CdTe Application Note: Characterization of X-ray Tubes

Figure 1. Direct output x-ray spectra from the Mini-X x-ray tube (Ag target – left, W target – right).

System Description

X-123CdTe

The X-123CdTe is a high performance X-Ray and Gamma Ray detector mounted on a thermo-electric cooler (Peltier type) together with the input FET to the preamplifier. Monitored by an integrated circuit, these components are kept at -30 °C and are enclosed in a hermetic package with a vacuum tight, light tight Beryllium window. Power and signal processing to the detector is provided by the integrated DP5 digital pulse processor, and PC5 power supply, which ensures quick, stable operation in less than one minute from power turn-on. In short, the X-123CdTe includes the detector, preamplifier, digital pulse processor and MCA, and power supply. It connects via USB, ethernet or RS-232 to your PC and is powered by either USB or the included +5 V AC power adapter.

Figure 2. EXVC collimator kit on the X-123CdTe.
 

Collimator Kit

Amptek has developed the EXVC Collimator Kit to collimate the primary X-ray beam. This system is comprised of the standard 1.5 inch extender box which slides inside a Collimator Housing. The Collimator Housing can accommodate up to two Tungsten collimator disks that are placed inside a bayonet holder in front of the detector. By selecting the appropriate Tungsten collimator disks, the user can reduce the incoming X-ray flux and allow the detector and electronics to process the X-ray spectrum. Seven different Tungsten collimator disks are provided with different size holes (ranging from 25 µm to 2,000 µm hole) in order to allow for a wide range of applications. The Collimator Housing is made out of stainless steel.

Figure 3. The EXVC Collimator Kit slides over the 1.5″ extender of the X-123CdTe.

The kit includes

• Stainless steel collimator housing
• Tripod and mounting plate
• Laser pointer
• Brass Spacer
• 5 Tungsten (W) Collimator disks:
  • 1 mm thick with 25 µm hole
  • 1 mm thick with 50 µm hole
  • 2 mm thick with 400 µm hole
  • 2 mm thick with 1000 µm hole
  • 2 mm thick with 2000 µm hole

 
All Tungsten disks are made of alloy HD17 (90% W, 6% Ni, 4% Cu). All Tungsten disks and spacers have a diameter of 0.625 inches. Optional: EXVC-W-SPACER This Tungsten (W) Spacer /Collimator is 36 mm thick with a 300 µm hole. It is designed to stop and collimate x-rays greater than 100 keV produced from high energy tubes.
 

Escape Peak Adjustment with XRS-FP Software

The XRS-FP software can be used to adjust the output spectrum for the escape peaks of the CdTe detector. This can be a significant effect for higher energy x-ray tubes that operate above the absorption edges of Cd and Te.

 
For more information please see the application note ANCDTE1: CdTe Measurement of X-Ray Tube Spectra: Escape Events.
 

Figure 4. Plot showing a tungsten (W) x-ray tube output spectrum taken with a CdTe detector after processing to remove escape events. The gray trace shows the original spectrum. The green trace illustrates the escape events in the original spectrum. These are subtracted from this original spectrum, then the correct energies are computed (by adding in the energy which escaped). The blue trace shows the corrected escape events, which are then summed with the gray trace. The dark black trace shows the final result of the processing with the events in their correct channels.

Repeat Measurements for Short and Long Term Characterization

The X-123CdTe makes a powerful tool for characterizing short and long term stability of X-ray tubes and controllers. The use of the EXVC Collimator placed in front of the X-123CdTe reduces the X-ray flux and avoids detector saturation. The detector can now look directly at the x-ray tube and take energy spectra at regular intervals to monitor its performance. The Amptek DPPMCA Display and Acquisition Software incorporates a routine that automatically saves a complete spectrum at user defined intervals, and in addition produces a file consisting of the “total counts” in each spectrum. This “total counts” file can then be plotted in a spreadsheet program.

 
Click here to download the DPPMCA software.
 

For short term stability investigations, Figure 5 shows the setup of the DPPMCA software. It is set up for a routine of 100 repeated spectra of 60 seconds each. The plot of the output X-rays (total counts file), as seen in Figure 6, shows that the X-ray tube and controller take about 12 minutes to warm-up before they stabilize.

Figure 5.

Figure 6.
 

For long term stability investigations, Figure 7 shows the plot of the output X-rays (total counts file) over 6 hours. The inset in the plot is a graph of one of the spectra saved. Between the total counts file and the full spectrum at each point, a total analysis can be made of the x-ray tube stability and integrity. The plot also shows the Average, Predicted Standard Deviation, Calculated Standard Deviation, and Current Stability. The total counts file enables these four calculations to be made easily in any spreadsheet program.

Figure 7.
 

A different way to measure stability of the X-ray flux of the tube is to set up the X-123CdTe to detect the X-ray fluorescence (XRF) produced from a stationary target. A typical angle of “tube-target-detector” is about 45 degrees. In this configuration the EXVC collimator kit might not be necessary.