NRRPT Radiation Fundamentals & Detection 4 — Questions and Answers
Question 1: A sealed radioactive source contains 10 mCi of Cs-137 (T½ = 30 years). Approximately how much activity remains after 90 years?
- 5 mCi
- 2.5 mCi
- 1.25 mCi (Correct answer)
- 0.625 mCi
Correct answer: 1.25 mCi
90 years = 3 half-lives; 10 × (1/2)³ = 10/8 = 1.25 mCi remaining.
Question 2: In a proportional counter, what is the primary advantage over an ionization chamber?
- Simpler circuitry requirements
- Gas multiplication amplifying the primary ion signal (Correct answer)
- Ability to operate without applied voltage
- Lower sensitivity to gamma radiation
Correct answer: Gas multiplication amplifying the primary ion signal
Proportional counters operate in the gas multiplication region where avalanche ionization amplifies the primary signal by factors of 10³–10⁵.
Question 3: The photoelectric effect is the dominant photon interaction in tissue at energies:
- Below about 100 keV (Correct answer)
- Between 1 and 10 MeV
- Above 1.022 MeV
- At all diagnostic energies equally
Correct answer: Below about 100 keV
Photoelectric absorption dominates at low photon energies (below ~100 keV in tissue) and is highly dependent on atomic number (Z³).
Question 4: Which detector is best suited for measuring low-energy beta emitters like tritium (H-3)?
- GM tube with thin mica end-window
- NaI(Tl) scintillation detector
- Liquid scintillation counter (Correct answer)
- Pressurized ionization chamber
Correct answer: Liquid scintillation counter
Liquid scintillation counting dissolves the sample in scintillant fluid, allowing nearly 100% geometric efficiency for weak beta emitters like tritium.
Question 5: What is the significance of the 'dead time' in a radiation detector?
- The time needed to warm up the detector before use
- The period after each pulse during which new events cannot be recorded (Correct answer)
- The decay time of the scintillation light pulse
- The delay between radiation exposure and TLD readout
Correct answer: The period after each pulse during which new events cannot be recorded
Dead time is the minimum time that must separate two events for both to be detected; counts are lost during this interval at high count rates.
Question 6: Which nuclear decay mode is characterized by emission of radiation from nuclear isomers (metastable states)?
- Isomeric transition (IT) (Correct answer)
- Internal conversion (IC)
- Electron capture (EC)
- Spontaneous fission (SF)
Correct answer: Isomeric transition (IT)
Isomeric transition occurs when a metastable nuclear isomer (indicated by 'm') decays to a lower energy state, typically emitting a gamma ray.
Question 7: The Compton edge in a gamma-ray spectrum represents:
- The full energy peak shifted by detector resolution
- The maximum energy transferred to the recoil electron in Compton scattering (Correct answer)
- The energy of backscattered photons from shielding
- The minimum detectable photon energy
Correct answer: The maximum energy transferred to the recoil electron in Compton scattering
The Compton edge is the maximum recoil electron energy, occurring when the photon is backscattered 180° and transfers maximum energy to the electron.
NRRPT Radiation Fundamentals & Detection 4
A sealed radioactive source contains 10 mCi of Cs-137 (T½ = 30 years).
Approximately how much activity remains after 90 years?