This Kratos monochromator adjustment procedure is used on the Kratos Axis, 165, Ultra and Nova monochromators.
Steps involved in optimizing the performance of the Al Monochromatic x-ray source:
First, load a sample bar containing a grid alongside a clean Ag foil sample at the same height.
- Using the Mg source image the grid to locate the height of the auto stage that gives the best focus. Move across to the Ag foil sample.
- Using the Mg anode find the BE position of the Ag 3d 5/2 peak. This method requires that the calibration of the energy scale of the instrument is correct.
- Using the BE position obtained with the Mg dual anode switch to the mono source and begin tuning the source position in conjunction with the crystal settings as detailed below.
- The image below shows the mono crystal adjustment mechanism. First undo the locking screw. (Rather than just loosening the locking screw, it sometimes helps to use the locking screw as a counter drive against the micrometers to enable a finer adjustment on the crystal mechanism). Micrometer A tilts the crystal left and right moving the mono spot in the x-axis on the auto stage. This can be carefully adjusted to optimize the count rate on its own (no interaction with other parameters). Similarly, there is a tilt adjustment on the mono source that changes the source to crystal distance that can be adjusted for maximum count rate at this point.
- The micrometer on top of the mechanism (marked “Do not adjust” in the image) changes the distance between the crystal and the sample. (After initial factory set up …i.e. for normal maintenance procedures…anode changes etc… this will not need re-tuning).
- Micrometer B tilts the crystal to move the mono spot along the y-axis of the auto stage (towards and away from the dual anode source). This micrometer always needs to be adjusted in conjunction with the linear movement of the mono x-ray source itself. When the mono source position is changed it moves the refracted beam on the sample so the tilt adjustment is required to move the mono spot back to the analysis position to maximize the count rate.
- The linear drive on the mono source itself should be adjusted in small increments. At each new position micrometer B is used to move the beam spot back to the analysis position (by maximizing the count rate). The best method is to tilt the crystal carefully through the maximum once or twice and make a note of the best count rate obtained.
Next, move the mono source a short distance (this will decrease the count rate). Now repeat the adjustment on Micrometer B to bring the count rate to a new maximum. If the new maximum is higher than the previous one continue the process moving the source in the same direction until the new maximum is lower than the previous one. Then move the source back to the previous setting and re-optimize.
If at the start of the procedure the second maximum is lower than the first, reverse the direction of movement of the mono source and continue the process in that direction to find the best “maximum” count rate.
Once the set up is close to optimum performance more improvement can sometimes be obtained by setting up the tuning using the 110um selected area instead of large area “hybrid” mode. (Starting with the 110um aperture is sometimes difficult due to the lower counting rates).
The locking screw should finally be re-tightened in conjunction with small final adjustments of the micrometers to maintain the maximum count rate. The locking screw may push the crystal position slightly as it is tightened.
Installing parts using Au seals.
When installing using Au wire seals it is essential to locate the seal in the correct position. The seal should be resting snugly on the shoulder formed above the main flange.
This way when the 2 surfaces come together the Au wire is ‘boxed” in and compressed.
If the Au seal is stretched or is not a tight fit on the shoulder and therefore “hangs” over the edge of the shoulder above the flange it will be pinched as the flanges are pulled together and NO SEAL CAN BE ACHIEVED NO MATTER HOW MUCH YOU TIGHTEN THE BOLTS!
Since the seal cannot be seen after mating up the flanges the only indication of whether the Au seal is in place is the gap between the 2 flanges when they are pushed together. There should only be a very small gap between the flanges <~1mm. If the gap is larger it means the Au seal is pinched.
Water Box Maintenance
With 15kV applied at the anode the water lines act as a conductive path to ground. The first point of contact to ground is at the outlet pipe from the solenoid valve (above the valve in the water box. (The water tube goes from there to the supply pipe on the anode). The other point is where the return line from the anode attaches to the flow switch (below the flow switch)
These are the most like places for corrosion due to the voltage difference. Check and clean out or replace these fittings to bring up the flow rate.
Also remember to check the in-line filter. If there is a drop in pressure across the filter as indicated by the pressure gauges, the filter may need to be replaced.
NEVER LOWER THE SLIDER ON THE FLOW SWITCH TO GET BACK INTO OPERATION IF THE X-RAY SOURCE HIGH VOLTAGE WON’T COME ON. OPERATING THE SOURCES AT LOWER FLOW THAN THE FACTORY RATING WILL CAUSE SHORTENED ANODE LIFETIMES.