Question

Could you explain why elements with atomic numbers higher than 50 have higher L-alpha and L-beta X-Ray Fluorescence than K-alpha and K-beta X-Ray Fluorescence?

Thank you

Answer 1

We have two diffractometers, one with Cu and one with Co anode. The Cu is the most widely used, but samples that are rich in Fe, Cr, Mn will fluoresce under the incident Cu Ka beam and create polychromatic radiation. Exactly what you don't want. It can lead to strange shaped and elevated backgrounds. The answer is to choose an element for the tube that is appropriate to your sample or experimental requirement; in this case Co for samples wth Fe. The resulting 2-theta values will be different for the same material on the two anodes, but the d-spacings will always be the same. So if you are reporting d-spacing then it does not matter which anode you use.

You also ask about why not Kb, well it has much lower intensity and would give very poor signal to noise if it were used as a main line (I'm not sure how you would eliminate Ka!!). Almost all diffractometers will have a method of monochromation to remove the Kb component, either a Ni filter on a Cu anode or Fe filter on Co anode, or a graphite secondary monochromator, or a primary monochromator or mirror.

Actually X-ray tubes are produced with different anode materials. The nature of the material determines the wavelength produced. For copper it is reasonably long 1.54056 Å, which gives good resolution in registration of the pattern. Another standard that has been used historically for powder diffraction is cobalt with the wavelength of 1.789 Å which is even better for resolution of the reflections. The groups running both powder and single crystal diffraction studies with a single X-ray source use usually Mo Kalpha with the wavelength of 0.71073 Å, see, for example, Cryst Growth&Design 2011 11 1238-1243.

Monochromatic X-rays are required in X-ray diffraction as it is evident from Bragg's law. Since the technique is used to identify and study material structures Copper K-alpha is used which is intense compared to K-beta for better resolution. Strictly speaking K-alpha also has two components i.e K-alpha-1 and K-alpha-2. If we roughly compare the intensity ratio of three wavelengths ( K-alpha-1:K-alpha-2: K-Beta )it will be 10:5:2. Copper is the most general purpose target and Fe,Cr,Mo etc are also used depending on the sample to be analysed and also on accuracy of the data to be generated.