Friday, August 19, 2011

EDS Pulse Processing Reset

The EDS detector pulse processing unit is shown in this first picture. On the left hand side it has several LED's that blink and change color depending upon the status of the EDS detector. The most important are the RATE and PREAMP LED's which are typically green in color. If they are red then the EDS detector is being swamped with events. As such the dead-time is very high and the detector is not working efficiently. This status information will be reflected in the dead-time bar in the EDS software.

The EDS detector window is opaque to visible light but is transparent to X-rays above C Kα. In practice there is a thin organic layer cryo-pumped on the EDS detector window which limits light element EDS sensitivity. This organic layer consists of diffusion pump fluid as well as organics that evolve from samples themselves. However, the EDS detector window, even with this cryo-pumped organic film, is still transparent to IR. As such, the IR lamp for the chamber scope will flood the EDS detector.

The second image shows the digital display for the EDS detector working normally. At the top right corner is the detector bias voltage which is always around 600 VDC as it is dependent upon the detector design. The top left corner shows the LN2 status. If the liquid nitrogen level were to drop, this would show some indication and eventually the EDS detector would be turned off. In the middle of the screen is a bar graph of the EDS detector signal. It shows a 4 kHz scale and a count rate of ~ 2.5 kHz. This photo was taken with a 30 kV beam on a brass sample stub, so X-rays were definitely being produced.


This third image shows the left hand side of the pulse processing unit during normal operation. The rate light is GREEN and BLINKING, as is the preamp LED. This is all normal functioning.

Photos were then taken with the chamber scope IR lamp on. The chamber scope is a very naive product. Since the ET-detector-- the secondary electron detector-- is not sensitive to IR, the chamber scope floods the SEM chamber interior with infrared radiation. A small CCD camera sensitive to the IR spectrum is open to the SEM chamber and then directly images the chamber interior. This is problematic for the BEI and EDS detectors which are both sensitive to IR. That is an acceptable complication as one is typically using the SEM in SEI mode together with the chamber scope to bring samples to an appropriate working distance.

This next image shows the EDS pulse processing digital display after the chamber scope IR lamp has been on for several seconds. Note in the top right corner that the detector bias has dropped to 20 VDC from 600 VDC rending the EDS detector incapable of operation. Also note that beneath the rate bar it says "Detector off 51". This indicates that the detector is functionally OFF and the pulse processing unit is at "51" counting down from "60" at which time it will attempt to turn back on. If the rate is still too high it will turn the detector off and count down again. If one were to look at the dead-time status bar in the EDS software it would be red and at or near 100%.

If one attempts to take a spectrum with the chamber scope IR lamp ON and the EDS detector switched off when it is saturated with counts-- one will either see nothing or a huge low energy background. The appropriate response is to stop EDS data acquisition, turn the chamber scope IR lamp OFF and let the EDS detector pulse processing electronics recover and turn the EDS detector bias back ON.

However, some users will close the EDS software when this happens. The result is that the EDS pulse processing electronics gets hung up. To fix this, close the EDS software and turn the pulse processing unit off and on. In this last picture the ON/OFF switch is seen on the bottom left of the back of the unit near the power cord.

The moral of this post is that the EDS detector always works: it's just a question of whether it's detecting X-rays or IR-- and whether the bias is turned off from it being soaked in IR. Everything you need to know is on the front panel of the pulse processing unit.

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