−
16
−
ME12044A
6.5.6
Adjusting the detector
In XAFS measurement, the energy of detected X-rays varies depending on the element under
measurement, unlike in powder X-ray diffraction, which uses characteristic X-rays. This requires
changing the voltage of the detector and the settings of the single channel analyzer.
The standard configuration of the R-XAS Series employs a semi-transmission-type proportional
counter for monitoring the intensity of incident X-rays, and a scintillation counter for counting the
intensity of X-rays transmitted through the sample. While the scintillation counter provides
detection efficiency of nearly 100% over a wide energy range, the detection efficiency of the
proportional counter varies depending on the type of gas with which it is filled. For XAFS
measurement, it is desirable to use an I
0
-detector with detection efficiency ranging from
approximately 10% to 30% for incident X-rays. Therefore, the gas best suited to the target energy
must be chosen. To make an adjustment using the Cu foil, employ the Ar-300 (filled with argon
gas) supplied with the equipment as a standard accessory.
Adjust the detector by selecting “Adjust”
→
“I
0
Counter” (or “I Counter”). For “HV scan,” set
the window of the single channel analyzer to the specified value, and gradually vary the voltage
imposed on the detector. This will change the gain of the detector and bring the pulse hight of the
output signal to within the range of the single channel analyzer. Enter the voltage of the peak
position detected by “HV scan” in HV of “PHA scan,” and initiate the PHA scan. The value of
the starting point of the peak obtained in the PHA scan will become the “Baseline” value, and the
width of the peak will become the “Window” value. When the scintillation counter and proportional
counter are compared, the PHA scan of the latter is found to be narrower in width. In addition, the
proportional counter exhibits escape peaks that are not seen on a scintillation counter. An actual
adjustment procedure using the Ar-300 will be described below.
(1)
HV scan
Enter HV (low) = 800, HV (high) = 1200, Step = 5, and click on [OK]. As the HV scan comes to
an end, a dialog box for range selection will be displayed, as in the case of the
ω
scan. Enlarge the
proximities of the peak selectively, and click on [OK]. However, note that the peak value found
here will be stored in the HV/PHA tab, but that the value will not be reflected in the PHA scan tab
in the following step. Therefore, it will be necessary to memorize the value.
If no peak is observed, it is likely that there is no X-ray entering the detector. Perform the
preparations again, starting from the adjustment of the monochromator.
Figure 6.11 HV Scan Tag
