RE: Combined neutron/x-ray refinements
Jon Wright wrote : PS: Any offers other than GSAS and multipattern fullprof for actually doing these fits? Yes ARITVE, for amorphous compounds : http://www.cristal.org/aritve.html Nevertheless, ARITVE can work also for crystallized compounds with simple profile shape (gaussian only), and no zeropoint, and etc : do not try ! I have tested a combined Rietveld refinement on calculated neutron and X-ray data for CuVO3. Good news, the neutron data inclusion does not seem to degrade the X-ray result ;-). However ARITVE works by using f/f**2 values, so that the X-ray data looks like neutron ones : no intensity decrease at large angle... Best Armel Le Bail - Université du Maine, Laboratoire des Fluorures, CNRS ESA 6010, Av. O. Messiaen, 72085 Le Mans Cedex 9, France http://www.cristal.org/
Re: [RE: Combined neutron/x-ray refinements]
If we have an atom that is seen by one radiation and not by the other there will be a degradation in the quality of the parameters by combining the refinement in the current fashion. Do you mean for example that we might degrade the parameters of a V atom by introducing neutron data ? I don't think this is true, but it is an interesting question. If we were to extrapolate this argument "ad absurdum" we could say that because some reflections (for a given radiation) do not give any information about some parameters (easy to demonstrate) then we would obtain better estimates for those parameters by removing those reflections from the least squares process. (Surely untrue :-) What is true is that if we introduce systematic errors by combining radiations, we may indeed degrade the result. For example if we have serious preferred orientation with a very small X-ray sample, it is probably unwise to introduce this biased information into the refinement of the neutron data, where there may be less bias because of the average over a much larger volume. But if the data is not biased, you must always (?) do better by including more data, with for example combined X-ray and neutron refinements. Surely, it would be better to use a new weighting function for the atomic parameters, that is dependent on the scattering lengths for each radiation. Playing around with weighting schemes is to enter dangerous territory. Alan H. Alan Hewat, ILL Grenoble, FRANCE [EMAIL PROTECTED] tel (33) 4.76.20.72.13 ftp://ftp.ill.fr/pub/dif fax (33) 4.76.48.39.06 http://www.ill.fr/dif/
Re: [Re: [RE: Combined neutron/x-ray refinements]]
Alan, I am not suggesting removing reflections. But, I think that we should make sure that we are combining the data in the best possible way. If we know have strong information on a vanadium position from X-rays and (extrapolate again) have only noise from neutrons, then stastically introducing the neutron data whilst no changing the best fit will degrade the least - squares approach to it. The final structure should fit all data, but are we approaching it optimally? I know that this is a can of worms, but it is good to think about what we are doing as combined refinements will continue to become less exotic. -Andrew -- Andrew Wills Centre D'Études Nucléaires de Grenoble "Alan Hewat, ILL Grenoble" [EMAIL PROTECTED] wrote: If we have an atom that is seen by one radiation and not by the other there will be a degradation in the quality of the parameters by combining the refinement in the current fashion. Do you mean for example that we might degrade the parameters of a V atom by introducing neutron data ? I don't think this is true, but it is an interesting question. If we were to extrapolate this argument "ad absurdum" we could say that because some reflections (for a given radiation) do not give any information about some parameters (easy to demonstrate) then we would obtain better estimates for those parameters by removing those reflections from the least squares process. (Surely untrue :-) What is true is that if we introduce systematic errors by combining radiations, we may indeed degrade the result. For example if we have serious preferred orientation with a very small X-ray sample, it is probably unwise to introduce this biased information into the refinement of the neutron data, where there may be less bias because of the average over a much larger volume. But if the data is not biased, you must always (?) do better by including more data, with for example combined X-ray and neutron refinements. Surely, it would be better to use a new weighting function for the atomic parameters, that is dependent on the scattering lengths for each radiation. Playing around with weighting schemes is to enter dangerous territory. Alan H. Alan Hewat, ILL Grenoble, FRANCE [EMAIL PROTECTED] tel (33) 4.76.20.72.13 ftp://ftp.ill.fr/pub/dif fax (33) 4.76.48.39.06 http://www.ill.fr/dif/ Get your own FREE, personal Netscape WebMail account today at http://webmail.netscape.com.
RE: Combined neutron/x-ray refinements
Jon Wright wrote: I guess the degradation which is found would come from parameters which are determined by both datasets and come out with different values in each separate refinement. Not necessarily. In order to get the ESD, the variance-covariance matrix is multiplied by chi^2, and the roots of the diagonal elements are taken. Therefore, if the chi^2 of the combined refinement is worse than that of the individual ones, the ESD will automatically be worsened. I think this is by far the commonest case. Also, by adding reflections that are insensitive to a given parameter my feeling is that you increase the esd on that parameter even if chi^2=1, but the proof of this is too tedious. Paolo
RE: Combined neutron/x-ray refinements
As chi^2 is a function of the number of data points included in the refinement, combined refinements have considerably improved values for a total chi^2 when compared with refinements carried out against individual data sets. Correspondingly the ESDs in the combined refinement output should be significantly lower than those obtained from a single data set refinement unless there is something drastically wrong with the application of combined refinement to the particular problem (e.g. preferred orientation, surface vs bulk etc). It is my experience that the combined refinement chi^2 is always lower than that obtained from using just (say) the neutron data. We have frequently collected data sets at both room temperature and 5 K using D2b. The room temperature data are refined simultaneously with lab X-ray data to give a chi^2 of 2.02 whilst the D2b data collected at 5 K refined as a single data set gives chi^2 of 4.53 (published in JACS, 1999, 121, 3958-3967). In my experience this improvement in chi^2 is typical. Eddie Cussen Inorganic Chemistry Laboratory, Department of Chemistry, University of Oxford, South Parks Road, Oxford, OX1 3QR United Kingdom E-mail: [EMAIL PROTECTED] tel: (..44)(0)1865-272602 Fax: (..44)(0)1865-272690 On Tue, 25 May 1999 [EMAIL PROTECTED] wrote: Jon Wright wrote: I guess the degradation which is found would come from parameters which are determined by both datasets and come out with different values in each separate refinement. Not necessarily. In order to get the ESD, the variance-covariance matrix is multiplied by chi^2, and the roots of the diagonal elements are taken. Therefore, if the chi^2 of the combined refinement is worse than that of the individual ones, the ESD will automatically be worsened. I think this is by far the commonest case. Also, by adding reflections that are insensitive to a given parameter my feeling is that you increase the esd on that parameter even if chi^2=1, but the proof of this is too tedious. Paolo
RE: Combined neutron/x-ray refinements
I guess the degradation which is found would come from parameters which are determined by both datasets and come out with different values in each separate refinement. If they come out differently it is because they are differently biased by different systematic errors in the data not described by the model. Not necessarily. In order to get the ESD, the variance-covariance matrix is multiplied by chi^2, and the roots of the diagonal elements are taken. Therefore, if the chi^2 of the combined refinement is worse than that of the individual ones, the ESD will automatically be worsened. You may also get a higher chi^2 with higher resolution data, so does that mean that the structure will be less well determined with hi res data ? I think not, because the correlation between structural parameters should then be smaller - even if you have more points to fit with the same number of parameters, and the peak shapes are less well described by the model. You should similarly do better if you have both X-ray and neutron data (in the absence of bias). Also, by adding reflections that are insensitive to a given parameter my feeling is that you increase the esd on that parameter even if chi^2=1, but the proof of this is too tedious. Tedious and also impossible ? (This sounds like a contradiction in terms) There is no case you can make based on pure statistics or the mathematics of refinement. The only way combined refinement can be worse is if you introduce bias through systematic error (which unfortunately may happen). ... in most cases that the ESD's are underestimated. Mainly because the ESD's are only correctly calculated if the model is CAPABLE of fitting the data. This is not usually true when systematic errors are important compared to statistical errors, since the model is usually not capable of describing these systematic errors fully - background, texture etc... The conclusion is that you should use combined refinements provided that one set of data does not contain important uncorrected systematic errors. Alan Hewat, ILL Grenoble, FRANCE [EMAIL PROTECTED] tel (33) 4.76.20.72.13 ftp://ftp.ill.fr/pub/dif fax (33) 4.76.48.39.06 http://www.ill.fr/dif/
RE: Combined neutron/x-ray refinements
On Tue, 25 May 1999 [EMAIL PROTECTED] wrote: Not necessarily. In order to get the ESD, the variance-covariance matrix is multiplied by chi^2, and the roots of the diagonal elements are taken. The justification for multiplying by chi^2 is to assume that the systematic errors are really just due to overestimated counting statistics and you can rescale the weight of each data point accordingly. A question arises as to whether you should rescale each pattern's esds according to the individual patterns chi^2 or do you have to use the overall chi^2 for both together? Thinking of an (over-determined) D20 data and an (under-determined) lab x-ray data set then it makes sense to rescale errors for the D20 data but not the x-ray (common sense?!). It seems as if the method for calculating the esd's is nonsense - surely one can only justify rescaling the weights on a per dataset basis. The systematic errors which are being accounted for in each dataset are different. Fullprof (multipattern) does give a chi^2 per pattern although I don't know how it gets the esd's, GSAS doesn't so I assume it degrades the esd's. (I read that the multiplication by chi^2 has no basis in statistics anyway :) So is it compulsory to multiply by the overall chi^2? If not then I see no reason for a degradation unless the individual fits get worse due to a disagreement over a parameter. Therefore, if the chi^2 of the combined refinement is worse than that of the individual ones, the ESD will automatically be worsened. I think this is by far the commonest case. Agreed although I'm interpreting it as an odd method for estimating an error. Is it set in stone? Also, by adding reflections that are insensitive to a given parameter my feeling is that you increase the esd on that parameter even if chi^2=1, but the proof of this is too tedious. Can you direct me to a text with this tedious proof? My feeling is that if the derivative of a data point w.r.t a parameter is small or zero then it does not affect the LSQ calculation unless it alters the chi^2. If the chi^2 is 1 then how do an extra bunch of zero derivatives affect an esd??? For example adding or excluding background regions shouldn't alter the esd's on positions provided the chi^2 is unchanged. Is there anything other than GSAS for doing combined fits anyway? Apologies to the list if I am displaying my ignorance, sometimes it's the quickest way to learn. Jon Wright PS: Sorry to pick at your comments Paolo, it's a shame I'm not at RAL at the moment. Could have discussed it out over a coffee...
RE: Combined neutron/x-ray refinements
On Tue, 25 May 1999, Alan Hewat, ILL Grenoble wrote: I guess the degradation which is found would come from parameters which are determined by both datasets and come out with different values in each separate refinement. If they come out differently it is because they are differently biased by different systematic errors in the data not described by the model. I was thinking of C-H (D) bondlengths from x-ray and neutron data. Don't they come out differently if you use spherical form factors for the x-ray data? I guess a neutron expert might look on this as an systematic error in the x-ray model :) Maybe not if one looks on the x-ray refinement as fitting of the electron density function, rather than the nuclear positions. For bonding studies it is the differences which are of interest! Jon Wright. PS: Any offers other than GSAS and multipattern fullprof for actually doing these fits?
RE: Combined neutron/x-ray refinements
On Tue, 25 May 1999, Alan Hewat, ILL Grenoble wrote: Mainly because the ESD's are only correctly calculated if the model is CAPABLE of fitting the data. This is not usually true when systematic errors are important compared to statistical errors, since the model is usually not capable of describing these systematic errors fully - background, texture etc... ...and weights. Lubo
Re: [RE: Combined neutron/x-ray refinements]
Dear All, Firstly, it was a pleasure to return to my email and read an interesting discussion on combined refinements. It is good to have aired some of the problems and limitations (e.g. are neutrons and X-rays seeing the 'same' sample?). With a simplistic view, this technique must be the way ahead as we are adding together more independent information. A good question to raise now, is how we should do it best. If we have an atom that is seen by one radiation and not by the other there will be a degradation in the quality of the parameters by combining the refinement in the current fashion. Surely, it would be better to use a new weighting function for the atomic parameters, that is dependent on the scattering lengths for each radiation. Ignoring the systematic errors that differ between neutrons and X-rays , perhaps the question that should be raised is how to best combine the information that each dataset holds. -Andrew --- Andrew Wills (Dr) Centre D'Études Nucléaires de Grenoble p.s./ Tao, the easiest way to add a list of atoms to a GSAS phase is by manual editing of the .exp file. I have a little program (DOS) that does this if you are interested. Get your own FREE, personal Netscape WebMail account today at http://webmail.netscape.com.
Re: Combined neutron/x-ray refinements
What Armel is describing is precisely the way we started. We used it when required, discussed over a few pints whether it was a good idea or not, and came to the conclusion that it was. Later we tried it also on refinements where the need was not so clear, and now it is routine. I have not read any real arguments against it. But may be I am blind to that. Further, my remarks related the oxides were more directed at the precision/esd of the oxide position. Of course I do know where the oxides are within say 0.05A but i want it an order of magnitude better. Remember that most structural distortions in metal oxides are due to displacements of the oxide sublattice and often you fail to observe that with X-rays. With regard to the "brown envelope" technique. I am not such a fan of that. It is very hard for a beam line scientist the know whether the data collected are as desired, have a good enough s/r ratio etc, which details to look for. In addition, the last few years when we went we had more samples that we good run sensibly. For that reason I use 'on the fly' refinements to check whether things are going OK. With respect to the additional costs, what are the daily running costs of say D2b or HRPD, 10,000-15,000 dollar orso? The travel costs are than a relative small part of the total cost involved. refinement was really fruitful (speaking of V atoms comes certainly to mind, but why not prepare an isomorphous sample without V ?-). Because you want to study the V containing sample of course! Now I will go to the beach and dive into that glorious caribean sea and wash my soul of all my recent sins. Best Jaap Jaap Vente Cinvestav-IPN Unidad Merida Departamento de Fisica Aplicada Carretera Ant. a Progreso km 6 Apartado Postal #73 Cordemex Merida, Yucatan, 97310 Mexico Fax: (..) 52 - 99 - 812917 Tel: (..) 52 - 99 - 812960 ext 246/233 e-mail: [EMAIL PROTECTED]
Re: Combined neutron/x-ray refinements
On Tue, 11 May 1999, Armel Le Bail wrote: I already suggested to install such an "automatic" powder diffractometer at ILL. As Alan wrote recently, this could be a question of manpower. I think that this is rather a local political question : it is not a very interesting job for a human being to be a simple sample loader... A scheme like this could open up NPD to a wider range of users which would inevitably mean new users...? The simple loader might get involved in a lot of collaborations by returing refinement results instead of just raw data. They would also be in a unique position to offer the odd favour here and there. So long as it doesn't mean getting up every six hours then I think it sounds like a fantastic job. In any case - since simaltaneous fitting is being discussed - I have a question about multipattern fullprof which is now available as a beta version. It appears you can assign the statistical weight to each pattern, and in the example x-rays and neutrons are weighted equally. I'm wondering what the best choice is. Each observation comes with a statistical weight from the experiment? Isn't it a sin to alter that? Jon Wright PhD Student, Dept. of Chemistry, Lensfield Road, Cambridge, CB2 1EW, UK
Re: Combined neutron/x-ray refinements
Let start to cut the crap and go to Armel's real reason not to use a combined refinement: Of course this is only kidding and provocative opinion, as usual. I love both radiations, indeed. However, I tend to think that a simultaneous refinement could eventually degrade the heavy atom position accuracy AND degrade the light atoms accuracy. The former being better from the X-ray only and the latter from the neutron only, not to speak of the various problems that hurt me like, for instance : Do not agree! If you work on any transition metal oxide, what I am doing all the time, you will find that the oxide position are basically not determined. OK I work a lot with heavy elements like lanthanide's and iridium and the like. But in general the x-ray are very insensitive to where the oxides are. Not the forget the oxide content! So they do not effect the positions as required by the neutrons. Things can be even more true the other way around. e.g when you have a random dirtribution of either Ti or Mn and another metal you can have scattering lengths of zero. - the need to refine two sets of cell parameters, inevitably slightly different if both data are of high resolution. Basically what you mean is that the neutron diffractometer is not properly calibrated. Well a combined refinement in which you refine the neutron wavelength is than the answer! Recently I ran two different samples on D2b, and refined lambda: sample 11.59362(5) sample 21.59354(5) well that is very similar isn't it. I have read people stating on this e-mail list that they don't trust cell parameters after the third decimal. - not exactly the same temperature True, but you can control that if you want to, and in general does bot seem to be the biggest pain. - bulk with neutrons, surface with X-ray What? Do you propose to throw all the single crystall structure determinations in the bin, out of the window or where else because it is a surface technique? Of course the penetration depth of x-ray is a lot smaller than that of neutrons, but it remains a bulk technique. Thank god other wise we would still be cleaving those large single crystals. I dare to say that you can study the bulk structure even with electrons, which have an even smaller penetration depth. Of course you have to be carefull as Brian Toby pointed out. But in those cases you either a non-homogeneous sample, or a phase transformation which does happen on the slow cooling central parts of the particles but not on the fast cooling outer regions of the particle. In those cased it is back to the preparation lab. Giving the results of 2 independent refinements seems better to me. But which journal is going to accept that! Finally Armel, in my view an attempt the split the Rietveld community in two, i.e. in house X-ray and central facility neutrons, is artificial. By teh way did you note that someone called Mark Weller was on your neutron list as well. As far as I know, he has been working in Southampton for some time now. So there is hope for ordinairy university based scientist to use neutrons! Best Jaap Jaap Vente Cinvestav-IPN Unidad Merida Departamento de Fisica Aplicada Carretera Ant. a Progreso km 6 Apartado Postal #73 Cordemex Merida, Yucatan, 97310 Mexico Fax: (..) 52 - 99 - 812917 Tel: (..) 52 - 99 - 812960 ext 246/233 e-mail: [EMAIL PROTECTED]
Re: Combined neutron/x-ray refinements
The neutron fit will be completely insensitive to the V positions and the x-rays insensitive to the D positions. (This is easily verified.) Before some-one else says they can see hydrogen with x-rays, Mike Glazer, Bill David and I saw vanadium with neutrons. So let's just say "insensitive" but not "completely". Otherwise I agree :-) Combined refinements are sometimes necessary and a good thing. 1. David, W. I. F., Glazer, A. M. and Hewat, A. W. (1979) Phase Transitions 1, 155-69. The structure and ferroelastic phase transition of bismuth vanadate (BiVO4). And its not even in ICSD :-( Alan Hewat, ILL Grenoble, FRANCE [EMAIL PROTECTED] tel (33) 4.76.20.72.13 ftp://ftp.ill.fr/pub/dif fax (33) 4.76.48.39.06 http://www.ill.fr/dif/
Re: Combined neutron/x-ray refinements
At 07:08 PM 5/10/99 +0200, Armel wrote: PS- take the Rietveld Round Robin PbSO4 X-ray pattern and omit the O atoms, you will have RB~15%. Make a Fourier difference and you will see if the "light" atoms are so light, when using good data. When I remember my crystallography courses 25 years ago, maybe, I see the decomposition of the structure factor by atom pairs. O atoms are involved not only in O-O pairs, but also in Pb-O and S-O pairs, fortunately. In many cases, like Li2TbF6, or LiSbWO6 and so on, I had no difficulty to see the Li atoms in the Fourier syntheses, by X-ray. Hence not a large need of neutrons, but in a few cases. Armel could "see" the Li atoms from X-ray data, and I found the H in AlSiO3(OH) (Schmidt et al., American Mineralogist 83, 881, 1998) with powder data in a 3-phase mixture. It was clearly visible in the difference Fourier and could be refined with reasonable distances and thermal factors. I think all would agree that this was an unusual case, and that one would not normally expect to find H atoms from X-ray powder data. Clearly it is important to have a variety of tools in your arsenal and use whatever of them you need. In addition, make certain that your scientific administrators and legislators understand the need for neutron facilities! Larry -- Larry W. Finger [EMAIL PROTECTED] Geophysical Laboratory Phone: +1 (202) 686-2410 X 2464 5251 Broad Branch Road N.W. FAX: +1 (202) 686-2419 Washington, DC 20015-1305, USA http://www.gl.ciw.edu/~finger/ Note NEW URL http://btgix8.bgi.uni-bayreuth.de/~lafi
Re: Combined neutron/x-ray refinements
On Mon, 10 May 1999, Armel Le Bail wrote: published combined X-ray and neutron refinements. Am I so far from the truth if I estimate the number of published works combining X-ray and neutron in a single refinement to, say less than 20 cases ? I would like to see, in the 10 next years, this number increase to, say, 1000, for being convinced that this is really a way that should be adopted. But I do not believe it. Such combined refinements will stay anecdotal (or elitist if you prefer), reserved to specific cases, or to those having easy access to both radiations. Armel, this discussion has gone too far from the starting point. The question really isn't "neutrons Yes/No", but if all of us have equal access to all sources. People from both NIST, Grenoble or RAL would, no doubt, answer yes, why not ? Well, if I were there I wouldn't hesitate for a moment, but if I am outside, there are plenty of time and financial but NO scientific constraints. I think the most of us would be happy if they could access neutron sources as easily as their in-lab machines. Do not agree, please. The days before GLASGOW are so boring ... Best, Lubo
Re: Combined neutron/x-ray refinements
this discussion has gone too far from the starting point. The question really isn't "neutrons Yes/No", but if all of us have equal access to all sources. People from both NIST, Grenoble or RAL would, no doubt, answer yes, why not ? Well, if I were there I wouldn't hesitate for a moment, but if I am outside, there are plenty of time and financial but NO scientific constraints. I think the most of us would be happy if they could access neutron sources as easily as their in-lab machines. Do not agree, please. The days before GLASGOW are so boring ... Sorry, Lubo, but I agree. However, when you speak of easy access to a lab machine, you should account for some little problems that sometimes lead to long breakdown. Breakdowns are democratically distributed, even at ILL with the long reactor shutdown. Here with a brand new Bruker D8 powder diffractometer, we had not access to the machine longer than 3 months since December 1998, due to problems with bad ceramic tubes, random stop of the stepping motors, files not saved at the end of measurements and so on. Youth problems, may be. Have the happy buyers of the D8 similar problems (the Bruker mailing list if completely silent) ?? Armel 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/
Combined neutron/x-ray refinements
I also use combined CW neutron and synchrotron refinements. A simple minded justification goes as follows. Most of the problems I work on are badly underdetermined -- at least by the crystallographic rule-of-ten (10 crystallographic observations for each structural variable). By changing scattering lengths, I get a second set of observations which gives me more observables. Thus, I agree strongly with all of Dr. Jaap Vente's points: 1) in general the refinement is more stable. 2) their is the possibility to study much more complicated structures than with only one of the techniques. 3) because you now have two really different sets of data your structural model is more reliable. 4) you can study compounds which contain elements that are difficult to locate precisely with one technique, think of vanadium oxides or manganese/iron oxides. Andrew Wills is correct that X-rays see the electronic distribution and neutrons see nuclei positions, but electrons distributions are pretty close to spherical (our form factors assume this) for high-Z elements and are usually well centered around the nucleus. One can make a good argument that displacement parameters (aka temperature factors) can be completely different for x-rays vs neutrons, but experimentally this is seldom true. In any case, for all but the simplest systems, with powder work we don't have the precision to tell. Besides, x-ray displacement parameters are pretty meaningless anyway :-). I do not know of any codes other than GSAS that do combined x-ray/neutron fits, but in GSAS all the experimental effects (orientation, absorption, etc) are segregated by dataset so one only needs to apply these corrections to the x-ray data. (Neutron data seldom have either problem). In any case, if you can't model them well, you can't use the data. The "weighting" problem is overstated. The data are weighted by how well you know them. Usually the x-rays do contribute more to the Chi2 than the neutron, but the algorithm will minimize the deviations in both appropriately. One could downweight the x-ray data artificially, since you will probably have worse precision on the more structurally accurate neutron data, but this will screw up the Chi2 value. The biggest problem for combined refinements is that you need to have exactly the same sample and the same conditions for both the x-ray and neutron work. Since single crystals are frequently grown under different conditions than bulk samples, the utility of combined x-ray single crystal - powder neutron refinements is limited. Alas, it is fairly common that someone makes a material, measures the x-ray diffraction and then scales up the synthesis for neutrons, but ends up with something different. Attempts to simultaneously fit one model to x-ray data from the first batch and neutron data from the second batch are a waste. Other issues can also arise. We recently had a case where a material seemed nearly pure by x-rays, but the neutron work showed that the centers of the large particles were still composed of unreacted starting material. The x-rays did not penetrate far enough to see the purity was only ~70%. It would probably be a good idea to check that the model obtained from the combined refinement agrees well with (possibly constrained) models using the individual datasets. Perhaps we could entice John Parise to write a message about how to do this. Finally, I should mention in response to Armel that at least here at NIST, most requests for time are scheduled within 2-8 weeks of when we get them (see http://www.ncnr.nist.gov/~toby/bt1.html). Brian H. Toby, Ph.D.Leader, Crystallography Team [EMAIL PROTECTED] NIST Center for Neutron Research, Stop 8562 voice: 301-975-4297 National Institute of Standards Technology FAX: 301-921-9847Gaithersburg, MD 20899-8562
