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From: Mike Meier [[email protected]] Newsgroups: sci.techniques.xtallography Subject: XRD Tubes Date: Wed, 15 Sep 1999 18:22:39 -0700 Organization: U.C. Davis Xref: daresbury sci.techniques.xtallography:5660 I would like to learn more about the modern x-ray tube, especially things which effect its useful lifetime. Can anyone recommend a book, article or web site? The reason I ask is that I am about to retire a two year old copper tube due mostly to the increase in the intensity of a tungsten_L_alpha peak. It is now at about 4% of the intensty of the copper_K_alpha peak and is creating problems for some of my users. The tube seems to be good in all other respects, and while I could use the Ni filter, the drop in intensity (about 50%) would be a problem for many of my other users. I realize that two years of useful life is not bad, but maybe there is something I can do to keep it alive longer. Also, what are the advantages of the ceramic tubes? I just received a quotation for one that costs about 30% more than my current tube. It's power rating is the same, 2.2 kW, but its warantee is longer. But if I still get the L_alpha peaks after 1.5 years, what would be the point of spending the extra money? Mike Meier [email protected] www.matsci.ucdavis.edu
From: [email protected] (Lachlan Cranswick)
Newsgroups: sci.techniques.xtallography
Subject: Re: XRD Tubes
Date: Thu, 16 Sep 1999 10:10:24 LOCAL
Organization: Daresbury Laboratory, UK
Xref: daresbury sci.techniques.xtallography:5662
Have the ceramic tubes been around long enough for people to
be able to draw adequate conclusions on them of there value
and robustness vs reliability vs glass tubes vs price?
The theory is that ceramic tubes should be better but I have
heard horror stories involving ceramic tubes - but this was
mainly due to dodgy generators - not the ceramic XRD tube.
----
For trace phase ID work in Australia (amongst other uses,
Rietveld, etc), the % amount of parastic Cu K beta lines
and Tungsten L was quite important for us (grain boundary
type phases that could overlap with the parasitic lines).
When the amount of Tungsten reached what for us was a
high level (~0.3%), it used to get donated to the
single crystal lab next door and a new tube purchased.
This meant going through around 1 glass Long Fine
Focus Powder XRD tube per diffractometer per year. But
that was the price for doing the work on these types of
phase systems.
(Appearance of Tungsten over time could vary with tube).
One possibility is to specify the performance you expect
in what you purchase and see what the vendor will go with.
In Australia, we had mainly Philips Powder XRD equipment and
Philips Australia were very good on this type of support if things
didn't come up to a spec we had both agreed upon.
Lachlan.
PS: Given the life-times of good XRD tubes, a lot of this
area can be folk-lore-ish as it is hard to go through a
statistically valid number of XRD tubes to draw scientific
conclusions(?)
-----------------------
Lachlan M. D. Cranswick
Collaborative Computational Project No 14 (CCP14)
for Single Crystal and Powder Diffraction
Daresbury Laboratory, Warrington, WA4 4AD U.K
Tel: +44-1925-603703 Fax: +44-1925-603124
E-mail: [email protected] Ext: 3703 Room C14
http://www.ccp14.ac.uk
From: Cristal [[email protected]] Newsgroups: sci.techniques.xtallography Subject: Re: XRD Tubes Date: Thu, 16 Sep 1999 19:18:51 +0200 Organization: Private Xref: daresbury sci.techniques.xtallography:5665 Lachlan Cranswick wrote : > The theory is that ceramic tubes should be better but I have > heard horror stories involving ceramic tubes - but this was > mainly due to dodgy generators - not the ceramic XRD tube. We have horrible story here for the ceramic tubes mounted on the Bruker D8 Advance. The decay is 15-25% per month, observed by careful measuring on a good standard. This cannot be considered as normal, so that the manufacturer has already replaced graciously 2 tubes since December 1998... If this was due to dodgy generators, we would expect the manufacturer to explain something, but we obtained no explanation up to now. Are we alone in that case ? It is extremely unpleasant to buy such a new expensive diffractometer, and to be in the situation of not being able to use it fully. -- Armel Le Bail - Universit� du Maine, Laboratoire des Fluorures, CNRS ESA 6010, Av. O. Messiaen, 72085 Le Mans Cedex 9, France http://www.cristal.org/
From: [email protected] (Diffract) Newsgroups: sci.techniques.xtallography Subject: Re: XRD Tubes Organization: AOL http://www.aol.com Xref: daresbury sci.techniques.xtallography:5666 The 'bible' for building and understanding eletron tubes is "Electron Tubes" by W.H. Kohl, Reinhold Publishing Corp, NY 1960 (out of print). Or you can try another classic "Handbook of electron tube and vacuum techniques" by F. Rosebury, AVS Classic series, AIP 1993. These books are from a bygone era but so is tube building. The tubes on the market these days change randomly in quality by the year since tube building is a very complex process few people really understand or try to understand. One manufacturer may be good today and become unreliable tomorrow. Regarding tube life, it so much depends how you treat your tube regardless what other people may tell you. It is a dynamic device and impatience is the killer. That means at least 30min warm-up time. For daily use, running a tube at 40-50% of its max. power rating should result in a tube life of at least 3 years on a decently made device. L. Keller CAMET Research, Inc.
From: Armel Le Bail [[email protected]] Newsgroups: sci.techniques.xtallography Subject: Re: XRD Tubes Date: Fri, 17 Sep 1999 10:04:25 +0200 Organization: Universite du Maine - France Xref: daresbury sci.techniques.xtallography:5668 > We have horrible story here for the ceramic tubes mounted > on the Bruker D8 Advance. The decay is 15-25% per month, > observed by careful measuring on a good standard. This > cannot be considered as normal, so that the manufacturer > has already replaced graciously 2 tubes since December 1998... Other hypothesis this morning : one tube could effectively have problems. But the main problem could have been due to the photomultiplier window. Displacing slightly the photomultiplier position led to 400% increase (!!). The story is still horrible but the fault is displaced. Any experience/opinion about that ? Armel Le Bail - Universite du Maine, Laboratoire des Fluorures, CNRS ESA 6010, Av. O. Messiaen, 72085 Le Mans Cedex 9, France http://www.cristal.org/
From: "Mark Bowden" [[email protected]] Newsgroups: sci.techniques.xtallography Subject: Re: XRD Tubes Date: Tue, 21 Sep 1999 11:39:43 +1200 Xref: daresbury sci.techniques.xtallography:5679 Mike Meier wrote in message [[email protected]]... >I would like to learn more about the modern x-ray tube, especially >things which effect its useful lifetime >(snip) >I realize that two years of useful life is not bad, but maybe there is >something I can do to keep it alive longer. Mike, A colleague of mine (Martin Ryan) looked at the life of conventional tubes a while back, and presented the results at a conference (AXAA - Australasian X-ray Analysts Assoc. '93 by recollection). Our system utilises a post-diffraction monochromator, so we aren't normally affected by tungsten lines. The outcome of Martin's work was that the intensity you get out of the tube is propotional to: 1. (kV-kVc)^1.5 where kV is the tube voltage and kVc is the critical voltage for excitation (approx. 8 for Cu tubes) and the exponent 1.5 is approximate. 2. mA On the other hand, tube life is proportional to kV * mA. So, to get the the longest life for a given intensity, choose a high kV and and low mA since you get more intensity from increasing kV and it has the same effect on tube life. Martin's guidelines were to go for about 75% of the rated tube power, selecting a kV to give 3-5 times kVc. Another factor to check is the filament current setting on the generator. The normal kV and mA settings refer to the voltage and current between the filament and target. Generators frequently have a second control to adjust the current which flows through the filament to heat it sufficiently so that it emits electrons which are subsequently accelerated to the target. The tube manufacturers normally recommend a filament current in their product literature. Too high a filament temperature might result in the unacceptable tungsten contamination you are observing. Hope this is of some use, Regards Mark Bowden Industrial Research New Zealand [email protected]
From: Frank May [[email protected]] Newsgroups: sci.techniques.xtallography Subject: Re: XRD Tubes Date: 21 Sep 1999 10:26:00 -0700 Organization: University of MO-St. Louis Xref: daresbury sci.techniques.xtallography:5682 I presently acquire tubes from Seifert which last for at least 2-3 years without objectionable W-L(alpha) lines. The system is 1981-vintage Scintag which was originally a PAD II. When I turn the system on, I slowly ramp up the power and when I turn it off, I ramp down the power and allow the tube to completely cool before turning off the cooling water. I suspect that having sufficient cooling water is a major contributor to long tube life, because my practice is to leave the system on continuously and running at 45 KV and 40 mA (1800 watts on a 2 KW rated tube). I had one episode where the tube life was shortened to about 1 year, but it was during the time when there were a significantly number of power failures in the lab. As a consequence, the tube was turned off from full power and cooling water removed, so the tube cooked itself to death. Anyone else have similar experience? ----------------- Frank May Department of Chemistry University of Missouri-St. Louis St. Louis, Missouri 63121 [email protected]