Q.18 Which is not a unit of radioactivity?
1. Gray (gy)
2. Roentgen (R)
3. Millicurie (mCi)
4. Pascal (p)
Correct Answer: 4. Pascal (Pa)
Pascal measures pressure (force per unit area), unrelated to radioactivity which quantifies decay rates, exposure, or absorbed dose. This MCQ tests differentiation between radiation-specific units and general physics units, common in GATE Life Sciences radiation biology sections.
Option Analysis
1. Gray (Gy)
SI unit of absorbed dose: 1 Gy = 1 joule/kg energy deposited by radiation in matter. Directly measures radioactivity’s physical effect on tissues.
2. Roentgen (R)
Legacy unit of radiation exposure: quantifies ionization in air (2.58×10⁻⁴ C/kg). Used for X/gamma rays; relates to radioactivity via emitted photons.
3. Millicurie (mCi)
Submultiple of Curie (Ci), the traditional radioactivity unit: 1 Ci = 3.7×10¹⁰ decays/sec (Bq equivalent). mCi = 10⁻³ Ci; directly measures source activity.
4. Pascal (Pa) (Not radioactivity)
SI unit of pressure: 1 Pa = 1 N/m². Used in fluid dynamics, mechanics; zero connection to nuclear decay or radiation dosimetry.
Unit of radioactivity questions test radiation physics fundamentals essential for GATE Life Sciences and medical entrance exams. Standard units measure decay activity (Ci/Bq), exposure (R), and absorbed dose (Gy), while Pascal belongs to mechanics. This guide clarifies distinctions with mnemonics and exam strategy.
Radiation Units Hierarchy
R-E-A-D mnemonic (from NRC):
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Radioactivity: Ci, Bq (decays/sec)
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Exposure: Roentgen (air ionization)
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Absorbed dose: Gray (J/kg)
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Dose equivalent: Sievert (Sv)
Pascal? Absent—it’s Pressure (N/m²).
Unit Comparison Table
| Option | Measures | Radiation-Relevant? | Definition |
|---|---|---|---|
| Gray (Gy) | Absorbed dose | Yes | 1 J/kg tissue |
| Roentgen (R) | Exposure | Yes | Ion pairs in air |
| mCi | Activity | Yes | 3.7×10⁷ decays/sec |
| Pascal (Pa) | Pressure | No | 1 N/m² force |
Conversion Quick Reference
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1 Gy = 100 rad (absorbed dose)
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1 Ci = 3.7×10¹⁰ Bq (activity)
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1 R ≈ 0.01 Gy in air (exposure→dose)
GATE Exam Pattern
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Weightage: 2-3 Qs in Biomolecules/Radiation Biology
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Traps: Confuse Gy (dose) vs Bq (activity); Pa mimics physics units
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Memory Aid: “Get Rid of Pascals—Ci/Bq/R/Gy radiate!”
Pro Tip: All options except Pa appear in radiation safety protocols. Pascal’s inclusion tests unit categorization across physics domains.
