Self-Assessment Quiz — Chapter 29

Test your understanding of the core concepts before moving on. Try to answer each question before checking the solutions at the end.

Q1. (Multiple Choice) What is the approximate total activity of radioactive isotopes in a 70 kg human body?

(a) 44 Bq (b) 440 Bq (c) 4,400 Bq (d) 8,700 Bq

Q2. (Multiple Choice) The single largest source of radiation exposure for the average American is:

(a) Cosmic rays (b) Radon inhalation (c) CT scans (d) Nuclear power plants

Q3. (True/False) Eating a banana significantly increases the total ${}^{40}\text{K}$ activity in your body.

Q4. (Short Answer) Name the three long-lived primordial radionuclides that dominate terrestrial background radiation, and give their approximate half-lives.

Q5. (Multiple Choice) Radon-222 is a health hazard primarily because:

(a) Radon gas is chemically toxic to lung tissue (b) Radon's alpha decay daughters deposit in the lungs and deliver intense alpha radiation to bronchial tissue (c) Radon produces neutrons through nuclear reactions in the body (d) Radon is absorbed into the bloodstream and irradiates the bone marrow

Q6. (Multiple Choice) The EPA action level for residential radon is:

(a) 1 pCi/L (37 Bq/m$^3$) (b) 4 pCi/L (148 Bq/m$^3$) (c) 10 pCi/L (370 Bq/m$^3$) (d) 20 pCi/L (740 Bq/m$^3$)

Q7. (Short Answer) The cosmic ray dose rate increases with altitude. Give the approximate dose rate at sea level and at jet cruising altitude (10,000 m), and explain the physical reason for the difference.

Q8. (True/False) The radiation dose to the nearest resident from a normally operating nuclear power plant is typically comparable to one chest X-ray per year.

Q9. (Multiple Choice) Which medical imaging procedure delivers the highest effective dose?

(a) Chest X-ray (PA) (b) Mammogram (c) CT abdomen/pelvis (d) Dental X-ray

Q10. (Short Answer) Define the difference between deterministic effects and stochastic effects of radiation. Give one example of each.

Q11. (Multiple Choice) The LD$_{50/60}$ (lethal dose to 50% of exposed individuals within 60 days, without treatment) for acute whole-body gamma irradiation is approximately:

(a) 0.5 Sv (b) 1 Sv (c) 3–5 Sv (d) 10 Sv

Q12. (True/False) Under the LNT model, a dose of 1 mSv carries exactly 1/1000 the cancer risk of a dose of 1 Sv.

Q13. (Multiple Choice) The ICRP nominal risk coefficient for radiation-induced fatal cancer is approximately:

(a) 0.5% per Sv (b) 5% per Sv (c) 15% per Sv (d) 50% per Sv

Q14. (Short Answer) State the three principles of radiation protection recommended by the ICRP, and explain each in one sentence.

Q15. (Multiple Choice) The radiation weighting factor $w_R$ for alpha particles is:

(a) 1 (b) 2 (c) 5 (d) 20

Q16. (Short Answer) An occupational dose limit of 20 mSv/yr (averaged over 5 years) is recommended by the ICRP. Calculate the implied additional lifetime cancer risk for a worker who receives exactly this dose for a 40-year career.

Q17. (Multiple Choice) Which personal dosimeter technology provides real-time dose readings?

(a) Film badge (b) Thermoluminescent dosimeter (TLD) (c) OSL dosimeter (d) Electronic personal dosimeter (EPD)

Q18. (True/False) The principal reason the LNT debate cannot be resolved is that scientists refuse to study the problem carefully enough.

Q19. (Short Answer) The "bomb pulse" of ${}^{14}\text{C}$ from atmospheric nuclear testing peaked around 1963. Explain why this pulse is decreasing with a half-life of approximately 16 years, even though the radioactive half-life of ${}^{14}\text{C}$ is 5,730 years.

Q20. (Multiple Choice) A contaminated area has a surface deposit of ${}^{137}\text{Cs}$ (half-life 30.17 years) and an environmental weathering half-life of 5 years. The effective half-life of the contamination is approximately:

(a) 2.5 years (b) 4.3 years (c) 17.6 years (d) 35.2 years

Q21. (Short Answer) A worker receives 0.5 mGy of alpha radiation to the lungs. Calculate the equivalent dose $H_T$ and the contribution to the effective dose $E$, given $w_R = 20$ and $w_T(\text{lung}) = 0.12$.

Q22. (Multiple Choice) The primary mechanism by which radon mitigation (sub-slab depressurization) works is:

(a) Chemical conversion of radon to a non-radioactive gas (b) Creating negative pressure below the foundation to prevent radon entry (c) Filtering radon from indoor air with activated charcoal (d) Sealing every crack in the foundation

Q23. (True/False) Normal operation of a nuclear power plant contributes more radiation dose to nearby residents than natural background radiation.

Q24. (Short Answer) Why is LiF preferred over CaSO$_4$ as a TLD material for personnel dosimetry? Answer in terms of the effective atomic number and its relation to tissue equivalence.

Solutions

Q1. (d) ~8,700 Bq. The body contains ~4,400 Bq of ${}^{40}\text{K}$, ~3,700 Bq of ${}^{14}\text{C}$, and smaller contributions from ${}^{210}\text{Po}$, ${}^{87}\text{Rb}$, and others, totaling approximately 8,700 Bq.

Q2. (b) Radon inhalation, at approximately 2.28 mSv/yr (37% of total). CT scans are second at ~1.47 mSv/yr (24%).

Q3. False. The body maintains potassium homeostasis — excess potassium from a banana is excreted within hours, keeping the total ${}^{40}\text{K}$ activity constant at ~4,400 Bq.

Q4. ${}^{238}\text{U}$ ($t_{1/2} = 4.47 \times 10^9$ yr), ${}^{232}\text{Th}$ ($t_{1/2} = 1.41 \times 10^{10}$ yr), and ${}^{40}\text{K}$ ($t_{1/2} = 1.25 \times 10^9$ yr).

Q5. (b) Radon itself is a noble gas that is inhaled and exhaled. The hazard comes from its short-lived alpha-emitting daughters (${}^{218}\text{Po}$, ${}^{214}\text{Po}$) that plate out on aerosols and deposit in the bronchial epithelium.

Q6. (b) 4 pCi/L = 148 Bq/m$^3$. The WHO recommends a lower reference level of 100 Bq/m$^3$.

Q7. Sea level: ~0.34 mSv/yr (~0.039 $\mu$Sv/hr). Cruising altitude: ~5 $\mu$Sv/hr (~100 times sea level). The atmosphere provides ~1,030 g/cm$^2$ of shielding; at 10 km altitude, most of this shielding has been removed, and cosmic ray secondaries (muons, neutrons, electrons) are far more abundant.

Q8. True. The annual dose from normal nuclear power plant operations to the most-exposed member of the public is typically <0.01 mSv, while a PA chest X-ray delivers ~0.02 mSv.

Q9. (c) CT abdomen/pelvis delivers approximately 10 mSv, compared to 0.02 mSv (chest X-ray), 0.4 mSv (mammogram), and 0.005 mSv (dental X-ray).

Q10. Deterministic effects have a dose threshold and increase in severity with dose (e.g., radiation sickness/ARS above ~1 Sv). Stochastic effects have no proven threshold; probability increases with dose but severity is independent of dose (e.g., radiation-induced cancer).

Q11. (c) 3–5 Sv without medical treatment. With modern supportive care, survival is possible up to ~6–7 Sv.

Q12. True. This is the defining feature of the linear no-threshold model: $R(D) = R_0 + \alpha D$, so risk scales linearly with dose at all dose levels.

Q13. (b) 5% per Sv (= $5 \times 10^{-2}$ Sv$^{-1}$), as estimated by ICRP Publication 103 (2007).

Q14. (1) Justification: Any activity involving radiation must produce sufficient net benefit. (2) Optimization (ALARA): Doses should be as low as reasonably achievable, considering economic and social factors. (3) Dose limitation: Individual doses must not exceed prescribed limits for planned exposure situations.

Q15. (d) $w_R = 20$ for alpha particles, reflecting their high LET and dense ionization damage to DNA.

Q16. Lifetime dose: $40 \times 20\,\text{mSv} = 800\,\text{mSv} = 0.8\,\text{Sv}$. Excess risk: $0.8\,\text{Sv} \times 5\%/\text{Sv} = 4\%$.

Q17. (d) Electronic personal dosimeters provide real-time dose and dose-rate readings with programmable alarms. TLDs, OSL dosimeters, and film badges are all passive integrating dosimeters read out after the wearing period.

Q18. False. The fundamental reason is statistical: the expected cancer excess at doses below ~100 mSv is smaller than the statistical fluctuation in the baseline cancer rate (~25%). Detecting a 0.5% signal against a 25% background would require millions of subjects — far beyond any feasible study.

Q19. The excess atmospheric ${}^{14}\text{C}$ is not disappearing through radioactive decay (which is negligibly slow on this timescale) but is being absorbed by the oceans and terrestrial biosphere. The ocean is a vast carbon reservoir that gradually incorporates the excess ${}^{14}\text{CO}_2$, diluting the atmospheric concentration with an effective mixing half-life of ~16 years.

Q20. (b) 4.3 years. $1/t_{\text{eff}} = 1/30.17 + 1/5 = 0.0332 + 0.200 = 0.2332\,\text{yr}^{-1}$, so $t_{\text{eff}} = 4.29\,\text{yr}$.

Q21. Equivalent dose: $H_T = w_R \times D = 20 \times 0.5\,\text{mGy} = 10\,\text{mSv}$. Contribution to effective dose: $E = w_T \times H_T = 0.12 \times 10\,\text{mSv} = 1.2\,\text{mSv}$.

Q22. (b) Sub-slab depressurization creates a slight negative pressure (~5–15 Pa) beneath the foundation slab, reversing the natural pressure gradient that draws radon-laden soil gas into the building. The radon is vented above the roofline.

Q23. False. Normal operations contribute <0.01 mSv/yr to the nearest resident, far less than natural background (~3 mSv/yr).

Q24. LiF has an effective atomic number ($Z_{\text{eff}} \approx 8.2$) close to that of soft tissue ($Z_{\text{eff}} \approx 7.4$). Because the photoelectric absorption cross section scales approximately as $Z^{4-5}$, a material with $Z_{\text{eff}}$ close to tissue absorbs energy at a similar rate across the photon energy spectrum. CaSO$_4$ ($Z_{\text{eff}} \approx 15.3$) significantly over-responds to low-energy photons (<100 keV) due to enhanced photoelectric absorption, requiring energy-dependent correction factors.