X-ray Techniques in Forensic Analysis: XRD, XRF and X-ray Imaging
UGC-NET Paper 2 Unit II notes on X-ray techniques: production, XRD with Bragg's law, EDXRF vs WDXRF, forensic applications, Indian labs. PYQ pointers and MCQ takeaways.
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X-ray techniques sit in Unit II of UGC-NET Forensic Science (subject code 82) because they cover three of the most commonly tested instruments in a modern crime lab: X-ray diffraction (XRD) for crystalline phase identification, X-ray fluorescence (XRF) for elemental composition, and X-ray imaging for radiographic examination of sealed exhibits. NTA likes this bullet because the underlying physics is short, the equations are clean, and the casework applications (paint, glass, soil, gunshot residue, counterfeit currency, gemstones, illicit drugs) recur across other units.
Treat XRD and XRF as a pair. Both rely on the same wavelength regime, both are non-destructive, both are widely available at Indian CFSLs. What separates them is the question they answer: XRD tells you what crystalline phase a sample is, XRF tells you what elements it contains. Lock that one distinction, learn Bragg's law to one decimal, and you have the bulk of the marks for this bullet.
- Bragg's law
- nλ = 2d sin θ. The condition under which X-rays scattered from successive crystal planes interfere constructively. Foundation of XRD.
- d-spacing
- The perpendicular distance between adjacent lattice planes in a crystal, expressed in angstroms. A diffraction pattern is essentially a list of d-spacings unique to a phase.
- Characteristic X-rays
- Sharp emission lines produced when an inner-shell vacancy in an atom is filled by an outer electron. Their energies are element-specific (Moseley's law) and form the basis of XRF identification.
- K-alpha line
- The characteristic X-ray emitted when an electron drops from the L-shell to fill a K-shell vacancy. Brightest line for most elements; the workhorse line in EDXRF.
- Continuous (Bremsstrahlung) radiation
- Smooth background X-ray spectrum produced when high-energy electrons decelerate in the anode. Element-independent. The 'hump' under the characteristic lines.
- EDXRF
- Energy-dispersive X-ray fluorescence. A solid-state detector resolves the entire emission spectrum at once by photon energy. Cheap, portable, faster.
- WDXRF
- Wavelength-dispersive X-ray fluorescence. An analyser crystal disperses fluorescence by wavelength (Bragg's law again). Slower, costlier, much higher resolution.
- Anode target
- The metal block (commonly Cu, Mo or Rh) that electrons strike inside an X-ray tube to produce the X-ray beam. Choice of anode sets the K-alpha wavelength used for excitation.
X-ray fundamentals: how the beam is produced
Sealed tube, two kinds of spectrum, one cooling problem.
X-rays for laboratory work are produced in a sealed X-ray tube. Electrons boiled off a tungsten filament are accelerated through 20 to 60 kV onto a metal anode. Two distinct spectra come out of the anode at the same time.
- Continuous spectrum (Bremsstrahlung). Electrons that decelerate inside the anode lose energy as a broad, smooth X-ray continuum. The short-wavelength cutoff is set by the tube voltage (λ_min = 12,398 / V, in angstroms and volts). This part of the spectrum carries no element information.
- Characteristic spectrum. Some incoming electrons knock out an inner-shell (K or L) electron of an anode atom. An outer-shell electron drops in to fill the vacancy and emits an X-ray photon at an energy fixed by the anode element. For a copper anode the dominant line is Cu Kα at 1.5418 Å. This is the line used for XRD work.
Two practical points NTA likes to test. First, the anode is water-cooled because more than 99 percent of the input electron energy becomes heat, not X-rays. Second, Moseley's law (√ν proportional to Z) is the physics behind XRF: the characteristic line energy depends only on atomic number, which is why XRF can identify elements regardless of chemical state.
XRD: principles and Bragg's equation
Constructive interference from lattice planes, indexed against ICDD cards.
X-ray diffraction (XRD) works because the wavelength of X-rays (around 1 Å) is comparable to the spacing between atomic planes in a crystal. When a monochromatic X-ray beam hits a crystalline sample at angle θ, reflections from successive planes interfere constructively only when path difference equals an integer number of wavelengths. That is Bragg's law:
nλ = 2d sin θ
where n is the diffraction order (usually 1), λ is the X-ray wavelength, d is the spacing between lattice planes, and θ is the angle of incidence (half the deflection 2θ recorded by the detector). The output is a diffractogram: a plot of intensity against 2θ, with sharp peaks at angles where Bragg's condition is met.
Every crystalline phase has its own unique set of d-spacings, catalogued in the ICDD PDF (Powder Diffraction File) database. Phase identification is a pattern-matching exercise: measure the d-spacings of the unknown, look them up, name the phase. A typical Indian forensic XRD bench (CFSL Pune physics division, for instance) runs a copper-anode source (Cu Kα, 1.5418 Å), a graphite monochromator and a scintillation or position-sensitive detector, scanning 2θ from about 5 to 80 degrees.
What XRD answers in forensic casework: what crystalline phase is this? Examples include distinguishing α-quartz from amorphous silica in a soil sample, confirming the pigment in a paint chip (rutile vs anatase TiO₂, lead chromate vs lead carbonate), identifying explosives (RDX, PETN, ammonium nitrate) by their lattice pattern, and confirming the active ingredient in seized tablets without dissolving them.
XRF: EDXRF vs WDXRF
Same photoelectric effect, two ways to sort the fluorescent photons.
XRF asks a different question from XRD: what elements are in this sample, and roughly in what proportion? A primary X-ray beam knocks out an inner-shell electron from the sample atoms. The vacancy is filled from an outer shell, and a secondary (fluorescent) X-ray is emitted at an energy characteristic of the element. Sort those secondary photons by energy or wavelength and you have an elemental fingerprint.
The two flavours of XRF differ only in how they sort the fluorescent photons.
| Feature | EDXRF (Energy-dispersive) | WDXRF (Wavelength-dispersive) |
|---|---|---|
| Sorting method | Solid-state Si(Li) or SDD detector resolves photons by energy in real time | Analyser crystal (LiF, PET, TlAP) disperses photons by wavelength using Bragg's law; goniometer scans |
| Resolution | ~ 130 to 150 eV (Mn Kα) | ~ 5 to 20 eV; much better separation of overlapping lines |
| Speed | Whole spectrum captured simultaneously, seconds per sample | Sequential or simultaneous on multiple channels; slower |
| Cost and portability | Lower cost, available as handheld units | Bench-top only, higher cost, requires stable power |
Forensic applications across exhibit types
One technique each, mapped to the question it actually answers.
Most casework uses XRD and XRF together. XRF tells you the elements; XRD tells you the crystalline phase those elements are arranged into. The exhibit categories below recur in NET MCQs and short-answer questions.
| Exhibit | Question to answer | Best technique | Why |
|---|---|---|---|
| Paint chip (hit-and-run) | Is the chip on the victim's clothing from the suspect vehicle? | XRF for pigment elements (Ti, Pb, Cr, Fe); XRD for the crystalline pigment phase | Layer-by-layer pigment chemistry plus phase is highly individualising |
| Glass fragment | Container glass vs window vs vehicle glass? | XRF for refractive-related oxides (Si, Na, Ca, Mg, Al); SEM-EDX for trace | Float vs container glass differ in trace element profile |
| Soil / sand | Did this soil come from the crime scene area? | XRD for mineral phases (quartz, calcite, feldspar); XRF for major oxides | Mineral assemblage is a strong geographic discriminator |
| Gunshot residue (GSR) | Did the suspect fire a weapon? |
Indian institutional context
Where in India this work actually happens.
For the institutional MCQ angle, three names cover most of what NTA asks.
- CFSL Pune (Physics Division). Houses both XRD and XRF benches for paint, glass, soil and counterfeit-currency casework. The Physics Division is the canonical Indian forensic home for crystalline and elemental examination.
- BARC (Bhabha Atomic Research Centre), Mumbai. Runs synchrotron-grade X-ray facilities (and accesses the Indus-2 synchrotron at RRCAT Indore for collaborative work). Trace-element work referred from forensic labs sometimes ends up here when sensitivity matters.
- NPL (National Physical Laboratory), New Delhi. The national metrology institute. Calibrates XRF and XRD standards used across Indian forensic labs and runs certified reference materials.
Also worth knowing for institutional MCQs: NFSU Gandhinagar has XRD and XRF in its Forensic Physics laboratory; the GSI (Geological Survey of India) runs XRD as its routine mineral-identification workhorse and sometimes assists forensic labs with soil cases; the AERB (Atomic Energy Regulatory Board) regulates the use of X-ray tubes and radiation sources in any forensic lab under the Atomic Energy (Radiation Protection) Rules, 2004.