Measurement of radioactivity MCQs With Answer

Measurement of radioactivity MCQs With Answer is an essential resource for B. Pharm students learning radiation measurement, dosimetry, and safety in pharmaceutical sciences. This concise guide covers core topics such as units of activity (becquerel, curie), detection systems (Geiger-Muller, scintillation, ionization chambers, semiconductor detectors), counting statistics, efficiency, dead time, background correction, and quality control of radiopharmaceutical assays. Emphasis on practical concepts like half-life calculations, decay constant, liquid scintillation counting, energy resolution, and ALARA principles prepares students for lab work and exam success. Clear MCQs reinforce concepts used in clinical radiopharmacy and regulatory compliance. Now let’s test your knowledge with 50 MCQs on this topic.

Q1. What is the SI unit of radioactivity measured as disintegrations per second?

• Becquerel
• Curie
• Rad
• Sievert

Correct Answer: Becquerel

Q2. Which detector is most commonly used for gamma spectroscopy because of good energy resolution?

• Geiger-Muller tube
• Scintillation detector (NaI(Tl))
• High-purity germanium (HPGe) detector
• Proportional counter

Correct Answer: High-purity germanium (HPGe) detector

Q3. The relation A = λN links activity (A) with decay constant (λ) and number of nuclei (N). What does λ represent?

• The half-life of the radionuclide
• The probability per unit time that a nucleus will decay
• The number of decays detected per minute
• The detector efficiency

Correct Answer: The probability per unit time that a nucleus will decay

Q4. If a radionuclide has a half-life of 10 hours, its decay constant λ is approximately:

• 0.0693 h⁻¹
• 0.1 h⁻¹
• 10 h⁻¹
• 0.693 h⁻¹

Correct Answer: 0.0693 h⁻¹

Q5. In counting statistics, the standard deviation for N counts follows which distribution?

• Gaussian with mean N and SD N
• Poisson with SD sqrt(N)
• Binomial with SD N/2
• Uniform with SD 1

Correct Answer: Poisson with SD sqrt(N)

Q6. Dead time in a detector affects measurements by causing:

• Increase in measured count rate at high activity
• Loss of counts and underestimation of true activity
• No effect on counting accuracy
• Only energy resolution changes

Correct Answer: Loss of counts and underestimation of true activity

Q7. A Geiger-Muller counter is best suited for which measurement?

• Precise gamma spectrometry with energy peaks
• High-sensitivity detection of ionizing events for surveys
• Alpha spectrometry with high resolution
• Liquid scintillation beta counting with quench correction

Correct Answer: High-sensitivity detection of ionizing events for surveys

Q8. Which unit measures radiation dose absorbed by tissue?

• Becquerel
• Gray
• Curie
• Bq/kg

Correct Answer: Gray

Q9. In liquid scintillation counting, chemical quench reduces:

• Background radiation only
• Detected light yield and counting efficiency
• Detector dead time
• Half-life of radionuclide

Correct Answer: Detected light yield and counting efficiency

Q10. The minimum detectable activity (MDA) depends on background, counting time, and:

• Detector efficiency
• Half-life of detector
• Manufacturer name
• Ambient temperature only

Correct Answer: Detector efficiency

Q11. Which interaction primarily contributes to the photopeak in gamma spectroscopy?

• Compton scattering
• Pair production
• Photoelectric effect
• Bremsstrahlung

Correct Answer: Photoelectric effect

Q12. The Compton edge in a gamma spectrum is caused by:

• Complete absorption of photon energy
• Maximum energy transfer in Compton scattering
• Electron capture decay
• Detector dead time

Correct Answer: Maximum energy transfer in Compton scattering

Q13. Which radionuclide decay emits a positron used in PET imaging?

• 99mTc
• 131I
• 18F
• 32P

Correct Answer: 18F

Q14. Activity measured in counts per minute (cpm) must be corrected for efficiency to obtain:

• Disintegrations per minute (dpm)
• Mass of sample
• Half-life directly
• Detector voltage

Correct Answer: Disintegrations per minute (dpm)

Q15. Which material is commonly used as a scintillator crystal in clinical gamma cameras?

• Silicon
• Sodium iodide doped with thallium (NaI(Tl))
• Carbon fiber
• Lead tungstate

Correct Answer: Sodium iodide doped with thallium (NaI(Tl))

Q16. Efficiency calibration of a detector requires known standards and consideration of:

• Sample geometry and photon energy
• Ambient lighting only
• Only detector brand
• Half-life of calibration source only

Correct Answer: Sample geometry and photon energy

Q17. The ALARA principle in radiation protection stands for:

• As Low As Reasonably Achievable
• All Levels Are Radioactive Anyway
• Always Limit Accumulated Radiation
• Average Level And Radiation Assessment

Correct Answer: As Low As Reasonably Achievable

Q18. Coincidence summing corrections are important in detector systems when:

• Counting very low energy X-rays only
• Two or more photons are emitted in cascade and detected simultaneously
• Using Geiger-Muller for alpha particles
• Background is zero

Correct Answer: Two or more photons are emitted in cascade and detected simultaneously

Q19. Which process results in conversion of nuclear transition energy directly into an orbital electron ejection?

• Internal conversion
• Beta decay
• Alpha decay
• Electron capture without radiation

Correct Answer: Internal conversion

Q20. Background radiation should be subtracted from sample counts to correct for:

• Dead time effects
• Natural and environmental counts not arising from the sample
• Only gamma-ray energies above 1 MeV
• Detector efficiency drift solely

Correct Answer: Natural and environmental counts not arising from the sample

Q21. Which counter uses scintillation in a liquid cocktail to detect beta emitters?

• Geiger-Muller counter
• Liquid scintillation counter
• Ionization chamber
• Semiconductor detector

Correct Answer: Liquid scintillation counter

Q22. The term “half-life” describes the time required for:

• The sample activity to double
• The activity to decrease by half
• Counts to reach background level
• The detector to reset

Correct Answer: The activity to decrease by half

Q23. Which correction is essential when converting counts per minute to activity in disintegrations per minute in a detector with known efficiency?

• Divide cpm by detector efficiency
• Add background counts only
• Multiply cpm by dead time
• Subtract half-life time

Correct Answer: Divide cpm by detector efficiency

Q24. In gamma spectrometry, energy resolution is frequently expressed as:

• Peak area
• Full width at half maximum (FWHM)
• Dead time
• Counting time

Correct Answer: Full width at half maximum (FWHM)

Q25. Which shielding material is most effective for attenuating gamma radiation?

• Plastic
• Lead
• Aluminum foil
• Wood

Correct Answer: Lead

Q26. For very low-level activity, increasing counting time improves precision because:

• The detector efficiency increases over time
• Statistical uncertainty (relative) decreases with more counts
• Half-life becomes longer
• Dead time increases linearly

Correct Answer: Statistical uncertainty (relative) decreases with more counts

Q27. A proportional counter differs from a GM tube by providing:

• Only qualitative detection
• Energy information about the ionizing event
• No sensitivity to alpha particles
• Infinite dead time

Correct Answer: Energy information about the ionizing event

Q28. Self-absorption in a sample affects measured activity by:

• Causing some emitted radiation to be absorbed before escaping the sample
• Increasing detector efficiency artificially
• Eliminating background completely
• Only affecting beta emitters in vacuum

Correct Answer: Causing some emitted radiation to be absorbed before escaping the sample

Q29. Branching ratio in radionuclide decay describes:

• The ratio of detector counts to background counts
• The fraction of decays following a specific decay mode or emitting a particular radiation
• The energy resolution of the detector
• Time between successive decays

Correct Answer: The fraction of decays following a specific decay mode or emitting a particular radiation

Q30. What is the main advantage of semiconductor detectors (e.g., HPGe) over scintillators?

• Lower cost
• Superior energy resolution
• No need for cooling
• Higher mechanical robustness

Correct Answer: Superior energy resolution

Q31. When calibrating a gamma spectrometer, which factor must be established for each energy of interest?

• Detector brand
• Efficiency at that photon energy
• Ambient temperature only
• Background chemical composition

Correct Answer: Efficiency at that photon energy

Q32. Cherenkov counting detects radiation by measuring:

• Heat produced in the sample
• Light generated when charged particles exceed phase velocity in a medium
• Radioactive decay constant directly
• Alpha particle mass

Correct Answer: Light generated when charged particles exceed phase velocity in a medium

Q33. In a multichannel analyzer (MCA), pulse height analysis enables:

• Counting only total pulses irrespective of energy
• Separation of pulses by energy to produce a spectrum
• Elimination of dead time
• Automatic sample dilution

Correct Answer: Separation of pulses by energy to produce a spectrum

Q34. Which radionuclide is commonly used as a calibration source for gamma detectors at 662 keV?

• 137Cs
• 99mTc
• 131I
• 14C

Correct Answer: 137Cs

Q35. What effect does increasing detector bias voltage generally have on a scintillation detector up to optimum?

• Decreases count rate to zero
• Improves pulse amplitude and resolution until plateau
• Converts gamma rays to alpha particles
• Removes background permanently

Correct Answer: Improves pulse amplitude and resolution until plateau

Q36. An ionization chamber is most suitable for measuring:

• Very low-level single alpha events
• High radiation dose rates and exposure measurements
• Precise gamma energy spectra
• Liquid scintillation quench curves

Correct Answer: High radiation dose rates and exposure measurements

Q37. Electron capture decay often results in emission of:

• Positrons only
• Characteristic X-rays and Auger electrons
• Neutrons
• Only gamma rays above 5 MeV

Correct Answer: Characteristic X-rays and Auger electrons

Q38. The activity of a sample after time t is given by A(t) = A0 e^{-λt}. If λ is large, activity decreases:

• Slowly over long periods
• Rapidly over short periods
• Not at all
• Only when shielded

Correct Answer: Rapidly over short periods

Q39. Which factor is NOT relevant when selecting a counting geometry for efficiency?

• Distance between sample and detector
• Sample container material and shape
• Photon energy of interest
• Detector brand logo color

Correct Answer: Detector brand logo color

Q40. In quality control of radiopharmaceuticals, radionuclidic purity assessment requires:

• Counting only background levels
• Gamma spectrometry to identify unwanted radionuclides
• Only pH measurement
• Visual inspection of vial cap

Correct Answer: Gamma spectrometry to identify unwanted radionuclides

Q41. Which phenomenon reduces accuracy by causing pulse pile-up at high count rates?

• Quenching in liquid scintillation only
• Pile-up leading to distorted pulse heights and lost counts
• Background subtraction
• Detector cooling

Correct Answer: Pile-up leading to distorted pulse heights and lost counts

Q42. What is the primary purpose of using a lead collimator in gamma imaging detectors?

• Increase detector sensitivity indiscriminately
• Define the direction and spatial resolution of incoming photons
• Cool the detector crystal
• Eliminate the need for calibration

Correct Answer: Define the direction and spatial resolution of incoming photons

Q43. Which method corrects for true coincidence loss in PET detectors?

• Dead time extension only
• Coincidence timing windows and prompt/random subtraction algorithms
• Removing NaI(Tl) crystals
• Using GM counter for PET instead

Correct Answer: Coincidence timing windows and prompt/random subtraction algorithms

Q44. What is the effect of temperature on semiconductor detectors like HPGe?

• Requires cooling (often with liquid nitrogen) to reduce electronic noise
• Works better at very high temperatures without cooling
• Changes half-life of radionuclides detected
• Only affects scintillation detectors

Correct Answer: Requires cooling (often with liquid nitrogen) to reduce electronic noise

Q45. The concept of “efficiency” in radioactivity measurement typically refers to:

• Ratio of detected counts to actual disintegrations
• Detector weight per unit volume
• Speed of decay per second
• Background count rate only

Correct Answer: Ratio of detected counts to actual disintegrations

Q46. In portable contamination monitors for surface checks, which mode often distinguishes alpha from beta/gamma?

• Pulse shape discrimination or separate detector windows
• Using only NaI(Tl) crystals
• Measuring half-life on the spot
• Counting time fixed at 1 second always

Correct Answer: Pulse shape discrimination or separate detector windows

Q47. Which decay emits an alpha particle?

• Beta-minus decay
• Alpha decay from heavy nuclei like 226Ra
• Electron capture only
• Positron emission

Correct Answer: Alpha decay from heavy nuclei like 226Ra

Q48. The term “specific activity” refers to:

• Activity per unit mass of a radionuclide sample
• Number of detector channels used
• Only activity measured in Bq without mass information
• Ambient background divided by time

Correct Answer: Activity per unit mass of a radionuclide sample

Q49. When performing gamma counting of a vial, corrections for attenuation by the vial and solution are required because:

• They create additional radionuclides
• They reduce the number of photons reaching the detector
• They increase detector dead time only
• They change the half-life

Correct Answer: They reduce the number of photons reaching the detector

Q50. What is the primary benefit of energy windowing in gamma counting?

• It changes the half-life of radionuclides
• Improves selectivity by accepting pulses in a chosen energy range, reducing background and interfering peaks
• Eliminates need for efficiency calibration
• Makes detectors insensitive to dead time

Correct Answer: Improves selectivity by accepting pulses in a chosen energy range, reducing background and interfering peaks