In case of XRD similar parameters, which are necessary to quantify retained austenite in steels, are widely acceptable (Standard ASTM E-975); moreover, anyone can find respective formulas together with needful material constants (!) to reproduce the mentioned reference data by himself. Unfortunately, this is not the case for the neutron diffraction where all practical details of relevant calculations become particularly desirable for the following reasons. ///////////////1) There is no published document of this sort containing data for various wavelengths of working monochromatic sources of neutrons; 2) The situation even more aggravates for the TOF (non-monochromatic) technique because each diffraction peak corresponds to a particular wavelength, whereas each specific (partial) flux of incidental neutrons notably depends on related WAVELENGTH RANGE. ///////////////// Of course, there is no whole information on underlying computations in regular papers, and I will infinitely appreciate any useful reference, advice or, more so, data.
In PowderCell you can select as radiation neutrons. However, only a fixed wave length is available adapted from the condition for available correction values. Thus, the velocity of the neutrons is fixed to be 2200 m/s so that a wave length of 1.7982 A results.
Additionally, the Rietveld refinement software like GSAS or FullProf provides an opportunity to create a 'dummy' pattern with any instrument including monochromatic or TOF neutron diffraction. Also the latest editions of PDF-4 database have an option to plot the neutron diffraction data if the crystal structure is known for the particular entry.
Here I attached two setting files for FullProf (pcr file) which calculate diffraction patterns of Al2O3 a.k.a corundum, for constant wavelength (CW) and TOF methods.
I don't go into details of pcr files because it's a bit complicated. Please be careful that the formats of two pcr files are different. (CW:single phase format, TOF:multiphase format)
If you want to calculate for your own sample, basically you only need to change unit cell information and lattice constants. Change lambda if you need simulation for CW method. For the case of TOF, three coefficients are required for a polynomial equation which converts TOF to d(hkl): zero, dtt1, dtt2. Each instrument has its own conversion coefficients for given experimental condition. If you want to know them, ask instrumental scientists. They'll give you the coefficients or standard sample data for coefficient calibration. Note that you need a dummy experimental data file even if you just want to simulate and don't have experimental results. For pcr files that I give, just make empty 'dummy.dat' in the same folder as pcr files.
After you execute pcr file with FullProf, you'll get a bunch of output files. You can see a simulated diffraction pattern (prf file) with WinPlotr (see manual). You can change the format of simulated patterns. A list of calculated intensities is written in .out file.
Comments to your claim.
1) I suppose there are two reasons for this.
- The number of neutron diffraction users are far less than lab x-ray.
- Common wavelengths such as Cu-Ka for lab x-ray are not well-defined for neutron.
2) Usually, you don't have to care about the incident flux distribution as an user. Data you bring home are already normalized. (or at least your local contact will tell you how to deal with it). Softwares such as FullProf or GSAS calculate the intensity which can be compared to normalized experimental results.
I think all published data are also normalized with flux distribution.
Dear colleagues, thanks a lot for useful information! I stil wonder whether a kind of old good RIR approach would be better in case of TOF because of its treatment with first-princile calculations is complicated and gives the impression of blackbox of hidden errors.
I agree that full profile refinement with TOF data has some tricky parts. Whatever approach one uses, I would say choose CW if you don't have problems, if asked by someone.
But CW method for neutron diffraction might be difficult to use from long term aspect due to nuclear policy change in some countries.
This advice assumes that you are enquiring about constant wavelength data which appears to be the case from your question. Time-of-flight calculations are a little more challenging, but not difficult if you have access to the many XRD programs which also do ND calculations
Do you have measured neutron data which you are processing or do you just wish to simulate ND patterns to compare with published data from another source?
Also, are your asking about single crystal data or powder data?
I will assume that you are dealing with neutron powder data at a single wavelength.
1) Most XRD Rietveld programs have options to do ND calculations as well as XRD. These programs will allow you to simulate ND patterns. And as has been suggested, the excellent PDF-4+ data base has a user-friendly way to simulate ND powder patterns as well as XRD.
2) The programs will need you to provide the CIF file for each structure of interest. The program will then calculate the simulated ND patterns at the wavelength that you nominate. In doing so, it will use neutron scattering factors rather than x-ray scattering factors.
3) In terms of the underlying principles, there is a vast literature on neutron diffraction which will guide you. For example, there is a nice introduction in the text Fundamentals of Crystallography by Giacvasso et al; and the old classic text Neutron Diffraction by G Bacon says it all. Additionally, via googling, there are many examples of ND applications to guide you. Also, the annual reports for neutron scattering facilities around the world will list ND papers for you to follow. Huge numbers of papers are published each year.
Let me know if you require further advice.
Brian O'Connor
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