CHP Radiation Measurement and Instrumentation 2 — Questions and Answers
Question 1: A thermoluminescent dosimeter (TLD) uses which physical process to record dose?
- Ionization of a gas cavity
- Trapping of electrons in crystal lattice defects (Correct answer)
- Optical absorption in silver halide grains
- Scintillation light emission from NaI
Correct answer: Trapping of electrons in crystal lattice defects
TLDs work by trapping electrons in crystalline defect sites; heating the crystal releases them as measurable light proportional to absorbed dose.
Question 2: The Bragg-Gray cavity theory applies to ionization chambers when the cavity:
- Is large enough to achieve charged-particle equilibrium
- Is small enough not to perturb the radiation field (Correct answer)
- Contains a gas at atmospheric pressure only
- Has walls made of tissue-equivalent material
Correct answer: Is small enough not to perturb the radiation field
Bragg-Gray theory requires the cavity to be small relative to the range of secondary electrons so it does not disturb the fluence of charged particles.
Question 3: Which detector type is best suited for measuring fast neutrons through proton recoil?
- BF3 proportional counter
- Proton recoil scintillator (EJ-301) (Correct answer)
- 3He ionization chamber at thermal energies
- LiF TLD with 6Li enrichment
Correct answer: Proton recoil scintillator (EJ-301)
Organic scintillators such as EJ-301 (NE-213) detect fast neutrons via elastic scattering off hydrogen nuclei, producing measurable proton recoil tracks.
Question 4: Dead time in a Geiger-Müller tube is the period during which:
- High voltage is ramping to operating voltage
- The detector cannot register a new event after a previous pulse (Correct answer)
- The fill gas slowly absorbs ionization electrons
- Background radiation exceeds the signal rate
Correct answer: The detector cannot register a new event after a previous pulse
Dead time is the interval following a pulse during which the detector is paralyzed and cannot produce a new independent output pulse.
Question 5: The energy resolution of a sodium iodide (NaI) detector at 662 keV (137Cs) is approximately:
- 1–2%
- 6–8% (Correct answer)
- 15–20%
- 30–40%
Correct answer: 6–8%
NaI(Tl) detectors typically achieve 6–8% full-width at half-maximum energy resolution at 662 keV, far inferior to HPGe but adequate for many field applications.
Question 6: When calibrating an air ionization chamber for exposure measurement, the calibration factor N_X converts:
- Absorbed dose in water to kerma in air
- Instrument reading to exposure in air (Correct answer)
- Count rate to activity of the source
- Charge collected to dose equivalent
Correct answer: Instrument reading to exposure in air
The exposure calibration factor N_X relates the electrometer reading (corrected for temperature and pressure) to the true exposure in roentgens or C/kg.
Question 7: A survey meter reads 2 mR/h at 1 meter from a point source. At 3 meters, the expected reading is approximately:
- 0.67 mR/h
- 0.22 mR/h (Correct answer)
- 6 mR/h
- 0.11 mR/h
Correct answer: 0.22 mR/h
By the inverse square law, intensity scales as 1/r²: 2 × (1/3)² = 2/9 ≈ 0.22 mR/h.
CHP Radiation Measurement and Instrumentation 2
A thermoluminescent dosimeter (TLD) uses which physical process to record dose?