The low-Z XAS hutch configuration consists of two separate Helium gas spaces. The front end / I0 section is supplied by SSRL house helium supply, while the low-Z sample box section is supplied by the Helium cylinder in the hallway behind the 4-3 blue electronics racks. The two sections are separated by a thin polypropylene film inside a clamped O-ring assembly as indicated in the picture.
The spaces are kept separate to speed up sample changing and Helium purge times. The polypropylene film will be damaged by the beam over a duration of 1-2 days. If a hole is formed in the film, the two spaces are no longer separate. This means that when a sample is changed, air is introduced into both the low-Z sample box AND the front end / I0 section. Air in the front end section can significantly change the I0 signal, as well as reduce the number of X-rays which reach your sample. This needs to be remedied by changing the polypropylene window.
To test for a leak between sections:
Remove one of the two paddles from the low-Z sample box and observe the flowmeter on the front/end I0 section. If the flow drops to zero, then there is a leak between the two sections, and you will need to change the polypropylene film.
To replace the polypropylene film and fix the leak between sections:
Move VERT and HORZ stage motors to reposition the low-Z sample box until the bellows is not bent vertically or horizontally. VERT is about 42, and HORZ is about 25, but check by eye and adjust if still bent.
Look behind the back of the low-Z sample box. There is a rail along the back of the box attached to the stage plate. There are two allen hex bolts going through this rail to hold the back of the low-Z box to the rail. Use a 5/32" Allen wrench to loosen each bolt a few turns. This will allow the box to slide upstream and downstream on the stage.
Remove the clamp for the O-ring between the two gas sections.
Slide the Low-Z sample box downstream to separate the two gas sections. Remove the O-ring and old polypropylene film. You may see a small cut in the polypropylene film where the beam burned through.
Roll out and tear off a new piece of polypropylene film from the box of SPEX 3520 Polypropylene. About 3" of length is enough. Do NOT touch the center of the film, only the edges - finger oil has Sulfur in it!
With one hand, lay the film flat against the flange to cover the hole in the front end / I0 gas section. Then push the O-ring against the film with the other hand.
Slide the low-Z box back upstream to seat the O-ring. Then clamp the flange / O-ring / film assembly using the clamp.
Test that the two gas sections are separated again. To do this, check that the He flowmeter is at a high value (it reads "H +F" above 500) and decreasing. The initial high value is because there is heavier gas (air) being purged out of the Front end / I0 section. It should eventually settle around 200-240 when only Helium is in the front end / I0 section. If the flowmeter value is already lower than this after clamping, this means there is a hole in the film. This often happens because the O-ring can cut the film. No worries! Just unclamp the assembly, replace the film, clamp and test again!
Once the separation of the two sections is confirmed, retighten the two allen hex bolts in the back of the low-Z box using the 5/32" Allen wrench. This ensures that the box moves properly with the stage when you move the HORZ motor. If it is kept loose, the bellows will tend to oppose this HORZ motion and make scans with the HORZ motor not reproducible.
That's it! Make sure the flowmeter settles back down to 200-240 and wait 5 to 10 more minutes to ensure no air is in the front end. Then you can turn on the X-rays and continue!