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Additional Spectra
Internal Silver or Multilayer Collimators for 500 µm and 680 µm Thick Detectors
Use of External Collimators with 7 mm2/300µm and 13 mm2/300µm Detectors
Rise Time Discrimination (RTD)
Resolution as a Function of Energy
Figure 1. X-ray fluorescence (XRF) of multi-element sample from109Cd.
Figure 2. Low (Z) element x-ray fluorescence (XRF) with 6 mm2/500 µm detector.
Figure 3. 241Am Spectrum.
The 5mm2/6mm2/13mm2/25mm2 X 500 µm and 5mm2/20mm2 X 680 µm detectors exhibit "edge effects" due to partial charge collection at the edge of the detector which produce a secondary peak.
Figure 4. This plot shows a comparison between a collimated detector and a detector without a collimator.
Although a small effect, approximately 1% of the counts of the 5.9 keV peak, an internal silver (Ag) or multi-layer (see below) collimator is now used on all 5mm2/6mm2/13mm2/25mm2 X 500 µm and 5mm2/20mm2 X 680 µm detectors in order to remove the secondary peak. An external collimator is always possible.
Collimators can be made from material other than Aluminum, like Copper, Tungsten, Silver or other, provided the fluorescence peaks from the collimator material do not interfere with the anticipated measurement.
In cases where fluorescence peaks produced from the edges of collimators need to be minimized or eliminated, a multilayer collimator can be made by progressively using lower Z materials. Each layer will act as an absorber to the fluorescence peaks of the previous layer. The final layer will be of the lowest Z material whose fluorescence peaks are of low enough energy to be outside the anticipated X-ray detection range.
Amptek has developed a state-of-the-art internal MultiLayer Collimator (ML). The base metal is 100 µm of tungsten (W), the first layer is 35 µm of chromium (Cr), the second layer is 15 µm of titanium (Ti), and the last layer is 75 µm of aluminum (Al).
X-ray events that are produced near the edges of a detector may result into partial charge collection. Hence, a typical "tail" is observed at the low energy side of an energy peak. This "tail" is often called the "Background".
In the case of the 55Fe, the ratio of the counts at the 5.9 keV peak to the counts at about 2 keV is called the "Peak to Background Ratio" (P/B).
The XR-100CR with a 7 mm2 uncollimated detector exhibits a P/B = 260. However, the same detector with an external Aluminum collimator having a 2 mm diameter hole has a P/B = 3150. See Figure 5.
The use of a collimator increases the signal to noise ratio of X-ray events that fall on the low energy tails of higher peaks, and thus increasing the counting statistics in observing such events. As shown in the figure, the 4.2 keV Silicon escape peak from the 5.9 keV is better defined with a collimated detector than with an uncollimated one.
Figure 5.
 Figure 8. RTD vs. No RTD |
The 7 mm2/300 µm and 13 mm2/300 µm detectors are partially depleted. From the total thickness of 300 µm, approximately the front 200 µm are totally depleted. The remaining 100 µm near the back contact are partially depleted. Electron-hole pairs created by X-Rays which interact with the silicon near the back contact of the undepleted detector are collected more slowly than normal events. These events can result in smaller than normal charge collection and may increase the peak to background ratio (P/B) in the energy spectrum. In partially depleted detectors (7 mm2/300 µm or 13 mm2/300 µm) these events pruduce secondary peaks that need to be removed. To remove the secondary peaks, both the PX2CR and the PX4 incorporate a Rise Time Discrimination circuit (RTD) which prevents these pulses from being counted by the MCA. See figure 8. All spectra shown in this specification were taken using RTD.
Figure 10. Resolution as a function of energy for various detector resolutions. For example, a detector with an 55Fe resolution of 145 eV FWHM will follow the red curve.
Example: Find the 55Fe row in the above table and locate the resolution of the detector (bold). The column with that resolution lists the resolutions of that detector for these comon energies.
Amptek X-Ray Chart (K and L emission lines)
Revised March 14, 2007