UPSC Combined Geo-Scientist, Mains · 2024 · Mains Paper III
Q1(e). Properties needed in a detector for X-ray fluorescence
Question
What are the properties vital for detectors to be used in X-ray fluorescence instrument?
Answer in brief
High detection efficiency over the X-ray range used, pulse height proportional to photon energy, good energy resolution, short dead time (linear response at high count rates), low noise and long-term stability.
Key points
- High detection (quantum) efficiency over the range used (about 1–40 keV); a thin, low-Z window (Be, polymer) admits soft X-rays of light elements.
- Proportionality: pulse height ∝ photon energy, allowing pulse-height selection (WDXRF) and energy dispersion (EDXRF).
- Good energy resolution (small FWHM): Si(Li) or silicon drift detector about 130–150 eV at Mn Kα (5.9 keV).
- Short dead time and linear response at high count rates: dead time about 1 µs or less.
- Low noise and low background; semiconductor detectors are cooled (liquid N₂ or Peltier) to lower thermal noise.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
CoversX-ray fluorescenceXRF detectorenergy resolutiondead timeSi(Li)proportional counter
All parts of Question 1
- 1(a) Percentage relative error of a balance reading5 marks
- 1(b) Partition ratio k′ and relative retention α5 marks
- 1(c) Transmittance from Beer’s law5 marks
- 1(d) Identifying a magnesium compound from its thermogram5 marks
- 1(e) Properties needed in a detector for X-ray fluorescence5 marks
- 1(f) Generation of the plasma in the ICP torch5 marks
- 1(g) Calorific value of a fuel5 marks
- 1(h) AAS and ICP compared: accuracy, interferences, detection limit5 marks