Over the years I have the great fortune to use a few different models of Nonius or Enraf-Nonius diffractometers from the CAD4 to the Kappa CCD. They have always been a great pleasure to use if not always simple to master.
Below is a tale as to why good diffractometer maintenance is so very important.
So the story starts………
After Months of down time, mainly because we (Robin, Merf and myself) we’re concentrating on the new machine (this was a new KappaCCD), I finally convinced Robin to let me pull the MACH3 to bits and to try and fix the shutter problem.
We gingerly dismantled many a-bit-and-bob until when Rob wasn’t in one morning I lost my patience and removed the shutter itself. Prior to this we could see the corrosion in the tube housing and were going to attempt some minor cleaning on it however, the following pictures show why the removal of the shutter was very necessary.
Its amazing how much crap forms after 10 years of continued use and it shows that the MACH3 was a well built piece of kit. Previously the Phi drive failed on this MACH3 and when Caas replaced it both him and Harry could not believe that 1) it was still running and 2) so much wear could appear on a gear.
|MACH3 shutter with small lead insert.||The “cleaned” shutter. See the difference?|
Corrosion can be seen quite clearly. Including the path the X-ray beam transversed.
Merf very kindly analyzed some of the green powder that was found in the tube housing and many connecting components using Powder XRD and came up with the interesting result of Nickel Nitrate Hydroxide Hydrate. A relatively good match to boot. Well done Merf.
After we reassembled the MACH3 and it became the smoothest shutter action on J floor next to the KCCD (J floor was located in the Faraday/Chemistry Tower on the UMIST, Sackville street campus).
The lead (Pb) inset which can be seen as the circular object in both pictures of the shutter perhaps clearer in the cleaned shutter shows considerable damage but was still present (mercifully, even after Merf’s explorations with his Swiss army knife) and counts running with shutter closed in-line beam at full strength were negligible.
Bearing in mind I took off the shutter I also took off the monochromator and its housing which also looked pretty grim.
After putting the whole system back together and the X-ray checks proved ok we undertook the slow job (for amateurs) of realigning the MACH3 which we also moved to a Cu tube as well. (By we I meant me and by amateurs I meant just me in between doing my PhD and co-running the service with Robin).
The two pictures of the actual shutter have a very interesting feature. Can you guess what it is? Well I’ll tell you! They both show a clear X-ray burn mark, for want of a better description.
> > Great John so what of it? > >
Well this is the important bit - it isn’t circular as you can see there is a flat edge to the burn. You could therefore assume that the shutter well, wasn’t completely closed when it was sticking so that the burn at that point missed the bottom of the shutter or at the very least missed 50% of the lead shielding.
So what did we learn?
Well the old MACH3 shutter only cared about being “open” and “not open” which is very different from caring about being “open” and being “closed”. The shutter was sticking because of the nickel salt furring up the mechanics and making the travel to the “I’m open” limit switch too slow. It was also stopping the shutter falling closed under gravity completely. Hence the the X-ray beam missing the lead and the bottom of the shutter.
What is also worth thinking about is on a MACH3 you place you head into the cabinet to align the crystal. No fancy digital video cameras.
Hmmmm, X-rays head - and what is worse my head!