NRRPT Detection and Measurement 2 — Questions and Answers
Question 1: Which detector type operates on the principle of collecting ion pairs produced by radiation in a gas-filled chamber?
- Scintillation detector
- Ionization chamber (Correct answer)
- Thermoluminescent dosimeter
- Semiconductor detector
Correct answer: Ionization chamber
Ionization chambers collect the ion pairs (positive ions and electrons) produced when radiation ionizes the fill gas, generating a measurable current.
Question 2: A Geiger-Mueller tube operating in the 'plateau' region of its voltage-response curve indicates:
- The tube is saturated and no longer counting accurately
- Optimal operating voltage where count rate is stable despite small voltage variations (Correct answer)
- The tube has entered proportional counting mode
- The applied voltage is too low to detect radiation
Correct answer: Optimal operating voltage where count rate is stable despite small voltage variations
The plateau region is the recommended operating range where the count rate remains relatively constant with small voltage fluctuations, ensuring reliable detection.
Question 3: What is the primary advantage of a NaI(Tl) scintillation detector over a GM tube for gamma spectroscopy?
- Higher sensitivity to alpha particles
- Ability to discriminate photon energies and perform spectroscopy (Correct answer)
- Lower cost and simpler electronics
- Better performance in high radiation fields
Correct answer: Ability to discriminate photon energies and perform spectroscopy
NaI(Tl) detectors produce light pulses proportional to photon energy, enabling gamma-ray spectroscopy and radionuclide identification, which GM tubes cannot do.
Question 4: The 'dead time' of a radiation detector refers to:
- The time required for the detector to warm up before use
- The period after a count during which the detector cannot register another event (Correct answer)
- The time between battery replacements
- The delay before the detector's display updates
Correct answer: The period after a count during which the detector cannot register another event
Dead time is the brief recovery period after each detected event during which the detector is insensitive to additional radiation, causing undercounting at high dose rates.
Question 5: A thermoluminescent dosimeter (TLD) measures absorbed dose by:
- Counting electrical pulses generated by ionization
- Measuring light emitted when heated crystals release stored energy (Correct answer)
- Detecting changes in chemical composition of a film emulsion
- Amplifying ionization current with a photomultiplier tube
Correct answer: Measuring light emitted when heated crystals release stored energy
TLDs store energy from radiation in crystal lattice traps; when heated, electrons return to ground state and emit light proportional to the absorbed dose.
Question 6: Which detector is most appropriate for measuring neutron dose equivalent in a mixed neutron-gamma field?
- Proportional counter with BF3 or He-3 fill gas (Correct answer)
- Standard GM tube
- NaI(Tl) scintillator
- Silicon PIN diode
Correct answer: Proportional counter with BF3 or He-3 fill gas
BF3 and He-3 proportional counters are neutron-sensitive via (n,α) and (n,p) reactions respectively, making them standard choices for neutron detection.
Question 7: When calibrating a survey meter, the calibration factor is applied to correct for:
- Battery voltage fluctuations during measurement
- Differences between the detector's actual response and the displayed reading (Correct answer)
- Temperature changes in the measurement environment
- Distance from the radiation source to the detector
Correct answer: Differences between the detector's actual response and the displayed reading
Calibration factors correct the instrument's displayed reading to match true dose rate values established with traceable reference sources.
NRRPT Detection and Measurement 2
Which detector type operates on the principle of collecting ion pairs produced by radiation in a gas-filled chamber?