Moxtek makes ultra-thin polymer X-ray windows that are often used in SEM and TEM microanalysis detectors. Moxtek windows are an excellent gas barrier and allow detector electronics to stay in vacuum while the sample chamber varies between vacuum and atmospheric pressure for sample loading and unloading. It is important to understand the pump down and vent rate tolerances of Moxtek's X-ray windows for proper equipment use design.
Moxtek windows are tested in a pressure chamber that delivers pressure to the front side of the window then returns to atmosphere. This simulates the stress seen by an in situ X-ray window by cycling windows between 0.19 atm and 1.2 atm of differential pressure.
Moxtek windows are tested to be able to withstand a minimum of 10,000 cycles and still maintain their hermetic properties. Figure 1 shows an example of the differential pressure delivered during 10 of these cycles. The hermetic seal is tested after the 10,000 cycles with a helium leak detector. The vent and pump down rates can be calculated by finding the average slope of both the vent and pump down portions over the area of the greatest rate of change.
Figures 2 and 3 show calculations for a vent rate and a pump down rate, respectively. The average slope of these sections yields 0.390 atm/sec for venting and 0.335 atm/sec for pump down.
The same calculations from Figures 2 and 3 were done for 500 cycles. Table I shows the mean and standard deviation of 500 cycles for both pump down and vent rates.
Moxtek ensures that its X-ray windows can tolerate a minimum of 10,000 cycles from 0.19 atm and 1.2 atm differential pressures. The average maximum vent rate for 500 cycles was 0.404 atm/sec and the average maximum vent rate for 500 cycles was 0.330 atm/sec. Moxtek recommends designing equipment with Moxtek windows to pump down no faster than 0.330 atm/sec and vent chambers no faster than 0.404 atm/sec.
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