User Tools

Site Tools


Frida-1 can read data from many instruments, see read-in. In more detail are explained:

  • TOFTOF – the procedure to read the data obtained from the time-of-flight instrument TOFTOF at the FRM II. The other instruments will be explained as requested.
  • x-y-dy text files – read simple text files. This is of course especially convenient when one wants to read data obtained at other neutron scattering instruments for which no read-in routines exist yet. After reading those files, all Frida-1 features such as absorption correction, Fourier transformation &c. are accessible. An example of reading a diffractogram is given in this tutorial.
Self Absorption Correction

First the good news: if you have a sample whose transmission is bigger than 0.9 (of course 0.9 is some kind of arbitrary limit!!!), the shading of the walls by the sample is minor, and you can say in a first approximation that your signal is that of your sample plus that of your container.

Now the bad news: if your sample has a transmission smaller than 0.9, there will be more neutrons that will “see” the first wall they found than the last wall of the container, because the sample has decreased the number of neutrons going through. The signal of your container is overestimated in the measurement of the pure empty can compared to the measurement of the sample+container. This overestimation depends on the scattering angle. One way to correct that is to use the ideas of Paalman and Pings wrote down in a classical paper (see J. App. Phys. 33 (8)2635, from 1962, when was usual not to perform simulations, but work out nice and long formulas ;-)). This is implemented in IDA by the formula (see also Bee p 142 where the notation is taken from):

Ssample(2θ,ω)=1/Assc * (Ssample+container(2θ,ω)-Arel*Scontainer(2θ,ω))

IDA will give you the 2θ dependent values of Assc and Arel. The first one corrects the absolute intensity, and the second the relative contribution of your container related to that of your sample, therefore if you get a number bigger than 1, you made something wrong… In case you set the transmission of the inner and outer can to 1 (which is the case for “normal” Al-cans at large wavelengths), Assc=Arel – in this case, Arel is not calculated by the routine. Now let's put hands into dough (Spanish expression ;-). Let's do the case of a cylindrical sample. We start with file 1, where your data are (no prior background subtraction):

1you're in your data file
_sacfor Self Absorption Correction
3 hollow cylinder
2.26 The outer radius of your sample (that is the inner radius of your outer sample container) in cm
0.05 The thickness of your sample layer
0.98 Transmission of the inner container.
0.05 Thickness of the inner container
0.98 Transmission of the outer container
0.05 Thickness of outer container
0 Fraction of backscattering traversing the sample: for backscattering machines only, where the neutrons go two times through the sample
4.641e-3 Number density of scatterers for of your sample, which is the density(g/cc)*602*#atoms/Molecular weight (602 comes from Avogadro's number
96928 Scattering cross section, which can be calculated from tables at: www.ncnr.nist.gov/resources/n-lengths/ . By the way, here do not divide by the number of atoms in your molecule, it has already being done in the calculation of density
746.33 Absorption cross section, also in the aforementioned web
y it is all right!!!
80Angular meshes
80Radial meshes. Some explanations about the two last values: to do the calculations the program will make a grid of angles and radius. 80 seems to be a reasonable value for both, but it depends on the case. However the bigger the values, the longer it takes!

You should now have two files with Assc, and Arel. Now we perform the calculations to obtain the corrected intensity, and now:

First step : dividing the S sample + container file by the *.Assc file 1) select the S sample + container file 2) type : oy (for operation on y) 3) type : / or 55 4) 2nd argument ? y2 5) choose the *.Assc file

Then you have the S sample + container / Assc

Second step : dividing the *.Arel file by the *.Assc file : 1) select the *.Arel 2) type: oy (for operation on y) 3) type : / or 55 4) 2nd argument ? y2 5) choose the *.Assc file

Then I have Arel / Assc

Third step : multiplying the S container file by the (Arel / Assc) file created in the second step : 1) select the S container file 2) type : oy (for operation on y) 3) type : * or 54 4) 2nd argument ? y2 5) choose the file created in the second step

Then we have S container * (Arel / Assc)

Fourth step : subtraction of the (S sample + container / Assc) file from the S container * (Arel / Assc) file : 1) select S sample + container / Assc file 2) type : oy 3) type : – or 52 4) 2nd argument ? y2 5) choose (S sample + container / Assc)

Then we have the S sample file after subtraction of the empty can with self absorption correction.