how to make a measurement XRD minimizing the fluorescence effect on copper anticathode due to the presence of iron in the sample?
maybe decreasing Kv and mA in the instrument?
Thanks in advance....
Dear Vincenco,
lowering the mA does not change your tube spectrum.
Lowering kVp down to 8,9kV will decrease/cancel your Cu K-alpha fluorescence output of your tube; . . . . but in your wavelength dispersive set up you need a strong characteristic line for XRD.
For reducing the Fe fluorescence background in your XRD pattern (which is mainly caused by the Cu- K-alpha radiation of your current tube) you have to use a Co-anode based x-ray tube.
Co K-apha is around 6,9keV und thus smaller than the Fe K-edge energy of about 7,1keV. Thus only minor Fe- K fluorescence will show up from your sample in this case, which is still caused by Co K-beta (7,6keV) and the bremsstrahlungs part of the tube spectrum above 7,1 keV.
Fe- or Cr-anode x-ray tube will be even better in this respect, but you have to think on the availability of such a tube and on the availble x-ray flux. Peakshift in 2-theta can be easily converted to Cu tube equivalent pattern.
Dear Vincenco,
a second solution to your experimental challenge is to use a balanced filter set consisting of e.g. 5,5µmCo and 5,0µmNi.
Such a filter set is also called a Ross filter.
In the attachment please find the transmission curves of the filters.
You will see that the energy regime around 8keV (this is your main XRD energy) is differently affected by these filters. So you have to perform two measurements.
a) having the 5µm Ni filter in front of the detector and performing the XRD measurement
and
b) having the 5,5µm Co filter in front of the detector and performing the XRD measurement using the same experimental parameters as in (a).
then
c) take the difference betweeen these two diffractograms (c) = (a)-(b) and you will have the net Cu K-Alpha XRD pattern; having the Fe fluorescence and other scatter components outside the filter window strongly reduced.
Please pay attention that the thicknesses of the filters have of to match within about 0,2µm (or even better) to the thickness values given above.
Dear Gerhard MArtens,
unfortunately I do not have the cobalt filter, I have only copper and nickel filters.
could you explain better what happens if I lower the KV Tube? I will not have a good signal?
I have one diffractometer in which are set to minimum values 20 kV and 5 mA (the defaults values for the measurement 40KV -40 mA), if I try to lowering them, could damage the tube?
The standard measure which is carried out is the following:
Tube 40kV - 40mA
Without Nickel filter (I will add in this case).
do you have any advice for the slits? diff- theta 2theta bragg brentano
thanks in advance
Riva V.
Dear Vincenzo,
with respect to the slits please use those which are recommended for your system.
The slits do not really affect your iron fluorescence background.
When lowering the tube voltage from 40kV to 20 kV you will significantly decrease the Cu K-apha flux which you urgently need for your experiments. By lowering kV and mA setting you could not damage your tube; but you will have much less amount of x-ray output.
The combination of Ni and Cu filter (at right thickness ratio) will not help you, because these two filters will set up an x-ray photon energy window between 8,34keV (Ni edge) and 8,99keV (Cu edge); but your Cu K-alpha line in your experiment is at about 8,0keV.
But instead of Co you may use Fe filter at right (balanced) thickness.
By the way; you can buy such filters for small money; very much smaller compared to the price of a Co tube.
http://www.kayelaby.npl.co.uk/atomic_and_nuclear_physics/4_2/4_2_1.html
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