RSO Viva Questions & Answers: Quantities, Units and Dose Limits

The viva-voce carries as much weight as the written paper in the AERB Level III RSO certification, and examiners almost always start with fundamentals: quantities, units and dose limits. If you are shaky here, the rest of the viva becomes harder. This chapter collects the questions candidates most often report being asked on these topics, with short answers you can say out loud in under a minute.

Use it actively: read the question, answer it aloud, then check. For the exam format, eligibility and registration steps, see our RSO Exam Guide.

Tip: examiners reward precise definitions with correct units. Say the definition, the SI unit, and one practical example, then stop.

Part A: Dosimetric quantities

Q1. Define absorbed dose. What is its unit?

Absorbed dose (D) is the mean energy imparted by ionising radiation per unit mass of matter: D = dE/dm. The SI unit is the gray (Gy), where 1 Gy = 1 J/kg. The old unit is the rad; 1 rad = 0.01 Gy. Absorbed dose applies to any type of ionising radiation and any material, so you should always state the medium (for example, "absorbed dose to water").

Q2. What is kerma, and how is it different from absorbed dose?

Kerma (Kinetic Energy Released per unit MAss) is the sum of the initial kinetic energies of all charged particles liberated by uncharged radiation (photons or neutrons) per unit mass. Its unit is also the gray. Kerma is energy transferred at the point of interaction; absorbed dose is energy deposited locally. The two differ because secondary electrons carry energy away from where they were set in motion, and some of that energy is lost as bremsstrahlung.

Q3. What are collision kerma and radiative kerma?

Total kerma is split into collision kerma, the part of the transferred energy spent in ionisation and excitation near the electron track, and radiative kerma, the part lost as bremsstrahlung. For low atomic number media such as water and air at diagnostic and cobalt-60 energies, radiative kerma is small.

Q4. What is charged particle equilibrium (CPE) and why does it matter?

CPE exists in a volume when every charged particle carrying energy out of it is replaced by an identical one carrying the same energy in. Under CPE, absorbed dose equals collision kerma. This is the basis of cavity theory and of converting ionisation chamber readings to dose. CPE fails near interfaces, in the build-up region of megavoltage beams, and at beam edges.

Q5. Define exposure and its units.

Exposure (X) is the absolute value of the total charge of ions of one sign produced in air when all electrons liberated by photons in a small mass of air are completely stopped, divided by that mass: X = dQ/dm. The SI unit is C/kg; the old unit is the roentgen (R), where 1 R = 2.58 × 10⁻⁴ C/kg. Exposure is defined only for photons in air, and is practically measurable only up to about 3 MeV.

Q6. How are exposure and air kerma related?

Collision air kerma equals exposure multiplied by the mean energy needed to form an ion pair in dry air per unit charge, W/e ≈ 33.97 J/C. So 1 R corresponds to roughly 8.76 mGy of air kerma.

Part B: Protection quantities

Q7. What is equivalent dose?

Equivalent dose (H_T) to a tissue or organ is the absorbed dose averaged over that organ, weighted by the radiation weighting factor: H_T = Σ w_R × D_T,R. Its unit is the sievert (Sv), where 1 Sv = 1 J/kg. The old unit is the rem; 1 rem = 0.01 Sv. It accounts for the fact that some radiation types cause more biological harm per unit dose.

Q8. Give the radiation weighting factors from ICRP 103.

Q9. What is effective dose and why do we use it?

Effective dose (E) is the tissue-weighted sum of equivalent doses in all specified organs: E = Σ w_T × H_T, in sieverts. It expresses non-uniform exposure as a single number linked to overall stochastic risk, so exposures from different situations can be compared with each other and with dose limits. It is a quantity for radiation protection of populations, not for estimating the risk to an individual patient.

Q10. Name the tissue weighting factors in ICRP 103.

The factors add up to 1.0.

Q11. Why can't we measure equivalent dose or effective dose directly?

They are defined inside the human body and depend on organ doses and weighting factors, so no instrument can read them directly. In practice we measure operational quantities, which are designed to give a reasonable, usually conservative, estimate of them.

Part C: Operational quantities and monitoring

Q12. What operational quantities are used for area monitoring?

Ambient dose equivalent, H*(10), for strongly penetrating radiation such as gamma rays, and directional dose equivalent, H'(0.07,Ω), for weakly penetrating radiation such as beta particles. Survey meters used around radiotherapy bunkers are calibrated in terms of ambient dose equivalent.

Q13. What operational quantities are used for individual monitoring?

Personal dose equivalent Hp(d): Hp(10) to estimate effective dose, Hp(0.07) to estimate skin and extremity dose, and Hp(3) to estimate dose to the lens of the eye. Here d is the depth in millimetres below a specified point on the body.

Q14. Where should a personnel monitoring badge be worn, and what if the worker uses a lead apron?

The badge is normally worn on the chest, at the front of the trunk, so it represents the whole-body exposure. When a lead apron is worn, the badge is worn under the apron to estimate the dose to the protected body. Where lens or extremity doses may be significant, for example in interventional procedures, additional dosimeters are used at the collar or on the hand.

Q15. What dosimeter is used for personnel monitoring in India, and how does it identify radiation type?

The TLD badge used in India contains CaSO₄:Dy discs placed behind different filters, for example a metal filter, a plastic filter and an open window. Comparing the readings under the different filters indicates the type and approximate energy of the radiation, and helps judge whether an exposure was real or the badge was contaminated or exposed while not being worn.

Part D: Dose limits and protection principles

Q16. What are the three basic principles of radiation protection?

Justification: any practice involving exposure must do more good than harm. Optimisation: exposures must be kept as low as reasonably achievable (ALARA), economic and social factors taken into account. Dose limitation: individual doses in planned exposure situations must not exceed the prescribed limits. Dose limits do not apply to medical exposure of patients.

Q17. State the occupational dose limits for radiation workers.

Note that ICRP recommends 50 mSv as the maximum in a single year for effective dose; the AERB value for India is stricter at 30 mSv. Examiners like candidates who know the difference.

Q18. What are the dose limits for members of the public?

An effective dose of 1 mSv per year; an equivalent dose to the lens of the eye of 15 mSv per year; and an equivalent dose to the skin of 50 mSv per year.

Q19. What limit applies to a pregnant radiation worker?

Once a worker declares her pregnancy, her working conditions must ensure that the dose to the embryo or fetus does not exceed about 1 mSv for the remainder of the pregnancy. This does not mean she must stop radiation work; it means the RSO must review her duties and monitoring.

Q20. What limits apply to trainees and apprentices aged 16 to 18?

For apprentices and students aged 16 to 18 who use radiation as part of their training, the effective dose limit is 6 mSv per year, with correspondingly lower limits for the skin and extremities. No one under 16 may be employed as a radiation worker.

Q21. What is the difference between stochastic effects and tissue reactions?

Stochastic effects, such as cancer and heritable effects, have a probability that increases with dose, with no assumed threshold; their severity does not depend on dose. Tissue reactions (formerly deterministic effects), such as skin erythema, cataract or infertility, occur only above a threshold dose, and their severity increases with dose. Dose limits keep tissue reactions from occurring and keep stochastic risk acceptably low.

Part E: Practical protection calculations

Q22. What are the three methods of external radiation protection?

Time, distance and shielding. Dose is proportional to time spent near the source; dose rate falls with distance (inverse square law for a point source); and shielding attenuates the radiation. In practice, distance is often the cheapest and most effective, for example using long tongs when handling a source.

Q23. State the inverse square law and give an example.

For a point source with no attenuation or scatter, dose rate is inversely proportional to the square of the distance: D₂ = D₁ × (d₁ / d₂)². If the dose rate at 1 m is 400 µSv/h, at 2 m it is 100 µSv/h and at 4 m it is 25 µSv/h.

Q24. Define half-value layer (HVL) and tenth-value layer (TVL).

HVL is the thickness of a material that reduces the intensity of a narrow beam to one half; TVL reduces it to one tenth. For narrow-beam exponential attenuation, TVL = HVL × log₂(10) ≈ 3.32 × HVL. The number of TVLs needed for a barrier is n = log₁₀(1/B), where B is the required transmission factor; for example, B = 10⁻³ needs 3 TVLs. Broad-beam TVLs used in bunker design are larger than narrow-beam values because of scatter build-up.

Q25. State the radioactive decay law and the relation between decay constant and half-life.

A = A₀ × e^(−λt), where λ is the decay constant. The half-life is T½ = ln 2 / λ ≈ 0.693 / λ. After n half-lives, the activity is A₀ / 2ⁿ; after 10 half-lives, it is about 0.1% of the original.

Q26. Give the half-lives of the common teletherapy and brachytherapy sources.

Q27. A Co-60 source has decayed for 5.27 years. What fraction of its activity remains, and what does this mean for treatment time?

One half-life has passed, so half the activity remains. For the same prescribed dose, treatment time roughly doubles. This is why Co-60 teletherapy sources are typically replaced after about one to two half-lives, when treatment times become too long for clinical practice.

Q28. What are the old and new units of activity, and how are they related?

The SI unit is the becquerel (Bq), one disintegration per second. The old unit is the curie (Ci): 1 Ci = 3.7 × 10¹⁰ Bq = 37 GBq. So 1 mCi = 37 MBq.

Q29. What is the exposure rate constant, and give its value for Co-60.

The exposure rate constant (Γ) is the exposure rate at a unit distance from a point source of unit activity. For Co-60 it is about 1.31 R·m²·h⁻¹·Ci⁻¹. Combined with the inverse square law, it lets you estimate the exposure rate near an unshielded source, which is a common viva follow-up in emergency scenarios.

Q30. Convert the old units to SI units: 500 mrem, 2 rad, 10 R.

500 mrem = 5 mSv. 2 rad = 20 mGy. 10 R = 2.58 × 10⁻³ C/kg, which corresponds to about 87.6 mGy of air kerma.

How to use these answers in the viva

Examiners rarely stop at the first answer. After "define equivalent dose", expect "why is w_R for alpha 20?" or "why is it 1 for photons at all energies?". After dose limits, expect a scenario: "a worker's badge reads 12 mSv in one quarter, what do you do?". Practise extending each answer by one level of reasoning.

Test yourself under exam conditions with our timed RSO mock tests, and compare study options on the plans page. For quick conversions and dose limit look-ups while you revise, use the free Radiation Safety Toolkit.

This chapter reflects ICRP Publication 103 and AERB guidance at the time of writing. Always check the latest AERB directives before your exam, as limits and procedures are revised periodically.