UPSC Combined Geo-Scientist, Mains · 2026 · Mains Paper III
Q1(f). X-ray fluorescence in X-ray diffraction
Question
What is the cause of fluorescence in X-ray diffraction? How does fluorescence phenomenon impact X-ray diffraction data?
Answer in brief
When the incident photon energy exceeds the K absorption edge of an element in the sample (Fe, Co, Mn with Cu Kα), core electrons are ejected and outer electrons fall back, emitting characteristic fluorescent X-rays in all directions. They raise the background and lower the peak-to-background ratio.
Key points
- Cu Kα (8.04 keV) lies above the K edges of Fe (7.11 keV), Co (7.71 keV) and Mn (6.54 keV). Photoionisation, not diffraction, is absorbed; the Kα photon emitted by Fe has a random direction.
- (i) high, sloping background; (ii) weaker peak/background, noisy weak peaks, poorer detection limit; (iii) strong absorption (high mass absorption coefficient) so the intensity from the bulk falls. Peak positions are not changed.
- use Co Kα or Cr Kα for Fe samples; a diffracted-beam monochromator or an energy-discriminating (Si drift) detector; a thin Ni or Fe filter.
The full model answer
Worked out step by step, as it should go in the answer booklet. The solution sheet follows it.
- Cause: Cu Kα (8.04 keV) lies above the K edges of Fe (7.11 keV), Co (7.71 keV) and Mn (6.54 keV). Photoionisation, not diffraction, is absorbed; the Kα photon emitted by Fe has a random direction.
- Effect: (i) high, sloping background; (ii) weaker peak/background, noisy weak peaks, poorer detection limit; (iii) strong absorption (high mass absorption coefficient) so the intensity from the bulk falls. Peak positions are not changed.
- Remedy: use Co Kα or Cr Kα for Fe samples; a diffracted-beam monochromator or an energy-discriminating (Si drift) detector; a thin Ni or Fe filter.
CoversXRD backgroundX-ray fluorescence in X-ray diffraction
All parts of Question 1
- 1(a) Classification of samples by size5 marks
- 1(b) Stationary-phase film thickness in gas chromatography5 marks
- 1(c) Percentage of amine from the absorbance of its picrate5 marks
- 1(d) Principle of vapour-generation AAS5 marks
- 1(e) Purge gases in thermogravimetric analysis5 marks
- 1(f) X-ray fluorescence in X-ray diffraction5 marks
- 1(g) Argon in ICP-MS; acids for arsenic5 marks
- 1(h) MnO₂ in pyrolusite by back titration5 marks