CdTe X-Ray & Gamma Ray Detector

Theory of Operation

X-rays & 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.

In order to facilitate the electron/hole collection process in the CdTe detector, a + 500 volt potential is applied. This voltage is too high for operation at room temperature, as it will cause excessive leakage, and eventually a breakdown. Since the detector in the XR-100T-CdTe is cooled, the leakage current is reduced considerably, thus permitting the high bias voltage.

The thermoelectric cooler cools both the CdTe detector 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 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.

A temperature monitoring sensor is placed on the cooled substrate to provide a direct reading of the temperature of the internal components, which will vary with room temperature. Once the internal temperature gets below minus 10 °C the performance of the XR-100CdTe will not change with a temperature variation of a few degrees. Hence, accurate temperature control is not necessary when using the XR-100CdTe inside the laboratory.

Vacuum Operation

The XR-100CdTe can be operated in air or in vacuum down to 10^-8 Torr. There are two ways the XR-100CdTe can be operated in vacuum: 1) The entire XR-100CdTe detector and preamplifier box can be placed inside the chamber. In order to avoid overheating and dissipate the 1 Watt of power needed to operate the XR-100CdTe, good heat conduction to the chamber walls should be provided by using the four mounting holes. An optional Model 9DVF 9-Pin D vacuum feedthrough connector on a Conflat is available to connect the XR-100CdTe to the PX5 outside the vacuum chamber. 2) The XR-100CdTe can be located outside the vacuum chamber to detect X-Rays inside the chamber through a standard Conflat compression O-ring port. Optional Model EXV9 (9 inch) vacuum detector extender is available for this application. See photograph of XR-100CdTe with extender and Conflat and components for vacuum applications.

XR-100CdTe X-Ray and Gamma Ray Detector

General
Detector type	Cadmium Telluride (CdTe) Diode
Detector areas	5 x 5 mm (25 mm2)
Detector thickness	1 mm
Energy resolution @ 122 keV,  57Co	<1.5 keV FWHM
Dark counts	<5 x 10-3 counts/sec @ 10 keV < E < 1 MeV
Detector window	Graphite: 1 mil thick (25 µm) Be: 4 mil thick (100 µm)
Preamplifier – Amptek custom design	Reset
Case Size	3.00 x 1.75 x 1.13 in (7.6 x 4.4 x 2.9 cm)
Case weight	4.4 ounces (125 g)
Total power	Less than 1 watt
Operation conditions	0°C to +40°C
Storage and Shipping	Long term storage: 10+ years in dry environment Typical Storage and Shipping: -20°C to +50°C, 10 to 90% humidity non condensing
	TUV Certification Certificate #: CU 72072412 01 Tested to: UL 61010-1: 2004 R7 .05 CAN/CSA-C22.2 61010-1: 2004
Inputs
Preamp power	±8 volts @ 25 mA
Detector power	+500 volts @ 1 µA
Cooler power	Current = 350 mA maximum Voltage = 4 V maximum
Outputs
Preamplifier: Sensitivity Polarity	0.82 mV/keV Negative signal out (1 kohm max. load)
Temperature monitor: Sensitivity	PX5: direct reading in K through software.

XR-100CdTe Connectors

6-Pin Lemo Connector Pin Out

CdTe Detection Efficiency

For 1 mm thick CdTe and 4 mil Be or 1 mil Graphite window (window dominates low energy response, 1 mm thickness defines high energy response).

Figure 6. Log-log plot of interaction probability between 1 keV and 1 MeV.

Figure 7. Linear plot of interaction probability between 10 keV and 250 keV. For more information on the efficiency of the CdTe detector see the AN-CdTe-001 application note. Efficiency Package: A ZIP file of coefficients and a FAQ about efficiency. This package is provided for general information. It should not be used as a basis for critical quantitative analysis.

PX5 Digital Pulse Processor, MCA, and Power Supply for the XR-100-CdTe

Power to the XR-100CdTe is provided by the PX5. The PX5 is DC powered by an AC adaptor and provides a variable Digital Pulse Shaping Amplifier (0.2 µs to 100 µs peaking time), the MCA function, and all necessary power supplies for the detector and preamplifier. The PX5 connects via USB, RS232, or Ethernet to a PC.

The XR-100CdTe/PX5 system ensures stable operation in less than one minute from power turn-on.

Figure 4. This diagram shows the internal connections between the AXR hybrid sensor and the electronics within the case, as well as the external connections to the PX5.

Figure 5. Throughput for various peaking times. The XR-100CdTe provides the best resolution between 1 and 6 µs peaking time.

Application Notes

• XR-100CdTe Efficiency Application Note (AN-CdTe-001)
• XR-100CdTe Charge Transport Application Note (AN-CZT-2)
• Medical X-Ray Tube Spectra for Mammography and Radiology
• X-Ray Tube Characterization
• CdTe Measurement of X-Ray Tube Spectra: Escape Events (ANCDTE-1 Rev. A1)
• CdTe Detectors for Quantitative X-Ray Fluorescence – PDF 1.1 MB
• Characterization of CdTe Detectors for Quantitative X-Ray Spectroscopy – PDF 500 k
• ) from Selected Nuclides³ (5x5x1 mm³Typical Countrate of XR-100CdTe 3x3x1 mm
• Modeling

Typical Countrate of XR-100CdTe 3x3x1 mm³ (5x5x1 mm³) from Selected Nuclides

Modeling

Please refer to the TO-8 detector drawing.

1. The TO-8 cover is made of Ni, 14 mm diameter, 0.250 mm thick.
2. The Detector element is located 1.27 mm behind the 100 µm thick Be window.
3. The detector cavity is vacuum.
4. The size of the detector is 5x5x1 mm³.
5. The top contact is Pt of 20 nm thickness (- side).
6. 200 nm Cd dead layer
7. CdTe depth +/- 50 µm
8. The bottom contact is In of 1 µm thick (+ side).
9. The detector is placed on a ceramic substrate of 0.75 mm thickness.
10. The thickness of the cooler is 2.75 mm.
11. The base of the TO-8 package is made of steel (Kovar), 1.5 mm thick.

Typical Kovar Composition

• Carbon – 0.02
• Silicon – 0.20
• Manganese – 0.30
• Iron – 53.48
• Cobalt – 17.0
• Nickel – 29.0

Do NOT use RTD when trying to compare theoretical results to actual measurements. The following applications notes may be useful when modeling the XR-100CdTe response

• Efficiency of XR-100CdTe Detectors
• Charge Trapping in XR-100CdTe Detectors

For an excellent guide to modeling solid-state detectors (SSD), please consider the following paper:

“A modeling tool for detector resolution and incomplete charge collection” by Jorge E. Fernández, Viviana Scot and Lorenzo Sabbatucci

The authors present an easy to use modeling tool that can be tailored to a specific detector. The tool can be used with three detector types: solid-state detectors, scintillators, and gas proportional counters. The authors used the Amptek CdTe detector as an example of peak shape.

Example Spectra

Figure 1.

Figure 2. 

Figure 3.

Figure 4. 

Figure 5.

CdTe Configurations

XR-100CdTe - PX5

A complete spectrometer including XR100CdTe detector and preamplifier with a PX5 digital pulse processor.

X123CdTe

X123CdTe is a complete spectrometer containing a CdTe detector, a preamplifier, a DP5 digital pulse processor, and a PC5 power supply. 

 


X123CdTe - Vacuum options

CdTe detectors can be operated in vacuum if desired, several options exist.  This version has the detector and preamplifier attached on a flexible cable inside the vacuum space.

X123-CDT230 (PA230 style)

X123CdTe is a complete spectrometer containing a CdTe detector, a preamplifier, a DP5 digital pulse processor, and a PC5 power supply. This version has the detector and preamplifier attached on a flexible cable. 

XR-100CdTe Mechanical Dimensions

1.5 Inch Extender (standard)

All dimensions are in inches except as noted ±0.0005.

Download the XR-100 STP File

CdTe Detector Module