric cooler cools both the CdTe diode detectors and the input FET transistor to the charge sensitive preamplifier. Cooling the FET reduces its leakage current and increases the transconductance, which in turn reduce the electronic noise of the system.
In order to further reduce the electronic noise, the feedback capacitor and part of the current feedback network to the preamplifier are also placed on the same substrate as the detector and FET. This minimizes parasitic capacitance at the input.
General | |
| Detector type | Cadmium Telluride (CdTe) Diode |
| Detector Area | 5 x 5 mm (25 mm2) |
| Detector Thickness | 2.25 mm (3 x 0.75 mm) |
| Energy Resolution (typical) | 1.5 keV FWHM @ 122 5 keV FWHM @ 662 |
| Detector Window | Be, 4 mil thick (100 µm) |
| Case Size | 3.75 x 1.75 x 1.13 in 9.5 x 4.4 x 2.9 cm |
| Case Weight | 4.4 ounces 125 g |
| Total Power | Less than 1 Watt |
Inputs | |
| Preamp Power | ± 8 Volts @ 25 mA |
| Detector Power | + 1400 Volts @ 1 µA |
| Cooler Power | Current = 350 mA maximum, voltage = 4 V maximum with <100 mV peak-to-peak noise |
Outputs | |
| Preamplifier | Sensitivity: 0.14 mV/keV (may varry for different detectors) Polarity: Negative Signal Out 1 kohm max. load |
| Temperature Monitor Sensitivity | 770 mV = -50 °C PX4: direct reading in K through software |
Connections | |
| Preamp Output | BNC coaxial connector |
| Power and Signal | 6-Pin LEMO connector (Part# ERA.1S.306.CLL) |
| Interconnect Cable | XR100T-CdTe-Stack to PX4: 6-Pin LEMO (Part# FFA.1S.306.CLAC57) to 6-Pin LEMO (5 ft length) |
Options | |
| Other detector sizes: 5 x 5 x 5 mm3 are available on special order. | |
| Other Be window thicknesses are available on special order. | |
| Components for vacuum applications. | |
| Collimator kit for high flux applications. | |
| See also XR-100T-CdTe X-Ray and Gamma Ray Detector specifications and XR-100CR specifications using Si-PIN for detection of low energy X-Rays with high resolution (149 eV FWHM @ 5.9 keV, 55Fe). | |
| Available in X-123 configuration. | |
6-Pin LEMO Connector Pin Out | |
| Pin 1 | Temperature monitor diode |
| Pin 2 | + H.V. detector bias, + 1400 Volt max. |
| Pin 3 | -8 Volt preamp power |
| Pin 4 | +8 Volt preamp power |
| Pin 5 | Cooler power return |
| Pin 6 | Cooler power 0 to +4 V @ 350 mA (max) |
| Case | Ground and shield |
Figure 4. Computed interaction probability.
Figure 5. Comparison with NaI detector.
Figure 6. Comparison with NaI detector
Figure 7. Energy resolution as a function of energy.
Figure 8. Uranium oxide spectrum compared to CZT and HPGe (log)
Figure 9. Uranium oxide spectrum compared to CZT and HPGe (linear)
Figure 10. Uranium oxide spectrum (high energy)
Figure 11. CdTe Stack diagram.
Figure 12. Connection diagram
Reference
Redus, R.H., A. Huber, J. Pantazis, T.Pantazis, "Multielement CdTe Stack Detectors for Gamma-Ray Spectroscopy," IEEE Trans. Nucl. Sci. Vol. 51, No. 5, p.2386, Oct 2004.
Miceli, A., Thierry, R. et al.,"Comparison of simulated and measured spectra of an industrial 450 kV X-ray tube," Nuclear Instruments and Methods in Physics Research A, 580 (2007) p.123-126.
Xr100T-CdTe-Stack Specifications (511 k)
Revised November 14, 2007