Q.12 Which metal is used for electron beam generation in scanning election nucroscopy?
1. Tungsten (W)
2. Magnesium (Mg)
3. Mercury (Hg)
4. Zinc (Zn)
Answer: Option 1 (Tungsten (W), ID: 68019147823).
Tungsten filaments in SEM electron guns are heated to ~2800K, emitting electrons via thermionic emission due to their high melting point (3422°C) and low vapor pressure, enabling stable beam generation.
Option Analysis
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1. Tungsten (W): Standard hairpin filament cathode; heated to emit electrons; cost-effective for routine SEM imaging.
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2. Magnesium (Mg): Low melting point (~650°C); oxidizes rapidly; unsuitable for high-temperature vacuum electron emission.
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3. Mercury (Hg): Liquid metal; used in lamps, not SEM guns; vaporizes uncontrollably under vacuum heating.
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4. Zinc (Zn): Low boiling point (~907°C); evaporates before reaching thermionic emission temperatures.
SEM Electron Gun Comparison
| Metal | Melting Point | SEM Use | Reason |
|---|---|---|---|
| Tungsten | 3422°C | Yes | Highest stability |
| Magnesium | 650°C | No | Oxidizes |
| Mercury | -39°C | No | Liquid/vapor |
| Zinc | 420°C | No | Low boiling point |
Metal electron beam generation scanning electron microscopy uses tungsten filaments as the standard cathode in SEM electron guns. GATE Life Sciences questions test this fundamental microscopy principle.
Tungsten Filament Physics in SEM
Hairpin design: 100μm tungsten wire bent into V-tip, heated to 2400-2800K. Thermionic emission follows Richardson-Dushman equation: J = A T² e^(-φ/kT), where φ (work function) = 4.5 eV for W.
Why tungsten?
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Highest melting point of all metals
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Low vapor pressure prevents column contamination
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~100 hours filament life before burnout
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Cost-effective vs field emission guns (FEG)
SEM Gun Types Comparison
| Gun Type | Cathode | Resolution | Cost | Applications |
|---|---|---|---|---|
| Tungsten | W filament | 20-50nm | Low | Routine biology samples |
| LaB₆ | Cerium hexaboride | 5-10nm | Medium | Higher resolution |
| Field Emission | W single crystal | <2nm | High | Nanotechnology |
GATE Life Sciences Application
Tungsten recognition: “Electron beam generation” + high melting point metal = tungsten. Links to your microscopy series with colony hybridization (molecular ID) → SEM (structural imaging).
Pro tip: Jaipur lab students—tungsten filaments available locally; change every 50-100 hours based on usage. Master scanning electron microscopy metal properties for guaranteed scores.
